4145 Heat Treatment 1

[MetalsInc]

The part is a 4145 tube (6" OD x 10" long x 1" wall). It will be austenitized and then needs to be heat treated to get a hardness of ~32HRc. Two options are being considered:
1) Quench to ~850F and hold for 1 hour - air cool.
2) Quench to ~700F and remove and air cool (martemper)

Both quenches would be done in a salt bath. An oil quench (with temper) is not an optionin this case. Any comments on the two processes? Any alternate suggestions? We don't currently use a salt bath so this is a new are to us.

 

[Carburize]

I would try option 2

 

[MetalsInc]

Is there a particular reason? I am unfamiliar with salt baths but I wonder how to know when the piece reaches 700F. Will it be a timed process? ie. do some trials, figure out the time, and then use the time in production.

I appreciate the help.

 

[swall]

What other temperatures, if any, can the salt bath be run at? If you could quench into 400F salt, then you could temper to the desired hardness --do you have a temper furnace available? Also, how will you austenitize the part?

 

[MetalsInc]

The salt bath could be run at any temperature in theory (we don't have one and would purchase on based on our process).

It is unlikely that the part will survive a traditional quench to martensite. I would love to do that and then temper, but it won't work in this case due to thermal shock.

We will austenitize in a protective atmosphere. A tempering furnace is available.

 

[EdStainless]

Tubing of this size is quenched all of the time. I have seen 4140 being in-line heat treated through induction coils and quench sprays.
You do need to watch the quench rate, but you can control that.
Are you trying ot hold tight tolerances?

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[swall]

If the intent of a quench into 800F salt and 1 hour hold is to austemper the part, you won't make it. The 1" section size is way too heavy to austemper.So,this leaves either the quench to 700F or some other option. Why do think that this part wouldn't survive a traditional quench to martensite? Are there holes or grooves in the tube?

 

[MetalsInc]

There's a coating on it. Throwing it into an oil bath would pop it off (I believe). I will certainly be trying it just to prove it to myself, but the thermal shock should be too much. Tolerances aren't much of an issue as it will be ground to size further down the line.

How do you think the martemper will do? Some pieces will be 0.5" wall.

 

[metengr]

I agree with martempering. After austenitizing at 1550-1600 deg F for 1 hour at temperature, quench in a salt bath held at 700 deg F, and hold for 10 minutes at this temperature. Typically, a salt bath provides a more rapid response for martempering heat treatment, but considering the section size I would use 10 minutes at temperature, you may need to experiment to optimize the holding time. Rapidly cool to room temperature to allow complete transformation of austenite to martensite, and temper at 975 deg F to achieve the 30-32 HRc.

Martempering has to be performed above the martensite start temperature for a short period to assure minimal temperature gradients before transformation of austenite to martensite. You can go with a modified martempering cycle, where you quench to below the martensite start temperature, and hold until thermal gradients are removed, and cool to room temperature. You will still need to temper to lower the hardness.

Austempering is an option for 4140 and higher carbon grades. However, you might still need to temper as a separate step to achieve the lower hardness range of 32 HRc.
Typically, for 4145, austempering will result in a minimum hardness of 35 HRc, which is slightly higher than what you are shooting for in your application.

 

[unclesyd]

A word of caution I would check the compatibility of your coating with the heat treating salt system you are planning to use. Also of concern is that you have the proper size of salt bath to keep down the inevitable temperature rise of the bath.

As I've stated before have a workable operation and safety plan in place and enforce both.

 

[swall]

I believe you will need an oil quench coming out of the 700F martemper salt quench.

 

[MetalsInc]

Thanks for the replies everybody - very helpful as always.

unclesyd - when you talk about compatibility I assume you mean chemically? I will have to investigate that. And I will certainly make sure the sizing can handle, with lots of room to spare, the heat input. I have not dealt with molten salts and am very concerned about operations in and out of the tank. I'm trying to get as automated (racks, cranes, carts) a system as possible.

swall - oil quenching will be fine to bring it from 700F to ~RT. That should be a limited thermal shock.

 

[swall]

Yes, less thermal shock. That is the whole idea behind marquench.

 

[redpicker]

Just to be sure you aren't overlooking to obvious, why don't you heat treat the material before machining. 4145 bar can be purchased heat treated 285-341 as 6.25 to 6.5 inch with hardness guaranteed at the surface and one inch below with excellent toughness properties.

Unless (duh!) you are trying to re-heat treat after applying a carbide coating that brazes at over 1900 F. There is a company in Indiana that does this and I believe their process is protective atmosphere austenitize around 1650F, a low temperature salt quench (around 450 F, I think) followed by an air cool, then tempered (I'd guess around 1100 F to get 32HRC). I know the Indiana company put in a lot of research into how they could re-heat treat the parts after applying the coating.