Phase Transformation and Mechanism on Enhanced Creep-life in P9 Cr-Mo Heat-resistant Steel

• Main Authors: YU, HSIAO-YAO, 余曉堯
• Other Authors: CHEN, PIN-YI
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/srk247

碩士 === 明志科技大學 === 機械工程系機械與機電工程碩士班 === 107 === This work explores mechanical properties, structural evolution, and mechanism of creep-life enhancement for widely used P9 heat-resistant steel.The 17-year-on-site used P9 alloy exhibit a higher tensile strength and a smaller elongation than the new P9 alloy from room temperature to 700oC.The P9 alloy also displays a typical ductile feature with a significantly necking profile.The P9 alloy shows phase transition sequences of α-Fe(bcc)→(Ac1~858℃)→α+γ-Fe(bcc+fcc)→(Ac3~894℃)→γ-Fe(fcc) upon heating and γ-Fe(fcc) →(Ms~352℃)→martensite(bct)→(Mf~300 ℃)→martensite(bct) upon cooling.The new P9-alloy tube mainly contains ~73.5% ferrite phase (α-Fe) and ~26.5% carbide M3C.However,the used P9-alloy tube shows four crystalline phases including ~45.9% ferrite, ~14.5% martensite, ~37.5% cementite (M3C) and ~2.7% carbide M23C6.The creep test indicates that the used P9-alloy tube has a longer creep-life (or better anti-creep ability) than the new tube.Activation energies of atomic diffusion for the new and used tubes are respectively 252.45 and 345.87 kJ/mol, indicating a decreased diffusion capability in the used tube. This work suggests that martensite laths, lath boundaries,and precipitates (such as carbides) play important roles to inhibit creep-deformation in the P9-alloy steel.