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对变压器硅钢片熔化极惰性气体保护焊(MIG)焊接过热区组织进行了分析,结果表明,焊接接头热影响区的脆化主要表现在:1.局部晶界的脆化。在应力作用下,裂纹首先在局部晶界成核,形成裂纹源。2.晶粒的脆化。过热区内魏氏组织形态的析出相强化了铁素体,引起了脆化,使晶内塑性变形能力急剧降低。形态分析表明,沿晶破断具有低塑性开裂特征,由于晶界的脆化不是全局性的,总的说来晶界尚具有一定阻止裂纹发展的能力,此外由于同一晶粒内裂纹是多源发展的,因而晶内的强化仍然是导致宏观脆性断裂的主要因素。沿晶开裂的作用只是在一定条件下可以作为源,引发邻近晶粒的解理穿晶断裂。脆性断裂产生在HAZ 的粗晶区中,裂纹具有锯齿形特征。锯齿台阶彼此平行,晶内各微裂纹也相互平行。晶内按魏氏组织形态分布的沉淀相是在一个中等冷却速度下产生的。焊后900℃加热处理可使已经析出的晶界与晶内沉淀相以及有可能产生的M—A 型针状组织等消失,消除了由沉淀强化导致脆性的组织因素,大大改善了接头的塑性。过热的粗晶区中,尽管在微区可能产生孪晶型马氏体,然而由于其量甚微,且均被高韧性相奥氏体所包围,它对宏观脆性并未产生多大作用。
The results show that the embrittlement of the HAZ in welded joints mainly includes: (1) embrittlement of local grain boundaries. Under stress, the crack first nucleates at the local grain boundary to form the source of cracks. 2. Grain embrittlement. In the overheated zone, the precipitated phase of Widmanstatten strengthens the ferrite and causes embrittlement, resulting in a drastic reduction of the intracrystalline plastic deformation capacity. Morphological analysis shows that the intergranular fracture has the characteristics of low plasticity cracking. Because the embrittlement of the grain boundary is not global, the grain boundary generally possesses the ability of preventing crack growth. In addition, since the same intragranular crack is a multi-source development Therefore, the intragranular strengthening is still the main factor leading to macro-brittle fracture. Crack along the crystal role only under certain conditions can be used as a source, triggering the cleavage of adjacent crystal transgranular fracture. Brittle fracture occurs in the coarse grain region of the HAZ and the crack has zigzag features. The jagged steps are parallel to each other, and the microcracks in the crystal are also parallel to each other. The intragranular precipitation of Widmanstatten is produced at a moderate cooling rate. Heat treatment after welding at 900 ℃ can eliminate the precipitated grain boundaries and intragranular precipitates and M-A-type acicular structures that may be generated, and eliminate the tissue factor caused by the precipitation strengthening and thus greatly improve the plasticity of the joint . In the overheated coarse-grained region, although the formation of twin-type martensite may occur in the micro-region, it does not contribute much to macroscopic brittleness due to its small amount and being surrounded by high-toughness austenite.