BioMax
+1y
The sanctioning bodies that do allow use of 4130 will in almost all cases require you to TIG weld the part and will not allow powder coat for a few different reasons. One as you stated, you cannot see small cracks emerging and two, because of 4130s mild heat treatability, the temperatures that powder is baked at is within 4130s tempering range. Also, don't think that a mild steel chassis will be ignored while the 4130 chassis gets examined after every race. Any decent team should examine any vehicle no matter what it is build out of.
I have never heard that a 4130 bike frame should only be raced for one year. That is true about aluminum, but I have always understood that 4130 can be run for up to 5 years and a Ti frame can be used for up to 9. The only thing that I can think of is that the frames are being built so thin that they should not be trusted after a year. I know that there are a few manufacturers that have special built racing frames for their team, but won't sell them to the public because of the liability. My statement wasn't about BMX race bikes it was about the dirt-jump bikes. They are heavier and can last for years.
I completely understand the "stronger is more brittle" point and in some cases that statement is completely true. But I think that where we are not in agreance is partly my fault. Mild steel is considered to be "tougher" than 4130 because it will bend before is cracks, just like the grade 2 bolt. Where 4130 excels is it's "memory." 4130 is somewhat springy while still maintaining a memory of what shape it was originally in. So on a dirt jump frame that will take numorous "hits" it will maintain it's shape for a long time. While the same frame built out of mild steel will only live for a short time before it finally gives up and bends. This memory is very important for certain applications like aircraft and Top Fuel cars. In a Top Fuel car the chassis is so flexible that the cross braces in the frame are on sliders to allow the frame to be stiff while having a controled amount of flex. Mild steel could not do this, it would flex, then stay there.
This whole discussion is proving my point that 99% of the builders out there do not need to concern themselves with 4130. If I remember correctly, NASCAR does not allow 4130 to be used on their chassis for the reason that you guys are pointing out, mild steel absorbs the impact better and therfore is safer for the driver. But on the flip side of that coin, a chassis that needs to last a lot longer than a few races 4130 would be a better choice if strength and longevity is a concern.