MOUTH-LIKE CRACKING IN A HIGH-STRENGTH MULTIPHASE STEEL AND ITS RELATIONSHIP TO FRACTURE TOUGHNESS
Author(s)
XUAN WANG, JICHENG GAO, YUN HANG, YI ZHENG, DEREK O. NORTHWOOD, CHENG LIU
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.
Keywords
high tensile strength, mouth-like crack, quenching-partitioning-austempering process, nano-scaled structure, crack initiation and propagation
Related Book
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