Dual Certification

The term "dual certification" is most commonly used to refer to the practice of certifying a material as meeting the requirements of a specification(s) as two different alloys.

The most common example of this type of "dual" certification is that of 304 and 304L (or 316 and 316L). Material melted to the lower carbon range and processed to maintain the minimum properties for the straight grade will qualify as both "L" grade and regular grade. This dual certification has been a standard practice for years since AOD refinement essentially eliminated the cost differential between regular and low carbon versions. Into the mid-70’s there was a significant premium charge (as much as 10%) for low carbon 304 and 316. It should be noted that dual certifying of this type is not permitted when it encroaches on the principle of grade substitution. For example, Type 321 stainless could meet the chemical and mechanical property requirements of 304, but since it also has the intentional addition of another element (titanium), it cannot be certified as 304.

Alloy

Carbon (Wt %)

Yield Strength (KSI)

Tensile (KSI)

304

0.08 Max

30.0 Min

75.0 Min

304L

0.030 Max

25.0 Min

70.0 Min

It is also common to see Alloy 2205 dual certified as both UNS S31803 and UNS S32205. The original broad chemistry limits for S31803 permitted a composition that could result in poor mechanical and/or corrosion properties. The S32205 chemistry limits fall within the original ranges, but are restricted to ensure better properties. The S32205 material also has a slightly higher minimum tensile. Dual certification was necessary, in part, because until recently only the S31803 alloy was included in the ASME B&PV Code.

We also see requests to dual certify plate to a bar specification. It is not possible to certify plate as bar, except when specifically permitted by the specification. ASTM Specification A484 covering stainless bar allows for the conversion of plate to flat bar. With the exception of ASTM B446 for Alloy 625, there is no such allowance for nickel alloy bar; although in many cases the differences between plate and bar product properties are minimal. AMS specifications for titanium bar products specifically prohibit the use of plate to produce bar. In addition, many (but not all) specifications define bar as being produced in widths under 10", while plate is produced as a product greater than 10" wide. In any event, the applicable tolerances alone would generally prevent plate from being fully certified as bar, since bar dimensional tolerances are much tighter than those for plate. However, when a customer’s requirement permits, it is possible to certify that the chemical and mechanical properties of a plate product satisfy the requirements for the corresponding bar specification. This can often alleviate availability or minimum quantity issues associated with flat bar requirements for nickel alloys.

Another form of "dual certification" is more appropriately called "multiple" certification. The most common situation for nearly all of materials we supply is to certify them to both an ASTM and an ASME material specification. This is possible because in most instances the ASME version is nearly identical, word for word, to the ASTM version. Another form of this multiple certification is to also certify the material to an AMS specification. For most of the materials we supply, the requirements (chemical, mechanical, method of manufacture, finish condition and tolerances) are the same (or similar enough) that the material can be produced to satisfy all the requirements of the ASTM and AMS specifications.