Welding Repair Material Selection and Process Explanation for Cast Iron Components

Cast iron parts are an established structural material—many components of mechanical equipment are made from cast iron—and yet, repair welding of these cast iron parts continues to be a frequently encountered issue.

1. Classification of Cast Iron Parts and Typical Materials

Cast iron is an iron-carbon alloy with a carbon content exceeding 2%, typically containing impurities such as silicon, manganese, sulfur, and phosphorus. Sometimes, other alloying elements are added to produce cast irons with tailored properties. Depending on the state and form in which carbon exists within the cast iron, it can be classified into gray cast iron, ductile (or spheroidal) cast iron, white cast iron, malleable cast iron, and vermicular graphite cast iron.

If gray cast iron HT150 (HT15-33) complies with GB/T 9439-2010, and more types of cast iron—such as ductile iron grades QT500-7 (QT50-5) and QT60-2—are used, then both gray cast iron and ductile iron become available.

Characteristics of Cast Iron Welding Processes and Selection of Welding Materials

The main challenges in welding cast iron parts

1) Welded joints are prone to forming white etching and hardened microstructures. Due to the rapid cooling rate of the weld metal—distinct from the slower cooling rate of gray cast iron—the molten portion of the cast iron base material can partially flow into the weld, increasing the carbon content in the weld. This leads to the formation of martensitic hardened structures in the heat-affected zone's semi-molten region and within the weld itself, while the base material may develop undesirable microstructures such as white etching, significantly degrading its mechanical properties.

When welding cast iron, appropriate pre-welding heating measures should be taken to slow down the cooling rate, adjust the weld's chemical composition, enhance its graphitization capability, and prevent the transformation of parent carbon into weld material, thereby avoiding the formation of martensitic structures. Whether graphite can precipitate during the cooling process of cast iron largely depends on the weld's chemical composition and the cooling conditions.

2) When welding cast iron, the weld joint is prone to both cold and hot cracking. Cold cracks typically occur below 400°C, resulting from the combined effects of cast iron's varying ductility and the tensile stresses imposed by welding constraints.

Thermal cracking is caused by low-melting-point eutectics and the effects of welding stresses during the solidification process; it is closely related to sulfur content and typically occurs in dissimilar weld metals that use nickel-based and low-carbon steel welding materials.

Characteristics of Cast Iron Welding Processes

The welding processes for cast iron parts are generally categorized into three types: hot welding, preheated welding, and cold welding, with each process requiring different welding materials.

1) The cast iron hot-welding process preheats the entire or partial cast iron component to 600–700°C, maintaining this temperature throughout the welding procedure. After welding, the joint is covered with red-hot asbestos powder or other insulating material and allowed to cool slowly, which promotes graphite precipitation. The advantages of this hot-welding method include reducing the temperature difference between the weld metal and the base material, lowering stress levels in the welded joint, preventing cracks, and avoiding white mouth formation and hard microstructures.

2) The semi-hot welding process for cast iron involves preheating the entire or part of the cast iron to 300–400°C and maintaining this temperature throughout the welding procedure. While this semi-hot method enhances construction conditions and reduces welding costs, it results in poor crack resistance during welding.

3) Cast iron cold welding typically does not require preheating before welding. However, if the ambient temperature is low or welding constraints are significant, preheating to 100–150°C is recommended prior to welding. Cast iron cold welding usually demands specialized welding materials and essential process measures.