Strengthening mechanisms in an ultrafine grained powder metallurgical hot work tool steel produced by high energy mechanical milling and spark plasma sintering

The strengthening mechanisms in an ultrafine grained (UFG) powder metallurgical (PM) tool steel produced by high energy mechanical milling (MM) and spark plasma sintering (SPS) have been investigated. In order to gain a proper insight to the Hall-Petch relation, bimodal grain size microstructures were produced by mixing different fractions of the nanocrystalline (NC) MM and the coarse grain (CG) gas-atomized powders to achieve different equivalent grain sizes. Dislocation strengthening (similar to 435 MPa) and Hall-Petch strengthening (similar to 388 MPa) are the major contributing mechanisms in strengthening the as-sintered steel. The presence of 34 vol% retained austenite (RA) had a negative influence on the strength (similar to 555 MPa) and highlights the need of heat treatments (i.e., quenching and tempering) to achieve a fully tempered martensitic microstructure with a dispersion of fine secondary carbides. Recovery of dislocations and defects accompanying the precipitation of carbides reduces the effect of grain size (i.e., martensite packet size) on the strength after tempering. However, dislocations acting as preferential sites for secondary carbides nucleation during tempering, provide a finer and more homogenous particles dispersion compared to the CG counterpart. In this condition, the strength is mainly governed by the carbides size and distribution (similar to 210 MPa) confirming that the indirect beneficial effects of MM can still be observed after heat treatment.