Welding cracks are the most serious type of welding defect.
Welding cracks are strictly divided into five main categories: cold cracks, hot cracks, reheat cracks, lamellar tearing, and stress corrosion cracks.
Today, we will mainly discuss cold cracks. So, what are cold cracks?
Cold cracks are cracks that appear at relatively low temperatures. How low is the temperature? Generally, it is below 300°C. Some people might ask, 300°C is already quite high, but compared to the melting point of steel (around 1500°C, which varies depending on the alloy elements) and the temperature at which hot cracks form, 300°C is considered a low temperature. Therefore, cracks formed when the welded joint cools to below 300°C (below the martensitic transformation start temperature) are commonly called cold cracks.
Cold cracks are divided into three main categories:
The first type is called delayed cracking. It does not occur immediately after welding, but after a period of time. This time may be a few hours, a few days, or even a few months.
How are delayed cracks formed?
They are mainly caused by the high content of diffusible hydrogen in the weld. In factories, some materials require low-hydrogen welding rods or wires to control the content of diffusible hydrogen, such as medium-high carbon steel and medium-high alloy steel. Low-hydrogen welding rods or wires are generally used to avoid the occurrence of delayed cracks.
Cold cracks
The second type of cold crack is quench hardening embrittlement cracking. This type of crack is mainly caused by martensite during the phase transformation process.
What is martensite? The rigorous academic definition is: a supersaturated solid solution of carbon in α-Fe. This definition is too complicated to understand. A simple analogy is: an office has four seats, but now there are more than four people who need to sit in this office. What to do? You can't drive them away, so you can only squeeze them in. This squeezing makes everyone uncomfortable. Therefore, martensite, relatively speaking, has high strength and high hardness. It is a hardened structure. When the welded joint cools rapidly, quench hardening cracks are likely to occur. Of course, martensite also comes in many types, such as lath martensite, twinned martensite, and plate-like martensite. Each type of martensite has different properties.
Generally, medium- and high-carbon steels and martensitic stainless steels are most prone to quench cracking. Therefore, when welding these materials, attention must be paid to the tendency for cracking.
The third type of cold crack is low-ductility cracking. This occurs when the shrinkage strain at low temperatures exceeds the material's own plastic limit.
For example, cast iron. As is well known, repairing cast iron by welding is very difficult. Why is this? Because cast iron is a very hard material, and the harder the material, the more difficult it is to weld and the more prone it is to low-ductility cracking. Therefore, when welding cast iron, it is essential to choose the appropriate cast iron welding rod or welding wire; otherwise, low-ductility cracking is very likely to occur.
That's all for this episode. In the next episode, we'll discuss another serious defect in welding: hot cracking.