Microstructure and Mechanical Properties of 5%Ni Low-temperature Steel MAG Multi-layer Welded Joint

doi:10.3901/JME.260501

Abstract

Abstract: 5%Ni steel has poor weldability due to its high content of alloying elements, which restricts its wide application in the field of pressure vessels. The microstructure and mechanical properties of 50 mm thick 5%Ni low-temperature steel for low temperature storage tank were characterized and tested by optical microscopy, scanning electron microscopy and tensile testing machine. The results show that the welded joint is well formed. The weld structure is austenitic dendrite. The heat affected zone(HAZ) is formed by welding thermal cycling. The HAZ is mainly composed of lath martensite, tempered sorbite and a small amount of M-A components. Multiple thermal cycles lead to the precipitation of carbides and segregation of carbon elements, which promotes the formation of M-A components. The average hardness of the weld zone is 238 HV₁₀, the peak hardness of the joint is 306 HV₁₀ in the coarse-grained heat affected zone, and the hardness of the joint in the subcritical heat affected zone is lower than that of the base material, and the hardness is 190 HV₁₀, which is related to tempering and softening. The average tensile strength of the joint is 674 MPa, the average yield strength is 510 MPa, and the average elongation is 24%. The fracture location corresponds to the subcritical heat affected zone of the joint. The impact energy in the micro-zone of the fusion line is the lowest, with an average value of 92 J. The impact energy of the fusion line is 33% lower than that at the +2 mm position of the fusion line. Obvious embrittlement occurs in the coarse-grained HAZ near the fusion line. Coarse lath martensite and M-A and chain carbides in the heat affected zone of coarse grains reduce the low temperature toughness of welded joints. The results are of positive significance for the popularization and application of MAG welding process in the field of low-temperature pressure vessel welding of 5% Ni steel.

Key words: 5% Ni steel, MAG welding, multi-layer and multi-pass welding, heat affected zone, low temperature toughness

CLC Number:

TG406

ZENG Zhiwei, HUANG Henan, ZHANG Luyun, BAO Jiayu, SUN Jialu, ZHANG Junwei. Microstructure and Mechanical Properties of 5%Ni Low-temperature Steel MAG Multi-layer Welded Joint[J]. Journal of Mechanical Engineering, 2026, 62(10): 219-230.

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URL: http://www.cjmenet.com.cn/EN/10.3901/JME.260501

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