Why is multi-pass welding required for welding duplex steel?

It is to control the heat input, in order to reduce and prevent the formation of a single-phase or excessive ferritic structure as well as coarse grains in the weld and heat-affected zone. Whether a multi-layer, multi-pass welding process is required depends on the thickness of the duplex steel as well as its specific chemical composition, to be determined through weld evaluation. Typically, a welding procedure qualification is carried out before welding, and the welding plan is determined based on this qualification.

I also have a question: why don’t materials such as 304 and 316 have such requirements?

It depends on the thickness; those that can be fully welded in one pass generally do not require multiple welding passes. Still the same thing: according to the weld evaluation: lol

I really don’t know this. Following the answer from above, I’ve managed to learn some knowledge about how this is achieved

Controlling heat input and output in duplex steel is aimed at preventing harmful phases in the weld metal and regulating the proportion of ferrite. Regular austenite is determined by the chemical composition of the base material and the welding filler metal. Of course it’s not about not having control

Some key points for welding duplex steel: Based on the results of tests conducted on duplex steel, the following lessons can be drawn regarding its welding: 1. If the customer requires compliance with the A923C standard, low-temperature impact testing, or ferrite content determination, the welding requirements for 2205 and 2507 are quite high. These requirements are reflected in the following aspects: 1) The shielding gas must be Ar + 2.5% N2; if the nitrogen content is less than 2.5%, the corrosion results will be several dozen to hundreds of times higher than acceptable. However, for thin sheets with a thickness of 6 mm or less, pure argon can also satisfy these corrosion test requirements； 2) The energy of the welding arc must be controlled; for 2205, it shall not exceed 2.5 KJ/mm, and for 2507 it shall not exceed 1.5 KJ/mm. Additionally, for a welding wire with a diameter of 2.4 mm, the current should not exceed 150 A, as otherwise it will also affect the test results ； 3) The welding cooling rate and interpass temperature have a significant impact on the determination of ferrite content. An excessive cooling rate leads to an overly high amount of ferrite, while a low cooling rate results in too little ferrite. Generally, it is more appropriate to use a steady welding speed within the range of 6–10, with an interpass temperature below 100°C. 4) In the cover welds of thick plates, multiple passes are required for welding; as a result, the ferrite content in the final cover weld tends to be high, due to the small amount of weld metal deposited and the rapid welding pace. Therefore, this final weld pass needs to have a certain deposit thickness ； 2. The use of pure argon, along with shielded metal arc welding, makes it easier to meet the corrosion requirements specified in GB4334.5; therefore, if there are no special requirements for the project, these two methods can be used in combination. However, other testing requirements must still be fulfilled in accordance with the relevant provisions in clause 1. 3. For some duplex steels that require Ni-based welding materials, it is difficult to pass the back-bending test during the process evaluation; therefore, double-sided welding should be used whenever possible. 4. When using flux-cored wires to weld duplex steel, the A923C method may also fail to yield satisfactory results, so caution is needed when this method is employed. 5. Relevant tests have not yet been conducted for solid wire and submerged arc welding; the wire has been purchased, and further testing can be carried out. 6. All of the above test results were obtained from the company’s routine dual-phase steel process tests; one can compare these results with the welding records.