Cracks are a serious problem in welded cast iron. Cracks in welded joints of cast iron may occur in weld metal or in base metal. The cracks of base metal usually appear near the crack zone, which may be longitudinal, transverse or oblique. Shandong cast iron guardrail manufacturer reminds us here that because of the poor plasticity of cast iron, hardly any plastic deformation can occur and the low strength, cracks will occur when the local stress is greater than the strength limit under the joint action of welding stress and the stress (tissue stress) of the casting itself. In severe cases, the weld metal will be separated from the base metal, that is, the weld will be separated from the base metal, that is, the so-called stripping.
If the weld strength is higher and the strength of base metal is lower, or the white cast iron shrinkage (1.6% - 2.0%) is larger than that of cast iron (0.9% - 1.8%) and the plasticity is poor, the peeling will occur. Cracks in weld metal are generally transverse cracks, sometimes longitudinal and oblique cracks. There is no common blue color when high temperature oxidation occurs at weld fracture. Cracks often produce crisp cracking sounds of metals. Usually the cracks occur after the dark red of the hot weld metal disappears, that is, below 600 C, until the temperature of the weld and the whole weld is homogenized. The temperature at which cracks are most likely to occur is below 400 C. Usually, the cracks that occur under the combined action of thermal stress and tissue stress are called thermal stress cracks.
(1) Preheating before welding and slow cooling after welding:
Preheating the whole or part of the weld before welding and slow cooling after welding can not only reduce the whiteness tendency of the weld, but also reduce the welding stress and prevent the cracking of the weld。
(2) Measures to reduce welding stress by arc cold welding:
选用塑性较好的焊接材料，如用 镍,铜,镍 铜,高钒钢等作为填充金属，使焊缝金属可通过塑性变形松弛应力，防止裂纹；用细直准焊条，小电流，断续焊，分散焊的方法可减小焊缝处和基本金属的温度差而减小焊接应力；通过锤击焊缝可以消除应力，防止裂纹。
Welding materials with good plasticity, such as nickel, copper, nickel-copper and high vanadium steel, are selected as filler metals to make the weld metal relax stress and prevent cracks through plastic deformation; the temperature difference between the weld and the base metal can be reduced by fine collimating electrode, small current, intermittent welding and dispersed welding; and the stress can be eliminated by hammering the weld to prevent cracks。
When the weld is cooled, it can compensate for the free shrinkage of the hindered bottom, thus avoiding excessive force leading to cracks.
(3) Adopt hot welding method and control temperature well。
When the temperature is higher than 600 C, some internal stresses are eliminated due to certain plastic deformation. Generally, thermal stress cracks will not occur when welding above 600 C.
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