Hardenability of different raw materials.+ 3

[ckiyak]

Hello guys, I have been working as heat treatment engineer since september 2017. This is my first experience in industry. I have problem with the hardness and tensile of some parts. The problem is, actually we are using different type of raw materials(15 different types for aısı4340 for example). All of them related to same spec. But you know there should be little difference between every raw material, something like elemental amounts. The problem is, planning department doesnt know anything about raw materials. Every aısı4340 is same for them. But sometimes hardness value should be lower than what spec want even i do the same treatment. What do you think, what should i check on raw material certificate to understand which raw material will be harder or higher tensile strength than others.

Thanks for your help.

 

[metengr]

You check the chemical composition range for key elements on the MTR which effect both hardness and hardenability. For example, carbon, manganese, nickel to name a few. Carbon content is the main contributor to increased hardness and strength of steels.

Secondary effects are grain size and prior processing steps.

 

[EdStainless]

There are formulas fro hardenability that take into account the minor variations in alloy content.
The other way to do this is to look at the as quenched hardness.
There are tables out there (for example in AMS2759-1F (below 220ksi UTS)) that show you what temper temperature to use based on the desired strength and the actual as quenched hardness.
Most of these alloy variations are probably intentional by the mills making them. know of a stainless mill that melts 32 variations of 304.....

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P.E. Metallurgy, Plymouth Tube

 

[ornerynorsk]

In addition to the material, process controls are also critical.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[bobjustbob]

Your raw material is one piece. You need to familiarize yourself with your internal steel requirements. When ordering you should be stating a specification. You need to review the incoming material certifications to ensure they are reporting a chemistry that meets that specification. Periodically you should pull a sample and have a chemistry done to compare their reported chemistry to what you get. Now, remember your spectrometer and their spectrometer will vary and the chemistry may vary somewhat from their report. So, just because the report says .58% Cr and you get .55% Cr, that does not mean the report is not valid. Different elements will vary differently. When I do a chemistry check, I concentrate on cmparing Mn, Cr, Ni, Mo, and V. I look at Si, Al, P, S, Cu and other trace elements to ensure they are within spec. You can get a C on an optical emission spectrometer, but if I have a concern the carbon should be checked using a combustion method like a LECO.

Once you are sure you are getting the steel they say you are you can start comparing alloying elements with results. You can look at individual elements like C for hardness but things like Mn, Ni, Cr and Mo while they will affect hardness you probably would loose any correlation in the noise. So, a tool like a carbon equivalent or DI would help to you look at the overall hardenability of the steel and compare results. If you see that when you get some 8630 with a CE<60 gives you low hardness with your process you can start to tighten up the internal specifications. For example, ASTM A352 Grade LCC has a pretty lenient range for chemistry, however, if we got a carbon equivalent much below 36 we could not meet the elevated tensile requirements that one of our customers had so we put a lower C and Mn restriction for our internal requirements to ensure we would always have that CE.

This is assuming you have tight controls on the rest of the process. Your quench and temperature control in your ovens will have huge impacts on your final results. If you do not have a firm grip on these, nothing you do to control chemistry is going to help. Like Ed stated above, as quenched hardness can help you determine the effectiveness of your quench and if you need to adjust temper.

 

[ckiyak]

Thanks for all answers guys. Answers helped me a lot.

 

[WKTaylor]

In addition to everyone else's great comments...

Heat treating steel is also 'art'. Time-temperature values in most specs are 'recommended'. An old heat treater begged me to refer to the spec [AMS2459/1] for HT 4330 forgings... but allow him wiggle-room-variation for actual tempering temperatures and soak-times.

He would then make minor adjustments to tempering temperatures based on the quench hardness of the accompanying sample-test coupons... to ensure the actual parts hit the middle of hardness/strength ranges. IF the quench-hardness was high, he would temper slightly higher than spec... whereas hardness too-low and he would temper slightly lower than spec... etc. Soak times were varied based on some long-time experience and outcome of metallurgical examinations.

The sample HT coupons were taken thru the same tempering process as the parts. The coupons were hardness and tensile-tested to ensure that the parts were in-range. Also, the coupons could be metallurgically sections/examined to ensure it was spec-compliant.

Regards, Wil Taylor

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