PSI - Issue 2_A

3402 G Sudhakar Rao et al. / Procedia Structural Integrity 2 (2016) 3399–3406 G.S. Rao et al. / Structural Integrity Procedia 00 (2016) 000–000 in the 20 MnMoNi 5 5 steel steel, respectively. The relative decrease of the reduction of area by a decrease in strain rate from 10 -3 to 10 -5 s -1 at 250 °C was  20 times larger in the 20 MnMoNi 5 5 steel.

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Ultimate tensile strength  UTS [MPa]

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400 Yield stress  0.2% [MPa] 450 500

20 MnMoNi 5 5

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YS UTS d  /dt

1E-5 s -1 1E-4 s -1 1E-3 s -1

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Temperature [°C]

Figure 1 (a): Variation of yield strength (σ 0.2% ) and ultimate tensile strength (σ uts ) with temperature at strain rates of 10 (b): variation of strain rate sensitivity index (m) with temperature at strain rates of 10 -3 and 10 -4 s -1 .

-5 s -1 to 10 -3 s -1 .

3.2. Effect of hydrogen on mechanical properties in the DSA range The effect of hydrogen charging on the tensile properties of 20 MnMoNi 5 5 at 250 °C and 288 °C for different strain rates between 10 -4 s -1 and 10 -1 s -1 are shown in Fig. 2 and the comparison with 22 NiMoCr 3 7 are shown in Fig. 3. A negative strain rate sensitivity effect on yield stress and ultimate tensile strength due to DSA is seen with and without hydrogen at both temperatures in both steels. Figure 2a & 3a shows a decrease in the ultimate tensile strength (softening) in the hydrogen charged samples. Furthermore, a marginal increase (hardening) of yield stress of a few MPa (~ 0.5-1 %) was observed after hydrogen charging that is in the typical specimen to specimen scatter range for RPV steels. A more pronounced but still moderate reduction of the ultimate tensile strength of a few 10 MPa (~ 5-10 %) and strain/work hardening rate occurred after hydrogen charging. The decrease in ultimate tensile strength was more pronounced at 288 °C and there appeared to be a maximum in softening and scatter at a strain rate of 10 -2 s -1 at both temperatures that might be related to the matching of dislocation and hydrogen mobility. Thus hydrogen would amplify the DSA reduction in work hardening at these temperatures. A similar behavior was also observed in the hydrogen charged 22 NiMoCr 3 7 steel, i.e. a marginal and moderate reduction of yield stress and tensile strength after hydrogen charging, respectively, Roychowdhury (2016).

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Figure 2: Variation of tensile properties at 250 and 288 °C for different strain rates with and without hydrogen in 20 MnMoNi 5 5 (a) ultimate tensile strength (σ UTS ), (b) uniform elongation (c) reduction of area. Figure 2b and 2c shows the effect of hydrogen on uniform elongation and reduction in area respectively between strain rates of 10 -4 and 10 -1 s -1 at 250 °C and 288 °C with and without hydrogen. The effect of hydrogen on the uniform elongation and reduction of area is much more pronounced than on yield stress and ultimate tensile strength. A drastic drop of uniform elongation and reduction of area and significant embrittling effects are seen after