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HRB vs HB
4

HRB vs HB

HRB vs HB

(OP)
I work in an automotive manufacturing facility. We get forgings in that have certain hardness specifications. I'm trying to wrap my brain around an issue we're having and cannot find any information. We have a hardness requirement in HB. We use a 2.5mm tip @ 187.5kg load. Upon inspection of the material we've found the hardness to be too high. We contacted the supplier and their records indicate that the material was well within spec during their testing. Their method for measuring hardness is the HRB scale. We've sent samples to two different independent facilities where the HB hardness was well out of specification in the same range that I measured. One of the independent labs measured in HRB but their results showed the hardness to be in specification. The specified tolerance of the steel is 167 - 229HB. The HRB scale shows 86 - 98.2. I guess my question is what is the significant difference between the two measurement methods and why would somebody chose one over the other?

Thanks,
Kevin

RE: HRB vs HB

The HB, or Brinell method, is good for production hardness testing as it can be done on large or heavy parts with a portable King brinell tester. To do HRB, or Rockwell, the part has to be placed on a table-top hardness tester.

RE: HRB vs HB

(OP)
There isn't much difference in the conversion, steel type, grain structure, etc. to make one chose either or? Both methods the part is mounted and polished and placed onto a table top hardness tester. Any insight to why a sample would measure 25 points above the upper limit of 229HB and using HRB the testing results show it right at or slightly below the upper limit of 98.2. If you convert 257HB to HRB you would get a value of 102HBR. For some reason my test piece that measures 255HB measures 97.8HRB in two different testing labs. Also a total of three testing labs have measured it in the Brinell method and their data matches mine. I'm so confused. I'm sitting on tons (literally like ten tons) of material and I need to know if we're going to burn up tooling if we run it. Are conversions based on a tolerance window so if the material measures higher than that window you'd use a different formula? I've searched all over and cannot find a reason to why my samples are measuring so far apart when using different methods.

Thanks for the help,
Kevin

RE: HRB vs HB

SAmerica-

Brinell hardness is measured using a 10mm dia probe and 3000kgf, while Rockwell B-scale is measured using a 1/16" dia probe and 100kgf. For the situation you describe, BHN would seem like the better metric.

However, you note that your inspection procedure involves a large batch of forgings. Typically, there are other factors that matter more with regards to the quality of steel forgings than just surface hardness. Things like forging grain size, material cleanliness, etc, can matter more. With steel forgings, what you should strive for in your as-delivered condition is a forging that has uniform metallurgical properties/quality and is free from retained stresses. If your forging samples are showing excessive hardness at their surface, then you may need to put the forgings thru a stress-relief and normalize after forging.

RE: HRB vs HB

Brinell hardness is more commonly required on forgings where I work over other methods. I've run into problems of hardnesses not matching when sufficient decarb is not ground off. All of the forging specs I've come across have wanted Brinell hardness on forgings.

RE: HRB vs HB

SAmerica--does your drawing call out the type of hardness measurement to be used on this part?

RE: HRB vs HB

when HRB is over 100, this method is not recommended. A scale is more suitable if you have to use Rockwell. Still, Brinell is more representive due to the bigger area it tests. If the probe is smaller than a grain/phase size...when hitting in the middle of grain, or soft phase, you will get a lower reading, when hitting grain boundary, or hard phase, hard particle, you will get a higher reading.

RE: HRB vs HB

tbuelna,

While a common Brinell test uses a 10-mm diameter indenter and 3000 kgf applied force, multiple other indenter sizes and applied forces are allowed within the testing standards like ASTM E10 or ISO 6506. SAmerica has provided the test parameters, 2.5-mm indenter diameter and 187.5 kgf applied force. Those are close to the values for Rockwell B testing (1.588-mm diameter and 100 kgf applied force).

SAmerica,

Since the two hardness methods are similar, you should expect similar results. It is confusing that you are seeing large variation. While ISO 18265 shows 102 HRB conversion for 257 HBW, that is outside the defined range and can be used as an estimate only. Since you are near the upper limit for Rockwell B testing, and since it is possible that some labs still use the banned steel indenter balls for Rockwell B testing, I recommend you consider the Brinell testing to be more reliable.

RE: HRB vs HB

If you are concerned about damaging tools during machining, perhaps you should section one of the forgings to take a look at the structure. You have not mentioned whether the forgings are annealed after forging, used in the as-forged condition, or subjected to a controlled air cooling after forging. There could be an inconsistent microstructure that may be associated with the hardness variation. If nothing else you will gain confidence as to whether you will burn up tooling.

RE: HRB vs HB

(OP)
Wow. Thanks for all of the responses. The material is SAE 1049S. It undergoes normalizing per TSH5101G. The forgings have an external and internal requirement which is the same (167 - 229HB or 86 - 98.2HRB). The external specification is well within spec however the internal specification is the one that's way out of specification using the Brinell method. The suppliers specification on their drawing is in HB or HRB. They choose to use HRB. The normalizing of the material is air cooled in a controlled environment. Unfortunately we don't have the means to analyze structure in the material. I believe the supplier is measuring their own. With the internal testing we're cross sectioning an area and measuring the hardness 3 - 5mm from the surface. It seems more of a normalizing issue than decarb issue. I just wish I could get some concrete data to show the supplier so we can have the parts reprocessed at no cost. Again, thanks so much for all of the responses so far. I'll be using this forum from now on. If you have any more insight regarding the information I just provided let me know!

Thanks,
Kevin

RE: HRB vs HB

(OP)
Are banned steel indenter ball obsolete?

RE: HRB vs HB

Is the internal hardness too low? If so you may need to cool from the normalizing temp with forced air

RE: HRB vs HB

(OP)
It's a large company with a very controlled normalizing process. They have been very reliable to us for many years. There may have been issues in their process of normalizing. I'm just wondering now if grain structure as described above is causing my data in HB to be so far out when the same sample is measured in spec using HBR.

RE: HRB vs HB

It certainly could be a normalizing issue, I agree. There are numerous reasons for differences between surface and core microstructure, that are associated with material, time at heat, size of load, cooling rate, starting and finishing temperatures, just to name a few. Since you are in automotive, your quality department should have a approval package or PPAP that may have a met lab report with microstructures, as well as a control plan. Ask the supplier if anything has changed recently. If you can get some microstructures and post them here, many of us on this forum may be able to offer their thoughts on the heat treating.

RE: HRB vs HB

(OP)
Our engineer asked about structure analysis and they said everything checked out fine. If I had a way to check it myself I'd like to see with my own eyes.

RE: HRB vs HB

Standards like ASTM E18 and ISO 6508 require tungsten carbide indenters, so the steel indenters are obsolete.

RE: HRB vs HB

Is the normalized structure and hardness part of the final product customer spec, or it is your spec that is required only for machinability purposes? You mentioned a concern about tool life, ultimately if you cannot machine the stock something has to change. Can you try machining a small lot, maybe there will be enough pieces to make a call on machining.

RE: HRB vs HB

Heat Treat facilities that perform normalizing operations typically stack as much material as possible on each furnace load to maximize productivity. This usually doesn't present any significant problems during the heating stages of the normalizing cycle. But it can create issues after the parts are pulled out of the furnace for air cooling. If they are not physically separated from each other so that they can properly air cool, then the forgings on the outside of the tray will cool faster than the forgings near the center of the load. And this can cause the forgings near the center of the load to cool so slowly that they anneal. For forgings between the center and outside of the load, you will often find a mixed microstructure that has been poorly normalized/poorly annealed. And these forgings may also show hardness variations within themselves, depending upon where they are tested. Take another look at the microstructure from two different areas of the same forging where significant hardness differences were detected. You will likely see a corresponding difference in the microstructures at these locations.

Maui

www.EngineeringMetallurgy.com

RE: HRB vs HB

It is important - really important - to understand that hardness conversions are performed per ASTM E140, which explicitly states that conversions are estimates that do not necessarily reflect the hardness values when directly measured. I have seen major deviations when using the conversion compared with actual measurements. Since you have found such a deviation, it is incumbent on your vendor to perform Brinell testing directly to determine compliance to your specs - do not accept their conversions.

RE: HRB vs HB

(OP)
Unfortunately our drawing allows the use of both methods (HB and HRB). They aren't converting the data. To them their making the argument that the parts are within the specified limits given their method of choice (which is the HRB method). Just mind boggling that it measures at the upper limit on the HRB method but way over on the HB method when taking actual measurements on the same sample. I'll have to talk with the QM and QE to see if a trial run can be performed. Guess we'll call this one a mystery.

Thanks again for all the help guys,
Kevin

RE: HRB vs HB

Quote (SAmerica)

...With the internal testing we're cross sectioning an area and measuring the hardness 3 - 5mm from the surface....

SAmerica, as CoryPad correctly pointed out, in your OP you mentioned you are using a 2.5mm dia probe to check hardness. And in a later post you mentioned you are performing the hardness check on a sectioned specimen at 3-5 mm from an adjacent surface. It may be possible that your results are affected by a combination of the probe diameter and the proximity of the indenture location to the edge of the specimen.

Maybe someone familiar with hardness test procedures can confirm if there are guidelines for edge margin of test points with various sizes/shapes of brales/probes/tips.

RE: HRB vs HB

2
According to ASTM E10 and E18, the minimum distance from center of an indentation to part edge shall be 2.5 times the indentation diameter.

RE: HRB vs HB

SAmerica,
Why do you believe that your 2.5 mm (diameter?) tip (indenter) with 187.5 KG load accurately provides the BHN when compared to the 10mm ball indenter and 3000KG load? It will not.

RE: HRB vs HB

weldstan,

Your question is important if the material is heterogeneous on the scale of the indentations produced by the Brinell and Rockwell B tests. The Brinell test they chose is very similar to the Rockwell B test, so the variations between those methods are confusing.

RE: HRB vs HB

I would be interested in more if the details of the Brinell test. According to ASTM E10, the device used to measure the diameter of the indentation has to be able to be able to resolve 0.0025 mm (about 0.00010 inches) and to have an error of less than 0.0025 mm over the range measured. While I am sure there is equipment out there that can meet these requirements, they are considerably different from what I expect when I think of a Brinell scope.

The reason I ask is that I have seen many different types of Brinell testers and not all of them (none, actually) would meet this requirement, although I haven't seen one certified for 187.5 KG / 2.5mm ball. The differences in results being seen here, in my opinion, cannot be attributed to steel vs. WC ball or even microstructurial differences. As pointed out by CoryPad, the loads and ball sizes of the two test methods are quite similar, so I would expect the results to be much more similar than what is being reported.

Without knowing anything more, I'd attribute the differences seen to testing errors.

RE: HRB vs HB

(OP)
I'd have to rule out testing errors since we did do a correlation with two independent laboratories who's results almost matched my measurements within +/- 2HB. One lab was using a 2.5mm and one was using a 10mm ball. We're in the works to switch over to HRB I just heard so hopefully we won't see. We have other parts that will exceed 100 HRB so will we need to use another Rockford method (HRC)?

Thanks again for all of the help. You guys are very knowledgeable,
Kevin

RE: HRB vs HB

Yes, you should also get an HRC indenter for your tester, especially since the upper limit is already close to the edge.

RE: HRB vs HB

(OP)
Fortunately we do have an HRC indenter but I'm sure we'll order an HRB one as well. It does make sense though with the measurement distance from the surface. I wonder why the customer called out an internal hardness in that area if the distance is shorter than 2.5 times the indenter diameter. At least now I can get closure to this issue.

RE: HRB vs HB

(OP)
The supplier did analyze the grain structure and found no significant abnormalities. Unfortunately we don't have the means to check this in our lab.

RE: HRB vs HB

If the heat treater is your supplier they should be able to send you photomicrographs of the microstructure. If they analyzed the grain structure it would be easy to tell someone it is 'fine grain' and they would be correct, but without telling you anything about the makeup of those fine grains. It looks like you have worked with some outside labs, have them section a part and check the structure. Again, you can post a photomicrograph here and one of us may be able to help. At my shop we routinely machine materials in this hardness range and structure can be a factor if the parts are not processed correctly.

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