Relative contribution of interstitial carbon vs pearlite to strength of plain carbon steels

[AndrewFinAustralia]

Hi All,

Am an honours-qualified Materials Engineer with 15 years experience, prior to teaching high school students since 2002. The content of the course I teach is at the first-year university level, excluding calculus, delivered over 2 years. With COVID-19, I'm teaching my senior class via streaming and online video. As part of preparing my materials, I stopped to question what I was teaching.

Seeking advice or papers for something so basic that was never covered in my training or professional life.

We all know that the general yield strength of a plain carbon steel increases with increasing carbon content.

It was always inferred that this was due to the interstitial hardening of iron by carbon, which as an undergraduate, you accept blindly.

However, the maximum solubility of carbon in iron is 0.03% The rest goes into cementite.

My supposition is that the increase in strength is due to increased volume of pearlite in the matrix and an associated 'grain size' or 'interlamellar-spacing' hardening, but I can not find any references to explain this - I've got a copy of Ashby and Jones here as Reed-Hill and other physical metallurgy texts. No information in any of them that I could see.

So, asking two questions if I may:

(1) Is my above supposition correct? and;
(2) Does anyone have a paper or reference to explain the actual strengthening mechanism in a plain carbon steel with increasing carbon content?

As I said, a textbook search here at home didn't find anything, and an internet search drew a lot of chaff and not much substance.

With thanks in advance,

A

 

[Dhurjati Sen]

Dear AndrewFinAustralia,

It feels wonderful to talk about metallurgy. I am a metallurgical engineer myself associated with industry (mainly oil and gas) since 1999.

You have searched the grand old Reed-Hill etc. mainly under Physical Metallurgy.

I suggest you have a look at Mechanical Metallurgy by Dieter Ch-6 Strengthening Mechanisms.

Also, some pointers.

We have read and still believe that as Carbon content increases, the strength of the steel increases. Let us follow the Fe-Fe₃C phase diagram. Solubility of Carbon in ferritic steels is 0.025% max. and hardness increases due to interstitiality, as you have correctly mentioned.

Also, as Carbon content increases, you get pearlite (a mixture of ferrite and cementite). As we all know, cementite is one of the hardest phases available and the grain boundaries between ferrite and pearlite also contribute to the overall strength.

So your supposition is absolutely correct. Your students are in good hands.

Dieter would help you with the rest.

Stay safe.

Regards.

DHURJATI SEN
Kolkata, India

 

[AndrewFinAustralia]

Thanks Dhurjati,

I actually had a copy of Dieter here as well - looked it up as per your suggestion, but it covered the Hall-Petch equation and general strengthening explanation for grain size modification.

Have you any idea of research that quantifies the strengthening mechanism in a plain carbon steel -> ie: is it more due to the hardness/stiffness/volume fraction of the cementite or is it due to the inflexible grain boundaries within the pearlite lamellae? Bulk deformation must involve fracture of the cementite/ceramic and deformation of the ferrite to accommodate the required movement.

There's one student that will ask... Trying to get my ducks in a row.

It's an esoteric question, I know, but one that has been studied out there somewhere.

All I can come up with is that, as the increase in strength is linear with carbon content to a maximum of 0.8% (euctectoid -100% pearlite), then the volume fraction of cementite/pearlite has an effect in hypoeutectoid steels.

Practically, the effects of increasing carbon are known. I'm trying to get a better handle on the 'why'.

Trust that the above makes sense and thanks again for your reply.

A

 

[AndrewFinAustralia]

Found an answer in part in an article on pearlitic wire.

Mainly Hall-Petch strengthening between the pearlite lamellae. Fracture and shear of the cementite occurs, but it's not the primary contribution to strength. - it does initiate microvoids which can reduce ductility.

I was going to link the article, but the author gets charged per view. Seems a bit unfair.

 

[Dhurjati Sen]

Also look up the chapter on dislocations.
Try the following approach.
When you carry out the tensile test, there are dislocation pile ups, dynamic strain aging, etc. that all lead to an increase in strength.
And all that is due to interstitiality, i.e. the carbon atoms.

DHURJATI SEN
Kolkata, India