UNS N08xxx Alloys in ASTM & ASME Specs

Around 1964 a decision was jointly made within ASTM committees A01 on Steel and B02 on Non-ferrous Metals and Alloys to incorporate coverage of the high nickel stainless steel alloys into the nickel alloy Subcommittee B02.07.

The chemistry, processing and use of these alloys were considered to be more complex than basic stainless steels, and these alloys were truly thought of as nickel alloys. The first alloy included was Alloy 800, soon followed by Alloy 825 and Alloy 20.

In 1974 the Unified Numbering System (UNS) was adopted (see the August 11, 2014 blog posting) and this group of materials, identified as nickel-iron-chromium solid solution strengthened alloys, was assigned the prefix of N08. As new alloys were developed and assigned UNS numbers, they were grouped together based on chemistry, characteristics and end use. However, some were actually iron-nickel-chromium materials rather than nickel-iron-chromium.

Fast forward again to 1992 or so, when the ASTM committees A01 and B02 agreed to follow the ISO definition of defining a nickel alloy. This definition required that a nickel alloy contain more nickel than any other constituent. If a new alloy contained more than 11% chromium and had more iron than nickel, it was to be considered for specification coverage as a stainless steel. It was also agreed that any N08xxx alloy with existing coverage in B02.07 specifications (identified with a “B” prefix) could remain, or could be transferred to A01 stainless standards (identified with an “A” prefix) depending on the preference of the producer. Because ASME adopts ASTM specifications, there was also agreement from ASME committee members even though this change would mean minor differences (less restrictive) in the rules for vessel construction.

Now we move to present day. N08020, N08367, N08700, N08800, N08810, N08811, N08904, N08925, and N08926 have been incorporated into the appropriate A01 stainless steel specifications. These alloys (except for N08904 or 904L) still have coverage in the Subcommittee B02.07 specifications and this is creating some confusion.

For non-ASME uses, ordering to either the A (stainless) or B (nickel) ASTM specifications presents no problem. Most of these materials are probably being supplied with test reports that reflect both standards. However, for ASME construction, ordering to the ASME SB specifications is necessary. This is because ASME has not yet adopted all of the revised ASTM A standards and, more to the point, the Tables in Section II, Part D for Design Allowable Values (DAV) have not been updated to reflect the new SA standards. The complete transition and adoption by ASME will probably take another two to three years at which time the process to remove these alloys from the B02.07 specifications can begin.

N08904 (904L) Material Specifications for ASME Pressure Vessels
ASME Specification Form Identical to ASTM Current ASTM Version
SB366 Fittings B366-04b B366-10a
SB625 Plate, Sheet, Strip B625-99 B625-14
SB649 Bar, Wire B649-95 B649-06 (2011)
SB673 Welded Pipe B673-91 B673-05 (2011)e1
SB674 Welded Tube B674-91 B674-05 (2011)
SB677 Seamless Pipe/Tube B677-99 B677-05 (2010)e1

904L is a special case. Through the 2013 edition of ASME Section II, the SB specifications covering 904L have not been updated to the new ASTM versions (see inset). This is because the current ASTM versions of these B specifications no longer include the alloy. Before the current ASTM versions can be adopted, the ASME must provide DAVs in Section II for 904L which reference the SA specifications.