PSI - Issue 23

Roman Petráš et al. / Procedia Structural Integrity 23 (2019) 209–214 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

210

2

applied the damage mechanism is environmentally related as well , Polák (1991) . The transition from cycle-related damage to time-related damage occurs when increasing the temperature of specimen exposure. The damage mechanism is mainly associated with the creep and environmental effects, Christ (2013). Generally, the austenitic stainless steels are prone to the intergranular failure at elevated temperature due to preferential grain boundary oxidation and its subsequent cracking. The model proposed is regularly termed as SAGBO – Stress Assisted Grain Boundary Oxidation. The mutual reaction of the diffusing oxygen with an alloying element leading to the oxide layer formation ahead of the crack tip at the grain boundaries. Accordingly, the embrittling effect is associated with the rapid cracking of the oxide developed resulting in intergranular crack propagation, see Kang et al. (1995). In the case of LCF loading in high vacuum the environmental effect is suppressed. Given that, the creep damage is expected to be dominant. The effect of the environment has been investigated on various materials subjected to different loading conditions, e.g. Christ (2007), Zauter et al. (1994), Christ et al. (2016). The main intention of this contribution is to shed the light on the principal differences in damage mechanism for LCF experiments conducted in the air and in high vacuum.

2. Experimental

2.1 Material

The material involved in the study of the environmental effect on the damage mechanism under LCF loading conditions was Sanicro 25 steel. Sanicro 25 grade was provided by Sandvik, Sweden in the form of cylindrical rod. The material was designed for use in re-heaters and super-heaters in power plants working at temperatures up to 700 °C. The chemical composition of the material in wt.% is 0.1 C, 22.5 Cr, 25.0 Ni, 3.6 W, 1.5 Co, 0.5 Mn, 0.5 Mn, 0.5 Nb, 0.23 N, 0.2 Si and the rest Fe. The specimens in the shape of cylinder having gauge length of 16 mm and diameter of 7 mm were fabricated. The specimens were heat treated by solution annealing at 1200 °C for one hour followed by cooling in the air before final machining. Subsequently, the gauge length was mechanically and electrolytically polished.

2.2 Mechanical testing

The LCF procedures were conducted utilizing standard servohydraulic testing machine MTS 880. One of the MTS system was facilitated with a vacuum chamber allowing to reach the pressure down to 5x10 -5 mbar. Symmetric tension-compression loading cycle with total strain control and constant total strain rate 2x10 -3 s -1 was applied at 700 °C using inductive heating system.

2.3 Study of the fatigue cracks

The crack nucleation mechanisms were identified by means of scanning electron microscope (SEM) Tescan Lyra 3 XMU FESEM equipped with focused ion beam (FIB). In depth-profile of the crack was revealed by FIB cutting. In order to determine the internal damage of the strained specimens, longitudinal cross-sections parallel to the stress axis were produced, polished and subsequently subjected to the SEM investigation.

3. Results

3.1 Cyclic stress-strain response Mechanical response of the material subjected to the LCF loading conditions at 700°C in a ir as well as in vacuum strained with total strain amplitude 7x10 -3 is depicted in Fig.1. Cyclic hardening/softening curves exhibit rather similar response to both straining conditions implemented. The tendency of rapid cyclic hardening followed by reaching the saturation of the stress is evident. Even though the character along with the hardening rate is very similar for both loading conditions; the LCF test in the air resulted in higher stress response and lower fatigue life.