Induction Surface Hardening 4140 alloy steel

black69wolf69 · Apr 11, 2012

Hi,

I am in charge of the induction heat treatment machine at my company and since induction heat treatment is quite new for us, we need to practice our technique on a couple dozen bars of 4140 before we can apply the correct technique to the bars that will eventually be shipped to the customer sometime next month.

All the bars I am working with have a diameter of 14mm (0.55") and length 400mm (14 3/4"). Most of them arrive in poor condition with lots of rust and some machining grooves on one end, which always cause excessive distortion on the grooved ends after heat treatment, but fortunately not elsewhere on the bar. Although this might seem like an obvious question, but should I consider removing any rust first prior to heat treatment?

Our induction heating machine uses a spray quench system from a coil to cool the heated parts after heating; the heated sections are generally only heated for a few seconds prior to being quenched with water from the orifices of the spray coil.

We are trying to achieve a hardness in the range of 35-40HRC, but so far I have managed to achieve this only one time, with a rotation speed around the axis of 1200rpm and a vertical movement speed of 10mm/s at 50kHz frequency (this can't be changed manually but the machine can handle frequencies up to 120kHz) and about 13kW of power (this is manually adjustable). The other problem is that the bars arrive with varying hardnesses along their lengths. On the most recent bar I heat treated, I took 2 hardness measurements on one end that came out to be 22 and 29, and on the other end I obtained 31, 32 and 24 on the Rockwell C scale. Is it possible to achieve a uniform hardness after one heat treatment, even if the original hardness values are so different?

Does anyone have any recommendations for the best method to achieve a hardness of 35-40HRC for 4140 steel by selecting the most appropriate combination of parameters and calculating how much distortion that can be expected as a result of heat treatment? None of the induction heat treating literature I have contain any equations for calculating the amount of distortion that can be expected; they only offer qualitative solutions for handling this issue. BTW the required case depth we need is around 1.5-2.5mm.

Also, since martensite is quite a brittle phase, should I consider tempering after heat treating? I know that if my hardness is too high, then I can temper to reduce the hardness as well as achieve the required combination of toughness and strength, whilst reducing stresses due to quenching - but even if I am able to achieve the required hardness after the first heat treatment, tempering at a lower temperature should be considered to reduce residual stresses and create tempered martensite. However, we only have the one induction heating machine for heat treating as well as tempering, so this machine would also need to be used for tempering. Any ideas on what power setting would allow me to achieve the tempering temperatures required to achieve my target hardness? I am assuming a low power setting, for example 2.5-5kW, but let me know if there is an equation that I can use to work this out.

Thanks in advance for all your help.

HuFlungDung · Apr 11, 2012

http://www.dfoggknives.com/hardening.htm

I posted that link as information not direct advice on what to do. A certain amount of time is required for the structure to transform on its way up to the critical temperature. It may be the case that your equipment is heating too short of a section too quickly.

black69wolf69 · Apr 11, 2012

Thanks for your link. I am quite familiar with the theory, but in this case, practice makes perfect. Previously I have encountered some problems due to too long a heating time or too high a heating temperature. As a result, the hardness in those areas was too low (like around 28-30 or so). I might consider slowing the vertical movement speed, but another problem that I have encountered is too great a fluctation in the amount of power applied, which probably explains the non-uniform hardness after heat treatment.

HuFlungDung · Apr 11, 2012

I don't see how you can get only 40Rc if you are reaching the critical temperature with enough time for transformation. Is the quench insufficient?

black69wolf69 · Apr 11, 2012

You are right; I generally get 48-50HRC or above, and the one time I did achieve around 40Rc was when the quench was not applied efficiently - that was when the previous operator was quenching the part, but I am taking over now so I will take a look at this issue.

It seems that in order to achieve 40Rc with the required microstructure and properties, an initial heat treatment that brings the surface temperature up to the critical temperature then quenching followed by tempering down to the desired target hardness is what will give me the optimal combination of hardness, toughness and strength. A 40Rc surface hardness the first time probably means the microstructure is something other than martensitic, perhaps bainitic, but possibly a combination of bainite and martensite.

johnoder · Apr 12, 2012

Since your goal is uniformity, how about starting with uniformly clean parts, like fresh from the bead blaster?

black69wolf69 · Apr 12, 2012

Thanks johnoder for your tip. I actually went ahead today and started with a clean bar mostly free of rust and I was able to achieve consistent hardness throughout the bar. Clearly removing rust and scale is necessary for achieving better results. I have to check if we have a bead blaster because I actually spent a lot of time sanding and air blasting away the rust, which is clearly not very efficient. Another method I have used in the past was a sulfuric acid bath containing about 1 part sulfuric acid (I think the concentration was about 1M) to 4 parts water, allowing the rusted parts to be fully immersed in the bath and then removing the rust and scale with a brush. However, it may be necessary to use a rust inhibitor after washing away the excess sulfuric acid with water in order to avoid running into the same problem again over time.

4GSR · Apr 12, 2012

Is there a reason for rotating the part 1200 rpm?

The Induction hardening units I've dealth with only rotated around 200 rpm max.

black69wolf69 · Apr 12, 2012

Actually, I'm just experimenting with different rotation speeds. A heat treatment expert with 30+ years experience who trained me on induction heat treating for a few months told me that the rotation speed should only be about 60rpm, although there's nothing wrong with slightly faster speeds of around 100rpm for example. A source of information I came across suggested 800-1000rpm during heating, and then dropping to 60rpm during cooling. In any case, I will try lowering the rotation speed and see what I get - too fast a rotation speed may mean that the heat is not penetrating to the required case depth.

I also made a discovery during tempering - I used the same rotation speed as during the initial heat treatment, but reduced the vertical movement speed and the power. Most of the hardness readings showed no change in hardness after tempering, but I did get some really erroneous values on one side of the bar - although this could have been due to the bar not being held in place properly thus producing the strange readings. Since temperature control is critical for tempering, i will only temper again once I have access to an infrared thermometer since trying to temper with no indication of temperature is really just very random and not recommended.

jimfnd · Apr 12, 2012

The problem with too fast a rotation speed is you just made your part a quench slinger or the impeller of a pump.

4GSR · Apr 13, 2012

Black69,

Something else you may want to think about, your traveling speed sounds like it is too fast, too. Also a 50kHz unit is not going to penetrate very deep on heating. I don't know of your 30+ year man told you this or not. And your tempering sequence I would suggest doing this in a oven, which is a slow heat build up and generally oven cool slowly. Induction heat is fast and cools fast, which is more or less a air quench. Which youre not really accomplishing anything torying to temper with induction heat.

Just some of the things I've learned many years ago with induction heat treating.

Ken

black69wolf69 · Apr 15, 2012

Hi Ken,

Thanks for your comments. I would also really rather use an oven to temper, but the problem is we don't have one right now, though I have already made a recommendation to purchase one. Since I only have access to an induction heating machine at this time, what should I consider doing in terms of tempering then? Perhaps self-temper or not temper at all? Yes, I was told that the penetration depth on a 50kHz unit is not very deep, perhaps only 1 mm or so - anyway, this is what we have so adjusting the power and/or current and traversing/rotation speeds is all we can do to achieve a greater penetration depth.

faizalrazali · Feb 25, 2018

Hello everyone...
Im new in here. i have some question to ask about induction tempering. there is any formula in order to calculate the rpm and time for metal to achieved required temperature. Now i want to do tempering on 4140 steel. The required hardness after tempering is 35-40 HRC. The frequency i used is 45KHz

Rickyb · Feb 27, 2018

black69wolf69 said:
Hi,

I am in charge of the induction heat treatment machine at my company and since induction heat treatment is quite new for us, we need to practice our technique on a couple dozen bars of 4140 before we can apply the correct technique to the bars that will eventually be shipped to the customer sometime next month.

All the bars I am working with have a diameter of 14mm (0.55") and length 400mm (14 3/4"). Most of them arrive in poor condition with lots of rust and some machining grooves on one end, which always cause excessive distortion on the grooved ends after heat treatment, but fortunately not elsewhere on the bar. Although this might seem like an obvious question, but should I consider removing any rust first prior to heat treatment?

Our induction heating machine uses a spray quench system from a coil to cool the heated parts after heating; the heated sections are generally only heated for a few seconds prior to being quenched with water from the orifices of the spray coil.

We are trying to achieve a hardness in the range of 35-40HRC, but so far I have managed to achieve this only one time, with a rotation speed around the axis of 1200rpm and a vertical movement speed of 10mm/s at 50kHz frequency (this can't be changed manually but the machine can handle frequencies up to 120kHz) and about 13kW of power (this is manually adjustable). The other problem is that the bars arrive with varying hardnesses along their lengths. On the most recent bar I heat treated, I took 2 hardness measurements on one end that came out to be 22 and 29, and on the other end I obtained 31, 32 and 24 on the Rockwell C scale. Is it possible to achieve a uniform hardness after one heat treatment, even if the original hardness values are so different?

Does anyone have any recommendations for the best method to achieve a hardness of 35-40HRC for 4140 steel by selecting the most appropriate combination of parameters and calculating how much distortion that can be expected as a result of heat treatment? None of the induction heat treating literature I have contain any equations for calculating the amount of distortion that can be expected; they only offer qualitative solutions for handling this issue. BTW the required case depth we need is around 1.5-2.5mm.

Also, since martensite is quite a brittle phase, should I consider tempering after heat treating? I know that if my hardness is too high, then I can temper to reduce the hardness as well as achieve the required combination of toughness and strength, whilst reducing stresses due to quenching - but even if I am able to achieve the required hardness after the first heat treatment, tempering at a lower temperature should be considered to reduce residual stresses and create tempered martensite. However, we only have the one induction heating machine for heat treating as well as tempering, so this machine would also need to be used for tempering. Any ideas on what power setting would allow me to achieve the tempering temperatures required to achieve my target hardness? I am assuming a low power setting, for example 2.5-5kW, but let me know if there is an equation that I can use to work this out.

Thanks in advance for all your help.

May I suggest you consult a metallurgist. From your description it is apparent that you have no idea what heat treatment is. The object is to obtain 100% martensite for some specific depth. To do this your as quenched hardness must always be in the 55-65 range. You then temper to the 35-40 range for the ductility you customer is expecting. In addition, if you are not doing this process vertically you will have straightness problems.

Miguels244 · Feb 27, 2018

johnoder said:
Since your goal is uniformity, how about starting with uniformly clean parts, like fresh from the bead blaster?

Yes...
If what's coming in is a mess, you can't expect different on the way out.

dian · Mar 1, 2018

i would have thought the hardness differences before hardening would have no effect on the result. wrong?

Sukhjot · Sep 11, 2020

Isn't 4140 not supposed to be water quenched? (blame the surface cracks)
From what I know, this should be polymer quenched. What are your observations?

4GSR · Sep 11, 2020

In induction heat treating you want a water quench, not oil. Generally the quenching fluid is water based with some kind of polymer, anti freeze, added. That how we did it back in the day. The cooling system would cool down the quenching fluid to below freezing before use. That was the reason for the anti freeze added. About a 50-50 mix. I'm sure this helped on eliminating cracking in the surface material. In all of the 4140 we induction heat treated I don't recall us have any problems with quench cracks. Got to remember they run continuous bar heat treat lines where a large steel bar enters the furnace from one end and when it exists the other end, it is cherry red, and immediately gets hits with a water based quench that brings the temperature down to ambient. Then it has to be tempered by passing thru another continuous feed furnace at the proper tempering temperature. I've seen both gas asperted and induction heat used by this method. Ken

Rickyb · Sep 11, 2020

Rickyb said:
May I suggest you consult a metallurgist. From your description it is apparent that you have no idea what heat treatment is. The object is to obtain 100% martensite for some specific depth. To do this your as quenched hardness must always be in the 55-65 range. You then temper to the 35-40 range for the ductility you customer is expecting. In addition, if you are not doing this process vertically you will have straightness problems.

This is 100% sound advice. If you cannot quench out at depth at about Rc 60 your equipment/process is not capable of doing this job.

Notice I said at depth, which means ~Rc 60 as quenched at a depth equal to your customers tempered spec. You don’t get a pass to measure this at the surface when setting up your process. Slowing the process to compensate for too high a coil frequency is just another nail in the coffin. Wait till you see the straightness effect of that. Vertical process will help, but...

My advice is send it out for heat treat.

1953chevB · Sep 11, 2020

here is some reference heat treat data.
Induction Hardening is like a black Art, the OP should be grateful for all sharing their knowledge.

Induction Surface Hardening 4140 alloy steel

Plastic

Diamond

Plastic

Diamond

Plastic

Diamond

Plastic

Diamond

Plastic

Aluminum

Diamond

Plastic

Plastic

Cast Iron

Diamond

Titanium

Plastic

Diamond

Cast Iron

Cast Iron

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