1. Hot Cracks
1.1 Solidification Cracks
1.1.1 Mechanism of Formation
During the later stages of solidification, a low-melting eutectic liquid film weakens the intergranular bonds, leading to cracking under tensile stress.
Commonly found in carbon steel and low-alloy steel welds with high impurity content, and in single-phase austenitic steel and nickel-based alloy welds.
1.1.2 Influencing Factors
High levels of sulfur, phosphorus, carbon, and silicon in the weld metal increase the tendency for solidification cracking.
Improper welding process parameters, such as excessive welding current and slow welding speed, which prolong the time the weld remains at high temperatures, also easily lead to cracking.
1.1.3 Preventive Measures
Strictly control the content of impurities such as sulfur and phosphorus in the base metal and welding materials.
Optimize welding process parameters, reasonably control welding current and speed, and avoid prolonged high-temperature exposure of the weld.
1.2 High-Temperature Liquefaction Cracks
1.2.1 Mechanism of Formation
The peak temperature of the welding thermal cycle causes remelting in the heat-affected zone and between multi-layer welds, resulting in cracks under stress.
Mainly occurs in the near-weld zone or between multi-layer welds of high-strength steels containing chromium and nickel, austenitic steels, and nickel-based alloys.
1.2.2 Influencing Factors
High levels of sulfur, phosphorus, silicon, and carbon in the base metal and welding wire significantly increase the tendency for liquefaction cracking.
Excessive welding heat input leads to excessively high temperatures in the heat-affected zone, resulting in coarse grains and reduced material plasticity.
1.2.3 Preventive Measures
Select welding materials with low sulfur and phosphorus content to reduce elements sensitive to liquefaction cracking.
Control welding heat input to avoid overheating the heat-affected zone, refine the grains, and improve material plasticity.
1.3 Polygonization Cracks
1.3.1 Mechanism of Formation
Under high temperature and stress, lattice defects in the solidified crystal front move and accumulate to form secondary boundaries. In this low-plasticity state, cracks occur under stress.
Mostly occurs in pure metal or single-phase austenitic alloy welds or near-weld zones. 1.3.2 Influencing Factors
The magnitude and distribution of residual stress in welded joints; the greater the residual stress, the higher the tendency for polygonal cracking.
The composition and microstructure of welding materials; for example, excessively high alloy element content may affect the movement and aggregation of lattice defects.
1.3.3 Preventive Measures
Use a reasonable welding sequence and process to reduce welding residual stress.
Select appropriate welding materials and control the alloy element content to avoid excessive aggregation of lattice defects.
2. Reheat Cracking
2.1 Mechanism of Formation
In thick-plate welded structures made of steel containing precipitation-strengthening alloy elements, cracks occur in the coarse-grained areas of the heat-affected zone during stress-relieving heat treatment or service.
It mainly occurs in the coarse-grained areas of the heat-affected zone of low-alloy high-strength steel, pearlitic heat-resistant steel, austenitic stainless steel, and nickel-based alloys.
2.2 Influencing Factors
The chemical composition of the steel, such as the presence of precipitation-strengthening elements like vanadium, molybdenum, and titanium, promotes reheat cracking.
Welding process parameters, such as welding heat input and preheating temperature, affect the grain size and residual stress distribution in the heat-affected zone.
2.3 Preventive Measures
Optimize the steel composition to reduce the content of precipitation-strengthening elements.
Reasonably control welding process parameters, such as appropriately increasing the preheating temperature and reducing the welding heat input, to refine the grains in the heat-affected zone.
3. Cold Cracking:
3.1 Delayed Cracking
3.1.1 Mechanism of Formation
Cracks with delayed characteristics under the combined action of hardened microstructure, hydrogen, and restraint stress.
It mainly occurs in the heat-affected zone of low-alloy steel, medium-alloy steel, medium-carbon and high-carbon steel, and in some cases, on the weld metal.
3.1.2 Influencing Factors
Hydrogen content in the weld; hydrogen is a key factor leading to delayed cracking; the higher the hydrogen content, the greater the tendency for cracking.
Restraint stress of the welded joint; the greater the restraint stress, the more easily cracks are produced.
3.1.3 Preventive Measures
Strictly control the hydrogen content of welding materials and use low-hydrogen welding materials.
Take preheating and post-heating measures to reduce the restraint stress of the welded joint. 3.2 Quenching Cracks
3.2.1 Mechanism of Formation
Discovered immediately after welding, mainly caused by the formation of hardened structures under welding stress.
Commonly found in welded joints of high-strength steel and ultra-high-strength steel.
3.2.2 Influencing Factors
Welding process parameters, such as excessive welding speed and cooling rate, easily lead to the formation of hardened structures.
The geometric shape and size of the welded joint; complex shapes and large thickness joints are prone to quenching cracks.
3.2.3 Preventive Measures
Optimize welding process parameters, control welding speed and cooling rate to avoid the formation of hardened structures.
Adopt a reasonable welded joint design to reduce stress concentration points.
3.3 Low Plasticity Embrittlement Cracks
3.3.1 Mechanism of Formation
When low-plasticity materials are cooled to low temperatures, the shrinkage force causes the strain to exceed the material's plastic reserve or the material becomes brittle, resulting in cracks.
There is no delay phenomenon; it mainly occurs in welded structures operating at low temperatures.
3.3.2 Influencing Factors
The low-temperature toughness of the material; materials with poor low-temperature toughness are prone to low-plasticity embrittlement cracks.
Residual stress in the welded joint; high residual stress leads to a higher tendency for cracking.
3.3.3 Preventive Measures
Select welding materials with good low-temperature toughness.
Optimize the welding process to reduce welding residual stress.
4. Lamellar Tearing:
4.1 Mechanism of Formation
Layered inclusions exist inside the steel plate, and stress perpendicular to the rolling direction during welding leads to lamellar tearing.
Commonly found in the manufacturing process of large oil platforms and thick-walled pressure vessels.
4.2 Influencing Factors
The quality of the steel plate; a high content of layered inclusions leads to a greater tendency for lamellar tearing.
Welding process parameters, such as welding heat input and welding sequence, affect the welding stress distribution.
4.3 Preventive Measures
Strictly control the quality of the steel plate to reduce layered inclusions.
Optimize the welding process, reasonably control the welding heat input and welding sequence to reduce welding stress.
5. Stress Corrosion Cracking
5.1 Mechanism of Formation
Delayed cracks produced in welded structures under the combined action of corrosive media and stress. Influencing factors include material type, type of corrosive medium, structural form, welding process, welding materials, and the degree of stress relief.
5.2 Influencing Factors
The corrosion resistance of the material; materials with poor corrosion resistance are prone to stress corrosion cracking.
The type and concentration of the corrosive medium; strong corrosive media accelerate crack formation.
5.3 Preventive Measures
Select welding materials with good corrosion resistance.
Adopt effective anti-corrosion measures, such as coating protection and cathodic protection.
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