Holding Temperature Between Martensite-Start and Martensite Finish 2

[bovs7]

Hi all,

I have a question on the fundamentals of the martensite start and finish temperatures, so perhaps if someone can just point me to a good reference that'd be great!

I understand that the amount of undercooling below the Ms temperature affects the ratio of martensite to austenite and there are calculations to predict this. What I wonder, however, is what happens if you quench to a temperature below the Ms, hold at this temperature for a period of time (effectively a temper) and then cool to room temperature.

Will the transformation continue further as you cool further (and therefore when calculating the undercooling it is basically the minimum temperature reached regardless of the processing path so long as you reach your Ms before forming pearlite and bainite)?

Will the martensite formed upon cooling to room temperature be therefore untempered martensite (meaning that if you quench and temper with only a small amount of martensite formed before cooling to room temperature, the majority of your steel is untempered martensite)?

 

[metengr]

The process you describe in a nutshell is a modified martempering process for steel. Look up and review martempering.

 

[bovs7]

Hi metengr,

My understanding of martempering is to rapidly cool to something above the Ms temperature so that a uniform, lower temperature austenite phase can be transformed to martensite with less stress while also avoiding any pearlite or bainite formation.

I guess by extension then, my question is- What happens if you conduct a martempering process but your initial quench is to just *below* rather than just above the Ms?


As a secondary question on martempering (as I've never been involved with this process before) - is it customary to temper a steel that has been hardened my martempering?

 

[metengr]

Look at the reference below

http://books.google.com/books?id=Kx...fied martempering versus martempering&f=false

 

[EdStainless]

For another variation read chapter 6 of that ref on Austempering.

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Plymouth Tube

 

[dbooker630]

At our facility we have routinely run a quenching process with SAE 4340H where the product is quenched to just below the midpoint of the Ms and Mf temperatures using hot marquench oil, product is held for about 20-30 minutes, then removed from the furnace. The product does have to be tempered. Key benefit has been to reduce sensitivity to quench cracking.