Article By: Paul Wideman
Originally Published In The May 2011 Issue Of Cycle Source Magazine
I’ve wanted to write this article for some time now. I’ve heard the debate waged that 4130 chromoly is the be-all end-all when it comes to fabrication in the racing and powersports industries. I’ve also heard the exact opposite; moly is a waste of money and is just for big dollar racing outfits. The scariest thing is that I see up and comers, and even some dudes that have been around a while and should know better, using the wrong material in the wrong application in our business. Some responsibility needs to be seen here, guys. If you don’t know the proper way to weld, normalize, anneal, stress relive, etc, you shouldn’t be using the material. Maybe a little more research and a little less willy-nilly fab work, huh? Nestled here in the river valleys of Eastern Missouri are some of the world’s premier Pro-Stock and Pro-Mod chassis shops. These guys knock out cars that are running 300 mph in the quarter mile, and they know their stuff. I’ve had a handful of these cats work for me from time to time, and I’ll tell you, no one can weld like these guys; at least no one in the motorcycle industry. We’ve had lengthy discussions about this very debate; would a chopper frame or frontend benefit from the use of Chromoly tubing? I’ve always gotten the same answer from these wizards of molten metal, but that is just a response from the average Joe; usually a man that is just repeating the knowledge he has heard from the instructor in weld school or the old timer that taught him the ropes.
I wanted to dig a little deeper, providing an in-depth description of what material should be used in a respective application. While this is more info than your run of the mill, beer drinking, shop BS session might provide, this is still a small article in a motorcycle magazine. As with so many of our tech articles, this is intended to give a fairly educated overview and urge you to research and learn more for yourself. DO NOT take this article as Gospel, as I am merely gathering research I have accumulated over the years and trying to assemble it all in an intelligent fashion. If you are reading this and you are in the least bit uncertain, PLEASE speak with a professional. Your local welding supply store can point you in the direction of an authoritative voice. The AWS is always an incredible source of info as well. Chromoly is the name commonly used to describe 4130 steel. Two of the alloying elements in the 41xx series of metals are Chromium and Molybdenum, hence the name Chromoly. While the entire series contains very similar elements, 4130 is what most people are referring to when the term Chromoly is used. Mild 1020 DOM tube is the name given to what many people simply call DOM, or seamless tubing. DOM means Drawn Over Mandrel. Essentially the tubing is rolled and welded, then a mandrel is pulled through the tubing to precisely size the inside diameter and wall thickness, as well as smoothing the weld seam. This cold working of the material also improves the mechanical properties of DOM.c
The strength of materials such as Moly and DOM can be measured in various manners. The two primary measurements are tensile strength and yield strength, both measured in ksi, or kilo-pounds per square inch. Tensile strength is the maximum amount of stress a material can withstand while being pulled or stretched prior to necking. Necking is when the material’s cross section deforms due to the aforementioned stress. Yield strength is the maximum amount of stress a material can withstand without deforming plastically. Whereas tensile strength is tested by pulling the material, yield strength is tested by applying pressure from the side. Think of pulling a piece of tubing as the tensile strength test and then standing on the middle of it while the two ends are suspended as the yield test. A material can deform during the yield test but return to its original shape prior to the yield point. This is deforming elastically. So as you are standing on your tube for your homemade yield test, it may always return to the original shape. But when your friend stands on it too, it bends farther, surpassing the yield point and deforming plastically, permanently changed. Again, as stated earlier, this is only a brief outline of what goes into this science, and there are many other ways to measure the strength steel and tubing, such as flexural strength.
For the most part Moly and DOM are very similar metals. A 1 ¼” x .120” wall of DOM will weigh exactly the same amount as its Chromoly counterpart. The tensile strength of Moly is approximately 90 ksi, while DOM is around 10% less at 80 ksi. Chromoly boasts a yield rate of around 80 ksi, while the DOM again falls a bit short at 70 ksi. These numbers do vary slightly. With this minimal amount of improvement from one material to the next, a great argument can be made that the weight savings gained using Chromoly are negligible compared to DOM in the motorcycle arena, as the amount of tubing used to build a frame or frontend is a drop in the bucket compared to a top fuel or funny car chassis. While the increased cost compared to the minute weight savings seems like enough reason to choose DOM over Moly, there is more to the debate here. OK, hang on to your seats, because it gets a little tricky here. When welding the joints of DOM, the Heat Affected Zone, or HAZ, will be of a Pearlite or Proeutectoid nature, which is a large grain structure and generally very ductile. When welding the same joint on Chromoly, the HAZ will form a Martensite micro-structure, which is extremely brittle. To bring the Moly up to the more desirable Pearlite, annealing is required. Any stress in the HAZ that has not been stress relieved or annealed will crack. This is why the NHRA only certifies a chassis for a certain period of time before it is required to step down a class, and be allowed a lower maximum speed. Many welders will preheat Moly prior welding to minimize thermal shock and reduce warpage, thus reducing stress in the HAZ. Where the Chromoly really accels is when it is properly heat treated. Heat treated 4130 will have a tensile strength of around 190 ksi and a yield strength of approximately 170 ksi!!! But, is your fabricator heat treating your frame? Essentially where this puts us is that un-heat treated 4130 will have just about the very same tensile strength and yield strength as the DOM, that is to say if they are made of the same material. But we fall short in two places; the first being that many fabricators use thinner wall Chromoly, because of the common misconception that Moly is the super material, so we need less to do more. Wrong. Thinner tubing equals less strength. Second, the lack of annealing or heat treating allows our HAZ to be brittle and VERY susceptible to work hardening; definitely a double whammy. Is annealing and stress relieving of a custom Springer needed to realize the true strength of a Chromoly structure? Yes, absolutely. Is it reasonable? I would argue not. The weight savings and strength gains are totally negated by the cost of material and labor, if done correctly, in our application. So, in short, 4130 is superior to DOM when heat treated and properly welded. If these steps are skipped, 4130 is far less desirable, and some would say dangerous. I am one of those that think it has no place in the motorcycle world. I am not saying 4130 has no place on your bike, because it certainly does; but only if the application calls for it. But sloppy work that cuts corners or bleeds of ignorance, that has no place around motorcycles. So, like I said earlier, if you are uncertain, ask an expert. Or just don’t do it. Until next month FTW, Paul- JFL