Study On Microstructure And Properties Of Heat Exposure Zone Of TMCP890 Steel

This paper, in order to promote Q890steel in engineering machinery, study weldability and welding process of low carbon bainitic steel TMCP890steel. Therefore in this paper, firstly, SHCCT-. maximum hardess test and Y-groove cracking test were used to evaluate the cold crack sensitivily of TMCP890steel. Secondly, welding simulation technique is used to replicate HAZ experiencing single and double welding thermal cycles, and effect of postweld heat treatment (PWHT) on HAZ microstructure and properties. Finally, through actual joints microstructure and properties reaserch effect of different heat inputs and PWHT temperatures.By drawing SHCCT diagram found that when the cooling time tg/5<18s, the microstructure of CGHAZ is low-carbon lath martensite and the hardness value between328-334HV10. When the cooling time t8/5>18s, CGHAZ appears lath bainite and hardness value decreases with tg/5increase. The results of maximum hardness test show that TMCP890welding cold cracking tendency smaller, for HAZ maximum hardness value is342HV10and a certain degree of softening in ICHAZ and SCHAZ. The results show that the empirical formula established by Michio Inagaki was more suitable to calculate the cooling time ts/5-Through simulated the relationship between the cooling time t8/5) preheat temperature and the hardness of coarse grain zone in HAZ, the results showed that, in compliance with the actual welding process conditions:heat input E10-20kJ/cm, preheat temperature To50～150℃, CGHAZ hardness values stable at318-335HV10. Y-groove cracking test show that there is no cold cracking when preheat temperature at60℃.Through simulated and contrasted HAZ properties of TMCP890and QT890steel which experience single thermal cycles by thermal welding simulation technique, show that TMCP890steel HAZ has no brittle zone, but QT890steel CGHAZ and ICHAZ is brittle zone.The reason is TMCP890steel inter-lath have a large number of residual austenite for adding more Copper which is austenite stabilizing element, but QT890steel has high carbon content, a large number of cementite precipitate at inter-lath and inside-lath for auto-tempered in CGHAZ which is brittle zone. M-A constitute have an important influence on toughness of ICHAZ. Through thermodynamic calculated the mass fraction of (Ti,Nb)(C,N) precipitation and particle size after Ostwald maturation, found (Ti,Nb)(C,N) particle could rapid dissolution when the heating temperature exceeds1200℃, and CGHAZ grian size rapid grow. LSW theory can calculate the particle size after Ostwald maturation in the welding process. As the cooling time t8/5increase, the particles increase, the CGHAZ grain size also increase.Through thermal simulation technology and instrumented impact researched microstructure and properties of TMCP890steel CGHAZ experience thermal cycles. The results show that CGHAZ grains further grow and retained austenite reduction after experience welding thermal cycle which peak temperature is1350℃(UACGHAZ), which leads to crack propagation energy and crack initiation energy decline, and the crack propagation energy decline is the main reason that leads to impact energy decline. CGHAZ impact energy decreased significantly after experienced welding thermal cycle which peak temperature is950℃(SCCGHAZ) and800℃(IRCGHZ), which were brittle zone. The reason is the phenomenon of "structure heredity" exsits in IRCGHAZ and SCCGHAZ, that a large number of massive M-A constitute distributed in the grain boundary lead to less energy for crack expanding.Using domestic GHS90wire, in three heat input (12kJ/cm,15kJ/cm,18kJ/cm) welding TMCP890steel. With the increasing of heat input, the tensile strength of the weld metals and joints showed a downward trend, but were higher than90%of the strength of the base metal. The impact toughness of weld metal decreased with the increasing of heat input, while the impact toughness of HAZ appeared a trendency of decrease after the first increase. As TMCP890steel uses low-carbon bainitic steel alloy design system, HAZ hardened and softened is not serious with three heat-input.Using three post-weld heat treatment temperatures(PWHT)250℃、480℃and600℃to the welded joints. With the increasing of PWHT, the tensile strength and impact energy of the joints showed a downward trend. When the PWHT increase250℃to600℃, the weld joints tensile strength from993MPa to755MPa, the weld metal impact energy from89J to16J. The reason is CGHAZ and FGHAZ impact energy decreased significantly after PWHT600℃×2h which impact energy is2.0J and16J through thermal simulation technology.