Those fine grained steels are micro-alloyed steels for pressure purposes as, for example, those given in EN 10028-3 or in EN 10222-4.
For these steels, the quantitative value of permissible membrane stress ( 2/3 Re/t and 5/12 Rm/20) stated in Annex I Section 7.1.2 does not apply. However an equivalent overall level of safety must be achieved.
When using a harmonised standard for pressure equipment (e.g. EN 13445), no further justification is needed for the quantitative values which have been used as regards Annex I section 7
The requirement to demonstrate an equivalent overall level of safety applies to the product itself, and to the measures taken to meet the essential safety requirements. The use of a “recognised” code is not, in itself, sufficient to demonstrate an equivalent overall level of safety.
What are fine grained steels?
Fine-grain steel generally has a smaller grain size than other types of steel and the grain size number can be determined according to the E 112 test methods. This is achieved by micro-alloying (using small amounts of alloying elements) which improves the properties of the steel. The steel properties that can be improved include strength, toughness, weldability and formability.
