Chromoly vs Mild Steel Cages: An Engineering Perspective
So, what's the deal with chromoly? What are the advantages and disadvantages when using it for a roll cage material? Is it really better than mild steel?
First things first. Chromoly is not some magical material - it is a type of steel. Steel is a general term for a metal that is comprised mostly of iron, mixed with some other elements, most commonly carbon. The other elements mixed in with the iron can give the steel many properties that are more desirable than pure iron. For instance carbon gives steel strength and hardness - mild steel uses carbon as the primary alloying element. Nickel and chromium can give steel better corrosion resistance - stainless steel is a steel alloy with high percentages of these two elements. Chromoly steel's two main alloying elements are chromium and molybdenum - that's how it got it's name.
To fully address the argument between mild steel and chromoly, we need to cover a little bit of basic materials science. We'll cover the concepts of density, stress, and elastic modulus. We'll also cover a little about welding.
Most people are familiar with the concept of DENSITY, so I won't belabor this; the density of mild steel and chromoly is almost identical. That means that for two pieces of metal the same size, one made of mild steel and one made of chromoly, the weight is essentially the same. So the fact that a component is made of chromoly does not in itself mean that it is lighter. Two otherwise identical cages, one made of chromoly and one made of mild steel will weigh the same. However, other properties of chromoly in relation to mild steel can allow the component to be designed lighter.
The next concept we need to cover is STRESS. Stress is the result of the load on a part and its geometry. It is defined as the load per area; that is, the force acting on an object divided by the cross sectional area, or in mathematical format, S=F/A. To put it in more familiar terms, you can think of it as how "spread out" the force is across a component. The higher the force or the lower the cross section, the higher the stress.
Just like a balloon pops when the pressure inside it gets too high, when the stress reaches a certain point in a material, the material will fail and the part breaks. A material's capacity to resist stress is a property of the material itself.
Let's say you have a piece of wood and a piece of steel that are the same size. If you put the same weight on each one, the stress in each piece will be the same, because the load is "spread out" over the same area for each of them. If more weight is continually stacked on, the stress will increase, and eventually the wood will break, but the steel will be able to hold the weight. This is because the steel has a higher stress capacity than the wood does. Now, if you take a larger piece of wood, then the same load will be spread out over more area, and the stress will drop below the wood's stress capacity, and it will be able to hold the load. This is the important part: the higher stress capacity of steel compared to wood means that less steel is required to hold the same weight.
The main advantage that chromoly has over mild steel is that its stress capacity is higher. This allows a smaller cross section of chromoly material to stay below its stress capacity for a given load, while mild steel will require more cross sectional area. In terms of roll cages, the chromoly tubing can have a thinner wall or smaller diameter. Because the density of the two steels is nearly identical, this means that the chromoly tube will be lighter, and a cage made from chromoly will weigh less.
The next concept is ELASTIC MODULUS. You can think of this like a spring. The stiffer the spring or, in engineering terms, the higher the spring rate - the more force it takes to compress it. The elastic modulus is the internal spring rate of a material. As any material is put under load, it will flex, just like a spring compressing. And just like a spring with a higher rate will compress less, a material with a higher modulus will flex less. Elastic modulus, like stress capacity, is also an intrinsic property of a material. Steel has a higher modulus than wood. If you try to bend a half inch piece of wood, it's fairly easy. But if you try to bend a half inch steel bar, it takes a lot more force.
Chromoly and mild steel have almost identical moduli. So for equally sized pieces of material, they will have the same stiffness. But here's the difference as it pertains to roll cages: a mild steel tube will be larger than a chromoly tube, so it will be stiffer. A mild steel roll cage will be heavier than a chromoly one, but it will also contribute more stiffness to the chassis.
The last difference between mild steel and chromoly is in how they are welded. Mild steel is pretty forgiving to weld. It's strength doesn't change significantly through the welding process. Chromoly, on the other hand, is called an air-hardening steel. After welding, it becomes very brittle, and this is especially important from a fatigue and impact standpoint. Over long term use under changing loading, the material can fail even if the loads stay below the normal stress capacity of the material. If you take a paper clip and bend it, it won't immediately break because the load is below its stress capacity, but if you bend it back and forth enough times, it will break. That is an example of fatigue failure. In a static structure like a building, the loading doesn't change much, so fatigue is not a major concern, but in a racecar, which experiences turns, acceleration, and braking, fatigue is a very important factor. I have personally seen chromoly frames crack because they were not welded properly, and in one case, the car almost broke entirely in half. Brittle materials are also less able to absorb impact loads, which of course is a concern with a structure specifically designed to absorb the impact of a car accident.
The only way to properly weld chromoly steels is to stress relieve them after welding. For large components, this is a complex process, involving precise temperature measurement and carefully calculated holding periods. However, for relatively small components like the tubes of a roll cage, a relatively simple process can make a significant difference - it won't be perfect, but it will really help. Heat the welded area up until it is glowing red and then allow it to cool slowly. There is a misconception that stress relieving is not necessary when using a TIG (sometimes called GTAW or heliarc) welding process. I'm not sure who first stated this, but I'm pretty sure he was selling TIG welders. One of the major benefits of TIG welders is that they have very precise heat application and as a result, they have narrow heat affected zones, or HAZ for short. The heat affected zone is the part of the metal involved with the weld that is heated enough to affect its material properties. The narrow HAZ of a TIG weld actually increases the temperature gradient during welding, because it is spread out over a narrower area. This makes the welds more brittle and therefore more susceptible to fatigue and impact failure, and makes post weld heat treating even more important than with other types of welding. It goes without saying that in an accident, your life could depend on your cage - make sure it is welded properly.
In conclusion, the material that you use for your roll cage is, as with nearly all things, a trade-off based on what is most important to you. Chromoly, with its higher stress capacity than mild steel, will allow you to use smaller, and therefore lighter tubing, making the overall cage weigh less. But the larger, thicker mild steel tubing will provide more stiffness to the chassis. Chromoly is more expensive and is also more difficult to weld properly.
References and Further Reading
Mechanics of Materials. Beer, Johnston, DeWolf. McGraw-Hill, 2002.
Engineer to Win. Carroll Smith. MBI Publishing, 1984.
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