What is Chrome Moly anyway? Well first off, If you just searched the internet for chrome moly, I have to tell you that the correct spelling is actually Chromoly. Chromoly is like steel on steroids. It contains Chromium and molybdenum a splash of carbon and mostly iron.
The most popular grade of chromoly tubing is 4130. Here is what 4130 means: The American Iron and Steel Institute (AISI) has a numbering system that uses 4 numbers to categorize steels.
The first number indicates the major alloying element.
Take 1010 plain carbon steel, for example. The 1 denotes plain carbon steel. The second number indicates the approximate percentage of the major alloying element. In the case of plain carbon steel the number is a zero because there is no major alloying element. The third and fourth numbers indicate the carbon content of the steel.
In the case of 1010 steel They indicate that the steel contains approximately 0.1% carbon, that’s 0.1% or point one percent carbon.
OK lets look at 4130 chromoly.
The 41 in 4130 indicate the main alloying elements are chromium and molybdenum and that the steel contains about 1% chromium and molybdenum combined. The last 2 digits indicate 0.3% carbon.
The reason 4130 chromoly is so freakin strong and the reason it performs so well when used in airplanes, bicycles, dragsters, etc. is because the combination of chromium, molybdenum, and carbon, along with proper normalized heat treatment , give 4130 chromoly tubing a combination of strength, toughness, and ductility.
4130 chromoly tubing has great properties and great strength to weight ratio. The trick is not to screw that up when you weld it.
There are some purists out there who insist that gas welding is the only way to go for welding chromoly tubing for small airplanes. The idea is that the slow heating and slow cooling that is part of the gig with gas welding, is necessary and when 4130 tubing is gas welded, it kind of gets stress relieved at the same time.
But with all the data available on TIG welding 4130 chromoly tubing without the need for stress relief in enough for me to feel good about TIG welding it. Even for small aircraft
I just make sure to only weld within established guidelines and I don’t use 4130 filler rod.