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.     

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 of leaded brass in a sulphuric acid environment

Free-cutting leaded brass is commonly used as sleeve fittings (also termed clamping ferrules) on polytetrafluoroethylene-lined flexible hoses for the filling and distribution of compressed gases, e.g., oxygen, nitrogen and carbon dioxide, for various industrial and medical applications. Some of the gas-filling and gas distribution facilities are located in the proximity of highly industrialized areas for the convenience of transportation, application and customer service. Therefore, the gas-filling and gas distribution gears are frequently exposed to the environment containing various chemical substances, which in the presence of ambient moisture and under the influence of mechanical and residual stresses in the material can effect an undesirable material degradation reaction. Stress corrosion cracking (SCC) has been identified to occur in C36000 Cu–Zn–Pb leaded brass ferrules under the synergistic reactions of a sulphuric acid production environment in a sustained tensile stress environment. The tensile stress was imparted to the material by the mechanical crimping process applied on the ferrules, and superimposed by cyclical high-pressure gas-cylinder-filling operations. The chemical species responsible for the SCC originated from the gaseous vapours and/or ionic derivatives of S-containing substances emitted from a neighbouring sulphuric acid production plant, which reacted with water and moisture condensates on the brass ferrule surfaces and effected the chemical corrosion reaction(s). SCC of the leaded brass ferrules gave rise to predominantly intergranular failures with fracture surfaces heavily decorated by corrosion products of various configurations. Most corrosion products were found to have embedded on the grain-boundary planes of the fracture surfaces, suggesting that grain-boundary short-circuit diffusion may have served as a viable mechanism for the SCC of C36000 leaded brass under the operating conditions of this case study.     

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.     

Stress corrosion cracking and hydrogen embrittlement of cold worked AISI type 304 austenitic stainless steel in mode I and mode III

Abstract

Comparative testing of cold worked AISI type 304 austenitic stainless steel in mode I and mode III under conditions of cathodic charging and chloride stress corrosion cracking (SCC) has been used to assess the role of hydrogen in SCC. The experimental results of these tests and those previously published have been used to deduce the mechanism and rate controlling step for SCC. The mechanism for chloride SCC in mode I is anodic dissolution of active slip planes containing hydrogen with the rate controlling step being the transport of hydrogen to these slip planes. The mechanism of SCC in mode III is tunnelling corrosion followed by overload again occurring on a plane of maximum hydrogen concentration.

MST/348     

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.     

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.     

Stress corrosion cracking of maraging steel weldments

Abstract

The stress corrosion characteristics of 18 wt-%Ni (MDN-250) maraging steel and its weldments made under different welding conditions have been investigated. The threshold stress intensity factor K _ISCC in stress corrosion conditions has been determined in 3.5 wt-%NaCl environment for the base metal and weldments. The fractured surfaces were analysed to study the types of fracture during stress corrosion cracking in base and weld metals. Fracture toughness tests were carried out and the results obtained from these tests have been compared with K _ISCC values.     

Stress corrosion cracking of Al-added mild steel

Stress corrosion cracking (SCC) experiments under constant load were conducted in boiling 55 % Ca(NO₃)₂ solution using Al-added low carbon steel. The as-normalized and decarburized specimens fractured intergranularly within 2 to 4 h, when held under constant loads of 0.44 and 0.90 of ultimate tensile strength (UTS), respectively. The precipitation of fine carbide and the segregation of P are probably responsible for the SCC of the decarburized specimen. An appreciable plastic deformation is a necessary condition for the occurrence of intergranular SCC. SEM fractography suggests that it is neither hydrogen-induced embrittlement (HE) nor stress-induced cracking, but anodic SCC (strain-induced SCC) which is involved in the SCC of mild steel in Ca(NO₃)₂ solution.     

Control of hydrogen cracking in the welded steel using microstructural traps

Hydrogen diffusion into steel can embrittle the material in H₂S environments, but this effect can be offset by suitable hydrogen trapping sites in the microstructure. Fine Ti(C,N) inclusions have been studied as the trapping sites in high strength low alloy (API X-70) welds, with Ti additions ranging from 0.004 to 0.16?wt.%. The trapping sites were investigated by electron microscopy and thermal desorption spectroscopy. Manganese sulphide particles were the main initiation sites for hydrogen induced cracking as expected. The optimum Ti addition was around 0.02% with no evidence of cracking in the weld. The estimated values of trapping activation energy for dislocations, microvoids, MnS and Ti(C, N) were approximately 25.9, 34.6, 65.1 and 87.6?kJ?mol^?1, respectively.