Journal of Jilin University(Engineering and Technology Edition) ›› 2025, Vol. 55 ›› Issue (8): 2530-2538.doi: 10.13229/j.cnki.jdxbgxb.20231449

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Evolution of thermal cycle， microstructures and mechanical properties of SA516Gr.70 steel prepared by friction stir welding

Wen-biao GONG1,2(),Zi-qi MIAO1,2,Heng CUI3,Xiu-ying WANG4,Wei LIU1,2()   

  1. 1.Key Laboratory of Advanced Structural Materials，Ministry of Education，Changchun University of Technology，Changchun 130012，China
    2.School of Materials Science and Engineering，Changchun University of Technology，Changchun 130012，China
    3.Changchun Railway Vehicles Co. ，Ltd. ，Changchun 130062，China
    4.The Challenge Petrochemical Machinery Corporation of Maoming （CPM），Maoming 525024，China
  • Received:2023-12-10 Online:2025-08-01 Published:2025-11-14
  • Contact: Wei LIU E-mail:1060952646@qq.com;liuwei920107@sina.com

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Abstract:

SA516Gr.70 steel has excellent strength and toughness and is widely used in the manufacture of pressure vessels， but the traditional arc welding is prone to lead to defects， such as crack and hydrogen embrittlement in the weld. In this paper， a W-25Re（%） alloy stirring tool was used to carry out friction stir welding of 4 mm thick SA516Gr.70 steel plate， and the thermal cycle curve during welding was measured by using a type K thermocouple to study the effect of thermal cycle on the microstructure and mechanical properties of the welded joint. The results show that defect-free welded joints can be obtained at a rotation rate of 800 rpm and a traverse speed of 90 mm/min. The maximum value of the peak temperature of the thermal cycle appeared in the advancing side （AS） reaching 1 018 ℃ and in the retreating side reaching 998 ℃. The microstructure of the weld nugget zone （WNZ） produces fine martensite and bainite， with an average grain size of 5.4 μm， which is a significant effect of grain refinement in the WNZ compared with the base metal. The microhardness of welded joints is negatively correlated with the distance from the center of the weld， and the microhardness of the WNZ reached 449.7 HV. Tensile test fractures are all in heat affected zone （HAZ）， and different areas of the joint show different fracture morphologies.

Key words: SA516Gr.70steel, friction stir welding, thermal cycle, microstructures, mechanical properties

CLC Number: 

  • TG456.9

Table 1

Chemical composition of SA516Gr.70 mild steel"

CMnSiCrCuNiMoNbTi
0.1741.1260.2310.1570.0120.010.060.010.011

Fig.1

Schematic of the tests"

Fig.2

Thermal cycle curve of each temperature measurement point of FSW joint"

Fig.3

Cooling rate of each temperature measurement point of AS and RS of FSW joint"

Fig.4

Weld surface morphology"

Fig. 5

Cross-sectional shape of SA516Gr.70 mild steel FSW joint"

Fig.6

EBSD orientation imaging of SA516Gr.70 mild steel FSW joint"

Table 2

Average grain size of each area of FSW joint"

BMSZAS-TMAZRS-TMAZAS-HAZ1RS-HAZ1
9.85.47.86.911.510.9

Fig.7

Percentage of grain boundary orientation in each region of SA516Gr.70 mild steel FSW joints"

Fig.8

FSW joint cross-sectional hardness contour map and hardness distribution curve"

Fig.9

Fracture location of SA516Gr.70 mild steel FSW joint tensile specimen and stress-strain curves of BM and test specimen"

Fig.10

Fracture morphology of SA516Gr.70 mild steel FSW joint tensile samples in each region"

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