How to choose the right stainless steel cored wire for welding?

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Selection of cored wire for stainless steel – The use of cored wire for stainless steel is primarily in gas-shielded welding, and gas-shielded welding with cored wire has the following characteristics. ①Compared to shielded metal arc welding, the deposition rate can be increased by 2 to 4 times, with a deposition efficiency as high as 90% (compared to only 55% for stainless steel electrodes), resulting in significantly improved production efficiency and excellent cost-effectiveness. ② It has a wide range of tolerance for current and voltage; compared to traditional active gas shielded welding stainless steel solid wires, it makes it easier to set welding conditions, facilitating semi-automatic and automated welding. ③ It has good slag removal properties, and the weld surface is shiny. Furthermore, spatter is minimal, the arc stability is excellent, and the pass rate for X-ray inspection is high. When welding with stainless steel cored wires, since the main alloying elements of the weld metal are the same as or similar to those of the base material, the weld’s corrosion resistance and heat resistance can basically be ensured. However, the contents of C, O, and N, which have a significant impact on the mechanical properties of welds, exhibit some special variation patterns during welding with flux-cored wires, and these should be taken seriously. In stainless steel welding, reducing the carbon content in the weld metal is one of the effective ways to improve its resistance to intergranular corrosion. However, when welding is carried out using a shielding gas containing CO2, the tendency of the weld metal to become carbonated is almost inevitable; therefore, solid stainless steel wires usually must be welded under inert gas protection. However, for stainless steel cored wires, even when pure CO2 is used as the shielding gas, the increase in carbon content in the weld metal can be kept at a very low level. In recent years, many ultra-low carbon (C≤0.04% or C≤0.03%) and extremely low carbon (C≤0.02%) stainless steel flux-cored wires have been widely used. The shielding gas used in most stainless steel cored wires is CO2 or a mixture of Ar and 20%~25% CO2. When using flux-cored wire to weld stainless steel, the oxygen content in the weld metal is relatively high compared to other welding methods. As the oxygen content increases, the impact toughness of the weld metal decreases. Most of the oxygen in the weld metal is dispersed among the crystal lattice in the form of oxide inclusions; these oxide inclusions originate partly from the oxides present in the powder and partly from the products of deoxidation reactions. An excessively high content of oxide inclusions can also be one of the reasons for the failure of stainless steel welded joints during cold bending. The oxygen content in the weld can be reduced as the basicity of the wire slag increases, but an increase in basicity also lowers the viscosity of the slag, thereby affecting the all-position weldability of the wire. Figure 1: Comparison of oxygen content in the weld metal of 308 stainless steel using different welding methods PART 01: Selection of welding wire for welding martensitic stainless steel. Martensitic steel can have its properties adjusted through heat treatment; therefore, to meet the required performance standards, especially for heat-resistant martensitic steel, the composition of the weld should be as close as possible to that of the base material. To prevent cold cracking, austenitic welding materials can also be used; in this case, the strength of the weld will inevitably be lower than that of the base material. When welding martensitic stainless steel, the following problems are likely to occur. ①Overheating zone hardening and cold cracking. Martensitic stainless steels have a particularly high tendency to harden; after being heated at high temperatures, they can develop hard and brittle martensite when cooled in air. Under the influence of welding restraint stress and diffused hydrogen, welding cold cracks can easily occur. ②Cracking in the overheated zone. In martensitic stainless steel, grain growth tends to be very severe in the overheated zone, which significantly reduces the plasticity and toughness of the joint, leading to embrittlement in that area. ③Softening of the heat-affected zone. Under prolonged heating at high temperatures, visible cracks are likely to appear in the softened zone of the heat-affected area, reducing the reliability of the joint. When the composition of the weld is similar to that of the base material, both the weld and the heat-affected zone become hardened and more brittle during welding. To prevent cold cracking, the base material often needs to be preheated, and heat treatment is required after welding to improve the properties of the joint. Since the thermal expansion coefficients of the weld metal and the base material are essentially the same, welding stresses can potentially be completely eliminated after heat treatment. Welding wires for welding Cr13-type martensitic steel should have harmful impurities such as S, P, and Si strictly controlled; wires containing elements such as Ti, N, Nb, or Al should be used to refine the grain structure and reduce hardness. When preheating or heat treatment of the workpiece is not permitted, it is advisable to use welding materials that can produce an austenitic weld structure. Such welds possess high plasticity and toughness, can relieve welding stresses, and allow for greater solubility of elements, thereby reducing the tendency to cold cracking. However, such joints with uneven material properties may develop thermal fatigue cracks under cyclic temperature conditions due to different coefficients of thermal expansion, leading to joint failure. Figure 2 Selection of welding materials for martensitic stainless steel PART 02 Selection of flux-cored wires for welding ferritic stainless steel. Ferritic stainless steel does not undergo any phase changes during heating and cooling; therefore, even if it is cooled rapidly after welding, no hardened structure will form. When welding this type of steel, the following problems are likely to occur. ①After being heated to above 900°C, the ferrite grains grow rapidly; even with rapid cooling after welding, it is impossible to avoid the sharp decline in toughness resulting from grain growth, as well as an increased tendency to intergranular corrosion. ②Ferritic steel inherently has a high chromium content, as well as higher levels of harmful elements such as C, N, and O. It possesses a high brittle transition temperature and is highly sensitive to notches. Therefore, the post-welding embrittlement phenomenon is relatively severe. ③During prolonged heating and slow cooling at 400–600°C, embrittlement occurs at 475°C, resulting in a significant decrease in toughness at room temperature. After prolonged heating at 550–820°C, phases tend to precipitate from the ferrite, which also significantly reduces its plasticity and toughness. When selecting cored wires for ferritic stainless steel, those with low levels of harmful elements (such as C, N, S, P, etc.) should be used in order to improve welding performance and weld toughness. The weld composition can use the same material as that of the Cr17 series, but cracks are likely to occur under high constraint; heat treatment can be applied after welding to restore corrosion resistance and improve the ductility of the joint. Austenitic high-Cr, Ni welding materials can also be used to reduce notch sensitivity and improve the crack resistance of the joint, such as 309(24-13) and 310(26-21) types of austenitic stainless steel cored wires. Austenitic weld metal is essentially as strong as the ferritic base material, but in certain corrosive environments, the corrosion resistance of the weld may differ significantly from that of the base material; this factor should be taken into account when selecting flux-cored wires.