INVESTIGATIONS ON THE TOPIC OF RESIDUAL STRESSES AND DISTORTION IN
FERRITIC-AUSTENITIC DUPLEX STAINLESS STEELS
A. Bracke, T. Eutebach and M. Pohl — Ruhr-University Bochum, Institute of Materials
ABSTRACT
Due to their high corrosion resistance and the good combination of their mechanical properties, fer-
ritic-austenitic duplex stainless steels are used in many technical fields (e.g. plant engineering, power
plant technology, environmental technology). The microstructure of these alloys consists of ap-
proximately the same amount of ferritic and austenitic constituents. Both phases are characterised
by different thermal strengths and thermal expansion coefficients. To obtain optimal corrosion resis-
tance of these materials, a solution heat treatment is required. As a result of such heat treatments,
the component shows thermally-induced distortion due to the specific interactions of these phases.
This phenomenon is highly undesirable, and it has not yet been possible to assess distortion empiri-
cally. In this context, corrosively-stressed centrifuges used in environmental technology are men-
1 MDR VER tioned as an example of this distortion problem. In order to assess distortion, the irreversible amount
of deformation of three typical duplex alloys is quantified for different thermal cycles which are
relevant to production. Depending on the temperature, permanent extensions and contractions of the
ing of o nedles sample material develop after the heat treatment. By means of a characterisation of the developing
microstructural changes and the specific inherent stress conditions of the phases, basic approaches
the structure is to assess the thermally-induced deformation behaviour can be obtained.
2 tn films of
nen Strength and INTRODUCTION
Increasingly great demands are made on materials used in constructive shaping in plant and material
engineering. The materials must show wide applicability with regard to their strength and corrosion
resistance. Considering these aspects, ferritic-austenitic duplex steels have been more and more es-
tablished on the market in the past years. Although production and processing of the relevant alloys
has been optimised several times, some problems in processing these alloys still remain due to their
complex transformation and precipitation behaviour (1). Most cases require a solution heat treat-
ment in order to remove possible precipitations. In the case of the 1.4462 forgeable alloy which had
been investigated plastic deformations and changes of the phase geometry were determined by
means of thermal alternating stress. In the course of the following investigations, the thermally-
induced distortion and the microstructural changes which are connected to this will first of all be
measured (2). Starting out from these data, it is possible to determine the phase-specific residual
stresses by developing an optimised FEM model (3,4). The results are to improve the general un-
derstanding of thermally-induced microstructural interactions in ferritic-austenitic duplex steels.
TEST MATERIALS
For the investigation programme, the 1.4462 duplex steel was chosen. The two-phase forgeable al-
loy ofthe X 2 CrNiMoN 22 5 3 type is the variant which has been most thoroughly investigated
and has partly been standardised already. The exact composition of the test material is listed in
Tab. 1.
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