Stainless steel refers to the amount of chromium is higher than 12% of the steel, chromium in the role of steel is able to form a layer of solid dense Cr2O3 film on the surface of the steel, so that the steel itself and the atmosphere or corrosive media isolation and protection from corrosion. On this basis, and then add a certain amount of Ni, Ti, Nb, W and other elements, can form a special corrosion resistance, resistance to high temperature oxidation or a certain degree of high temperature strength and other properties of various types of stainless steel.
Stainless steel according to its microstructure can be divided into five categories: ferritic, martensitic, austenitic, austenitic + ferrite and precipitation hardening stainless steel. Austenitic stainless steel is usually organized as pure austenite at room temperature, and some are austenite + a small amount of ferrite, and these small amounts of ferrite help prevent thermal cracking. Austenitic stainless steel because of good weldability, in the chemical industry, petroleum containers and other industries are more widely used.
Austenitic stainless steel has good weldability, but when the welding material or welding process is not correct, the following defects will occur: intergranular corrosion, stress corrosion cracking, thermal cracking.
According to the above welding characteristics of stainless steel, in order to ensure the quality of the joint, the following welding process should be used:
1. Pre-welding preparation. It is necessary to remove all kinds of contamination that may carbonize the weld metal. Welding bevel and welding area should be de-greased and de-watered with acetone or alcohol before welding. Carbon steel wire brushes shall not be used to clean bevel and weld surfaces. Slag and rust removal should be grinding wheel, stainless steel wire brush.
2. Welding electrodes must be stored in a clean warehouse. When using the welding rod should be placed in the welding rod cylinder, do not directly touch the welding rod flux skin with your hands.
3. Welding thin plate and less constrained stainless steel weldments, you can choose titanium oxide type flux-skin welding rod. Because the arc of this electrode is stable, and the weld is beautifully shaped.
4. For vertical and vertical welding position, calcium oxide flux cored electrodes should be used. Its slag solidification faster, the melted weld metal can play a certain supporting role.
5. gas shielded welding and submerged arc automatic welding, should be used in chromium and manganese content than the base material of the wire, to compensate for the welding process of alloying elements of the burn.
6. In the welding process, the weldment must be kept at a low interlayer temperature, preferably not exceeding 150℃. Stainless steel thick plate welding, in order to accelerate the cooling, can be sprayed from the back of the weld or compressed air blowing weld surface, but the interlayer must pay attention to clean up, to prevent compressed air contamination of the weld zone.
7. When manual electric arc welding, the welding current should be selected within the current range specified in the welding rod manual. Due to the stainless steel resistance value is larger, near the clamping end of a section of the electrode is susceptible to the role of resistance heat and red, in the welding to the second half of the electrode should be accelerated melting speed, so that the weld depth of fusion is reduced, but the melting speed is too fast and will result in the unfused and slag and other defects. From ensuring the corrosion resistance of the joint considerations, also requires the selection of smaller welding current, reduce the welding heat input, to prevent overheating of the welded heat-affected zone.
8. Narrow welding path technology should be used in operation technology, try not to swing the welding rod when welding, and improve the welding speed as much as possible under the premise of maintaining good fusion.
9. stainless steel weldments after welding to do passivation rust treatment, passivation mechanism can be used to explain the thin film theory, that is, passivation is due to the role of the metal and oxidizing substances, the role of the metal surface to generate a very thin, dense, good coverage performance, firmly adsorbed on the metal surface passivation film. This film into a separate phase exists, usually oxidized metal compounds. It plays the role of the metal and corrosion media completely separated from the role of preventing metal and corrosion media contact, so that the metal basically stop dissolving to form a passive state to achieve the role of corrosion prevention.
Post time: May-14-2024