Rating Non-Metallic Inclusions in Steel: Key Points

The quantity, morphology, and distribution of non-metallic inclusions directly affect the fatigue and toughness properties of steel. GB/T 10561 and ASTM E45 specify microscopical rating against standard charts, dividing inclusions into types A, B, C, and D and assigning severity levels. The reliability of the rating depends heavily on sampling and specimen-preparation quality.

Inclusion Classification

Type A covers sulfides (such as MnS), which deform plastically into elongated stringers; type B covers alumina-type inclusions (such as Al₂O₃), which align along the rolling direction as discontinuous stringer chains; type C covers silicates, which are deformable and appear as dark elongated stringers; type D covers globular oxides, which are undeformable and dispersed as rounded particles. For rating, each type is subdivided into thin and heavy series by width, and severity levels (0.5 to 3.0) are assigned at 100× against the standard chart.

Sampling and Examination Area

The examination plane shall be a longitudinal section parallel to the rolling direction, with a polished examination area of about 200 mm², and the sampling location follows the product standard. During rating, the entire examination area is scanned field by field, and the level is assigned by the agreed method, such as ASTM E45 Method A (worst-field method). Examination is performed in the unetched condition; etching destroys inclusion morphology and introduces artifacts.

Preparation Considerations

The core of preparation for inclusion rating is to keep inclusions intact—free of pull-out and tailing. Cutting shall avoid overheating, grinding shall proceed through adequate steps, and polishing should use a short-nap cloth with diamond suspension for a limited time. Brittle oxides tend to fracture and fall out during prolonged polishing, forming false pores that must be distinguished from genuine globular inclusions. Automated grinding/polishing equipment with matched consumables markedly improves inclusion retention and rating consistency.

Practical Tips

• Classify inclusions as types A/B/C/D and rate against the GB/T 10561 or ASTM E45 charts.
• Use a longitudinal section examined unetched, with an area of about 200 mm².
• Keep inclusions intact during polishing; avoid pull-out and tailing that cause misjudgment.
• The rating method follows the product-standard agreement; the actual operating procedure shall prevail.