HRB vs HB 4

[SAmerica]

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

 

[tbuelna]

...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.

 

[CoryPad]

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.

 

[weldstan]

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.

 

[CoryPad]

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.

 

[redpicker]

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.

 

[SAmerica]

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

 

[mrfailure]

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

 

[SAmerica]

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.

 

[Maui]

Did you bother to evaluate the microstructure?

Maui

http://www.EngineeringMetallurgy.com

 

[SAmerica]

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.

 

[dbooker630]

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.