Reading a microsection — white layer, diffusion zone, and what the photo isn't telling you

A microsection is one slice through one part. Several things change what you see without changing what is real on the die.

Section orientation. A transverse cut through a long thin punch reads the case differently than a longitudinal cut along its axis. Vendors take the easier cut. If the part has a critical bearing surface, ask for the section through that feature.

Edge rounding. During grinding and polishing the brittle compound layer can chip at the section edge and the polish wheel can round the corner. A photo with a missing or thinned compound layer at the edge can be a prep artifact rather than a real case feature. Struers and Buehler protocols specify a thin electroplated nickel or copper layer on the surface before mounting to preserve the corner.

Scratches that fake porosity. A worn polishing cloth leaves parallel scratches that operators mistake for Porensaum channels. Real porosity runs roughly perpendicular to the surface and sits in the outer compound layer only. Polish scratches run at a consistent angle across the whole field.

Hot mounting that overtempers the case. Standard phenolic hot mounts run about 180°C for 5-10 minutes and do not affect the case. Aggressive thermosets and longer cycles can heat the part above the substrate's original temper temperature, dropping core hardness on the traverse without touching the production die. Cold mounting in epoxy eliminates the risk; competent labs default to cold mount on nitrided work.

Smearing. Grinding without proper coolant or with too coarse a paper drags the brittle compound layer across the surface, fattening the apparent layer and faking phase uniformity. ASM Handbook Vol. 9 and Struers nitriding protocols specify a very fine final polish (1 µm diamond minimum, often 0.04 µm colloidal silica) for this reason.