The EN ISO 15608 Material groups are similar but not 1:1 comparable with the P-no groups acc. to ASME BPVC Section IX.
The main difference between the two is that the ISO system allows any material within the scope of the reference standard to be assigned to a certain group, whereas the ASME system assigns a P-No. only to materials listed in the ASME BPVC Section IX or that have the same UNS number as listed materials.
The two most commonly used grouping systems are provided by EN ISO 15608 and ASME BPVC Section IX, QW-422, EN ISO 15608 classifies materials into ‘groups’ and ‘subgroups’, identified by a numeric value, whilst the ASME BPVC Section IX assigns P-numbers (P-No, where ‘P’ is for parent material), as shown in Table below.
Comparison of material groups EN ISO 15608 vs ASME BPVC Section IX
| Material Group | EN ISO 15608 | ASME BPVC Section IX QW-422 |
|---|---|---|
| Steels | 1.1 – 11.3 | P-No. 1 – P-No. 15F |
| Aluminium and aluminium alloys | 21 -26 | P-No. 21 – P-No. 26 |
| Copper and copper alloys | 31 – 38 | P-No. 31 – P-No. 35 |
| Nickel and nickel alloys | 41 – 48 | P-No. 41 – P-No. 49 |
| Titanium and titanium alloys | 51.1 – 54 | P-No. 51 – P-No. 53 |
| Zirconium and zirconium alloys | 61 – 62 | P-No. 61 and P-No. 62 |
| Cast irons | 71 – 76 | Not applicable |
EN ISO 15608 Material groups
Due to the wide range of alloys available and the different standards covering them, assigning an alloy to a particular group within EN ISO 15608 may prove difficult. However, three standards exist that may be useful for determining the grouping for a material of interest. ISO 20172, ISO 20173 and ISO 20174 list many different grades of material and the group in which they sit. These standards can be very useful in determining the relevant group and therefore the range of qualification.
ASME BPVC Section IX, QW-422 P-Number
The ASME grouping is made using the alphanumeric designation P-No for the base metal. This grouping is based on comparable characteristics of the material such as weldability, mechanical properties and composition.
The base metals are listed in the table QW-422, which allows determination of what the P‑No of a certain material is, depending on its specification according to ASTM or its UNS number designation. Only materials accepted for use in the construction of pressurised components according to the ASME BPVC are listed in QW-422, hence they can be assigned a P-No. If a metal does not appear in the table QW-422 it is considerate ‘unassigned’ and cannot be grouped under a P-No. If an unlisted material has the same UNS number as a listed material, the unlisted material belongs to the same P-No as the listed one.
