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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 Reasons To Anneal Steel

Posted June 29, 2010 10:05 AM by Milo

Here are 5 reasons to anneal steel.

  1. To alter the grain structure;
  2. To develop formability;
  3. To improve machinability;
  4. To modify mechanical properties;
  5. To relieve residual stresses.

The annealing process is a combination of a heating cycle, a holding period or "soak" at temperature, and a controlled cooling cycle. Atmospheric controls are generally used to protect the steel from oxidation.

The temperatures used and the cooling rates are carefully selected to correspond with each steel grade's chemical composition in order to produce the results desired.

For bar steels used in our precision machining shops, there are three kinds of annealing that may be encountered:

  1. Subcritical Anneal
  2. Solution Anneal
  3. Spheroidize Anneal

Subcritical Anneal

A subcritical anneal is the metallurgical name for what is termed a process anneal or stress relief anneal in North American commercial practice. It consists of heating the steel to a temperature close, but below, the steel's lower critical temperature or Ac1. This simple anneal reduces stress and hardness in the material and makes modest changes in its microstructure. Steel mills often employ this to improve cold shearing or cold forming. This is sometimes used between cold forming operations to reduce hardness.

Solution Anneal

Lamellar Pearlite photo

Solution annealing is referred to in commercial practice as 'LP Anneal' or Lamellar Pearlite Anneal. Lamellar pearlite is the microstructure that predominates when doing this kind of anneal. The cycle for this anneal involves heating the material above the critical range (Ac3) and holding the steel (soaking) at that temperature for a length of time followed by slow cooling below the critical range (Ar1) temperature. This cycle reduces hardness and reprecipitates the carbide phase as lamellar pearlite. Controlling the time and temperature gives the metallurgist a means to alter the resulting lamellar pearlite structure, and refine the ferritic (as rolled) grain size.

LP anneals are usually applied to medium carbon (0.40-0.65 weight %) plain carbon and alloy steels for precision machining in order to reduce hardness and improve machinability.

Spheroidize Anneal

Spheroidized Photo

Spheroidize annealing is the term that describes a thermal process which results in a globular or spheroidal type of carbide after heating and cooling. There are several types of spheroidize cycles which we will write about in a future post.

Spheroidized microstructures are desireable for machinability and improved surface finish when machining higher carbon steels. Spheroidized microstructures are also preferred when the steel is to be severely cold worked: cold extruded, cold upsetting, or bent. Most bearing steels are first spheroidize annealed prior to machining.

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#1

Re: 5 Reasons To Anneal Steel

06/29/2010 3:11 PM

Thank you Milo. Interesting reading.

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#2

Re: 5 Reasons To Anneal Steel

06/30/2010 3:27 AM

Good stuff, I wish I had the facilities to play with some of this stuff, I'll have to make do with wood for now.
Del

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#3
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Re: 5 Reasons To Anneal Steel

06/30/2010 8:06 AM

Del, I wish I had the talent that you did working with Wood. It's a wash.

Milo

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#7
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Re: 5 Reasons To Anneal Steel

07/01/2010 11:55 AM

Don't try annealing wood

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#9
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Re: 5 Reasons To Anneal Steel

07/01/2010 12:21 PM

Ha!
Interesting you should say that. There is much talk of 'heat treating wood' and fire hardening it. The few very limited experiments I have tried have made no difference, but some people swear by it, and plenty of native people use heat on bows and spear tips etc.
I'm not convinced that some of it is the placebo effect...I find rubbing my bows with ladies underwear stiffens them up a bit..
Del
(Sorry we're way off topic here Milo)

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#4

Re: 5 Reasons To Anneal Steel

07/01/2010 1:58 AM

Hi Milo,

great stuff!

Please enrich with data of etching to get the fotos and the related magnification of these.

Is it possible to extend the information to cover maraging steels too?

RHABE

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#5

Re: 5 Reasons To Anneal Steel

07/01/2010 2:10 AM

As a research engineer, I did a little on-site investigations concerning annealing. I have written a few papers on residual stresses. I am now retired, but I did numerouus temperature -elongation studies of vaious steel grades and studied shear angles of steel grades and many other metals from titanium to gold and silver, but mostly steel and aluminum.

I was our companies expert on residual stresses and slitting plus a few ther areas.

The elongation temperature curve data going thru the trabsformation stage is amazing

PEbobimm

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#6
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Re: 5 Reasons To Anneal Steel

07/01/2010 7:52 AM

Welcome to CR 4 Guest.

I am certain that our community would be interested in your work. If you register and contact me by (clicking on my name above to send private email) we can help you share your experience. Thanks.

Milo

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#8

Re: 5 Reasons To Anneal Steel

07/01/2010 12:21 PM

Thanks for discribing Reasons to anneal steel. We often anneal T.P SHEARING BLADES in coal furnace after red heated remain blades in coal ashe till cool

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