PSI - Issue 5

J. Belzunce et al. / Procedia Structural Integrity 5 (2017) 1275–1282 J.Belzunce et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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All these microstructures correspond to tempered martensite. The profuse carbide precipitation that takes place during the tempering stage is clearly seen. As the tempering temperature increases, internal residual stresses are released, restauration and recrystallization phenomena take place, dislocation density and martensite lath boundaries decrease. Additionally, carbides globulize and their size increases, given rise to a more uniform carbide distribution [5]. Moreover, as tempering temperature increases, the steel hardness and strength decrease, while elongation decreases. The hydrogen desorption at room temperature of each steel grade is shown in Fig. 2. These figures represent the total hydrogen content of each sample versus the exposure time at room temperature. The initial hydrogen content of each grade, C H0 , corresponds to the first points of the curves. The residual hydrogen, C Hf , is the hydrogen strongly trapped in the steel microstructure, hydrogen content after a long exposure at room temperature, being the diffusible hydrogen the amount that is able to get out from traps and diffuse out of the steel in long terms (C H0 -C Hf ). In line with the microstructural changes previously observed, when the tempering temperature increases, the initial and residual contents of hydrogen decrease (there are a lower density of traps and strong traps). Fig. 2 shows the results obtained with the four steel grades. 3.2. Hydrogen desorption curves

C H0 =1.9ppm C Hf =1.5ppm C H0 -C Hf = 0.4ppm

(a)

(b)

C H0 =1.3ppm C Hf =0.6ppm C H0 -C Hf = 0.7ppm

C H0 =0.6ppm C Hf =0.2ppm C H0 -C Hf = 0.4ppm

C H0 =1.2ppm C Hf =0.3ppm C H0 -C Hf = 0.9ppm

Fig. 2. Hydrogen desorption curves for (a)42CrMo4 and (b)2.25Cr1Mo. Initial (C H0 ), residual (C Hf ) and diffusible (C H0 -C Hf ) hydrogen content.

The hydrogen contents measured in these steel grades after thermal pre-charging and along the desorption process do not differ much from the values measured when steel specimens are evaluated under very high hydrogen pressure) which is a testing methodology closer to normal final applications [6, 7]. 3.3. Tensile tests on smooth specimens Tensile tests on smooth hydrogen pre-charged specimens were performed on each steel grade. No significant difference was observed between tests carried out at different displacement rates. Fig. 3 shows the embrittlement index corresponding to mechanical strength and reduction in area as a function of the steel hardness (HB), obtained under a displacement rate of 0.04mm/min. The EI related to the strength remains almost constant with an approximate value of 2%, and regarding the reduction of area, EI is 0% for hardness values lower than 200HB, but above this value, EI has a clear tendency to increase with hardness. Anyway, it could be said that tensile properties of hydrogen pre-charged smooth specimens remained practically unaffected independently of the rate of load application.