Anyone ever worked with HRx scale? 1

[RichLeimbach]

We're using the HRx scale to do hardness checks on some sintered bronze materials which are too soft to use other, more standard scales. The HRx scale uses a 1/4" indenter and a 15kg load. I've been unable to find much info on this scale, but I'd like to get a better physical feel for the variation we are seeing within our parts.
My question is: does anyone have any experience with using this scale? How would a +/- 5 point spread with this compare with a +/- 5 spread in another scale? Trying to get a feel for how in control our process is. Any help or links are appreciated.

 

[boo1]

ASTM E18-00, Standard test methods for Rockwell hardness and Rockwell Superficial hardness of metallic materials.

Lists hardness standards the Rockwell scales A - K, N, and T, and where specified has allowed the use a "hardmetal" tungsten carbide ball as the indenter as an alternative to a hardened steel ball indenter.

 

[NickE]

Im not sure about the rockwell x-scale.... usually the harder materials produce a lower spread in the rockwell measurement... (in my experience at least)

http://www.gordonengland.co.uk/hardness/index.htm

has a bunch of information about hardness testing....

sorry I cant answer your question...

One thing that may help: Do you calibrate your machine regularly? If so then you can maybe use data from a daily check sample (or several) if you use certified check samples on a regular basis you can observe the machine and operator variation....



Nick
I love materials science!

 

[NickE]

Edit to the above.....

I found the x-scale Its a superficial scale..
15,30,45 kgf total force...

Usaually (again in my experience or Your Mileage May Vary)

I find there to be more variation in the superficial scales due to the much lower indenter penetration depths...




Nick
I love materials science!

 

[boo1]

The Superficial Rockwell Scales lists the 15x, 30x, and 45x they are:

Scale Indenter Major Minor Applications
Load Load
15X 1/4" ball 15 kg 3 kg Used for very soft materials
30X 1/4" ball 30 kg 3 kg Used for very soft materials
45X 1/4" ball 45 kg 3 kg Used for very soft materials

Converting Scale:
Sometimes it is necessary to test in one scale and report in another scale. Conversions have been established that have some validity, but it is important to note that unless an actual correlation has been completed by testing in different scales, established conversions may or may not provide reliable information. (Refer to ASTM scale conversion charts for non-austenitic metals in the high hardness range and low hardness range.) Also refer to ASTM standard E140 for more scale conversion information.



The Rockwell method measures the permanent depth of indentation produced by a force on an indenter. First, a preliminary test force (pre-load or minor load) is applied to a sample using a diamond indenter. This is the zero or reference position that breaks through the surface to reduce the effects of surface finish. Then, an additional test force (or major load) is applied to reach the total required test force. This force is held for a predetermined amount of time to allow for elastic recovery. The additional test force is then released and the final position is measured against the preliminary position and converted to a hardness number. Preliminary test forces range from 3 kg (used in "Superficial" Rockwell scale) to 10 kilograms (used in "Regular" Rockwell scale) to 200 kilograms (macro scale - not part of ASTM E-18; see ASTM E-1842). Total test forces range from 15 through 150 kilograms (superficial & regular) to 500 through 3000 kilograms (macro). A variety of indenters may be used: a conical diamond with a round tip for harder metals, and ball indenters ranging from 1/16" to 1/2" for softer and softer materials.

As a general guide to selecting a Rockwell scale, the operator should select the scale that specifies the largest load and smallest indenter possible to do the job without exceeding defined operating conditions and accounting for conditions that influence the test result. These influencing conditions include test specimens which are below the minimum thickness for the depth of indentation.

 

[RichLeimbach]

Boo1,
Thanks for the info.

Nick,
Thanks for the excellent site. We've been seeing a pretty wide range in our values on this part, and while everything look functionally OK, we're leery of accepting such a wide range because in other scales it usually indicates some bad process variation. If we could get a physical feel for how much this means, I think we'd be more comfortable. I'll keep looking...

 

[TVP]

Rich,

A huge source of variation in hardness testing of sintered components is porosity. I doubt very much that you will be able to see variation less than +/- 5 Rockwell points unless the parts are sintered to near their theoretical density. We use very light load Brinell hardness for specifying high-porosity PM parts, such as 1 mm ball and 10 kgf. You should definitely invest the time and effort into calibrating the tester for the light loads and deflections that you will see when using the superficial X scale. I highly recommend the new Wilson 2000 series from Instron.

 

[RichLeimbach]

TVP,
We're actually seeing variation of +/- 10 on this scale (40-60), which seems a bit excessive to us since we are used to the HRc scale where we typically spec +/-3 or +/-4 for the vast majority of our parts. We're trying to use this test a a quick check to ensure that the porosity and material remain consistent.
We've got 2 calibration standards for our Rockwell tester in the HRx range and seem to at least be getting consistent results with them using our current equiptment. Thanks for the tip on the light load Brinell testing, we might give this a try as well.

 

[EngineerDave]

Interesting topic:

I'll through two more questions in the mix:

Is anyone aware of a prediction life for the indenters? I know this would be extremely difficult to predict, since the indenter is used to test many types of materials. But if for instance the same indenter was used with the scale and loading on the same material, are there any predictive estimates out there on life.

Also, on the portable units. Does anyone have any success with these? There is Krautkramer Benson that makes one type and I was wondering if anybody else had some experience with these or recommendations.