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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4. Conclusions

The hydrogen embrittlement susceptibility of 42CrMo4 and 2.25Cr1Mo steels subjected to various heat treatments was evaluated in this work and the following conclusions can be drawn.  As the tempering temperature of quenched and tempered steels increases, residual stresses are released, restauration and recrystallization phenomena take place, dislocation density and martensite lath boundaries decrease, carbides precipitate, globulize and their size increases, given rise to a more uniform carbide distribution. Due to all these microstructural changes, the steel microstructure gains capacity to store and retain hydrogen (the initial and residual hydrogen contents increase), but the amount of diffusible hydrogen decreases.  Thermal hydrogen pre-charging performed at 450ºC under a hydrogen pressure of 19.5 MPa was seen to be a suitable method to study hydrogen embrittlement of steels using pre-charged specimens. Initial hydrogen contents of 0.6-1.9 ppm and residual hydrogen content after a large stay at room temperature of 0.2-1.4 ppmwere measured in the steel grades used in this work.  Tensile tests on smooth specimens are not suitable to evaluate HE. Tensile properties of hydrogen pre-charged smooth specimens remained practically unaffected, independently of the rate of load application. Nevertheless, when the triaxiality of the specimen was increased by means of a sharp notch or even better by a crack, the deleterious effects of hydrogen were better seen. Hydrogen accumulates in those areas of high triaxiality such as crack or notch tips, which fosters the possibility of reaching a critical hydrogen concentration that triggers premature failure. Additionaly, the lowest rates of load application gave rise to the highest embrittlement indexes, due to the fact that hydrogen atoms have more time to diffuse and attain the process zone.  The effects of hydrogen were especially significant in the fracture toughness tests, where at least a 50% decrease of the J Q crack initiation parameter was measured, as a consequence of the presence of internal hydrogen. Nevertheless, the J resistance curves of the grades with the lowest strength (with significant internal hydrogen contents) are quite high and ductile fracture mechanism maintained. By the contrary, the grades with the highest strength showed intergranular fracture, and in the case of 42CrMo4, with yield strength of 1086 MPa, fracture toughness dramatically decreased.

Acknowledgements

The authors would like to thank the Spanish Ministry of Economy and Competitivity for the support received for the development of the research project MAT2014-58738-C3 (SAFEHIDROSTEEL) and L. Borja Peral the financial aid given by FEDER and the Asturias government, through the project FC-15-GRUPIN14-001.

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