Chemical Processing - January 2008 - (Page 30) Material selection guide Design temperature, °F Cryogenic -425 to -321 -320 to -151 -150 to -76 Low temperature -75 to -51 -50 to -21 -20 to 4 5 to 32 Intermediate temperature Carbon Steel Material Stainless Steel 9 Nickel 3 ½ Nickel 2 ½ Nickel Plate SA-240-304, 304L SA-353 SA-203-D SA-203-A SA-516-55, 60 SA-516-All SA-516-All SA-53B or SA-106B SA-105 SA-234-WPB Pipe SA-312-304, 304L SA-333-8 SA-333-3 SA-333-6 SA-333-1 or -6 Forgings SA-182-304, 304L SA-522-1 SA-350-LF3 Fittings SA-403-304, 304L SA-420-WPL8 SA-420-WPL3 Bolting SA-320-B8 with SA-194-8 SA-320-L7 with SA-194-4 SA-193-B7M with SA-194-2H SA-350-LF2 SA-420-WPL6 33 to 775 SA-516 All SA-193-B7 with SA-194-2H 776 to 875 Elevated temperature 876 to 1,000 1,001 to 1,100 1,101 to 1,500 Above 1,500 C- ½ Mo 1 Cr-½ Mo 1 ¼ Cr-½ Mo 2 ¼ Cr- 1 Mo Stainless Steel Alloy 800 Alloy 800HT Alloy X SA-204-B SA-387-P12- Cl1 SA-387-P11- Cl2 SA-387-P22- Cl1 SA-240-347H SB-424 SB-443 SA-335-P1 SA-335-P12 SA-335-P11 SA-335-P22 SA-312-347H SB-423 SB-444 SA-182-F1 SA-182-F12 SA-182-F11 SA-182-F22 SA-182-347H SB-425 SB-446 SA-234-WP1 SA-234-WP12 SA-234-WP11 SA-234-WP22 SA-403-347H SB-366 SB-366 SA-193-B8 with SA-194-8 SA-193-B5 with SA-194-3 Table 1. Temperature provides a good basis to select materials for use in non-corrosive service. Sources: References 1 and 2. zone corrosion in some corrosive processes [3]. However, L grade stainless steels have lower strength than straight (non-L) grade stainless steels and the ASME code penalizes the design 15% to 20% with additional shell thickness and lower flange rating [4, 5]. What’s important to understand here is that a lot of the weldable forms of stainless steels (Types 304/316) produced today in the U.S. come dual certified as Type 304/304L or Type 316/316L. These steels have the higher strength of straight-grade stainless >> Generalized corrosion Figure 1. Carbon steel corrodes uniformly, so a corrosion allowance can be used. Photo courtesy of Matco Associates, Inc., Pittsburgh, PA. 30 • January 2008 steels and have the superior resistance to sensitization during welding of the L grade stainless. This is because they’re now made in a melt furnace process that substitutes nitrogen for carbon. Nitrogen strengthens the steel (like carbon) but won’t promote sensitization during welding. Fabricators often will purchase dual certified materials but will use the lower strength values of the L grade material in their calculations if you specify L grade material on your data sheet. This results in unnecessarily adding extra wall thickness and possibly crossing into a higher flange rating. 5. Properly use corrosion allowance. This allowance adds extra thickness to account for uniform metal loss over the equipment’s expected service life. The key word here is uniform. Mild carbon steel uniformly corrodes due to the galvanic cell potential of the interlaced ferrite-cementite grain structure, called pearlite (Figure 1). Specifically, there are millions of anodic (ferrite) and cathodic (cementite) sites that in the presence of moisture provide the four necessary elements for corrosion (anode, cathode, metallic bridge, and electrolyte). Alloyed materials in aggressive service will also uniformly corrode because their strong protective oxide layer is breached. Specifying a corrosion allowance for these situations is appropriate. However, many alloys, such as austenitic stainless steels, duplex stainless steels, nickel alloys and titanium, are more resistant to uniform corrosion and tend to corrode locally — that is, pit or crack. So, it’s less appropriate to specify a corrosion allowance for these materials in relatively benign processes. Furthermore, as the thickness of the stainwww.chemicalprocessing.com http://www.chemicalprocessing.com
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