Formation of fine grained globular microstructures during isothermal annealing of PM T42 high speed steel: Role of nanometric proeutectoid carbides

Abstract

The microstructure of powder metallurgy T42 high speed steel has been analysed after isothermal annealing. The heat treatment parameters (austenitising temperature, annealing time and annealing temperature) have been chosen aiming at obtaining a moderate hardness (50 HRC) adequate for structural applications. It has been demonstrated that it is possible to attain the targeted hardness through a globular pearlitic matrix microstructure at temperatures markedly lower than A₁ and by short time annealing cycles. The key factor for the generation of such microstructure is the precipitation of a homogeneous distribution of nanometric tungsten rich M₆C proeutectoid carbides in the austenite. The presence of these carbides avoids the growing of lamellar pearlite independently of the annealing time.