I was once told by someone that 4140 could not be color case hardened but I don't remember if it was Oscar Gaddy or someone else. There is always lots of false information around these secretive techniques. I would suggest that you contact Turnbull's and any other guys out there who do color case work and just ask them straight out if it can be done. Most of them will be glad to help you out, just as long as you don't want to know what their receipe is. Bob