Ways to determine inner hardness of surface induction hardened 4140?

[amillertobe]

I have a supplier that offers 4140 in different diameters, but the bars are surface induction hardened to 2mm depth 60HRC. I don't need that surface hardness, quite the contrary, but my designation is material that has HRC around 30, but between 20-40 would be sufficient. Is there any way to determine/estimate the inner hardness of this material other than purchasing the rod and doing HRC testings?

[JRouche]

Is there any way to determine/estimate the inner hardness of this material other than purchasing the rod and doing HRC testings?

I think thats it.. I use 4140PH and it is about 30 on the scale.. How much do you need? JR

[MrWhoopee]

Pre-hardened 4140 (4140QT) and ETD-150 will both be about Rc 28-32 as received, ETD-150 has better machinability. 4140 does not contain enough carbon to reach Rc 60, maximum hardness is usually given as Rc 45-48. It can be case-hardened to reach higher hardness but tends to become brittle.

Here's someone who states that he can achieve Rc 48-50 (Post #5)
https://www.practicalmachinist.com/v...-steel-244895/

Induction hardening involves fast, localized heating before the quench. Core hardness may not be any higher than annealed bar.

[rickyb]

I have a supplier that offers 4140 in different diameters, but the bars are surface induction hardened to 2mm depth 60HRC. I don't need that surface hardness, quite the contrary, but my designation is material that has HRC around 30, but between 20-40 would be sufficient. Is there any way to determine/estimate the inner hardness of this material other than purchasing the rod and doing HRC testings?

A quick answer is no. Not without knowing the specific chemistry. With the actual chemistry for that heat of material and a ton of assumptions you could calculate the the hardness through the bar. Rc60 surface says you have upper end chemistry but also says you have a dead soft core with no martensite. There are other possibilities like pre-hard material before induction hardening which then automatically gives you core hardness.
Your question looks like you have a specific shear strength as your criteria of acceptance and hardness is your way to quantify it. That depends on microstructure as well as hardness. I’m from the camp that tempered hardness doesn’t mean squat without knowing the microstructure.

[Bented]

Cut a sample then do multiple RH tests ,from the outer surface to the core, this will tell you all that you wish to know.

Naturally hobbyists will proclaim that most home shops do not have Rockwell Hardness test equipment but you asked the question.

Any claim by a material manufacturer that you can not measure directly is a 'claim"

[Sparky_NY]

Isn't 4140 PH hard all the way through to RC28-32? I didn't think it had a case that was harder than the core. Now I am curious.

[Mcgyver]

Isn't 4140 PH hard all the way through to RC28-32? I didn't think it had a case that was harder than the core. Now I am curious.

even if heated through, I've wondered how the quench speed effects hardness inside vs out. I mean that the difference in the speed of the quench medium effects things, how much slower is the centre of a 2" bar than the exterior? Stands to reason the centre will not be as hard, but I don't know by how much.

instead of buying the HT stuff, you could get annealed and heat treating it yourself after roughing to shape....then any exposed surfaces will be hard, not just those that started as the exterior of the bar

[rickyb]

Isn't 4140 PH hard all the way through to RC28-32? I didn't think it had a case that was harder than the core. Now I am curious.

Probably not, but it depends. It depends on the material dia(or size), the hardenability of that heat of steel and the severity of the quench. Without tuning the quench to the material the results will be hit and miss. Does the supply house do this? Who knows.
Where I worked, volume allowed us to buy entire heats of steel. This then allowed us to specify mid range chemistry where we had reasonably control. No low end softer parts and no high end parts that were too hard and could quench crack. The process controls are not easy, set and forget, so many places don’t do this as they could.

that said, there are good reasons not to through harden. A softer core causes compressive residual stresses on the surface. This does wonders for fatigue life and is very desirable in many applications. A through hardened part would not fare as well in the same duty cycle. Here you have identical material and processing cost yet there is a significant performance difference.

[I make chips]

2mm depth. "the case" as it's called in induction terminology is about right for most cased and sometimes chromed surfaces such as hydraulic cylinder rod. Again with the induction terms as I sold it for years, 10 kHz will deliver that case depth. Anything deeper than that 2mm case depth will be in the 'as cast' condition. So the 4140 under that case will be in the rC 20's or essentially dead soft.

So here's the deal. 4140 will harden to around 60 rC. BUT.. It must be immediately tempered to relieve the residual compressive stresses in the case to prevent cracking. To maintain the most hardness, the rod is put in a furnace or oven at 400F to relieve those stresses. Now you have a very hard surface with a nice ductile core.

Want to harden those moly alloys? Heat it to 1550-1600F bright orange and quench it in oil or polymer quenchant. (a subject for a different discussion) Don't use water as that will crack it.

Download this free phone app which is the heat treaters bible. It will tell you the temperature you need to attain to bring that hard case hardness down to whatever level you need.
https://www.asminternational.org/web...ttreatersguide

Easy test for case depth:
Take a section of whatever case hardened bar you have and glass bead or sand blast the end of the piece. The case will be visually apparent as the soft core will blast more deeply than the hard case.

[Sparky_NY]

Probably not, but it depends. It depends on the material dia(or size), the hardenability of that heat of steel and the severity of the quench. Without tuning the quench to the material the results will be hit and miss. Does the supply house do this? Who knows..

Pretty unlikely that a supply house does the heat treat of 4140PH, far more likely done at the steel mill that produces the alloy. All this talk of properly quenching and such makes it sound like the steel mill folks are rank beginners, pretty doubtful a home shop guy can have the expertise they do.

The OP stated the material was surface induction hardened to 60RC so it sounds like it is not the common 4140PH. The supplier should be able to provide the mill spec sheet for the material which would clear up a lot of questions.

[A.K. Boomer]

Cut a sample then do multiple RH tests ,from the outer surface to the core, this will tell you all that you wish to know.

Naturally hobbyists will proclaim that most home shops do not have Rockwell Hardness test equipment but you asked the question.

Any claim by a material manufacturer that you can not measure directly is a 'claim"

More and more im liking this guys advice,,,



other than that --- with a round piece is it not possible to get flex ratings? hardened pieces resist flex more than annealed --- so one with a skin hardening should be a little more flexy than one hardened totally through... ?
.

[amillertobe]

The supplier is a small retailer. It might be that the rods are case hardened by carburizing or nitriding or similar process, hence the surface can easily be induction hardened. The are sold to be used as machinery shafts and similar, precision ground. Just thought because this retailer was very close to me. Found a supplier that sells normalized and QT 4140 at 38HRC which is a bit hard-ish, but suffices, and I might give it a little heat at the end to bring it down just a little bit if it turns out necessary. I think, though, that extra hardness may actually be beneficial, and the toughness curve looks appealing.

[A.K. Boomer]

Sometimes charts don't make sense to me --- this is one of those times, can somebody explain to me what's going on here?

the hardness to temp rating is polar opposite of what I would expect...

[Dan Dubeau]

Sometimes charts don't make sense to me --- this is one of those times, can somebody explain to me what's going on here?

the hardness to temp rating is polar opposite of what I would expect...

Tempering temperature, not temperature before quenching. The higher temperature you temper at the more hardness you lose. The graph is starting at full hardness.

[A.K. Boomer]

Tempering temperature, not temperature before quenching. The higher temperature you temper at the more hardness you lose. The graph is starting at full hardness.

Thank you - I kinda was wondering if that's what they were talking about so thanks for clearing it up.

Toughness sure goes through a weird phase does't it?