Degree of purity (material)
The degree of purity of a metallic material is the specification of the content of inclusions in the material that influence its performance properties. The specification of the degree of purity, which is particularly important for steel, describes the content of non-metallic inclusions in the material. In contrast to the determination of the superficial degree of purity , in which the purity of the surface of a component is assessed with regard to adhering particles or the existing wetting with fluids, this is an assessment of inclusions inside the material. Occasionally, the purity of a chemical is also referred to as the degree of purity in chemistry .
Evaluation of the degree of purity
The degree of purity of a material is determined by examining a certain proportion of the material and determining the number of inclusions found and making it available in a key figure. There are several methods of doing this. On the one hand there is the possibility of carrying out corresponding examinations with ultrasound , on the other hand mostly optical and occasionally electron-optical methods are used.
Microscopic degree of purity
When determining the microscopic degree of purity, metallographic sections are created in accordance with specifications and the sectioned surface is searched for non-metallic inclusions in the microscope . Inclusions found are then classified according to their appearance, color, length and width and classified into classes. Depending on the method used, the numbers found for each class are then processed further mathematically so that one or more indicators are determined that characterize the degree of purity of the material. A small number stands for a low proportion of inclusions and thus higher stress resistance of the component made from it.
A distinction is made between mean value methods and maximum value methods:
- With the mean value methods, large and small inclusions are rated. A certain degree of purity can be achieved with these methods either by finding a few large inclusions or many small inclusions.
- With the maximum value methods, only the largest inclusions found in a section are used to determine the purity value.
In order to obtain an objective evaluation, many of these procedures have been standardized. In Germany, DIN 50602 or its successor standard DIN EN 10247 was and is widely used. Recently, the Stahl-Eisen-Prüfblatt 1571 is an evaluation method that largely complies with the discontinued DIN 50602. Internationally, the evaluation according to ASTM E45 or ISO 4967 is often used. Each of these standards also contains several methods of evaluation, which are intended to meet different requirements for the usability of the material.
| standard | DIN 50602 | DIN EN 10247 | ASTM E45 | ISO 4967 | SEP 1571 |
|---|---|---|---|---|---|
| Number of incremental samples | 6th | 6th | 6th | - | 6th |
| Area of the incremental samples | 100 mm² | 100 mm² | 160 mm² | 200 mm² | 100 mm² |
There are also standards that describe the purely statistical evaluation of the inclusions found, but do not provide any information on the classification or the appearance of the inclusions. ASTM E2283, for example, specifies the statistical evaluation of inclusions found according to the Gumbel distribution .
Macroscopic degree of purity
While particularly small inclusions from a few micrometers in length to a few millimeters in length are detected using the microscopic method, larger inclusions are determined using macroscopic methods: