The BillaVista 60

Bomb Proof Dana 60
Part 1a- The Tech Behind the Talk
Steel and Material Strength
By BillaVista

The BillaVista-60 Super Dana 60 Front Axle Project.

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The title of this part, "The Tech behind the Talk", says it all.  Part 1a will examine in some detail the subject of "Materials Science", focusing primarily on steel and other ferrous metals. It will attempt to educate the reader on the composition of steel, it's properties and how to properly interpret them, as well as examine the many different types of steel and their uses, and much, much more.  As usual, I also hope to dispel some of the vast amount of misinformation that exists surrounding the subject.  The purpose of all this is threefold:

1) First and foremost to educate the reader, so that they may be better armed to wade through all of the marketing hype involved with 4x4 products.  Armed with this knowledge the reader should be better able to determine for themselves who builds and markets the "best" or "strongest" parts, which manufacturer's really know what they're doing, and who is just "blowing smoke" and trying to capitalize on the latest marketing buzz-words like "alloy", and "chrom-moly".

2) As a secondary goal, I hope to present enough information to assist the reader in selecting material most appropriate for their own construction and fabrication projects, at least within the confines of the disclaimer.  At the very least I hope to illuminate the subject enough to motivate the reader to further research or at least begin to ask the right questions.

3) Part 1b is a special section that focuses on a subject that is near and dear to my heart, as well as being one of the least understood yet most frequently asked about topics of all time - axle shaft technology.

Why would I write this, and why should you care?
1) Breaking things sucks!
2) Paying more for something than is necessary sucks!
3) Getting conned by flashy advertising that can't be backed up - sucks!
3) Something not being as safe as it should be, sucks!
4) Misinformation sucks!

It is the intention of this article to address these issues by arming you, the reader, with the ability to calculate where possible, and judge where required, what would be a reasonable material choice for your project, the best material for the parts you wish to buy, etc.

Why steel, in particular?
Simply because, in my humble opinion, it is the greatest material mankind has for construction. It is cheap, strong, readily available, easily cut, joined, and formed. Wood can be light and stiff, but not very strong. The best aluminium is strong and light, but very difficult to join. Titanium is superb in terms of strength to weight ratio and stiffness – but it’s incredibly expensive, difficult to obtain, and even more difficult and expensive to machine properly. There’s no way you’re ever going to perform a battery-weld field-fix on a part made from 7075-T6 aluminium or titanium! In the end – we come back to steel – from mild carbon to some of the more exotic alloy steels – pound for pound it is the most righteous material available for our needs.


This article is written by the layman (me) for the layman. I am not an engineer, physical chemist, physicist, or metallurgist. Neither am I a machinist, millwright, or certified welder. I am a shade-tree fabricator – just like the majority of you. I can cut, shape, and join metal – subject to the limitations of my limited and non-professional tool collection. All of which is to say – USE THE INFORMATION HERIN WITH CAUTION - AND AT YOUR OWN RISK. This article is not an engineering text, it is not a specific guide to anything – it is background knowledge that you will have to apply yourself, in a manner in which you choose.


Throughout this article I have borrowed heavily from many texts, and 3 in particular.   I encourage everyone with any interest in the subject to purchase and read these fine books, as they are chock full of excellent information (much more than I can cover here), and well written.  They are:

Engineer to Win : The Essential Guide to Racing Car Materials Technology or How to Build Winners Which Don't Break
by Carroll Smith
Paperback - 280 pages (April 1985)
Motorbooks International; ISBN: 0879381868 ;
Machinery's Handbook Tool-Box Edition
by Erik Oberg (Editor), Christopher J. McCauley (Editor), ricca Heald, Franklin Day Jones, Henry H. Ryffel
2640 pages 26 edition (April 15, 2000)
Industrial Press, Inc.; ISBN: 0831126256 ;
High Performance Hardware : Fastener Technology for Auto Racers and Enthusiasts
by Forbes Aird
Paperback - 192 pages 1 Ed edition (April 1999)
Berkley Pub Group; ISBN: 1557883041 ;

Much technical data was also gleaned from the following sites:

I am also greatly indebted to the following technical experts, most of them professional engineers, all of them gentlemen, and valuable members of the forums.  Without their selfless sharing of technical facts and information I would never have been able to understand all of this:

Thanks to:

Gordon, PIG, Ed Stevens, Dave Kamp

And very Special thanks to:

Robin Ansell, Goat1 and lt1yj


Part 1 - The Tech behind the talk is laid out as follows:

Section 1 - Steel basics: what it is, where it comes from, its structure, and why we use it so much

Section 2 - The concept of stress and strain,

Section 3 - The critical definitions - malleability, hardness, toughness, etc.

Section 4 - The steels of interest to us - mild, carbon, alloy steel etc.

Section 5 - Steel manufacturing - alloying, cold rolled, hot forged, etc.

Section 6 - Heat treating and hardness

Section 7 - Design principles - the flow of stress, how removing material can increase strength, shapes are strong, etc.

Section 8 - Fatigue

Section 9 - Dispelling myths / FAQ - hollow vs solid, DOM vs HREW, square vs round, is pipe only for poop?

Section 10 - Equations

Section 11 - Tables

Section 12 - Glossary

Section 13 - Sources and Notes

Section 1: Steel basics

What is steel?
Steel is a metal. The Merriam-Webster online dictionary defines it a metal as:
any of various opaque, fusible, ductile, and typically lustrous substances that are good conductors of electricity and heat, form cations by loss of electrons, and yield basic oxides and hydroxides;

Yea, great – but really – what is a metal, in terms of what we care about? Metals are materials that:

Steel, in my humble opinion, is the king of all metals, having the best of all these properties - especially considering its weight and cost.

So, in summary, steel is a metal that can easily be formed into useful parts, can resist high levels of load or force without breaking, and can change shape (bend) in response to a load and spring back to original shape.

Why steel?
How is it that steel has such wondrous properties?  The answer to this question alone could fill several books.  I will attempt to give a brief answer, something we can refer back to when we need, in order to help our understanding.  My answer certainly won't make metallurgists of you!

As, noted, the reason steel is so strong and yet flexible and formable is very complicated, but the answer all boils down to it's atomic structure and the building blocks this atomic structure leads to, that all interconnect and build on one another until we have the actual steel.

Let me explain a bit further.

Not surprisingly, the answer lies deep within its atomic structure. All I will say here, is that metals are made up of atoms that have strong flexible atomic bonds, lie in very close formation and that are arranged in a regular and repetitive fashion into crystal unit cells of various shapes which are, in turn, built up, Lego-like, on a regular and repetitive 3-dimensional lattice structure into crystals of metal called “grains”. The nature of these atomic bonds, and the resulting crystal lattice structure, is what lends metals, and particularly steel, it’s combination of strength, hardness and malleability.  In a gross over-simplification, we can say that steel atoms bond and form crystals, small crystals, or "grains" of metal join together to form a crystal lattice structure, much like bricks join together to form a wall. 

Continuing our brick wall analogy: the nature of the crystal lattice structure - i.e. how regular the grains are in shape and orientation - determines the properties (and chiefly of interest to us, the strength) of the metal.  In much the same way as the shape and orientation of the bricks in a wall determine the properties or strength of the wall.  Compare an old-fashioned stone wall constructed from stacked boulders and stones to a modern wall of precisely arranged rectangular concrete blocks.

There are many factors that determine the size, shape, and orientation of the grains and crystal lattice structure, many of which we can manipulate.  It is these factors and their manipulation which really interests us.  The factors range from the atoms themselves i.e. the composition or "alloy" of the steel, to the shape and orientation of the grains - affected by things like forging, heat treating, cold working etc.

One more brick-wall analogy: The bricks, their size, shape, and orientation to one-another determine not only how strong a wall is - but also in what way it is strong.  For example, an old fashioned rock wall may have a great deal of strength if we bear down on it from above - i.e. set something heavy on it and it will easily support the load.  However, lean against it sideways and it may easily topple over.  In much the same way, the different ways in which we manipulate and treat steel can impart many different properties or "strengths" from compressive strength to shear strength - more on this later.

Ultimately, it's all in the atomic and inter-atomic bonds. How we forge the steel from ore, what alloying elements we add to it, how we "work" it, and even how we machine and weld the final product will all have an impact on the atoms/grains/crystal lattice structure - and therefore strength and properties of the steel.  Understanding how this happens and why is chiefly the goal of this article.

As a point of trivia, there is no such thing as a “molecule” of steel. In fact, metals, as a family of elements, are distinguished from other elements in part because their crystalline structure is made up of individual atoms (they are monatomic) as opposed to molecules.

Where does steel come from?

Steel is not a naturally occurring substance - it is entirely man made.  Steel is chiefly a combination of two naturally occurring elements: iron and carbon (along with small amounts of other elements - depending on the steel in question).  The process by which man makes steel, would, again, fill several volumes.  Here is my amateur synopsis:

Iron is mined from the ground in the form if a reddish-brown rock called iron-ore.  This ore is then smashed up, strained, filtered, chemically treated etc, until ultimately it is melted in huge blast furnaces into something called pig iron.  The process uses coke (a type of coal), which in turn imparts large amounts of carbon to the pig iron.  As a result, pig iron itself is full of impurities, brittle, and unmachinable - practically useless.  Except - it is the raw material from which all other irons and steels are made.  Pig iron is so produced in either huge vats of molten material, or it is cast into ingots (in fact, pig iron got it's name because the ingots or "chunks" produced were thought to have resembled piglets).

Pig iron is then refined into either metallic iron or steel using specialized furnaces and processes.  The distinction between the two is that metallic iron has between 2-6% carbon content, and steel has <2% carbon content.  Of course metallic iron is further refined into, and categorized as, many different types of iron - from grey cast iron to nodular iron; in much the same way as steel is further refined into and categorized from low carbon steel to alloy steel.  We'll explore all of these in detail later. Metals like iron and steel, being largely composed of elemental iron, are known as "ferrous" metals after the chemical symbol for elemental iron - Fe.

A final word about carbon.  carbon is critically important to our whole discussion because it is the presence of carbon that turns the element of iron that is naturally soft and weak, into the strong, rigid materials we know as iron and steel. Precisely how this is so is beyond the scope of this article,  suffice to say:

The strength, hardness and toughness that make the ferrous based metals useful to us are profoundly influenced by the remarkable sensitivity of the physical and chemical properties of iron crystals to relatively small percentages of carbon dissolved within their matrixes (actually, the sensitivity is to the movement of dislocations within the crystal space lattice). This sensitivity to dissolved carbon is in fact, the very basis of ferrous metallurgy. [1]

A poor, but perhaps useful metaphor may be the use of fibre-mat and resin in fibreglass work.  The bulk raw material of fiberglass is the fibre matting (as iron is to steel) - but by itself the matting is of no practical use.  Not until we add the resin to it to make fibreglass (as we add carbon to iron to make steel) do we get a useful product.  In both cases, neither raw material is much use alone, but combine them and we really have something.  Similarly, though carbon may only be present in small quantities, just as the amount of hardener added to fibreglass resin has a profound effect on the material, so does the small amount of carbon present in useful metallic iron and steel.

Section 2: Stress and Strain

You may have noticed that we have already bantered around a bunch of terms that we really should define - just so we're on exactly the same page. And there's many more terms to come.  So lets lay out some initial definitions now.

Strength - a measure of how strong something is! ha ha!  Seriously, this is a very important definition, as this entire series of articles is, in large part, just about how strong things are.  Not only that, but the truth is not nearly as simple as my little joke above would lead you to believe.

In the broadest of terms, when we speak of a substance or products strength, we are talking of it's ability to resist an external force or load, without deforming, breaking or rupturing.  Technically we say a materials strength is the greatest stress it can endure without rupturing (by rupturing - I mean the atomic bonds coming completely apart.)

There are many specific kinds of strength, from the "pure 3" - tensile, compressive, and shear to complicated combinations such as torsional and bending. We will examine each of these in detail throughout the article.

Stress.  We all know instinctively that, generally speaking, if we have 2 things made from the same stuff, that the larger will be "stronger".  In order for us to be able to discuss the strength of material, and particularly to compare different materials, we need a way to compare strength without constantly referring to how big something is.  That is, we need to be able to eliminate size as a factor.  We do this by employing the concept of stress.  Stress is a force or load applied, divided by how big the part is, in other words force per unit of cross sectional area. It is common to measure the force applied in pounds and the cross sectional area in square inches.  Thus, the unit for stress is pounds per square inch, or PSI 

Using the concept of stress, we can now compare relative strengths regardless of size.  Say we have 2 steel bars, one is 1 square inch in cross sectional area, the other 1/2 sq. in.  If they are both made from a material having a breaking strength of 10,000 psi, one will break when 10,000 lbs is applied, the other when 5,000 lbs is applied, despite that they are made from the same material.  Conversely, if we have 2 products of unknown or difficult to determine size but we know that one is made from a material with a yield strength of 75,000 psi and the other 100,000 psi, we know that the second is made of a stronger material (regardless of comparative size), and we can also say that the first would have to be 25% greater in size to be of equal strength.  This logical approach can lead us to quite accurately determine that, if the sizes are similar, the second is definitely the stronger product.

Strain - closely allied to the concept of stress - the concept of strain allows us to quantify or describe how a part or material responds to an applied force or load.  Quite simply: Strain is a change in shape or dimension in response to a stress.  It is usually expressed in percent elongation (%) Percent elongation is the difference in length between the original length of a test specimen (often 2" in length) and that same specimen after it has been ruptured by a tension load.

In other words, materials STRAIN to resist a STRESS (much like people do to!).

Stress and strain are two separate, useful concepts but the real power of these concepts is only realized when the 2 concepts are combined.  There is a defined relationship between stress and strain, discovered by an English bloke in 1680, that states the strain of any material is proportional to the stress within it. It is known as Hooke’s law and is simply stating what you already know – “the harder I push on this, the more it will bend……”. What’s important though, is that, up to a point, this relationship is proportional, or linear…meaning that if I push twice as hard, it will bend twice as far.  The point up until which this is true is called the elastic limit.  Realize that Hooke's law applies not only while a load (stress) is applied, but also when it is removed.  Meaning, as long as we are within the elastic limit of the material (i.e. the stress is less than the elastic limit of the steel) strain is always proportional to stress, meaning if stress (load) is zero then so too is strain (distortion).  What we have just described is one of the most righteous properties of steel.  That is, it is "elastic" in nature - which is a fancy way of saying: up to a point, we can bend it by applying a load, then if we remove the load it will "spring" back to exactly the same shape and size it was originally.  We call this elastic deformation.  Of course, we can apply a stress that exceeds the elastic limit of the steel, and if we do, the steel changes from elastic in nature (able to spring back) to plastic in nature - meaning the stress applied, once it passes the elastic limit, will result in a permanent change in shape of the metal.  This we call plastic deformation.  Another quote from Mr Smith is in order:

A solid is considered to be elastic if, after a change of shape due to an external load, the body returns to its original size and shape when the load is relaxed.  Plasticity, in the metallurgical sense of the word, is the ability of a metal to be deformed beyond its range of elasticity without fracture; the result is a permanent change in shape.  These two related properties are the most significant of all the characteristics of the family of metals.  Plasticity gives us the ability to form metals into useful shapes and elasticity allows us to use metal fabrications as load-bearing members in our structures. [2]

You can begin to see now, how with a little knowledge, and an idea of the stress/strain diagram we can solve forever the arguments of whether something is stronger than something else, whether it is "too brittle" or will "bend before it breaks" etc.

Let's examine a sample, theoretical graph of stress vs strain, called a stress/strain diagram

*Diagram concept adapted from "High Performance Hardware". Forbes Aird; Berkley Pub Group, 1999 p.14

There are a few important concepts to note here:

Comparative stress/strain diagrams

Now that we have a good idea of all that the stress/strain diagram illustrates, it is extremely educational to examine some comparative stress/strain diagrams.

*Diagram concept adapted from "Engineer to Win". Carroll Smith; Motorbooks International, 1985 p.41

Examining this diagram carefully, we can learn much about the properties of different materials, and ultimate ly that materials suitability for a given part.

Note: Again, this diagram is for illustrative purposes, the actual scale of numbers used are not real, but are for illustration only - the relative shape and size of the curves between the different materials is real however.  It is the concepts that are important.

A good understanding of what is really going on here can help us understand much of the "common wisdom", as well as the persistent myths that are out there regarding steel and iron parts - from the "grade 8 bolts are too brittle" nonsense to other commonly held misconceptions about cast iron and steel.

The differing yield strengths of the different materials are indicated by the blue arrows, and the ultimate strengths the red arrows.


These last 2 points are the root of what I see as one of the most often misquoted and most poorly understood concepts of material strength - the interrelationship between ductility and strength. 

Take for example the classic misconception/myth: "Grade 5 bolts are a better choice than Grade 8 because the grade 8 are too brittle and will snap, while the grade 5 will bend before breaking". 

By examining the above stress/strain diagram and imagining the grade 5 bolts represented by the "mild steel" curve and the grade 8 bolt by the "alloy steel' curve (accurate enough for our purposes) we can clearly see both the underlying "truth" in the myth, as well as the great error that ultimately leads it to be an incorrect statement and a very poor guide for bolt choice in all but a very few cases.

First the "truth".  We can see from the curve, that indeed, the grade 5 bolt will deform more (exhibit greater strain) between yield point and rupture.  However, the myth neglects two critical factors that become abundantly clear when looking at a stress/strain diagram:

And finally, one more kernel of "truth" to the myth.  The one place the grade 5 may be superior is in the area of "impact strength" or "toughness".  By impact strength we mean the ability to withstand a sudden impact or shock load. An example might be when a snowplow running through powder at 30 mph suddenly hits an 8" concrete curb (ouch!) The loads imposed by such a sudden impact are exponential, off the chart, and hard to calculate or predict accurately.  Remember that we said that the total area beneath the curve is an indication of the materials toughness, and that toughness was the ability to absorb energy?  This ability to absorb energy is exactly what we need in the case of a severe impact shock load, and we can see that the area beneath the "grade 5" curve is greater than beneath the "grade 8" curve.  Having said that though - the grade 5 bolt subjected to a shock load is ONLY a better choice because it may allow you to limp home without the plow (or whatever) separating from the truck - the bolt will almost certainly still have passed the yield point and has technically "failed" and therefore requires replacement.

This case study is but one example of how we can apply a true and accurate understanding of materials, specifically stress/strain diagrams, to either our own part fabrication or our judgement of which part is really "strongest", "best", or "most suitable".

Essentially, the choice of a material for any given structural duty or load bearing part boils down to a considered compromise or trade off between how high and steep the curve is (the yield strength) and how long it is (how much it can deform before rupturing), and the height between yield and ultimate strength (how much more stress it can take since yielding before rupturing)

Ultimately, we can label a stress/strain diagram slightly differently to help us understand the qualitative and quantitative differences between different materials.  I have chosen to do so like this:

One final point - remember - it is the whole stress/strain curve that really describes how a material will react under load - the size, shape, etc. - NOT just any singular value (this, in fact shows the fallacy we indulge in when we blithely compare products by quoting just a single property - like ultimate tensile strength or the like).  Compare, if you will, the following 2 curves for 2 different, completely hypothetical materials. Note from the curves that, though they share very similar values in yield strength and even similar ultimate strength, they are VERY different materials and will react under load in very different ways - making them potentially much more or less suitable for any given use.

Study the diagram, and once you completely see and understand this, you are ready to proceed, young grasshopper!

Section 3 - The Critical Definitions

Now, it would be impractical for us to try and communicate about different materials and parts simply by comparing stress/strain diagrams, despite the great amount of information they portray; not to mention how complicated it is to collect the data to plot an actual graph.  As such, we have a whole vocabulary surronding metal parts that we use to describe the properties of various steels, and the parts made from them.  Here are some of the most important definitions.  Note how they can all be derived from, illustrated by, or relate back to the stress/strain diagram.

Hardness – is the property of resisting penetration. Normally, the hardness of steel varies in direct proportion (i.e. as one gets bigger so does the other and vice versa) to its strength – the harder it is, the stronger it is, and vice-versa.

Brittleness – is the tendency of a material to fracture without changing shape. Hardness and brittleness are closely related. The harder (and therefore stronger) a metal is, the more brittle it is likely to be. Materials that are too brittle will have very poor shock load resistance.

Malleability – is the opposite of brittleness. The more malleable a material, the more readily it can be bent or otherwise permanently distorted. As hardness was closely related to strength, so then is malleability. Generally, the more malleable a metal, the weaker it is.

Ductility – much like malleability, ductility is the ability of the material to be drawn (stretched out) into thin sections without breaking. The harder and stronger a metal is, the less ductile, and vice versa.

Toughness – The ability of a metal to absorb energy and deform plastically before fracturing. It is usually measured by the energy absorbed in an impact test. The area under the stress-strain curve in tensile testing is also a measure of toughness.

You can see how there is a trade-off between a metal’s malleability/ductility and its hardness/strength (which makes perfect logical sense – since malleability is the ease with which we can form it, by applying force, and strength is its ability to resist force). There are a whole group of people employed in a field called “physical metallurgy” whose job it is to figure out how to use things like alloying, heat treatment, and cold-working to skew this relationship (malleability/strength) in our favour, so that hopefully we can develop materials that are both strong and malleable.  It will, of course, come as no surprise that their best efforts cost the most money - aint it always the way! We'll be looking at what they've come up with so far in a little while.

Before we begin our examination of specific steels and treatments, there are a couple more principles we need to get under our belts - the modulus of elasticity and the concept of stiffness.

Modulus of elasticity and the concept of stiffness.

Intuitively we all know that load bearing parts must be strong, must be built from material that is strong.  But there is more to it than that.  We already defined strength as the greatest stress a material can endure without rupturing. But that's not all - for given just that definition - we could decide to build load bearing parts out of soft materials that are strong - like copper and rubber - they can tolerate large loads without rupturing.  However, we know instinctively they wouldn't be suitable for building bridges, buildings, crankshafts, frames and axles - they are too flexible and while not rupturing under the load - they would deform too much.  So - what we need is a material that while strong, will also resist deforming under load - we call this property STIFFNESS.  We need load bearing parts made from strong (so they don't break) and stiff (so they can bear the load without excessive deformation) materials.

How do we determine what materials are suitably "stiff" as well as being strong?  The problem is complicated by the fact that how stiff something is, is a function of not only the material, but also the shape. For example, take a piece of paper - not very stiff is it?  Now roll it into a tube and press it together lengthwise  - pretty stiff eh? In much the same way as we use the concept of stress to isolate strength due to material from strength due to size; so we need a way to isolate stiffness due to structure (shape) and stiffness due to material.  Another English guy figured it our for us. In 1800 English physicist Thomas Young discovered that he could rewrite Hooke's law to read "for any material, stress divided by strain is equal to a constant".  What this deceptively simple statement hides is the very fabric of all structural design engineering - namely that elastic materials, such as steel, all have a unique "constant of elasticity" that describes the materials elasticity - it's ability to spring back into shape after a stress is removed. This constant of elasticity is a measure of how stiff the material is - the larger the constant, the larger the stress than can be applied without exceeding the elastic limits of the material, and the stiffer the material. 

This "constant" is unique and constant for each material, and is known as the Young's modulus or the "modulus of elasticity" of the material.  In order to be able to compare materials, in much the same way as there is s standard way to measure yield strength (stress required to produce 0.2% elongation), there is a standard way to calculate Young's modulus.  Young's modulus is defined (and calculated) as the the stress required to double the length of a test specimen.  Obviously this is a theoretical value, useful only for comparing materials stiffness, as no structural material on earth can actually be doubled in length without rupturing.  The modulus of elasticity for all steels is about 30, 000, 000 psi !!!!

Stop!  Read that again!  Yes, that's right - the modulus of elasticity is the same for ALL steels.  This means they are all comparably stiff, they will ALL resist bending or twisting about the same amount - from cheap pipe to expensive cr-mo tubing.  The difference between them is what happens when they bend - in other words, the stress/strain curves are different. The better, more expensive steels, due to their much higher yield points and greater elastic ranges, will be able to easily shrug off the load, wheras the lesser material may yield (take a permanent set, or bend) or actually rupture.  This is very important, and hugely misunderstood concept, so I'll repeat it.  All steels will bend or twist the same amount under the same load - the difference is in how they handle this loading - good steel will "spring back", poorer steel will bend permanently or break.

Mathematical proof of this comes from the equation used to determine how much a tube will deflect under a given load. The equation is:

P*L / (E*I)


P = the load (force) placed on the tube (lbs)
L = the length from where the tube is supported to where the load is applied (in)
E = modulus of elasticity (same for all steels)
I = Moment of Inertia.

In comparing 2 tubes, the only factor in this equation that can change is the Moment of Inertia, I

The formula for calculating I for any tube is:

I = (0.049*OD^4) - (0.049*ID^4)


OD = the outside diameter of the tube
ID = the inside diameter of the tube

Note that the equation does not take into account anywhere the type of steel the tube is made from.  The factors that effect how much the tube will deflect, or bend, are just the OD and ID of the tube (or OD and wall thickness if you prefer, it amounts to the same thing); with OD being the much more powerful determining factor.

Now, you might reasonalby ask - why build anything of expensive steels then? Where are the equations to deal with that?  Remember, the tube will deflect the same amount, but whether it survives that deflection is where we get into the difference between steels, using the concept of yield strength.

How much load the tube can handle before yielding (changing shape permanently) is calculated as:

Ld = 2*I/OD*Fy/L


I = I = Moment of Inertia
OD = the outside diameter of the tube
L = the length from where the tube is supported to where the load is applied (in)
Fy = the yield strength of the steel in question.

This last factor, Fy, is where the different steels will have hugely different values - from 30,000 psi for A-53 welded pipe to 240,000 psi for quenched and tempered 4340 Cr-Mo tubing.

I should point out that these equations are shown simply for the purpose of understanding and illustrating the concepts discussed: DO NOT USE THEM for calculations, as they may not include critical elements such as design factor, impact loads, safety margins, and fatigue factors! (that's what professional Engineers are for :-)

Mr Smith summarizes quite nicely:

"We do not build structures from materials with low moduli of elasticity [non-stiff, flexible] simply because such structures would sag under any reasonable load...We do not make structures from weak materials simply because such structures would break under load.  Together the two properties of stiffness and strength define the physical properties of a solid material.  For instance:

And finally, before moving on to the next section, we shall now cover some of the other more commonly encountered and more important definitions.  Other definitions are covered in the glossary:

The mixture of any element with a pure metal. However, there are several elements regularly occurring in plain carbon steel as manufactured, such as carbon, manganese, silicon, phosphorous, sulphur, oxygen, nitrogen and hydrogen. Plain carbon steel is therefore an alloy of iron and carbon and these other elements are incidental to its manufacture. Steel does not become alloy steel until these elements are increased beyond their regular composition for a specific purpose, or until other metals are added in significant amounts for a specific purpose.

Alloying Elements
Chemical elements added for improving the properties of the finished materials. Some alloying elements are: nickel, chromium, manganese, molybdenum, vanadium, silicon, copper.

Alloy Steel
Steel is considered to be alloy steel when the maximum of the range given for the content of alloying elements exceeds one or more of the following limits: Manganese 1.650/0, silicon,.60%, copper,.600/0, or in which a definite range or a definite minimum quantity of any of the following elements is specified or required within the limits of the recognized field of constructional alloy. Steels: Aluminium, chromium up to 3.9~, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, or any other alloying element added to obtain a desired alloying effect.

Carbon Steel
Steel is classified as carbon steel when no minimum content is specified or required for aluminium, boron, chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium, or any other element added to obtain a desired alloy effect; when the specified minimum for copper does not exceed .40% or when the maximum content specified for manganese does not exceed 1.650/0; silicon .600/0; copper .60%.

Cold Finishing
The cold finishing of steel, generally used for bars and shafting, may be defined as the process of reducing their cross sectional area, without heating, by one of five methods: Cold rolling, Cold drawing and grinding, Turning and grinding, Cold drawing, or Turning and polishing.

Cold Rolling (Cold Finishing)
A forming process in which metal is rolled or drawn through dies, usually at room temperature. This produces a product with certain advantages over hot rolled steel, such as tighter tolerances, increased properties, improved finish and straightness.

The ability of a material to be plastically deformed without fracturing

Elastic Limit
The greatest stress which a material is capable of developing without a permanent deformation remaining upon complete release of the stress.

Endurance Limit
Also known as fatigue limit, a limiting stress, below which metal will withstand without fracture an indefinitely large number of cycles of stress. If the term is used without qualification, the cycles of stress are usually such as to produce complete reversal of flexural stress. Above this limit failure occurs by the generation and growth of cracks until fracture results in the remaining section.

The phenomenon of the progressive fracture of a metal by means of a crack which spreads under repeated cycles of stress.

Fatigue Resistance
The ability of a metal to withstand repeated and varying loads.

Metals or alloys that contain appreciable amounts of iron.

A hot working operation generally involving plastic deformation of metal at high temperatures into desired shapes with compressive force.

Fracture Toughness
The ability of a material at a given temperature to resist further crack propagation, once a crack has started.

The ability of a metal to resist penetration, defined in terms of the measurement (Brinell, Rockwell,Scleroscope, Vickers, Knoop etc.)

This relates to the ability of steel to harden deeply upon quenching and takes into consideration the size of the part, the method of quenching and the analysis and grain size of the steel. Carbon steels are considered as shallow hardening and various alloy and tool steel grades are considered deep hardening or through hardening.

Increasing the hardness by suitable heat treatment, usually involving heating and cooling. When applicable, the following more specific terms should be used: age hardening, case hardening, flame hardening, induction hardening, precipitation hardening, and quench hardening.

Heat Treatment
An operation or combination of operations involving the heating and cooling of a metal in the solid state for the purpose of obtaining certain desirable conditions or change in properties or metallurgical structure. Heat treating operations include annealing, normalizing, quenching and tempering, etc.

Hot Rolled
Hot rolled products are those products that are rolled to finish at temperatures above the recrystallation temperature.

Impact Toughness
The ability of a material to resist fracture under an impact.

Mechanical Properties
The properties of a material that reveal its elastic and inelastic behaviour where force is applied, thereby indicating its suitability for mechanical applications; for example, modulus of elasticity, tensile strength, elongation, hardness and fatigue limit.

Modulus of Elasticity
Measure of stiffness. The ratio within the limit of elasticity of the stress to corresponding strain. The stress in pounds per square inch is divided by the elongation in fractions of an inch for each inch of the original gauge length of the specimen. The modulus of elasticity for cold rolled steel is 29,500,000 psi and for other steels varies between 28,600,000 and 30,300,000 psi.

Plastic Deformation
Deformation of a material that will remain permanent after removal of the load which caused it.

A process of rapid cooling from an elevated temperature by contact with liquids, gases or solids.In the heat treating of steel, the step of cooling metals rapidly in order to obtain martensite by immersing or quickly cooling the steel in a quenching medium. The quenching media may be water, brine, oil, special solutions, salts or metals; and the intensity of the quench is determined by the temperature, volume and velocity of the media. In the case of air hardening tool steels the quenching medium is air at room temperatures.

Residual Stress
Macroscopic stresses that are set up within a metal as the result of non uniform plastic deformation or thermal gradients. Stresses of this nature are caused by cold working or by drastic gradients of temperature from quenching or welding.

Rockwell Hardness
A method of measuring the hardness of materials (resistance to penetration). Rockwell measures the hardness by pressing an indentor into the surface of the steel with a specific load, then measuring how far the indentor was able to penetrate. There are a number of Rockwell tests the most common is Rockwell B.

A term applied to the operation of shaping and reducing metal in thickness by pressing it between rolls which compress, shape and lengthen it following the roll pattern. Steel is either hot rolled or cold rolled depending upon the product being manufactured,

A complex iron oxide formed on the steel surface during the hot rolling operation or formed on steel parts which are heat treated in the presence of oxygen.

A solid solution of iron and carbon.  An iron-base alloy, malleable in same temperature range as initially cast, and containing carbon in amounts greater than .05% and less than about 2.00%. Other alloying elements may be present in significant quantities, but all steels contain at least small amounts of manganese and silicon.

Deformation produced on a body by an outside force.

Stress Relieving
A process of reducing residual stresses in material by heating to a suitable temperature and holding for a sufficient time. this treatment may be applied to relieve stresses inducted by casting, quenching, normalizing, machining, cold working or welding.

The state of or condition of a metal as to its hardness or toughness produced by either thermal or heat treatment and quench or cold working or a combination of same in order to bring the metal to its specified consistency. A condition produced in a metal or alloy by mechanical or thermal treatment and having characteristic structure and mechanical properties.

Tensile Strength
The maximum load in pounds per square inch that the sample will carry before breaking under a slowly applied gradually increasing load during a tensile test. The ratio of maximum load to the original cross-sectional area.

The ability of a metal to absorb energy and deform plastically before fracturing. It is usually measured by the energy absorbed in a notch impact test such as the Charpy or Izod Impact Test. The area under the stress-strain curve in tensile testing is also a measure of toughness.

Ultimate Strength
See tensile strength.

Work hardening
An increase in resistance to deformation (hardness and strength) caused by cold working.

Yield Point
The yield point is the load per unit area at which a marked increase in deformation of the specimen occurs without increase of load during a tensile test.

Yield Strength
The point at which a material exhibits a strain increase without increase in stress. This is the load at which a material has exceeded its elastic limit and becomes permanently deformed.  Stress corresponding to some fixed permanent deformation such as .1 or.2% offset from the modulus or elastic slope.

Young's Modulus
Same as modulus of elasticity.

Section 4 - The Steels of interest to us.

OK - hopefully we now have a good comprehensive understanding of what the different properties of materials are - from elastic range and yield strength to ductility and ultimate strength.

Now what we need to do is develop and understanding of how and why some materials have these differing properties, followed by looking at the actual and relative properties of different irons and steels we will encounter, and finally which we should choose (or demand our suppliers and manufacturers choose) and why.

There are really three groups of metals that are of greatest interest and use to us, the builders and wheelers of hardcore 4x4s.  They are all ferrous metals, meaning they are iron based and are magnetic.  The groups are:

We will now examine each in a bit of detail. 

Cast Irons

Ahhh - yes, the much maligned, hugely misunderstood cast iron.  Let me get this off my chest right now. The term CAST IRON does not refer to a specific material with properties that we can discuss. It is a GENERIC TERM for a whole group of ferrous metals that are made up of iron, silicon, and carbon.  The name has 2 parts - CAST referring to the fact that these metals can be readily poured into moulds (cast) when molten, to make parts, and IRON for the chief element that makes them up. It is sometimes (though rarely) necessary, and therefore just barely acceptable, to use the term "cast iron" when referring to the material from which something is made - but ONLY if we do not know the more specific type of cast iron.  It is NEVER acceptable (though vast numbers of derelicts and miscreants are guilty of it!) to shorten this further and refer to any material by calling it 'cast' and using the word cast as a noun.  In our world, "cast" is a verb, and is the method of making a part by pouring molten metal into a mould.

There, I feel better, don't you?

Now, there are several different types of cast irons that we should know about, and within each type there are often several different "grades." The different types of cast iron are distinguished by the form which the carbon takes - be it carbides, graphite, flakes, nodules, etc. The different types are:

Gray Iron
Composed of iron and silicon and carbon, with it's carbon content in the form of very thin interconnected flakes of graphite Gray iron possesses excellent castability and machinability so that complex parts can be readily cast and economically finish machined. The material has modest tensile strength values, good wear resistance, and good resistance to galling. It is economical to produce, cast and finish. Some examples of its use include: machine tool bases, ways and housings, disc brake rotors, cylinder blocks and heads.

White Iron
Having it's carbon content in the form of granules of iron carbide (due to low silicon content and rapid cooling when cast); white iron is very hard and brittle and virtually unmachinable.  It is therefore of little practical use to us, despite having high compressive strength and good resistance to wear and abrasion.  It is, however, often the starting point for malleable iron.

Malleable Iron
Malleable Iron has most of its carbon content in the form of irregularly shaped lumps or nodules of graphite mixed in the matrix (lattice) of iron.  These nodules of graphite are not connected to one another though.  Malleable iron is created by careful and precise heat treatment of solid white iron castings.  The result is a cast iron that is extremely tough (toughest of all cast irons) and that can have (depending on the exact heat treatment) tensile strength and/or ductility similar to a mild carbon steel.  It is not as easy to cast or machine as gray iron and therefore is usually only used to cast relatively simple parts.  It is, however, often used for: hand tools (e.g. C-clamp, pipe wrench), brackets, hangars, axle housings, drive yokes, connecting rods, brake callipers, etc.

Nodular (Ductile) Iron
Named "Nodular Iron" because all of it's carbon content appears as tiny spherical nodules of graphite, and with carbon content of up to 10%, nodular iron combines the best properties of gray iron and malleable irons.  It has both excellent castability as well as very good machinability while being the most ductile of all cast irons and having very high tensile strength (for a castable metal).  With carefully controlled changes in chemical composition (it can be alloyed with other elements such as nickel, molybdenum, vanadium, etc - much the same as steel)  and/or heat treatment it can be used to manufacture very strong, stiff, tough components.  The trucking and transportation industries have been quick to appreciate nodular castings as lighter and stronger replacements for complex steel weldments and for their ability to produce complex structural shapes - both cored and solid - that are strong, light, and cheap.  Examples include such critical components as crankshafts, gears (including ring and pinion sets), and front suspension steering knuckles. [4]

One of the most popular examples of nodular iron applicable to us must be the famous Ford 9 inch rear axle with the highly sought after nodular housing.  Note also that some popular Dana axles (center sections) are cast from malleable iron (e.g. Dana 30) and some from Nodular Iron (e.g. Dana 44, 60)

Carbon Steels

It stands to reason that we cannot possibly describe or compare something unless we have a common language and a way to ensure we're talking about the same thing. In other words, a way to make sure we are comparing 'apples to apples".  This is, in fact, one of the areas most full of error that I encounter on the web - people are not comparing "apples to apples".  Hopefully, after reading this section, we will all be able to do so more accurately.

Steel Designations. 

Numbering systems currently in use today for steel have been developed over the years by various groups, including: trade associations, engineering societies, standards organizations, and private industry groups.  Some examples are those developed by the American iron and Steel Institute (AISI), Society of Automotive Engineers (SAE), American Society for Testing and Materials (ASTM), American National Standards Institute (ANSI), Steel Founders Society of America, American Society of Mechanical Engineers (ASME), and the American Welding Society (AWS).  You can see how it would be easy to fall into the trap of comparing apples to oranges!  Not to worry, for our use, it's pretty easy as we generally stick to SAE/AISI designators.  If you have or come across a steel with a different designator, there are many good trade publications that contain tables showing equivalent designations.

All carbon steels, the types we are interested in, (at least in North America) are designated using a standard four digit numbering system developed in cooperation between the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE).  For example, we are often used to seeing different steels referred to as 1020 or 4130 or 4340.  The numbers are not arbitrary, they have specific meaning, and can tell us a lot about the steel in question. The first 2 digits of the designation are the "classification" of the steel. Carbon steels all belong to one of four (4) classifications, only two of which are any use to us - the 10xx and the 15xx.   The four classifications of carbon steel are:

10xx—nonresulfurized carbon steel. Basic structural "low-carbon" or "mild" steel.
11xx—resulfurized carbon steel. Free machining steels, inherently brittle.
12xx—resulfurized and rephosphorized carbon steel.
15xx—nonresulfurized, high-manganese carbon steel. Basic carbon steel used for low-cost forgings.

The last two digits of the standard four digit designator indicate the approximate carbon content of the steel in tenths of a percent. For example, SAE 1020 contains approximately 0.20% carbon (actually from 0.17% to 0.23%).

Recall that the higher the carbon content the higher the ultimate tensile strength—and the lower the ductility.

Sometimes a suffix H is attached to a AISI/SAE number to indicate that the steel has been produced to prescribed hardenability limits. For example 1541H is a commonly used carbon steel in the manufacture of axle shafts.

There are a great many different specific types of carbon steel. The following information, taken from Carroll Smith's "Engineer to Win" and from the 24th Edition of the Machinery's Handbook (not the most modern, but the one I happen to own!) covers all or most of the types that are likely ever to be used by us to make parts from or used by manufacturers we might buy from.

SAE 1010-1015
This is the most common of the low carbon or mild steels. It is generally available as hot-rolled or cold-rolled sheet and it is used to form ERW tube in wall thicknesses below 0.065". Both formability and weldability are excellent. Like all of the low-carbon steels, 1010-1015 does not respond to heat treatment. Its strength levels are moderate and it was never intended to be used as a primary structure—lawn furniture, trailer frames and tooling only!

SAE 1018-1020
This is a very popular grade of low-carbon structural steel. It is available as hot-rolled or cold-finished bar, as ERW tube in wall thicknesses of 0.063" and up, as cold-drawn-seamless and DOM tube. It welds and forms very well and, while it does not respond to heat treatment, it can be case hardened by carburizing. I use it for just about everything other than suspension links— usually as DOM round tube or as cold-rolled sheet.

SAE 1025
This is the best of the low carbon steels. To the best of my knowledge it is now available only as seamless round tube—and that rarely. Before 4130 was developed, 1025 was the standard aircraft structural tubing. We don't use it simply because it is difficult to find and, if a tube fabrication deserves something better than 1020, it deserves to be made from 4130 and heat treated.[5]

And here's what the 24th edition of the Machinery's Handbook has to say on the matter:

Carbon Steels,-- SAE steels 1006, 1008, 1010, 1015:  These steels are the lowest carbon steels of the plain carbon type, and are selected where cold formability is the primary requisite of the user.  They are produced both as rimmed and killed steels.  Rimmed steel is used for sheet, strip, rod, and wire where excellent surface finish or good drawing qualities are required, such a body and fender stock, hoods, lamps, oil pans, and other deep drawn and formed products.  This steel is also used for cold heading wire for tacks, and rivets and low carbon wire products. Killed steel (usually aluminium killed or special killed) is used for difficult stampings, or where non-aging properties are needed.  Killed steels (usually silicon killed) should be used in preference to rimmed steel for forging or heat treating applications.

These steels have relatively low tensile values and should not be selected where much strength is desired.  Within the carbon range of the group, strength and hardness will rise with increases in carbon and/or with cold work, but such increases in strength are at the sacrifice of ductility or the ability to withstand cold deformation.  Where cold rolled strip is used the proper temper designation should be specified to obtain the desired properties.

With less than 0.15 carbon, the steels are susceptible to serious grain growth, causing brittleness, which may occur as the result of a combination of critical strain (from cold work) followed by heating to certain elevated temperatures.  If cold worked parts formed from these steels are to be later heated to temperatures in excess of 1100 degrees F., the user should exercise care to avoid or reduce cold working.  When this condition develops it can be overcome by heating the parts to a temperature will in excess of the upper critical point, or at least 1750 degrees F.

Steels in this group, being nearly pure iron or ferritic in structure, do not machine freely and should be avoided for cut screws and operations requiring broaching or smooth finish on turning.  The machinability of bar, rod and wire products is improved by cold drawing.  Steels in this group are readily welded.

SAE 1016, 1017, 1018, 1019, 1020, 1021, 1022, 1023, 1024, 1025, 1026, 1027, 1030: Steels in this group, due to the carbon range covered, have increased strength and hardness, and reduced cold formability compared to the lowest carbon group.  For heat treating purposes they are known as carburizing or case hardening grades.  When uniform response to heat treatment is required, or for forgings, killed steel is preferred; for other uses, semi-killed or rimmed steel may be indicated, depending on the combination of properties desired.  Rimmed steels can ordinarily be supplied up to 0.25 carbon.

Selection of one of these steels for carburizing applications depends on the nature of the part, the properties desired, and the processing practice preferred.  Increases in carbon give greater core hardiness with a given quench, or permit the use of thicker sections.  Increases in manganese improve the hardenability of both the core and case; in carbon steels this is the only change in composition that will increase case hardenability. The higher manganese variants also machine much better.  For carburizing applications SAE 1016, 1018,  and 1019 are widely use for thin sections or water quenched parts.  SAE 1022 and 1024 are used for heavier sections or where oil quenching is desired, and SAE 1024 is sometimes used for such parts as transmission and rear axle gears.  SAE 1027 is used for parts given a light case to obtain satisfactory core properties without drastic quenching.  SAE 1025 and 1030, while not usually regarded as carburizing types, are sometimes used in this manner for larger sections or where greater core hardness is needed. 

For cold formed or headed parts the lowest manganese grades (SAE 1017, 1020, and 1025) offer the best formability at their carbon level.  SAE 1020 is used for fan blades and some frame members, and SAE 1020 and 1025 are widely used for low strength bolts.  The next higher manganese types (SAE 1018, 1021 and 1026) provide increased strength.

All of these steels may be readily welded or brazed by the common commercial methods.  SAE 1020 is frequently used for welded tubing.  These steels are used for numerous forged parts, the lower carbon grades where high strength is not essential. Forgings from the lower carbon steels usually machine better in the as forged condition without annealing, or after normalizing. [6]

Some sample uses of 10xx and 15xx steels, taken from the Machinery's Handbook:

Steel Use
1020 Camshafts, Fan Blades, Welded Tubing, Wrist Pins
1030 Brake Levers, Gear Shift Levers, Key Stock, Seamless Tubing
1035 Bolts and Screws

Axles, Brake Levers, Camshafts, Connecting Rods, Carbon Steel Forgings, Studs

1045 Axle Shafts, Crankshafts, Carbon Steel Forgings, Ring Gears, Spline Shafts
1060 Clutch Disks, Clutch Springs, Lock Washers, Snap Rings, Valve Springs, Thrust Washers
1070 Clutch Disks, Plow Beams
1080 Agricultural Steel, Plow Discs, Plow Shares
1085 Clutch Disks, Leaf Springs, Mower Knives, Bumper bars
1095 Harrow Discs, Harrow Rake Teeth, Coil Springs
Adapted from: "Machinery's Handbook" 24th Edition. Erik Oberg, Franklin D. Jones, Holbrook L. Horton, Henry H. Ryffel, Robert E. Green; Industrial Press Inc., 1992 p.382-4

Alloy Steels

Alloy steels are steels that have had finite and precise amounts of alloying elements added to them during their manufacture.  Alloying Elements are chemical elements added for improving the properties of the finished materials. Some alloying elements are: nickel, chromium, manganese, molybdenum, vanadium, silicon, copper.   Small, precise changes in the exact chemistry of the steel can change the mechanical properties quite drastically.  Generally, alloying elements are added to steel to maximize some particular mechanical property(ies).  Of course, nothing in life is free, and there is always a price to pay.  The more alloyed a steel is, the narrower it's appropriate use, as it becomes more and more specialized (narrow in focus)  There is also a trade off in the reduction of other properties: as hardness and strength go up due to alloying with chromium and molybdenum - ease of welding and ductility go down, and of course cost goes up - in some cases WAY up.

Steel is considered to be alloy steel when the maximum of the range given for the content of alloying elements exceeds one or more defined limits. 

Common alloying elements and their effects are:

Element Effect
Aluminium Deoxidizes and restricts grain growth
Boron Increases hardenability
Carbon Increases hardenability and strength
Chromium Increases corrosion resistance, hardenability and wear resistance
Lead Increases machinability
Manganese Increases hardenability and counteracts brittleness from sulphur
Molybdenum Deepens hardening, raises creep strength and hot-hardness, enhances corrosion resistance and increases wear resistance
Nickel Increases strength and toughness
Phosphorus Increases strength, machinability, and corrosion resistance
Silicon Deoxidizes, helps electrical and magnetic properties, improves hardness and oxidation resistance
Titanium Forms carbides, reduces hardness in stainless steels
Tungsten Increases wear resistance and raises hot strength and hot-hardness
Vanadium Increases hardenability

Alloy steels are also designated using a standard four digit numbering system, very similar to that used for the carbon steels.  However, the first two digits indicate the major alloying element or elements, whereas the last two digits again indicate the approximate carbon content of the steel in tenths of a percent. For example, SAE 4340 contains approximately 0.40% carbon. The first two digits of an alloy steel's designation indicate the alloying elements and their percentages as follows:

13xx - Manganese-1.75 %
40xx - Molybdenum-0.20% or 0.25%
41xx - Chromium-0.50%, 0.80% OR 0.95% PLUS Molybdenum 0.25%
43xx - Nickel-1.83% PLUS Chromium 0.50% or 0.80% PLUS Molybdenum 0.25%
44xx - Molybdenum-0.53 %
46xx - Nickel-0.85% or 1.83% PLUS Molybdenum 0.20% or 0.25%
61xx - Chromium-0.60% or 0.95% PLUS Vanadium 0.13% or 0.15%
86xx - Nickel-0.55 % PLUS Chromium 0.50% PLUS Molybdenum 0.20%
87xx - Nickel-0.55% PLUS Chromium 0.50% PLUS Molybdenum 0.25%
88xx - Nickel-0.55% PLUS Chromium 0.50% PLUS Molybdenum 0.35%
92xx - Silicon-2.00%

Note that the hugely popular designation "Chrom-moly" steel has nothing to do with the shiny stuff on bumpers and hubcaps, but is in fact a reference to the fact that the steel in question has major alloying elements of Chromium and Molybdenum. That's why I prefer to write the abbreviation as Chrom-Moly, without the "e" on chrome. It is also popularly abbreviated as Cr-Mo, Cro-Mo, etc. Note that 41xx and 43xx alloy steels can and frequently ARE both refereed to as "Chrom-Moly Steel", though obviously the 43xx also has significant Nickel added.

Some characteristically enlightening, if rather opinionated, insight from my hero on the most common/popular alloy steels follows:

SAE 4130
Best known of the family of CHROME-MOLY steels, 4130 is often considered, in racing circles, to be the ideal steel for all high-strength/high-stress applications. IT IS NOT! In thin sections (that is, in tube or sheet form) its unique combination of excellent tensile strength, toughness and response to mild heat treatment combined with its good formability in the annealed condition and its outstanding welding characteristics make it virtually unbeatable for fabrications subject to high stress levels. It is critical that all welds be stress relieved. I prefer the use of OXWELD 32 CMS welding rod with 4130 for the simple reason that it both normalizes and heat treats well in conjunction with 4130. Many welders prefer to use a stainless rod, but the high nickel content of stainless welding rods means that the weldment will not respond well to heat treatment. Since I believe that not heat treating 4130 fabrications is DUMB (if you don't heat treat you end up with an expensive part with the same strength as 1020—and brittle weld areas). Smith's law says to use the heat-treatable rod for EVERYTHING. I heat treat 4130 fabrications to Rock-well C Scale 26 to 30 and no higher. This results in an ultimate tensile strength of about 130,000 psi with sufficient ductility that I do not have to worry about brittle parts. The other side of the 4130 coin, often unknown to (or at least unappreciated by) the racer, is that it possesses poor deep-heat-treating characteristics and has an inborn dislike of varying cross-sections. These characteristics make 4130 a poor choice for machined or forged parts—it doesn't forge very well anyway. It also doesn't machine very well, at least in the normalized condition—too gummy. Those people who make hubs, steering knuckles and the like from 4130 are kidding themselves—and their customers. It doesn't make very good shafts, either, as in drive shaft, or axle, or torsion bar.

SAE 4140
This is a deep hardening chrome-moly steel with excellent impact resistance, fatigue strength and general all-around toughness. It is commonly used for small-aircraft forgings. I use it in bar form for all of the little gub-bins and small parts that we are always machining. It doesn't weld as well as 4130 but it does weld satisfactorily. Welded to 4130 tube or sheet, with Oxweld 32 rod, a 4140 machined component can be heat treated to the same spec as 4130.

SAE 4340
This is the nickel-chrome-moly deep-hardening steel that we SHOULD use, in its vacuum-melted configuration, for our hub forgings, drive shafts, axles and the like. Its tensile strength, toughness, fatigue resistance, excellent deep-heat-treating characteristics and very high tolerance of stress reversals (which is just another way of saying that it has excellent fatigue resistance) make it just about unbeatable. It is also weldable (with care and a lot of pre-and post-heat) and eminently forgeable. In use it should be heat treated to the 180,000—200,000 psi range, maximum—although it can be taken to 220,000 psi without significant loss of toughness. The hardness range between Rockwell C Scale 46 and 48 should be avoided with this steel as it becomes brittle in this range.

SAE 4340 MODIFIED (300M)
This is, as you would expect, very similar to 4340... The addition of a trace of vanadium and an increase in the silicon level (while they have no notable effect on the hardness, strengths, or ductility of the resultant alloy) work miracles in the toughness and resistance to fatigue, producing a steel which, in the 270,000 to 300,000 psi range, is the toughest, most impact resistant and most fatigue resistant of the usually available steels. Unfortunately, even in the normalized condition, it is a bear to machine. Reducing the hardness by reducing the level of heat treatment or by tempering has the curious effect of reducing the tensile strength WITHOUT increasing the ductility or toughness. The most common use of this outstanding steel, also known as 300M, is for military and commercial aircraft landing gear. We use it for hubs, for drive shafts, axles and torsion bars—when we can afford it (or when we cannot afford NOT to use it). At its normal, heat-treated hardness level of Rockwell C 52/56 it is hard enough that we can and do run roller bearings directly on its surface. While the material is great, the heat treating is tricky. There is a very real danger of surface decarburization which can only be avoided by copper plating prior to heat treat. The ONLY heat treat specification for 300M is MIL H 6875, but there are tricks to every trade. MIL H 6875 calls out a two-hour quench at 575 degrees F. Doubling the quench time to four hours will notably increase the ductility and fatigue resistance of the finished product. Another trick is to absolutely forbid the heat treat shop to perform Rockwell or Brinell hardness tests on the actual part, supply a "test coupon" of the same material and cross-section wired to each part and insist that the coupon be heat treated along with the part and that all hardness tests be done on the test coupon. This is a good idea with ALL parts heat treated much above Rockwell C 40.

These are usually known by trade names such as Hi-Tuff and Stress Proof. They contain up to about 3 % silicon and are, as the names suggest, tough as hell. They are popular for stock car and off-road racing axles—and the alloys are very suitable for these applications. They are not as good as 4340 M or even 4340, but they are also a damned sight cheaper and, especially where the minimum weights imposed are high, the fact that a part with the same strength and fatigue resistance can be made lighter by using a better steel may be a lot less significant than the cost difference. However, these steels are tough only because of the high silicon content, which is mainly in the form of longitudinal fibers or strings of silicon. This limits the efficient (and safe) use of the alloys to parts with minimal section changes and virtually no transverse machining (we don't want to cut the longitudinal strings that make the stuff tough to start with, do we?). They also don't like being bent very much because that may rupture the silicon strings. Mind you, I have made a lot of street car antiroll bars from Stress Proof with excellent results and pretty severe bends—but in this case the bends are almost, by definition, in lightly stressed areas.[7]

Mechanical Properties

The following tables illustrate mechanical properties of various different types of steels of varying types - from hot rolled 1020 to cold finished 4340 chrom-moly alloy steel.  They are taken from a variety of sources[8].  In addition to the actual specific numbers / data they contain, I believe they clearly illustrate some key concepts:

      Ultimate Strength   Modulus Of Elasticity      
Material Type Condition / Treatment Tension (psi), T Compression % of T Shear % of T Yield Point (psi) In Tension, E (millions of psi) In Shear, %  of E  % Elongation Reduction of Area %) Impact Strength (Izod) ft-lb
Gray Iron Class 20 Class 20 20,000 360-440 % 160%   11.6 40%      
  Class 40 Class 40 40,000 310-340% 140%   17 40%      
  Class 60 Class 60 60,000 280% 100%   19.9 40%      
Malleable Iron     40-100,000     30-80,000 25 43%      
Nodular (ductile) Iron     60-120,000     40-90,000 23        
Cast Steel Carbon   60-100,000 100% 75% 30-70,000 30 38%      
  Low Alloy   70-200,000 100% 75% 45-170,000 30 38%      
Magnesium     37-55,000   19-27,000 26-44,000 6.5        
Carbon Filament     280,000       110        
Aluminium 6061-T6   48,000   34,000 40,000 10.5        
  2024-T3   65,000   34,000 59,000 10.5        
  7075-T6   83,000   48,000 73,000 10.5        
Titanium     50-135,000     40-120,000 15-16.5        
Steel ASTM A-53 pipe Grade A 48,000 100% 75% 30,000 30 38%    
    Grade B 60,000 100% 75% 35,000 30 38%      
  1020 Cold Drawn Bar 82,000 100% 75% 70,000 30 38% 20% 65%  
  1020 Hot Rolled Bar 69,000 100% 75% 40,000 30 38% 38% 52%  
  1020 Normalized (1600*F) 64,000 100% 75% 50,250 30 38% 36% 68% 86.8
  1020 Annealed (1600*F) 57,000 100% 75% 42,750 30 38% 37% 66% 91
  1030 Quenched & Tempered (400*F) 123,000 100% 75% 94,000 30 38% 17% 47%  
  1030 Quenched & Tempered (800*F) 106,000 100% 75% 84,000 30 38% 23% 60%  
  1030 Quenched & Tempered (1200*F) 85,000 100% 75% 64,000 30 38% 32% 70%  
  1025 (low carbon)   60-103,000 100% 75% 40-90,000 30 38%      
  1045 (medium carbon)   80-182,000 100% 75% 50-162,000 30 38%      
  1095 (high carbon)   90-213,000 100% 75% 20-150,000 30 38%      
  1095 (high carbon) As-Rolled 140,000 100% 75% 83,000 30 38% 9% 18% 3
  1095 (high carbon) Normalized (1650*F) 147,000 100% 75% 72,500 30 38% 10% 14% 4
  1095 (high carbon) Annealed (1450*F) 95,250 100% 75% 55,000 30 38% 13% 21% 2
  1095 Quenched & Tempered (400*F) 216,000 100% 75% 152,000 30 38% 10% 31%  
  1095 Quenched & Tempered (800*F) 199,000 100% 75% 139,000 30 38% 13% 45%  
  1095 Quenched & Tempered (1200*F) 122,000 100% 75% 85,000 30 38% 20% 47%  
  4130 RC-30 136000 (81-179,000) 100% 75% (46-161,000) 30 38%      
  4130 Normalized (1600*F) 97,000 100% 75% 63,250 30 38% 26% 60% 63.7
  4130 Annealed (1585*F) 81,250 100% 75% 52,250 30 38% 28% 56% 45.5
  4130 Hot Rolled & Annealed Bar 86,000 100% 75% 56,000 30 38% 29% 57%  
  4130 Hot worked & Annealed Tube 85,000 100% 75% 65,000 30 38% 20%    
  4130 Cold Drawn & Normalized Bar 98,000 100% 75% 87,000 30 38% 21% 52%  
  4130 Cold Worked & Normalized 95,000 100% 75% 75,000 30 38% 15%    
  4130 Water quenched @ 1550*F Tempered at 1000 1000*F 146,000 100% 75% 133,000 30 38% 17% 50%  
  4340 RC-44 208000 (109-220,000) 100%