Micro analysis 4

[metboss]

Hi all,

Does Microstructure evaluation & chemical analysis have any correlation to higher hardness value(Material low alloy steel 4130).
My question is how we can correlate hardness based on chemical analysis and microstructure evaluation? Will high carbon content or any other element affect hardness vaue?
What is microstructure evaluation? How one can deduce hardness from it?

Regards

 

[kumkumvijay]

The chemistry is the basic building block for getting microstructure and hardness.
Microstructure and hardness is linked to each other very much.

The chemistry can give different microstructure (hardness) based on heat treatment process.

Surface Hardness:
Quenching and tempered SAE 4130 - surface hardness - around HRC 60 ( Martensite microstructure)
Annealing SAE 4130 : surface hardness - around BHN 235 ( Ferrite & Pearlier microstructure)
Surface hardness have direct relationship with carbon content.Increase in carbon increase the martensite content of the microstructure on the surface Which increase the hardness value.

Core Hardness:
core hardness determined by alloying elements Ni,Cr,Mo,B: Alloying elements increase the hardenability ( thats make the metal able to produce martensite microstructure away from the surface)

Its not exactly possible to say the exact hardness value by seeing microstructure always but by experience, its possible to guess.

 

[metboss]

Kumkuvijay thanks for your response.

Could you re-confirm above your quote regarding surface hardness and microstructure for aneealed & Q+T low alloy steel?
I believe quenched and tempered low alloy steel have hardness 235 BHN and microstrures would be ferrite + perlite.

Regards

 

[kumkumvijay]

Hello Metboss
sorry, HRC 60 is in carburized condition.

Q&T: BHN 235-240
Annealed: BHN 150-170
Normalized: BHN 160-190
Thanks

 

[CoryPad]

4130 after quenching and tempering would have a martensitic structure, not ferrite plus pearlite.

 

[metboss]

If surface hardness value comes more than 310 BHN, then what type microstructure(martensite??)is highly likely? Based on obtained microstructure, after evaluation, can we come to comclusion that material is not heat treated properly?

 

[tbuelna]

I would agree with CoryPad. A low alloy steel such as 4130 should be primarily martensitic after a typical heating, quenching and tempering cycle. However, there may still be some small percentage of retained austenite in the structure after heat treatment. As for chemistry, it is possible that there will be some loss of carbon (decarburization) in the surface of the material resulting from the heat treatment process. And this can affect the heat treated material properties in the surface layer.

 

[CoryPad]

Above 310 HB, the structure most likely is martensitic. Observing the microstructure should allow you to determine if the parts were heat treated correctly.

 

[kumkumvijay]

To come to the conclusion, parts are not heat treated properly, you have to mention the specification given to the supplier.
The hardness is more related to the severity of quenching.
More sever quenching --> More Martensite--> High Hardness
Less Sever Quenching--> Less Martensite--> Less Hardness

To control the severity of quenching , important things to be considered are:
1. dimensions of the part- customer in put
2. Quenching media, temperature, volume used, part temp before quenching - supplier input.

For 4 inch dia part water-quenched from 1575ºf; tempered at 900ºf will have hardness of BHN 241
For 1/2 inch dia part water-quenched from 1575ºf; tempered at 900ºf will have hardness of BHN 331.

To come to the conclusion, HT is wrong, you must refer your specification given to HT supplier. What i have specified in my previous post is for standard range. This will give a general idea of making your requirement to your HT supplier. Exact values is depends on the size of the product.

 

[redpicker]

A microstructure analysis can tell you what heat treatment a specific material (in this case 4130) received, or, if the heat treatment is known, perhaps it can tell if the heat treatment was effective.

A lot depends on the section size, particularly with 4130. With thin sections, say less than 0.5 inch, achieving a predominately martensitic structure on water quenching after fully austenitizing. With heavier sections, say > 2 inches, the as-quenched structure will have a higher percentage of non-martensitic transformation products. These non-martensitic transformation products are lower in hardness than martensite and, like martensite, will soften with tempering, but not as much as martensite will. It can be difficult to distinguish between tempered bainite and tempered martensite.

To be able to determine if the material was heat treated properly, you have to know what the heat treat specification requirements are. In general, the heat treatment is performed to achieve specific mechanical properties. If those properties are not met, the microstructure can explain why, but the microstructure is not going to be able to tell you if the material will meet any specific mechanical properties.

For example, with 4130, a normalizing treatment can be expected to produce a microstructure composed of ferrite and pearlite, and result in a hardness of, say 262 HBW maximum. If the hardness checks out to be 363 HBW and the microstructure reveals a large percentage of untempered martensite, it would be likely the material was not allowed to cool in still air from the austentitzing temperature. To be able to draw this conclusion, however, you have to know both the required heat treatment (normalizing) and required mechanical properties (262 HBW max). Just knowing the surface hardness is out of spec does not tell you the problem and just looking at the microstructure is not going to tell you what the hardness is.

rp