13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Mechanisms for cleavage fracture in duplex stainless steels Guocai Chai1,2* and Peter Stenvall1 1Strategic Research, Sandvik Materials Technology, 811 81 Sandviken, Sweden 2Engneering Materials, Linköping University, 581 83 Linköping, Sweden * Corresponding author: guocai.chai@sandvik.com Abstract Duplex stainless steels that have two phases with different mechanical and physical properties exhibit micro deformation heterogeneity and localized cracking or fracture in nature. In this study, the cleavage fracture behaviors in two super duplex stainless steels have been investigated in the following three conditions: at low temperatures, the material with spinodal decomposition and that with other precipitates and defects. By both impact toughness and fracture toughness tests, the ductile to brittle transition and cleavage behaviors have been investigated. The fracture is analyzed using a SEM with EBSD. The fracture mechanisms are focused; mainly the parameters or factors that cause the occurrence of cleavage in the materials. Since the austenitic phase in the material in these conditions is still tough, a coupling effect will be discussed to explain the cleavage in the austenitic phase observed. Local approach to fracture is used to assess or discuss the influence of local hardness, precipitates and stress concentration on the formation of cleavage fracture in the materials in different conditions. Keywords Duplex stainless steel, cleavage, local approach method, spinodal decomposition, fracture 1. Introduction Multiphase materials due to their microstructures and excellent properties are becoming more attractive for both engineering applications and academic interests. Duplex stainless steels (DSS) are a group of steels that consist of approximately equal volume of austenite and ferrite. Due to a good combination of excellent corrosion resistance and high mechanical properties, they are increasingly employed in various industries [1-4]. Duplex stainless steels have two phases with different mechanical and physical properties such as modulus of elasticity, yield strength and deformation hardening rate, and therefore exhibit micro deformation heterogeneity [5-13]. As a result, both stress and strain are not uniformly distributed at the phases and the actual load sharing on the microscopic scale is dependent on the property mismatch and microstructural features. It is believed that the difference in the elasto-plastic properties between the phases and the coupling effect, i.e., the load and strain sharing between the phases, is largely responsible for the varying elasto-plastic deformation mechanisms with varying plastic strain ranges in DSSs [8]. The phase-specific stresses, i.e., the total stresses that the constituent phases are subjected to is the sum of macrostresses, corresponding directly to the applied stresses, and microstresses due to micromechanical responses. The importance of micromechanical interactions under mechanical load has been recognized [8, 9]. Recently, the micro yielding and damage behavior of the austenitic and ferritic phases in duplex stainless steels have been studied by in-situ X-ray and neutron diffraction and multiscale modelling [8-13]. Fracture in duplex stainless steel is a local process in nature. Several works on local approach to fracture have been done to discuss the fracture in duplex stainless steels [14-17]. However, less
RkJQdWJsaXNoZXIy MjM0NDE=