hardness sample preparation 1

[cv1]

When preparing a welded sample for hardness surveys we have normally used a surface grinder to prepare both faces of the sample to ensure a flat parallel surface. The working surface is then further prepared (120 - 240 - 320 - 800 - 1200 wet grit) prior to etching for the hardness survey (Vickers HV10).

On recent samples of quenceh and tempered steel it was "demanded" of us to not use surface grinding on the working face but to dry linish using a 120 grit belt then the wet process as per above. The basis of this was that the surface grinding does so much more "damage" than the linishing it shoud not even be considered. As a metallurgist I find this arguement to be somewhat nonsensical. Can anyone offer information or suggestions as to which is the more correct preperation method?

 

[metman]

cv1,
Many years ago I practiced the art of metallography as a metallurgist. I am also a machinist and mechanical designer so I can empathize with your concerns. Even if the surface grinding process employs coolant, you could have grinding damage. For shafts used as inner races for bearings in a planetary gearbox we had to use LSGT (Low Stress Grinding Techinique) to eliminate untempered and overtempered martensite generated from normal grinding practice. Cincinatti Gear Co. had a proprietary procedure for this but with appropriate testing one can establish their own procedure. Testing invlolves double etching with two different reagents and macro examination. In essence LSGT requires removing minimal material per pass with adequate coolant of appropriate chemistry.

The demand of a 120 grit dry belt. Is this a moving belt sander? If so I see a problem as you do if it is dry. If it is hand stroking on a stationary 'belt', then it is probably ok and would not generate as much heat as a surface grinder even with the most appropriate grade of wheel used on the surface grinder. With moderate pressure on the sample and very short duration on a moving belt sander, you can produce minimally damaged surface that will be abraded away in the subsequent wet process but I do not understand the requriement for doing it dry.

It might not be worth the trouble but you could purposely damage a speciman by surface grinding that would produce twinning and phase transformation damage and use it for comparison with what you feel is a good technique starting with surface grind then the wet 120...1200 and use textbook examples to prove your point. What does Buheler say? Jesus is The TRUTH

 

[CoryPad]

metman mentioned temperature, but another factor influencing damage is force. Grinding with a relatively large, rigid ceramic disk introduces significantly more force than a moving thin substrate (paper, polymer film) with integral abrasives. If you use bulk grinding, you will need to remove more material with the fine grinding/polishing to get the true microstructure.

 

[cv1]

Some furthur information on our technique for surface grinding. We remove a maximum of 0.1mm (0.004&quot at a time with the final pass being 0.05mm (0.002&quot. The wheel is 150mm diam and rotates at 6250 rpm and the sample is under coolant (water / cutting oil emulsion flush). The dry grinding is done on a high speed belt linisher with intermittant water cooling (hand dipped samples).

 

[metman]

cv1,
0.1mm=0.004" is a big bite. even 0.002" is a relatively heavy cut so that the force as CoryPad says affects the structure. 0.002 depth of cut would be tolerable since you have coolant IF the lateral cut per pass is relatively small say .5mm or maybe even less. To reiterate Cory's comment, "If you use bulk grinding, you will need to remove more material with the fine grinding/polishing to get the true microstructure."

However, back to your original comment that you are hardness testing rather than analyzing microstructure but still you can induce work hardening with grinding forces. Jesus is The TRUTH

 

[Viktor]

Please note the following.

If the hardness of the material is measured as HB, then the damage imposed by grinding cannot change it at all – different level of the depth of RS due to grinding and ball penetration. Almost the same can be said about R hardness measurements. If you use HV hardness test then grinding can make the difference.
Viktor

http://viktorastakhov.tripod.com

 

[TVP]

Viktor is referring to the standard Brinell test which uses a 10 mm ball and the indentation load is 3000 kgf. The area of deformation when using these parameters is quite extensive. However, the Brinell test is applicable over a range of ball diameters and indentations loads (as small as 1 mm and 10kgf), so the same precautions apply when using these other conditions. As cv1 mentioned in his/her original message, their test method is HV10.