As one of the most harmful welding defects, welding cracks seriously affect the usability and safety and reliability of the welding structure. Welding cracks are classified into four types: hot cracks, cold cracks, reheat cracks, and lamellar tears. Through the study of the first two lessons, everyone took a look at the causes and preventive measures of hot and cold cracks. In this lesson, we come to know the reheat crack.
Reheat cracking refers to cracks caused by re-heating (stress-relief heat treatment) of the welded joint in a certain temperature range after welding. Such cracks mostly occur in welded joints of low-alloy high-strength steels, pearlite heat-resistant steels, austenitic stainless steels, nickel-based alloys, etc., especially the coarse-grained regions in the heat-affected zone. Reheat cracks are analyzed from different angles and present different characteristics.
From the material point of view
Metal materials containing certain precipitation strengthening elements, such as low-alloy high-strength steel, pearlite heat-resistant steel, austenitic stainless steel, and some nickel-based alloys have high reheat crack sensitivity. Carbon steel and solid solution strengthened metal materials are generally No reheat crack was formed.
From the conditions of formation
The formation of cracks occurred in the process of "reheating" (heat treatment or service at a certain temperature), and the welding zone had a large residual stress accompanied by stress concentration. Sensitive temperature range of reheat cracks for certain materials: Sensitive temperature for precipitation-strengthened low-alloy high-strength steel is about 500-700 ℃; for austenitic stainless steel and high-temperature alloys, the sensitive temperature is about 700-900 ℃ Inside.
From the crack location and propagation path
Reheat cracks occur in the coarse grained area of the heat-affected zone of the weld. The cracks propagate along the coarse grain boundaries of the austenite on the side of the base metal of the fusion line (cracking along the grain). Reheat crack.
To prevent reheat cracks, we need to take measures from many aspects, such as raw materials and welding specifications.
Strict control of raw materials. Select a welding material with low reheat crack sensitivity (strictly control the content of S, P, V, Nb and other elements), and the lower limit of the weld metal strength.
Develop reasonable welding specifications. We must reduce the welding line energy as much as possible and control the preheating interlayer temperature. These two aspects determine the cooling conditions of the weld metal and have a greater impact on the microstructure of the weld zone. Generally, a small line of energy is used for multiple layers and the cooling rate of the weld zone is appropriately increased, which is beneficial to improve the microstructure, improve the impact toughness, and prevent the occurrence of hot cracks.
Take appropriate warm-up measures. It can soften the hardness of the hardened layer, improve crack resistance and toughness.
Control the welding process and reduce the amount of small defects. Strict implementation of welding specifications to reduce small defects; reduce the amount of deposited metal, the use of narrow gap welding is also an effective measure to control reheat cracks.
Control of welding residual stress. The use of half-way heat treatment or high-frequency ultrasonic impact method can reduce welding residual stress.
During the post-weld heat treatment process, the speed of heating and cooling is controlled to expand and contract slowly and uniformly, and reduce the occurrence of reheat cracks.
Because reheat cracks are generated during heat treatment or operation, the reheat cracks are concealed to a certain extent, and the accidents caused by them are unpredictable. Therefore, we must consider the occurrence of reheat cracks in the early stages of equipment design, manufacturing and inspection.