Determination of non-metallic inclusions in steel is a key method for evaluating steel cleanliness, judging the level of steelmaking processes, and predicting the mechanical properties of steel (especially fatigue properties). At present, the main determination methods are divided into microscopic assessment methods (the most commonly used, focusing on morphology and distribution) and separation analysis methods (focusing on composition and total content). The following is a detailed determination guide based on national standards (GB/T 10561, GB/T 20123, etc.):
I. Microscopic Assessment Method (Standard Rating Chart Method)
This is the most common method in industrial production. Polished metallographic specimens are observed under an optical microscope, and inclusions are compared with standard rating charts.
1. Standard Basis
GB/T 10561-2005 Steel — Determination of content of non-metallic inclusions — Micrographic method using standard diagramsEquivalent to ASTM E45.
2. Specimen Preparation
- Sampling: Cut from representative locations of the steel product (e.g., transverse section), avoiding overheated structures caused by sampling.
- Mounting: Metallographic mounting is usually required to protect specimen edges.
- Grinding and Polishing: Proceed through coarse grinding, fine grinding to mechanical polishing. Key requirement: The surface must be bright and scratch‑free, and smearing is strictly prohibited, otherwise fine inclusions will be obscured.
3. Inspection Procedure
- Magnification: Normally 100× (10× objective + 10× eyepiece).
- Field Selection:
- General inspection: Select 100 fields uniformly on the cross-section of the specimen.
- Special requirements: May increase to 200 or 300 fields.
- Observation Area: Normally avoid edges (1–2 mm away from edges), unless specifically inspecting surface decarburization or surface inclusions.
- Classification of Inclusions: According to morphology and optical properties, inclusions are divided into four types:
- Type A (Sulfides): Gray/black, highly ductile, appearing as single particles or chains.
- Type B (Alumina): Poor deformability, black, irregularly angular, aligned in the rolling direction.
- Type C (Silicates): Highly ductile, elongated, light in color (white/gray).
- Type D (Globular Oxides): Non-deformable, round or elliptical, randomly distributed.(Note: Type DS single globular inclusions and other types are also included.)
4. Assessment Method
- Grade Rating: Compare inclusions in each field with standard rating charts (grades 0.5, 1.0, 1.5, 2.0, …) and record the highest grade for each type of inclusion.
- Statistical Methods:
- Worst‑field Method: Record the highest grade of the given type of inclusion observed in all fields (applicable to high‑requirement steels such as bearing steel).
- Average‑grade Method: Sum the grades of 100 fields and divide by 100.
II. Image Analysis Method (Automatic Quantitative Analysis)
With technological development, manual rating has been gradually replaced by automatic image analyzers, yielding more objective results and richer data.
1. Principle
The specimen surface is scanned by an image acquisition system. Inclusions are separated from the matrix through gray‑threshold segmentation, and various parameters are calculated using software.
2. Main Parameters
- Area Fraction: Percentage of the total inclusion area relative to the field area (the most commonly used quantitative indicator).
- Number Density: Number of inclusion particles per unit area.
- Size Distribution: Proportion of inclusions in different size ranges (e.g., <1 μm, 1–5 μm, >5 μm).
- Morphological Parameters: Aspect ratio, circularity, etc.
3. Advantages
- Capable of detecting micro‑inclusions at the micrometer or even sub‑micrometer level.
- Data are traceable, avoiding errors from human subjective judgment.
III. Separation Analysis Method (Chemical / Physical Separation)
Used when the chemical composition or total content of inclusions needs to be accurately determined.
1. Electrolytic Extraction Method (Commonly Used)
- Principle: Use the steel sample as the anode for electrolysis in a non‑aqueous solution. The metal matrix dissolves, while non‑conductive inclusions precipitate on the filter membrane.
- Subsequent Analysis:
- Chemical Analysis: Dissolve inclusions on the filter membrane with acid and determine the content of elements such as O, S, Al, and Ca.
- Scanning Electron Microscopy (SEM) Analysis: Directly observe the morphology of inclusions on the filter membrane and determine their composition with EDS (Energy Dispersive Spectrometer), e.g., MgO‑Al₂O₃ spinel, CaO‑SiO₂‑Al₂O₃, etc.
2. Acid Dissolution Method
Dissolve the steel matrix with acid, filter and collect residues for analysis. However, this method may dissolve some soluble inclusions (such as certain sulfides), resulting in lower accuracy than the electrolytic method.
