As the resistance coefficient of stainless steel is much higher than that of low carbon steel, the electrode and the base metal in the welding area are easy to be heated and melted during welding, and the matrix around the melting area is overheated, resulting in uneven deformation and coarse grains in the welding area.
The linear expansion coefficient of stainless steel is large, the thermal conductivity is small, the heat is not easy to transfer, the penetration depth is large during welding, the welding heating makes the structure expand, and the cooling produces large shrinkage deformation and tensile stress, which is easy to cause hot cracks.
After stainless steel welding, intergranular corrosion is easy to occur in the welding heat-affected zone.
The reason is that in the welding heat affected zone, in the sensitization temperature range (450℃&8mdash; 850 ℃), the matrix is partially poor in chromium, which is difficult to passivate, resulting in a significant decline in corrosion resistance.
Therefore, the steel is preferentially corroded in the corresponding corrosion environment, and the grain boundary of the steel is widened due to corrosion.
At this time, the plasticity and strength of the corroded parts have been seriously lost, cracks and brittle fracture occur during cold bending, and there is no metal sound on the corroded parts.
Preventive measures for stainless steel welding
1. Control welding current:
The welding specification of stainless steel shall be less than that of low-carbon steel, and the current is about 80% of that of low-carbon steel.
2. Accelerate welding speed:
Use a faster welding speed as much as possible to reduce the width of the heat-affected zone, shorten the residence time of the weld in the sensitization temperature range, make the weld in a stable state, and refine the weld microstructure.
3. Reasonable selection of welding materials:
Appropriate welding materials and protective atmosphere shall be selected during welding.
The chemical composition of the welding wire has an important influence on the corrosion resistance of the welding seam.
The welding rod shall have the chemical composition similar to that of the base metal, which can make the weld metal and the base metal have similar chemical composition, and is generally considered to achieve corrosion resistance.
4. Cleaning before and after welding:
The surface of materials must be cleaned before welding, and welding slag shall be removed after welding.
Stainless steel welding method
|Welding process||Martensitic stainless steels||Ferritic Stainless Steel||Austenitic stainless steel||Applicable plate thickness (mm)|
|Argon arc welding||B-C1||A||A~B2||＞0.8|
|Submerged arc welding||B||C5||A~B6||＞6|
In the above table, A, B, C, 1, 2, 3, 4, 5, 6 has the following meanings:
A: Easy to weld, widely used;
B: In some cases, the weldability or weld quality is poor, but it is also widely used;
C: Poor weldability or weld quality, hardly used;
1) The materials with high C content are easy to cause ground temperature cracking during welding and need preheating;
2) SUS310 and 347 steel often cause high-temperature cracking during welding;
3) The use of shielding gas mixed with H2 is easy to cause porosity and ground temperature cracking at the weld;
4) Pulse welding shall be used for welding above 2.0mm, generally above 3.0mm;
5) The grains in the welding part are coarse and easy to become brittle;
6) Although there is no problem with low-carbon steel and stable stainless steel, the welding parts of 304, 316 and other ordinary austenitic stainless steels with carbon content are prone to intergranular corrosion.