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
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