Abstract Scope |
Modified Medium-Mn steel has been widely investigated as a promising candidate alloys for the third generation of advanced high strength steel and wear-resistant steel used in automotive industry, mining machinery and railway switches, because of the combination of strength and ductility, superior work hardening capacity and economic efficiency. In practice, the welding process is usually indispensable for the fabrication and application of the steel structure product. However, the modified medium-Mn steel has inferior weldability due to its high carbon and alloying elements content which easily results in the precipitation of carbide at grain boundary and the embrittlement of heat affected zone (HAZ). In addition, there is no mature application or commercial welding wire for medium-Mn steel at present. Aiming at these problems, this study is to investigate metal inert gas (MIG) multi-pass welding of thick medium-Mn steel plate with austenitic stainless steel and nickel-based filler wire. The microstructural evolution and distribution of alloying elements in these two welded joint were examined carefully. The correlation between carbide precipitates and mechanical properties, including microhardness, tensile strength and impact toughness, of the welded joint, was also discussed.
The modified medium-Mn steel plate provided by Baosteel Inc. (China) was used as the base metal (BM). The microstructure of experimental steel is composed of equiaxial austenite and annealing twins with an average grain size is about 19.8 µm measured by linear intercept method, and no precipitates are observed at grain boundaries or inside the grains according to the optical microscope (OM) scanning electron microscopy (SEM) images. Before welding, the experimental steel plates were machined to the dimensions of 240 mm80 mm12 mm with groove angle of 60o, and the faying surfaces were grounded and cleaned with abrasive paper and alcohol. The butt welding was performed under the MIG mode using a CMT welder. The filler materials selected were 307Si stainless steel and NiCrMo-3 nickel-based wire both with diameter of 1.2 mm. Three-layer and four-pass welding was conducted to prepare the joint of medium-Mn steel with a thickness of 12 mm, the inter-pass temperature was controlled below 100 oC. The metallographic specimens were prepared by standard processes, and then BM and HAZ were etched with a solution of 4% Nital for 30 s, the weld metal (WM) with 307Si and NiCrMo-3 wire was electrolytic etched using volume fraction 10% oxalic acid, to reveal the microstructure. X-ray diffraction device (XRD), Electron probec micro-analyzer (EPMA) and scanning transmission electron microscopy (STEM) were employed to observe the microstructure, elemental distribution and carbide precipitates. Mechanical properties were measured via microhardness, tensile and Charpy impact tests. Impact specimens were machined from four different positions into standard V-notch samples. The V-notch position of impact specimens was designed at the center of WM, fusion line (FL), 2 mm outside of FL and 4 mm outside of FL, respectively, which was referred as WM, FL, HAZ1 and HAZ2, respectively.
The microstructure in HAZ of medium-Mn steel welded joint with 307Si wire and NiCrMo-3 wire, respectively, is clumpy austenite without any phase transformation, which can also be confirmed by XRD results. Apparent coarse-grain HAZ was observed in both welded joints. However, there is an obvious fine-grain region between HAZ and WM in welded joint with NiCrMo-3 wire, which is related to the nucleation of liquid-solid transition at FL and the diffusion of Nb element from WM to HAZ. The EPMA analysis results show that there was carbon migration near the fusion line, which was attributed to the high content of Cr in WM and high temperature. We also found the segregation of C and Cr at grain boundaries in HAZ which may be chromium carbide precipitates. TEM analysis was utilized to further confirm these precipitates. According to TEM and SAED patterns analysis, the irregular block-shaped precipitates is M7C3 with the size of about 200 nm, which is rich in Cr, Mn and C. The variation trend of microhardness first increases and then decreases gradually from WM to HAZ and then to BM in these two welded joints. The higher microhardness of HAZ than BM was attributed to the precipitation of carbide at grain boundaries. The tensile strength of medium-Mn welded joint with NiCrMo-3 and 307Si wire is 675 MPa and 572 MPa, approximately 85.7% and 72.6% welding efficiency, respectively. The two joints with NiCrMo-3 and 307Si wire both failed in HAZ. Charpy V-notch impact test results reveal that the impact toughness at FL of welded joint with NiCrMo-3 wire (104 J) and 307Si wire (96 J) are lower than that of BM (134 J), which was caused by the inhomogeneous microstructure, complex composition and serious stress concentration. The impact toughness in HAZ1 of welded joint with NiCrMo-3 wire (136.3 J) is higher than that with 307Si wire (107 J), because the discontinuous blocky-shaped precipitates are randomly distributing along the grain boundaries in HAZ of welded joint with 307Si wire.
Obvious grain coarsening occurred in HAZ for both welded joints. The carbon migration of BM and the dilution of alloying elements in WM existed in the vicinity of FL. C and Cr segregated at grain boundaries in HAZ, the carbides were determined as M7C3 phase. Although, all the welded joints failed in HAZ during tensile tests, the tensile strength of welded joint with NiCrMo-3 wire is much higher than that with 307Si wire. The impact toughness around the fusion line is the worst for these two welded joints, which was caused by the inhomogeneous microstructure and high residual stress. The microhardness in HAZ was higher than that in BM for both welded joints, attributed to the precipitation of M7C3 phase at grain boundaries. The tensile strength and impact toughness of welded joint with NiCrMo-3 wire are superior to the welded joint with 307Si wire. This was mainly caused by the higher heat input of welded joint with 307Si wire, resulting in the difference of morphology and distribution of the precipitates.
Keywords: Medium-Mn steel, MIG welding, Microstructural evolution, Carbide precipitate, Mechanical property |