Creep Rupture Properties of an Austenitic Steel with High Ductility under Multi-axial Stresses.

Abstract

Using tubular specimens of austenitic steel SUS310S with high ductility, creep rupture tests are conducted in tension, torsion and combined tension-torsion stress states at 700°C. It is found that the maximum principal stress determines the multi-axial creep rupture life of the steel. The reasons are assumed to be that many voids nucleated in the early stages of creep, and that at any creep stress states this steel showed larger deformations and continuous nucleating of voids during creep testing. Furthermore, by observing on the scanning electron micrographs, the specimens ruptured at various stresses are found to exhibit different fracture modes. These creep fracture modes and their formatting factors are discussed in detail. It is assumed that at a multi-axial stress state, the von Mises equivalent stress promotes creep deformation and nucleation of creep voids, meanwhile the growth of the voids should be restrained by the large creep deformation. It is further suggested that in a specimen with existing of the mean stress and a smaller equivalent stress, a brittle intergranular fracture should occur easily.KEY WORDS: creep; multi-axial stress; creep rupture life; fracture; ductility. Table 1. Mechanical properties of the steel used. ly. These tests were performed in air using a tensile-torsional creep test machine. The temperature was kept at 700Ϯ 1°C and was monitored with a thermocouple attached on the external surface of each specimen. Tensile creep elongation and torsional creep rotation angle were measured continuously, using a dial gauge and a rotary encoder device, respectively.For the ruptured specimens, creep fracture surfaces and their longitudinal sections were observed with a scanning electron microscope (SEM). Results Creep Rupture LivesThe von Mises effective strain e¯of the tubular specimens was calculated from the tensile elongation and the torsional rotation angle measured during creep testing. Creep curves of the steel at 4 stress states (torsional, tensile and combined stress states), showing the relation of the effective strain and time, are plotted in Fig. 2. It is found that the torsional creep specimens exhibited larger creep deformations than the others. And at each stress state, the effective strain to rupture showed a decrease tendency with decreasing the applied stress. The relation between the effective strain and time to rupture is clearly shown in Fig. 3. Among the 4 stress states, the effective strain to rupture exhibited larger values in torsion and smaller values in tension. Creep Fracture ModesBy observing the creep fracture surfaces with SEM, the creep fracture modes were examined in detail. It was found that different fractures were exhibited in the specimens under different stress conditions. Regardless of the fracture modes, it was also found that creep voids formed homogeneously in every ruptured specimen by observing its longitudinal section.Firstly, all of the present specimens in torsional creep were found to exhibit a ductile transgranular fracture in a shear mode. ...

Initiation and Growth Behavior of Creep Voids in an Austenitic Steel with High Ductility under Multi-axial Stresses.

“…14) As shown in Fig. 7, the fractions of creep voids on grain boundary lines for the specimens ruptured at the 4 stress states and under a given maximum principal stress reached almost the same.…”

Analysis of Inelastic Behavior for High Temperature Materials and Structures

“…Tubular specimen subjected to tension usually exhibit much shorter lifetime and lower ductility if compared to the case of pure torsion. This stress state effect has been observed for copper in Kowalewski (1995) and for austenitic steels in Niu et al (2002); Trivaudey and Delobelle (1993), for example.…”

Tension-Torsion Creep Fracture Behaviors of Notched Specimens of Austenitic Steel SUS310S

“…Figure 2 shows the two types of FEM models. The mechanical properties shown in Table1 and the tensional and torsional creep properties obtained in creep rupture tests on smooth specimens (8) of the steel SUS310S were used in the FEM analyses.…”

“…Some of them have been made on the creep rupture properties of the materials loaded in torsion, combined tension-torsion, internal pressure or bending stress states, and so forth (1) - (3) . It has been known that the multi-axial creep strengths are influenced not only by the materials characteristics but also by the multi-axial stress components, such as the von Mises effective stress, the maximum principal stress and so on (1), (4) - (8) . However, effects of the multi-axial stresses on the creep fracture behaviors of materials at complex stress states, which are very important for the safety and the efficiency in designing of high-temperature components, have not been made clear.…”