Mr. Ghomashchi et Cm. Sellars, MICROSTRUCTURAL CHANGES IN AS-CAST M2 GRADE HIGH-SPEED STEEL DURING HOT-FORGING, Metallurgical transactions. A, Physical metallurgy and materials science, 24(10), 1993, pp. 2171-2180
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.