Austenitic stainless steel has good corrosion resistance because it contains high chromium content and can form a dense oxide film.
When Cr18% and Ni8% are contained, a single austenitic structure can be obtained. Therefore, austenitic stainless steel has good corrosion resistance, plasticity, high temperature performance and welding performance.
However, under different working conditions, austenitic stainless steel welded joints often face some special problems, which are easy to cause construction defects, such as intergranular corrosion, stress corrosion cracking, knife corrosion, welding hot cracking, embrittlement of the α phase and so on.
01. Corrosion analysis of welded joints
Intergranular corrosion of welded joints
Intergranular corrosion is one of the most important corrosion problems of austenitic stainless steel. Once intergranular corrosion occurs, its strength is almost lost when it is severe, and intergranular fracture will occur when a certain stress is applied.
The main cause of intergranular corrosion of austenitic stainless steel welded joint is chromium carbide precipitation.
When austenitic stainless steel is sensitized in the temperature range of 500 ~ 800 ℃, the diffusion rate of carbon from supersaturated solid solution to the grain boundary is faster than that of chromium.
Near the grain boundary, carbide (Cr, Fe) 23c6 is synthesized with chromium and precipitated at the grain boundary, forming the phenomenon of chromium deficiency near the grain boundary.
When the chromium content in this area decreases below the threshold content required for passivation (w(CR) 12.5%), corrosion in this area will be accelerated and intergranular corrosion will be formed.
Intergranular corrosion in the sensitization temperature zone of the heat-affected zone occurs in the peak heating temperature range of 600 ~ 1000 ℃ in the heat-affected zone.
The reason for intergranular corrosion is still the precipitation of chromium carbide at the austenite grain boundary.
The main preventive measures to reduce and prevent intergranular corrosion include:
① Adopt process measures such as small specifications (small current, large welding speed) and multi-pass welding;
② Try to reduce the carbon content in the base metal and welding materials, and use welding materials with a C content of less than 0.03%;
③ The weld is changed from austenite single phase to austenite plus ferrite dual phase. The diffusion rate of Cr in ferrite is faster than that in austenite.
Therefore, chromium diffuses more quickly to the grain boundary in ferrite, which reduces the phenomenon of chromium deficiency at the grain boundary of austenite;
④ Adding Ti, Nb and other elements with higher affinity to carbon than chromium to steel and welding materials can form stable compounds with carbon, so as to avoid chromium deficiency at the grain boundary of the austenite.
Stress corrosion of welded joints
Stress corrosion of stainless steel is the most damaging corrosion behavior.
There is no deformation when cracking.
Accidents are often sudden and the consequences are serious.
There are many factors that affect the stress corrosion cracking of stainless steel under service conditions, including composition, structure and condition of the steel, type of medium, temperature, concentration, tensile properties, size and structural characteristics.
Measures to reduce and prevent stress corrosion cracking mainly include:
① Avoid strong assembly, mechanical impact and arc burn, and reduce deformation and stress in cold working;
② Strictly control the impurities in the environment and the environment (especially chloride, fluorine, etc.);
③ Reasonable selection of material (base metal and welding material): avoid grain coarsening and hardened martensite structure;
④ The weld is well formed without any stress concentration (such as undercut);
⑤ Reasonably organize the welding sequence to reduce stress;
⑥ Anti-corrosion treatment: add corrosion inhibitor to coating, lining or cathodic protection.
02. Sensitivity analysis to thermal cracking of welded joints
The hot cracking of austenitic stainless steel is mainly crystalline cracking, which is produced during the solidification of weld metal and liquid metal.
At this time, there is primary crystal in the melting point eutectic, mainly between the dendrites. There are three main causes:
① S, P and C form low-melting eutectics with Ni (for example, the melting point of NIS + Ni is 644 ℃) to weaken the grain boundary strength;
② Austenitic stainless steel has a large distance between liquidus and solidus, long crystallization time, strong dendritic directionality and easy segregation of impurity elements;
③ Steel has small thermal conductivity and large coefficient of linear expansion, which is easy to produce tension.
The main measures to avoid hot cracks in welding include:
① Strictly control the sulfur and phosphorus content in the base metal and welding material;
② The duplex structure of about 5% ferrite is produced in the weld, which disturbs the direction of the columnar austenite crystal;
③ Technological measures: use alkaline electrode and small specifications (low current, fast welding) to avoid thermal cracks.
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03. Control of ferrite content in welded joints
The ferrite content in the weld metal of austenitic steel is not only related to the formation of α (σ) phase embrittlement and thermal resistance, but also directly affects the hot cracking resistance of the joint.
After the workpiece is heated at high temperature for a certain time, the brittle σ phase will precipitate.
The longer the heating time, the longer the high temperature residence time and the greater the precipitation, which will seriously affect the mechanical properties of the joint.
From the point of view of thermal crack resistance, a certain amount of ferrite is required in the weld metal, but the lower the ferrite content, the better from the point of view of α phase embrittlement and thermal resistance.
Therefore, for welded joints with high temperature resistance requirements, the ferrite content must be strictly controlled. In some cases, austenitic weld metal must be used.
