low alloy steel microstructure 4

[priyam07]

Hi, I have here a very interesting microstructure of a low alloy steel with 0.3 carbon in quenched and tempered condition. I dont have much information on the heat treatment temperatures. Usually I would expect to see a tempered martensitic microstructure but this one is kind of mixed with light etching phase what look likes retained austenite. The bulk hardness is about 46HRC. Any idea what happened here and what is the light etching phase is?
Thanks and Happy Holidays

 

[metengr]

Looks more like islands of untransformed ferrite rather than retained austenite. The austenitization temperature was either too low or too short.

 

[swall]

Was this possibly induction hardened?

 

[priyam07]

Not induction hardened!

Only the hardness make me doubt about ferrite as it is 46 HRC. With that amount of ferrite it is amazing the average is that high.

 

[mrfailure]

The hardness also stumps me. Also, it looks to me more like fine pearlite grains with ferrite grain boundaries.

Are you completely sure about both the chemistry and the hardness?

 

[metengr]

Did you perform microhardness testing and convert to HRC scale? I could believe the 46 HRC scale hardness is the bulk hardness.

 

[priyam07]

Yes, did double check the chemistry and hardness. Hardness done using macro Rockwell machine, i.e. bulk hardness.

 

[metengr]

Hardness done using macro Rockwell machine, i.e. bulk hardness.

. This hardness is not indicative of the light etching phase, which is why it is bulk hardness and not microhardness. You need a microhardness test performed on the light light etching phase. I would bet it will be much lower, and is ferrite.

 

[mrfailure]

What metengr is saying is, did you use a microhardness tester to measure Vickers or Knoop hardness specifically of the structure you showed? Hardness can vary for numerous reasons - for example, you may have an unexpected hardened surface but the core would be softer and exhibit a different microstructure.

 

[priyam07]

Thanks metengr and mrfailure, What I meant by average hardness is through hardness across the specimen thickness and it is very consistent.
I did try with 25g load micro-Vickers hardness on the white phase and the martensite. The hardness on the martensite is about 700 HV and on the white phase the indent is too big even at 25g load (~600 HV, I would not assume it is correct)

 

[Lyrl]

The white microstructure component is definitely ferrite by the shape.

Undissolved ferrite (from too low an austenitizing temperature or too short an austentitizing time) tends to be more blocky whereas quench ferrite (from the insufficient alloys in the steel to transform or from too slow of a cooling rate) tends to be more stringy. It's a subtle difference but I lean towards the pictured ferrite being more blocky, so I think a too-cold or too-short cycle is the most likely cause.

As far as the 46 RC bulk hardness, I believe your alloy is capable of 50s RC if fully transformed to martensite, meaning the 46 RC is actually a reduction from the hardness you would get with a better cycle (fixing the cause of the partial martensite, whether it is from incomplete austenitation or from too slow of a quench).

How thick is the part's cross-section? Are your Rockwell tests done on the surface of the part? Where is the pictured microstructure section taken from?

 

[priyam07]

Thanks Lyrl!

This is a cast part and the section cut for analysis is about an inch thick. The hardness was done on the cross section and the micro as well.

 

[redpicker]

Oh, it's a casting!

Interdendritic ferrite.

A good homogenizing treatment should eliminate it, but higher austenitizing temps will help, too.

 

[priyam07]

Thanks redpicker and all, I think so too.

 

[MagBen]

Microstructure showed 20-30% of while phase, if that was ferrite, how could the hardness be 46 Rockwell C?
Why not fresh martensite originated from retention of austensite? which is not uncommon even for low carbon alloys (e.g. 0.3% carbon), especially when the austenitizing temperature is high and grain size is big before martensitic transformation.
A deep etching or a high magnification micro could reveal a latch structure if this was the case.

 

[metengr]

MagBen
Because the matrix was tempered martensite.

 

[MagBen]

Yeah, but what puzzled me was even with 30% soft phase it was still 46C, and consistently! The contribution to hardness was fundamentally from the hard phase only?
My suspicion was the white phase could be un-tempered martensite, tempering again could eliminate this phase, and the hardness could be decreased.