Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.     

小锥度试样在研究应力腐蚀裂纹形核中的应用

尝试利用小锥度试样测量304不锈钢应力腐蚀裂纹的形核时间,以确定裂纹形核在整个断裂过程中的贡献.结果表明,用小锥度试样可以有效测量应力腐蚀裂纹的形核时间.对304不锈钢,在沸腾MgCl₂溶液中应力腐蚀裂纹的形核时间与应力之间满足t_i=4310_exp(-0.0097σ),而断裂时间与应力的关系为t_f=6964_exp(-0.0095σ),说明应力腐蚀断裂时间的大部分来源于裂纹的形核过程.断口形貌与外加应力有关,随外加应力增大,断口的穿晶解理部分减少.     

Improvement of the resistance to stress corrosion cracking in austenitic stainless steels by cyclic prestraining

Austenitic stainless steels are known to be sensitive to stress corrosion cracking (SCC) in hot chloride solutions. The aim of the present study is to find improvements in the SCC behavior of 316L-type austenitic stainless steels in 117°C MgCl₂ solutions. Previously, the authors have proposed the “corrosion-enhanced plasticity model” (CEPM) to describe the discontinuous cracking process which occurs in SCC. This model is based on localized corrosion (anodic dissolution, and hydrogen absorption)-deformation (dislocations) interactions (CDI). From the framework of this model, it is proposed that a prestraining in fatigue at saturation decreases the SCC sensitivity. This idea is experimentally confirmed for both crack initiation and crack propagation, through the analysis of the SCC behavior by slow-strain-rate tests of single and polycrystals after different prestraining conditions.     

The effects of microstructure,strength level,and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.     

A unified mechanism of stress corrosion and corrosion fatigue cracking

A mechanism of stress corrosion cracking (SCC) is outlined in which anodic dissolution at film rupture sites relieves strain hardening and reduces the fracture stress at the crack tip. Experimental evidence is cited to suggest that relief of strain hardening occurs by interaction of subsurface dislocations with divacancies generated by the anodic dissolution. A transgranular crack propagates by accumulation of divacancies on prismatic planes which then separate by cleavage under plane strain conditions at the crack tip. At appropriate metallurgical and chemical conditions, anodic dissolution and/or divacancy migration may be enhanced at grain boundaries, leading to an intergranular failure mode. Evidence is also available to indicate that cyclic loading relieves strain hardening. Relief of strain hardening by combined cyclic loading and corrosion accounts for the higher incidence of corrosion fatigue cracking (CFC) without the requirement of any critical dissolved species. Data on fatigue of stainless steel at elevated temperature in both vacuum and air provide additional support for the proposed mechanism.     

Crack paths,microstructure, and fatigue crack growth in annealed and cold-rolled AISI 304 stainless steels

To assist in the understanding of micromechanisms for corrosion fatigue crack growth in metastable austenitic steels, the relationships between the crack paths and the underlying microstructure were investigated for annealed and cold-rolled (CR) 304 stainless steels that had been tested in a deaerated 3.5 pct NaCl solution, air, and vacuum. Corrosion fatigue in the deleterious environments (3.5 pct NaCl and air) was brittle and occurred primarily by {001}_γ and other unidentified, quasi-cleavage (QC), accompanied by preferential cracking along {111}_γ twin and grain boundaries. In contrast, fatigue cracking in vacuum was ductile, fully transgranular, and noncrystallographic. Transformation to alpha prime (α′-) martensite by fatigue was found to be essentially complete in the CR steel, which contained ε-martensite, and in the annealed steel tested in vacuum, but was substantially less in the annealed steel tested in air and 3.5 pct NaCl solution. These results, taken in conjunction with the crack growth and electrochemical reaction data, support hydrogen embrittlement (HE) as the mechanism for corrosion fatigue crack growth in 304 stainless steels in 3.5 pct NaCl solution. Martensitic transformation appears not to be the only responsible factor for embrittlement. Other microstructural components, such as twin and grain boundaries, slip bands, and cold work-induced lattice defects, may play more important roles in enhancing crack growth rates.     

The effect of cathodic polarization on the corrosion fatigue behavior of a precipitation hardened aluminum alloy

Fatigue experiments were conducted on polycrystalline and monocrystalline samples of a high purity Al, 5.5 wt pct Zn, 2.5 wt pct Mg, 1.5 wt pct Cu alloy in the peak-hardened heat treatment condition. These experiments were conducted in dry laboratory air and in 0.5N NaCl solutions at the corrosion potential and at applied potentials cathodic to the corrosion potential. It has been shown that saline solutions severely reduce the fatigue resistance of the alloy, resulting in considerable amounts of intergranular crack initiation and propagation under freely corroding conditions for polycrystalline samples. Applied cathodic potentials resulted in still larger decreases in fatigue resistance and, for poly crystals, increases in the degree of transgranular crack initiation and propagation. Increasing amounts of intergranular cracking were observed when applied cyclic stresses were reduced (longer test times). The characteristics of cracking, combined with results obtained on tensile tests of deformed and hydrogen charged samples, suggest that environmental cracking of these alloys is associated with a form of hydrogen embrittlement of the process zones of growing cracks. Further, it is suggested that stress corrosion cracking and corrosion fatigue of these alloys occurs by essentially the same mechanism, but that the often observed transgranular cracking under cyclic loading conditions occurs due to enhanced hydrogen transport and/or concentrations associated with mobile dislocations at growing crack tips.     

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.     

Influence of Simulated Outside-Reactor Irradiation on Anticorrosion Property of Austenitic Stainless Steel

The influence of γ-ray irradiation on the properties of inside-reactor stainless steel structures was studied by simulating the working condition of pressurized water reactor （PWR） first circuit and the outside-reactor y-ray irradiation. The result shows that the simulated outside-reactor irradiation （irradiation dose 4. 4 ）〈 104 Gy） has no influence on anticorrosion properties of solutionized SUS304 austenitic stainless steel, including intergranular corrosion （IC） and stress corrosion cracking （SCC）. Anticorrosion properties （IC, SCC） of sensitized SUS304 austenitic stainless steel are reduced by simulated outside-reactor irradiation. The longer the sensitized time is, the more obvious the influence is.     

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.     

硫化氢环境下两种不锈钢的应力腐蚀开裂行为

用U形弯试样浸泡和慢应变速率拉伸实验研究了3Cr17Ni7Mo2SiN和00Cr22Ni5Mo3N(2205)不锈钢在硫化氢介质中的应力腐蚀开裂(SCC)行为.2205不锈钢的SCC萌生孕育期较长,在pH较低的饱和H₂S溶液中具有明显的SCC敏感性,其SCC敏感性随溶液pH值的升高或H₂S含量的降低而迅速降低.3Cr17Ni7Mo2SiN的SCC孕育期均低于2205不锈钢,在pH ≤ 4.5、H₂S的质量浓度 ≥ 10³mg·L^-1的H₂S介质中均具有明显的SCC敏感性,其SCC敏感性受pH值和H₂S含量变化影响较小.3Cr17Ni7Mo2SiN的SCC以沿晶裂纹萌生,扩展后转变为穿晶应力腐蚀开裂;2205不锈钢近表面处首先发生奥氏体-铁素体相间氢致开裂,并促进SCC萌生,其SCC为穿晶应力腐蚀开裂.     

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     

A strain-based fracture model for stress corrosion cracking of low-alloy steels

The stress corrosion cracking (SCC) behavior of 4135 steel under different heat treatments is analyzed in an attempt to relate microstructural characteristics with macroscopic measurements of SCC resis-tance, especially the very impressive improvements associated with changes from intergranular (IG) to transgranular (TG) fracture paths. Considering that local hydrogen embrittlement at the crack tip causes SCC processes, a local cracking criterion, based on a critical strain depending on hydrogen concentration, is assumed to control the process. Stress corrosion cracking is viewed as a discontin-uous series of unstable crack extensions through the locally embrittled regions. The model developed on this basis explains the macroscopic behavior observed at the threshold situation and partially at stage II propagation and clarifies the role of the metallurgical variables in each of the types of fracture detected.     

高温水环境下温度对316L不锈钢应力腐蚀开裂的影响

在高温水环境中,采用慢应变速率拉伸实验方法研究了温度对316 L不锈钢应力腐蚀开裂的影响规律,并通过扫描电镜(SEM)对试样断口形貌进行分析.结果表明:在高温水环境中,温度为200~345℃时316 L不锈钢具有应力腐蚀开裂敏感性;材料脆性指标随温度升高而增大,应力腐蚀开裂敏感性增强,断口分析与之吻合;250℃是316 L不锈钢发生应力腐蚀开裂的敏感温度,断口边缘形貌呈现明显脆性断裂特征.     

Crack size effects on the chemical driving force for aqueous corrosion fatigue

Small crack size accelerates corrosion fatigue propagation through high strength 4130 steel in aqueous 3 pct NaCl. The size effect is attributed to crack geometry dependent mass transport and electrochemical reaction processes which govern embrittlement. For vacuum or moist air, growth rates are defined by stress intensity range independent of crack size (0.1 to 40 mm) and applied maximum stress (0.10 to 0.95 Φ_ys). In contrast small (0.1 to 2 mm) surface elliptical and edge cracks in saltwater grow up to 500 times faster than long (15 to 40 mm) cracks at constant δK. Small cracks grow along prior austenite grain boundaries, while long cracks propagate by a brittle transgranular mode associated with tempered martensite. The small crack acceleration is maximum at low δK levels and decreases with increasing crack length at constant stress, or with increasing stress at constant small crack size. Reductions in corrosion fatigue growth rate correlate with increased brittle transgranular cracking. Crack mouth opening, proportional to the crack solution volume to surface area ratio, determines the environmental enhancement of growth rate and the proportions of inter- and transgranular cracking. Small cracks grow at rapid rates because of enhanced hydrogen production, traceable to increased hydrolytic acidification and reduced oxygen inhibition within the occluded cell.     

Mechanism of brittle fracture in a ductile 316 alloy during stress corrosion

The ductile f.c.c. 316 alloy is shown to exhibit brittle transgranular (and intergranular) stress corrosion cracking in a 153°C MgCl₂ solution at free corrosion potential. Tests on smooth and pre-cracked specimens are performed to identify the mechanisms of fracture. Transgranular cracking is related to both a discontinuous microcleavage mainly on {100} planes and a microshearing on {111} planes. A new physical modelization is proposed to explain the brittle transgranular cracking. It is based on the influence of the localized anodic dissolution on the enhancement of the plasticity at the crack tip. The formation of dislocation pile-ups and the conditions of restricted slip induce a brittle microcracking. The crack propagation is then limited and arrested by the strong effect of relaxation in the ductile 316 alloy. Such a model is discussed as a function of the main factors governing the transgranular stress corrosion cracking sensitivity of ductile f.c.c. single-phase materials.     

A mechanistic study of transgranular stress corrosion cracking of type 304 stainless steel

Transmission electron microscopy (TEM) was utilized to characterize the deformation substructure of 304 stainless steel tested for transgranular stress corrosion cracking (TGSCC) in 45 wt pet MgCl₂ at 155 °C. The TEM characterization was conducted in thin foils prepared from the fracture surface and from a series of known depths below the fracture surface. The results indicate that the stacking fault energy (SFE) of the material immediately ahead of the crack tip is lowered, with the deformation mode at small distances (a few microns) in front of the growing crack front being entirely coplanar while at larger distances homogeneous. The reduction in the SFE is attributed to absorbed hydrogen formed during the cathodic reaction. Based on this and previous observations of transgranular stress corrosion characteristics of aus-tenitic stainless steels in chloride environments, a “hydrogen-induced cleavage” model is proposed. This model is essentially a modification of a model based on enhanced structural reactivity associated with Lomer-Cottrell locks proposed by Robertson and Tetelmann in 1962.²⁷     

pH值对高温高压水中304L不锈钢应力腐蚀开裂的影响

在高温高压水环境中,采用慢应变速率拉伸试验方法,研究了不同pH值对304L不锈钢应力腐蚀开裂行为的影响规律,并通过扫描电镜对试样断口形貌进行观察与分析.结果表明:在300℃时,304L不锈钢在弱酸性和弱碱性溶液中的应力腐蚀开裂敏感性较大,且酸性越强,敏感性越大.在中性溶液中,304L不锈钢的强度和塑性损失较小,应力腐蚀敏感性较小,断口分析与之吻合.     

The effects of cathodic charging on the acoustic emission generated by intergranular cracking in sensitized 304 stainless steel

It has been reported that the mode or type of crack propagation (transgranular, intergranular, or microvoid coalescence) in 304 stainless steel could be identified by analysis of the acoustic emissions generated during crack propagation. Different crack propagation modes were produced in double cantilever beam specimens of 304 stainless steel using combinations of heat treatments and cathodic charging. The acoustic emission generated was measured, analyzed, and correlated with metallographic results. Intergranular cracking or separation was observed in three experimental conditions: (1) sensitized samples tested in air at room temperature, (2) sensitized samples which were cathodically charged prior to testing in air at room temperature, and (3) sensitized samples which were simultaneously and continuously cathodically charged while being tested at room temperature. The particular acoustic emissions generated by intergranular separations were identified by careful analysis of the acoustic emission waveforms. The amount of intergranular cracking and the acoustic emissions detected were found to be strongly dependent on the experimental test conditions. The amplitude, duration, and, hence, the energy carried in the waveforms of the emissions from intergranular separations were found to decrease dramatically when there was a constant supply of hydrogen,i.e., during continuous cathodic charging. The results are consistent with the lowering of the cohesive energy along the grain boundary; however, other mechanisms are also plausible. Formerly with the Physics Department, University of Denver.