School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
Cite this article:
ZHANG Min,JIA Fang,CHENG Kangkang,LI Jie,XU Shuai,TONG Xiongwei. Influence of Quenching and Tempering on Microstructure and Properties of Welded Joints of G520 Martensitic Steel. Acta Metall Sin, 2019, 55(11): 1379-1387.
As a low carbon martensitic precipitation hardening stainless steel, G520 steel has been widely used in heavy load and corrosion-resistant components such as compressor impeller due to its high strength with reasonable toughness, ductility and corrosion resistance. Although heat treatment usually presents a tendency to promote a improvement of mechanical properties, it may cause unpredictable changes in the microstructure and properties of high strength steel weldment, which is extremely complicated and normally very sensitive to heat. Based on this scenario, the influence of quenching and tempering on the mechanical and microstructural properties of G520 steel weld metals obtained by shielded metal arc welding (SMAW) was studied in this work. Tensile test, impact test and metallographic examination by OM, XRD, SEM and EBSD were performed for mechanical and microstructural characterization. The results indicate that, the welded joints after quenching (at 850 ℃, oil cooling) and tempering (at 520 ℃, air cooling) have better strength and toughness than the pre-weld quenching and tempering. Moreover, the quenching and tempering treatment of the weld metal, breaks down the columnar microstructure into smaller martensite sub-blocks. Meanwhile, it form a certain amount of inversion austenite at the prior austenite grain boundary and the boundary of the lath martensite. As above, the proportion of the large angle grain boundary is increased, which effectively improves the toughness of the weld metal.
Fund: Supported by National Natural Science Foundation of China(51974243);Natural Science Foundation of Shaanxi Provincial Department(2019JZ-31);and Science and Technology Program of Xi'an, China(201805037YD15CG21(16))
Table 1 Chemical compositions of materials applied (mass fraction / %)
Fig.1 Schematic of welded joint (unit: mm)
Welding layer
d / mm
I / A
U / V
v / (mm·s-1)
Obverse side 1~2
3.2
120~135
25~35
2.3~3.0
Reverse side 1~2
3.2
120~135
25~35
2.5~3.3
Reverse side 3
4.0
155~167
25~30
3.3~3.8
Obverse side 3~4
4.0
155~167
25~30
3.3~5.0
Obverse side 5
4.0
155~167
25~30
2.8~3.3
Reverse side 4~5
4.0
155~167
25~30
3.3~4.3
Table 2 Welding parameters of G520 steel plate
Fig.2 Schematics of heat treatment process (AC—air cooling, OC—oil cooling, FC—furnace cooling, Ac1—start temperature of austenite transformation)(a) quenched-tempered before welding (1#) (b) quenched-tempered after welding (2#)
Fig.3 OM images of welded joints quenched-tempered before (a, c, e) and after (b, d, f) welding (BM—base metal, HAZ—heat affected zone, WM—weld metal)(a, b) fusion line (c, d) WM (e, f) HAZ
Material
σs / MPa
σb / MPa
δ / %
Fracture location
Quenched-tempered parent metal
1069.0
1097.5
21.4
-
Quenched-tempered before welding
917.0
1070.5
15.0
Weld metal
Quenched-tempered after welding
1055.4
1118.0
16.3
Weld metal
Table 3 Tensile strength of parent metal and welded joints quenched-tempered before and after welding
Fig.4 Impact toughnesses of welded joints quenched-tempered before (1#) and after (2#) welding
Fig.5 SEM images of impact fracture of weld metals (a, b) and fusion line (c, d) in welded joints quenched-tempered before (a, c) and after (b, d) welding
Fig.6 EBSD orientation imaging maps of weld metals quenched-tempered before (a) and after (b) weldingColor online
Fig.7 XRD spectra of parent metal and weld metals quenched-tempered before (1#) and after welding (2#)
Fig.8 Misorientation angle distribution figures of grain boundaries in weld metal zone of quenched-tempered before (a) and after welding (b)
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