The role of retained austenite in stress corrosion cracking of steels

The ability of retained austenite to affect stress corrosion cracking susceptibility has been examined in two steels, containing mechanically stable and unstable retained austenite of varying amounts and distributions. While limited improvements due to this constituent were observed in both threshold (where it may act as a compcting energy trap) and in growth rates (where it can act as a mechanical barrier and hydrogen sink), these were neither consistent nor more effective than other heterogeneities. Contrary to popular belief, retained austenite does not appear to be a consistently effective microstructural constituent, at least if the aim is to improve SCC performance at strength levels up to about 1000 MPa.     

The role of retained austenite in stress corrosion cracking of steels

The ability of retained austenite to affect stress corrosion cracking susceptibility has been examined in two steels, containing mechanically stable and unstable retained austenite of varying amounts and distributions. While limited improvements due to this constituent were observed in both threshold (where it may act as a compcting energy trap) and in growth rates (where it can act as a mechanical barrier and hydrogen sink), these were neither consistent nor more effective than other heterogeneities. Contrary to popular belief, retained austenite does not appear to be a consistently effective microstructural constituent, at least if the aim is to improve SCC performance at strength levels up to about 1000 MPa.     

Effects of silicon additions and retained austenite on stress corrosion cracking in ultrahigh strength steels

A study has been made of the effects of silicon additions and of retained austenite on the stress-corrosion cracking (SCC) behavior of commercial ultrahigh strength steels (AISI 4340 and 300-M) tested in aqueous solutions. By comparing quenched and tempered structures of 4340 and 300-M i) at equivalent strength and ii) at their respective optimum and commercially-used heat-treated conditions, the beneficial role of silicon addition on SCC re-sistance is seen in decreased Region II growth rates, with no change in K’_ISCC. The beneficial role of retained austenite is demonstrated by comparing isothermally transformed 300-M, containing 12 pct austenite, with conventionally quenched and tempered structures of 300-M and 4340, containing less than 2 pct austenite, at identical yield strength levels. Here, the isothermally transformed structure shows an order of magnitude lower Region II SCC growth rates than quenched and tempered 300-M and nearly two orders of magnitude lower Region II growth rates than 4340, K ISCC values remaining largely unchanged. The results are discussed in terms of hydrogen embrittlement mechanisms for SCC in martensitic high strength steels in the light of the individual roles of hydrogen diffusivity and carbide type.     

The role of crack tip strain rate in the stress corrosion cracking of high strength steels in water

Creep tests have been performed on fracture mechanics specimens of as-quenched 4340 and 3.5NiCrMoV rotor steel to confirm the importance of crack tip strain rate in causing stress corrosion cracking. By allowing creep in a noncracking environment, dry air for the high strength steels tested, cracking did not occur when water, the corrosive solution, was later added to the system. Thus, it is possible to inhibit stress corrosion in spite of conditions otherwise conducive to crack growth. Conditions necessary to restart cracking were also tested. The importance of this result in terms of the mechanism of stress corrosion and difficulties in measuring K_ISCC is discussed. I. O. Smith, formerly Associate Professoraf with the Department of Mining and Metallurgical Engineering, University of Queensland     

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.     

Initiation of stress corrosion cracking for pipeline steels in a carbonate-bicarbonate solution

The linearly increasing stress test (LIST) was used to study the stress corrosion cracking (SCC) behavior of a range of pipeline steels in carbonate-bicarbonate solution under stress rate control at different applied potentials. Stress corrosion cracking, at potentials below -800 mV(SCE), was attributed to hydrogen embrittlement. Stress corrosion cracking, in the potential range from about-700 to -500 mV(SCE), was attributed to an anodic dissolution mechanism. In the anodic potential region, the SCC initiation stress was larger than the yield stress and was associated with significant plastic deformation at the cracking site. The relative SCC initiation resistance decreased with in-creasing yield strength. In the cathodic potential region, the SCC initiation stress was smaller than the yield stress of steel; it was approximately equal to the stress at 0.1 pct strain(@#@ Σ_0.1pct) for all the steels. The original surface was more susceptible to SCC initiation than the polished surface.     

Martensite and retained austenite in hot-rolled,low-carbon bainitic steels

The microstructure and occurrence of a microconstituent, consisting of martensite and retained austenite in hot-rolled plates of low-carbon bainitic steels was studied by electron microscopy and microprobe analysis. The results of the studies showed that the formation of the martensite-austenite constituent is controlled by the composition of the steel and by the cooling rate of the plates following hot-rolling. The mechanisms involved in the formation of the martensiteaustenite constituent are discussed.     

The effect of retained austenite on the fracture toughness of high-speed steels

The aim of this work was to find the quantitative dependences between fracture toughness K_lc and the volume fraction of retained austenite in the matrix of quenched high-speed steels. The tests were carried out on three model alloys of a different content quotient of Mo: W which, after quenching, were gradually supercooled up to ? 196°C and then tempered at 450°C. Also the measurements of the content of retained austenite in the vicinity of the surface of a sample fracture were carried out. It was determined that after tempering at 450°C the fracture toughness of the matrix of high-speed steels is directly proportional to the content of retained austenite in it. Every 1 % by volume of retained austenite increases the fracture toughness K_lc of the matrix by about 5%, despite the fact that most probably it is completely transformed into fresh martensite in front of a propagating crack. Higher fracture toughness of the matrix of high-speed steels rich in molybdenum should be explained exlusively by a larger content of retained austenite. Transformations in the martensitic part of the matrix of the alloys richer in molybdenum clearly reduce the advantageous effect of retained austenite on this steel feature.     

Stress corrosion cracking of stainless steels

The similarities and differences in the stress corrosion cracking response of ferritic and austenitic stainless steels in chloride solutions will be examined. Both classes of materials exhibit a cracking potential: similar transient response (to loading) of the potential in open circuit tests or the current in potentiostatic tests and similar enrichment of chromium and depletion of iron in the film associated with localized corrosion processes. The ferritic steels are more resistant to localized corrosion than are the austenitic steels, which is responsible for the difference in the influence of prior thermal and mechanical history on cracking susceptibility of the two types of steel. Similarities in the fractography of stress corrosion cracks and those produced by brittle delayed failure during cathodic charging of the ferritic steels indicate that hydrogen embrittlement is involved in the failure process.     

Clean steels for steam turbine rotors—their stress corrosion cracking resistance

This work presents the results of a comprehensive study concerning stress corrosion crack growth rates in steam turbine rotor steels exposed to hot water. The effects of stress intensity, temperature, and dissolved gases in the water have been investigated. Special attention has been given to the influence of impurities and alloying elements in the steel such as P, S, Mn, Si, Mo, and Ni, and to the effect of yield strength and fracture toughness on the growth rates of stress corrosion cracks. The results of this study clearly show that there exists a threshold stress intensity of about 20 MNm^−3/2 above which the invariably intergranular stress corrosion cracks grow at a constant, stress-independent velocity. This plateau stress corrosion crack growth rate isnot affected by the oxygen and carbon dioxide concentration in the water. The temperature and the yield strength of the steel have a strong influence on the growth rate of stress corrosion cracks. In contrast, there isno effect of the steel composition within the range investigated, neither of the impurity elements such as P and S, nor of the major alloying elements such as Mn, Si, Mo, and Ni. Steels with low fracture toughness due to temper embrittlement do not exhibit faster stress corrosion crack growth rates in water than nonembrittled steels. No direct relationship between intergranular temper embrittlement and intergranular stress corrosion crack growth in water can be demonstrated.     

残留奥氏体对中碳双相钢冲击性能的影响

采用不同的热处理工艺研究了残留奥氏体对中碳双相钢冲击韧性的影响。利用金相显微镜、扫描电镜、透射电镜和摆锤式冲击试验机,对不同试样的显微组织与冲击韧性进行观察、检测和分析。试验结果表明：中碳贝氏体钢的冲击性能显著高于Q/P马氏体钢（室温冲击功是57J对应15J,-40℃冲击功是33J对应9J）,可能的原因是贝氏体钢中薄膜状残留奥氏体,对裂纹扩展的阻止效应更显著。     

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

Grain boundary segregation in austenitic stainless steels and its effect on intergranular corrosion and stress corrosion cracking

This paper reports a study of grain boundary segregation, intergranular corrosion, and intergranular stress corrosion cracking in austenitic stainless steels. The results show that phosphorus, nitrogen, and sulfur all segregate to grain boundaries in these materials and that they can affect one another's segregation through site compctition. In particular, the results demonstrate that phosphorus segregation can be lowered by the presence of nitrogen and sulfur in the steel. Also, if manganese is present in the steel, sulfur segregation will be greatly decreased as a result of formation of manganese sulfides. Phosphorus, sulfur, and nitrogen will not initiate intergranular corrosion in the modified Strauss test, although if corrosion is initiated by chromium depletion, these elements might enhance the corrosion process. Phosphorus segregation does enhance corrosion in the Huey test, even in steels that have not undergone grain boundary chromium depletion, although there does not appear to be a precise correlation between the depth of corrosion penetration and phosphorus segregation. Intergranular stress corrosion cracking in 288 °C water at a pH of 2.5 and electrochemical potential of OV_SHE can occur in these steels even in the absence of chromium depletion if sulfur is present on the grain boundaries. Phosphorus segregation appears to have very little effect.     

Effects of strain states on stability of retained austenite in medium Mn steels

Based on uniaxial tensile and plane strain deformation tests,the effects of strain states on the stability of RA(retained austenite)in medium Mn steels,which were subjected to IA(intercritical annealing)and QP(quenching and partitioning)processing,were investigated.The volume fractions of RA before and after deformation were measured at different equivalent strains.The transformation behaviors of RA were also investigated.The stability of RA differed across two different transformation stages at the plane strain state:the stability was much lower in the first stage than in the second stage.For the uniaxial tension strain state,the stability of RA corresponded only to a single transformation stage.The main reason was that there were two types of transformations from RA in the medium Mn steel for the plane strain state.One type was that the martensite originated in the strain-induced stacking faults(SISF).The other type was the strain-induced directly twin martensite at a certain equivalent strain.However,for the uniaxial tension state,only the strain-induced twin martensite was observed.Dislocation lines and dislocation tangles were also observed in specimens deformed at different strain states.In addition,complex microstructures of stacking faults and lath-like phases were observed within a grain at the plane strain state.     

Stress corrosion cracking of stainless steels in NaCl solutions

The metallurgical influences on the stress corrosion resistance of many commercial stainless steels have been studied using the fracture mechanics approach. The straight-chromium ferritic stainless steels, two-phase ferritic-austenitic stainless steels and high-nickel solid solutions (like alloys 800 and 600) investigated are all fully resistant to stress corrosion cracking at stress intensity (K₁) levels ≤ MN • m-^3/2 in 22 pct NaCl solutions at 105 °C. Martensitic stainless steels, austenitic stainless steels and precipitation hardened superalloys, all with about 18 pct chromium, may be highly susceptible to stress corrosion cracking, depending on heat treatment and other alloying elements. Molybdenum additions improve the stress corrosion cracking resistance of austenitic stainless steels significantly. The fracture mechanics approach to stress corrosion testing of stainless steels yields results which are consistent with both the service experience and the results from testing with smooth specimens. In particular, the well known “Copson curve” is reproduced by plotting the stress corrosion threshold stress intensity (AT_ISCC) vs the nickel content of stainless steels with about 18 pct chromium. Formerly with the BBC Brown Boveri Company, Baden, Switzerland     

Experimental determination of the stability of retained austenite in low alloy TRIP steels

The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the M^σ_s temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A single-specimen temperature-variable tension test technique (SS-TV-TT) has been employed, which allowed to link the appearance of yield points in the stress-strain curve with the mechanically-induced martensitic transformation of the retained austenite. The results indicated that the M^σ_S temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400°C rather than 375°C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb-containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size.     

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.     

Microstructural effects on the stress corrosion cracking behavior of medium and high strength steels

The effects of variations in microstructure and strength level on the stress corrosion cracking susceptibility of three medium to high strength steels, H13, 300M, and HY-130, in 3.5 pct NaCl have been systematically studied. Superimposed on the expected inverse dependence of K_ISCC on yield strength was more than an order of magnitude reduction in crack growth rate, with no strength penalty. These results have been analyzed in terms of the possible relative roles of different microstructural features, in particular retained austenite, whose detailed behavior is the subject of a companion paper.