Complete Workflow for Rating Non-Metallic Inclusions in Steel

Applicable Samples and Objectives

This workflow applies to the qualitative classification and quantitative rating of non-metallic inclusions in rolled products, forgings and continuously cast slabs. Inclusions directly affect fatigue life, isotropy and surface quality. The objective is to classify and rate Type A (sulfides), Type B (alumina), Type C (silicates) and Type D (globular oxides) inclusions against standard rating charts on an unetched polished surface.

Complete Workflow

1. Sampling and Cutting

   The examination plane is usually longitudinal (parallel to the rolling/forging direction), through the center or at representative locations specified by the product standard, with a polished area of about 200 mm² (e.g. 20 mm × 10 mm). Sample with a precision cut-off machine under ample coolant to prevent thermal alteration of inclusion morphology.

2. Mounting

   Mount small specimens for convenient handling and edge examination; mounting temperature and pressure must not deform the specimen or damage the inclusions.

3. Grinding and Polishing

   This is the critical step of inclusion rating. Grind stepwise with SiC papers from P240 beyond P1200; polish with diamond from coarse to fine (9 μm, 3 μm, 1 μm), optionally finishing with a short 0.05 μm colloidal silica polish. Throughout the process prevent inclusion pull-out, comet tails and embedment of polishing media; the specimen remains unetched at all times.

4. Surface Quality Check

   Before examination, check the polished surface under bright field on a metallographic microscope: inclusions should have intact outlines with no pull-out pits or tails; otherwise return to fine polishing.

5. Metallographic Microscope Examination

   Systematically scan the entire polished surface at 100×. Bright field distinguishes inclusion types and colors; dark field or polarized light assists in identifying transparent oxides (e.g. Al₂O₃ appears bright in dark field). Using the worst-field method, compare each field with the standard rating charts and record the most severe rating of each inclusion type (fine/heavy series); alternatively, apply the extreme-value method to record the occurrence of the most severe ratings.

6. Determination and Rating

   Classify by morphology and characteristics (see table below); each type is divided into fine and heavy series by width, with ratings from 0.5 to 3.0 in steps of 0.5. Oversize inclusions or special types are noted separately per the standard annexes.

7. Recording and Reporting

   The report shall state the test method, number of fields, results for each type and rating, typical field micrographs and sampling locations.

Inclusion classification characteristics:

Key Control Points

• No etching at any stage: etching dissolves or alters inclusions and invalidates the rating.
• The polished surface must be flat without relief: sulfides differ greatly in hardness from the matrix and easily develop relief, so fine polishing must use light pressure and short times.
• Sampling is always longitudinal and must be stated; transverse planes distort the length of Type A/C inclusions.
• The scanning path must cover the whole examination area, moving continuously field by field without skipping or selecting.
• The rating charts are fixed at 100×; convert the field size when the image magnification differs.
• Operators should be calibrated periodically against the standard charts to ensure consistency.

Common Deviations and Remedies

• Pull-out pits misrated as Type D: re-polish; re-prepare the sample if pull-out is extensive.
• Comet tails stretching Type B strings so they are misjudged as Type A: reduce polishing pressure and lubricant; re-check by morphology and color.
• Disagreement on the fine/heavy series boundary: measure the width with a micrometer scale and apply the standard limits strictly.
• Half a rating or more difference between operators: recalibrate against the standard charts and increase the number of fields, reporting the worst result.
• Surface scratches contaminating the rating: scratches are excluded; re-polish to remove them before rating.

Reference Standards

• GB/T 10561 Steel — Determination of content of nonmetallic inclusions — Micrographic method using standard diagrams
• ASTM E45 Standard Test Methods for Determining the Inclusion Content of Steel
• ISO 4967 Steel — Determination of content of non-metallic inclusions — Micrographic method using standard diagrams

The above is a methodological example for technical communication and process discussion, not actual measured data from a specific customer; the actual test plan, sampling locations and acceptance limits are subject to sample evaluation and the procedure confirmed by both parties.