Thermo-mechanical Fatigue of Austenitic Steel

P. J. Szabó - J. Ginsztler

TU Budapest, Dept. Materials Science and Engineering ,Budapest, Hungary

Most of the materials applied in the power industry are generally subjected to low-cycle thermo-mechanical fatigue due to the increased loading. Some structural elements of the power plants, such as weldings, pipes, joints, etc., act as stress-concentrators, i.e. the local mechanical stress exceeds the outer stress at these points. That's why it is important to know how the material behaves at a given high amplitude mechanical and/or thermo-mechanical periodic loading.

This phenomena was modeled under laboratory conditions. A simple thermo-mechanical fatigue machine was developed in order to produce thermo-mechanically fatigued samples. Samples were made of AISI 316 austenitic stainless steel, which is very often used in the power industry. Samples were heated to a certain temperature by electric current, and then water quenched. One cycle therefore means one heating and one quenching.

Since the thermal expansion of the samples was restricted, mechanical stress arose on the samples. Cyclic repetition of this led to the failure of the material. Samples were kept "failured" when the average crack length reached the so-called engineering value, i.e. 0,1 mm. The full lifetime of the material was then defined as the number of thermo-mechanical cycles belonging to the failured state.

After that another set of samples were fatigued up to a certain part of the lifetime. The propagation of the damage was then investigated by classical metallography, tensile test and surface analysis. The change of the microstructure was followed by scanning and transmission electron microscopy.

It was observed that cracks appeared after the first 500 cycles. After 4000 cycles, however, the material was generally fully damaged, since the average crack length reached the 0,1 mm. Cracks generally propagated along the grain boundaries. The dislocation density increased continuously, and at higher lifetimes the dislocations formed band-structure. On the grain boundaries carbide precipitation appeared. These carbides were so small that they were only seen by transmission electron microscopy.

References:

J. Ginsztler, A. Penninger, L. Szeidl, P. Várlaki: Stochastic Modelling of Stress Processes in Power Plant Boil Walls, Int. J. of Pressure Vessel and Piping (62) 1996 p. 119-124.
J. Szabó-T. Ungár.: Investigation of the Dislocation Structure and Long-Range Internal Stresses Developing in an Austenitic Steel During Tensile Test and Low Cycle Fatigue, Periodica Polytechnika, vol. 40. no. 2. pp. 113-120, 1996.

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