Under the combined effect of welding stress and other embrittlement factors, the bonding force of metal atoms in the local area of ​​the weld joint is destroyed and the gaps formed by the new interface are called welding cracks. They have sharp notches and large aspect ratio characteristics.

(1) Characteristics of thermal crack defects in steel pipes: During welding, the cracks generated when the weld and heat-affected zone metal cool to the high temperature zone near the solidus are thermal cracks.
- Causes: This is the result of the combined effect of the liquid layer formed by the low-melting eutectic during the cooling and crystallization of the molten pool and the solidification under the action of tensile stress. Increasing the effect of any one aspect can promote the formation of thermal cracks.
- Preventive measures: Control the content of harmful impurities in the weld, that is, the content of carbon, sulfur, and phosphorus, and reduce the formation of low-melting eutectic in the molten pool. The content of sulfur and phosphorus in the weld metal is generally less than 0.03%. The mass fraction of carbon in the welding wire does not exceed 0.12%. Basic welding rods or flux should be used for welding important components. Control welding parameters, appropriately increase the weld shape coefficient, and avoid deep and narrow welds. Use multi-layer and multi-pass welding, preheat before welding and slow cooling after welding, correctly select welding joint forms, arrange welding sequence reasonably, and try to use symmetrical welding. Use an arc closing plate to lead the arc pit to the outside of the weldment, so that even if an arc pit crack occurs, it will not affect the weldment itself.

(2) Characteristics of cold crack defects in steel pipes: Welding cracks generated when the welded joint is cooled to a lower temperature (200~300 °C for steel) are called cold cracks.
- Causes: Cold crack defects mainly occur in medium carbon steel, low alloy steel, and medium alloy high-strength steel. The reasons are: the welding material itself has a greater tendency to harden; a large amount of hydrogen is dissolved in the welding pool; the welded joint generates a large restraint stress during the welding process.
- Preventive measures: Before welding, strictly dry the welding rod and flux according to the requirements to reduce the source of hydrogen. Strictly clean the dirt, moisture, and rust on the groove and both sides, and control the ambient temperature. Select high-quality low-hydrogen welding materials and their welding processes. When welding low-alloy high-strength steel with strong hardenability, use austenitic stainless steel electrodes. Correctly select welding parameters, preheating, slow cooling, post-heating, and post-weld heat treatment. Choose a reasonable welding sequence to reduce welding internal stress. Properly increase the welding current and slow down the welding speed to slow down the cooling rate of the heat-affected zone and prevent the formation of hardened structure.

(3) Characteristics of reheat crack defects in steel pipes: Cracks caused by reheating the weldment within a certain temperature range after welding (such as stress relief heat treatment or multi-layer welding) are called reheat cracks.
- Causes: Reheat cracks generally occur in the area of ​​1200~1350 near the melting point line. The heating temperature for reheat cracks in low-alloy high-strength steel is approximately 580~650 °C. When the steel contains more alloying elements such as chromium, molybdenum, and tungsten, the tendency of reheat cracks increases.
- Preventive measures: Control the chemical composition of the steel pipe and weld metal, and appropriately adjust the content of elements that have a great impact on reheat cracks (such as chromium, vanadium, and boron). Reduce joint stiffness and stress concentration, and grind the weld and its interface with the steel pipe smooth. Use high heat input for welding. Increase the preheating and post-heating temperatures. Take process measures to reduce welding stress during welding, such as using small diameter electrodes, welding with small welding parameters, and not swinging the electrode during welding. When relieving stress tempering, avoid the sensitive temperature zone that produces reheat cracks. The sensitive temperature varies with the type of steel.