Carbide-free Bainite/Martensite (CFB/M) Duplex Phase Steel

Abstract

To achieve energy and raw materials conservation, the world focuses attention upon researching and developing structure steel with ultra high tensile strength. Compared with ordinary structure steel, ultra-high strength low alloy (UHSLA) steel has higher tensile strength (R_m≥1500 MPa) but lower toughness. Because of low alloy elements, simple hot working process and relative low cost, UHSLA is widely used in spaceflight, aviation and conventional weapons industries. The process of ultimate heat treatment of this type of steel involves “quenching + tempering” or “austempering + tempering”, Austempering + tempering produces tempered martensite or low bainite + tempered martensite duplex phase microstructure, usually used for making component under a larger stress at room temperature (like aircraft undercarriage, gun barrel and bulletproof steel plate). However, during the process of application, because UHSLA steel has not high enough toughness, brittle fracture often occurs, which results in short service life (Fan et al, 2006). The components made of UHSLA steel for dynamic and huge impact load have higher requirement of fatigue life. But medium or high carbon ultrahigh strength low alloy steel is usually low in toughness. It is shown in the recent research that UHSLA steel with tempered martensite structure often breaks in brittle manner due to insufficient toughness. When fatigue cycle is about 10⁷ or more, the initiation of fatigue fracture change to inside materials and produce model ladder S-N curve, which results in further lowering fatigue limit (Wang et al, 2002).

The experimental materials used in Chapter 7 are listed in a appendix at the end of Chapter 7.