304 vs. 304L Stainless Steel

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,

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...

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.

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.

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.