QUESTION:
Our piping specification requires that duplex stainless-steel castings be supplied per ASTM A890. A valve company took exception, and indicated that duplex valve bodies and bonnets would be supplied per ASTM A995. What is the difference between these specifications?
ANSWER:
ASTM A890 "Castings, Iron-Chromium-Nickel-Molybdenum Corrosion Resistant, Duplex (Austenite-Ferrite) for General Application" was released in 1988, the first ASTM standard that covered the modern duplex stainless-steel casting grades. As such, many piping specifications include references to ASTM A890 and its various duplex grades for piping components such as valves. Unfortunately, ASTM A890 has some shortcomings when used to purchase pressure-retaining components.
ASTM A890 references ASTM A781 "Castings, Steel and Alloy, Common Requirements, for General Industrial Use" for coverage of general requirements. ASTM A781 refers back to the individual product specification (A890) for tensile test requirements. ASTM A890 lists tensile property requirements, but does not require that the tensile tests actually be performed. Tensile tests are only required if Supplementary Requirement S32 is specified on the purchase order. In order to obtain a test report showing the actual chemistry and tensile test results, Supplementary Requirement S12 must also be imposed. Marking of heat numbers on castings is not required unless specified separately on the purchase order.
In 1998, ASTM released ASTM A995 "Standard Specification for Castings, Austenitic-Ferritic (Duplex) Stainless Steel, for Pressure-Containing Parts." At first glance, this specification appears very similar to A890. Any casting that meets A995 also meets A890. However, A995 imposes some additional requirements:
§ A995 includes fewer grades (1B, 2A, 3A, 4A, 5A, and 6A) than A890 (1A, 1B, 1C, 2A, 3A, 4A, 5A, and 6A), and the composition limits for the common alloys are listed with higher precision in many cases.
§ A995 requires that weld repairs be made using procedures and welders qualified per ASTM A488. A890 does not.
§ A995 provides a definition for major weld repairs and requires post-weld heat treatment (i.e., re-solution heat treatment) after all major weld repairs. A890 only requires post-weld heat treatment if Supplementary Requirement S33 is imposed on the purchase order.
§ A995 references ASTM A703 "Steel Castings, General Requirements, for Pressure-Containing Parts," instead of A781. A703 imposes several other requirements that are not included in A890 nor A781:
§ Each heat of material must be tensile tested.
§ Each casting must be hydrostatically tested.
§ Each casting must be marked with a heat number or with a serial number traceable to a heat number.
§ Certification must be provided listing the chemical analysis of the heat and the mechanical property test results.
In 2000, ASME issued SA995, which is identical to the 1998 revision of ASTM A995. ASTM A995 was reapproved without changes in 2003.
Once a specification has been adopted by ASME, a progression begins toward implementation of the included grades in various portions of ASME codes:
§ P-numbers are assigned in Section IX of the ASME Boiler and Pressure Vessel Code.
§ Mechanical properties vs. temperature, allowable stresses vs. temperature, and maximum temperature limits are included in the appropriate tables in Section II Part D of the ASME Boiler and Pressure Vessel Code.
The material may be adopted in ASME codes for specific products or applications, including:
§ ASME B16.34 Valves - Flanged, Threaded, and Welding-End
§ ASME B16.5 Pipe Flanges and Flanged Fittings: NPS ½ Through NPS 24
§ ASME B31.1 Power Piping
§ ASME B31.3 Process Piping
Whether and when a particular specification/material is included in any of the above codes depends on numerous factors. However, one thing is clear-materials are not added in these codes unless the parent specification requires tensile testing to ensure that the product being supplied actually meets the minimum mechanical properties upon which the design is based. For this reason alone, many specifications that don't require actual tensile testing are not represented in these codes. Examples include ASTM A276 stainless-steel bar and ASTM A743 and A744 castings. A890 grades will not be added for the same reason. When duplex stainless steels are added to these codes, the parent specification will be A995/SA995.
This is the reason why valve companies prefer to specify duplex stainless steels per A995/SA995 for control valves. Customer piping specifications that specify duplex stainless steels per ASTM A890 are essentially out of date, and should be revised to reference ASTM A995 or ASME SA995 for these materials.
Source: valve magazine Winter 2005
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