The quality of steel is essential for the automotive, metalworking, and construction industries. To ensure the highest product standards, an accurate and reliable workflow for the inspection of non-metallic inclusions is essential. In this article, we show you the best way to analyze your steel, from low carbon to stainless and high alloyed, based on the requirements of the various standards and methods (ASTM E 45, ISO 4967, and the most recent EN 10247).
1. Which are the parameters to rate the quality of your steel and what do they indicate?
Steel always has non-metallic inclusions. The amount of inclusions defines the cleanliness of the steel. But even an ultra-clean steel includes some non-metallic particles. Furthermore, clean (max. 60 ppm [0,006%] sum of all elements) and ultra-clean (max. 30 ppm [0,003%] sum of all elements) steels are low in oxygen, sulfur, hydrogen, nitrogen, and phosphorus. The following parameters are used to rate inclusions and indicate the quality of steel:
- Width, length, and height;
- Aspect ratio;
Significance of the steel inclusion color in terms of composition:
- Light gray = sulfide inclusions
- typically highly deformable, variable aspect ratio, rounded contours, and isolated;
- Black =
- aluminate inclusions: typically non-deformable, aspect ratio < 3, angular, and grouped or
- globular inclusions: typically non-deformable, aspect ratio ≤ 3, angular or circular, and isolated;
- Black/dark gray = silicate inclusions
- typically highly deformable, aspect ratio > 3, pointed ends, and either isolated grouped.
2. What is the purpose of the new EU standards for the inspection of steel quality?
To ensure a high-quality product, special industry standards have to be followed when rating steel inclusions. The most common standards in use are the ASTM E45, ISO 4967, DIN 50602 (although this standard has been officially withdrawn), as well as the newly implemented EN 10247.
- Benefits of the EN 10247 standard
- easier to implement for automated image analysis
- offers comprehensive definitions for inclusion classification, even with respect to manual evaluation
- Challenges of the EN 10247 standard
- comparison with results obtained via other standards is difficult
- interpreting the physical meaning of the inclusion rating leads to some controversy
3. How to prepare correctly a steel sample for the quality inspection
The sample preparation has a big influence on the quality of the results, particularly with automated inclusion rating solutions. During sample preparation, polishing can cause defects. To ensure the best and cleanest sample preparation, the following steps are recommended.
- Final polishing with a 1 µm diamond paste on a low nap cloth
- Use anhydrous suspensions and non-water-based (e.g., alcohol-based) lubricants
- Use oxide suspensions (colloidal silica or alumina) with soft steels to remove scratches
- Ultrasonic cleaning is not recommended, as it can distort inclusion size
- In case of problems during the preparation, simply follow the rules of ASTM E768
4. How to ensure accurate and reliable quality inspection results
There are different Leica systems which enable reproducible and precise inspection of steel quality. Each system meets different needs. Leica Microsystems offers the