Fracture Micro Mechanism of Cryogenically Treated Ledeburitic Tool Steel

Fracture micro mechanism of cryogenically treated Cr-V ledeburitic tool steel was studied on fracture toughness testing specimens, by using the scanning electron microscopy and microanalysis. Experimental steel has been processed at different combinations of cryogenic temperatures (from the range -75 to -269 degrees C) and tempering regimes, producing different microstructures (martensite, retained austenite, carbides), hardness- and fracture toughness values (from the ranges 700-1000 HV and 13-20 MPa x m(1/2), respectively). Conventionally quenched the same steel was considered as a reference. Generally, the obtained fracture surfaces manifest combined low-energetic ductile/cleavage crack propagation mode. The low-energetic ductile mode is associated with the presence of small globular carbides (size < 0.3 mu m) that are produced by cryogenic treatments. On the other hand, cleavage mode is more pronounced with increased matrix stiffness, which is caused by the precipitation of nano-scaled transient carbides within the martensite. Also, differences in role of crack propagation between various carbides were determined. These differences are caused by crystallography of these phases as well as by their size. While small globular carbides (cementite) and dominant amount of the eutectic carbides (cubic MC-phase) assist more probably (by 50-60%) in ductile micro mechanism the coarser secondary particles (hexagonal M7C3-phase) are much more prone to cleavage cracking. This tendency increases with decreasing steel hardness since the matrix becomes more plastic, and the carbides cannot deform together with the matrix as they are much more brittle.

Automated summary

This study investigates the fracture micro mechanism of cryogenically treated Cr-V ledeburitic tool steel using scanning electron microscopy and microanalysis. The research finds that small globular carbides produced by cryogenic treatments enhance ductility, while nano-scaled transient carbides within the martensite are more prone to cleavage cracking.