Influence of residual stresses and strains generated by cold drawing on hydrogen embrittlement of prestressing steels

Hydrogen embrittlement plays an important role in environmental cracking of prestressing steel wires. The ammonium thiocyanate test (ATT) was implemented as a standard for determination of their susceptibility to environmentally assisted fracture. However, ATT reveals neither how hydrogen induced fracture (HIF) goes on in steel nor the roles of important manufacturing and service factors. To this end, the knowledge of the residual stresses and plastic strains in wires due to cold drawing, as well as the hydrogenation from harsh environments, are the keys to successful predictions of wire lives. This paper advances previous analyses of HIF in cold drawn prestressing wires via numerical modelling, firstly, of the cold drawing process to obtain the distributions of residual stresses and plastic strains, and secondly, of the stress-strain assisted hydrogen diffusion in wires towards creation the conditions for HIF nucleation. Generated results prove the relevant role of residual stress-and-strain field in hydrogen diffusion in the wires, as well as their possible consequences for HIF.     

Effects of tungsten on the hydrogen embrittlement behaviour of microalloyed steels

The effects of tungsten (W) additions (0, 0.1, 0.5 and 1 wt.%) on the hydrogen embrittlement behaviour of microalloyed steels were systematically investigated by means of slow strain rate tests on circumferentially notched cylindrical specimens, and the mechanism of hydrogen-induced embrittlement was discussed. W addition is found to increase the activation energy of hydrogen desorption. Microstructural features affect the hydrogen embrittlement behaviour and fracture modes of microalloyed steels. It is suggested that the hydrogen-induced embrittlement in the studied microalloyed steels with different W additions is caused by the combined effects of decohesion and internal pressure in the presence of hydrogen.     

Effect of residual stresses on hydrogen permeation in iron

The effect of residual stresses on electrochemical permeation in iron membrane was investigated. Four thermal and mechanical treatments were chosen to obtain different surface states in relation to the residual stresses.Residual stresses were determined by X-ray diffraction (XRD) using the Macherauch and Müller method. The results were completed by the microhardness measurements. For all iron membranes, compressive residual stresses were obtained.Electrochemical permeation experiments using a Devanathan and Stachurski cell were employed to determine the hydrogen permeation behaviour of the various iron membranes. The latter was charged with hydrogen by galvanostatic cathodic polarization in 0.1 M NaOH at 25 °C. The experimental results revealed that hydrogen permeation rate increases with increasing residual stresses introduced in iron membranes.     

Role of microstructure, composition and hardness in resisting hydrogen embrittlement of fastener grade steels

The degree of hydrogen embrittlement for several fastener grade steels has been determined. While microstructural alteration resulted in some improvement in resistance to hydrogen embrittlement, the overriding factor contributing to susceptibility of the steel was strength. The degree of susceptibility of the microstructures to hydrogen embrittlement, ranked in increasing order, is as follows: fine pearlite, bainite, tempered martensite. The effects of alloying were also assessed by comparing results from different fastener grade steels with similar microstructures. In most cases, the alloy chemistry had little effect, presumably due to trap saturation associated with this testing technique.     

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.     

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.     

Effect of post-weld heat treatment on the microstructure and hydrogen permeation of 13CrNiMo steels

The hydrogen trapping characteristics of 13CrNiMo martensitic steel weld metals, with different austenite contents resulting from different post-weld heat treatments, have been analysed. Scanning electron microscopy and X-ray diffraction have been used to study stable austenite resulting from intercritical tempering of these soft martensitic stainless steels weld metals. Austenite contents up to 25 vol.% have been obtained. Hydrogen diffusion and permeation coefficients have been obtained from an analysis of the permeation rate of hydrogen through these materials. A decrease of the hydrogen apparent diffusion coefficient is observed when the tempering temperature is increased in the range ; this decrease is attributed to changes in the martensitic matrix as well as to the increase of austenite content. The role of the austenite phase on trapping is discussed.     

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Prestressing steels occasionally fail by a process named “stress corrosion cracking”. This process has not been fully elucidated and several theories exists in order to explain the cases in which real structures have collapsed. This paper briefly mentions the different theories and identifies the progress in understanding whether it is necessary to use a testing method, which is able to separate the different steps and mechanisms contributing to the failures.This paper presents the methodology used for inducing controlled localized attack to study the susceptibility of the high strength steels resistance to stress corrosion cracking (SCC). The method is designed to study the growth of cracks initiated from a mechanical notch; the crack is not produced by fatigue.It consists of several stages: coating of the bar with epoxy resin, generation of a small notch, constant load and controlled potential test in the media, mechanical test in air and fractographic study. It allows us to calculate the crack propagation rate and the fracture toughness in the same test.Finally, it has been possible to apply the surface mobility mechanism (SMM) in order to identify the SCC mechanism that operates.     

Corrosion-induced hydrogen embrittlement in aluminum alloy 2024

The present paper focuses on the observed corrosion-induced embrittlement of alloy 2024 and tries to answer the key question on whether the observed embrittlement is attributed to hydrogen uptake and trapping in the material. Hydrogen is produced during the corrosion process and is being trapped in distinct energy states, which correspond to different microstructural sites. The formation of a hydrogen-affected zone beneath the corrosion layer is supported by fractographic analysis. Removal of the corrosion layer leads to complete restoration of yield strength but only partial restoration of ductility. Additional heat treatment to release the trapped hydrogen leads only to complete restoration of ductility.     

Stress corrosion cracking and hydrogen embrittlement susceptibility of an eutectoid steel employed in prestressed concrete

The stress corrosion and hydrogen embrittlement behavior of AISI 1080 steel employed in concrete prestressing tendons was studied with different experimental techniques. A simulated concrete pore solution, with and without contaminants such as chloride, sulfate and thiocyanate ions was used. For comparison purposes the standard 20% ammonium thiocyanate solution was also employed. Polarization curves, slow strain rate tests and fracture mechanics tests were used to evaluate the influence of parameters such as potential, temperature (between 0 and 100 °C), and tempering temperature of the steel. The results have shown that the fracture mechanism of the stress corrosion cracking process is associated with hydrogen action.     

The effects of sacrificial coatings on hydrogen embrittlement and re-embrittlement of ultra high strength steels

This paper describes an investigation of electrodeposited Zn-14% Ni and aluminium-based SermeTel 1140/962 coatings as possible replacements for cadmium. Slow strain rate tests were performed to measure the extent of direct hydrogen embrittlement of a high strength steel substrate as a result of the coating process and of hydrogen re-embrittlement caused by coating corrosion. The level of re-embrittlement was shown to depend on both the electrochemical potential of the coating and its barrier properties. Zn-14% Ni coatings caused the most re-embrittlement as they had the most active potential and contained through-thickness defects which left the steel exposed to hydrogen uptake. The microstructure of the high strength steel was also shown to be an important factor affecting the extent of embrittlement. AerMet 100 steel was more resistant than 300M steel and this was attributed to the presence of reverted austenite surrounding the martensite laths in AerMet 100, which trapped absorbed hydrogen and prevented a critical hydrogen concentration being reached in the more susceptible martensite phase.     

The effects of cold rolling and sensitisation on hydrogen embrittlement of AISI 304L welds

The notched tensile strength (NTS) of 304L stainless steel welds subjected to cold rolling (20% thickness reduction) and sensitisation treatment (600 °C/10 h) was measured in a MgCl₂ (40 wt.%) solution at 80 °C. The NTS loss, which is used for comparing the susceptibility to hydrogen embrittlement (HE), was consistent with the extent of embrittled area on the fracture surface of various welds. The HE susceptibility of the weld decreased after cold working but increased with the additional sensitisation treatment of the cold-rolled welds. Besides, cracks tended to propagate along the skeletal boundaries for all welds tested in the MgCl₂ solution.     

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。     

Effect of microstructure on the hydrogen trapping efficiency and hydrogen induced cracking of linepipe steel

The hydrogen trapping efficiency in different microstructures is compared, and the critical hydrogen flux for hydrogen induced cracking (HIC) is determined for API X65 grade linepipe steel. By controlling the start cooling temperature (SCT) and the finish cooling temperature (FCT) in thermomechanically controlled process (TMCP), three different kinds of microstructure such as ferrite/degenerated pearlite (F/DP), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) are obtained. A modified ISO17081(2004) standard method is used to evaluate the hydrogen trapping by measuring the permeability (J_ssL) and the apparent diffusivity (D_app). Microstructures affecting both hydrogen trapping and hydrogen diffusion are found to be DP, AF, BF and martensite/austenite (M/A) constituents. The hydrogen trapping efficiency is increased in the order of DP, BF and AF, with AF being the most efficient. HIC is initiated at the local M/A concentrated region when the steel has such microstructures as F/AF or F/B. Although the trapping efficiency of bainite is lower than that of AF, bainite is more sensitive microstructure to HIC than to AF.     

Modelling the effects of tool-edge radius on residual stresses when orthogonal cutting AISI 316L

Tool-edge geometry has significant effects on the cutting process, as it affects cutting forces, stresses, temperatures, deformation zone, and surface integrity. An Arbitrary-Lagrangian–Eulerian (A.L.E.) finite element model is presented here to simulate the effects of cutting-edge radius on residual stresses (R.S.) when orthogonal dry cutting austenitic stainless steel AISI 316L with continuous chip formation. Four radii were simulated starting with a sharp edge, with a finite radius, and up to a value equal to the uncut chip thickness. Residual stress profiles started with surface tensile stresses then turned to be compressive at about 140 μm from the surface; the same trend was found experimentally. Larger edge radius induced higher R.S. in both the tensile and compressive regions, while it had almost no effect on the thickness of tensile layer and pushed the maximum compressive stresses deeper into the workpiece. A stagnation zone was clearly observed when using non-sharp tools and its size increased with edge radius. The distance between the stagnation-zone tip and the machined surface increased with edge radius, which explained the increase in material plastic deformation, and compressive R.S. when using larger edge radius. Workpiece temperatures increased with edge radius; this is attributed to the increase in friction heat generation as the contact area between the tool edge and workpiece increases. Consequently, higher tensile R.S. were induced in the near-surface layer. The low thermal conductivity of AISI 316L restricted the effect of friction heat to the near-surface layer; therefore, the thickness of tensile layer was not affected.     

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.     

In situ electrochemical nanoindentation: A technique for local examination of hydrogen embrittlement

Nanoindentation combined with AFM (NI-AFM) has been used to study the effect of electrochemically in situ charged hydrogen on the deformation of small volumes of nickel and copper single crystals. Hydrogen reduces the unstable elastic plastic transition load (pop-in) in nickel, but does not have any effect on copper. It has been shown that the activation energy for the onset of plasticity (dislocation nucleation) is reduced by dissolved hydrogen. This is because hydrogen reduces shear modulus and stacking fault energy in nickel, whereby the former results in hydrogen-enhanced decohesion (HEDE) and the latter in the hydrogen-enhanced plasticity (HELP) mechanism.     

Environmental embrittlement of automobile spring steels caused by wet-dry cyclic corrosion in sodium chloride solution

The susceptibility to environmental embrittlement (EE) of automobile spring steels was investigated using six different steels. Slow strain rate tensile test and thermal desorption spectroscopic analysis were applied to specimens subjected to wet-dry cyclic corrosion tests in a NaCl solution. Experimental results revealed that the reduction in ductility after the corrosion tests was pronounced with increasing strength level. This degradation was closely associated with the resistance to pitting corrosion. Consequently, the hydrogen absorbed in steel and the corrosion pit as a geometric damage were responsible for the EE of spring steels. The hydrogen in rust layer had no significant influence on the EE.     

A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.