P20S Microstructure Analysis after Quenching and Tempering

[JeremyBonelloB]

Good evening,

I am currently working on understanding the phases present in a quenched and tempered sample of P20S (1.2312) tool steel.

The quenching process was done from the austenitizing temperature and oil quenched for a number of seconds.

After quenching what I understand is that the present phases would be martensite and retained austenite. (55HRC)
After tempering which was done at 520oC for half an hour, from the information I gathered from different solid sources I managed to put together that the retained austenite transformed into martensite whilst the martensite transformed into tempered martensite. (42HRC)

What I am confused about is all the white inclusions present in the microstructure which is the microstructure after tempering.
Am I correct to say that there are only 2 phases present which are martensite and tempered martensite? Or does there still remain retained austenite?

 

[metengr]

First, are you sure there is no bainite besides what you believe is possibly retained austenite? Do you have a CCT or even a TTT diagram for this alloy to check phase and hardness?
Retained austenite upon quenching may not transform to fresh martensite during tempering, it dendends on the chemistry and actual cooling rate. If you want to ensure minimal retained austenite, a subcooling treatment can be performed prior to tempering.

Have you performed microhardness testing to evaluate the microstructure?

 

[EdStainless]

I suggest a good reference book on color etching also.
30 min temper sounds very short, even at that temp.
Have you tried double temper?
I would give it overnight in LN and then temper again.
But you really need transformation curves.
Your quench may not have been aggressive enough.

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P.E. Metallurgy, Plymouth Tube

 

[JeremyBonelloB]

Hello guys, Yes I have the CCT diagram and the whole point of this was to design a heat treatment process ourselves in order to apply to an application.

I performed a single temper

The CCT Diagram with regards to relative hardness (in red) displays a transformation from Austenite to martensite with retained austenite as the Martensitic finish line is not available at room temperature.

Please note I am a beginner when it comes to metallurgy so I have alot to learn

 

[EdStainless]

In most alloys you want to avoid retained austenite for a couple of reasons.
First off it is softer, and then there is the annoying tendency for it to later transform into untempered martensite.
What was the starting condition? Annealed or had it been pre-hardened?
I have only ever seen this material sold pre-hardened.
This alloy needs to be annealed before you harden it.
They also warn you to temper before the parts cool to room temp, and I have usually seen 2 hr temper times.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

This material is a tool steel. To avoid retained austenite you either increase the severity of the quench or perform a sub cool treatment and temper for a second time at lower temperature.

 

[JeremyBonelloB]

The condition was annealed in my case.
The heat treatment we were allowed to do is very limited.

Am I correct to assume that we have Martensite and tempered martensite as phases?

 

[metengr]

Tempered martensite, yes. Untempered martensite, I doubt it, and I think you hit the bainite region of the curve if your retained austenite transformed upon cooling from tempering. Fresh marteniste would be confirmed with microhardness testing, as I mentioned above.

 

[Maui]

Jeremy, what did you etch your sample in and for what length of time? If you double the length of time that your sample is in the etching solution, do the white colored areas remain or do they disappear?

P20 is typically offered in the pre-hardened condition at either 300 BHN or 340 BHN hardness. It is unlikely that you will be able to achieve a hardness of 55 HRC in this material at quench. After soaking for 30 minutes at an austenitizing temperature of 1500F - 1550F and oil quenching the tempering response of this material should fall along these lines:

https://www.crucible.com/eselector/prodbyapp/hotform/csm2d.html

A tempering time of 2 hours should be used, not 30 minutes.

Maui

 

[JeremyBonelloB]

Thank you all for the help