Stress corrosion cracking of cold-worked austenitic stainless steels

Stress corrosion cracking (SCC) of AISI 304L and AISI 316L stainless steels, cold-worked under various conditions (i.e. at different degrees of deformation obtained by drawing and rolling at room temperature and at liquid nitrogen temperature) has been carried out in H₂O containing 1000 ppm Cl^? at 250°C and in a boiling MgCl₂ solution. The effect of heat treatments at 400 and 900°C on the SCC of previously cold-worked steels has also been studied. Particular attention was directed towards heat treatment at 400°C. In steels deformed at room temperature, it increases the SCC resistance. By contrast, for steels deformed at liquid nitrogen temperature, heat treatment at 400°C reduces the SCC resistance if carried out for short periods of time (1–6 h). Hardness measurements, structural analyses via X-rays, scanning and transmission electron microscopy (SEM and TEM), as well as modified Strauss tests, seem to prove that reduced stress corrosion resistance is not to be related to the chromium-rich carbides precipitation which could have been accelerated by the presence of α′-martensite. Instead, they tend to suggest that perhaps this phenomenon is connected to an increase in the level of internal micro-stresses which are generated by a reciprocal re-ordering of the α′ and γ structural phases.     

High voltage electron metallography of stress corrosion cracking of austenitic stainless steels

High voltage electron metallographic studies of stress corrosion cracks nucleated on miniature U-bend specimens of austenitic stainless steels support an anodic dissolution mechanism of cracking. For all the alloys examined crack nucleation occurs at slip traces and a characteristic slotted dissolution morphology is observed in crack tip regions. The crystallography of this attack as determined by stereo-microscopy and trace analysis, reveals the slots to be bounded by {110} planes whilst growing in 〈111〉 directions.Cracking occurs through the network of corrosion slots at the crack tip and the well known features of stress corrosion fracture surfaces can be interpreted by this mechanism.     

Microstructure and stress corrosion cracking in simulated heat-affected zones of duplex stainless steels

The effects of nitrogen content and the cooling rate on the reformation of austenite in the Gleeble simulated heat-affected zone (HAZ) of 2205 duplex stainless steels (DSSs) were investigated. The variation of stress corrosion cracking (SCC) behavior in the HAZ of 40 wt% CaCl₂ solution at 100 °C was also studied. Grain boundary austenite (GBA), Widmanstatten austenite (WA), intergranular austenite (IGA) and partially transformed austenite (PTA) were present in the HAZ. The types and amounts of these reformed austenites varied with the cooling rate and nitrogen content in the DSS. U-bend tests revealed that pitting corrosion and selective dissolution might assist the crack initiation, while the types and amounts of reformed austenite in the HAZ affected the mode of crack propagation. The presence of GBA was found to promote the occurrence of intergranular stress corrosion cracking. WA, IGA and PTA were found to exhibit a beneficial effect on SCC resistance by deviating the crack propagation path.     

Prediction of the remaining lifetime of stainless steels under conditions of stress corrosion cracking

The prediction of the lifetime of metal structures and equipment under conditions of stress corrosion is very complicated because of the complexity of this process of degradation. Recently a new method, based on the so‐called corrosion elongation curves, has been found, which can be used to predict the time to failure under these conditions. By upgrading of these curves (and thus obtaining Upgraded Corrosion Elongation Curves – UCEC's) it has been possible to obtain a precise definition of the time needed for the initiation of the corrosion crack, and for its stable growth. It is upon this basis that diagrams for the prediction of remaining lifetime (DPRL's) have been developed. DPRL's can also be used to predict the values of various critical parameters which have to be achieved if a stress corrosion crack is to occur.     

Effect of alloy element contents on caustic stress corrosion cracking of several stainless steels

The stress corrosion cracking behavior in caustic solutions (200 g/l sodium hydroxide, 10 g/l sodium chloride) of three austenitic (18Cr-10Ni-2.5Mo, 20Cr-25Ni-4.5Mo, 27Cr-31Ni-3.5Mo) and three duplex (23Cr-4Ni, 22Cr-5Ni-3Mo, 25Cr-7Ni-4Mo-N) stainless steels was examined. U-bend and Slow Strain Rate (SSR) tests were performed at 200–250°C. The negative influence of nickel in the lower range content for the 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo has been shown; when the nickel content is significantly increased (>30%), as in the case of the steel 27Cr-31Ni-3.5Mo, an increase of SCC resistance has been detected. The negative effect of molybdenum, mainly on the behaviour of duplex stainless steels, has also been evidenced. The duplex stainless steels show better caustic SCC resistance than austenitic stainless steels type 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo. The best behaviour has been found for the less-alloyed steel 23Cr-4Ni.     

Impact of temperature excursion on stress corrosion cracking of stainless steels in chloride solution

The impact of a temperature excursion on the subsequent stress corrosion crack growth at the normal operating temperature has been investigated for 321 stainless steel (UNS32100) and 316L stainless steel (UNS31603) using precracked compact tension specimens. Although the data are preliminary the indication is that once crack growth has initiated in 321 SS at the elevated temperature, 130 °C in this study, the crack growth may be sustained at the lower temperature (40 °C), at least over the exposure time of about 700 h. However, the growth rate of 316L SS at the lower temperature was significantly lower than for 321 SS and tended to zero after 2000 h. For the 316 SS a temperature transient should not impact on structural integrity, provided it is short in duration.     

The correlation between mechanical and electrochemical parameters of stress corrosion cracking of austenitic stainless steels

Extension measurements of exposed specimens of austenitic stainless steels in hot magnesium chloride solutions are interpreted with the potential-time curves. For austenitic stainless steels, it is very difficult to determine the yield point; it is necessary to study the stress-elongation curves at different elongation rates and to known the creep behaviour of the steels. In dead load stress corrosion tests the elongation-time curves allow the incubation time of stress corrosion cracks to be distinguished from the propagation time. The propagation time is more important than incubation time for the classification of the susceptibility of austenitic stainless steels to stress corrosion cracking, because the incubation time is more dependent on experimental procedure than is the propagation time. The stainless steel classification obtained was compared with a new test in which a load is applied and immediately taken off; after a rapid fall the potential-time behaviour provides information about the crack velocity.     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.     

Chloride-induced stress corrosion cracking of Duplex stainless steels. Models,test methods and experience

Stress corrosion cracking (SCC) induced by chlorides frequently causes problems in applications where standard austenitic stainless steels are being used. Often this problem can be solved by the use of duplex stainless steels. In this report the mechanisms for SCC have been surveyed, and the cause for the high SCC resistance of duplex stainless steels has been discussed and evaluation of test methods for SCC and how duplex stainless steels respond to them, as well as practical experience of duplex stainless steels. The study shows that no single mechanism can be attributed to the good resistance to SCC of duplex stainless steels. Probably a synergistic effect of electrochemical and/or mechanical effects is responsible for the good performance. Test methods for SCC often give relatively good correspondence with real applications, but ranking is often doubtful, and comparisons of different material types should be made with caution. Numerous cases with SCC on standard austenitic stainless steels have been solved by the use of duplex stainless steels.     

Effect of W on stress corrosion cracking susceptibility of newly developed Ni-saving duplex stainless steels

Effect of W on stress corrosion cracking behavior (SCC) of Ni-saving duplex stainless steels (Fe18Cr6Mn3Mo0.4N (2.13, 5.27)W, in wt%) was investigated in 4 M NaCl solution using a slow strain rate test method. The change in the W content from 2.13 to 5.27 wt% marginally affected the tensile properties of the investigated DSSs. Alloying W clearly improved the pitting initiation resistance and repassivation tendency of the investigated alloys, but the SCC susceptibility was not remarkably decreased by addition of W. The slight enhancement in the SCC resistance of the alloy containing 5.27 wt% W was revealed to be correlated with the accelerated galvanic corrosion between the ferrite and austenite phases as a result of the W partitioning preferentially into the ferrite phase which could encourage the propagation of pitting.     

Passivity breakdown and stress corrosion cracking of stainless steel

Passivity breakdown of stainless steel is an important initial process for starting stress corrosion cracking. It was found that small amount of impurities in environments affects the initiation process, but do not affect the propagation process of SCC. The environmental effect on the initiation process is rationally explained by introducing “bound water model” of passive film and HSAB (Hard and Soft Acid and Base) rule. Background of the issue including bound water model and HSAB rule was discussed.     

Controlling grain boundary energy to make austenitic stainless steels resistant to intergranular stress corrosion cracking

Intergranular corrosion and intergranular stress corrosion cracking are the two localized corrosion mechanisms that are of concern to the typical applications of austenitic stainless steels in industries. Until recently, the common understanding was that a higher frequency of random boundaries increases the susceptibility, caused by a sensitization heat treatment or by operating temperatures, of austenitic stainless steels to both intergranular corrosion and intergranular stress corrosion cracking. A recent study demonstrated that extreme randomization of grain boundaries leads to a considerable improvement of resistance to both sensitization and intergranular corrosion. This work is a continuation of Ref. 1 and relates the effects of grain boundary randomization to intergranular stress corrosion cracking: the results show a trend consistent with earlier observations on intergranular corrosion. It is shown that there is improvement in resistance to intergranular stress corrosion cracking with extreme randomization of grain boundaries.     

Effects of surface preparation on pitting resistance, residual stress, and stress corrosion cracking in austenitic stainless steels

Surface finishing treatments such as shot blasting and wire brushing can be beneficial in improving the integrity of machined surfaces of austenitic stainless steels. These operations optimize in-service properties such as resistance to pitting corrosion and stress corrosion cracking (SCC). In this study, ground steel surfaces were subjected to a series of sand blasting and wire brushing treatments. The surfaces were then characterized by their hardness, surface residual stress state, and resistance to stress corrosion and pitting corrosion. Some samples were selected for depth profiling of residual stress. It is found that surface hardening and the generation of near-surface compressive residual stress are the benefits that can be introduced by sand blasting and brushing operations.     

The fractography of stress corrosion cracking in carbon steels

The stress corrosion cracking of carbon steels in a number of environments (nitrates, hydroxides, carbonates, chlorides and liquid ammonia) has been studied by scanning electron microscopy. The crack path depends on both heat treatment and environmental conditions. In annealed steels it appears that solutions which produce thick corrosion products induce intergranular cracking, while in solutions where adsorption induced inhibition occurs or thin films form, cracking is transgranular. Although the available fractographic evidence is ambiguous, it appears to rule out either hydrogen embrittlement or brittle films as being important in crack propagation. However, the fracture surface morphologies are consistent with the cracks propagating by anodic dissolution, possibly aided by mechanical tearing.     

Activation pH and susceptibility to hydrogen assisted stress corrosion cracking of martensitic stainless steels in sulphate solutions

The investigation of two different high strength stainless steels by the slow strain-rate technique has shown the existence of critical pH values (activation pH) for stress corrosion cracking in sulphate-containing media which depend on the steel composition. The slow strain-rate test indicates a sharp decrease of specimen ductility at the activation pH the value of which can also be determined by independent electrochemical methods.     

The austenitic manganese-chromium steels resistant to stress corrosion cracking

The basic type of MnCr steels (composition 0.05Mn19Cr13) was found to be highly resistant to chlorides. Additions of titanium, niobium and vanadium did not impair resistance to stress corrosion cracking, but addition of nickel in excess of 0.5% appreciably impaired the resistance. The chemical composition of the stabilized steels was selected in such a way as to make the ferrite content as low as possible. The two stabilized steel grades ability to passivate was limited, also because of the presence of manganese; this makes these grades suitable only for less aggressive environments. The ferrite content can be controlled, or the chromium content increased for better corrosion resistance, only by adding another strong austenite-former. Considering the adverse effect of nickel on the resistance to stress corrosion cracking, attention was given to manganese-chromium grade modified with nitrogen and molybdenum.