Tip #9: Sample Preparation
Checking the results of the heat-treatment operation in a production environment is challenging. Proper preparation of sample mounts is critical in that one often finds the placement of multiple fasteners in a single mount necessary.
Tips for Proper Sample Preparation
1. Don’t let the volume of samples or speed at which they need to be generated catch you unprepared. Do not sacrifice quality for expediency.
2. Wet grind fasteners prior to placement into the mount. Extreme care must be taken to avoid burning (tempering back) the surfaces. Fasteners are often held by pliers or other gripping devices, so be sure to apply even, steady pressure on a wet grinder. Excessive sparking or discoloration of the surfaces is a clear indication that surfaces have gotten too hot.
3. Macroetch with 2-3% nital prior to mounting to check for grinding burns and to look for uniformity of case (if case hardening).
4. Do not overload the mount. Mounts with too many fasteners tend to bulge and/or have rounded surfaces, making grinding/polishing difficult and producing scratched or poorly prepared surfaces. Some fasteners in crowded mounts are invariably too close to the edge of the mount to be adequately prepared or considered representative parts.
5. Use both proper grinding/polishing materials and the proper sequence/number of steps so as to eliminate scratches from all surfaces. Do not compromise quality for speed.
6. Microhardness test using the proper load. Too often, near-surface or shallow case depths are measured using 500-gf loads when it is inappropriate to do so. (Remember the “diving board” effect – as you approach the edge with too heavy a load, deflection occurs and the reading is erroneous.)
7. Etch lightly first to reveal pattern irregularities and to estimate total case depth.
8. Look at the microstructure as well as the case depth both at 100x and higher (400-1,000x) after proper etching.
9. Record results on permanent forms or in computer databases.
10. Retain sample mounts for the appropriate length of time (typically 1-5 years).