Understanding and Avoiding Intergranular Fracture Characteristics of Hadfield/Hot-Press-Forming Multi-Layer Steel Sheets

An austenitic Hadfield steel shows highly sustained strain hardening as well as excellent tensile strength and ductility, but its yield strength is relatively low (400 to 500 MPa). In order to overcome this shortcoming, two multi-layer steel (MLS) sheets were fabricated by the hot-roll-bonding of the Hadfield steel with two kinds of martensitic hot-press-forming (HPF) steels. Carburized and decarburized layers were formed near Hadfield/HPF interfaces by the C diffusion from the high-C Hadfield (1.2 pct) to low-C HPF (0.23 and 0.35 pct) layers. The tensile test results indicated that the two MLS sheets were fractured right after the yielding with almost no plastic deformation because the intergranular fracture appeared in the carburized and HPF layers. This intergranular fracture was caused by Cr7C3 carbide precipitation and C segregation at prior austenite grain boundaries as well as considerably high residual stresses generated in both Hadfield and HPF layers. The tempering at 473 K (200 °C) was adopted for relieving residual stresses, while most of grain-boundary carbides remained. The 473 K (200 °C)-tempered MLS sheets showed improved tensile properties over the non-tempered MLS sheets, and well satisfied a rule of mixtures.