Rapid cooling of 45 carbon forgings

[PeterCharles]

I'm going to show some ignorance here.

We use some 0.45% carbon components machined from forgings. Normally the forgings are slowly cooled, even annealed following forging. If the forgings were allowed to cool to, say, 350 deg C then quenched to speed up cooling how would this affect the hardness of the forgings?
It's being suggested that by doing this there would be a saving in forging costs and with modern machining tools there would not be any increase in machining costs.
Normally I might try some practical trials, but this is not possible at the present moment.

Any comments?

 

[redpicker]

It depends on the cooling rate to 350 C, the alloy content of the forging steel, and how intricate the part and/or machining is.

With a plain carbon steel, with little or no alloying elements, most of the transformation will be complete by the time it gets to 350 C. With a higher alloyed steel, maybe not.

Even at 350 C, however, quenching can lock in some residual stresses that could interfere with the machining, either by moving during the machining or by distorting the part (up to causing cracking of the material) after machining.

It is quite possible that everything would be fine, but without knowing more details, it could as easily cause a lot of problems.

rp

 

[PeterCharles]

Sorry if I'm a bit vague, but this is something just floated to me.
The steel, just a plain carbon steel eg C45, 080A42 etc.
The cooling rate no idea, say off the hammer, onto a mesh belt conveyor with a variable speed drive, then off the end into a quench tank. (I'm just guessing here, not sure what they have in mind)
We're not talking about high precision machining, tolerances say +- 0.003".

Is there any data around on resulting hardness after quenching from these sorts of temperatures? Or am I looking to do practical tests myself? (which I would like to do but currently can't).

 

[metengr]

There should be no change in bulk hardness upon rapid cooling from 350 deg C for plain carbon steels. Residual stress from thermal gradients upon rapid cooling even from 350 deg C is another matter.

 

[unclesyd]

Annealing is normally accomplished by heating to 1450F and furnace cooled at 20F per hour to 1200F then air cooled. As stated above 350F is well below the cooling cutoff. If your parts are not too large cooling while covered with Vermiculite would work just fine as the cooling rate from forging temperature would be quite slow.

 

[dbooker630]

I agree with metengr that residual stress could be an issue. Otherwise I would not expect any metallurgical effects.

Rather than install a quench system that you have to maintain, why not install a series of large fans to speed up cooling? You could start forced air cooling at a higher temperature, say 425C, which should not produce the acicular structure that machines don't like.

 

[redpicker]

We're not talking about high precision machining, tolerances say +- 0.003".

Depending on the geometry of the forging and the geometry of the machining operations, this could be considered fairly tight. But, there are chances with everything. Being a plain carbon steel, and assuming a failry uniform geometry (and L/D ratios of less than 3), you are probably going to be OK. For long, slender parts, the water cooling from 350 C (660 F) is likely to bow them, and unless stress relieved, will bow more when machined (even if straightened).

rp

 

[PeterCharles]

Speaking to a machining subcontractor from a machining point of view he'd be happy to machine the forgings at Rc30, even Rc35.
So, could the forgings be cooled from the hammer to give Rc30/35 and so reduce forging costs due to quicker cooling?
Or is someone spinning me a line....

 

[TVP]

I would agree that using modern machining techniques, machining a carbon steel forging with a ferrite + pearlite + bainite + martensite microstructure at 30-35 HRC will not pose any issues with tool wear. As the others have mentioned, residual stresses due to quenching would be a concern. Would this type of microstructure be desirable for the end use?

 

[dbooker630]

The short answer to your question is yes, you can quench a .45%C forging to 30-35 HRC. How consistent the hardness and microstructure will be directly from the hammer is another story. Will the variation in hardness and microstructure cause your machining source and end user any issues? It does not appear that you are using microalloy material, and depending upon the hot work imparted there will be variation in grain size and temperature prior to quench. I still prefer the use of fans for consistency, if hardness or structure is not an issue put them right at the press and start controlled air cooling right away. Just my thoughts...

 

[redpicker]

The cost savings are likely to result from eliminating a heat treating cycle (ie, the annealing treatment) and/or allowing forgings to be processed at a location that does not have provisions for slow cooling to ambient.

That is, it is not cooling the forgings more quickly that provides the cost savings, but not requiring a slow cool permits the parts to be processed on equipment that can process them more economically (specifically, equipment that does not have provisions for slow cooling to ambient). Quenching them off the forge would not save any money and there are several metallurgical reasons it would not be beneficial.

With C45, if the machining is done with tool steel machining tools, the lower hardness greatly increases tool life; the lower the better. However, with carbide machining tools, tool life is unaffected up to around 35 HRC or so. What I'm saying is that in the past, the slow cooling may have been economically justified because of the lower machining costs. If the machining is now done with carbide tools, the economics may change.

rp

 

[mfgenggear]

Hmmmm is this steel equivlant to lets say c-1045 steel?

Did you say the final hardness required?
the configuration & hardness will dictate machining sequence.

This may be over kill but I like to Normalize , Harden &
Temper for forgings after rough machining.

#1) to remove residual stress from forging.
#2) at 30-35 HRc is easier/better for machining
material dead soft is not condusive for
machining. it tears and has a bad finish.

 

[arunmrao]

In order to reduce the incidence of residual stresses by water quenching,how about considering fast air cooling. You can have blowers setup along the line and cool the forgings fast. This would not cause steep thermal gradients in the forgings,thus eliminating residual stresses.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[PeterCharles]

Let me recap as this has developed a bit.

1) The part is a forging
2) The material is a 0.4-0.45 % carbon steel
3) I can get the forging machined at Rc30-35
4) I can use the final part at Rc30-35

So if I understand correctly -
a) from the hammer, load the forging onto some conveyor
b) the conveyor is fitted with fans to provide an air blast to cool the forging
c) the rate of cooling is controlled and determines the hardness of the forging
d) it is possible to achieve a final hardness of Rc30-35 by this process.

As a result there would be cost savings from -
i) no additional annealing after cooling from forging
ii) no additional heat treatment after machining to increase the hardness/strength to the required level

I presume the initially there would need to be some experimentation with the cooling conveyor to achieve the RC30-35 hardness, and machining trials to check any resultant distortion??

Please fee free to correct any misinterpretation's of the comments made.

And thanks to all who have contributed to the discussion (and my education)

 

[dbooker630]

A, B, and C are definite, I have my doubts about D.

I suspect the air cooled hardness will be on the low side if not below the 30-35 HRC range. Unalloyed .45%C steel may reach the mid 20's HRC, and you should be able to skip additional annealing.

If your application still needs the refined structure that the annealing provided, in the future you may want to consider a microalloyed 1045 material that you can cool directly from the press without additional heat treatment.

 

[redpicker]

I agree with dbooker630, I doubt you would achieve 30-35 HRC with air cooling. I suspect this hardness range just represents the maximum the machine shop can process without having problems related to the hardness of the material and the forge shop is willing to guarantee they will not exceed this range with their standard cooling. In reality, I would expect the hardness to be much lower.

You seem to have changed your question and I am no longer sure what you are asking. If your original process required a heat treatment after machining, I doubt you will be able to eliminate that with this material.

rp

 

[PeterCharles]

The thread has developed because of the answers to the original question.
Lets face it, times is hard! This started off following up comments on chopping down costs after the forging hammer. Clearly, if the forgings can be service ready from the hammer, there are furthers savings.

I suspect there is no real future in this.
Just buy cheap forgings from China, then a bit later get the Chinese to machine them cheaply as well.

 

[arunmrao]

RC 30 -35 was postulated as water cooling at 350C was considered. also it was agreed that machining at 30-35RC was not a challenge. However, concerns were raised abot residual stresses.

Hence air cooling was proposed,which might result in hardness of 20-24RC. Probability of residual stress is reduced and machinability is good.



Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)