STRENGTH AND TOUGHNESS OF T250 MARAGING STEEL JOINT HYBRID-TREATED WITH LASER WELDING AND AGING
Kun LI1,Jiguo SHAN1,2(),Chunxu WANG3,Zhiling TIAN3
1 Laser Processing Research Center, Department of Mechanical Engineering, Tsinghua University, Beijing 100084 2 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Tsinghua University, Beijing 100084 3 Institute for Special Steel, Central Iron & Steel Research Institute, Beijing 100081
Cite this article:
Kun LI,Jiguo SHAN,Chunxu WANG,Zhiling TIAN. STRENGTH AND TOUGHNESS OF T250 MARAGING STEEL JOINT HYBRID-TREATED WITH LASER WELDING AND AGING. Acta Metall Sin, 2015, 51(8): 904-912.
Maraging steels are leading members of the ultra-high strength steel family due to a combination of two solid state reactions: martensitic transformation and subsequent ageing. These steels can be hardened by the precipitation of refined Ni3(Ti, Mo) intermetallic particles. They have been widely used in the military and aerospace applications such as solid rocket motor cases and submarine shells due to their high strength and toughness. The T250 maraging steel has used Ti as one of the primary strengthening elements to replace Co, which decreases the cost of maraging steels. Its properties are comparable to the standard Co-bearing grades in the 1.4~2.1 GPa strength levels. It possesses good weldability without porosity in the weld and other weld defects. However, the combination of strength and toughness of welded joints is the main problem which has not been solved well via different welding methods so far. In this work, T250 maraging steel plate with 2 mm thickness was hybrid-treated with laser welding and aging treatment. The strength and toughness of welded joints aged before and after welding were investigated. The microstructures of parent metals and welded joints were observed with OM and SEM. Chemical compositions in parent metals and weld zones were analyzed with EPMA. The tensile strength and static toughness were acquired with the auxiliary device of Gleeble machine and could represent strength and toughness of the welded joints. The results show that the tensile strength and static toughness of the welded joint aged before welding are 62% and 28% that of the aged parent metal, respectively. However, the tensile strength and static toughness of the welded joint aged after welding reach 98% and 71% that of the aged parent metal, respectively. The weld metal is the key zone to influence the strength and toughness of the welded joints. Ni3(Ti, Mo) precipitates in the weld metal are the intrinsic reason resulting in that the strength and toughness of the welded joint aged after welding are superior to that of the welded joint aged before welding. Ni3(Ti, Mo) precipitates are beneficial to the strength and static toughness in the elastic deformation stage, and it has a dual effect on the static toughness in the plastic deformation stage of the welded joints. The reverted austenite has a negligible effect on the strength and static toughness in the elastic deformation stage, while it is detrimental to the static toughness in the plastic deformation stage of the welded joints.
Table 1 Tensile strength and static toughness of parent metals and welded joints hybrid-treated with welding and aging
Fig.3 OM images of as-received parent metal (a), aged parent metal (b), welded joint aged before welding (c), welded joint without aging (d) and welded joint aged after welding (e)
Fig.4 SEM images of as-received parent metal (a, b), aged parent metal (c, d), welded joint aged before welding (e, f), welded joint without aging (g, h) and welded joint aged after welding (i, j) at low (a, c, e, g, i) and high (b, d, f, h, j) magnification
Fig.5 BSE images of Ni (a), Mo (b) and Ti (c) in welded joint aged after welding
Material
Location
Ni
Mo
Ti
Al
Cr
Fe
As-received parent metal
Cell center
18.96
3.00
1.49
0.110
0.30
76.14
Cell boundary
19.01
3.01
1.52
0.090
0.28
76.09
Aged parent metal
Cell center
19.00
3.02
1.49
0.120
0.29
76.08
Cell boundary
19.02
3.01
1.50
0.100
0.31
76.06
Welded without aging
Cell center
18.38
2.77
1.32
0.097
0.28
77.15
Cell boundary
19.46
3.12
1.53
0.099
0.30
75.49
Aged after welding
Cell center
17.20
2.34
1.08
0.099
0.30
78.98
Cell boundary
20.67
3.48
1.72
0.096
0.29
73.74
Table 2 EPMA analysis of chemical compositions in parent metals and welded joints hybrid-treated with welding and aging
Fig.6 OM images of cross sections of welded joints aged before (a) and after (b) welding (BM—base metal, WM—weld metal, PMZ—partial melted zone, DER—deep erosion region)
Fig.7 Microhardness of welded joints hybrid-treated with welding and aging (HAZ—heat affected zone)
Fig.8 SEM images of PMZ (a, c) and DER (b, d) in welded joints aged before (a, b) and after (c, d) welding
Fig.9 SEM images of fractures in as-received parent metal (a), aged parent metal (b), welded joint aged before welding (c) and welded joint aged after welding (d)
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