Heat Treat Data for AISI 4620 steel 7

[mfgenggear]

Just looking for any information on heat treating the above steel.
Through hardening procedures, & carburizing (case depth .008-.012) core hardness ? if final case hardness at 58 min.
plus any data on mar quenching or mar tempering if it's possible.

Thank You

 

[TVP]

There are several different process sequences that can be used depending on the necessary core and case properties needed. For maximum case hardness, in the range of 60-63 HRC, you can directly quench from the carburizing temperature of 1700 F (925 C) into agitated oil, and then temper at 150 C. The core hardness will be around 28-40 HRC for typical oil temperatures in the range of 160-195 F (70-90 C), depending on gear section thickness, load size, and agitation. Using a higher temperature oil like Houghton Mar-Temp 355 around 300 F (150 C) will probably produce core hardness more like 20-30 HRC. For better core toughness, the temperature after carburizing should be reduced to ~ 1540 F (835 C) prior to quenching. Tempering at 450 F (230 C) will reduce the case hardness to ~ 58-59 HRC, with core hardness of 30-40 HRC. Hot oil quenching may reduce the core properties further. Some of this data is in ASM Heat Treater's Guide.

 

[mfgenggear]

TVP

thank you kindly for your reply.
the subject part will be a thin wall aircraft application, gear & spline.

did some research with the ASM Volume 4 Handbook good stuff.
and there was some data with the AMS2759/7 specification.
and the Timken Practical data for metallurgist.

what was missing was the data for core hardness achievable with mar quench.

normally have plenty of excess stock on SAE 9310 steel. but
this job will require near net configuration which is bad news.
It appears mar quench will be unacceptable. The part requires 27 Min. HRc. core hardness. 58 Min HRc case Hardness after machining.
some test will be performed to verify the core hardness.

If mar quench fails then will try direct quench method or may require to use a restraining fixture.
sorry for such a vague first posting but must protect customer confidentiality.

I appreciate the feedback.

 

[tbuelna]

mfgenggear-

I would assume that since the finished(?) carburized case depth required for your gear and spline tooth flanks is only .008" to .012", your gear and spline teeth must have a fairly small cross section (DP or module). The problem you always face with carburized gears/splines is quench distortion. The amount of case that must be removed during finish grinding to correct quench distortions can easily result in a finished surface hardness of far less than Rc58. There is also the possibility that the thickness/hardness profile of the finish ground case will be highly uneven along the tooth flank surfaces.

With a "thin-walled" part requiring carburized gear/spline teeth, I would suggest performing the carburizing, quenching and tempering operations on a part that has only been machined with the surfaces that require carburizing, and as much material as possible is left everywhere else. Having thick sections during quenching will minimize distortion. Copper plate masking can be used to prevent the surfaces that require finish machining from being case hardened.

Lastly, since your gear/spline teeth only require a finished case depth of .008" to .012", why not consider using nitriding? You can easily get the case depth and core strength you need with nitriding and an alloy like Nitralloy N. Nitriding does not require a quench, so you won't have to deal with quench distortions.

 

[TVP]

If the part has thin section then you may be able to use marquenching and adjust the temperature and agitation to achieve 27 HRC consistently in the core. We have some applications where we get better core properties, but it will obviously depend on the section size. I definitely recommend contacting Houghton or Heatbath/Park Metallurgical to discuss your particular application.

 

[mfgenggear]

Terry

Thank You for the kindly reply.
The part is made to a customer print, so the parameters of the parts are set.
this part material if to thick, it does not core harden all the way through. this is why I have to harden to near net size.
I am limited to .500 max wall thickness..

but It would be possible to do leave the other areas with plenty of excess stock if I carburize & anneal.
then semi finish machine, then harden & temper.

TVP

Thank You for that suggestion
will give that a try.

 

[tbuelna]

mfgenggear-

If your design requires a finished case surface of at least Rc58 with an Rc50 point (effective case) at .008" to .012", then HT distortion might be a big problem. With a relatively shallow finished case requirement, you will not have much allowance for stock removal during finish grinding. If your gears/splines have wide faces (high L/D) then it might be helpful to run a sample part through the full HT cycle and see how it distorts. You can then adjust the rough machined profile of your gear/spline teeth to compensate for HT distortions.

You did not mention how (or if) the finished case hardness and depth profile will be verified. Usually, the case properties of gear teeth are checked at the mid point of the tooth profile section, and allowances are made for greater variation in the tip and root areas. And as I mentioned, with high L/D ratio gears/splines, there can be quite a bit of variation in the finished case profile along the length of any particular tooth, so where along the tooth face you check the case properties can make a difference.

Good luck to you.
Terry

 

[kumkumvijay]

Hello

My opinions are as below

 

Core Hardness:

What is the definition from your customer for core? Generally for gear & spline, the pitch dia is considered as core for hardness measurement.  Once you know the distance from the surface to the hardness measurement location for core, you can check the hardenability data from the steel mill certificate if you are using H grade SAE4620. If you don’t use H grade then you can try the SAE hardenability calculator to know the hardness from surface to core curve. If you have thin section (0.5 inches) then you have quenching effect from multiple direction and possibility for getting higher hardness.

 

You might have found already that If you keep 870C for 1 hr then you will get around 0.012-0.013 inches case depth for SAE 4620.

 

Reducing Distortion:

1.       Anneal the part before HT cycle.

2.       Plug quenching is widely used for spline components to reduce distortion. For gear press quenching can be used. This will definitely reduce distortion.

 

Case Hardness and core hardness:

1.       Press quenching the oil flow, quantity and time can be controlled and which can help to balance the quenching effect and martensitic formation from case to core ( hardness increase).

2.       Due to carbon enrichment on the case and easy martensitic formation, you can achieve the case hardness HRC58 min with the hot quenching oil.

3.       If you ensure the alloying element in lower side of the spec from the steel mill , it will reduce the hardenability and reduce the chances of getting higher hardness in the core. SAE hardenability calculator can help you for this calculation.

4.       Hot quenching also reduce the quenching severity and hardness increase on the core.

 

I am not sure, your customer is asking for effective case depth (ECD) or total case depth (TCD). Generally for carburizing process TCD is preferred for gear application in automobile file customers.

 

[mfgenggear]

Terry

thank you for kind replies.
the case depth & case hardness will be verified at the gear & spline P.D.
the gear will have pre grind data, the spline will be finished.
most likely minimum stock will be removed from the profiles to maintain
case depth & hardness. generally the case hardness depth will be held to a minimum depth from the surface.
please be advised that this is common processing from my experience.
I just needed any past carburize procedure for this material.
one was answered by TVP, the core hardness is a concern maintaining the
minimum hardness of 27 HRc through the entire cross section of the parts.

Kumkumvijay

thank you for your suggestions.

core hardness must be through the entire cross section of the geometry.
case depth is effective case depth at the 50 HRc standard cut off.

Please advise a link for the SAE calculator.

Thank you

 

[tbuelna]

mfgenggear-

AMS2759/7 describes 4620 as a "low hardenability steel", and provides certain considerations with regard to its heat treat processing. You have to read the spec carefully to get a complete understanding of what it implies.

I've also attached a couple relevant pages from AGMA 2004-B89 that provide guidance for processing carburized gears. There are tables for case depth vs. DP, and min core hardness vs. DP for carburized gears of various steel alloys. There is lots more useful information in this AGMA spec, so I'd recommend giving it a read.

Good luck to you.
Terry

 

[kumkumvijay]

Hello Terry
Unfortunately i have no link for the SAE hardenability calculator. Its was bought from SAE.

I found a link which can help you more.

http://www.emjmetals.com/PDF/BBook/Mechanical Properties and Hardenability.pdf

Page -19, You can see the hardenability data for SAW 4620 water and oil quenched. you have mentioned that the core hardness is min HRC 27. you have mentioned that 0.5inches is the thickness of the part. So the center of the core which is far away from the quenching surface will be at 0.25 inches. In steel mill certificate, it will be around J4 location. As per the oil quenching chart, you can get min HRC25 at the core with min chemistry band.

To ensure min HRc 27 at core, ensure teh alloying elements in the steel lies in the middle of the band including carbon.

Timken handbook page-64 : SAE4620RH graade meet your expectation exactly by min HRc27 at J4 location.

 

[mfgenggear]

thanks guys.

 

[Maui]

If it is decided that press quenching is necessary to hold the required tolerances, the following paper describes this process for those unfamiliar with it:

http://www.asminternational.org/students3/-/journal_content/56/10192/16265619/BOOK-ARTICLE

Maui

http://www.EngineeringMetallurgy.com

 

[tbuelna]

kumkumvijay-

Thanks for the link to the Timken handbook, it's an excellent reference. I have seen the handbook before, but I did not have a copy of it.

You make a good point about the effect of alloy composition on hardenability. If mfgenggear's customer will not allow for a slightly lower core hardness requirement in less critical areas of the part or areas with thicker sections, then one option would be to order the 4620 material with tighter chemistry control (RH), or other types of acceptance tests, to ensure less variation in core hardness of the heat treated part.

 

[kumkumvijay]

Hello Tbuelna
Nice to know that my link and details are useful.
Thanks