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.     

Hydrogen-facilitated corrosion and stress corrosion cracking of austenitic stainless steel of type 310

The effects of hydrogen précharge and stress on anodic dissolution for Type 310 austenitic stainless steel (ASS) have been investigated. An experiment determining the effect of hydrogen on stress corrosion cracking (SCC) was carried out in a boiling 42 pct MgCl₂ solution and in a 2.5 mo/L H₂SO₄ + 1 mol/L HC1 solution. The results showed that both hydrogen and stress would increase the dissolution rate, and the effects of hydrogen and stress on the dissolution rate were synergistic rather than simply additive. Hydrogen lowered the threshold stress and the shortened fracture time of SCC in a boiling MgCl₂ solution by a factor of 1/5 and 10, respectively.     

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.     

Solidification and solidification cracking in nitrogen-strengthened austenitic stainless steels

The solidification behavior of three heats of nitrogen-strengthened austenitic stainless steel was examined and was correlated with solidification mode predictions and with hot cracking resistance. The heat of NITRONIC* 50 solidified by the austenitic-ferrite mode, and the NITRONIC 50W and NITRONIC 50W - Nb heats solidified by the ferritic-austenitic mode. This behavior was in good agreement with predictions based on Espy’s formulas for Cr and Ni equivalents. Both the NITRONIC 50W and NITRONIC 50W + Nb welds contained primary delta-ferrite, with the latter weld and the NITRONIC 50 weld also containing some eutectic ferrite. Solute profiles in austenite near the eutectic ferrite showed decreasing Fe and increasing Cr, Ni, Mn, and Mo relative to austenite in the dendrite cores. Numerous Nb-rich precipitates were found on the eutectic ferrite/austenite interfaces and within the eutectic ferrite. The precipitates were mainly Nb(C, N), with some Z-phase, a Nb-rich nitride, also detected. One instance of the transformation of eutectic ferrite to sigma-phase was observed to have occurred during cooling of the NITRONIC 50 weld. Hot cracking was seen in the NITRONIC 50 and NITRONIC 50W + Nb welds and resulted from the formation of a niobium carbonitride eutectic in the interdendritic regions. In the absence of Nb, the NITRONIC 50W heat formed no observable eutectic constituents and did not hot crack. The presence of hot cracks in the NITRONIC 50W + Nb weld indicates that solidification by the ferritic-austenitic mode did not counteract the effects of small Nb additions.     

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     

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.     

The effect of high temperature low cycle fatigue on the corrosion resistance of austenitic stainless steels

The effect of high temperature (650 °C) low cycle fatigue on the corrosion behavior of five austenitic stainless steels (Types 304, 316L, 321, and Incoloy Alloys 800 and 800H) has been investigated. For comparison, corrosion tests were also performed on samples of as-received material as well as material which had been solutionized and material which was sensitized at 650 °C. It was observed that cyclic loading at high temperature reduces the corrosion resistance to a much greater extent than does just the exposure of unstressed material to elevated temperatures. Formation of chrome carbides during cycling and depletion of chromium from the matrix is responsible for the decrease in corrosion resistance. Of the alloys tested, Type 304 exhibited the lowest corrosion resistance. Superior corrosion resistance of the other alloys was due to the following: (a) a lower carbon content, (b) a higher chromium content, and (c) the presence of a strong carbide forming element (stabilized material).     

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.     

Dispersion strengthening austenitic stainless steels by nitriding

The internal nitridation of thin sections of austenitic Fe−Cr−Ni−Ti alloys containing up to 2 pct Ti was studied over the temperature range 1600° to 2210°F in order to develop a method of strengthening the alloys through the introduction of a dispersoid of stable titanium nitrides. The interparticle spacing (IPS) of the nitrides was found to increase linearly with depth from the external surface; the effects of various parameters on the rate of change of IPS vs depth are presented. The mechanical properties of these alloys at room and elevated temperatures were markedly improved by internally nitriding. Useful mechanical properties were obtained up to 2200°F, with typical properties at 2000°F of 10 to 20 ksi 0.2 pct offset yield strength and 15 to 25 ksi ultimate tensile strength, but section thickness was limited to about 10 mils because of the increase in IPS with depth and the long nitriding times needed for thicker material. In order to produce a small interparticle spacing in a heavier section, internally nitrided 5 mil strip was consolidated by hot roll bonding and evaluated at a 60 mil thickness by tensile and rupture testing at 2000°F. It is demonstrated that the approach taken in this work offers a feasible technique for making a high temperature alloy having useful engineering properties.     

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.     

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.     

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.     

合金元素对铁素体不锈钢耐应力腐蚀开裂性能的影响

通过42%沸腾氯化镁U型弯曲试验,结合扫描电子显微镜、X射线衍射分析,研究了合金元素Ni、Cu、Mo对铁素体不锈钢耐氯化物应力腐蚀开裂性能的影响规律。结果表明,Ni、Cu元素能够提高铁素体不锈钢应力腐蚀开裂的敏感性,单独添加Mo元素不会降低铁素体不锈钢耐应力腐蚀开裂的能力,但是钢中Mo、Cu元素同时存在时,应力腐蚀开裂的敏感性大大增加。钢中析出的ε-Cu在氯离子环境中形成点蚀,引起了铁素体不锈钢应力腐蚀开裂。     

Creep-fatigue interaction in austenitic stainless steels

Low cycle fatigue failures occur by the initiation and controlled growth of a surface crack. The development of crack propagation models, based on continuum mechanics, have enabled successful predictions of fatigue life at both room and elevated temperatures. This paper attempts to extend such models to cover the situations in which creep damage, introduced during periods of stress relaxation, influences the rate of growth of the surface fatigue crack. Equations predicting fatigue life as a function of hold period are in good agreement with experimental data, for Type 304 stainless steel, Type 316 stainless steel and Incoloy-800.     

Instabilities in stabilized austenitic stainless steels

The effect of aging on the precipitation of grain boundary phases in three austenitic stainless steels (AISI 347, 347AP, and an experimental steel stabilized with hafnium) was investigated. Aging was performed both on bulk steels as well as on samples which were subjected to a thermal treatment to simulate the coarse grain region of the heat affected zone (HAZ) during welding. Aging of the bulk steels at 866 K for 8000 hours resulted in the precipitation of Cr₂₃C₆ carbides, σ, and Fe₂Nb phases; the propensity for precipitation was least for the hafnium-stabilized steel. Weld simulation of the HAZ resulted in dissolution of the phases present in the as-received 347 and 347AP steels, leading to grain coarsening. Subsequent aging caused extensive grain boundary Cr₂₃C₆ carbides and inhomogeneous matrix precipitation. In addition, steel 347AP formed a precipitate free zone (PFZ) along the grain boundaries. The steel containing hafnium showed the best microstructural stability to aging and welding. Formerly with Exxon Research and Engineering Company.     

Studies of the orientations of fracture surfaces produced in austenitic stainless steels by stress-corrosion cracking and hydrogen embrittlement

Orientation studies have been made on several different austenitic stainless steels, using photogrammetric and electron channeling techniques. The fracture facets produced by SCC in boiling aqueous MgCl₂ (155 °C) were large and relatively flat in the case of type 310 steels, and the fracture plane was found to be at or near {100}. The transgranular stress-corrosion fractures in type 304 steels were more complex, and there was considerably more scatter in the orientation determinations. However, the orientations of the fracture facets in these steels were clearly not {100}, but fell into two distributions, one near {211} and the other near {110}. Electron diffraction studies from the fracture surfaces indicated the presence of α′ and martensites in the type 304 but not in the type 310 cases; the possibility that this was responsible for the differences in fracture planes is discussed. Studies were also made of a type 304 specimen which had failed by SCC at 289 °C. No martensitic phases were detected at the fracture surfaces in this case, and the fracture facets were large and flat, similar to those for type 310. Cleavage-like fracture surfaces were also produced in type 304 steels by hydrogen embrittlement, using both gaseous hydrogen and cathodic charging, but the facets were too small for precise orientation determination. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly Professor of Metallurgy, University of Illinois.     

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.     

The role of retained austenite in stress corrosion cracking of steels

Metallurgical and Materials Transactions A - The ability of retained austenite to affect stress corrosion cracking susceptibility has been examined in two steels, containing mechanically stable and...