Stainless steel welding process for medium frequency induction heating equipment

Stainless steel in medium frequency induction heating equipment can be divided into ferritic stainless steel, martensitic stainless steel and austenitic stainless steel according to different metallographic structures. Steel that can resist atmospheric corrosion becomes stainless steel. Among stainless steels, austenitic stainless steel is currently the most widely used steel because it has excellent corrosion resistance, heat resistance, plasticity and good weldability than other stainless steels.

Weldability of stainless steel

1. Corrosion resistance of welded joints

(1) Overall corrosion, under the action of any corrosive medium, there will always be corrosion on the surface of any stainless steel, which is called overall corrosion.

(2) Intergranular corrosion easily occurs in austenitic stainless steel. Intergranular corrosion occurs at grain boundaries, so it is called intergranular corrosion. This corrosion can occur in the heat-affected zone, weld seam or weld line and is one of the most dangerous forms of failure of austenitic metals.

2. Hot crack

Hot cracks are a kind of defect that is relatively easy to occur during stainless steel welding, including longitudinal and transverse cracks in the weld, arc crater cracks, root cracks in backing welding, and interlayer cracks in multi-layer welding. Austenitic stainless steels are more susceptible to hot cracking.

Stainless steel welding process

1. Manual arc welding

1) Preparation before welding. When the thickness of the plate is >= 3mm, the bevel should be opened, and the inside of the knife should be cleaned with acetone for 20-30mm on both sides of the bevel, and lime powder should be applied to prevent the splash from damaging the metal surface. 2) Selection of welding rods

3) The commonly used welding process, the use of small specifications can prevent intergranular corrosion, thermal cracks and deformation. The welding current is 20% lower than that of low-carbon steel. In order to ensure stable arc combustion, DC reverse connection is used, and the short arc is slow. Fill up the arc crater, and weld the surface in contact with the medium last. During multi-layer welding, the temperature between layers should be controlled. Forced cooling can be used after welding. Do not start arcs outside the groove. Correct by cold working.

2. Argon arc welding

When stainless steel is welded by argon arc, due to the good protection effect, the alloy elements are not easy to burn and the transition coefficient is high, so the weld is well formed, there is no slag shell, and the surface is smooth, so the welded joint has high heat resistance and good mechanical properties. It is used to weld 0.5-3mm stainless steel sheets. The composition of the welding wire is generally the same as that of the weldment. The protective gas is generally industrial pure argon. The welding speed should be appropriately fast and try to avoid lateral swing. For stainless steel with a thickness greater than 3mm, MIG welding can be used. The advantages of MIGA welding are high productivity, small heat-affected zone of the weld, small deformation of the weldment, good corrosion resistance, and easy automation.

3. Gas welding

Because gas welding is convenient and flexible, it can weld welds in various spatial positions. For some stainless steel parts such as thin plate structures and thin-walled pipes, gas welding can sometimes be used if there is no corrosion resistance requirement. In order to prevent overheating, the welding tip is generally smaller than when welding low-carbon steel of the same thickness. The gas welding flame should use a neutral flame. The welding wire should be selected according to the composition and performance of the weldment. 

In order to increase the corrosion resistance of stainless steel, surface treatment should be carried out after welding, and the treatment methods include polishing and purification. Welding parameters: including welding current, tungsten electrode diameter, arc length, arc voltage, welding speed, shielding airflow, nozzle diameter, etc.

(1) The welding current is determined by the size of the welding current. The greater the current, the greater the diameter.

(2) The diameter of the welding electrode is determined according to the size of the welding current. The larger the current, the larger the diameter.

(3) Welding arc voltage, arc length range is about 0.5-3mm, corresponding arc voltage is 8-10v.

(4) Welding speed: The current size, the sensitivity of the weldment material, the welding position and the operating factors should be considered in the selection.