Title: "RELATIONSHIP BETWEEN MICROSTRUCTURE AND MECHANICAL BEHAVIOUR IN DUPLEX STAINLESS STEELS AND SIMULATION OF CRITICAL COOLING CONDITIONS"
Author: MARIA PILAR ESTEBAN PASCUAL
Date: 2009-04-30
Directors: Dr. Isabel Gutiérrez Sanz and Dr. Amaia Iza Mendia
Abstract:
Duplex stainless steels have an austenitic-ferritic microstructure that gives them a very good combination of mechanical properties and corrosion resistance, especially to stress corrosion cracking in chloride containing media. Such good properties rely on a two phase microstructure comprised of a mixture with approximately equal amounts of austenite and ferrite. However, a number of undesirable phases such as carbides, nitrides and intermetallic compounds may appear in the δ ferrite and at δ/γ interfaces if the manufacturing processes are not carefully controlled. Among these secondary precipitates, σ and χ are particularly dangerous because they can cause a dramatic deterioration of the impact toughness and corrosion resistance even at contents below 1% vol.
Consequently, one of the most critical stages in the production of duplex stainless DSS is the water quenching after the solution treatment in the biphasic temperature range. In this way, the cooling rate has to be fast enough to avoid intermetallic precipitation in the product.
For this reason, there is a great deal of interesting in knowing the cooling rate gradient along the section of a DSS product during the water quench process, particularly in heavy section pieces where significant cooling rate differences between skin and core are experienced. In the present work, finite element modelling has been used for simulating different quenching process, particularly, industrial water quenching from the solution temperature for 2205 DSS bars of 225-480 mm diameter.
For the finite element model evaluation, continuous cooling tests have been carried out in the laboratory. These tests, which reproduced the industrial quenching process, allowed for obtaining temperature-time profiles in different positions of the quenched pieces. Therefore, the valid model is that which reproduces the experimental cooling profiles in the inner of the widest range of different sections of product.
The model requires several process parameters and the thermo-physical parameters that define the material, which were obtained from the bibliography. Moreover, the heat transfer coefficient h between the piece and the quenchant is the most important parameter of the modelling. Initially, the h-value is unknown and in the present work, it has been defined by the value which provides the optimum reproduction of the experimental cooling curves.
In addition, another interesting topic is to determine the minimum cooling rate sufficient to avoid phase precipitation during cooling for a given composition of a 2205 DSS. As a first attempt, calorimetry has been used in this work. However, in the case of stainless steels, the formation of intermetallic phases takes place in the same temperature range as δ→γ transformation, thus this technique is not suitable for this purpose. Instead of using calorimetry, the critical cooling rate has been determined experimentally. In this way, controlled continuous cooling tests have been performed in the laboratory and the detection and distinction of the first precipitates of σ and χ phases has been done metallographycally. In this sense, both phases were identified by composition with the EDS technique and by crystallography with the EBSD technique.
Finally, the mechanical behaviour of several 2304 duplex stainless steels without any intermetallic phases σ and χ was investigated. The influence of the phase balance, the microstructural anisotropy and the rolling texture on room temperature tensile properties and on impact toughness at subzero temperatures, were analysed.
Special attention has been given to the mechanical properties of two constituents, ferrite and austenite, with the purpose of finding a law of mixtures that is able to predict the global yield strength of the biphasic steel. Mechanical anisotropy is explained on the basis of both, the microstructural anisotropy and the ferrite rolling texture. The absorbed energy and the lateral expansion were related to the fraction of ductile fracture and the presence of delamination on the fracture surface of the Charpy specimens. The fracture characterization of the tested specimens showed that damage mainly propagates and grows through the ferritic matrix while austenite aggregates act as obstacles. In this sense, the amount and distribution of δ/γ interfaces strongly affect the final failure of the steel. On this basis, it is expected that variations in the percentage of hot reduction applied to a duplex stainless steel implies a different mechanical behaviour which is also studied in this thesis.
