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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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5 Engineering Aspects of Austenitic Grain Size

Posted June 15, 2010 9:34 AM by Milo

Austenitic Grain size is an important aspect in the specification and selection of steel for our engineering projects.

Here are five points to help you make sense of austenitic grain size and its influence on our work.

1) The variation in mechanical properties, response to simple heat treatment, and many other properties cannot be explained solely by chemical composition.

2) While the matter of grain size does not take away from the importance of chemical composition of steels, it does shed light on how the steel may be expected to perform in fabrication or service.

3) The method most widely used in steel is called the McQuaid Ehn test, and it uses plate IV in section 10 of ASTM E112 Standard Test Methods for Determining Average Grain Size. A full discussion of the procedure can be found in Annex A3, Paragraph A.3.2.1.2 (and subsequent) of ASTM E 112.

4) In ASTM E 112, Grain Sizes can range from grain size number 1 (coarse grain) to grain size number 8 (fine grain).

5) Impact strength at a given hardness, yield strength, % elongation, distortion in heat treatment, machinability,and ability to be plastically deformed are all material characteristics that are influenced by grain size.

While the origins of grain size determination were visual and chart based, today many laboratories use software to perform automatic image analysis. Utilization of semi-automatic digitizing tablets or automatic image analyzers to measure grain size is described in ASTM Test Methods E 1382.

For more information about the role of fine grain size in your machine shop, click here.

For more information about coarse grain size in your machine shop, click here.

Image of microscope reticle courtesy of Klarmann Rulings Inc.

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Re: 5 Engineering Aspects of Austenitic Grain Size

06/16/2010 12:26 PM

Thank you Milo. very informative.

any idea why the grain boundaries form?

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Re: 5 Engineering Aspects of Austenitic Grain Size

06/16/2010 1:05 PM

Simplest answer, they run into other grains growing with a different orientation. From liquid to austenite, the grains originate around something that nucleates them. Nucleation is proportional to the number of nucleation sites.This video shows the process: link

So to make fine grained steels, we add a grain refiner (like aluminum, vanadium, or columbium niobium) to innoculate the liquid steel and to nucleate many sites to promote the formation of the initial grains. More nucleation sites, more grains, smaller grains, since as the grains grow, they can only grow until constrained by a neighboring grain.The space between is filled with non-grain material, that is material "divorced out" of the solution as it coalesces into the crystal of the grain.

In a super pure melt, where there are "No Nucleants" using very slow cooling you can get to very coarse grains 'single crystal.' that is how they make some rotor vane parts for jet engines. the material in the grain boundary would be a cause for failure.

That is my understanding of the initial formation of the austenite from liquid (casting).

Transformation of austenite to ferrite-pearlite is another subject and I'll save that for another post. Great question.

Milo

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Re: 5 Engineering Aspects of Austenitic Grain Size

06/16/2010 1:11 PM

Thank you. such a well written response.. ga.

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Re: 5 Engineering Aspects of Austenitic Grain Size

06/16/2010 2:50 PM

Interesting! I look forward to learning more in your next installment...

Dick

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Re: 5 Engineering Aspects of Austenitic Grain Size

06/17/2010 2:29 PM

I agree with chris, very nice valuble information as a designer

p911

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