Microscopic Examination Method Using Standard Rating Charts

(GB/T 10561‑2005)

I. Principle

Polished steel specimens are examined under an optical microscope at 100× magnification. Non-metallic inclusions in the field of view are compared with standard rating charts and graded by type, so as to evaluate the cleanliness of the steel. Applicable to:Structural steels, bearing steels, tool steels, stainless steels and other steel grades.

II. Specimen Preparation

Sampling Location

• Generally take transverse sections.
• Avoid surface decarburized layers and edge-damaged zones (distance from edge ≥ 1–2 mm).

Mounting

• Small specimens are recommended for hot mounting to prevent edge chipping.

Grinding and Polishing

• Grind stepwise up to 2000# abrasive paper.
• Mechanically polish to a mirror finish; smearing is strictly prohibited, otherwise fine inclusions will be obscured.
• The final surface shall be free of scratches and deformed layers.

III. Classification of Inclusions

(Four main types + special type)Classified by morphology, optical properties and deformation behavior:

• Type A: SulfidesGray or black, deformable, appearing as elongated strings or chains.
• Type B: AluminaBlack, angular, non-deformable, aligned in the rolling direction.
• Type C: SilicatesLight gray, deformable, appearing as elongated shapes.
• Type D: Globular oxidesRound or oval, non-deformable, randomly distributed.
• Type DS: Single globular inclusionsLarge-sized, spherical or near-spherical, usually exogenous inclusions.

IV. Inspection Procedure

• Magnification: 100× (10× objective + 10× eyepiece).
• Number of fields:
  • Routine inspection: 100 fields.
  • Special requirements: 200 or 300 fields.
• Observation method:Scan the entire specimen section uniformly, avoiding repetition or omission.Record the highest grade of each type of inclusion in each field.

V. Rating Method

1. Comparison with standard rating chartsStandard grade series: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 …
2. Two statistical methods
   • Worst-field method (commonly used):Record the highest grade observed for each type of inclusion across all fields.Applicable to bearing steels, gear steels and other grades requiring high cleanliness.
   • Average-grade method:Total grades of 100 fields ÷ 100.Applicable to general structural steels.

VI. Advantages and Disadvantages

Advantages

• Simple and rapid operation.
• Low cost.
• Unified standards, suitable for daily factory quality control.

Disadvantages

• Relatively subjective.
• Insensitive to very fine inclusions.
• Cannot provide chemical composition information of inclusions.

VII. Application Scenarios

• Quality monitoring during production.
• Re-inspection of incoming materials.
• Evaluation of process improvement effects.

Not used for precise scientific research or failure analysis (SEM/EDS required).