Show you welding defects-hot cracks
Welding cracks, as one of the most harmful welding defects, seriously affect the usability and safety and reliability of the welded structure, which requires our attention.
According to the crack formation temperature, welding cracks are divided into hot cracks and cold cracks. In the last lesson, we learned about the causes of cold cracks and preventive measures. In this lesson, we will study thermal cracks in detail.
First, what is the welding heat crack?
Welding thermal cracks are mostly generated at high temperatures close to the solid phase line, and are characterized by distribution along the grain boundary. Sometimes, they can form along the "multilateralized boundary" at temperatures lower than the solid phase line. It is usually produced in the weld metal, and may also be in the heat affected zone structure near the weld fusion line (base metal). According to the crack generation mechanism, morphology and temperature range, welding thermal cracks can be divided into four types: solidification cracks, liquefaction cracks, polygonal cracks, and deplasticized cracks.
What is the difference between hot cracks and cold cracks?
1. Different temperature and time
Thermal cracks generally occur during the crystallization of the weld. Cold cracks generally occur when the weldment is cooled to 200-300 ° C. Some will appear immediately after welding, and some can be delayed for several hours to several weeks or even longer. Therefore, cold cracks are also called delayed cracks.
2.The generated parts and directions are different
The majority of thermal cracks occur in the weld metal, either longitudinally or laterally. Sometimes thermal cracks also extend into the base metal. Most of the cold cracks occur in the base metal or fusion line, most of them are longitudinal cracks, and few are transverse cracks.
3.Different appearance characteristics
The hot crack section has obvious oxidation color. The cold crack fracture is bright and has no oxidation color.
4.Different metallographic structure
Hot cracks are cracked along the grain boundaries. Cold cracks penetrate through the grains, that is, through-grain cracking, but some also crack along grain boundaries.
3. How are thermal cracks formed?
The first is the segregation of low melting point eutectics. Sulfur (S) and phosphorus (P) impurity elements in steels have severe macrosegregation due to metallurgical reactions, often forming a liquid film in the center of the weld.
The second is the effect of welding stress. The tensile stress caused by uneven heating and cooling during the welding process promotes the destruction of the liquid phase film and cracks.
The third is some other factors. Different materials have different thermophysical properties, resulting in different welding stresses; different welding methods and process parameters have different welding heat inputs; and the formation coefficient of the weld has a large effect on segregation.
How to prevent the occurrence of hot cracks?
1.Metallurgical measures
The metallurgical measures are mainly to limit the chemical composition of the weld. In order to reduce the tendency of the weld to form a low melting point eutectic, the content of sulfur (S) and phosphorus (P) is limited as much as possible; the carbon content of the weld is increased, and the Manganese content. The second is to change the structure of the weld. It is impossible to completely eliminate harmful impurities or even prevent it from forming low melting eutectics at all. Therefore, in order to prevent cracks under tensile stress, special alloying elements are often added to the weld metal to adjust the chemical composition of the weld metal and form a two-phase structure in the weld to disrupt the crystallization direction of the weld metal. , So that the low melting point eutectic can not be concentratedly distributed, thereby reducing the formation of thermal cracks.
2.Process measures
1) Decrease the fusion ratio. Decrease the fusion ratio to reduce the dilution. When multi-layer welding is used to weld the first layer of the weld, the proportion of the base metal fused to the weld is large, which increases the carbon content, sulfur, and phosphorus content, and easily generates thermal cracks.
2) Limit overheating. Overheating of the molten pool can easily cause thermal cracking. Reduce the line energy and use a small welding current and a small welding speed. Can not reduce the line energy by increasing the welding speed to ensure the formation of the weld.
3) Choose a reasonable welding sequence and welding direction to reduce the restraint. Usually, the convex weld with smaller size can reduce the sensitivity of cracks.
4) The basic electrode and flux are used. This is because the slag of the alkaline electrode and the flux has strong desulfurization ability.
In short, the fundamental way to prevent hot cracks is to reduce the number of low melting eutectics, take appropriate measures to reduce welding tensile stress, reasonably control the weld formation coefficient, reduce welding current, and thus reduce the occurrence of hot cracks.
