Specify Delta Ferrite in Welds?

The delta ferrite content indicates the proportion of magnetisable, body-centred structure of the iron atoms in the stainless steel, which otherwise has a surface-centred structure.

The formation of ferritic proportions in stainless steel production is influenced by its alloying constituents. But heat treatment such as welding can also lead to a transformation in the stainless steel structure to delta ferrite. The delta ferrite content has an influence on the material properties of the stainless steel, primarily on its corrosion resistance. But how should the content be specified in stainless steel for pharmaceutical equipment or welding seams?

The delta ferrite content is known through the so-called Basel Standard 2, which was created in the 1990s from factory standards of the Basel chemical industry. Here, the ferrite content was specified at < 0.5%, which from today's perspective doesn't make sense. It was thought that the low delta ferrite content would minimise the formation of rouge in stainless steel, but this has not proved to be the case. On the contrary, the delta ferrite content should not be too low, otherwise the welding of the stainless steel becomes more difficult and violet to brown discolouration can occur on the weld seam.

As a rule, the delta ferrite content in stainless steels (such as 316 L) for the pharmaceutical industry is < 5%, which ensures good workability and sufficient corrosion resistance. This knowledge has become widespread in the pharmaceutical industry in recent years, so that only very rarely plant components are required to comply with Basel Standard 2. But what about weld seams? Here, there have been repeated requirements in recent times whose scientific justification is unclear.

The requirement for a maximum ferrite content for welds of <1%, for example, can be useful when processing 90% nitric acid at temperatures above 70°C. A ferrite limit for welds of <5%, on the other hand, may be useful and can be guaranteed with normal 316L steel and the usual orbital welding process.