High hardness influence on parts 6

[DCMexpert]

Hello,

I have some carburized parts (AMS 6265 material) that exceed the hardness limit (gear area). Te requirement is max.63 HRC and the parts have around 64 HRC at .003 inch measured with HV0.5.My question is: Is it a small difference like 1-2 points HRC so detrimental to the parts? I am interested on studying this subject

Please share your opinions from your experience and knowlogies.

Thank you in advance!

 

[redpicker]

The question is why is the hardness out-of-spec? Yeah, you are only talking a Rockwell point or two, but that can be significant at these higher hardnesses. The key to properly carburizing this material is tight control of the carbon potential in the furnace atmosphere. If that was not done, you are probably safer to just scrap the parts and start over.

AMS 6265 (AISI 9310) has a very high hardenability and retained austenite can be a problem if the carbon content is too high. One way of combating the retained austenite problem involves re-austenitizing at a temperature just above the AC1, which can result in a carbide network at the grain boundaries. Another way is to cool the parts to cryogenic temperatures, which can convert the retained austenite to martensite. If tempering is not performed after the cryogenic treatment, you can have a lot of untempered martensite in the case, which could also cause problems. Both of these problems result from too high of a carbon content in the case and they both can drastically reduce the fatigue resistance of the carburized part.

 

[MintJulep]

If the metallurgical factors nicely explained by redpicker are so detrimental to this part are entirely dependent on how this part is loaded and used.

The designer of this part apparently decided that they were, because he assigned a limit of 63 max.

Hopefully this limit was determined by careful analysis.

Or perhaps it was simply applied because that was the limit of the last part that looked generally similar.

 

[Lyrl]

If you have access to a metallurgical lab that enables measuring hardness at .003" below the surface, I imagine you have some nital or something comparable on hand to etch the parts and evaluate the microstructure. Checking for carbide networks (which can cause problems as indicated by redpicker) would be a good idea when you're looking at hardness that high.

If the microstructure looks good (fully martensitic, or martensite with only fine, well-dispersed carbides and no carbide networks), the impact of the too-high hardness with its extra risk of brittleness is relatively small. I would retemper the parts at 25°F higher than they originally got to lower the hardness a tad and minimize any risk of problems.

 

[DCMexpert]

The microstructure looks good, there are some carbides but not networks. I know we have to respect customer requirements, this is an exception because I have informations that the process went well and there were no problems. Still I don't know why the hardness is so high, that's why I wanna investigate for the future.

On the other hand we have customers who do not specify a maximum hardness. If this is a big problem, why some customers are specifing a maximum limit and others not???

I also suspect this can be a requirement put on the drawing by Engineering Department just to be extra safe (we had some issues with them regarding the requirements from the drawing, because they don not know very well how heat treaments are made and how hard it is to control the process). And we all know that hardness values can be a little different when measured with other equipment (HRC, HV, HK) and converted, or if measured by an automatic machine or humans...there are some measuring errors that appear.

 

[Lyrl]

I've been in commercial heat treat for over 10 years and totally get where you're coming from on the design engineers who seem clueless about why they do certain things. But sometimes design engineers who don't know what they are doing copy from a design engineer who did know what they were doing: having a maximum hardness is important in applications where brittle failure due to excessive carbides is a possibility.

Parts with no maximum hardness may be in applications that never experience impact loading, so brittle failure is unlikely, or in applications where getting the absolute fastest turnaround or paying a few cents less for heat treat is more important than producing the absolute best quality part.

In your part, the hardness is higher than expected for a fully martensitic structure in 9310 material because of the presence of carbides. While the fine dispersed ones you describe are not detrimental, for the future, if possible adjust the planned process to reduce the carbides.

You say everything in the process went well this time. So what if something goes slightly off (oxygen probe goes bad, for example, and carbon potential in the furnace is higher than planned) and you end up with carbide networks and bad parts? Better to have a little breathing room with a normal process not having any carbides.

Sometimes the geometry makes this difficult or impossible (e.g. required hardness in sharp tooth roots means almost certainly the tooth tip corners will end up with carbides), but most parts can be carburized well without any carbides.

 

[weldstan]

One of my first experiences involving failure analysis (45 years ago) was where a 1/2 point increase in Rockwell Superficial hardness produced 25% failure rate in forming of parts and 1 pt hardness produced 90% failure rate. Minor Hardness deviations from a specified minimum or maximum can lead to catastrophic results.

 

[CoryPad]

The problem with the Rockwell C scale is that one-point steps are big at the high end of the range (60+ HRC). For example, from 20 HRC to 30 HRC, each 1 point HRC step is equal to an increase in tensile strength of ~ 20 MPa. From 50 HRC to 60 HRC, each point is worth ~ 60 MPa. If you use a universal scale like Vickers, then tensile strength is linear to hardness over the entire Rockwell C scale.

One reason to apply an upper limit to hardness is to insure proper tempering, so that fatigue life and impact resistance are preserved.

 

[tbuelna]

Since this is a carburized gear surface, there are a couple things to consider. First I checked AGMA 926-C99 for recommendations of carburizing AMS 6265. The case depth varies with tooth size (DP or module), but the recommended range of surface hardness is comparable to Rc58-64. So I don't think your result is an issue, except for your customer's requirements. The effective case depth is determined by the perpendicular distance from the tooth surface at half the tooth whole depth where the equivalent of Rc50 hardness exists.

Hardness testing for carburized gears is normally performed on a co-processed test specimen that is representative of the tooth shape. Are you conducting your tests on a specimen like this? The characteristics of the case as-carburized will vary slightly from the tooth tip to the root fillet. Typically, the most significant difference is the reduced case depth at the root fillet versus the flank surface. Does your customer provide explicit instructions on how the testing is to be performed?

Most carburized gears are finish ground, which means some of the outer case layer is removed. What is of concern with the surface contact fatigue capability of carburized gear teeth is the depth/hardness profile of the case after finish grinding. For this reason the amount of stock removed during finish grind is usually tightly controlled to both a minimum and maximum amount. For a .025" case depth as-carburized, grind stock removal might be something like .002"-.006" along the working portion of the flank surface, a slightly lesser amount on the root fillet surface, and slightly more at the tip relief surface.

 

[mfgenggear]

The effective case depth is normally measured at the P.D. & at the root diameter of a gear.

amount of case depth removed at grind is crucial. so that after grinding the case BP hardness is held.
there has only being a few cases with my customers that maximum hardness was specified.
as long as there were no carbide network present and no austenitic grain structure present or maximum %.
and that it passes their requirements or specification. normally AMS2759/7.

 

[metman]

As Lyrl suggests, why not retemper the parts at 25°F higher for this particular instance since your carbides are not an issue and your customer requirements will be fulfilled. Even a small amount of carbide network cam be tolerated depending on the application but this is not the case in this instance. (Pun intended)

Design for RELIABILITY, manufacturability, and maintainability

 

[Maui]

Tbuelna, the OP appears to hail from Romania, so the criteria that is used there for establishing the effective case depth may be different from what is used in the United States. A hardness value of 513 Vickers (or equivalently 50 HRC) is typically used in the United States to establish the effective case depth in gear teeth, while 550 Vickers is more commonly used in Europe for this purpose.

DCMexpert, what exactly does the customer specification that you are using for reference say regarding the hardness requirement? Does it list a maximum allowable value of 63 HRC for hardness at a depth of 0.003" below the gear tooth surface? Or does it say that a maximum [italic]surface[/italic] hardness of 63 HRC is allowed? If you are measuring a hardness of 64 HRC at a depth of 0.003" below the surface with a Vickers indenter and the specification stipulates that the surface hardness can't exceed 63 HRC, then taking a direct Rockwell C reading on that same surface will result in a value that is less than 64 HRC. This is because the diamond indenter used in the direct Rockwell C test will penetrate perpendicular to the case carburized gear surface and go deeper into the case than the Vickers diamond indenter resulting in a hardness value that represents an average over the depth of penetration. And that hardness value will be lower than the hardness measured at a depth of 0.003".

Maui

 

[tbuelna]

Thanks Maui. I do appreciate that many people posting questions here reside in counties other than the US.

The OP noted the gears were made from AMS 6265 (double vacuum melt 9310), so it's likely the gears were for a demanding application. In the OP it appeared to me there was some concern about having to scrap some expensive carburized gears due to the case properties on the test speicmen being slightly out of spec. So I asked some questions about how the case hardness inspection was performed. Was a separate test specimen used? If so, was the test specimen an accurate representation of the gear teeth? If a separate test specimen was used, was it processed identical to the gears in HT, including any masking, quench fixturing, etc? Was there a requirement to take the case hardness measurement in a specific location on the tooth profile? All of these things can affect the condition of the carburized case.

Given the small amount the inspected hardness value was out of spec, in order to keep from having to scrap a batch of expensive gears I would re-do the inspection in a way that takes full advantage of the requirements.

 

[DCMexpert]

Sorry for not answering anymore...had very busy days.

tbulena - Microhardness was measured on a finished/grounded part, not on a coupon ar sample.

Maui - Effective case depth definition is 50HRC, measured by Vickers 500 grams (513 HV0.5) and the requirement was 63HRC max. at 0.003". Measured directly on the surface with Rockwell is possible to be less than 63, but still does not comply customer requirements...and as I sad before, the microstructure looks very good (no carbides or austenite in excess).

weldstan - this is very interesting, having so many failures at 1/2 or 1 point difference in hardness. Are you sure that what lead to these failures was only the hardness? I am more interested on failure analyses and how small deviations from requirements can lead to those failures.

 

[mrfailure]

Stan makes a very good point: Think of the Vickers scale as a linear scale. The difference between 20 and 21 HRC is 5 HV, while the difference between 63 and 64 is 28 HV. So yes, being off in the upper range by only 0.5 or 1 Rockwell point can lead to failure.

 

[weldstan]

DCMexpret,
Oh yes. In the subject case, we were dealing with parts which were cold formed from cold rolled sheet supplied by the steel mill with which I was employed. The results were unmistakable. This was one of my first lessons learned after University (and not learned therein) and I am forever grateful for the experience.

But the same can be said of items in stress corrosion cracking service where one or 2 points Rc can lead to success or failure in service.

 

[tbuelna]

DCMexpert- Thanks for the reply.

So you performed the hardness inspection on an actual part that was sectioned. Were there any other requirements regarding how to perform the hardness test besides what you noted above? Was the location on the tooth profile specified for this inspection? Did you try performing this inspection on a couple different teeth around the gear? The reason I ask is that you noted the case hardness inspection was performed on a finish ground gear, and the remaining case after finish grinding can vary in depth/hardness at each tooth around the gear. The amount of case removed at each tooth will depend on the amount of distortion produced during HT. The sections of the gear that have greater HT distortion can require more case stock removal during finish grind, and this would affect the hardness profile of the remaining case. So if you haven't done so already, and it is permitted by the customer, you might try checking several of the teeth around the gear to see if you get an acceptable result.

 

[DCMexpert]

Weldstan - I don't have experience in failure analyses....I trust your experience on this. As Mrfailure noticed very well, there is a bigger difference in hadness in the upper limit when converted to HV, so is more critical.

Tbulena - the part was measured at 0 - 90 - 120 degrees (on 3 gears from the ssame part). On the tooth tip we had the biggest hardness (it's possibil that it wasn't grounded on that area), but also we obtained above 63 HRC on pitch area.
There where some differences between the results, meaning that at 0 degrees we had 63 HRC and at 120 degrees we had 63.8 HRC, both measured on PD for example (on the PD the part was grinded for sure). So, I could say that some of the gears from the part where ok and some not...does this mean that the part si good or not?!

 

[Maui]

DCMexpert, I suggest that you share these results with your customer. Ask them if they are acceptable on a deviation. They may say yes.

Maui

 

[mfgenggear]

DCMexpert

The hardness is measured at the root dia and at the gear p.d.that is normal practice.
the tooth tips are not measured unless ask for. if your customer does not ask, or the specification
does not require it, for the major dia to be tested then it is not required.

if root dia or the major dia are not ground, the results may be higher.
a finished part can be erratic on the test. it is standard practice to have a .005 inch deeper case depth
and hardeness on un ground surfaces.

a good suggestion is to contact your customer with the results. they will make the final decision.
if these parts are acceptable.

this is my opinion and only to my experience. I have never had parts rejected because it was to hard.
in fact it has been the opposite. as long as the transformation fron austinite to martensite was within specification requirements. and there was no visible carbides. or other defects.