Weldability and microstructure evolution of powder metallurgy high chromium cast iron/low carbon steel welded by gas tungsten arc welding
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摘要:
以粉末冶金高铬铸铁和低碳钢为原材料，采用多道次手工钨极氩弧焊，研究了焊接电流对焊缝组织演变和力学性能的影响，提出了焊接接头组织演化模型，并探讨了其断裂机理。结果表明，焊接接头的抗拉强度在焊接电流为140 A时达到538.1MPa，分别是烧结高铬铸铁和低碳钢抗拉强度的95.3%和97.4%。由于二次回火和合金元素扩散/偏析的作用，焊接接头的显微硬度在水平方向上由高铬铸铁一侧向低碳钢一侧逐渐降低，而在垂直方向则呈M形分布。当焊接电流为140 A时，焊缝的熔合区主要由奥氏体和回火马氏体组成，在熔合区与低碳钢母材之间存在一个单一奥氏体柱状晶区；在熔合区与烧结高铬铸铁母材之间，存在一个柱状高铬铸铁区域，该区域的碳化物为粗糙树枝状，沿基体晶界分布。
Abstract:Powder metallurgy high chromium cast iron (PM HCCI) and low carbon steels (LCS) were welded by multi-pass manual gas tungsten arc welding (GTAW). The effects of welding current on the microstructure evolution and mechanical properties of the weld joints were systematically investigated, the microstructure evolution model of the weld joints was proposed, and the fracture mechanism of the solders was also discussed. In the results, the tensile strength of the welded joints reaches 538.1 MPa at 140 A welding current, which is 95.3% and 97.4% of the tensile strength for PM HCCI and LCS, respectively. The microhardness of the welded joints decreases from HCCI side to LCS side in the horizontal direction, whereas in the vertical direction, the microhardness distribution is of M-shape due to the secondary tempering and alloying element diffusion/segregation. The fusion zone (FZ) mainly consists of the austenite and tempered martensite at the welding current of 140 A, and there is a single austenite columnar crystals zone between FZ and LCS; while there is a HCCI columnar crystal zone between FZ and the sintered HCCI, in which the coarsen carbides with branches are distributed along the matrix’s grain boundary.
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图 1 焊缝的几何形状（a）和试样拉伸尺寸（b）
Figure 1. Geometry of the weld (a) and the dimensions of tensile samples (b)
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图 2 低碳钢、高铬铸铁和焊接接头在不同焊接电流下拉伸强度（a）、断裂延伸率（b）和拉伸断裂样品（c）
Figure 2. Tensile strength (a), elongation (b), and tensile fractured samples (c) of HCCI, LCS, and the weld joints at different welding currents
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图 3 焊接接头在焊接电流为140 A时的显微硬度：（a）取点位置；（b）水平方向；（c）垂直方向
Figure 3. Microhardness of the welded joints at 140 A welding current: (a) testing points distribution; (b) horizontal direction and; (c) vertical direction
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图 4 焊接接头高铬铸铁侧在不同焊接电流下的显微硬度
Figure 4. Microhardness of the welded joints on the high chromium cast iron side at different welding currents
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图 5 不同焊接电流下焊接接头的拉伸断口显微形貌：（a）130 A；（b）、（c）140 A；（d）160 A
Figure 5. Fracture surface SEM images of the welded joints at different welding currents: (a) 130 A; (b), (c) 140 A; (d) 160 A
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图 6 低碳钢–高铬铸铁氩弧焊焊缝宏观形貌（a）和各部位显微组织（b）～（i）
Figure 6. Macroscopic morphology (a) and the microstructure of each parts (b)～(i)
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图 7 焊接接头显微组织示意图
Figure 7. Schematic diagram of microstructure distribution model for PM HCCI/LCS weld joints
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图 8 焊接接头高铬铸铁一侧在不同焊接电流下的显微组织：（a）、（d）130 A；（b）、（e）140 A；（c）、（f）160 A
Figure 8. Microstructure of the welded joints in the HCCI side at different welding currents: (a), (d) 130 A; (b), (e) 140 A; (c), (f) 160 A
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图 9 焊接电流为140 A时高铬铸铁–焊缝界面能谱分析
Figure 9. SEM image and EDS analysis of the fusion zone and HCCI interface with the welding current of 140 A
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图 10 焊接电流为140 A时焊缝–低碳钢界面能谱分析
Figure 10. SEM image and EDS analysis of the fusion zone and LCS interface with the welding current of 140 A
表 1 三种实验材料的化学组成（质量分数）
Table 1 Chemical compositions of the experimental materials
% 材料组分 C Cr Ni Si Mn Fe 粉末冶金高铬铸铁 2.67 15.40 1.22 0.77 0.63 余量 低碳钢 0.12 0.02 0.01 0.22 0.29 余量 焊丝 0.07 ― 0.20 0.90 1.60 余量 
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表 2 多道次手工钨极氩弧焊和热处理工艺参数
Table 2 Process parameter of manual multipass GTAW welding and heat treatment
焊丝直径 / mm 钨电极直径 / mm 氩气纯度 / % 气体流量 / (L·min−1) 焊接电流 / A 热处理工艺 2 2.4 99 5 130, 140, 150, 160 500 ℃, 2 h
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表 3 焊缝的元素组成（质量分数）
Table 3 Compositions of the welded zone
% 检测位置 Cr C Fe ① 1.58 10.06 88.36 ② 6.61 8.75 87.75 ③ 3.12 7.61 85.29
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