Welding Stainless 446

By goodyr1 Date 12-07-2005 23:56

446 is a first generation ferritic stainless steel that only contains chromium as a ferrite stabilizer and appreciable carbon. It also is subject to intergranular corrosion after welding unless a PWHT is applied. It also exhibits low toughness. Austenitic filler metals such as Types 309 and 312 are generally used to weld the ferritic steels to other types of stainless steel and to mild or low alloy steels. ERNiCr-3 can provide sound joints when welding ferritic stainless steels to themselves. All ferritic stainless steels are susceptible to severe grain growth at temperatures above 1700F, which will decrease it's toughness. 446 does not contain any stabilizing elements such as niobium that is useful in high heat aplications, though it does contain a higher carbon limit. I think I would question the suitability of the 446 as the first choice when exposed to the high temperatures of above 2100F. In the case of the 446, chromium partitioning can cause slight reduction in corrosion resistance. 446, with its high level of chromium, can exhibit chromium carbides at the ferritic grain boundaries, and the depletion of chromium from the matrix makes the steel susceptible to intergranular corrosion and embrittlement when rapidly cooled from high temperatures.

By Gratz Date 12-08-2005 15:04

We have used 446 for quite a long time now. It's getting quite difficult to get (we had to wait close to one year for 1/2" pipe). I noticed that several companies don't make the 446 and have replaced it with other alloys. We will try the ERNiCr-3 and see how this reacts in our application.

Thanks for the advise

By - Date 12-10-2005 16:11

I agree with goodyr1. The cause for any problems are going to be associated with the HAZ of the 446 ferritic base metal. Many of the ferritic base metals and applicable filler metals contain low percentages of copper, so I don't think that is a concern. Copper is generally added to stainless steels for increased corrosion resistance in certain environments, but decreases susceptibility to stress-corrosion cracking and adds age hardening effects. So, copper should not be a concern here. The concern with ferritic steels is the excessive grain growth during welding. Grain growth is not as much a concern in austenitic steels, but is a big concern in the ferritics. The fact that the ferritics start out with a lower level of toughness makes them vunerable to embrittlement in the HAZ. A stabilized grade of ferritic steel such as a 439 (or any other 2nd generation ferritic) will pin the grain boundaries and slow the grain growth during welding. The lack of solubility of nitrogen in the ferritic structure results in the precipitation of chromium nitrides in addition to the chromium carbides. Thus, these precipitates can cause sensitization and the loss of corrosion resistance. Unfortunately, the rate of transport or diffusion in the ferritic grades is such that you cannot cool an unstabilized grade of ferritic steel (446) fast enough to avoid carbide and nitride precipitation. I don't doubt that you've been using the 446 for a long time, but I am a bit surprised that you've been getting good service at the temperatures you have been exposing the steel to, especially in the HAZ. Since matching the 446 chemistry to a matching 446 filler metal would require a proprietary mix, I agree that the nickel alloy, ERNiCr-3 is a good choice. With your inability to obtain the 446 pipe within a year, it might be a wise choice to switch to a stabilized grade, which is much more readily available and prrobably more suitable to your application. It might require running another procedure, but it would sure cut down on having to wait a year to get the material. Just a thought.

Chuck

By GRoberts (***)

Date 12-13-2005 15:51

Chuck,
I'm not sure if your references to copper were directed towards my post, but I possibly wasn't clear. I was referring to copper as a contaminant, not an alloying element.

Anyways, when austenitic stainless is exposed to copper at elevated temperatures (such as molten copper), it penetrates the grain boundaries and causes cracking. The reference I have only refers to austenitic stainless, so that may be why ferritic stainless is successful in molten copper service. Anyway, I would highly recommend against using austenitic filler in copper exposed service at high temperatures due to this phenomenon. I do not believe nickel alloys have this problem, so ERNiCr-3 is a good recommendation as stated earlier.

By - Date 12-13-2005 17:57

No, I wasn't referring to your post directly. Actually, I was referring to filler metals. Maybe I wasn't clear on that. Sorry... To be honest, I think copper will penetrate the grain boundaries of the ferritics, too. I don't have any concrete documentation comparing the effects of copper as opposed to austenitics for every possible scenario, but at the 2100F operating temperature, I would think it would be just as detrimental. Either way, I'm not sure that I would be using an unstabilized ferritic in this high heat application...ferritic or austenitic.

Chuck

By GRoberts (***)

Date 12-10-2005 00:45

Be careful about using austenitic stainless. I don't have any references with me here, so I can't give you the details or scope, but copper can penetrate the grain boundaries of austenitic stainless and cause cracking. I'm not sure if that applies to nickel alloys as well, but this could possibly be the reason you are stuck using ferritic stainless steel in your application.