Sintering of AISI M2 Tool Steel Processed in High-Energy Planetary Mill

This work aimed to evaluate the effect of pre-sintering annealing heat treatments and sintering times in AISI M2 high-speed steel powders processed by high energy milling. Turning chips were obtained from an AISI M2 drill bit that was annealed during 2 hours at 900°C, under argon atmosphere, before machining. Subsequently, the chips were milled during 10 hours in a high energy planetary mill with a power ratio of 10:1, also under argon atmosphere. Half of the powder mass was annealed at 650oC during 30 minutes under argon atmosphere after milling. Three different samples were prepared, consisting of: non-annealed powder, annealed powder and a mixture 1:1 of annealed and non-annealed powders. All powders were compacted by uniaxial pressing before sintered. Compressibility curves were obtained for all samples. Sintering process was conducted at 1200°C during 1, 2 and 3 hours and samples were cooled inside the furnace. The annealed powder sample presented the best compactation behavior, due to its restored ductility, followed by the 1:1 mixture of annealed and non-annealed powders. The microstructure of sintered samples displayed a ferritic matrix surrounded by carbide networks at grain boundaries. Higher sintering times resulted in carbon impoverishing, leading to lower volume fractions of carbides and hence reducing its hardness. Non-annealed powders showed higher dependency of sintering time to reduce their porosity. The best results were obtained for the annealed powder with shorter sintering time, since it presented low volume fraction of porosities and smaller grain sizes.