Main Problems When Welding Cast Iron:
- The weld joint is prone to white and hardened structure. Because the rate of cooling of the weld metal is fast, which is different from the rate at which the cast iron is cooled in the sand, Part of the cast iron base metal is melted into the weld, so that the carbon content of the weld is increased, the semi-melting zone of the heat affected zone and the weld are easy to produce martensite hardening structure, and the base metal easy to generate chill structure and other poor performance structure.
- When the weld chemical composition is cast iron, in order to ensure that the carbon in cast iron precipitates in the form of graphite, To avoid the formation of white cast iron, we should take appropriate preheating measures before welding, slow down the rate of cooling, adjust the chemical composition of weld, enhance the graphitization ability of weld and prevent the carbon in the base metal from transferring into the weld to produce martensite. The precipitation of graphite during cooling mainly depends on the chemical composition of welding seam and cooling conditions.
- Cast iron welding, welding joints easy to appear cold cracks and hot cracks. The cold crack usually appears below 400 °C, which is caused by the poor plasticity of cast iron and welding restraint stress.
- The hot crack is caused by the welding stress in the process of eutectic and crystallization with low melting point, and is related to the content of sulfur. It occurs mostly in the heterogeneous weld metal with nickel-based and low carbon steel welding materials.
Cast Iron Welding Process Features:
The welding process of cast iron is generally divided into hot welding, semi-hot welding and cold welding, and the welding materials are different in different welding processes.
- Cast iron hot welding process is to preheat the whole or part of the cast iron to 600 ~ 700 °C, and maintain the temperature in the welding process, after welding while red hot state covered with asbestos powder or other insulation materials, slow cooling, conducive to graphite precipitation. The advantage of hot welding method is to reduce the temperature difference between weld and base metal, thus reducing the stress level of welding joint, which is helpful to prevent cracks, avoid chilling and hardening structure.
- Cast iron semi-hot welding process is the whole or part of the cast iron preheated to 300 ~ 400 °C, and keep the temperature during the welding process. The semi-hot welding method improves the construction condition and reduces the welding cost, but the crack resistance of the weld decreases.
- Cast iron cold welding process is generally not preheated before welding. When the ambient temperature is lower or the welding constraint is larger, it can be preheated to 100 ~ 150 °C. Special welding materials and necessary process measures are often used when cast iron is cold welding.
Welding Material Selection for Cast Iron Welding:
- Cast iron hot welding and cast iron semi-hot welding can choose cast iron core homogeneous weld welding materials, such as EZC (Z248), EZCQ (Z258) electrode; Or use low carbon steel core of heterogeneous weld welding materials, such as EZFe-2 (Z100), EZC (Z208), EZCQ (Z238) electrode.
- The cold welding of cast iron is generally made of heterogeneous weld materials, that is, the chemical composition of the weld is different from that of the base metal, including three kinds of welding materials: steel base, nickel base and copper base. Such as steel based electrodes EZV (Z116, Z117), EZFe-Fe), nickel based electrodes EZNi (Z308), and EZNiFe (Z408).
The weld hardness of nickel-based electrode is low, the chill structure in the semi-melting zone is thin and discontinuous, and the color of the weld is close to that of cast iron, so it is widely used, but the production cost is high. EZNi-1 (Z308) electrode is a pure nickel core, Graphite coating, weld metal has a certain strength and plasticity, the width of the semi-melting zone is only 0.05mm, after welding can be cut, mainly used for cutting surface welding. EZNiFe-1 (Z408) electrode is a nickel-iron alloy core with graphite coating. The crack resistance of weld metal is better than that of pure nickel and nickel-copper electrode. Its width of half-melting zone is 0.1mm and its tensile strength is high.