How can we ensure that the stainless steel weld metal obtains a duplex microstructure?
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The alloying elements in steel are the main factors in forming a duplex microstructure. The effects of alloying elements on the microstructure can be divided into two categories: austenite formers: Ni, N, Cu, Co, C, Mn. Ferrite formers: Cr, Nb, Ti, Si, V, Mo. Ferritic structure appears when the ratio of carbon to nickel content in stainless steel is greater than 1.8. Therefore, to ensure that the weld metal obtains a duplex structure when welding stainless steel, the key is to select welding materials containing a relatively high amount of ferrite formers. When welding 1Cr18Ni9Ti stainless steel, A132 electrodes are often chosen, as these electrodes contain a certain amount of Ti and Nb; the weld metal thus has a duplex structure, granting it high resistance to thermal cracking and corrosion. Practice has shown that when the mass fraction of ferrite in the weld structure is 2%–3%, it is sufficient to prevent the formation of thermal cracks. Welding electrodes used for 18-8 stainless steel can ensure that the surfacing metal contains 3%–8% ferrite by mass; therefore, such electrodes possess strong resistance to thermal cracking. When welding austenitic stainless steel or the root pass in multi-layer welding, electrodes with a higher ferrite content (5%–10% by mass) can be used, such as the Cr22Ni9 type electrode A122, which has a higher ratio of Cr to Ni. However, it is unnecessary to have a higher content of ferrite in the weld metal, as excessive ferrite can cause brittleness in the weld metal, especially in welded structures that operate at high temperatures; generally, the ferrite content should be kept below 5%.
Single-phase austenitic stainless steels such as 0Cr25Ni20 have a much higher tendency to thermal cracking during welding compared to 1Cr18Ni9Ti stainless steel, with thermal cracks most likely to occur in the root bead and at the arc crater. However, this type of steel cannot rely on adding a small amount of ferrite to improve its crack resistance. To form ferrite in the weld, a large amount of ferrite-forming elements must be added, which causes the composition and properties of the weld to differ significantly from those of the base material, thereby failing to meet the requirements for using the joint. Furthermore, more ferrite will also make the joint brittle. The main measures to prevent the formation of thermal cracks when welding single-phase austenitic steel are: 1) appropriately increasing the carbon content to form a certain amount of carbides in the weld, resulting in a dual-phase structure together with the austenite phase. It is generally believed that carbon is the main element responsible for hot cracking, especially in the welds of 18-8 type steels; when the mass fraction of carbon increases from 0.06%–0.08% to 0.12%–0.14%, the tendency for hot cracking increases significantly ； If it increases by another 0.18%–0.20%, the tendency toward thermal cracking becomes greater. Therefore, for 18-8 stainless steel, efforts are always made to reduce the carbon content in the welds in order to ensure sufficient crack resistance. However, in single-phase austenitic stainless steels, the carbon content is relatively high – high enough to cause thermal cracking – and it is no longer possible to limit this content. At this point, if the carbon content is increased further to maintain an appropriate amount of carbide eutectics in the weld, these eutectics, due to their low melting point and good fluidity, distribute dispersedly during the crystallization process in the molten pool. This helps to refine the austenite grains, and it fills the gaps between the grains promptly at the moment when the molten metal contracts and the thin layers between the grains are broken, thereby preventing cracks from forming. 2) Adding an appropriate amount of Mn and Mo metal elements to the weld can improve heat crack resistance; for single-phase austenitic stainless steel grades such as 25-20 and 15-36, 6%–7% by mass of Mn or 2%–5% by mass of Mo can be added. For another example, electrode A412 of the 0Cr25Ni20Mo2 type contains 2% to 3% Mo by mass. 3) Strictly control the content of harmful impurities such as S and P in the weld metal. For example, when welding 25-20 type cast steel parts, if a welding rod or wire with the same composition as the base material is used, cracks can be effectively prevented without taking any additional measures, as long as the phosphorus content in the weld does not exceed 0.015%.