Hydrogen embrittlement susceptibility of laser-hardened 4140 steel

Slow strain rate tensile (SSRT) tests were performed to investigate the susceptibility to hydrogen embrittlement of laser-hardened AISI 4140 specimens in air, gaseous hydrogen and saturated H₂S solution. Experimental results indicated that round bar specimens with two parallel hardened bands on opposite sides along the loading axis (i.e. the PH specimens), exhibited a huge reduction in tensile ductility for all test environments. While circular-hardened (CH) specimens with 1 mm hardened depth and 6 mm wide within the gauge length were resistant to gaseous hydrogen embrittlement. However, fully hardened CH specimens became susceptible to hydrogen embrittlement for testing in air at a lower strain rate. The strength of CH specimens increased with decreasing the depth of hardened zones in a saturated H₂S solution. The premature failure of hardened zones in a susceptible environment caused the formation of brittle intergranular fracture and the decrease in tensile ductility.     

Predicting the effects of material properties gradient and residual stresses on the bending fatigue strength of induction hardened aeronautical gears

This paper proposes an experimental methodology to characterize complex parts presenting various gradients using aeronautical induction surface hardened spur gears. A 3D fatigue model taking into account residual stresses, microstructure variations, and surface roughness is then proposed for the prediction of the bending endurance limit. The model is based on the well-known Crossland criterion; calibrated with representative axial and torsion laboratory specimens. The results are compared with testing performed on a custom-made single tooth bending fatigue (STBF) rig. Fracture surface analysis using electronic microscopy is used to investigate the crack initiation sites. It is shown that residual stresses can have a significant impact on bending fatigue and that two induction treatments can present very different fatigue resistance even if the shape and depth of the hardened layer is identical in the root. The proposed methodology could be adapted to other geometries and surface treatments.     

Mechanism of dislocation channel-induced irradiation assisted stress corrosion crack initiation in austenitic stainless steel

The mechanism by which dislocation channeling induces irradiation assisted stress corrosion cracking was determined using Fe–13Cr15Ni austenitic stainless steel irradiated with protons to a dose of 5 dpa and strained at high temperature in both argon and simulated boiling water reactor normal water chemistry environments. Straining induced dislocation channels that were characterized by digital image correlation and confocal microscopy. Dislocation channels were found to be either continuous across the boundary, discontinuous, or discontinuous with slip in the boundary. Discontinuous channels were found to contain the least amount of strain but have the highest propensity for initiating cracks. Discontinuous dislocation channel–grain boundary intersections were shown to have the highest local stress. TEM in-situ straining of irradiated steels and atomistic simulation of dislocation–grain boundary interaction provided supporting evidence that channels that were unable to transfer strain underwent cracking. The inability of channels to relieve stress, by either slip in the adjacent grain or in the grain boundary, resulted in high local stresses and increased susceptibility to stress corrosion cracking initiation.     

Analysis of the torsional strength of hardened splined shafts

The current study presents a finite element modeling framework to determine the torsion strength of hardened splined shafts by taking into account the detailed geometry of the involute spline and the material gradation due to the hardness profile. The aim is to select a spline geometry and hardness depth that optimizes the static torsion strength. Six different spline geometries and seven different hardness profiles including non-hardened and through-hardened shafts have been considered. The results reveal that the torque causing yielding of induction hardened splined shafts is strongly dependent on the hardness depth and the geometry of the spline teeth. The results from the model agree well with experimental results found in the literature and reveal that an optimum hardness depth maximizing the torsional strength can be achieved if shafts are hardened to half their radius.     

Internal friction analysis of lath martensite in press hardened steel

Internal friction (IF) measurements were carried out on a press hardened steel (PHS) after continuous annealing, press hardening and bake hardening. The IF peaks of the PHS with a lath martensite microstructure were analysed by comparison with previously published data. This was supplemented by comparison with the IF spectra of the same steel with a ferrite–pearlite microstructure after deformation at room temperature, and after recrystallisation annealing and quenching. The relation between the IF peaks of PHS, and the γ-peak, Snoek peak and Snoek-Kê-Köster peak observed for ferritic steel is discussed.     

Hydrogen-induced cracking in a very-high-purity high-strength steel

It is shown that intergranular fracture along prior-austenite grain boundaries in a ultrahigh-strength quenched and tempered 4340-type steel is not suppressed by eliminating all segregated embrittling elements. This leaves open the question of how brittle intergranular cracks are nucleated in hydrogen in the absence of such impurities.     

Numerical study on hydrogen permeation of ferritic steel evaluated under constant load

Hydrogen permeation of two kinds of ferritic steel with different level of 2nd phase particles are compared utilising electrochemical permeation technique evaluated under various loads. This study indicates that tensile stress in plastic domain induces expansion of the interface between the 2nd phase particles and steel matrix, and formation of blister crack. Such microstructural-discontinuity provides additional hydrogen traps, resulting in slower diffusion and lower steady-state permeation flux. These phenomena appear predominantly in steel containing coarse-sized inclusion and precipitate having sharp extremities. The numerical analysis also supports that the steel containing coarse-sized 2nd phase particles with sharp extremities exhibits higher parameters of hydrogen capture rate per irreversible traps and blister crack formation rate than the steel containing fine-sized ones with spherical shape.     

Fatigue behaviour and life predictions of case-hardened steels

This paper presents experimental and analytical results on fatigue behaviour of case-hardened steel. Fully reversed strain-controlled constant amplitude axial fatigue tests were performed on through-hardened case, through-hardened core and case-hardened steel specimens. Surface versus sub-surface cracking and the role of residual stresses and their relaxation are discussed. Multi-layer models of the case-hardened specimens were used to predict crack nucleation sites as well as fatigue lives, and the predictions corresponded well with the experimental results. Linear elastic fracture mechanics (LEFM) was also used to conduct fatigue crack growth analysis to further explain the experimental observations from the fracture surfaces of the case-hardened specimens. A fatigue strength estimation method based on hardness and inclusion size was used to estimate the fatigue limit of the materials investigated. Fractography of fracture surfaces and crack nucleation location are also presented.     

低碳钢焊接接头碱脆破裂的研究

本文叙述了用多种试验方法测试评定低碳钢焊接接头对碱脆破裂的敏感性。提出低碳钢焊接结构防止碱脆破裂的基本措施是消除焊后残留应力。     

A polycrystalline model for the description of ratchetting: Effect of intergranular and intragranular hardening

In this work, numerical simulations for predicting uni- and multi-axial ratchetting behaviors are carried out, using a polycrystal plasticity model. In this multi-scale modeling, the single crystal behavior is based on crystallographic slip (intragranular scale), whereas the polycrystal behavior is obtained from an explicit transition rule to calculate the local stresses and strains (intergranular scale). A systematic study is performed to show the effect of intergranular and intragranular hardening on the ratchetting behavior. For illustrative purposes, two examples are presented: the model is applied to simulate the experimental results from the literature for a 316 austenitic stainless steel and for a 1026 carbon steel. It was demonstrated that the polycrystalline model was successful in describing the inelastic behavior of the two considered materials adequately.     

Simulation of time-discontinuous chemically-aided intergranular fracture

In this work, based on experimental observations, a model for the pressure dependent diffusion and accumulation of hydrogen ahead of a propagating intergranular crack front is developed. In the model, the pressure dependency of the diffusion is incorporated into the activation energy of an Arrhenius form of the material's diffusivity tensor along high-diffusivity grain boundaries. Inherent to the model is that large amounts of hydrogen can be absorbed into the crack's process zone, due to the high triaxial stress in that region, which produces a larger effective diffusivity locally. Further ahead, the effective diffusivity quickly decreases. The combined effect forms a barrier to further rapid penetration, which leads to the mentioned accumulation of the hydrogen in the process zone. Theoretical aspects of the model are discussed and numerical simulations of the transient distributions of hydrogen and subsequent crack front propagation are performed to illustrate the main characteristics of the model.     

Simulation of dislocation glide in precipitation hardened materials

Precipitation hardening is a widely used method for increasing the critical resolved shear stress (CRSS) of a material. Our simulations offer a flexible means to calculate the CRSS as a function of many parameters involved, e.g. the average precipitate size. For this, one or more dislocations are simulated while gliding through obstacle fields of arbitrary type or spatial arrangement. The elastic self-interaction is fully allowed for. Unlike analytical approaches and simulations known from literature, our method covers both shearing and circumventing of obstacles in a single model. To start with, the obstacles used in this contribution were chosen to be spheres with a constant obstacle stress inside; the distribution of the radii and the spatial arrangement suit the case of Ostwald-ripened particles. This corresponds to the case of the nickel base superalloy NIMONIC PE16 where a dislocation has to create an antiphase boundary in order to shear the long range ordered precipitates. Typical examples for dislocation arrangements are presented, and the results for various obstacle concentrations and mean radii are compared with published results.     

Quantitative fractography of 2205 duplex stainless steel after a sulfide stress cracking test

This paper presents the results of SSC (Sulfide Stress Cracking) investigations of duplex stainless steel 2205 resistance to cracking failure under the combined action of tensile stress and aqueous environments containing hydrogen sulfide according to the NACE Standard TM0177-96. The investigations were carried out on 9 test pieces that were loaded with a tensile stress ranging from 1.02 × the yield stress (YS) to 0.72 × YS. The tests were terminated either when the test specimen failed or after 720 h (30 days) – whichever occurred first – in accordance with the standard requirements. Only two of the specimens examined, one loaded with 1.02 × YS and the second one with 0.72 × YS, failed during the test. The resulting fracture surfaces were subjected to qualitative and quantitative fractographic examinations. Quantitative fracture analysis included an estimation of such parameters as linear roughness index R_L, fractal dimension D_F and overlap index O_L.     

The influence of the temperature on the stress‐strain curves of a bearing steel and a case hardening steel

In the present paper the influence of the temperature and strain rate on the stress strain behaviour of two different steels were investigated. Two microstructures were considered: pearlitic and austenitic. Tensile tests with the bearing steel 100Cr6 and the case hardening steel 20MnCr5 were accomplished at various temperatures. For this purpose the Ludwik equation was used to describe the stress‐strain curve. The parameter of the constitutive equation was determined for each steel and microstructure. Especially for the austenitic state the parameters of the used material law were described as a function of the temperature.     

Failure analysis of impeller made of FV520B martensitic precipitated hardening stainless steel

This paper presents the failure analysis of a centrifugal compressor impeller. The impeller made of FV520B martensitic precipitated hardening stainless steel, fractured after service for 4 months in moist H₂S condition. Efforts were made to analyze the failure impeller in both experiment and FE methods. Sulfide Stress Cracking (SSC) was the primary failure mechanism for this investigated impeller. The martensite structure, the H₂S medium, and the applied stress consisted of the SSC system. This paper exhibits the details of investigation and suggests remedial measures to improve performance of this type impeller under H₂S corrosive environment.     

对非连续增强铝基复合材料的腐蚀、应力腐蚀断裂与氢脆研究的评述

本文就国内外对非连续增强铝基复合材料,主要是SiC、Al_2O_3颗粒和晶须增强铝基复合材料的腐蚀、应力腐蚀断裂和氢脆的研究现状和主要结论进行了介绍和评价,并为今后的进一步研究提供了一些思路。复合材料的腐蚀形态与铝合金相似,但复合材料的增强体对应力腐蚀断裂和氢脆的、作用不同研究者有不同的结论。     

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.     

7xxx系铝合金氢脆的研究现状及发展趋势

全面评述了7xxx系铝合金氢脆(HE)问题的国内外研究现状,讨论了氢脆机理以及氢脆的影响因素,并介绍了几种重要的测试方法.针对目前研究工作中尚未从理论上揭示氢脆本质这一突出问题,指出了今后7xxx系铝合金氢脆问题的研究方向,同时结合前期的研究工作,提出了具体的相关建议.     

Generation of intergranular strains during high temperature creep fatigue loading of 316H stainless steel

Abstract

This paper investigates the development of intergranular strains and stresses in AISI type 316H austenitic stainless steel during cyclic loading at high temperature. Isothermal cyclic creep tests at 650°C with 2 h displacement controlled creep dwells at maximum strain are conducted with in situ neutron diffraction monitoring at a time of flight facility. The evolution of intergranular strains in five independent {hkl} grain families were successfully measured during consecutive cycles. The grain family with {111} lattice planes normal to the loading direction exhibited linearly reversible behaviour with cyclic load whereas the {200} planes deformed in a non-linear manner forming a hysteresis loop. Intergranular strains during the first dwell remained unchanged with time, but relaxed with time during later dwells. The start of dwell intergranular strains increased from cycle 1 to cycle 2, but markedly less moving from cycle 2 to cycle 3.     

Embrittlement of austenitic stainless steel welds

To prevent hot-cracking, austenitic stainless steel welds generally contain a small percent of delta ferrite. Although ferrite has been found to effectively prevent hot-cracking, it can lead to embrittlement of welds when exposed to elevated temperatures. The aging behavior of type-308 stainless steel weld has been examined over a range of temperatures 400–850C for times up to 10,000 hr. Upon aging, and depending on the temperature range, the unstable ferrite may undergo a variety of solid state transformations. These phase changes affect creep-rupture and Charpy impact properties.