Carburizing Surface hardness
Carburizing Surface hardness
(OP)
We manufacture many aircraft gears which material is AMS6265, AISI 9310 Steel, We do not have in house capability
for Carburizing and Hardening, al of it is farmed out.
We are having a recent problem with a part which
requires .022 efective case depth, 90,5 HR15N at .005 depth
however the verification is done with Vickers,via a cut Met Lab sample Mounted.
The parts are verifing 15N 89 at .002 depth. however it is meeting the hardness at .005 after is ground with 15N superficial.
The procedure as specified per AMS 2759/7 effective hardness
at 50 HRc cutoff.
I have been advised by the supplier that the problem occured because of loss of carbon potential.
This job is run in Endothermic Carburizing Furnace.
what I am aware of is air leak, dew point adjustment, adjustment of gas mixture(methane)
can you please asvise me some finer points in the matter above.
Thanks in advance
for Carburizing and Hardening, al of it is farmed out.
We are having a recent problem with a part which
requires .022 efective case depth, 90,5 HR15N at .005 depth
however the verification is done with Vickers,via a cut Met Lab sample Mounted.
The parts are verifing 15N 89 at .002 depth. however it is meeting the hardness at .005 after is ground with 15N superficial.
The procedure as specified per AMS 2759/7 effective hardness
at 50 HRc cutoff.
I have been advised by the supplier that the problem occured because of loss of carbon potential.
This job is run in Endothermic Carburizing Furnace.
what I am aware of is air leak, dew point adjustment, adjustment of gas mixture(methane)
can you please asvise me some finer points in the matter above.
Thanks in advance





RE: Carburizing Surface hardness
Yes, an low carbon potential will result in a low case hardness. It would be possible for a loss in carbon potential would give you a low case hardness.
And, yes, an air leak would cause the loss of carbon potential, which would affect the dew point. At first, the heat treater could try to adjust the DP at the Endo Generator and, if unsuccessful, would make natural gas addition.
A possible result, particularly with 9310, is that the gas addition raised the carbon potential too high, resulting in retained austenite in the case. If this occured near the end of the cycle, it might only be a couple of thousanths deep. If this is the problem, they probably could be salvaged by a low temperature (cyrogenic) treatment. If the low surface hardness is due to a low carbon content, the cyro treatment won't help.
rp
RE: Carburizing Surface hardness
I guess I am trying to verify that what the
Supplier Informed me of, "is correct". I am trying to prevent further such Instance by getting more Knowledgeable in this area.
Please excuse my Inexperienced in these matters
what is "adjust the DP at the Endo Generator"
what was puzzling me was at .005 depth the case hardness was correct? should I be asking for pictures of the microstructure to verify the correct martensitic transformation?
Thanks
RE: Carburizing Surface hardness
Yes, absolutely request photomicrographs if not the specific samples examined, as well as the hardness survey so that you can examine the case profile.
Low carbon potential is certainly a possibility along with retatined austenite at the surface, but with a report and/or test sample, you will have a better idea of what happened. You can post the report on this thread and perhaps one of us can explain it if necessary.
RE: Carburizing Surface hardness
The type of carburizing you are talking about, Endothermic Carburizing, is performed by heating the parts in a controlled atmosphere furnace. The atmosphere inside the furnace is composed of, among many different gases, carbon monoxide, which is produced by the "Endothermic Generator". The "quality" of the atmosphere depends on the ratio of carbon monoxide to carbon dioxide. It is difficult, however, to measure these concentrations accurately. In practice, the H2O content of the atmosphere is measured as the H2O is one of the "many different gases" that is produced by the endothermic generator and the H2O content can be easily measured by measuring the Dew Point (DP).
So, when running a Endo carburizing system, if you start to lose carbon potential in the furnace, the first step would be to adjust the output of the generator to produce gas with a lower Dew Point. If that does not correct the situation, then a natural gas (methane or propane, generally) is made to the furnace. This can be tricky, since it may result in the furnace going "BOOM" (I know from personal experience).
And, yes, the microstructure of the case needs to be examined to determine the cause of the low hardness.
rp
RE: Carburizing Surface hardness
Second, I might question what flows they are running in the furnace now, and what they have historically run. That's not normally a huge proprietary issue with a trusted supplier. Our sintering process uses hydrogen (purchased) & nitrogen (self-generated). With th escalating cost of process gases such as hydrogen and propane, there is an industry-wide effort to cut processing gasses as close to the line as possible and increase consumption of the gases we generate.
RE: Carburizing Surface hardness
On relatively short case hardening jobs like yours in 9310, in order to get the hardness level at depth such as you mention the heat treater must flood the furnace with enriching gas throughout the cycle. The reason for this is that the short case depth job is all non-equilibrium carburizing, meaning that the surface of the material can only buildup a limited carbon potential. The surface material potential never reaches the potential of the surrounding furnace atmosphere. Even in a perfectly tight furnace, there must be a large available supply of enriching gas. Most likely this is line natural gas. Boost-diffuse cycles do not work well for short carburizing jobs. Stay with a straight carburizing cycle flooded with natural gas (high furnace carbon potential) . You may then tend to get some retained austenite at the surface which must be deep frozen out. Some people freeze right after quench and before temper, some snap temper first and then freeze. There is some risk of cracking intricate parts by freezing right after quench. Either way an additional temper must be perfomed after deep freeze to transform any converted untempered martensite to tempered martensite. Keep in mind that high furnace carbon potentials will over time tend to soot up a furnace and the furnace will need to be regularly burned out. This is running the furnace up to temperature and holding for lengthy periods with only air in the furnace. I would recommend a micro be made and reviewed and more discussion of carburizing process details with the heat treater. Modern heat treats have automatic dew point control at the endo generator and carbon probe potential control at the furnace.
RE: Carburizing Surface hardness
transverse from the surface at .005 by Vickers. Which method actually buys off the parts. Most likely the direct 15N on finish ground part. The check on the sample is to give an idea of whether at grind stock removal the hardness will be sufficient. It should be possible to pass both in your example with 9310. I prefer using knoop for these case hardness at depth checks.
RE: Carburizing Surface hardness
wanted to report the Independent lab report the
following results
Microhardness test
Knoop hardness @ 500 grams
Location OD gear
.005 90.7 HR15n 90.7HR15n
.022 51.5 HRc 50.0 HRc
.025 50.0 HRc
Requirement .005 90.5 HR15N Minimum
core 35 HRc 38.0 HRc
effective
case depth .025" .022"
requirement .022-.030 .022-.030
It conforms
The supplier results
at .005 57 HRc 58 HRc
does not conform
however after grinding gear
at surface 90.7 HRc
I am waiting to receive microcraphs
Thank you all
RE: Carburizing Surface hardness
last line should read
photomicrographs
Thanks
RE: Carburizing Surface hardness
As far as the test lab results, if your 15N hardness is allowed to be converted from Knoop or Vickers then I agree that you are OK. I would confirm the allowable conversion. All parties have to agree on the scale to be used.
If the near surface case is homogeneous then it might be permissible to convert from the lighter load. The 15N takes more of the structure into account. The micros should tell the rest of the story.
RE: Carburizing Surface hardness
Thank You for your reply.
The hardness at surface is my mistake should be 90.7 HR15N
and not HRc
Quote "but are the 57-58 HRC @ .005" supplier results converted from HR15N"
The 57-58 HRc was converted from Knoop/Vickers
It is allowed.
Take Care
RE: Carburizing Surface hardness
RE: Carburizing Surface hardness
Thank you for your reply
I am to meet with the supplier on this friday
about this matter an other issues.
The supplier is is using knoop/vickers microhardness on a
cut & mounted, polished & etched sample from the carburized gear teeth & journals . it is visually verified & micro harness tested per AMS2759/7, it is converted from knoop/vickers to HRc, It did not conform.
an actual gear was returned to us and it was precision ground then returned to the supplier for HR15N hardness test. and this is what I am trying to avoid in the future.
this did conform to the hardness requirement.
I never question their converting method.
but I shall find out.
the Independent lab used a cut, mounted and polished test sample and tested per the AMS 2759/7 and it conforms.
I do believe there is a problem with the suppliers micro hardness testing proceedure. This is going to be one of the Items disscussed in our meeting.
still waiting for the Micrographs.
Take Care
RE: Carburizing Surface hardness
.engineeringht
Please review and advise
Thanks
RE: Carburizing Surface hardness
thanks
RE: Carburizing Surface hardness
Thanks
RE: Carburizing Surface hardness
That said, it looks like there may be some retained austenite in the near-surface area. That is not uncommon for 9310. I can't be sure just looking at the scanned image as what I am seeing might just be some lightly etching martensite. As mentioned above, if the low hardness is a result of retained austenite, a cyrogenic treatment will fix it.
Was a cyrogenic treatment used in the processing? It is often used as a matter of course when carburizing 9310. Because of the alloy content of 9310, you can get retained austenite with case carbon contents over around 0.7 %, which can make achieving 60+ HRC difficult without a cyrogenic treatment.
rp
RE: Carburizing Surface hardness
RE: Carburizing Surface hardness
RE: Carburizing Surface hardness
RE: Carburizing Surface hardness
The Independent Lab reported this as RA at the surface.
My meeting with the supplier has been postponed till
7-10-08. so all issues will be disscussed & addresed.
Yes this part was deep freeze but this test sample was not.
The surface will be machined off to .005 depth, The RA will be removed at a post operation. parts will be acceptable.
I want to thank you all for the comments and help.
this board is one of the best if not the best.
RE: Carburizing Surface hardness