The comments by Robert are very thorough. Some additional comments that have come to mind:
A classmate of mine worked on producing a computer program to do a hardenability calculation, based on alloy % to hardenability relationships in our textbooks, back in the late 1970's! So such equations based on compositions were available even then. (And were no doubt based on regression analyses done in the previous couple of decades!) There are multi-elemental synergistic interactions as well, but I am sure that the original poster's concerns have been shared by others, and that there must be commercial packages that estimate heat treat response, and equally importantly tempering response. THESE ARE NOT NEW PROBLEMS. In fact they are old enough that the solutions to them may have been incorporated into work processes and the reasons for these solutions may have been lost to time and retirements and corporate re-organizations. I have seen this sort of knowledge loss within my career. "Oh, it's because we have always done it this way."
The grade designations you describe don't have the H suffix. I'm pretty out of date here, but the idea was that the compositions of the H grades had been controlled somewhat more tightly to avoid co-variations (for instance, low Ni in the same heat as low Cr) that would both swing the hardenability in one direction. This has the effect of narrowing the possible variation in the heat treat response that one could get. A raw steel supplier's interest may start at providing a product that meets the letter of the spec while minimizing alloy costs, which can result in "low-side" content of all of the elements added. The heat treater's interest is to provide a part that meets property spec. The H grades, and any other requirements that you may impose on your supplier, are your agreement with them to supply what YOU want. You can always negotiate to buy a product with a specific narrower hardenability range within the boundaries of, say, 8630/8630H. This increases your chance of hitting a heat treat target, and still meets your own client's chemistry requirement.
Likewise to hardenability, tempering response varies with alloy content, and slightly differently than the hardenability response. Again I am out of date on this but as I recall an element that is not a carbide former but affects quenching response, such as Ni, will not have the same degree of effect on tempering response as a carbide former, which affects the decomposition of the alloy carbides present. This type of relationship should be included in any calculational package that you might find to help predict holistic response to the combination of quench and temper.
Finally, you can ask your client why a narrow range of hardness is specified. Some but not all designers will be able to articulate the reasons for what the target is. XXX strength minimum but with a fracture toughness of YYY minimum? If you show that you can meet fracture toughness at a slightly higher hardness, would the part still be able to be accepted? Maybe fracture toughness isn't the limiting factor, it could be avoiding an embrittlement range or environmental effect. There might be wiggle room somewhere.
I hope these are illuminating.
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Paul Tibbals, PE
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Original Message:
Sent: 01-21-2021 17:51
From: Robert Werber
Subject: Improving first pass yield for HT of low alloy steel
Vanessa,
Correlating chemistry to mechanical properties is quite complex particularity if you are an end user. Most master-melters have slightly different chemistry aims for a given alloy based on their equipment and available feedstocks. Adding to this variation is the heat treating, machining and testing of samples. These all add to the scatter in the data analyzes.
Having said this I do not want you to get the impression that it is impossible nor worthwhile to attempt. It is necessary to determine what data is worthwhile to spend your time and efforts to analyze. Since every situation is different I can only suggest the following guidelines.
- Review that data and its source to identify blocks of data with the greatest integrity and potential to yield maximum economic benefit.
- Perform initial statistical analysis to identify data errors and outliers.
- Perform multiple regression analysis to identify the effect of elements, suppliers, heat treatment, etc. on the mechanical properties of interest.
- Sit back and think about how your conclusions match up with metallurgical theory and practice.
- Perform some Statistically Designed Experiments (DOE) in your manufacturing processes to verify what you found is actually significant.
- Integrate your results carefully and slowly into production and quantifiably track any benefits/problems.
I have had successes and failures correlating mechanical properties with composition. I recommend starting off small, learn as you go and above all enjoy the journey of discovery! If you would like some additional insight into statistical analyzes and some real-world examples then check out my website: Planish-Inc.com
Best Wishes,
Robert Werber, PE
Planish-Inc.com
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Robert Werber
Oregon City OR
(503) 557-2906
Original Message:
Sent: 01-19-2021 09:26
From: Vanessa Molina
Subject: Improving first pass yield for HT of low alloy steel
Hello,
I am working with some new specifications that call out very tight hardness and tensile ranges, for low alloy steel (8630 and 4340 large-ish forgings, among others). Our HT crew is trying to hit a specific Brinell number on every HT load, but they often miss and have to either temper longer, or perform a full re-austenize and quench prior to re tempering.
I would like to improve our first pass yield, and I think reviewing the chemistry from each bar of material and adjusting our tempering parameters accordingly will be key. The issue I'm having, is that my background is titanium and nickel, and I only have a basic understanding of steel chemistry vs HT time and temp.
I am starting a log to capture chemistry vs HT time and temp vs hardness outcome, but while I gather all that, can anyone recommend resources (books, class, articles, etc.) that may help my efforts?
I appreciate your time. Thank you,
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Vanessa Molina
Chief Metallurgist and Lab Manager
AFGlobal Corp
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