Tempering of martensite with retained austenite

[krautkramer]

Hi,

What changes will happen to retained austenite in a martensite matrix during tempering of martensite in an alloy steel @740 C-760 C (ie, below Ac1 temperature)?Will retained austenite transform to martensite (untempered) during tempering or retained austenite will stay as it is during tempering??Is there any possibility for a transformation from retained austenite to pearlite/bainite during tempering??I am confusedAny help would be highly appreciated.

PS:I am not looking for a cryogenic treatment to transform all retained austenite to martensite and further tempering solution to this problem.

Thanks

 

[krautkramer]

,119 views,no reply (right or wrong)so far.I never thought that this was a much complicated question!

 

[Bavid]

During heating, as in tempering, the retained austenite may form pearlite but will NOT form martensite. Martensite formation is diffusionless and shear-dominated, and therefore some combination of quenching and/or deformation is needed. In short, it is not possible to 'heat' a phase and convert it to Martensite. Or Bainite, for that matter since it too requires a fast quenching rate to form, although not as severe a rate as martensite.

 

[Astronuc]

Hi,

What changes will happen to retained austenite in a martensite matrix during tempering of martensite in an alloy steel @740 C-760 C (ie, below Ac1 temperature)?Will retained austenite transform to martensite (untempered) during tempering or retained austenite will stay as it is during tempering??Is there any possibility for a transformation from retained austenite to pearlite/bainite during tempering??I am confusedAny help would be highly appreciated.

PS:I am not looking for a cryogenic treatment to transform all retained austenite to martensite and further tempering solution to this problem.

Thanks

Is one referring to martensitic steels, or alloy steels in general.

Martempering occurs at rather low temperatures. A steel could be quenched into the martensite region then brough up into a temperature to promote bainite.

Here is some information from various sources:

The effect of tempering depends on the alloy composition, soak time (time at temperature), and quench rate.
http://www.azom.com/article.aspx?ArticleID=313

Some steels are deliberately austenitic, ferritic or martensitic, or some combination, e.g., duplex. Their processing will depend on composition, e.g., content of carbon and elements that promote stabilization of one of the microstructures, e.g., austenite.


Tempering Processes/Technology - ASM International
http://www.asminternational.org/pdf/spotlights/tempering.pdf

Austenitic and Ferritic Stainless Steels in Practical Applications: Part One
http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=156

Martensitic Stainless Steels
http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=199

AUSTEMPERING is the isothermal transformation of a ferrous alloy at a temperature below that of pearlite formation and above that of martensite formation. Austempering of steel offers several potential advantages: Increased ductility, toughness, and strength at a given hardness (Table) Reduced distortion, which lessens subsequent machining time, stock removal, sorting, inspection, and scrap The shortest overall time cycle to through-harden within the hardness range of 35 to 55 HRC, with resulting savings in energy and capital investment Steel is austempered by being: Heated to a temperature within the austenitizing range, usually 790 to 915 °C (1450 to 1675 °F) Quenched in a bath maintained at a constant temperature, usually in the range of 260 to 400 °C (500 to 750 °F) Allowed to transform isothermally to bainite in this bath Cooled to room temperature The process is described in detail by the inventors E.S. Davenport and E.C. Bain in U.S. Patent 1,924,099.

From: ASM Handbook Volume 4, Heat Treating (ASM International), Published: 1990, pp. 152-163
http://www.asminternational.org/por...a7e0e64e18110VgnVCM100000701e010aRCRD#details

Austempered ductile iron is produced by heat-treating cast ductile iron to which small amounts of nickel, molybdenum, or copper have been added to improve hardenability. Specific properties are determined by the careful choice of heat treating parameters. Austempering involves the nucleation and growth of acicular ferrite within austenite, where carbon is rejected into the austenite. The resulting microstructure of acicular ferrite in carbon-enriched austenite is called ausferrite. Even though austenite in austempered ductile iron is thermodynamically stable, it can undergo strain-induced transformation to martensite when locally stressed. The result is islands of hard martensite that enhance wear properties.

Austempering process description.
http://www.advancedcast.com/austempering-process.htm

The austempering process
http://www.appliedprocess.com/process
Examples
http://www.appliedprocess.com/adi
http://www.appliedprocess.com/as

Marbain process
http://www.fisherbartonsp.com/products/products.php

Some other information
http://www.msm.cam.ac.uk/phase-trans/2005/Stainless_steels/stainless.html


A practical application: Bainite and austempered ductile iron combined in high-strength steel
http://www.asminternational.org/por...toid=86f6ba653fcda310VgnVCM100000621e010aRCRD

 

[krautkramer]

Thanks Bavid and Astronuc for the reply. That means, during tempering cycle (ie, heating), retained austenite will not transform to martensite(ie, no possibility for lattice shear and the temperature is well above Ms temp.). Still the possibility of Austenite decomposition to ferrite and carbide during tempering heating cycle depends on tempering temperature and holding (soaking) time. The martensite will get tempered during tempering cycle. During cooling, below Ms Temperature, the remaining conditioned retained austenite (if it is available, much of carbon depleted from retained austenite during tempering thus Ms Temperature of alloy further raised) will transform to hard and brittle martensite. So, I think, I need one more tempering cycle to temper the secondary martensite.

 

[Bavid]

If it is plain carbon steel, Austenite is way too unstable to exist at the tempering temperature. I obviously do not know the tempering time you are using, but if you steel is plain carbon, you can very quickly convert ALL of the retained austenite to ferrite+pearlite, so when you quench for another cycle it is possible that there is no austenite left to form the 'secondary martensite' you mention.