Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel

The effects of electroplated and hot-dip zinc coatings on the fracture of low-alloy steel AISI 4140 bars tempered to hardnesses in the range Rc 33 to 49 were studied. Either electroplated or hot-dip zinc coatings decrease resistance to stress corrosion cracking,i.e., they reduceK _sc, the threshold stress intensity for stress corrosion cracking in 3.5 wt pct NaCl solution. AboveK _scelectroplated-zinc coatings do not appear to affect the crack-growth rate, although the incubation period prior to the onset of crack growth is reduced. Hot-dip zinc coatings increase stress corrosion crack growth rates slightly because of the additive effect of internal dissolved hydrogen. Hot-dip zinc coatings reduce the critical stress intensity for fracture in the absence of a corrosive environment because of embrittlement by internal hydrogen which is released from traps during hot-dip coating and confined by the inter metallic coatings which form on the steel surface in the hot dip bath. A simple fracture mechanics analysis indicates that either increasing diameter or the presence of a zinc coating lowers the critical hardness at which the stress corrosion cracking of structural bolts can occur.     

Stress history effect on incubation time for stress corrosion crack growth in AISI 4340 steel

The effect of stress history on stress corrosion cracking of AISI 4340 steel in an aqueous environment has been studied with the use of double-cantilever beam specimens. The stress history effect was found to influence the incubation time period with changes in the stress intensity. When the stress intensity was decreased, the incubation time period was dependent on the △K and finalK _f during stress corrosion testing. When the stress intensity was increased, the incubation time period was independent of the applied stress intensity. However, the stress history effect did not influence the steady-state crack growth rates. In this report, the stress history effect is explained by using the hydrogen embrittlement mechanism.     

Studies on the influence of metallurgical variables on the stress corrosion behavior of aisi 304 stainless steel in sodium chloride solution using the fracture mechanics approach

Stress corrosion data on a nuclear grade AISI type 304 stainless steel in a boiling solution of 5M NaCl+ 0.15M Na₂SO₄+ 3 mL/L HC1 (bp 381 K) for various metallurgical conditions of the steel are presented in this article. The metallurgical conditions used are solution annealing, sensitization, 10 pct cold work, 20 pct cold work, solution annealing + sensitization, 10 pct cold work + sensi-tization, and 20 pct cold work + sensitization. The fracture mechanics approach has been used to obtain quantitative data on the stress corrosion crack growth rates. The stress intensity factor,K ₁, andJ integral,J ₁, have been used as evaluation parameters. The crack growth rates have been measured using compact tension type samples under both increasing and decreasing stress intensity factors. A crack growth rate of 5 X 10^-11 m/s was chosen for the determination of threshold para-meters. Results of the optical microscopic and fractographic examinations are presented. Acoustic signals were recorded during crack growth. Data generated from acoustic emissions, activation energy measurements, and fractographic features indicate hydrogen embrittlement as the possible mechanism of cracking.     

Influence of austenitizing temperature on stress corrosion in 4330m steel—The role of impurity segregation in stress corrosion cracking of high strength steel

Measurements of the threshold stress intensity for stress corrosion cracking (SCC), K_ISCC, and crack growth rate,da/dt, in distilled water were made, respectively, on bolt-loaded WOL and precracked three-point-bending specimens of a 4330M steel. A significant improvement of resistance to SCC was obtained by increasing quenching temperature and it is due to a reduction of segregated impurities of P and S at prior austenite grain boundaries. Intergranular cracking tendency increases with inter-granular concentration of impurities and the fracture mode changes from intergranular separation along prior austenite grain boundaries to transgranular quasi-cleavage as the segregated impurity becomes low enough. The combined effects of hydrogen and intergranular impurities on reducing intergranular cohesion and the time for approaching the critical concentration of hydrogen are dis-cussed in terms of a dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack. Formerly with Shanghai Jiao Tong University, Shanghai, China     

Hydrogen embrittlement, grain boundary segregation, and stress corrosion cracking of alloy X-750 in low-and high-temperature water

The nature of intergranular stress corrosion cracking (SCC) of alloy X-750 was characterized in low-and high-temperature water by testing as-notched and precracked fracture mechanics specimens. Materials given the AH, BH, and HTH heat treatments were studied. While all heat treatments were susceptible to rapid low-temperature crack propagation (LTCP) below 150 °C, conditions AH and BH were particularly susceptible. Low-temperature tests under various loading conditions (e.g., constant displacement, constant load, and increasing load) revealed that the maximum stress intensity factors (K _{p max}) from conventional rising load tests provide conservative estimates of the critical loading conditions in highly susceptible heats, regardless of the load path history. For resistant heats, K _{P max} provides a reasonable, but not necessarily conservative, estimate of the critical stress intensity factor for LTCP. Testing of as-notched specimens showed that LTCP will not initiate at a smooth surface or notch, but will readily occur if a cracklike defect is present. Comparison of the cracking response in water with that for hydrogen-precharged specimens tested in air demonstrated that LTCP is associated with hydrogen embrittlement of grain boundaries. Equivalent activation energies for stage II LTCP rates (11.3 kcal/mol) and hydrogen diffusion (11.5 kcal/mol) indicate that hydrogen diffusion to the peak stress region ahead of a crack is the rate-controlling process. Auger analysis showed that variability in LTCP resistance is associated with phosphorus and sulfur segregation to grain boundaries. Above 150 °C, an increase in fracture resistance and decrease in the degree of hydrogen enrichment precludes rapid intergranular cracking. The stress corrosion crack initiation and growth does occur in high-temperature water (>250 °C), but crack growth rates are orders of magnitude lower than LTCP rates. The SCC resistance of HTH heats is far superior to that of AH heats as crack initiation times are two to three orders of magnitude greater and growth rates are one to two orders of magnitude lower.     

The stress intensities for slow crack growth in steels containing hydrogen

A test technique has been developed to determine the stress intensity for slow crack growth in hydrogen precharged steels. Measurements on several grades of maraging steel and a 300M steel show that hydrogen contents on the order of 2 ppm reduce the stress intensity for slow crack growth by 50 pct or more of theK _Ic values. At equivalent hydrogen contents the 300M steel was more severely embrittled than the mar aging steels. Comparison of the present results with aqueousK _Iscc data indicates that the amount of hydrogen “picked up by the steels in stress corrosion increases with increasing yield strength. Formerly with International Nickel Co.     

Effect of threshold stress intensity on fracture mode transitions for hydrogen-assisted cracking in AISI 4340 steel

Fracture mode transition in hydrogen-assisted cracking (HAC) of AISI 4340 steel has been studied from an equilibrium aspect at room temperature with 8.6-mm-thick double cantilever beam (DCB) specimens. The threshold stress intensity,K _th , necessary for the occurrence of HAC and the corresponding fracture surface morphology have been determined as a function of hydrogen pressure and yield strength. The K _th increases with decrease in hydrogen pressure at a given yield strength and also with decrease in yield strength at a given hydrogen pressure. AsK _th increases, the corresponding HAC fracture mode changes from the intergranular (IG) and quasi-cleavage (QC) modes to the microvoid coalescence (MVC) mode. The experimental results indicate that the critical hydrogen concentration for crack extension in the IG mode is higher than that for crack extension in the MVC mode. The fracture mode transition with varying hydrogen pressure and yield strength is discussed by simultaneously considering the micromechanisms for HAC and the hydrogen pressure and yield strength dependencies ofK _th .     

Stress corrosion cracking of austenitic steels—Region II behavior

A fracture mechanics study of stress corrosion cracking (scc) of cold worked AISI 310 austenitic steel, and an experimental metastable austenite, was conducted in hot aqueous solutions of 44.7 wt pct MgCl₂ and the results compared with previous studies on AISI 316 steel. Attention was directed towards Region II behavior where crack propagation rate (v) was independent of stress intensity (K_I). The apparent activation energy of Region II was found to be in the range ~65 to 75 kJ/mol, independent of the relative proportions of intergranular and transgranular cracking. Also, electron diffraction studies of fracture surfaces showed that α′-martensite formation was not a pre-requisite for scc, although it may influence crack propagation rates. Cracking was discussed in terms of a hydrogen embrittlement model under hydrogen transport control in the austenite lattice. However, adsorption (chemisorption) effects on repassivation and dissolution behavior could not be eliminated from consideration. Alan J. Russell, Formerly Research Student, University of British Columbia.     

Near-threshold corrosion fatigue crack growth behavior of type 422 stainless steel at controlled maximum stress intensities

The K_max-controlled near-threshold fatigue crack growth behavior was investigated on 422 stainless steel in a boiling NaCl solution. During the test, there was a transition from corrosion fatigue to stress corrosion cracking. The transition occurred at very high load ratios (R=-0.91) and at very lowAK levels (≤2.1 MPa√m). The characteristics of stress corrosion cracking (SCC) were manifested by time-based crack growth rather than cycle-based crack growth, by crack extension under static loading, and by change in fracture mode. In corrosive environments, the small ripple loading imposed on structural materials should be recognized for engineering designs and failure analyses.     

A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel

A new test specimen configuration, designated the T-notch double cantilever beam (TNDCB), was developed, calibrated and employed for a fracture mechanics study of stress corrosion cracking (SCC) of cold worked Type-316 austenitic stainless steel exposed to hot aqueous solutions of 44.7 wt pct MgCl₂. The effects of stress intensity (K _I ), temperature (T) and electrochemical potential (E) upon the crack velocity (v) and fractography were investigated. The stress intensity (K _ISCC ) below whichv became immeasurably small was ∼12 MN·m^−3/2. Above this value, three regions of behavior were observed. Region I exhibitedK _I dependent cracking followed by Region II which exhibitedK _I independent cracking and an apparent activation energy of 63 to 67 kJ/mol, followed by Region III where cracking again became dependent uponK _I . The relative proportions of intergranular and transgranular crack paths were markedly dependent upon bothK _I andE, and less sensitive toT. Crack velocity was insensitive to small changes inE with respect to the free corrosion potentials (E _corr), but could be terminated by an applied active potential of ∼−0.35 V_SCE. The pH within the propagating crack was estimated to be <1.0 atE _corr, rising to ∼4.5 at −0.35 V_SCE. The mechanism of SCC was discussed with respect to film rupture events caused by crack tip plastic deformation, adsorption controlled processes on the metal surface, and hydrogen diffusion in the metal lattice. Alan J. RUSSELL, formerly Research Student, University of British Columbia     

A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys

Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,K _ISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,_ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.     

Hydrogen induced cracking in a low alloy steel

Stress corrosion cracking behavior of 300M steel under various heat treated conditions was studied. Threshold stress intensity was slightly dependent upon the martensitic substructure, the K_Ic value, and amounts of retained austenite. The prior austenite grain size exerted the maximum influence. The crack growth rate was directly related to the number of constraint points and, hence, the prior austenite grain size. Increasing the prior austenite grain size increased theK _Iscc, although Stage II crack growth rate also increased. The actual crack growth rate in Stage II was intermittent and decreased slightly with increasing applied stress intensity.     

Effect of Variable Stress Intensity Factor on Hydrogen Environment Assisted Cracking

Fitness-for-service evaluations of engineered components that are subject to environment assisted cracking (EAC) often require analyses of potentially large crack extensions through regions of variable stress intensity. However, there are few EAC data and models that directly address the effects of variable stress intensity factor on EAC crack growth. The model developed here is used to evaluate stress corrosion cracking (SCC) data that were obtained on a high-strength beta-titanium alloy under conditions of variable crack mouth opening displacement (CMOD) rate. SCC of this Ti alloy in ambient temperature, near-neutral NaCl aqueous solution is thought to be due to hydrogen environment assisted cracking (HEAC). As the model equations developed here do not admit to a closed form solution for crack velocity as a function of applied stress intensity factor, K, a semiquantitative graphical solution is used to rationalize the crack growth data. The analyses support a previous suggestion that the observed crack growth rate behavior can be attributed to the effect of crack tip strain rate on rates of mechanical disruption and repair of an otherwise protective crack tip oxide film. Model elements introduced here to HEAC modeling include (1) an expression relating corrosion-active surface area to crack tip strain rate and repassivation rate, (2) an expression relating the critical grain boundary hydrogen to the applied stress intensity factor, and (3) an expression relating CTSR to both applied and crack advance strain rate components. Intergranular crack advance is modeled assuming diffusive segregation of corrosion-generated hydrogen to grain boundary trap sites causing embrittlement of the fracture process zone (FPZ). The model equations developed here provide a quantitative basis for understanding the physical significance of K-variation effects and, with additional development, will provide an engineering tool for analysis of crack growth in a variable K field.     

热镀锌板镀层表面缺陷及其消除措施

介绍热镀锌钢板镀层表面存在的诸如划伤、气刀条痕、辊印、波纹和条纹等缺陷的产生原因及其消除措施,同时还侧重介绍热镀锌钢板今后需要研究的锌渣缺陷、镀层表面质量与漆膜质量之间关系的课题。     

The threshold stress intensity for hydrogen-induced crack growth

The crack growth rates and threshold stress intensities,K _TH, for a 3 1/2 NiCrMoV steel (0.2 pct proof stress 1200 MPa) have been measured in a hydrogen environment at various temperatures and hydrogen pressures. Fractographic evidence and the observation of alternating fast and slow crack growth nearK _TH suggests that the crack advances by the repeated nucleation of microcracks at microstructural features ahead of the main crack. Transient crack growth is observed following load increases just belowK _TH. Using the idea, from unstable cleavage fracture theory, that for fracture a critical stress must be exceeded over a critical distance ahead of the crack, and assuming that this critical stress is reduced in proportion to the local hydrogen concentration (in equilibrium with the external hydrogen atK _TH), a theoretical dependence ofK _TH on hydrogen pressure is derived which compares well with the experimental evidence.     

A unified theory for some effects of hydrogen source,alloying¦elements,and potential on crack growth in martensitic AISI 4340 steel

The effects of hydrogen on crack growth in martensitic AISI 4340 steel are shown to be fundamentally the same whether the hydrogen is supplied as molecular gas, through stress corrosion, or by electrolytic charging. At a given yield strength differences observed in the values of threshold stress intensity for crack growth are proposed to be linked to the degree of dissociation of the hydrogen near the crack tip, and hence to the concentration of hydrogen developed in the critical crack-tip region. Over a range of yield strength values, an upper bound of threshold stress intensity is developed in molecular hydrogen gas and a lower bound on exposure to atomic hydrogen from cathodic charging during or prior to testing. The open circuitK _Iscc values of the steel fall always within the upper and lower bounds, but the values ofK _Iscc may be moved to the lower bound by coupling to magnesium (cathodic charging) or to the upper bound by coupling to copper (anodic polarization). Variations in the concentration of carbon or manganese in the steel at a fixed yield strength produce effects on the value ofK _Iscc similar to the effects produced by cathodic or anodic polarization. With the lower concentrations of carbon or manganese the steel acts as if it were coupled to copper and at the higher concentrations as if coupled to magnesium. Carbon and manganese are therefore proposed to shift the positions of local anodes and cathodes and so influence the proportions of molecular and atomic hydrogen which reach the critical crack-tip region. The proposal is supported by data which show that only cathodic polarization affects the threshold stress intensity of the lowest carbon and manganese steelsK _Iscc is lowered) whereas only anodic polarization affects the higher carbon or manganese steels(K _Iscc is raised).     

The stability behaviour of hard material coatings on alloyed steel substrates

The stability behaviour of hard material coatings made by CVD on different alloyed and carbon steel substrates depends on the properties of both substrate and coating. SEM in-situ investigations on the tension loaded surface of bended samples may be used to observe crack nucleation and growth in brittle hard material coatings such as TiC, TiCN on low alloyed and carbon steels. From the crack distance distributions some important parameters of mechanical stability such as the threshold value of fracture σ_th, the Weibull-parameter β and the ratio of the strength distribution function F and a characteristic length D may be estimated. The threshold value for fracture in the coatings σ_th decreases with increasing coating thickness for all steel-coating combinations. The threshold value also decreases if a heat treatment is made on the steel-coating system. This is due to the relaxation of the internal stress in the coating. The mean strength of the TiC-coatings decreases and the strength variance (measured by the Weibull-parameter β) increases with increasing coating thickness and therefore the mechanical stability of coatings decreases with growing thickness. The ratio of the strength distribution function F and the characteristic distance between defects D as a measure of crack density depends on the deformation behaviour of the steel as well as on the strength and the defect structure of the coating.     

Investigation of stress corrosion cracking of the cast and forged steel in water

The aqueous stress corrosion behavior of cast steel and forged steel of the same heat has been examined. It was shown that the activation energy of crack growth of both cast and forged steels was identical,i.e., Q = 5540 cal/mol, and was comparable with the apparent diffusion activation energy of hydrogen in the steel. Theda/dt for cast and forged steel increased under both cathodic and anodic polarization conditions. Correspondingly, the steady-state hydrogen permeation flux increased steeply with the increase of polarizing current under either cathodic or anodic polarization. The influences of the polarization upon theda/dt and the hydrogen permeation flux were similar. TheK _ISCC of the cast steel was larger than that of the forged steel. This may be due to the observation that the steady-state permeation flux for the forged steel was twice as large as that of the cast steel. For both cast and forged steels the fracture modes were clearly dependent uponK _I at the crack tip, and a transition from dimple to quasi-cleavage or intergranular was observed on the fracture surfaces with decreasingK _I . Formerly Students at Beijing University of Iron and Steel Technology