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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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3 comments

Five Ways Coarse Austenitic Grain Size Affects Your Machine Shop

Posted August 20, 2010 12:01 AM by Milo

Austenitic Grain Size is a material characteristic that is usually reported on test reports and certification documents for the steel materials that we machine in our shops.

Coarse Austenitic Grain Size is a result of NOT ADDING grain refining elements to a heat of steel. Because these Grain refining elements have not been added, the steel has a "Coarse Austenitic Grain Size."

This is Coarse Grain Austenite. You like it for machining.

Typically this practice is applied to free machining grades such as 11XX and 12XX steels. These steels are sold primarily for their ability to be machined at high production rates.

What does Coarse Austenitic Grain Size imply for the parts that you make?

  1. Better Machinability- Coarse Grained Steels are more machinable and provide longer tool life than Fine Grained Steels. (The elements added to make the Austenitic Grain size fine create small, finely dispersed hard abrasive particles in the steel)
  2. Better Plastic Forming- than Fine Grained Steels
  3. More Distortion in Heat Treat- than Fine Grained Steels
  4. Lower Ductility at the same hardness- than Fine Grained Steels
  5. Deeper Hardenability- than Fine Grained Steels

Coarse Austenitic Grain Size will show up on the test report as an ASTM value of 1-5. Values of 5 and higher are called Fine Grained Steels, and are the result of additions of Aluminum, Vanadium, or Niobium in North American commercial practice for most Carbon and Alloy steels.

The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods For Determining Average Grain Size.

A nice discussion can also be found HERE.

While we think that chemistry may be the controlling factor for machining performance of the steel in our machines, the contribution of austenitic grain size is also important. As long as you are ordering your free machining steels (11XX and 12XX series) to Coarse Grain Practice, Austenitic Grain Size should not be an issue in your shop.

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which originally appeared here.

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#1

Re: Five Ways Coarse Austenitic Grain Size Affects Your Machine Shop

08/21/2010 10:36 AM

Milo, in my line of business, we don't use free machining steels or steels with large grain sizes for a variety of reasons (primarily corrosion and ductility issues.) so I don't know a whole lot about them from personal experience. But something you said above seem to be in conflict with something else you said and I'm a bit confused what you mean. I'd appreciate it if you could try to clarify.

You mention that large grain sizes improve formability, but two sentences further down, you state that it is detrimental to ductility. These two statements would appear to me to be contradictory. Can you expand a bit on this point? Are you talking about cold notch sensitivity?

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#2
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Re: Five Ways Coarse Austenitic Grain Size Affects Your Machine Shop

08/22/2010 7:46 AM

The grain structure IMO gives it better machinabilty. The same grain structure makes it less ductile at the same hardness.

For ductility the grain stucture is contrary.

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Re: Five Ways Coarse Austenitic Grain Size Affects Your Machine Shop

08/23/2010 10:21 AM

Rorschach, Headers.

Thanks for your thoughtful comments.

This is a complex area, and the untangling of effects of Grain size from chemistry and other thermomechanical effects is not always easy.

I think that we can agree that in general formability decreases as grain size gets larger, since cracks can form and propogate more easily along grain boundaries.We can see this especiallty when the grain size is large relative to the size of the workpiece- ie castings, where the workability is extrememtly low.

In our machine shops the austenitic coarse grain steels that we machine tend to cold form better than fine grained steels because the silicon in the deoxidation of the fine grained steels acts as a ferrite strengthener. A similar issue happens with Aluminum killed steels too.

So I would say that your point about formability regarding grain size is on point, but in the case I was writing about, these steels do tend to form better and with less tool wear than silicon killed fine grained steels. ( the tool wear issue not som much abrasion as in machining as much as the silicon killed steels require higher forces to 'move.')

Thanks for the thoughtful engagement.

Milo

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