The ABC’s of Arc Welding

Q and A: Rust on a stainless steel weld

Question:
  We delivered 304-type stainless steel joints welded with the flux-cored wire, DW-308. Two weeks later, our customer complained that rust was forming on the surface of some of the weld beads. What causes this problem? In addition, we found that the remaining wire of the DW-308 would stick to a magnet. We thought that DW-308 would not stick to a magnet. What has happened with the wire?

Answer:
It is often said that stainless steels do not rust. This is not true; however, they are less likely to generate rust compared to conventional carbon and special steels. It is helpful to understand the way that each type of steel forms rust.

The case you described of rust forming on the beads was likely a type of rust that forms on dissimilar metals like a combination of carbon steel and stainless steel. This type of rust can be caused by such external factors as carbon steel powders enerated by nearby grinding and brushing with carbon steel wire brushes, adhering to the surface of the stainless steel weld beads. Therefore, we guess that some carbon steel powders that had adhered to the surface of the DW-308 weld beads caused the rust in your case.

Figures 1 and 2 show examples of bead appearance. The rusty bead in Figure 1 had been brushed with a carbon steel wire brush, while the other metallic, lustrous one in Figure 2 had been brushed with a stainless steel wire brush. The former one clearly has brown rust even on the base metal. You can therefore understand that stainless steel wire brushes are essential for brushing stainless steel welded joints.

You might have thought that the magnet and the wire were attracted to each other due to some ingredients contained in the DW-308 wire. The DW-308 wire indeed does react magnetically because of the stress-induced martensite on the surface of the wire. The martensite structure is formed by work hardening the surface of the wire during the process of drawing into the designated diameter. The stress-induced martensite, however, has nothing to do with rusting.

The DW-308 weld metal also behaves magnetically. This phenomenon is caused by a small amount of ferrite contained in the weld metal. With the desire to keep the cracking susceptibility of the weld metal as low as possible, DW-308 is designed to contain small amounts of ferrite in the weld metal. The ferrite tructure also has nothing to do with rust.

If the rust problem described above is not so serious, it can be wiped off with a sponge or cloth soaked with neutral detergent or soapy water. Careful washing with water is necessary afterward so as not to leave any neutral detergent or soapy water. When it is serious, use an exclusive cleaning solution for stainless steel or 15% diluted nitric acid. Polishing with sand paper or brushing off with a stainless steel wire brush is also effective. When followed by cleaning with soapy water, it will be perfect for obtaining beautiful welds.

Figure 1: Rusty appearance of a weld bead after brushing with a carbon steel wire brush

Figure 2: Metallic, lustrous appearance of a weld bead brushed with a stainless steel wire brush

<> References <>

(1) E. Kumagai: Kobe Steel Yohsetsu Dayori, Gijutsu Guide, Vol. 39, 1999, 11 (No. 355), p.11
  (2) http://www5.mediagalaxy.co.jp/hokusei/teire.html

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