抗晶间腐蚀奥氏体不锈钢的晶界工程

Sensitization by chromium depletion due to chromium carbide precipitation at grain boundaries in austenitic stainless steels can not be prevented perfectly only by previous conventional techniques, such as reduction of carbon content, stabilization-treatment, local solution-heat-treatment, etc. Recent studies on grain boundary structure have revealed that the sensitization depends strongly on grain boundary character and atomic structure, and that low energy grain boundaries such a~ coincidence-site-lattice (CSL) boundaries have strong resistance to intergranular corrosion. The concept of grain boundary design and control has been developed as grain boundary engineering (GBE). GBEed materials are characterized by high frequencies of CSL boundaries which are resistant to intergranular deterioration of materials, such as intergranular corrosion. A thermomechanical treatment was tried to improve the resistance to the sensitization by GBE. A type 304 austenitic stainless steel was cold-rolled and solution-heat-treated, and then sensitization-heat-treated. The grain boundary character distribution was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction The frequency of CSL boundaries indicated a maximum at the small reduction. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material. A high density of annealing twins were observed in the thermomechanical-treated specimen. The results suggdst that the therrmomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface. The effects of carbon content and other minor elements on optimization in grain boundary character distribution (GBCD) and thermomechanical parameters were also examined during GBE.     

Grain boundary segregation in Al-Zn-Mg alloys—Implications to stress corrosion cracking

Prior studies on grain boundary segregation in Al-Zn-Mg have shown that free Mg is present along the grain boundaries in these alloys under all heat treatment conditions. In this paper, a physical basis for the observed segregation profiles is provided through the vacancy-pump model developed by Anthony. The implications of the observed segregation to stress corrosion cracking (SCC) and other environmentally induced embrittling phenomena (where hydrogen could possibly play a dominant role) are discussed in terms of a possible Mg-H complex formation. Formerly with Martin Marietta Laboratories.     

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.     

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.     

Microstructure and microsegregation effects in the intergranular corrosion of austenitic stainless steel

Intergranular corrosion in austenitic stainless steel was studied using transmission electron microscopy of corroded thin foils and electron probe microanalysis of bulk specimens. In the sensitized material, since carbides remained unattacked in corroded grain boundaries after exposure to a boiling copper sulfate-sulfuric acid solution, the location and severity of corrosion could be directly observed in relation to individual carbide particles for various precipitate morphologies. After the sensitized material was exposed to a potassium dichromate nitric acid solution, carbides were consistently absent from corroded grain boundaries as the particles themselves apparently became susceptible to attack in this environment. Chemical composition inhomogeneities were measured for nickel and chromium in the commercially annealed material and found to become more pronounced upon sensitization heat treatments. Such inhomogeneities can result in chemical composition differences across grain boundaries, which in turn can lead to electrochemical action that may adversely affect intergranular corrosion behavior. N. C. BARBI, formerly Graduate Student, Rensselaer Polytechnic Institute, Troy, N. Y. 12181     

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.     

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.     

Grain refinement of wrought austenitic stainless steels by rapid heating

Rapid heating after cold deformation has been investigated as a grain refinement technique for austenitic stainless steels. The method shows considerable promise for obtaining average grain diameters in the 3 to 10 μm range without causing carbide precipitation in the alloy. This technique has been used here to obtain a wide range of grain sizes in type 316 steel for property measurements. The hardness, tensile strength and 0.2 pct offset yield strength were found to vary linearly with the reciprocal of the square root of the average grain diameter.     

A mechanism for hydrogen-induced intergranular stress corrosion cracking in alloy 600

Intergranular stress corrosion cracking (IGSCC) of Alloy 600 in high-temperature, deaerated water or steam has been termed “hydrogen-induced IGSCC.” We believe these cracks are initiated by the nucleation of a high density of bubbles on the grain boundary under the combined action of the applied stress and high pressure methane formed from carbon in solution reacting with hydrogen injected by corrosion. The bubbles then grow together by grain boundary diffusion to give local failure. This is supported by transmission electron microscope (TEM) observations of two-stage replicas, which show the subsurface formation of closely spaced (0.2μm) bubbles along boundaries, and their growth into fine cracks before they open to communicate with the corroding atmosphere. The kinetics are examined and shown to be in quantitative agreement with several experimental observations. This mechanism involves no grain boundary dissolution of the metal, the only role of corrosion being the injection of hydrogen at a high fugacity. It predicts an activation energy roughly equal to that for grain boundary dψusion of nickel in Alloy 600.     

A mechanism for hydrogen-induced intergranular stress corrosion cracking in alloy 600

Intergranular stress corrosion cracking (IGSCC) of Alloy 600 in high-temperature, deaerated water or steam has been termed “hydrogen-induced IGSCC.” We believe these cracks are initiated by the nucleation of a high density of bubbles on the grain boundary under the combined action of the applied stress and high pressure methane formed from carbon in solution reacting with hydrogen injected by corrosion. The bubbles then grow together by grain boundary diffusion to give local failure. This is supported by transmission electron microscope (TEM) observations of two-stage replicas, which show the subsurface formation of closely spaced (0.2μm) bubbles along boundaries, and their growth into fine cracks before they open to communicate with the corroding atmosphere. The kinetics are examined and shown to be in quantitative agreement with several experimental observations. This mechanism involves no grain boundary dissolution of the metal, the only role of corrosion being the injection of hydrogen at a high fugacity. It predicts an activation energy roughly equal to that for grain boundary dψusion of nickel in Alloy 600.     

Duplex grain boundary precipitation in austenitic stainless steels containing aluminium and titanium

This paper considers the grain boundary precipitation reactions which may occur during the ageing of stainless steels based on the composition Fe-20%Cr-25%Ni which contain additions of aluminium and titanium. It is shown that the formation of the equilibrium precipitate phase, associated with the composition of the alloy, causes the subsequent formation of adjacent regions of chromium-rich ferrite. In addition, it is shown that this ferrite is unstable, and may decompose to a more stable intermetallic phase. The choice of final intermetallic phase, together with the multiphase crystallographic orientation relationships obeyed during the precipitation reaction and the local precipitate phase compositions, are discussed with respect to the overall reaction sequence.     

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).     

Grain boundary structure and chromium segregation in a 316 stainless steel

Analytical electron microscopy (AEM) has been used to examine the relationship between grain boundary structure and the segregation of chromium in a sensitised AISI 316 stainless steel. Fifty grain boundaries have been analysed. Among the boundaries there was a threefold difference in full width half maximum (FWHM) of the chromium concentration profiles. This variation is interpreted in terms of the different long-range stress fields created by different combinations of structural units in each boundary. From the narrowness of their profiles, it is deduced that the σ = 3, 11, 13a, 13b and 2ga boundaries are favoured while the σ = 9 boundary is non-favoured. There was no correlation between FWHM and grain boundary chromium concentration. Additionally, we observe that it is not pre-requisite for the boundary plane to be close to {111} for a chromium carbide to nucleate. There is also no correlation between the boundary normal and either the FWHM of the chromium concentration profile or the grain boundary chromium concentration.