Alex_22

M2 steel 1190 C 1 hr and 620 C double tempering for 2 hrs. Etched using 5% Ni for 3 min. Around 100 um white region with big grains at the outside of the materials, the hardness is 200 HV. The brown region hardness is 750 HV. What could be this soft white region with big grains? How should I reharden it? Thanks!