Several years ago I took sample pieces that I had completed to a close friend who does military aircraft testing ....the parts were tested and showed 1.2-1.5 mm surface depth hardening and I was informed that all had passed mil spec. S-6090A which is the military aircraft spec for low carbon steel hardening.......and that is good enough for me.........

I have also hardened some aircraft parts over the years and these parts require testing and certification before return to service (paper trail from a certified testing lab).....color is not on the list of importance....depth and quality is.......these parts are magnafluxed and red dye tested as well, before certification showing any imperfections.......


Doug