Damage due to hydrogen embrittlement and stress corrosion cracking

Damage of metals due to the influence of hydrogen and to stress corrosion cracking is quite frequent and leads to dangerous failures as well as to loss of property and large compensational payments by insurance companies. One reason for this, is that some designers and engineers seem to lack sufficient knowledge of the basic mechanisms of these phenomena and accordingly often have only vague ideas how to prevent such failure causes. Although the basic concepts can be found in a number of good text books it seems worthwile to recall them in a short comprehensive paper.     

Monitoring hydrogen embrittlement cracking using acoustic emission technique

Acoustic emission during delayed failure of hydrogen-charged low-alloy high-strength steel has been investigated. Tests were carried out at room temperature using standard ASTM three-point bend specimens. It was found that the cumulative acoustic emission counts rose slowly in discrete steps with increasing time in the initial stage of the embrittlement process, whereas it rose rapidly in the later stage prior to fracture. It was also observed that the initial embrittlement phase consisting of microcrack nucleation is characterized by low-amplitude (35–55 dB) signals only, whereas the rapid crack growth region is marked with high-amplitude (60–100 dB) signals. These observations indicate that such a change in the pattern of cumulative counts together with the level of amplitudes of the generated signals can be used to detect the so called incubation period for hydrogen embrittlement. This kind of early detection of critical cracks may help towards better fracture control.     

Characteristics of hydrogen embrittlement,stress corrosion cracking and tempered martensite embrittlement in high-strength steels

Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diffraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel.     

Optimization of the simulation of stress-assisted hydrogen diffusion for studies of hydrogen embrittlement of notched bars

The stress-strain assisted hydrogen diffusion in metals under variable loading is concerned as a key element of elucidation of hydrogen embrittlement (HE). The suitability of simplified treatments of hydrogen diffusion in notched solids under monotonic loading is addressed comparing various 1D and 2D modeling approaches with the purpose to assess if generated approximate solutions can provide acceptable results along the diffusion depth towards prospective rupture sites, so that quite more expensive simulations may be eluded. For different geometry-and-loading cases, respective time-depth domains are revealed where certain simplified procedures can be fairly suitable to carry out calculations of metal hydrogenation for the purposes of HE analysis and control, while the choice of the optimum strategy for the stress-strain assisted diffusion simulations in notched members is case- and purpose-dependent.     

Stack cracking by hydrogen embrittlement in a welded pipeline steel

Arrays of cracks, parallel to the original plate rolling direction, were produced in a X65 microalloyed steel by hydrogen embrittlement of pipeline sections containing a weldment. A region of the heat-affected zone of the weldment was shown to have a lower yield strength (soft zone) than the surrounding material and cracking was concentrated in this throughthickness zone to produce the effect known as stack cracking. In situ cathodic hydrogen charging of tensile specimens under load led to failure by linking the rolling-plane cracks with transverse cleavage cracks, which were often initiated at inclusions. All cracking was predominantly by cleavage and failure occurred in tension in short times by hydrogen embrittlement when the applied tensile stress was above about half the uncharged yield stress. The influence of microstructure, hydrogen pressure and tensile loading conditions on the location of stack cracks and the mode of fracture is discussed.     

A comparison of hydrogen embrittlement and stress corrosion cracking in high-strength steels

The purpose of this study was to compare the known behavior of hydrogen embrittled highstrength steel to the characteristics of environmentally induced failure where hydrogen is continuously generated at the specimen surface. The incubation time for the initiation of slow crack growth was accelerated by prestressing for a fixed time below the lower critical limit. These results obtained on high-strength steel in a stress corrosion environment were directly comparable to behavior of hydrogenated specimens. These data along with hydrogen diffusivity measurements and the insensitivity of the incubation time and crack growth rate to specimen thickness indicated that the stress corrosion process was controlled by the distilled water-metal surface reaction.     

Improved resistance to hydrogen embrittlement by tailoring the stability of retained austenite

ABSTRACT

The susceptibility of hydrogen embrittlement (HE) in GCr15 bearing steel under two types of heat treatments: quenching and tempering (QT) and pre-quenching and austempering (PQA) were investigated. Results showed that PQA-treated specimen have higher mechanical stability of the retained austenite (RA) compared with QT-treated specimen. The experiment of indentation test and hydrogen bubbles suggested that PQA-treated specimen was less susceptible to hydrogen than the QT-treated sample. Subsequently, hydrogen permeation tests revealed significant differences in diffusion coefficient and the number of hydrogen trapping sites between QT and PQA specimens. It was demonstrated that PQA is an appropriate heat treatment to tailor the stability of the RA and enhances the resistance to HE of GCr15 bearing steel.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

Evaluation of stress corrosion cracking and hydrogen embrittlement in an API grade steel

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of pipeline steels in contact with soil was investigated. Different soils were prepared in order to determine their physical, chemical and bacteriological characteristics. Slow strain rate testing was carried out by using aqueous extracts from soil samples and NS4 standard solution. Stress vs. strain curves of API 5L grade X46 steel were obtained at different electrode potentials (E_corr, 100 mV below E_corr and 300 mV below E_corr) with 9 × 10⁻⁶ s⁻¹ and 9 × 10⁻⁷ s⁻¹ strain rate. In addition, the hydrogen permeation tests were carried out in order to evaluate the susceptibility of hydrogen penetrates into theses steels. The results demonstrated the incidence of cracking and their dependence on the potential imposed. In that case, cracking occurred by stress corrosion cracking (SCC) and the hydrogen embrittlement (HE) had an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant reduction of ductility.     

Stress corrosion cracking and hydrogen embrittlement cracking of welded weathering steel and carbon steel in a simulated acid rain environment

Abstract

The stress corrosion cracking (SCC) and hydrogen embrittlement cracking (HEC) characteristics of welded weathering steel and carbon steel were investigated in aerated acid chloride solution. The electrochemical properties of welded steels were investigated by polarisation and galvanic corrosion tests. Neither weathering steel nor carbon steel showed passive behaviour in this acid chloride solution. The results indicated that weathering steel had better corrosion resistance than carbon steel. Galvanic corrosion between the weldment and the base metal was not observed in the case of weathering steel because the base metal was anodic to the weldment. However, the carbon steel was susceptible to galvanic corrosion because the weldment acts as an anode. Slow strain rate tests (SSRT) were conducted at a constant strain rate of 7.87 × 10⁷ s^-1 at corrosion potential, and at potentiostatically controlled anodic and cathodic potentials, to investigate the SCC and HEC properties in acid chloride solution. The welded weathering steel and carbon steel were susceptible to both anodic dissolution SCC and hydrogen evolution HEC. However, weathering steel showed less susceptibility of SCC and HEC than carbon steel at anodic potential because of Cr and Cu compounds in the rust layer, which retarded anodic dissolution, and at cathodic potential due to the presence of Cr, Cu, and Ni in alloy elements, which inhibit the reduction of hydrogen ions. SEM fractographs of both steels revealed a quasicleavage fracture in the embrittled region at applied anodic and cathodic potentials.     

Effect of low temperature on resistance of stable austenitic steel to embrittlement by hydrogen

Abstract

Tensile straining of a ‘stable’ austenitic stainless steel at subambient temperatures has revealed deformation induced transformation to martensite reaching a maximum at about 200 K. Although the particular steel concerned is only marginally embrittled by hydrogen charging at ambient temperature, the transformation to martensite coincides with increasing embrittlement at lower temperatures. The recovery of a resistance to embrittlement below 215 K is attributed to the decreasing transport of hydrogen by moving dislocations as the temperature is further decreased.

MST/1701