Determining grain size for Tempered Martensite 1

[Naruwan]

Dear all,
I am having problem determining the grain size for AISI 4130 since it's microstructure will be of tempered martensite. I read it from ASTM E112 that I had to heat it up to 857 degree celsius, hold it there for an hour and quench it in water.

Is anyone able to tell me what it is all about? If we are going to heat it up to degree celsius, are we not modifying the microstructure of the steel?

Thanks!

 

[metengr]

Your probably interested in the prior austentite grain boundaries. There are chemical etchants that can highlight the prior austenite grain boundaries for low alloy steel. You can search for them.

 

[Naruwan]

metengr, thanks.
I think you are talking about about using Picric acid to etch. However I was wondering why after the heat treatment and after polishing, I am still seeing tempered martensite?

 

[TVP]

Naruwan,

If you heated 4130 steel to 857 C, held it for 1 hour at that temperature, and then quenched it into water, then the microstructure is as-quenched martensite, not tempered martensite. Martensite, whether as-quenched or tempered at typical temperatures, will have a similar appearance when etched with nital. As metengr noted, it is likely that your requirement is for determination of the prior austenite grain size, not the martensite grain size.

 

[Naruwan]

TVP, thanks! I had tried heat it up to 857 celsius and water quenched it. Here's a picture of the microstructure attached. I had already etched with 2% nital and the structure within the grains seemed to be tempered martensite to me.

 

[CoryPad]

Is this granular steel? Shot, machining chips, or similar? I don't understand why you are concerned with grain size, your piece size is already smaller than most grains in bulk polycrystalline solids.