PSI - Issue 2_A

G Sudhakar Rao et al. / Procedia Structural Integrity 2 (2016) 3399–3406 G.S. Rao et al./ Structural Integrity Procedia 00 (2016) 000–000

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specimens were carried out at 250 °C and 288 °C for 20 MnMoNi 55 steel and at 288 °C for 22 NiMoCr 3 7 steel between strain rates 10 -4 s -1 and 10 -1 s -1 . Cathodic hydrogen charging of the tensile samples was done in galvanostatic mode at room temperature in solution of 1N H 2 SO 4 and 30 mg/l As 2 O 3 for 7h, exposing only gauge section at a current density of 10 mA/cm 2 . Prior to hydrogen charging, the gauge section of the sample was polished with 1000 grit paper and ultrasonically cleaned for 5 minutes. To reduce the effusion of the hydrogen from the surface, after charging the sample was polished with 1000 grit paper and elctrolytically coated with Cu at 5V for 8 10 minutes in CuCN (22.5g/l), NaCN (33.7g/l) Na 2 CO 3 (15g/l) and Na 2 S 2 O 3 (0.2g/l) solution (pH=12). All the samples were taken to the tensile testing within 15-20 minutes after charging. Test temperature was reached typically within a period of 60-90 minutes. At the desired temperature the sample was held for 15 minutes prior to start of the test. Fracture morphology of both steels after fracture was analyzed by light microscopy and scanning electron microscopy (SEM). In a previous investigations on the 22 NiMoCr 3 7 steel, the total hydrogen content of the steel after 7h charging at 10mA/cm2 was found by hot gas extraction to be about 5.5 wppm at the beginning of the heating phase of the tensile tests, Roychowdhury (2016). Owing to similar microstructure and grain size, we expect a similar level of initial hydrogen concentration in the 20 MnMoNi 5 5 steel. Hydrogen content in the high-temperature tensile tests is expected to be significantly lower due to partial effusion of trapped hydrogen during heating and at test temperature, despite copper coating. The total hydrogen contents hereby decrease with decreasing strain rates due to the longer test duration at the test temperature of 250 or 288 °C. The mean diffusion distance x = (2·D·t) 1/2 of hydrogen is about 4 to 8 mm at 288 °C in 1 hour, Roychowdhury (2016). The time for heating and performing the tensile test varied between 1.3 (10 -2 s -1 ) to 2 hours (10 -4 s -1 ), a sufficient amount of lattice hydrogen is thus still available for interactions with the dislocations in the high-temperature tensile tests down to strain rates of 10 -5 s -1 . The release and desorption rate is strongly dependent on the nature (trap energy) and concentrations of traps and can be significantly retarded in case of high density of strong traps. Furthermore, during deformation new traps are produced. These effects result in a complex overall behaviour and the lattice hydrogen activity can thus significantly vary during a test. 3. Results 3.1. Dynamic strain aging behavior of 20 MnMoNi 5 5 steel The 20MnMoNi 55 steel revealed the typical DSA features in tensile tests as discussed by Hänninen (2001). The variation of yield stress  0.2% and ultimate tensile strength  UTS with temperature at strain rates of 10 -3 to 10 -5 s -1 is shown in Fig.1a. A peak in both yield stress and tensile strength and maximum in work hardening rate is observed between 200 and 400 °C that is shifted towards lower temperatures with decreasing strain rate. Similarly, a minimum in ductility and reduction of area (with a relative drop of reduction of area of ~ 10 %) has been observed between 150 and 350 °C. In the peak temperature range, the ultimate tensile strength is increasing with decreasing strain rate (negative strain rate sensitivity). The calculated strain rate sensitivity index m (  = K  d  /dt m ) at different true strain values 0.2%, 1%, 2.5% and 5% and temperatures is shown in the Fig. 1b. The strain rate sensitivity index m becomes negative between 200 and 300 °C with a minimum at 250 °C. Furthermore, serrated plastic flow was observed in this region. The strain rate/temperature range with serrated flow is shifted to lower strain rates/lower temperatures with decreasing temperatures/strain rates, respectively. The activation energy for the disappearance of serrated flow was found to be 1.7±0.3 eV, which is approximately equal to the sum of the activation energy for diffusion of C or N in bcc iron (~ 0.8 eV) and their binding to the dislocation cores (~ 0.8 eV), Hänninen (2001). Thus, the DSA and serrated plastic flow in this steel can be attributed to the interaction of dislocations with both interstitial C and N. Very similar DSA effects were observed in the same temperature-strain rate ranges in the previously tested 22 NiMoCr 3 7 steel with low DSA susceptibility, whereas the amplitude of DSA effects on strength and ductility at 250 and 288 °C were significantly smaller due to the lower free C and N contents in this steel, Roychowdhury (2016), Ritter (2016). The hardening by DSA as characterized by the relative increase of the normalized ultimate tensile strength  UTS (T) / G(T) from 25 to 288 °C at strain rates of 10 -4 and 10 -3 s -1 was a factor of ~ 1.5 to 4 higher