Hydrogen embrittlement of Microalloyed structural steels

The effect of Cr, Mo, V, Nb, Mn, their carbides and different microstructures on the corrosion and hydrogen uptake of two fine-grained microalloyed steel grades has been investigated. In practice, fine-grained structural steels appear to be more or less susceptible to hydrogen induced cracking, probably because of various strong interactions between the absorbed hydrogen and the various alloying elements or their carbides, acting as traps. The effect of microstructure was examined by cathodic polarization, by measuring the reduction of area, and by SEM examination of the fracture surface. In addition, hydrogen permeation transients for one microalloyed steel grade and Armco iron were measured, and the resistance of both steel grades to sulphide stress-cracking according to the NACE Standard TM-01-77 was also determined.     

Hydrogen embrittlement of high strength pipeline steels

A comparison was made between three API grade pipeline steels (X60, X80 and the X100 grade) from the point of view of their susceptibility to hydrogen embrittlement. The main aim was to determine whether the development of higher strength materials led to greater susceptibility to hydrogen embrittlement. This was achieved by straining at 2.8 × 10⁻⁵ s⁻¹ after cathodic charging. The results showed that there is a distinct susceptibility to loss of ductility after charging and this tends to increase with the strength level of the steel at a charging current density above 0.44 mA mm⁻². All three steels exhibited fine cracks parallel to the major rolling direction after charging and an increasing amount of brittleness on the fracture surface.     

Hydrogen embrittlement susceptibility and permeability of two ultra-high strength steels

Slow displacement rate tensile tests were carried out in a saturated H₂S solution to investigate the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of two ultra-high strength steels (PH 13-8 Mo stainless steel and T-200 maraging steel). Hydrogen permeation properties were determined by an electrochemical permeation method. The results of permeation tests indicated that over-aged specimens showed a lower diffusivity/hydrogen flux and higher solubility than those solution-annealed. The great increase in reverted austenite (irreversible hydrogen traps) together with numerous precipitates at the expense of dislocations (reversible) in the over-aged specimen led to such a change in permeability. Ordinary tensile tests indicated that four tested specimens had roughly the same yield strength level. Hence, the hydrogen embrittlement susceptibility of the material could be related to their permeation properties. The uniform distribution of strong hydrogen traps in over-aged specimens instead of weak traps in the solution-annealed impeded the hydrogen transport toward the strained region, thus, the resistance to sulfide stress corrosion cracking was improved in over-aged specimens.     

回火马氏体钢中氢的扩散行为及其氢脆敏感性

通过TDS方法研究了氢在18Cr2Ni4W和25CrNi2MoVNb两种低温回火马氏体钢中的扩散行为,同时结合慢应变速率拉伸实验研究了这两种马氏体钢的氢脆敏感性。结果表明,与18Cr2Ni4W钢相比,25CrNi2MoVNb钢因碳含量较高和晶粒显著细化作用使抗拉强度从1300 MPa级提高到了1500 MPa级后,其氢脆敏感性也明显增加。通过试样充氢后放置试验,测定氢在25CrNi2MoVNb钢和18Cr2Ni4WA钢中的扩散系数分别为7.87×10-7 cm2/s和3.99×10-7 cm2/s。可见,氢在25CrNi2MoVNb钢中更容易扩散,因而在充入相同可扩散氢时,25CrNi2MoVNb钢性能损失更大。     

Hydrogen embrittlement of ferritic steels: Observations on deformation microstructure,nanoscale dimples and failure by nanovoiding

While hydrogen embrittlement of ferritic steels has been a subject of significant research, one of the major challenges in tackling hydrogen embrittlement is that the mechanism of embrittlement is not fully resolved. This paper reports new observations and interpretation of fracture surface features and deformation microstructures underneath the fracture surface, providing a mechanistic view of failure catalyzed by hydrogen. Linepipe grade ferritic steels were tested in air with electrochemically pre-charged hydrogen and in high-pressure H₂ gas. The fracture surface features were studied and compared using high-resolution surface-sensitive scanning electron microscopy, and the deformation microstructures just beneath the fracture surfaces were studied using transmission electron microscopy. Significant dislocation plasticity was observed just beneath both ductile and quasi-brittle fracture surfaces. Further, the dislocation activity just beneath the fracture surfaces was largely comparable with those observed in samples tested without hydrogen. Evidence for hydrogen-enhanced plastic flow localization and shear softening on the sub-micron scale was observed very near the final fracture surface (<2 μm) in the tensile samples. The quasi-brittle fracture surfaces were found to be covered with nanoscale dimples 5–20 nm wide and 1–5 nm deep. Based on analyses of conjugate fracture surfaces, most of the nanodimples appear to be “valley-on-valley” type, rather than “mound-on-valley” type, indicating nanovoid nucleation and growth in the plastically flowing medium prior to ultimate failure. Based on these observations, an alternative scenario of plasticity-generated, hydrogen-stabilized vacancy damage accumulation and nanovoid coalescence as the failure pathway for hydrogen embrittlement is proposed.     

Hydrogen embrittlement of construction materials

Hydrogen embrittlement (HE) is considered to be part of stress corrosion cracking (SCC) problem and as a phenomenon of its own. In the first part of the review the present day methods for an identification of the role of HE in SCC are discussed and a general approach for any construction material is proposed to solve this question. The second part is concerned with those aspects of the HE theory which; are still not clear (acoustic data, the disposition of the fracture zones by HE ahead of the crack tip, critical hydrogen concentration) and with some newly proposed experiments.     

Hydrogen transport and embrittlement in 300 M and AerMet100 ultra high strength steels

This paper describes how hydrogen transport affects the severity of hydrogen embrittlement in 300 M and AerMet100 ultra high strength steels. Slow strain rate tests were carried out on specimens coated with electrodeposited cadmium and aluminium-based SermeTel 1140/962. Hydrogen diffusivities were measured using two-cell permeation and galvanostatic charging methods and values of 8.0 × 10⁻⁸ and 1.0 × 10⁻⁹ cm² s⁻¹ were obtained for 300 M and AerMet100, respectively. A two-dimensional diffusion model was used to predict the hydrogen distributions in the SSR specimens at the time of failure. The superior embrittlement resistance of AerMet100 was attributed to reverted austenite forming around martensite laths during tempering.