MICROSTRUCTURAL CHANGES IN AS-CAST M2 GRADE HIGH-SPEED STEEL DURING HOT-FORGING

High speed steels have a complex carbide pattern in the as-cast state which has to be modified to achieve the desired properties of adequate toughness, hot hardness, and wear resistance. The effects of hot forg ing and postdeformation annealing on carbide distribution and morpholo gy in M2 grade high speed steel were studied, and it was shown that ho t forging accelerates the spheroidization rate of M6C carbide with lit tle effect on coarsening. The mechanism responsible for such accelerat ion is dominated by mechanical disintegration of M6C carbide plates, w hile diffusion-controlled spheroidization was not significant. For MC carbide particles, coarsening was the dominant mechanism, but it was n ot possible to ascertain whether diffusion had been unaffected by defo rmation or even increased by a factor that could be as high as 10,000 times. Annealing after deformation accelerated spheroidization which w as attributed to the damaging of carbide plates during forging rather than an increase in diffusion rate, since the matrix was almost substr ucture-free in the annealed condition, i.e., lack of short-circuiting paths for diffusion.