Quote:
Originally Posted by rory_a
My understanding was that 4130 was stronger than steel (with never hearing of heat treating it either), therefore you can run a thinner tube wall and tube, netting a much lighter chassis. At least that's the way they sell it for VW drag chassis - at more than twice the cost.
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Well, they're both steel. 1018 or 1020 for example are regular mild carbon steels, and 4130 is a higher carbon content steel with chromium, molybdenum, and some other good stuff mixed into the metal composition. BUT.. 4130's strength is all about how it's been heat treated (quenched/tempered).
If you compare their yield strengths in the normalized condition, 1020 is 50,000psi and 4130 is 63,300psi (data from my machine design book). Sure, it is a bit stronger, but in my opinion that fairly small percentage gain in strength is not worth the cost/hassle of dealing with it. If, however, you properly heat treat 4130, you can achieve a pretty fantastic yield strength like 150,000+psi. But to do this, you have to stick it in an oven, heat to something like 1600*F for a good while, take it out and immediately quench in water or oil bath, and then reheat to a few hundred degrees for a set length of time to relieve some stress and regain some ductility. The end product's mechanical properties are all about that voodoo magic that happens at the heat treatment shop.
So, yes, you can design lighter weight parts out of 4130 that will carry the same load as a 1020 part, but only by heat treating it. This is why smaller parts like control arms, welded uprights, shafts, etc. are very frequently made out of 4130, but very big parts will typically not be. Generally, the bigger the part, the more expensive it is to heat treat. The shop you use has to be able to physically fit it into their ovens & quench baths.
Also, parts can distort a bit during heat treatment as things heat/cool and the stresses in the metal are relieved, so for something like a frame where you have suspension mount points you want to keep in alignment across a long distance (like from opposite corners of the car), it could be hard to control the end result.