Steel – the All-Rounder That Has to Pass Many Tests

April 29, 2010

Steel is a real all-rounder. However, each application requires a specifi c sort of steel grade. Without steel there would be no Olympic stadiums, wind energy plants, bridges, skyscrapers, trains, planes, cars, razor blades or knives for medical and home use – at least, not of the quality and design we know today. Buderus Edelstahl GmbH in Wetzlar, Germany is one of the world’s top producers of special steel. The company’s own accredited testing laboratory guarantees exceptionally high quality standards – and stereomicroscopes and compound microscopes from Leica Microsystems support the lab team. Holger Wörner, Head of the Testing Laboratory at Buderus Edelstahl, explains the quality control specifications and the roles played by light microscopy and image documentation.

Fig. 1: Holger Wörner, Head of the Testing Laboratory at Buderus Edelstahl GmbH, Wetzlar, Germany

Mr Wörner, do you see any other industrial materials with the potential for taking over the role of steel in the foreseeable future?

Definitely not. There are a few special applications today where ceramic materials or titanium alloys are used instead of steel to save weight. But they are comparatively expensive and often more prone to cracking. On the contrary, steel is conquering new areas of application due to the improved strength and mechanical properties or the potential for being used in smaller cross sections offered by new variations of alloys. Even a slight change of the alloy may significantly alter the properties of the finished product. Every year, steel manufacturers develop countless new types of steel for specific requirements. Steel is not replaceable in the foreseeable future.

Many of your products are safety-relevant components. What does this imply for your quality control?

If we wouldn’t comply with the highest standards of quality, we would not rank among the world’s best companies. As an accredited testing laboratory, we have to meet stringent specifications and are therefore able to vouch for the reliability, objectivity and accuracy of our results – irrespective of whether we inspect our own products during the production processes or, for instance, inspect damage and write appraisals on behalf of a customer. Quality control begins from the very moment the steel is melted.

The chemical composition of the melt, the so-called melting lot, is of fundamental importance for the properties of the material for the entire duration of the production process. Only non-destructive examinations can provide the engineer with the necessary information for assessing whether each volume element of the component will bear the required load when in use. The interior of the component is examined with ultrasound to detect any defects, the surfaces are subjected to magna flux or dye penetrant testing to check for the fi nest cracks. Mechanical inspection, purity testing and structural analysis under the microscope of samples from defined test positions of the components yield further data on the properties of the products. The type and number of samples at the various test positions depend on the component, of course. To obtain further information on process influences we also analyze specific samples in the scanning electron microscope.

Generator and turbine shafts weighing up to 70 tons are complex one-off parts from our opendie forge. They are rigorously inspected during the long manufacturing process. If defects are not found until the end, we not only lose money, but also risk losing our reputation with the customer. Most of the series-produced components from our closed die forge or the rolling mill, such as vehicle gear components, also have to meet high quality specifications. The customer has to be sure that the entire series is O.K.

Quality testing is considerably time-consuming. An ultrasonic inspection of a large generator shaft takes between three and five days, depending on the number of test positions. Everything has to be documented – even structural effects under a critical size have to be registered. Mechanical and microscopic testing usually takes us one to two days. Most of the time is spent on taking samples and preparing them for inspection. All the test results related to the criteria defined by the customer are finally documented in a certificate that is given to the customer together with his component.

Fig. 2: Mechanical testing laboratory at Buderus Edelstahl. © Buderus Edelstahl GmbH

What role do light microscopy and image analysis play in the quality control of your steel products?

High-quality light microscopes and image analysis software are mainly used for products – whether rolled or forged – for which the microscopic purity is important. After all, the German word ‘Edelstahl’ that is part of our company name refers to the purity, i.e. microscopic inclusions. Another standard inspection is the examination of the microstructure we carry out at different stages of production. Microstructure examinations are not only useful for detecting defects, but also for finding the causes within the production chain. A deviation in tensile strength or toughness can be identified via the microstructure. In the case of a defect in a forged or rolled product, the microscope image can tell us the point in time when the defect was originated.

Fig. 3: Steel inclusion ratings at 100x magnification (© Buderus Edelstahl GmbH) – from left to right:
– Example of oxidic inclusions
– Example of oxidic inclusions
– Examples of sulfidic inclusions

... and for damage inspection?

For damage inspection, examinations with a light microscope, and sometimes also a scanning electron microscope, are particularly important. Further information can be provided by chemical analysis. When a component fails, we are able to find out the reason for the failure. In about 95 per cent of cases, it’s not the steel itself that is defective. Sometimes it is being incorrectly used or the failure is due to an engineering error such as wrong dimensioning of the component.

We can trace almost everything with light microscopy. When inspecting a fracture, we use a stereomicroscope after an initial visual check, which already reveals a lot to the expert. The stereomicroscope often shows the starting point of a fracture. Light microscopic examinations of samples taken from the fracture area show whether the fracture was caused by an unacceptable inclusion, for instance. For this purpose, we produce microsections for structural examinations and purity inspections in order to get to the bottom of the problem. If necessary, we do an additional point analysis under the scanning electron microscope. Naturally, the component is also always subjected to mechanical-technological tests to check for criteria such as tensile strength and toughness.

Any material can fail as a result of fatigue due to its microstructure, load or environmental conditions. Even if the material is always under a cyclic load in the elastic range – so that there are no apparent signs of permanent distortion-, submicroscopically small cracks may eventually form the gradually growth via inclusions in the component, grain boundary disorders and fractures at unfavorably oriented grains and may develop into cracks of a critical size. The ensuing decrease in cross section then ceases to bear the load and there may be a sudden forced rupture. A danger like this can develop unnoticed over many years. That’s why appropriate regular maintenance and monitoring of safety-relevant structural components is essential.

 

Fig. 4: Grain size determination of a special steel sample (© Buderus Edelstahl GmbH) – from top left to bottom right:
– Microscope image with 200x magnification
– Reconstruction
– Measurement
– Results of the grain size distribution

How important is a reproducible preparation technique and what contrasting methods and magnifications do you use?

Reproducible starting conditions are particularly important for the automated purity tests. For microsections, we clamp the material in sample holder plates and grind them. They are then fine-ground and polished on another machine according to standard parameters. A properly prepared section has no scratches or grooves, nor are there any signs of chipping.

We mainly use brightfield microscopy, in some cases interference contrast. The magnifications vary from 50x to 1,000x. Purity testing is done at 100x or 200x, microstructure examinations between 50x and 1,000x.

What about "reproducible microscopy", in which automated microscopes are controlled by software and application-specific settings are reproducible?

Automated microscope systems are mainly an advantage for purity testing, which we perform according to the latest standards. Here it is essential that all the settings are reproducible. The grain sizes are also measured automatically and therefore reproducibly, as this is a heat-dependent variable. We are a leading manufacturer of gear steel, especially for large gears in wind energy plants. These large gears have to withstand tremendous dynamic loads. It is therefore important that we inspect the grain size reproducibly and document it before delivering the material to the customer.

Do you use microstructure examinations for the selective development of new alloys as well?

If a customer wants to make a product for which there is no ideally suitable type of steel grade, it is naturally up to us to design the right steel grade. Varying the amounts of various trace elements that are added to the melt along with carbon, chromium or nickel is the key to creating the demanded properties of the steel. After hot forming, such as forging or rolling, and subsequent heat treatment, an examination of the microstructure shows us whether we have achieved the result we were aiming at or have to change process parameters.

The development of a new steel grade is always a combination of mechanical inspection to determine the quality values for the engineer in the application and structural assessment. If we detect poor toughness properties in the component, we examine the microstructure and grain size. We use the structural examination as a basis for altering the relevant process parameters for reworking or new production – and then inspect the effectiveness of the measures we have taken. The microstructure is the link between the manufacturing parameters and the properties.

When we analyze the microstructure (phases, grain size, purity) we can look in two directions, to the past and to the future. The microstructure tells us how the steel has been originated. And we can predict the mechanical properties it is likely to have.

Fig. 5: Phase analysis of a ferritic-perlitic (© Buderus Edelstahl GmbH) – from left to right:
– Microscope image
– Visualization of the different phases

How important is image documentation for you?

Image documentation has always been indispensable in metallography. Many customers like to have image documentation of the microstructure and we generally provide this when examining cases of complaint or damage. Image documentation is equally important for our research and development activities. For example, database systems help us to file microstructure states of the various steel types and quickly retrieve them when needed. The image documentation comprises overview photographs and representative photographs taken at high magnifications that show the microstructure and, if possible, the homogeneity of the entire component.

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