In general, light microscopy is one of the basic steel examination techniques besides mechanical-technological tests. It accompanies the steel samples from preparation to automated image analysis examinations. Light microscopy yields information on the steel microstructure.
The structure of the samples may vary due to numerous heat treatments. With the aid of light microscopy, we are able to perform quantitative and qualitative analysis of the structure, using parameters such as grain size, shape and distribution of different phases or the content and distribution of non-metallic inclusions for sample assessment. The microstructure of the samples gives us information on macroscopic properties which is useful for the specific development of new steel qualities.
Metallography is basically concerned with microstructures and their illustration. Image documentation and archiving is therefore an indispensable part of our work. Our image databases contain knowledge and valuable experience we can refer to whenever necessary.
Inclusion rating is one of the most common metallographic examinations that industrial steel is subjected to. Non-metallic inclusions in the form of oxides and suphides are found in small quantities in every sort of steel. The shape, composition and distribution of the inclusions are caused by production processes such as melting, deoxidation techniques, alloying constituents, casting and moulding techniques. Inclusions usually have a negative influence on the properties of the material. Depending on the intended purpose, conditions of use and required properties of the final products, it is possible and desirable to differentiate inclusions according to quantity, shape, size, chemical composition and properties.
For steel inclusion rating we use automatic image analysis as well as manual tests as it facilitates the otherwise time-consuming quantitative rating methods.
Steel inclusion rating to EN 10247
The method defined in the European standard is based on the analysis of a defined inspection surface of a polished, unetched metallographic section. The sample must have been taken from a prescribed part of the product. The inspection surface, which must measure at least 200 mm2, is scanned for non-metallic inclusions at a suitable magnification. The standard magnification is 100x, although 50x and 200x can also be used. The inclusions are evaluated by comparing them with a chart of standard pictures. The standard is valid for particles with a length of 3 to 1,410 μm and a width of 2 μm and above. Subject to arrangement also for with a width less than 2 μm.
Fig. 3: Dillinger Hütte makes heavy plate that may be up to 400 mm thick, 5.2 m wide and 36 m long. Steel from Dillingen in the Saarland region spans bold bridges, withstands the forces of water in oil platforms, transports oil and natural gas, shapes modern skyscraper and large hall architecture. The so-called slabs from the continuous casting and blocks are rolled out on the rolling stands to the required length and thickness at a temperature of 1,200 °C with a force of 11,000 tons. Courtesy of Dillinger Hütte.
In connection with the European standardisation of steel specifications, there was an urgent requirement to define a new standard method for steel inclusion rating. The European Committee for Iron and Steel Standardisation ECISS resolved the elaboration of the new EN 10247 as long ago as 1988, and it was published as a preview standard in 1998. The valid standard was published in July 2007. In Germany, the DIN 50602 could still be applied as a valid standard during a two-year interim period.
The European standard is based on the examination technique used up to now in practice. The standard evaluation methods are similar to the previous ones, too. For users, however, it offers easier and clearer classification of the inclusions due to unambiguous inclusion definitions and rules of particle joining. The new chart of standard pictures on the basis of mathematical relationships and the physical dimensions of the measured values also make results easier to compare. Because the chart of standard pictures is based on mathematical algorithms and clear inclusion assessment rules, the new standard enables image analysis systems to be used as well. Their manufacturers were actively involved in the work of the VDEh work group.
More and more users are gradually realising the benefits of the new standard, and its application is becoming more and more common, partly to get new empirical values according to the EN 10247 standard for quality assurance. However, every new standard takes a while to get used to. Acceptance will therefore increase with every new evaluation and every exchange of experience between customer and steel supplier.
Quality tests make sure that product properties meet the specifications of the customer – that's what the customer expects. The type and extent of these tests are either defined in binding regulations or arranged between the customer and the supplier in the contract.
Steel has been the basis for technical progress for more than 3,000 years. Without steel, the pioneering technical inventions such as the steam engine, railway train, automobile, communication engineering, space travel and computer technology wouldn’t have happened. New types of steel are being developed and existing types being improved all the time. To take just one example: Whereas it took 7,000 tons of steel to build the Eiffel Tower in 1889, it would only take us 2,000 tons nowadays. And there is no end to the development in sight. Not to forget that steel can easily be recycled and is therefore a sustainable material. Steel is exciting and ultra modern and has a lot of potential for future applications.