Measuring ferrite content in austenitic molten zone

Controlling the ferrite content of a deposit of nominally austenitic stainless steel makes it possible to control :

• the risk of hot cracking: the amount of solidification ferrite remaining after cooling presumes the solidification mode, the ferrite-austenite mode being the most favourable;

• the risk of embrittlement at high temperatures: excess residual ferrite favors the development of a continuous "sigmatizable" network;

• mechanical properties, particularly impact strength, at low temperatures: excess residual ferrite favors the development of a continuous brittle path;

• in some cases, corrosion resistance, with the ferritic phase appearing as a preferentially attacked heterogeneity (nitric medium, for example).

This justifies the need to measure ferrite content reliably.

However, according to ISO 8249: "There is currently no universal opinion on the best experimental method for ensuring absolute measurement of the ferrite content of molten metal, either destructively or non-destructively".

In fact, several measurement methods for approximating this quantity coexist, but their results are not always consistent. These methods differ not only in terms of the physical principle used in the technique, but also in terms of whether they are destructive or non-destructive, the dimensions of the surface or volume to be analyzed or examined, the units used, and their ease of use.

Typically, manufacturers of consumable products express the ferritic tendencies of their inputs on prediction diagrams, industrialists practice metallographic counting during welding procedure qualification (MOS) and inspect non-destructively during manufacture using magnetic devices, just like inspectors.

All those involved in the integrity of a welded stainless steel assembly need to be able to agree on the ferrite content. These people may be :

• the manufacturer of welding consumables ;

• the manufacturer of the welded structure ;

• an organization responsible for developing codes or regulations of interest to public safety;

• an insurance company;

• any other organization that may be financially liable in the event of failure of the welded construction.

It is therefore essential that any method for determining ferrite content is reproducible, and that comparisons between measurements by different methods are possible.

The problem...