Originally Posted by
phabib
I'm still rebuilding my first 6x6 so I'm far from an expert, but as I looked to answer that same question for my Max, I kept hearing that 4140 was the stuff to get. The thing about 4140 is that it needs both pre-heat and post-heat when welding or it can crack.
Yesterday I talked to a mechanical engineer at work who also used to race dirt bikes and ATVs so he has both the theoretical and practical knowledge. He said that the 4140 without hardening is about twice as strong as the mild steel, but when properly heat treated it becomes about 3x as strong. He also said that back when he was racing and wanted every possible advantage he made a lot of stuff out 4140 or 4340 that could just as well have been made from mild steel. He suggested that my axles might be in that category. He said that the only time he could see an axle breaking would be if I got stuck, spun the wheels at full speed, then only 1 wheel got traction and took all the power at once. His advice was don't do that.
Since the chrome molly also requires special care and skill to weld, he suggested that the mild steel would also likely be better because mild steel won't be prone to failure the way improperly welded or heat treated alloy would be.
If strength is really an issue, he said to just bump up the material one size instead.
After all that advice, I'm still not sure if I'll listen or go ahead with the 4140 anyway.