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Four types of stainless steel and the role of alloying elements. Stainless steel can be divided into four types: austenitic, martensitic, ferritic, and duplex stainless steel. This classification is based on the microstructural pattern of the stainless steel at room temperature. When low-carbon steel is heated to 1550°F, its microstructure changes from the ferrite phase at room temperature to the austenite phase. During cooling, the microstructure of low-carbon steel transforms back into ferrite. The austenitic structure present at high temperatures is non-magnetic, and compared to the ferritic structure at room temperature, it has lower strength but better toughness. When the Cr content in steel exceeds 16%, the ferritic structure at room temperature is stabilized, allowing the steel to remain in a ferritic state across all temperature ranges. Therefore, it is called ferritic stainless steel. When the Cr content is greater than 17% and the Ni content is greater than 7%, the austenite phase is stabilized, allowing it to remain in the austenitic state across a range from low temperatures to almost the melting point. Austenitic stainless steels are commonly referred to as the “Cr-Ni” type, while martensitic and ferritic stainless steels are simply called the “Cr” type. The elements in stainless steel and filler metals can be divided into austenite-forming elements and ferrite-forming elements. The main austenite-forming elements are Ni, C, Mn, and N, while the ferrite-forming elements are Cr, Si, Mo, and Nb. Adjusting the element content can control the ferrite content in the weld. Austenitic stainless steels are easier to weld and yield better welding quality compared to stainless steels with a Ni content of less than 5%. Austenitic stainless steel welds possess excellent strength and toughness, and generally do not require preheating before welding or heat treatment after welding. In the field of stainless steel welding, austenitic stainless steels account for 80% of all stainless steel used; therefore, the focus of this article is on the welding of austenitic stainless steels. How to choose the right stainless steel welding material? If the base material is the same, the primary principle is to \"match the base material\". For example, when welding 310 or 316 stainless steel, the appropriate welding material should be selected. When welding dissimilar materials, the principle of selecting a base material with a high content of alloying elements should be followed. For example, when welding 304 and 316 stainless steels, 316-type welding material should be selected. However, there are also many special cases that do not follow the \"match the base material\" principle; in such cases, it is necessary to \"refer to the welding material selection table.\" For example, type 304 stainless steel is the most common base material, but there are no welding electrodes for type 304. If the welding material needs to match the base metal, how should one choose a welding material for welding 304 stainless steel? When welding 304 stainless steel, 308 type welding material is used, as the additional elements in 308 stainless steel can better stabilize the weld area. 308L is also an acceptable option. L denotes a low carbon content; 3XXL stainless steel has a carbon content of ≤ 0.03%, whereas standard 3XX stainless steel can have a carbon content of up to 0.08%. Since L-type welding materials and non-L-type welding materials fall under the same category, manufacturers should give special consideration to using L-type welding materials, as their low carbon content can reduce the tendency for intergranular corrosion. In fact, if manufacturers wish to upgrade their products, L-type welding materials will see wider use. The use of L-type welding materials can reduce the tendency to develop intergranular corrosion. Manufacturers who use GMAW welding methods also consider using 3XXSi-type welding materials, as silicon improves wetting properties. When the welded joint has a high bulge or when the weld pool fails to bond properly at the root of fillet or lap joints, using gas shielded welding wire containing Si can wet the weld and increase the deposition rate. To improve the wetting of the welding material in GMAW welding, wires containing Si are used, such as 308L Si or 316L Si. If carbide precipitation is a concern, 347-type welding materials containing a small amount of Nb can be chosen (last question). How to weld stainless steel and carbon steel? To reduce costs, a corrosion-resistant layer is welded onto the surface of carbon steel for some structural components. When welding base materials without alloying elements to those with alloying elements, weld material with a higher alloy content is used to balance the dilution rate in the weld. When welding carbon steel with 304 or 316 stainless steel, as well as when welding other different types of stainless steels (Table 2), 309L welding material is generally considered a suitable choice. If a higher Cr content is desired, type 312 should be chosen. It should be noted that the thermal expansion rate of austenitic stainless steels is 50% higher than that of carbon steel. During welding, differences in thermal expansion rates generate internal stresses, which can lead to cracks; therefore, it is necessary to select appropriate welding materials or specify suitable welding procedures. When welding carbon steel and stainless steel, warping deformation caused by different coefficients of thermal expansion requires greater compensation. What are the appropriate pre-welding cleaning procedures? When welding with other materials, first use a chloride-free solvent to remove oil, marks, and dust. Furthermore, when welding stainless steel, the primary thing to pay attention to is avoiding contamination by carbon steel, which could affect its corrosion resistance. Some companies store stainless steel and carbon steel separately to avoid cross-contamination. When cleaning the area around the groove, use special grinding wheels and brushes designed for stainless steel. Sometimes, secondary cleaning of the joint is also required. Since electrode compensation is more difficult during the welding of stainless steel than when welding carbon steel, it is very important to clean the joints. What is the correct post-weld cleaning procedure? Why do stainless steel welded parts rust? First, let’s recall that the reason stainless steel does not rust is that the reaction between Cr and O forms a dense oxide layer on the surface of the material, which provides protection. Stainless steel rusts due to the precipitation of carbides (see the last question) and the heating that occurs during welding, which causes iron oxides to form on the surface of the welded piece. In the as-welded state, even a perfectly welded joint can develop undercut at the rusted areas on the boundary of the weld heat-affected zone within 24 hours. Therefore, in order to regenerate the new chromium oxide, stainless steel must be polished, pickled, brushed, or washed after welding. It should be emphasized that grinders and brushes must be used exclusively for their intended purposes. Why is stainless steel welding wire magnetic? Stainless steel with a fully austenitic structure is non-magnetic. However, the relatively high temperatures during welding cause grain growth in the microstructure, increasing the crack sensitivity after welding. To reduce the susceptibility to hot cracking, welding material manufacturers add ferrite-forming elements to the welding materials (Figure 4). The ferrite phase refines the austenite grains, thereby increasing crack resistance. To avoid hot cracks, most austenitic welding materials contain a small amount of ferrite. The image shows the ferrite phases distributed on the austenitic matrix in 309L welding material. Magnets do not attract austenitic weld metal, but a slight pulling force can be felt when holding a magnet. However, this also leads some users to mistakenly think that the product is mislabeled or that the wrong welding material is being used (especially when the label on the packaging has been torn off). The amount of ferrite in the welding material depends on the service temperature of the application. For example, excessive ferrite reduces toughness at low temperatures. Therefore, the ferrite content of type 308 welding material used for LNG pipelines ranges from 3 to 6, whereas that of standard type 308 welding material is 8. In short, welding materials may appear similar, but even slight differences in composition can sometimes lead to significant differences. How to weld duplex stainless steel more easily? Typically, the austenite phase and the ferrite phase in the structure of duplex stainless steel each account for about 50%. The presence of the ferrite phase can improve strength and stress corrosion resistance, whereas the austenite phase can enhance toughness. The combined effect of the two phases results in superior properties for duplex stainless steel. There is a wide range of duplex stainless steels, with the most common grade being 2205: it contains 22% Cr, 5% Ni, 3% Mo, and 0.15% N. Figure 5: Duplex stainless steel combines the advantages of ferrite and austenite. The image shows the microstructure of a duplex weld, with austenite phases (the white areas) distributed within a ferritic matrix. When welding duplex stainless steel, an excessive amount of ferrite can cause problems (the heat from the arc causes the atoms in the ferritic matrix to reorganize). To this end, the welding material needs to provide more austenite-forming elements, usually 2-4% more Ni than the base material. For example, the flux-cored wire used for welding 2205 stainless steel contains 8.85% Ni. After welding, the ferrite content in the weld ranges from 25-55% (it may be higher as well). Note: The cooling rate after welding must be slow enough to allow austenite to re-form, but it cannot be too slow, as this will lead to the precipitation of intermetallic phases; nor can it be too fast, as this will result in an excessive amount of ferrite in the heat-affected zone. Be sure to follow the welding procedure and welding material selection manuals provided by the manufacturer. Why is it necessary to adjust parameters constantly when welding stainless steel? The main reason why welders adjust welding parameters (voltage, current, arc length, inductance coefficient, pulse width, etc.) at any time when welding stainless steel is the mismatched composition of the welding materials. Chemical composition is very important; differences in composition between batches can lead to significant variations in welding behavior, such as poor wetting and slag removal. The diameter of the welding material, surface cleanliness, pouring properties, and spiral shape can all affect the welding behavior in GMAW and FCAW. How to control carbide precipitation in austenitic stainless steel? At temperatures of 800–1600°F, when the carbon content exceeds 0.02%, carbon diffuses and migrates toward the austenite grain boundaries, where it reacts with chromium to form chromium carbides. If Cr is heavily bound by C, the corrosion resistance will decrease. At this point, if exposed to a corrosive environment, intergranular corrosion will occur, causing the grain boundaries to be eroded (Figure 6). Intergranular corrosion occurred in the weld heat-affected zone of a tank containing corrosive media. The use of welding materials with low carbon content or specially alloyed materials can reduce the tendency for carbide precipitation and enhance corrosion resistance. To control carbide precipitation, low-carbon welding materials are used to ensure that the carbon content in the weld metal remains as low as possible (not exceeding 0.04%). C can also be fixed by adding Nb and Ti elements; compared to Cr, elements Nb and Ti have a greater affinity for C. Welding material type 347 is designed for this purpose. How to prepare for selecting welding materials? First, it is necessary to collect information on the end-use applications of the welded parts, including the operating environment (especially the operating temperature, the presence of corrosive agents, and the desired level of corrosion resistance) as well as the expected service life. Information on the mechanical properties required under service conditions is also important, such as strength, toughness, ductility, and fatigue resistance. Most leading welding material manufacturers provide guidebooks for selecting welding materials. Here, the author would like to reiterate that it is recommended to consult the welding material application manuals or contact their technical experts. They will help us make a more accurate choice of stainless steel welding materials.