| Due to the excellent combination of strength with toughness and good weldability, high strength low alloy (HSLA) steels have been applied in various fields in modern industry. KMN steel, belonging to the family of HSLA steels, is imported from Italy and used for the manufacturing of centrifugal compressor impeller. However, the carbon equivalent of KMN steel is relatively high, showing cold crack tendency. In practice, micro crack appeared in heat-affected zone (HAZ) of welded KMN steel impeller, lowering the performance and security. Aiming to optimize the welding of KMN steel, the influence of welding heat input and post weld heat treated temperature on micro structure and properties of H AZ in gas tungsten arc welding (GTAW) joint were investigated. Meanwhile, crack healing by "heating-ultrasonic vibration impact method" and friction stir method were carried out.It revealed that, welding heat input greatly affected the microstructure and properties of HAZ in KMN steel joint. When heat input was0.25kJ/mm, some lath martensite formed in HAZ, causing the hardness in HAZ as high as460HV. Also the impact toughness was better than KMN steel base metal. With increasing of welding heat input, the portion of martensite reduced, which decreased the hardness and improved the impact toughness in HAZ. When heat input was higher than0.46kJ/mm, there was little martensite and the hardness became stable. Increasing welding heat input to0.67kJ/mm, microstructure in HAZ was lower bainite and acicular ferrite, showing excellent toughness (182.5J/cm2). When increasing welding heat input further to0.77kJ/mm, upper bainite formed in HAZ and coarsening of grain deteriorated the toughness.Post weld heat treatment (PWHT) of KMN steel joint at550-700℃for1h was carried out. Results showed that, with the increase of tempering temperature, hardened microstructure transformed to tempered sorbite, and the hardness in HAZ gradually decreased. Heat treated below610℃, hardness decreased slowly, behaving tempering resistance. Tempered at550and580℃, impact toughness in HAZ deteriorated obviously. XRD test revealed that the transformation of residual austenite was the main reason for toughness deterioration. Increasing tempering temperature higher, impact toughness was improved for the dispersion strengthening of precipitated alloy elements, and the coarsened carbides.Crack healing of KMN steel was carried out by "heating-ultrasonic vibration impact method". When heating the crack at800and1000℃, rich-Fe oxide was formed in crack, and there was no metallurgical bonding between this oxide and base metal. When healing at1000℃, decarbonization happened in crack and white massive ferrite formed in base metal near to crack. Local melt was observed in crack. Crack healing of aluminum by friction stir technology suggested that friction stir processing is suitable to heal the superficial crack, and friction stir welding could heal the internal defects. Moreover, friction stir technology could refine healed microstructure, and improve the properties of healed workpiece. If high performance stir tool was used, crack healing of steel would be realized by friction stir technology. |
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