AOD, DUAL CERTIFICATION, AND CHEMISTRY CONTROL

Most stainless steels, and a few higher nickel alloys, are available with different levels of

carbon. For resistance to intergranular corrosion, a low carbon is preferred, usually 0.03% carbon maximum in stainless steel. This is referred to as an ‘‘L’’ grade stainless steel, e.g., 304L and 316L. With respect to aqueous corrosion resistance, the lower the carbon, the better. For high-temperature service the opposite is true, and some minimum amount of carbon is required for both tensile and creep rupture strength.

The argon-oxygen decarburization (AOD) process for refining stainless steel was introduced in the 1970s. This made profound changes in how existing grades were produced as well as permitting totally new grades to be developed. Three of these changes are worth

discussing—carbon, sulfur, and precise control of chemistry.

Prior to the AOD, carbon could not be removed in the refining process without also removing chromium. Low-carbon grades could only be produced by starting with low-carbon raw materials, specifically low-carbon ferrochrome. The expense of low-carbon ferrochrome meant that the L grades were inherently more expensive. The AOD now permits refining carbon to very low levels, even with starting stock of higher carbon.

Industrywide specifications such as the American Society for Testing and Materials (ASTM) were written prior to the introduction of this new melting process. For example, ASTM A 240 for 304 stainless steel, UNS S30400, calls out 0.08% carbon maximum, no minimum, 30,000 psi minimum yield strength. Low carbon 18-8, 304L, S30403 is limited to 0.03% carbon, with a consequent lower limit for yield strength, 25,000 psi minimum. In addition, there is a 304H, meant for high-temperature use, with carbon specified as a range,

0.04–0.10%, and annealing and grain size requirements. This constitutes three separate grades. It is more economical if the mills can melt steel to only two, not three, different levels of carbon and dual certify. Consider 304, UNS S30400. As the carbon is specified only as a maximum, it might be possible to melt 304 to 0.03 maximum carbon. Lower carbon would also result in lower than the 30,000 psi yield strength required. However, using the AOD it is now possible to add a very small, precisely controlled amount of nitrogen. This does not harm intergranular corrosion resistance, but it does tend to increase room

temperature tensile properties. With care in annealing practice, it is possible to produce 304

with low enough carbon to meet the 304L specification yet with high enough yield strength

to meet 304 requirements. As this metal meets all specified requirements of both 304L and

304, the mill test report will show both S30403 and S30400, i.e., dual certified.

S30403/S30400 is appropriate for corrosion service but not for high-temperature mechanical

properties. For useful creep rupture strength some minimum amount of carbon is

required, typically 0.04%. The situation was addressed a few years ago by adding a number

of H grades to ASTM A 240, with controlled carbon for high-temperature strength. The

stainless steel 304H, S30409 has carbon specified at 0.04–0.10% for high-temperature

strength. In addition, there are grain size and minimum anneal temperature requirements.

The 304, S30400 has no requirement for minimum carbon, control of grain size, or annealing

temperature. Therefore any 304H containing no more than 0.08% carbon will meet 304

requirements and may be dual-certified with 304. One should note that dual-certified

304L/304 is suited only for aqueous corrosion service but would have rather low strength

at high temperature. Likewise dual-certified 304/304H is meant for high-temperature service

but may be unsatisfactory for welded construction in a wet corrosive environment. In practice,

there is rather little actual S30400 produced as sheet or plate at this time. Most is dual

certified, one way or another.

Like carbon, sulfur can now readily be refined to very low levels, typically less than

0.005%. Compare this with typical ASTM A 240 levels of 0.030% S maximum. Usually,

stainless steel intended for plates is refined to a low-sulfur level to improve hot workability.

The plate is generally formed and welded, with little machining by the customer. Low sulfur
is quite detrimental to machinability. As bar products are commonly meant to be machined,
most stainless bar actually must be resulfurized to some level, about 0.02%, for improved
machinability. When a plate is intended to be drilled for a tubesheet, machinability becomes
important and a resulfurized grade, still within the old 0.030 sulfur maximum, may be
chosen.
It is the precise control of chemistry, in particular nitrogen, that has permitted development
of the superaustenitic 6% molybdenum grades. The ability to closely control nitrogen
as an alloying addition has also tremendously improved the weldability of duplex stainless
steels. Whereas formerly the only duplex stainless steel used in North America was 3RE60,
and that only in tubing, today several grades of duplex stainless plate, pipe, and bar products
are used in significant and growing amounts.
Chemistry control also means that the producer will minimize the use of expensive
alloying elements. To remain competitive, mills now melt to the bottom end of the allowable
range. One consequence for the user is that 316L, specified as 2.00–3.00% Moly, now has
a melt range of about 2.00–2.10% Mo. Through the 1970s the typical Mo level of 316L
was about 2.3%. In addition, the average nickel content has dropped from just below 12%
in the 1970s down to around 10.2% Ni as currently produced. The result has occasionally
been that 25-yr-old 316L equipment, replaced in kind, gave unexpectedly short life. In part,
of course, this is due to increased corrosive conditions from recycling, rather than dumping,
water.

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