Effect of polishing on micro vickers hardness

[smurali1]

Hello,

I have a colleague who claims that the micro vickers hardness will be highly influenced by extent of polishing done (rough polishing vs. very fine polishing/mirror polishing). He claims the effect is as high as 30HV1 between "semi-polished" and "fully polished" in the same part and same measurement point.

I feel the difference may not be due to polishing but may be due to some other issues - like he did not chose the exact same point again, or error in the machine, different person measuring etc.,

Am I right?

Any comments

Thanks

SPM

 

[swall]

I suppose that severe polishing could have an effect if the material was one that tended to work harden easily and the Vickers load was low (i.e. less than 100g). Otherwise, I would suspect some other cause for the variability.

 

[EdStainless]

Surface roughness can have a big impact on HV, but I have only seen it with rough surfaces. Once you get under aprox 25 microinch it shouldn't much effect. Higher loads are less sensitive ot this.
What material are you working with?
How are you ginding/polishing?

Don't look at single readings. You should be taking 10 or 12 to get a good feel for scatter.

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Corrosion never sleeps, but it can be managed.

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[TVP]

Any microhardness testing should be performed on samples that have been polished. This means all mechanical damage from grinding must be removed, producing a smooth, reflective surface, free of residual stresses, etc. Polishing is achieved by using diamond pastes with a nominal size of 6 micrometers or finer. Surfaces that have scratches or other mechanical damage remaining after grinding (usually with SiC papers with a final grit of 1000-1200) will most definitely affect the microhardness measurements.

 

[smurali1]

Thanks for all your inputs.

We ran a simple experiment wherein in we polished a specimen (3 no.s) to just emery polish and measured the micro vickers. Subsequently we polished the same pc's with diamond paste+silk cloth and measured micro vickers again. The difference was ranging from 0 - 20HV1.

So, we concluded that the polishing difference does not influence the micro vickers values.

Once again thanks for all your inputs.

 

[gciriani]

To compare microhardness measurements, the charge applied (weight used) has to be the same and has to be quoted in the reading. I noticed that you wrote HV1; does that denote a charge of 1 kg?

In my industry (consumer-goods metal accessories), it is customary to use HV0.1 or HV0.05 (i.e. a charge of 100g or 50g). With a 100g charge, the depth reached by the pyramid-shaped, diamond penetrator would be 3.9 micrometers for 250 HV0.1 hardness.

If the mirror polishing is performed with excessive smear of metal in the valleys of the previously rough surface, you can see why there could be a change in hardness. Therfore I believe that you performed the polishing correctly, while your friend may have skipped the emery, performin buffing directly on a rough surface. Buffing gives the best mirror appearance, but it achieves so by exploiting the malleability of the material and smearing it around.

 

[TVP]

gciriani,

Yes, HV1 means 1 kgf indentation load. The consensus standards such as ASTM E 92 and ISO 6507-1 describe this methodology.

smurali1,

When the indentation load is 1 kgf, it is generally considered to be MACROhardness not MICROhardness. As you have discovered, surface roughness is not nearly as important for Vickers macrohardness as it is for Vickers microhardness. I would strongly recommend that you test fully polished specimens when using any indentation load below 1 kgf.

 

[swall]

TVP--I was of the opinion that macro Vickers is the 10Kg load, which the Europeans seem to prefer to Rockwell.

 

[CoryPad]

DIN EN ISO 6507-1 Metallic Materials -- Vickers hardness test -- Part 1: Test method defines three tests:

F [≥] 49,03 N is the Vickers Hardness Test
1,961 N [&le;] F < 49,03 N is the Vickers Small Force Hardness Test
0,09807 N [&le;] F < 1,961 N is the Vickers Microhardness Test

There are several standardized forces for each group. In the conventional test, the forces are HV5, HV10, HV20, HV30, HV50, & HV100. In the small force group, there are HV0,2 , HV0,3 , HV0,5 , HV1, HV2, & HV3. In the micro group, there are HV0,01 , HV0,015 , HV0,02 , HV0,025 , HV0,05 , & HV0,1.

In the US, the standards for Vickers testing include:

ASTM E 92 Standard Test Method for Vickers Hardness of Metallic Materials
for applied forces of 1 kgf to 120 kgf

and

ASTM E 384 Standard Test Method for Microindentation Hardness of Materials for forces from 1 gf to 1000 gf.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[grampi1]

Our experience is similar to TVP for microdur. Polishing makes a significant difference. Less than 300 grit is a waste of time.

 

[gciriani]

Perhaps the colleague who wrote +/-30HV1 for micro hardness variations, didn't mean 1 kg of force but something else. I've noticed in more than one docment, written by the surface-hardening community, notations like HV25 when the testing force was unmistakeably 25 g.

One possibility is that the person who wrote HV1 actually meant HV.1; subsequently the dot went lost. In case of a superficial hardness treatment, there is a decrease of hardness with depth. Thus a polishing of the surface would take away enough material to cause a sizeable micro hardness change.