The first developed austenitic grades of stainless steel had a large amount of carbon and led to the formation of chromium carbides of the type Cr23C6, primarily along the grain boundaries. As a result of these precipitates a chromium-depleted zone forms. When these precipitates are located along the grain boundaries, accelerated intergranular corrosion of the chromium depleted regions can occur.
The solution to avoid it consisted in adding another alloying element which would preferentially combine with the excess carbon in the steel, such as titanium and niobium which were particularly suitable for this task and resulted in the development of the titanium stabilised types.
As production technology advanced new and more economical methods were developed to reduce the amount of carbon from the steel, such as AOD (Argon Oxygen Decarburisation) and VOD (Vacuum Oxygen Decarburisation) both methods make carbon levels below 0.03% possible.
From this basic introduction, the stabilised and low carbon austenitic stainless steels are two options with different properties…
… related to corrosion:
- Pitting corrosion resistance: titanium has been found to have a detrimental effect on it.
- Crevice corrosion resistance: there is no difference between both types of austenitic steels regarding this type of corrosion.
- Stress corrosion cracking resistance: titanium has a negative effect on it. Stress corrosion cracks usually initiate from the base of corrosion pits located in highly stressed regions.
- Knife-line attack: this type of corrosion is only encountered with stabilised stainless grades when welded.
… related to mechanical properties:
Titanium has a negative effect on surface finish, so producers have reduced the carbon and nitrogen contents of their stabilised grades to very low levels in order to reduce the amount of titanium required.
- Resistance to high temperatures: care must be taken with low carbon types; these are not recommended for high temperature conditions because their strength at higher temperatures is considerable lower than that of the stabilised steels.
- Impact properties: additions of titanium to Steel result in the formation of large precipitates of carbon and nitrogen which decrease the impact toughness acting as crack initiators. In stainless steel long products, it becomes clear that the low carbon grade will be far more suited for cold heading applications than the stabilised grades.
… related to processing:
- Machinability: stabilised steels compared with the low carbon titanium free variants alter their machinability because these hard particles increase tool wear and reduce machining rates.
Polishability: the presence of the hard titanium carbonitrides of the stabilised steels, make them unsuitable for polishing, due to the comet trails that form during this process. It does not occur in low carbon steels.
Non-metallic inclusion content: due to the presence of titanium in the stabilised grades of stainless Steel, their inclusion content will be higher than those of the low carbon ones.
Welding: both grades can be readily welded using common techniques.
Due to improvements in the production of stainless steels, low carbon variants have replaced the titanium stabilised grades. In addition to minimising the possibility of intergranular corrosion, low carbon content grades improve the effect of surface treatments compared to stabilized variants.
Stabilised grades continue to be used as “traditional” grade.
As a conclusion, it can be stated that the use of stabilised is only justified when high temperature strength is a consideration.