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Speaking of Precision

Speaking of Precision is a knowledge preservation and thought leadership blog covering the precision machining industry, its materials and services. With over 36 years of hands on experience in steelmaking, manufacturing, quality, and management, Miles Free (Milo) Director of Industry Research and Technology at PMPA helps answer "How?" "With what?" and occasionally "Really?"

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.40 Carbon Steel Brinell Hardness vs. Tempering Temperature

Posted October 05, 2010 8:55 AM by Milo

Hardness of Quench and Tempered alloy steels is a function of the tempering temperature. The higher the tempering temperature, the lower the hardness.

This is called an inverse relationship.

And it's why some people call tempering "drawing."

The temper "draws the hardness out of the steel."

Normalized at 1600F, Quenched in oil 1550 F, Tempered 2 hours

These curves are a rough approximation of the as tempered brinell hardness for the grades shown. For example, I have other data for 4340 that shows 440 BHN at 800F; 410 at 900F; 380 at 1000F; 340 at 1100F, and 310 at 1200F temper temperature.

Fire! Can't do a blog on heat treat without a picture of fire.

Your mileage may vary, in other words, but this graph is close enough for 'considered judgment.'

Additional 4140 data that I have from my notes suggests 397 BHN at 800F; 367 at 900F; 335 at 1000F; 305 at 1100F and 256 at 1200F.

If you have better data from your process – USE IT.

Better yet, if you have time, send a sample to your heat treater for a pilot study.

In the absence of data from your process, the above figure and data will give you "a place to stand" in understanding what is possible when heat treating .40 carbon alloy steels- the steels most commonly encountered in our precision machining shops for Automotive, Aerospace, Agricultural and general applications.

Here is a video from PMPA member company Nevada Heat Treating to give you an inside look at what goes on at a heat treat service provider.

Permalink:

http://pmpaspeakingofprecision.com/2010/10/05/40-carbon-stee…ng-temperature/

Editor's Note: CR4 would like to thank Milo for sharing this blog entry.

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Re: .40 Carbon Steel Brinell Hardness vs. Tempering Temperature

10/05/2010 11:32 AM

Milo,

How to find out if the carbon content in the steel is the right amount I wanna use for a job, is it given by the heat number?

Further, is there a way (rule of thumb) to know how to quench a steel properly and how to soften (release?) once it's over done? What are the best methods for heat treatment and quenching, including back-yard methods?

Over the years I have become more confused when it comes to these issues not to mention how to heat-treat stainless steels, if there's a way.

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Re: .40 Carbon Steel Brinell Hardness vs. Tempering Temperature

10/05/2010 12:02 PM

Lets tackle thos questions Isti80.

How to find out if the carbon content in the steel is the right amount I wanna use for a job, is it given by the heat number?

The heat number format varies by supplier. In the US AISI SAE grade system, the last two digits of the grade designator are the mean carbon content. so in 4140, the average carbon content is 0.40.

An experienced hand can estimate the amount of carbon in the steel by sparktesting. I'll do a post on that in the near future.

Further, is there a way (rule of thumb) to know how to quench a steel properly and how to soften (release?) once it's over done? What are the best methods for heat treatment and quenching, including back-yard methods?

The problem most "field Expedient Heat treaters" have is getting temperature of the workpiece uniformly to temperature throughout the section. If the temperature is not uniform, the developed microstructure will be spotty too. Time at temperature is every bit as important as Temperature. Commercially, we use the rule "an inch an hour" for soak to assure we get uniform transformation.

Another problem can be differential loss of carbon on the surface (decarb) and or pickup of carbon if they bury the workpiece in the 'embers' and some of it is not in the embers. This usually takes a long time, but if someone leaves the pieces in the hearth too long, they can add or remove carbon in the skin of the part which will affect hardness achieved ...

For safety sake, Oil to quench most "unknown" materials is preferred. Water removes heat too agresssively for the higher carbon grades and can cause cracking. You might want to look at this post on CR4: http://cr4.globalspec.com/blogentry/13519/7-Causes-of-Quench-Cracking-in-Steel

For tempering, I posted a temper color chart. It is on my Blog here

Over the years I have become more confused when it comes to these issues not to mention how to heat-treat stainless steels, if there's a way.

Stainless steels are not all heat treatable, the austenitics, for example will not heat treat. That is another subject for another time.

Thanks for the query.

Milo

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