Crack cause analysis of high strength duplex stainless steel piston rod
ZHENG Kai1,2,3, ZHONG Zhenqian*1,3, CAO Wenquan2, LIN Shuangping1,3, GAO Qun1,3
1. NCS Testing Technology Co., Ltd.,Beijing 100081,China; 2. Institute for Special Steels,Central Iron and Steel Research Institute Co.,Ltd.,Beijing 100081,China; 3. Failure Analysis Center, Centre Iron and Steel Research Institute Co.,Ltd.,Beijing 100081,China
Abstract：The cracking of one high strength duplex stainless steel piston rod occurred during storage. The causes were analyzed in experiments by the means of metallographic microscope, scanning electron microscope (SEM), tensile testing machine, impact testing machine and hardness tester. The results showed that both transverse and longitudinal strength of piston rod was high. The maximum longitudinal tensile strength was up to 1 145 MPa. The Brinell hardness was HBW347, which was 21.8% higher than the upper limit of design requirement (HBW241-285). However, the transverse toughness was relatively low. The lowest percentage elongation after fracture was only 2.0%, which was only 14.3% of the lower limit of standard requirement (i.e., 14%). The average impact absorbed energy was only 5.8 J, which was 10.5% of the lower limit specified in the standard (i.e., 60 J). The alloying elements in piston rod material met the standard requirement. The structure was tempered martensite + thin strip ferrite. But the content of hydrogen was up to 5.4×10-6 (mass fraction). High hydrogen content caused hydrogen induced delay microcrack internally, which was the main reason to cause longitudinal cracking. The stripped ferrite deteriorated the transverse mechanical property of piston rod, which had certain promoting effect on the cracking. It was suggested to optimize the smelting and heat treatment process. Meanwhile, the dehydrogenization annealing should be also added before processing of piston rod to reduce the hydrogen content in raw material, thus to reduce the tendency of hydrogen induced delay cracking at the greatest extent.
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