Effect of microstructure on induction hardening steel 4

[NETDB]

We buy our AISI 4140 material from two different suppliers.
For one particular application we are trying to make a component induction case hardened (since we have the induction machine in house) instead of fully quenched and tempered (which is how we do it today, with a heat treatment supplier). Since the component only requires hardness in the outer areas (>55HRC) it only seems fair to try to do it in house with our induction hardening machine.
The point is, I recently found out that the material from the two suppliers are not provided with the same microstructure. We buy the material from both of them in the "as rolled" condition.

From supplier A you get the picture below (bainite? and perlite from what it looks like) due to unncontrolled cooling. All the material I analyzed from them looks like that.

From supplier B we get the below microstructure (fine perlite and ferrite). I think is odd that we get this microstructure from hot rolled condition without later heat treatment (should be messy microstructure due to uncontrolled cooling like supplier A, in my opinion).

Question is, what is the effect of these microstructures during induction hardening? Meaning: does the microstructure of supplier B favors a deeper case? The initial microstructure of the material matters at all since during hardening it will get austenitized?
The hardness of bulk material from both suppliers are the same. Shouldn't supplier A have a higher hardness?
Also, do you know any good color etchants to differ bainite from martensite using light microscope (without polarized filter or sensitive tint plates)? The ones that I tried with sodium metabisulfite don't seem to work very well.
Thanks!

 

[Maui]

I would suggest going back to your suppliers to ask them what the final thermal treatment cycle consisted of for each order. The microstructure from supplier B looks like ferrite with fine to coarse lamellar pearlite. The pinpoint gray dots are likely non-metallic inclusions. This type of microstructure would most likely be the result obtained from an annealing operation. I have no idea what supplier A did to obtained the microstructure that appears there.

What hardness did you yourself measure on samples A and B?

Maui

 

[remetaper]

Micro A is in hardened + tempered condition and NOT in as rolled condition because this grade cooled after hot rolling shows a microstructure of perlite and ferrite. (unless a special cooling process had been applied) --- Micro B is in annealed condition or slow cooled after hot rolling. A Brinnel hardeness could give more precise information about both conditions. In any case,a induction hardening should NOT apply in material unless a H+T can be perform before induction. The material A offers better performance and is better condition to recieve a induction hardening.
Steels that have been hardened and tempered with fine structure and with small carbides respond better than annealed steels with large and spheroidized carbides because these carbides are very low to dissolve when the steel is heated. This results in difficulties to obtain a satisfactory hardening response in the SURFACE with poor resistance in fatigue strength or wear resistance. Thus, even if a normalized condition could offer better results than annealed one, the best and safe metallurgical choice is hardened+tempered conditon.

 

[NETDB]

Thank you for your answers!
The hardness we get from Supplier A is around 220-230HV and Supplier B around 200-210HV.
Great explanation on the carbides, that was what I was looking for.
I find it hard to believe Supplier A would sell as the material in the "as rolled" condition (cheaper) but deliever in the hardened and tempered condition (more expensive). Plus, when we buy round bars in the "hardened and tempered" condition, for 30HRC, they have a fine structure as below.

 

[Maui]

A hardness of 220 - 230 HV in sample A corresponds to a hardness of approximately 96 Rockwell B (which is less than 20 Rockwell C). This hardness is not consistent with that of a hardened and tempered microstructure for AISI 4140.

 

[remetaper]

Maui, most of induction heating applications, require a condition of hardening + tempering heat treatment of steel in order to obtain a tempered martensitic structure whose Hrc value ( or Rp 0.2 an Rm )will depend on final use. The reason why of this structural condition were explained in my previous mail. Steel A shows a tempered martensite structure probably obtained by : (1) a controlled temprature at last step of rolling mill + a fast cooling in cooling bed+ final tempering or ( 2) a standard hardening and HIGH temperature tempering heat treatment enable to obtain low value of Hardness. Considering that few steel makers apply the first process, I think that the second ( 2) is the more probable. Thus , I presume that the supplier A have chosen a downgrading of this steel and prefers to sell in " as rolled " condition because a his repetition of H+T + straightening is expansive. In other words, a hardened and tempered steel delivers as rolled. This behaviour is not a novelty.

 

[remetaper]

Maui, I was forgetting. In order to complete these evaluations, you should verify what both certificates of Supplier A and B declare about the hardeness and/or mechanical properties in delivered conditon , size and chemical composition as well. The hardeness you have found verifying steel A (i.e. HV ( Vickers) = 210 -230) is too low for "as rolled " condition. This grade in such condition would offers HB(Brinell) between 230-250. In any case, the structure of steel A is not typical of an "as rolled" because its acicular structure is NOT certainly generated by an air cooling after hot rolling or a annealing.

 

[Martinos]

Hello. I have read your discussion and I have to write that the origin of the acicular structure is what I am interested in.

Hardness level about 220 HV? Definitely ferrite and pearlite, but why coarse acicular?

Standard hardening and high temperature tempering? I do not know why the result should be such an acicular microstructure after proper austenitising, quenching and high tempering... I really do not believe that regular quenching and high tempering were done on specimen A. It has something to do with rolling temperature and cooling velocity.

Anyway, I worked with this steel many times and I have never seen something like that. Standard as received ferrite + lamellar pearlite microstructure (like specimen B) is sufficient for full QT heat treatment as well as for surface induction hardening.

Is this coarse acicular microstructure (A) something beneficial to subsequent heat treatment against common microstructure B? I am not sure...

 

[mrfailure]

Until you told us the hardness, I was thinking Structure A was a quench-and-temper structure, but it looks instead like it was a mis-heat treated microstructure of some kind (overtempered?). I would expect more internal residual stress with that structure so it needs to be avoided, with note of all of the other comments above. Structure B is the expected structure for in the as-annealed condition as the general structure has more ductility while the surface has the intended fatigue resistance from your induction hardening application.

 

[dbooker630]

Structure A looks like Widmanstatten orientation, indicative of fast cooling. Structure B looks like it slow cooled or normalized. Structure B will not fully respond to induction hardening because of the free ferrite. Assuming that you can machine it, structure A may respond better than B. But quenched and tempered is certainly preferred over either structure.

 

[Maui]

Netdb, did you contact your steel suppliers and ask them to provide you with material certifications for the steel that you purchased from them? These certifications should tell you what condition the material was provided in. Why exactly are you induction hardening these components? Is it to improve wear resistance?

Remetaper, if sample A had been austenitized and tempered at a relatively high tempering temperature then the resulting microstructure that is shown should be much darker in appearance after etching. If samples A and B were etched in the same acid solution for the same length of time then The microstructure that is shown for sample A is not consistent with material that was tempered at a relatively high temperature.

Like Martinos, I am curious to find out what was done to generate this microstructure. It is very unusual for this grade of steel.

Maui

 

[remetaper]

Maui, all my evaluations was done considering the available information. Therefore, darkness or not darkness of micro cannot be evaluated if the time or kind of etching are not available. In any case, the structure of sample A is coming from a fast cooling + tempering. In my previous mail,I suggested (1) that the origin of that acicular structure ( tempered martensite or upper banite(?) ) were generated by a special process in rolling mill followed by a fast cooling and tempering. Just to have and Idea , a process similar to HSLA product. Pay attention on: I said "similar" not "same" because HSLA are different kind of grades ( microalloyed and so on.. ). Therefore, it will be very usefull to have the certificates of steel producer. The my second (2) hypothesis is more probable. Anyway, I confirm what I wrote in my previous mails ( downgrading, hypothesis 1-2, carbides, ecc...) waiting for more info by Netdb. In same time, I complety agree with the evaluattion of Dbooker630 and I want to repeat that the best choice for a good induction hardening is H+T condition . I suggest to forget other strucure such as perlitic -ferritic structures because have poor resistance to fatigue and wear resistance.

 

[Martinos]

Of coarse, sorbitic QT (core) microstructure is the greatest basis for surface hardening, but:

a) specimen A is definitely not in QT state, is coarse and acicular (Widmanstätten) this does not belong to proper austenitising, quenching and high tempering : definitely not sufficient microstructure from the side of fatigue, also notch toughness level has to be worsened.
Origin of this microstructure is, of coarse, fast cooling, I believe after hot rolling. I really do not believe that high tempering was done on the material A after hot rolling. Morphology does not indicate this: Widmanstätten would be broken. I assume microstructure is a result of faster cooling from hot roling end temperatures.

b) in commercial field the price of quenched + high tempered and subsequently surface hardened component will not be as competitive as price of normalised or as rolled and surface tempered component (concurrently the fatigue and wear resistance can be sufficient for most of the applications)

In my applications I would definitely prefer to have QT sorbitic micro before surface hardening, but if I can choose the one from above shown micros as an input for surface hardening, I would prefer B instead of A even there is some free ferrite in the microstructure, as I am not sure what can I expect from microstructure A and also I do not want to have core microstructure Widmanstatten on coarse primary grains.

 

[NETDB]

Dear all,
Sorry it took me a while to respond, I was gathering some infos.
Supplier A's certificate only says that the material is "hot rolled". The hardness they say is 308HB.
I agree with Martino's statment "a".Since both suppliers have similar prices, purchase department chooses which one to go to based first on delivery time and logistics. Like I said, it was not that long ago that we noticed the difference in the microstructures (which is not a big problem for us since most of the parts made from this material are fully quenched and tempered anyway, so the inicial microstructure is not that relevant). In that case, based on your feedbacks I would say that supplier A have a better condition for induction hardening.
@Maui, yes, we are hardening them to improve wear resistance and hardness (this parts are used in grinders).

The coarse acicular microestructure with perlite was the thing getting me. I was thinking bainite from fast cooling after hot rolling. I was looking for some color etching to help me figure it out, but not having a polarized filter in my OM is a bummer.
The microstructures below are from a different component that was induction hardened from supplier A. First is the quenched end and second is the interface bewteen quenched region and the core (core presented the same acicular microstructure we've seen). What do you think?

 

[Maui]

The hardness we get from Supplier A is around 220-230HV and Supplier B around 200-210HV.

Supplier A's certificate only says that the material is "hot rolled". The hardness they say is 308HB.

There appears to be a substantial difference between what supplier A listed on their material certification for hardness and the hardness you actually measured on their material when it arrived at your facility. I would recommend questioning them about this.

If it were up to me and I was austenitiizng, quenching and tempering this material prior to induction hardening it I would want to use the annealed material provided by supplier B. Based on the microstructure and the hardness that was provided, they appear to have done a competent job of annealing this material. And I would expect the heat treat response to be more predictable and consistent using material in the annealed condition.