Mechanical Properties of a Thermally-aged Cast Duplex Stainless Steel by in Situ Tensile Test at the Service Temperature

Abstract

Z3CN20.09M cast duplex stainless steel (CDSS) has been used for primary coolant water pipes in pressurized water reactors due to its excellent mechanical properties. Such pipes operate at an elevated service temperature (~320 °C) and experience issues of thermal aging embrittlement. In situ tensile tests were conducted to investigate the deformation mechanisms of Z3CN20.09M CDSS after long-term thermal aging at 475 °C for up to 2000 h in both optical microscope and scanning electron microscope at 320 °C. For the 320 °C tests, the tensile stress and other mechanical properties, e.g. the yield stress and the ultimate tensile strength, increase during the thermal aging process and recover to almost the same level as the unaged condition after annealing heat-treatment, which is caused by the formation and dissolution of precipitation during aging and anneal heat-treatment, respectively. For the slip mechanism, straight slip lines form first in the austenite phase. When these slip lines reach the austenite/ferrite interface, three kinds of slip systems are found in the ferrite phase. During the fracture process, the austenite phase is torn apart and the ferrite phase shows a significant elongation. The role of the ferrite phase is to hold the austenite matrix, thus increasing the tensile strength of this steel.

Three-Dimensional (3D) Microstructure-Based Modeling of a Thermally-Aged Cast Duplex Stainless Steel Based on X-ray Microtomography, Nanoindentation and Micropillar Compression

“…For example, at the applied strain of 20%, the tensile stress is ~900 MPa for the unaged condition and ~1100 MPa for the aged condition for the tensile loading Both in Figures 8a and 8b, it can be concluded that the aged at 475 • C for the 2000 h condition has a higher value in tensile stress than the as received condition. Such phenomenon was also found in the real in-situ tensile experiments of this steel [9]. As the mechanical properties of the austenite phase remain the same after long-term thermal aging [8,34,36], such an increase seen in bulk behavior can be explained by the hardening of ferrite phase during the aging process [8,34].…”

“…In Figure 8, the simulated stress is higher than the tensile experiments conducted on the same material that was published before [5,9]. This can be explained by two reasons.…”

“…Thus, researchers can combine the constitutive relationships obtained at different aging conditions with the same initial 3D microstructure to simulate the mechanical behavior at different aging conditions, which will be helpful to investigate the effect of thermal aging individually. It is already well established that the evolution on mechanical properties on CDSS among different aging conditions is caused by the spinodal decomposition and the G-phase precipitation [7][8][9][32][33][34].…”

“…The input mechanical properties obtained from the nanoindentation and micropillar compression tests were all conducted at room temperature. It has been reported that working at high temperatures will cause degradation on the mechanical properties [5,9,37,38], so as to reduce the tensile stress. Secondly, many researchers [39][40][41][42][43][44] have reported the phenomenon of "size effect" from micropillar compression tests, i.e., the compression stress of the material increases with the decrease of the diameter of the micropillar.…”

“…Such pipes made of CDSS are designed for a 40-year service life under high-temperature (~300 • C) and high-pressure (~15 MPa) conditions [5]. During service, these pipes experience issues of both frequently thermal impact loading and thermal aging embrittlement [6][7][8][9]. In order to get a deeper understanding of the relationship between microstructures and mechanical properties and to get some information that could not be achieved in typical macro tests, many researchers have conducted tensile tests on the micro-scale of duplex steels with both the unaged and aged conditions [5,[9][10][11][12][13].…”