Evaluation and Grading of Non-Metallic Inclusions in Steel
Aug 08, 2026
Evaluation and Grading of Non-Metallic Inclusions in Steel
Non-metallic inclusions are unavoidable heterogeneous substances formed during steel smelting, deoxidation and solidification processes, including oxides, sulfides, silicates and composite inclusions. Although the content is low, they seriously affect the isotropy, toughness, fatigue resistance, wear resistance and welding performance of steel materials. Excessive or oversized non-metallic inclusions will cause stress concentration, reduce structural continuity, and easily induce internal cracks and fatigue failure during processing and service. Therefore, accurate microscopic inspection and standardized grading of non-metallic inclusions are essential core items for steel quality evaluation, material acceptance and high-standard engineering material certification.
1. Classification of Non-Metallic Inclusions in Steel
According to international and domestic steel inspection standards, non-metallic inclusions are mainly divided into four major categories, which are classified and evaluated separately in actual grading tests:
Type A (Sulfide Inclusions)
Mainly manganese sulfide and other sulfide composite phases, showing gray strip or elongated distribution along the rolling direction. Sulfide inclusions have certain plasticity, which easily form long strip distribution after rolling, reduce the transverse toughness and ductility of steel, and affect the cold bending and stamping performance of materials.
Type B (Alumina Inclusions)
Dense hard oxide inclusions mainly composed of alumina, appearing as chain-like, granular or clustered distribution. Such inclusions are hard and brittle, cannot be deformed during rolling, and are easy to become the source of micro-cracks, seriously reducing the fatigue life and impact toughness of steel.
Type C (Silicate Inclusions)
Plastic silicate composite inclusions, distributed in irregular strip and block shape. Silicate inclusions have uneven morphological distribution, which will cause inconsistent local mechanical properties of steel and affect the processing stability of high-precision steel products.
Type D (Globular Oxide Inclusions)
Dispersed spherical or near-spherical fine oxide particles, with small particle size and random distribution. Although single particle size is small, excessive quantity will still reduce the compactness and structural uniformity of steel matrix.
2. Core Principle of Inclusion Rating Inspection
Non-metallic inclusion evaluation adopts metallographic microscope microscopic analysis method. After standard sampling, grinding and polishing of steel samples, the field of view observation, length statistics, width measurement and quantity counting are carried out under standard magnification. The quality grade is determined according to the morphological characteristics, maximum length, total distribution length and particle density of various inclusions.
The grading judgment is based on the comparison between actual detection parameters and standard rating charts. The higher the grade, the more serious the inclusion pollution degree, and the worse the internal microstructure purity of steel.
3. Standard Inspection and Grading Process
In order to ensure the accuracy and consistency of inclusion grading results, we implement standardized closed-loop testing procedures in strict accordance with ISO 4967 and ASTM E45 international standards:
Standard Sampling
Select representative positions from batch steel plates, steel pipes or profiles for sampling. The sampling surface is perpendicular or parallel to the rolling direction according to standard requirements to ensure that the inclusion distribution state can be truly reflected.
Sample Preparation
Complete cutting, inlaying, grinding and mirror polishing to ensure no scratches, corrosion and artificial defects on the metallographic surface, avoiding interference with microscopic observation.
Microscopic Observation and Scanning
Observe multiple fields of view under standard magnification, traverse the whole inspection surface, record the morphology, size and distribution of A/B/C/D four types of inclusions one by one, and retain original metallographic pictures.
Data Statistics and Grade Evaluation
Count the maximum single length, total cumulative length and particle distribution density of each type of inclusion, compare with standard rating diagrams, determine the pollution grade of each type of inclusion respectively, and form a comprehensive purity evaluation conclusion of steel material.
Result Review and Report Issuance
The test data and pictures are reviewed professionally, and a standardized inclusion grading test report is issued to realize traceable quality evaluation.
4. International Grading Judgment Basis
Our non-metallic inclusion evaluation fully complies with mainstream international industry standards:
ISO 4967 – Steel determination of non-metallic inclusions content – Micrographic method using standard diagrams
ASTM E45 – Standard test methods for determining the inclusion content of steel
According to the standard system, each type of inclusion is divided into different grade levels. The grading indicators cover thin series and thick series, which respectively evaluate fine dispersed inclusions and large-size concentrated inclusions, realizing multi-dimensional accurate evaluation of steel internal purity.
5. Engineering Impact of Inclusion Grade
The grade of non-metallic inclusions directly determines the application level of steel materials:
Low-grade inclusion pollution represents high material purity, uniform internal structure, excellent toughness, fatigue resistance and processing performance, suitable for high-end equipment, bridge engineering, pressure vessels, marine steel and dynamic load structural parts.
Excessive inclusion grade will lead to decreased impact toughness, poor fatigue resistance, easy cracking during bending and stamping, and unstable welding performance, which cannot meet high-standard engineering and long-life service requirements.
6. Quality Control Summary
Non-metallic inclusion grading is an important technical index to measure the internal metallurgical quality and structural purity of steel. Scientific and standardized microscopic evaluation can effectively identify the internal micro-defect level of steel, accurately judge the material quality grade, and provide reliable acceptance basis for steel delivery and engineering application.
Through strict metallographic testing and standardized inclusion grading control, we ensure that all exported steel materials have high internal compactness, uniform microstructure and excellent mechanical stability, and continuously provide high-purity and high-reliability steel raw materials for global high-standard engineering projects.
