Letter 16007

304 vs. 304L Stainless Steel [New York] 

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As the Chairman of our apparatus advisory committee, I am charged with developing specifications for all new fire apparatus for the Syracuse Fire Department. We currently are finishing up the specifications package for a new large heavy rescue vehicle, and a question has arisen about the body material. Some manufacturers we have talked to advise us to specify 304L (low carbon) stainless steel, as they feel that the welds are of higher quality and better durability. Other potential bidders have said that regular Grade 304 is "just as good" and they propose to take exception if we specify 304L. What do you think? Is there (as some have told us) "virtually no difference", and if there is a difference, should it be a factor in our decision?

Thank you,

David Reeves
Syracuse Fire Department - Cicero, NY, USA

When you have welding included in the manufacturing process, there is a difference between 304 and 304L. The difference is what happens to the HAZ (heat affected zone), there is a phenomena termed "chrome carbide precipitation", this happens at a temperature from approximately 850 degrees F to 1200 degrees F. The base metal will reach this temperature during welding. This is a "time at temperature" phenomena, so, the thicker the material, the higher the carbon level, the higher the temperature in the range, and the longer in the range, the more susceptable your material is to CCP. The chrome will combine with carbon and precipitate as chromium carbide, this leaves and "iron rich" area adjacent to the welds that corrodes more easily than fully annealed austenitic stainless steel. It is possible to heat treat and redistribute the carbon molecules with 304ss, but not practical with a large weldment. 304L has a low carbon content, this very much minimizes, or eliminates CCP, and is much more practical for large weldments. If you don't want your vehicle to "rust" in the heat affecte! d zone, specify the 304L and request certified mill test reports. I would also recommend you have the welds pickeled if they don't perform a blending operation on them. There are many common substances that can attack the HAZ on 304, chlorinated city water is one, road chlorides is another common one, I think they use calcium chloride in your area?

304L is readily available in sheet, pipe, tube, and structural shapes, availability should not be an excuse with a vendor. I hope I helped with your decision.

Best regards,

Bradford Maas
food tech - Northfield, MN

Thank you. Your reply confirms our choice to specify 304L.

Captain Dave Reeves
Syracuse Fire Department - Cicero, NY, USA

As a metal finishing facility in Syracuse under the protection of the Syracuse Fire Department, I feel compelled to respond. The previous responder was 100% correct, however, I've always heard the phenomenon referred to as "chrome depletion" in that the chrome precipitates out as chromium carbide (from the carbon content) in the welded / heated areas. If you want to see what happens to a standard carbon stainless chemical tank in a corrosive environment, come in and look at some of our welded tanks. The stainless looks good everywhere but the welded corners (which are covered with rust). Bottom line: if your supplier can build a truck without welding, no problem! Good luck...

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

Thanks Milt . . . why the heck didn't I ask you first?

Captain Dave Reeves, SFD
Syracuse Fire Department - Syracuse, NY, USA

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