The main problems of 18Cr austenitic stainless steel welding and preventive measures

As austenitic stainless steel contains high chromium, can form a dense oxide film, so it has good corrosion resistance. When containing 18% Cr, containing 8% Ni, you can get a single austenite organization, so austenitic stainless steel has good corrosion resistance, plasticity, high temperature performance and welding properties. However, under different working conditions, austenitic stainless steel welded joints often face some special problems, including intergranular corrosion, stress corrosion, knife corrosion, welding thermal cracking, alpha phase embrittlement and other construction defects.  

1. Corrosion analysis of welded joints

Intergranular corrosion of welded jointsIntergranular corrosion is one of the most important corrosion problems of austenitic stainless steel, once intergranular corrosion occurs, its strength is almost lost when serious, and the application of a certain stress will produce fracture along the crystal. The cause of intergranular corrosion of austenitic stainless steel welded joints is mainly due to chromium carbide precipitation caused. Austenitic stainless steel in the 500 ~ 800 ℃ temperature range for sensitization treatment, supersaturated solid solution of carbon to the intergranular boundary diffusion than the diffusion of chromium faster, near the grain boundary and chromium combined into (Cr, Fe) 23C6 carbide, and precipitation precipitation at the grain boundary, the formation of the grain boundary near the phenomenon of poor chromium, when the chromium content of the region is reduced to the limit of content required for passivation (W (Cr) 12.5%) or less, it will accelerate the corrosion of the region and form intergranular corrosion. Heat-affected zone sensitization temperature zone intergranular corrosion occurs in the heat-affected zone heating peak temperature in the range of 600 ~ 1000 ℃, the cause of intergranular corrosion is still the austenite grain boundary precipitation of chromium carbide caused. Reduce and prevent intergranular corrosion of the main preventive and control measures include.① take small specifications (small current, large welding speed), multi-pass welding and other process measures;② minimize the base material, welding material in the elemental carbon content, the use of C content of less than 0.03% of the welding material;③ make the weld from a single austenite phase changed to austenite plus ferrite dual phase, C r in ferrite diffusion rate than in austenite, therefore, chromium in ferrite faster diffusion to the grain boundary, to reduce the austenite grain boundary poor chromium phenomenon; ④ in steel and welded materials to add Ti, Nb and other elements with a stronger affinity for carbon than chromium, can be combined with carbon into a stable compound, thus avoiding the austenite grain boundary resulting in poor chromium.Stress corrosion of welded jointsStress corrosion cracking of stainless steel is the most harmful corrosion behavior, cracking without any deformation, the accident is often sudden and serious consequences. Stainless steel in use conditions produce stress corrosion cracking influence many factors, including the composition of steel, organization and state, the type of medium, temperature, concentration, stress nature, size and structural characteristics.Measures to reduce and prevent stress corrosion are mainly.① avoid strong assembly, mechanical impact and arc burns, reduce cold deformation and stress;② Strict control of impurities in the medium and environment (especially chlorides, fluoride, etc.);③ reasonable selection of materials (base material, welding material): to avoid grain coarsening and hardening of the martensite organization;④Welding seam is well formed and does not produce any stress concentrations (such as edge biting, etc.);⑤ Reasonable arrangement of the welding sequence to reduce stress;⑥Corrosion protection treatment: coating, lining or cathodic protection and other locations to add corrosion inhibitors.

2. Thermal crack sensitivity analysis of welded joints
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Austenitic stainless steel thermal cracking is mainly crystalline cracking, is generated during the solidification of the weld metal liquid metal, when the melting point eutectic presence of a crystal, mainly between the dendrite. The causes of its generation are mainly the following three aspects.① S, P, C, etc. and Ni form a low melting point eutectic (such as NiS + Ni melting point of 644 ℃), weakening the strength of the grain boundary;② austenitic stainless steel liquid phase line and solid phase line distance, crystallization time is long, dendritic directional, easy to produce impurity elements of segregation;③ steel thermal conductivity is small, the coefficient of linear expansion is large, easy to generate stress.The main measures to prevent and control welding thermal cracking include.① strict control of the base material and weld material harmful system base material and weld material harmful impurities sulfur, phosphorus content;② welding seam with about 5% ferrite biphasic organization, disrupting the direction of austenite columnar crystal;③ process measures: the use of alkaline welding rod, small specifications (small current, fast welding), to prevent the production of thermal cracking.Picture

3. Control of the ferrite content of welded joints

The amount of ferrite content in the weld metal of austenitic steel is not only related to the generation of α (σ) phase embrittlement and thermal strength, but also directly affects the thermal cracking resistance of the joint. The longer the heating time, the longer the high-temperature residence time, the more precipitation, will seriously affect the mechanical properties of the joint. From the thermal cracking resistance, the weld metal contains a certain amount of ferrite, but from the α-phase embrittlement and thermal strength to consider, the lower the ferrite content required, the better. Therefore, for high-temperature strength requirements of welded joints, the ferrite content must be strictly controlled, and in some cases, the weld metal must be austenitic.Wilson Pipeline  is a leading manufacturer and distributor of flange fittings material from  alloy steel  stainless steel to carbon steel.products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings. ASME B16.5  ASME B16.9 ASME B16.11 EN1092,ANSI ASTM,GOST,JIS,DINWilson Pipeline products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.If you want to have more information  you want to share your enquiry with us, contact us at sales@wilsonpipeline.com