By Jay Shah, Technical Team, Beta Diamond Products
Metallographic sample preparation follows six stages in fixed order: sectioning, mounting, grinding, polishing, etching, and examination. Each stage removes the damage introduced by the previous one while introducing less of its own. Done right, the finished surface shows the true microstructure of the material. Done wrong, it shows you artifacts, and you will make engineering decisions based on damage your own lab created.
This guide walks the full sequence for a typical steel sample, and ends with the exact starter shopping list we give labs setting up from scratch.
Stage 1: Sectioning
Cut a piece small enough to mount, usually under 40 mm, from the region you need to examine. Use an abrasive cut off wheel or a precision diamond wafering blade, always under flood coolant. Heat is the enemy. A dry or starved cut can temper, burn, or even rehard the cut face of a steel, with the altered zone extending a millimeter or more below the surface. Match the blade to the material: metal bonded blades for metals and general work, resin bonded for ceramics, carbides, and other hard brittle materials.
Stage 2: Mounting
Encapsulate the cut piece in resin so it is safe to handle, fits the polisher, and keeps its edges flat. Hot compression mounting cures a thermoset resin around the sample at roughly 150 to 180 degrees C under pressure, finishing in about ten minutes with excellent edge retention. Castable cold mounting pours a two part epoxy or acrylic at room temperature, protecting heat sensitive materials and allowing many mounts at once. We compare the two in a separate article. Either way, label the mount on the back. Future you will be grateful.
Stage 3: Grinding
Grind on silicon carbide papers in descending coarseness, 240, 400, then 600 grit ANSI, with water running over the paper. One minute per paper or until the previous paper's scratches are gone, rotating the sample 90 degrees between grits so leftover scratches stand out against the new direction. Moderate pressure only. Wash the sample and your hands when grinding ends; one stray grinding particle carried to a polishing cloth will scratch every sample that cloth ever touches again.
Stage 4: Polishing
Polish with diamond in descending micron sizes, 9, optionally 6, then 3, then 1 micron, each on its own dedicated cloth, each step erasing the scratches and subsurface deformation of the last. Charge the cloth, keep it moist with lubricant, use moderate pressure, and clean the sample between steps. For demanding work, finish with a sub micron oxide polish, colloidal silica or fine deagglomerated alumina. The most common failure here is contamination carryover, and the cure is boring discipline: one micron size per cloth, forever.
Stage 5: Etching
The polished surface is a mirror that reveals almost nothing. Etching attacks grain boundaries and phases at different rates, turning invisible structure visible. For carbon steels, 2 percent nital swabbed or immersed for five to thirty seconds is the workhorse. Etch lightly first; you can always etch again, but an over etched surface goes back to the 1 micron wheel.
Stage 6: Examination
Examine on a metallurgical microscope with reflected light, starting at 50 or 100x to survey. Scratches, drag lines, or smeared featureless areas each point to a specific prep stage that failed, and our troubleshooting articles cover how to read those clues.
The starter checklist: what a new lab actually orders
When a lab sets up sample prep from scratch, this is the first cart we build with them, in the order the process runs:
- One diamond wafering blade, sized to the saw, metal bond for metals or resin bond for ceramics and carbides.
- Mounting supplies, one route to start: phenolic powder if a mounting press is available, a cold epoxy kit if not.
- SiC grinding papers in 240, 400, and 600 grit ANSI. If your papers are European P graded, the equivalents are P280, P800, and P1200, and yes, those numbers are that different. See our conversion chart article.
- Diamond suspensions at 9, 3, and 1 micron, adding 6 micron for harder materials, matching the 9, 6, 3, 1 sequence most labs run.
- Three polishing cloths minimum: a hard woven cloth for 9 and 6 micron, a low nap cloth for 3 micron, a low to medium nap cloth for 1 micron, each permanently dedicated to its micron size from day one.
- Optional final step: a soft final polishing pad plus a bottle of low crystallizing colloidal silica or 0.05 micron deagglomerated alumina.
And because a lab starting out is exactly who should test before buying: every item on that list is available from us as a free sample. Build the method once, build it right, and the microscope will stop lying to you.