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Integration of automatic analysis in the determination of grain size of steel

The evaluation and determination of the grain size of steels is fundamental within the metallurgical characterization work since it can lead to the rejection of material for not complying with the requirements of the purchase specifications. The importance of grain size is well known because it has a remarkable effect on the mechanical properties of the material.   Fine-grained steels have greater tensile strength, greater ductility and become less distorted during quenching, being, in turn, less susceptible to cracking. In contrast, coarse-grained steels present opposite properties. In any case, fine-grained structures are not always needed in steels, there are for example some applications in which it a rougher grain is preferable. These structures can reach a higher level of hardness, often desirable for carburization processes. Something similar happens with those steels that undergo cold work processes or in high temperature applications, improving their creep resistance.
Microestructura ferrítico-perlítica de un acero estructural.
The Diagnosis and Failure Analysis team of AZTERLAN has verified on many occasions that the deviation in the target grain size can have a direct relationship with the thermal process it has undergone, which may not have been propper, either due to a too high temperature or to an excessive furnace time. Grain size characterization is a completely standardized process. The UNE EN ISO 643:2020 standards (internationally) and ASTM E-112:13 (at the USA) are the most widespread standards regarding product purchase specifications. Although the UNE EN ISO 643:2020 standard is only applied to steels, the ASTM E-112:13 standard is more generic, and it also includes non-metallic materials whose structures have similarities with the test pattern images defined for metallic structures. These standards include the methods of evaluation, the typology of steels that can be analysed, their corresponding methods of attack and the way in which the results are categorized and expressed. The grain size is cataloged according to the G index, the higher the G index the finer the grain size, and vice versa. As a summary, the evaluation methods can be classified as follows:

Grain size assessment software

Manual examination methods were, until not many years ago, the only way to determine the G index. However, nowadays advanced automated microscopy software can be found on the market. Among others, these softwares offer to metallurgical technicians and end customers some interesting functionalities such as:

Histograma resultado de análisis de tamaño de grano de acero estructural muestra la correlación entre porcentaje de recuento respecto a distribución de tamaño de grano
All these characteristics provide a greater reliability of the results. The following advantages can be highlighted:
Representación cromática de los distintos tamaños de grano en un acero inoxidable austenítico detectados en la micrografía analizada.
However, given the wide variety of steel grades (and in their different heat treatment states), the use of these softwares is not exempt from the need for expert technical knowledge and from a rigorous interpretation and control by qualified personnel. In the case of AZTERLAN, the metallurgical knowledge and extensive experience of our team of accredited technical professionals makes it possible to get a high precision in the characterization and in the presentation of results, eliminating potential processing errors by means of the application of tools such as control prefilters and post-processing of threshold and sensitivity.
This way, the integration of the most advanced analysis technologies, combined with metallurgical rigor and technical knowledge specialization become the best guarantee for a correct and reliable metallurgical characterization.
El área de Servicios Tecnológicos de AZTERLAN cuenta entre sus capacidades con software de análisis metalúrgico que facilita la elaboración de distintos tipos de análisis.
Ivan Ojeda
Iván Ojeda

Metallographic Analysis


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