Purpose
Support incoming inspection, process comparisons and defect analysis. Define section orientation relative to the rolling direction or casting location.
Route
Sampling map → representative section → mounting or clamping → progressive grinding and polishing → observation as required → documented review.
Checkpoints
Do not treat an arbitrary field as representative of an entire component. Preserve location and orientation records and minimize preparation damage.
Typical inspection scenarios
QT500-7 ductile (nodular) iron casting
- Challenge
- Rating graphite morphology and nodularity requires preparation that never dislodges, deforms or contaminates the graphite; the graphite is soft while the matrix is hard, so excessive pressure in rough grinding easily pulls out or smears the graphite, leading to misjudged nodularity ratings.
- Preparation & method
- Section on a wet abrasive cut-off machine; grind with light pressure throughout, using SiC papers from P240 to P1200 with thorough rinsing between steps to prevent abrasive embedment. Polish on a short-nap cloth with 1 μm diamond paste, using short times and light pressure to keep the graphite intact. First rate graphite form, nodularity and size against the reference charts in the unetched condition, then etch with 4% nital to reveal the pearlite/ferrite matrix.
- Reference standards
- GB/T 9441 Metallographic test for spheroidal graphite cast iron; GB/T 7216 Metallographic test for gray cast iron; ISO 945 Cast irons — Classification of graphite microstructure
HT250 gray cast iron brake disc casting
- Challenge
- Graphite morphology (types A/B/C/D/E) and length class directly determine the strength grade; different cooling rates at thick and thin walls mix graphite types, so rating must use representative fields; graphite is soft and weakly bonded to the matrix and pulls out easily during preparation, forming artifacts mistaken for pores or shrinkage.
- Preparation & method
- Sample representative thick- and thin-wall locations on a wet abrasive cut-off machine with ample coolant to prevent structural change; mounting is generally unnecessary, with cold mounting only when edges must be protected. Grind with SiC papers from P320 to P1200 under moderate pressure to avoid pulling out graphite, and polish briefly with 3 μm then 1 μm diamond — never over-polish, which dislodges graphite. In the polished, unetched condition, first rate the graphite type (uniform type-A flake preferred) and length class at 100× per GB/T 7216; then etch with 4% nital and examine at 500× for pearlite content of the matrix and the distribution of phosphide eutectic and carbides; measure hardness on a Brinell tester (HBW 10/3000).
- Reference standards
- GB/T 7216 Metallographic examination of gray cast iron; ISO 945-1 Microstructure of cast irons — Part 1: Graphite classification by visual analysis; GB/T 231.1 Metallic materials — Brinell hardness test
Scenarios are method examples, not customer test reports; confirm the process per specimen.
MICROCRE learning notes · confirm the method for the specimen.