Retained Austenite on 20MnCr5 material

[JohnMagna]

Have a forging that gets machined, Carburized, Hardened and Temper, then machined some more on a few dimensions. The customer requirements are <25% retained austenite in the functional area's, They are evaluating this optically @ 500X to photo standards all the way to the very surface. Using a few ISO documents we have found that performing a Vickers microhardness @ .04mm from the surface, if that meets the minimum 58Hrc then the part is considered good. Customer is having difficulty recognizing this method. Any suggestions?
I suspect the only way to reduce the RA is to cryogenic (freeze) the parts to convert the RA to Martensite.
Thanks in advance.

 

[Maui]

The closest SAE equivalent grade to 20MnCr5 is 5120, although it is not an exact match. The only area where you should find retained austenite is in the carburized case. A cryogenic treatment at -120F followed by one normal tempering cycle should eliminate any visible presence of retained austenite in the microstructure.

Maui

 

[EdStainless]

If you are seeing that much RA it makes me think that you have the surface C too high.
Either less C or a longer diffusion step?
But to hit the hardness you may need to freeze them.
If the customer wants >58HRC and <25% RA then that is the spec.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[JohnMagna]

The specification I am working too requires .7% carbon when carburizing. We hit the hardness without Freezing, but from the .04mm depth in to the surface we can have 35-45% retained austenite, customer wants 25% max. in the this area (surface to .04mm depth) This material with the carburization puts the Martensite finish below our oil temp of 120F, so we will always have some RA on the near surface.
Just looking to see if there is another way besides freezing.
Thanks

 

[Maui]

With the material you are using, the only other option to reduce the presence of retained austenite in the carburized case is to alter the heat treating recipe by either lowering the austenitizing temperature or by raising the tempering temperature. But making either of these changes will lower the surface hardness, which you don't want to do. A deep freeze followed by one regular tempering cycle would appear to be your only practical option.

 

[TVP]

In my experience, the automotive industry routinely carburizes 20MnCr5 to achieve surface hardness > 58 HRC with less than 25% RA (usually much less than this) without using a sub-zero/freezing step. Surface carbon is usually in the range of 0.7 to 0.9%. One critical factor is the geometry of the part, specifically the smallest cross section(s) that are measuring 35-45% RA. If this part has very thin sections, then it is exceedingly difficult to keep the surface carbon low enough to prevent high RA. Do you have carbide formation as well?

With regards to ISO standards, these have nothing to do with acceptance criteria for RA, unless you are referring to ISO 6336-5, in which case Table 5 section 10.5 specifically limits RA to be 25% maximum for MQ & ME Quality.

If you are looking for a method to process existing parts to meet the 25% RA limit and don't want to use sub-zero treatment, shot peening is another possibility. Otherwise, you will need to work on the carburizing cycle: temperature, carbon potential, time, atmosphere (ammonia addition at the end of the cycle is commonly used to reduce RA).

 

[Maui]

The most frequent objection that I hear from the automotive sector when we discuss implementing a cryo treatment step in the processing of their components is that they don't want to introduce microcracks into the part surface that could lead to fatigue failures in service. Although I understand their concern, I have never once seen an example of this occurring in our processing.

Maui

 

[JohnMagna]

TVP--What do you consider thin cross section? None of this I would consider thick in cross section.
The threaded area's are .1mm to .6mm the thickest area would be 1.3mm. As I stated we are bound by specification to .7% Carbon, so we Aim for .75 to ensure .7 min. We do have some carbides but we have adjusted for that and they are under the maximum. ISO 6336-5 was 1 document that uses the micro-hardness as a deciding factor at .5-1.0 mm from the surface. We are right in the mean for the Effective Case Depth so our time @ 1725F with a Carbon of .7 seems correct.
How do you apply ammonia at the end of the cycle and what does it do?
Thank you for the input and discussion.

 

[JohnMagna]

Cycle is: heat up to 1725F with .75Carbon atmosphere approximate time to get to 1725F is 1.5hrs, Soak/Boost @ .90Carbon @ 1725F for .5hrs
Cool to 1475F @ .75C hold for .5 hrs Oil quench 150-200F. Wash then Temper 350F 2hrs.

Gas is from Endo Generator, Generator and furnace all controlled with Atmosphere Engineering Computer system software.

Customer spec. has a minimum .70 Carbon call out, simulated Carbon with a .75 set point is showing a .68Carbon for the simulation.
Effective case depth has been at the mean target, case hardness has been on the high side 58-63 and we see 64-65 on some parts.
I have attached a photo of the RA & Carbides, this was rated at 35% optically by the customer.

What suggestions to improve the current cycle parameters?

Thanks

 

[TVP]

olds,

Sorry for the delay in responding, regular work has been busy the past week. Anyway, if I understand you correctly, you are saying that this part has a cross-section that is a maximum of 1.3 mm? That is extremely small for a carburized part. Most carburized parts are shafts or gears with a specified case depth of 0.5 to 1 mm, so if the entire part is only 1.3 mm thick, then the specified case depth must be quite small too, maybe 0.2-0.3 mm only? For parts this small, it will be extremely difficult to produce a case microstructure with < 25% RA using conventional gas carburizing. With low pressure (vacuum) carburizing, you can introduce the carbon very efficiently using pulsed acetylene, so very thin parts are often processed using this type of equipment.

Anyway, looking at your recipe, I think that your pre-heat cycle is part of the problem. A typical pre-heat at the process temperature would use a much lower CP of say 0.45% rather than 0.75%. Since the preheat is the longest part of your cycle, the parts will be absorbing carbon during this entire preheat sequence. Also, there is very little time for diffusion at the lower carbon potential (0.5 hours at 1475 F and CP = 0.75). This is compounded by the relatively low temperature of 1475 F, meaning that the diffusion rate is even slower, concentrating more of the carbon in the case and producing more RA. Typical temperature prior to quenching for 20MnCr5 is 1490-1545 F, so I would increase the temperature of that last stage. If you are concerned about distortion, I would increase the oil temperature and decrease the agitation rate. 150F is a very low oil temperature, and if this is a medium or fast oil, it will induce more distortion.

Best of luck, and let us know what happens going forward. By the way, I could not view the image, not sure if it is my work computer/IT settings, or something with the link.

 

[JohnMagna]

Solution was a couple of items: 1--Carbon on Diffuse portion was lowered to .65 as the customers spec. had been changed and they had not flowed that down to us. Also changed Temperature in diffusion cycle from 1475F to 1565F, from 30 minutes to 60 minutes as per customer recommendations. % RA and Carbides were reduced and accepted by the customer. The higher quenching temp did cause the core and case hardness to go up 2-3 RC points which then required the customer to just put a Minimum requirement on their print. So no cryogenics and the customer is happy about %RA and lack of Carbides.