atomic gas limits in low alloy steels 3

[texag]

Hello,

I am looking for information regarding acceptable allowances for hydrogen and nitrogen in low alloy steels such as 41XX, 43XX, or 86XX. There is quite a bit of information on how these atomic gases are introduced during the steelmaking process, and what effects they have on the quality of the produced steel, but I have found no information regarding recommended limits. It is generally understood that too much hydrogen can cause problems such as hairline cracks, blistering, and embrittlement; but it is also understood that controlling hydrogen content during the steelmaking process is difficult and expensive. The limit of hydrogen and nitrogen then becomes a balance between steel quality and economics. My question to this forum is what is an appropriate limit for hydrogen in steel? Nitrogen? Also, if the service condition is such that free hydrogen may be present, does this affect the answer? I would assume it does. Do any industry specifications control atomic gas limits in steel or is that the responsibility of the purchaser?

Any advice or shared experience on this topic would be much appreciated.

Texag

 

[metengr]

texag;
This is an excellent question. Not knowing your specific background, I thought you might find this information useful for starters;

http://www.keytometals.com/page.aspx?ID=CheckArtic...

The bottom line is steel purity requirements are based on application or end use.

 

[texag]

metengr,

Thanks for the link, I had seen the keytometals page on hydrogen in steels. My background is engineering for a wellhead equipment manufacturer. Naturally my question relates to hydrogen limits for steels used to make equipment for pumping down and flowing back, a well. Hydrogen could be present in fluids going downhole, or in acid gases (such as H2S) coming back out, for example. It is well known that elemental hydrogen can be present in flowing wells, which could diffuse into the metal, which could cause abrupt failure of equipment under tensile stress. In cases where that is a possibility, I think it is obvious the steel should be as free from hydrogen as possible, but again, what is a practical limit?

texag

 

[metengr]

texag;
I was not sure if the link worked for you based on your response. The article is titled

Clean Steel: Part One :: KEY to METALS Article

There is a table that lists the following; steel product, maximum allowed impurity and maximum inclusion size. Again, steel cleanliness is based on permitted inclusion size for specific performance in service. I believe this is self explanatory.

 

[texag]

metengr,

The information in that article is in agreement with other sources I talked to. Thank you for pointing me to it.

texag

 

[steelmtllrgst]

I will give you some information from experience, hopefully it is helpful! Regarding nitrogen, there is concern with high nitrogen causing hot ductility issues in the grades mentioned, but for most steel production methods you should be less than 120 ppm N and the potential for problems is small. Unless your supplier is intentionally adding nitrogen or using it as a stirring gas during liquid processing then the typical product nitrogen should not cause issues. Many specifications do not have a given N limit.

For hydrogen, the maximum allowable content is actually quite controversial. You are discussing some different types of hydrogen issues in your original post: hydrogen in the material from the start and hydrogen from the environment. I can comment only on hydrogen that is present in the steel as a result of liquid steel processing. Many researchers will suggest that a limit of 2.0 ppm is reasonable as it can be achieved with normal processing and at less than 2.0 ppm the occurence of hydrogen cracks is reduced or eliminated. However, there are a lot of other things that should be considered to avoid hydrogen flaking such as material cross-section, composition, and post-forge thermal handling. It is certainly possible to have hydrogen cracks in just about any low alloy steel grade of significant cross-section even at H contents of 1.0 ppm when the material is not handled correctly after hot-working.

My main point is that a 2.0 ppm H limit is common for the grades you have listed, but will not alone guarantee the material to be free from hydrogen cracks.

 

[mcguire]

texag
Very few metallurgists know the answer to your question. The hydrogen in any of the steels you cited, in the annealed condition is always less than 1 ppm. Hydrogen is extremely insoluble in the BCC iron matrix. The only way to get more hydrogen into the steel is to create traps, such as carbides, grain boundaries, vacancies and dislocations. If you worked out the math the carbides and dislocations cannot raise the solubility above several ppm. Dislocations however increase exponentially with carbon content in un-tempered martensite. John Hirth in the 1980 memorial Lecture spelled this all out very clearly, but researchers did not want to give up the grants their pet theories had brought them so it was ignored.
In about 2000, when mixed dislocation density as a function of carbon content and cold work was accurately measured, it became possible to see that hydrogen contents of between 100 and 1000 ppm were to be expected in as-quenched martensitic alloy steels. The level becomes much greater with cold work, about 2000 ppm in 4340 as-quenched with 0.10 deformation.

The exothermic binding energy and diffusivity of hydrogen is such that even at room temperature and ambient humidity, the equilibrium concentration will be attained rather quickly, regardless of what it had been when manufactured. So, if you are worried about hydrogen, you cannot limit it by a content specification. You must reduce dislocation density. This is done by tempering over 1000F. This coincidentally reduces hardness to about Rc22 in alloy steels, which, magically, is the working limit on hardness for martensitic steels in hydrogen-embrittling environments.

If you want more strength without increasing susceptibility, that is a whole other question.


Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[mcguire]

Correction: "if you worked out the math on carbides and GRAIN BOUNDARIES...

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[steelmtllrgst]

Mr. McGuire, are you suggesting that vacuum treatment of steels is a pointless measure to prevent hydrogen flaking???

 

[mcguire]

No. Hydrogen flaking involves the cooling of massive sections with little ability to infiltrate hydrogen from the environment. The amount of hydrogen in the melt could be quite high without counter-measures, such as vacuum, and not escape during solidification and cooling, resulting in its being available to cause cracking once martensite forms and is stressed by temperature gradients and phase changes.
What I am saying is that a martensitic steel processed to have low hydrogen, will reach an equilibrium content of hydrogen eventually, and that this level depends on its microstructure and ambient conditions, not its prior hydrogen level as manufactured. Having specified and received a low hydrogen content doesn't buy you a thing once a component goes into service.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[steelmtllrgst]

OK, thank you for the clarification. I think it is always important to note the very different aspects of environmental hydrogen induced cracking versus hydrogen flaking. Although caused by the same element, they are essentially unrelated when discussing prevention.

Cheers!

 

[TVP]

Nice discussion gents. I concur w/ steelmtllrgst about unrelated strategies for prevention, which is not always completely understood.