Effect of Copper in Low Alloy Steels

[redpicker]

Since I got so much good information on the Calcium issue, I thought I'd go to the well again and see if I can get some information on Copper.

I process low alloy steels (mostly Cr-Mo steels of various carbon contents; AISI 41XX or derivatives). We consume heat lots from the steel mills and essentially all is quenched and tempered to develop optimum strength and impact resistance. We are asked to meet ever increasing strength and impact levels and, in some cases, meet severe corrosion testing (NACE TM-01-77 Method A) at increasing strength levels.

Quite by accident, I have noticed that we realize improved processing with heats that have copper in the 0.15-0.25% range. At least, I think it is the copper content that is responsible for the improved processing. We will get material from both EF steel mills and BOF steel mills, and I see better processing with the EF heats. The EF heats are nearly all re-metled scrap with 0.15-0.25% Cu while the BOF heats are nearly all from blast furnace iron and have essentially no copper. As near as I can tell, the other elements are present in the same amounts (even calcium, I have discovered).

Now, the EF mills tell me that I don't have to worry about copper having an adverse effect on any of the properties I am interested (hardness, tensile strength, impact strength, and H2S resistance). They warn me that there are some tramp elements (tin and aresnic, for example) that can become a problem with EF heats, but as long as the copper is below 0.35%, it doesn't hurt.

The BOF guys, however, tell a different story. They say their steels are superior to EF steels specifically because of the lower copper. They point to European standards that have a 0.25% max copper and suggest that this implies that it is recognized that the lower the copper level, the better the strength and impact combination and, by implication, the H2S resistance.

I have talked to experts in H2S testing and they tell me that the copper content sholdn't affect the H2S resistance. However, all of these "experts" are from an EF background.

So, I need some existing research or other data that would help to clear things up. I would like to require our BOF mills to intentionally add copper in the 0.20% range to improve our processing, but I am meeting resistance from management that is concerned that it would be detrimental to either the impact strength or H2S resistance.

 

[TVP]

redpicker,

I'll respond in more detail tomorrow, but I don't believe that Cu in the amount of 0.15-0.35 mass % is responsible for any real improvement in strength/toughness of Q&T Cr-Mo steels. The testing with which I am most familiar shows improvements with austenite grain size refinement, and that the best performers were in fact Japanese BOF mills. It is absolutely true that As, Sn, etc. are quite detrimental, and allowing/specifying Cu seems like an open invitation for the tramp elements to appear.

 

[metengr]

redpicker;
I deal with boiler/pressure vessel steel plates. We normally will specify a maximum of 0.2% Cu for steel used in boiler tubing and feedwater heaters (the T22 grade).

I have not seen any data on this family of steels (1 to 2.25% Cr and 0.5-1.0% Mo)and in my dealings with steel mills that would indicate adverse effects of copper on mechanical properties. I have known that for steels used in the nuclear power industry, residual copper content must be as low as possible to avoid radiation induced embrittlement of fracture toughness properties. This was especially true for ESW in reactor vessels that used copper coated weld wire.

Regarding copper content in EAF produced steels, the only data I have seen indicates limited hot work ability (namely, forge welding) for copper contents in excess of 0.3%. It also has been reported that higher copper contents, other than to increase atmospheric corrosion resistance, can result in reduced surface quality because of the adherent mill scale. This would effect ferrous, high pressure feedwater heater tubes.

 

[redpicker]

Thanks for the responses.

I should say that I don't really expect any improvement in the toughness with the addition. I am purposely avoiding describing the processing improvement for confidentiallity reasons. I recognize that specifying a higher Cu content could "open the door" for higher levels of undesireable elements, which would have to be controlled.

Any recommendations as to what limits As and Sn should be held to? Any other tramp elements I should be concerned about?

I can say that the improvement I am expecting is really a minor issue, but it could improve our capacity, so if the adverse effects are not detrimental to the product (radiation induced embrittlement is of no concern). The surface quality issue is important, but as we are using both EF and BOF steels with acceptable results, I don't think these levels would pose any problems.

Thanks

rp

 

[EdStainless]

Have you looked at other trace elements? I am thinking of things like B, Al, N, O, Ti, Zr, Nb, and such.
It isn't that these change properties much, but they can change heat treatment response.

As for limits on As, Sn, Sb, In, Zn, Cd, well I would start by asking what the lower limit of detection is in the analysis. These lower melting point items can realy screw up grain boundaries.

The other factor in steel making is how the mill is post-melt/pre-casting processing. How are they making the final alloy additions? What it the ladle refinement process?

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[metengr]

Any recommendations as to what limits As and Sn should be held to? Any other tramp elements I should be concerned about?

Yes, you can run the calculations for the temper embrittlemnt J-factor based on hypothetical limits for tramp elements. Normally 150 or less is specified. Many reputable mills have their own specific tolerances regarding P in addition to tramp elements, such as As, Sn, Sb, lead and copper. P and Sn have the most direct impact on susceptibility to TE in steels and in weld deposits (which is the x-factor). You just need to ask up front or provide the mill with your specific chemical composition requirements and make sure that they can agree to achieve these ranges and report them on the MTR.

 

[TVP]

redpicker,

After reading the additional comments from metengr and EdStainless and re-reading your original post, I think I have a better understanding of your situation. First, with regards to the effect of copper on H₂S resistance and toughness, I think you will not find any reference that specifically addresses this. My thoughts are that it will have no detrimental effect on either when present in the amounts mentioned previously. Copper is a strong ferrite strengthener, which is one of the reasons it is used in the powder metal industry, but does not form any stable carbides. In this regard it is quite similar to nickel. It is also similar to nickel in that it is a austenite stabilizer. Neither of these things is bad when it comes to toughness or H₂S resistance. For authoritative references I would search Google Scholar and the various books on physical metallurgy (Cahn, Sinha, etc.).