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304 vs. 304L Stainless Steel

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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,

Captain Dave Reeves, SFD
Syracuse Fire Department - Syracuse, New York


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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 susceptible 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 affected zone, specify the 304L and request certified mill test reports. I would also recommend you have the welds pickled 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, Minnesota


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

Captain Dave Reeves, SFD
Syracuse Fire Department - Syracuse, New York


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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, New York



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

Captain Dave Reeves, SFD
Syracuse Fire Department - Syracuse, New York




April 30, 2011

Q. I have experienced said rusting of welds on some stainless steel sculptures I've been making. I'm curious, is this effect superficial only, or does the chromium depletion occur through any area that's been heated sufficiently? I'm welding 1/4" to 3/4" 304 rod and have found light surface rust after less than a year outdoors. Thanks very much in advance!

Mark Malmberg
- Orinda, California, USA

First of two simultaneous responses -- May 6, 2011

A. Mark,
The loss of corrosion resistance in the HAZ is due to the restructuring of the metal atoms enabled by the near-melting point temperatures. So yes, it does occur throughout the heated area, but it also only really matters on the surface where corrosion potentially occurs. A good passivation treatment will restore a protective chrome oxide layer.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois


Second of two simultaneous responses -- May 6, 2011

A. I'll put it another way. Chromium depletion / chromium carbide precipitation only occurs in an area that is sufficiently heated. The area around a weld called the "heat affected zone" is sufficiently heated (thousands of degrees) during welding to cause the formation of chromium carbide by the interaction of chromium in the stainless steel and carbon in the stainless steel. Since stainless steel is "stainless" in large part due to the ability of chromium in the metal to form stable and corrosion-resistant chromium oxides on the metal surface, any chromium that has been precipitated within the metal as a carbide reduces the amount of chromium that is free to develop an oxide on the surface thereby lowering corrosion resistance. L grade stainless has a low carbon content, therefore there is less carbon available to interact with the chromium during welding which also means that there is more chromium still available to form stable oxides after welding. That is why L grade stainless is preferred if you are welding. Even without L grade stainless, you can restore much of the corrosion resistance by mechanically polishing the welded area or preferably by using a stainless steel pickling paste. My preference is to both use L grade stainless and then follow up with a pickling paste. If you do that, the welded area is really just as corrosion resistant as the non-welded areas.

Jon Barrows, MSF, EHSSC
Springfield, Missouri


July 11, 2011

A. The problem with chromium depletion is not so just rusting as a serious lack of corrosion resistance. Chromium is what provides the resistance, and when it combines with carbon (as already described by others) it cannot perform the task of making the steel corrosion resistant. The loss of corrosion resistance is called sensitization, and occurs at welding heat, when chromium carbide forms.

It is significant that the problem happens primarily in the heat affected zone. The significant loss of corrosion resistance on a narrow strip on each side of the weld means corrosion attack will concentrate here. A type of cracking, with dense multiple branches, called IGSCC (Intergranular Stress Corrosion Cracking) is the serious threat to welded joints of 304 (316 is not much better). The whole weld can come off. The loss of resistance is so high that even normal minor impurities in water are a danger.

In L grades, the Carbon is restricted to about 0.03% and this makes the problem almost disappear. (Non-L grades such as 304 have typically 0.08% C.) Electrodes should be E308L/ER 308L for 304L.

In multi-pass welds, each pass can sensitize the previous layer, though mostly this is not serious.

Mohan Babu
- Mumbai, India

June 1, 2012

Q. How can you tell the difference between 304 stainless and 304L Stainless Steel?

Oscar Magana
- Oxnard, California, USA


November 20, 2012

Q. Can SS 304 be welded with SS 304L? If so what are the measures to be taken during welding (TIG or EB)to ensure the weld quality as that of welding 304 with 304 or 304 L with 304 L?

Ramesh Patnaik
- Hyderabad, Andhra Pradesh, India

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