304 stainless steel is a widely used metal material in various industrial applications. This material exhibits good weldability, and many welding methods have been employed for its joining. However, research and application of flash welding for this material remain relatively limited. In this article, we will briefly discuss the use of flash butt welding machines for stainless steel.
Flash butt welding is a resistance welding technique that achieves joining by heating the contact resistance between the workpieces. At the contacting end faces of the two workpieces, there are minute contact points. During welding, a high current passes through these points, rapidly increasing the temperature and melting the metal, forming liquid “bridges.” These liquid bridges explode under the influence of high-temperature metal vapor, creating a flashing effect. As the bridges continuously form and burst, a suitable temperature field is established at and near the contact surfaces, covering the end faces with a liquid metal layer. At this stage, an upset force is applied to close the gap, expelling the liquid metal layer and allowing the underlying solid metal to bond tightly.
In practice, flash butt welding shows promising potential for large-scale welding of stainless steel bars or large-section profiles. Moreover, as a high-alloy austenitic steel, studying its flash weldability holds significant importance.
304 stainless steel is an 18/8 stainless steel with a chromium content exceeding 18%. During welding, chromium oxide (Cr₂O₃) readily forms, which can weaken the joint strength. Compared to carbon steel, austenitic stainless steel has lower electrical conductivity, lower thermal conductivity, and a higher coefficient of thermal expansion. These properties lead to significant welding stresses, which become more pronounced with increased workpiece thickness and heat input. Post-weld brittleness and susceptibility to cold cracking make fusion welding particularly challenging. However, flash butt welding offers certain advantages. The continuous generation and bursting of metal bridges during flashing facilitate heat transfer, resulting in a smaller temperature gradient in the heat-affected zone compared to fusion welding. Additionally, during the upset stage, brittle materials and oxides are expelled as flash burrs, forming a dense welded joint.
Agera has developed a series of flash butt welding machines (ranging from 100 kVA to 800 kVA) specifically for 304 thick plates and large-diameter bars. During development and testing, it was found that secondary voltage and upset allowance significantly affect the tensile strength of the welded joint. For 304 stainless steel bars with diameters of 6–34 mm, optimal results are achieved with a secondary voltage of 6–8 V and an upset allowance of approximately 30–40% of the diameter. Under these conditions, tensile fractures occur away from the weld zone. Proper adjustment of welding current helps reduce peak temperatures at the joint, preventing chromium carbide precipitation and minimizing intergranular corrosion.
For welding ultra-thick plates, the skin effect causes higher current density at the surface than at the core, creating a temperature gradient from surface to center. Preheating before flashing, moderately reducing welding current, and ensuring proper heat conduction can improve joint penetration. Additionally, increasing upset speed and force helps reduce joint hardening and prevents void formation.
The above discussion is based on the technical insights of Agera’s flash butt welding engineers. If you wish to learn more about flash welding of stainless steel, we welcome you to engage with our R&D and manufacturing team for further exploration.
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