New magnetic techniques for microstructural characterisation of steels

There is no doubt that steel is one of the materials that has largely contributed to the technological and economical development of the twentieth century. Its mechanical and magnetic properties are determined by its chemical composition and the microstructure obtained in its manufacturing process. Traditionally, it has been necessary to mechanically destroy the material in order to analyze its microstructure by means of a microscope, i.e. to get a small sample, to polish it and to attack it with chemical compounds. Nowadays, significant progress is being made to magnetically obtain information about steel’s microstructure. Besides, due to their non-destructive nature, magnetic techniques allow us to skip destructive mechanical techniques.

In this context, the aim of the doctoral thesis was to design an electronic system capable of determining microstructure variations in steels by means of magnetic non-destructive techniques. In the research a thorough analysis of the signals obtained by means of these techniques was made, which led to the definition of several useful parameters for the characterisation of the microstructure and mechanical properties of steels. These new techniques are based on the following principle: The steel is formed by microscopic regions called magnetic domains. When a magnetic field is applied to the material, these domains tend to grow and their walls find microstructural obstacles in their movement, such as dislocations, grain boundaries, or precipitates, which hinder their growth.

The thesis proposes a measurement system that provides several representative parameters of the movement of the magnetic domain walls. By means of this system the magnetic domains of the material themselves are used as internal sensors that record the characteristics of the microstructure. With this method it is possible to determine whether the material has a high or low dislocation density, the way in which dislocations arrange themselves, whether the material has grain boundaries or precipitates etc.

In order to evaluate the system’s sensitivity, measurements were made on low carbon steel samples with various microstructures. Its sensitivity to plastic deformation was analysed and parameters with enough resolution were obtained to quantitatively investigate the evolution of the microstructure during the thermal treatment applied to the cold rolled steels. Specifically, during the metallurgical processes of recovery and recrystallization. It is remarkable that by means of these techniques recovery processes, which are not detectable by means of traditional techniques such as hardness measurements or optical metallography, can be monitored.

This doctoral thesis opens up new technological possibilities in the field of magnetic non-destructive testing techniques applied to microstructural characterization of steels. Some significant results have been published in international journals, such as Acta Materialia and Materials Science Forum.