The pitting corrosion behavior of 13Cr steel was investigated in simulated boiler waters containing chloride ions (Cl

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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. ...

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