MOUTH-LIKE CRACKING IN A HIGH-STRENGTH MULTIPHASE STEEL AND ITS RELATIONSHIP TO FRACTURE TOUGHNESS

Abstract

The effect of microstructure on crack growth behaviour in steels has always been a subject of considerable research interest. Based on a quenching and partitioning process (Q&P), and the transformation of a nano-scaled bainite in advanced high-strength steels, a novel quenchingpartitioning- austempering process (Q-P-A) has been developed for manufacturing a multiphase microstructure in a medium carbon steel (55Mn2SiCr). The processing sequence consists of the following steps: austenitizing at 900°C for 0.5 h; controlled quenching and cooling to 200°C, i.e. slightly below Ms (the start temperature for martensite transformation) for 5 s; austempering at 170°C for 5 min; up-heating to 250°C for 120 min; final air cooling to room temperature. An ultimate tensile strength (UTS) above 2 GPa, as well as an acceptable elongation of 3%, is obtained due to a multiphase formation comprising prior martensite (PM), bainitic ferrite (BF), retained austenite (RA) and nanoscaled structure ((BF + RA_(+C))nano). Mouth-like cracks are observed on the fracture surface and the crack arrest behavior is investigated. When a microstructural cluster with (BF + RA_(+C))nano fully covered PM is formed, a mouth-like crack can be formed and a superior crack resistance can be obtained. The crack initiates from the PM boundary and propagates along the interface between the PM and (BF + RA_(+C))nano over a distance of a few millimeters and before being arrested in the (BF + RA_(+C))nano. This behaviour is mainly attributed to the uniform distribution of film RA and needle BF with nano-level spacing in the (BF + RA)nano. The stress concentration energy at the crack tip can be absorbed by the martensitic transformation of the film RA. The results are important when designing a multiphase microstructure for a commercial high-strength steel.