On High Performance Steels for Fasteners

Abstract

The consumption of steel for fasteners in China is about 9 million tons annually, accounting for approximately one-fifth of worldwide usage. High strengthening is the main development trend of high performance of fasteners. As strength increase, the sensitivity of both fatigue failure and delayed fracture increases, and the processing technology varies. These are the main problems need to be addressed. Fasteners of property classes 8.8-12.9 are currently the most widely used high-strength fasteners. The match between fatigue life with strength is commonly the paramount consideration. We have developed a series of steels involving (i) SCM435 steel wire for key fasteners of engine with ultra-narrow composition (ΔC≤0.01%), slight hardness variation (ΔHRC≤ 2) and a qualified fatigue performance of 1×107 cycles; (ii) 10.9 and 12.9 fastener steels with large size, low cost and high hardenability for wind tower; (iii) A286 alloy wire with desirable 650 °C durability. In addition, we developed ML35 wire free of spheroidizing and microalloyed steels for slender bolts free of quenching & tempering process. The use of ultra-high strength fasteners ≥13.9 is necessary to achieve equipment light weighting. We have developed steels for 13.9 and 15.9 fasteners with an excellent resistance to delayed fracture. An austempering process was applied to 14.8-17.8 fasteners, which enables the reduction of the alloy content. A novel strengthening method needs to be introduced to develop fasteners in excess of class 19.8 fastener. We have developed 19.8 fastener steel with good fatigue and delayed fracture resistance. Secondary hardening plays a key role in the improvement of strength.