Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

Grain-boundary corrosion of type 304 stainless steel by cesium oxides

When austenitic stainless steel (300 series) is exposed to cesium oxides in the tempera-ture range from 450° to 700°C the grain boundaries are attacked preferentially. The penetration of cesium oxides into the grain boundaries of AISI Type 304 stainless steel has been studied as a function of time and temperature. These investigations have established that the penetration kinetics are linear in time, the activation energy for the process is 19 kcal/mole, and the rate of penetration is fairly insensitive to carbide precipitation and precipitate composition and morphology. The kinetics of the process are approximately an order of magnitude faster than those observed for some reactor (U, Pu) oxide fuel elements clad with Type 304 stainless steel under fast-flux irradiations, and the results are discussed qualitatively.     

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M₂₃C₆ was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti₄C₂S₂ which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M₂₃C₆ and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M₂₃C₆ and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.     

Ferrite formation in neutron irradiated type 316 stainless steel

Magnetic measurements were made on Type 316 stainless steel specimens that had been neutron irradiated to fluences of 1.8 × 10₂₂ neutrons/cm₂ at 425°C and 3.5 × 10₂₂ neutrons/ cm₂ (E > 0.1 MeV) at 500 and 600°C. A significant increase of magnetization was observed for the irradiated specimens compared to the unirradiated specimens. The shape of the magnetization vs field curves showed that the irradiated specimens contained many small superparamagnetic particles. The magnetic particles are assumed to be the ferrite phase although other possibilities cannot be excluded. The amount and distribution of the magnetic phase varied with pre-irradiation and post-irradiation heat treatment. The maximum value of magnetization was equivalent to 3.6 pct ferrite in a specimen annealed 100 h at 760°C before irradiation and 1 h at 500°C after irradiation at 425°C.     

Ferrite formation in neutron irradiated type 316 stainless steel

Magnetic measurements were made on Type 316 stainless steel specimens that had been neutron irradiated to fluences of 1.8 × 10₂₂ neutrons/cm₂ at 425°C and 3.5 × 10₂₂ neutrons/ cm₂ (E > 0.1 MeV) at 500 and 600°C. A significant increase of magnetization was observed for the irradiated specimens compared to the unirradiated specimens. The shape of the magnetization vs field curves showed that the irradiated specimens contained many small superparamagnetic particles. The magnetic particles are assumed to be the ferrite phase although other possibilities cannot be excluded. The amount and distribution of the magnetic phase varied with pre-irradiation and post-irradiation heat treatment. The maximum value of magnetization was equivalent to 3.6 pct ferrite in a specimen annealed 100 h at 760°C before irradiation and 1 h at 500°C after irradiation at 425°C.     

Tensile and fracture properties of type 316 stainless steel after creep

The effects of creep on the mechanical properties of type 316 stainless steel were studied. Tensile and Charpy specimens were machined from the oversize specimens crept at 750 °C and 103 MPa. The ambient fracture energy was found to deteriorate rapidly after creep. The ambient yield stress was increased moderately, but the tensile ductility was severely reduced. The effects of intergranular carbides alone on mechanical properties were studied with specimens thermal aged without load. These carbides were shown to cause a moderate reduction in fracture energy and tensile ductility but had little effect on yield stress. Extensive grain boundary separations were observed on the fracture surfaces. SEM studies showed that these grain boundaries were covered with micro voids initiated by the dense intergranular carbides. Frequently, large dimples on grain boundary joined up and initiated shear fracture into the grain. In the crept specimens additional microstructural changes in the form of intragranular carbides and subgrain boundaries were observed. Both are responsible for the increase in yield stress and the further reduction in tensile ductility and fracture energy. The intragranular carbides also modified the size and density of the dimples on the fracture surfaces. Formerly with Metallurgy and Materials Science Division, Brookhaven National Laboratory, Upton, NY 11973.     

Microstructural stability of thermal-mechanically pretreated type 316 austenitic stainless steel

Studies of the microstructural stability of Type 316 austenitic stainless steel were performed for a wide range of thermal-mechanical pretreatments in the limited aging temperature range of 550° to 760°C. The pretreatments were selected in order to investigate the effects of varying solution treatment temperature, amount of cold reduction by rolling, initial grain size, and initial precipitate distribution. Large variations in both phase stability and recrystallization behavior can be effected by appropriate pretreatments. Cold work accelerates precipitation of M₂₃C₆ carbide and the intermetallic compounds (Laves, χ, and σ phases). Both the amount and kinetics of σ phase formation are especially enhanced by recrystallization occurring in the aging temperature range. It is suggested that this occurs due to ready σ nucleation at slowly moving (recrystallizing) grain boundaries together with enhanced growth rates due to diffusion along the boundary. Fine grain size enhances phase instability by providing additional nucleating sites and decreased diffusion paths for precipitate forming elements, but in the grain size range studied (ASTM No. 3.5 to No. 13) the effect is not as significant as the effect of cold work, particularly when recrystallization occurs during the aging treatment. Fine grain size and pretreatments which precipitate the carbides prior to the final cold working step enhance recrystallization kinetics relative to solution treated and cold-worked materials. This is apparently due to stabilization of the cold-worked substructure in the solution treated samples by precipitation of carbide and Laves phases on the dislocations and stacking faults.     

Control of annealing twins in type 316 austenitic stainless steel

The twinning frequency expressed as the number of coherent twin boundaries per grain has been investigated as a function of grain size in type 316 and 316L commercial austenitic stainless steels with different carbon contents as well as doped with 30ppm of boron, and subjected to various thermomechanical treatments. Experiments have established that the number of coherent twin boundaries per grain (TB/grain) for grain sizes larger than 0.1 mm, increases linearly with grain size according to the equation: TB/grain = A + K_td, where A and K_t are constants and d is the true volume grain diameter. In the ultrafine and fine grain size range from 1.5 to 100 μm the number of twins per grain is the lowest one and virtually independent of grain size. Both the increase in carbon content by 0.01% and doping with 30 ppm of boron, significantly suppress the formation of twins. Also, the rate of twins formation depends on whether twins are formed during pure grain growth or simultaneous recrystallization and grain growth. The results obtained are explained in terms of the classical Fullman-Fisher theory of twins formation.     

Discontinuous creep deformation in a type 316 stainless steel casting

A static cast ingot of type 316 stainless steel was tested at 811, 866, and 922 K. Creep curves for this material under certain temperature and stress conditions show instantaneous increase in strain. In the open literature this increase has been called “strain burst” or “jump”. The number of “bursts” per curve varied from 0 to 3, and the instantaneous strain was as large as 2 pct. The bursts generally occurred at high stresses and at the two lower test temperatures, 811 and 866 K. The rupture time for specimens showing bursts was lower than for specimens without them. The creep-rupture strength of the static casting was approximately 40 MPa lower than that for wrought material. The rupture time of the casting was a strong function of specimen location. The fine grained structure near the ingot edge produced longer rupture time compared with the coarse grain structure. Microprobe analysis showed significant variation in chromium, nickel, and molybdenum across the dendritic regions. The inhomogeneous distribution of the potent creep-strengthening element molybdenum has been suggested as responsible for the strain bursts.     

Strain aging and load relaxation behavior of type 316 stainless steel at room temperature

The strain aging and load relaxation behavior of type 316 stainless steel (SS) at room temperature were studied. It is shown that rapid aging occurs in 316 SS at room temperature to an extent that affects the load relaxation behavior of the material. Qualitatively, the aging behavior was found to agree with those reported earlier for Fe-Ni-C-alloys, and the observed aging characteristics could be explained by using an earlier proposed vacancy-interstitial mechanism. The load relaxation behavior is analyzed in terms of Hart’s state variable model. Effects of strain aging and strain hardening on the load relaxation behavior and the scaling of the relaxation curves are determined. It is shown that aging can be accounted for by a time-dependent change in a model parameter, which is dependent on the mobile dislocation density and the dislocation mobility. In addition, a dependency on plastic state of the same parameter previously held constant was found. It is concluded that this phenomenon, which in 316 SS could be rationalized in terms of increasing forest dislocation density, is likely to be more general, and a provision for it should be made in the state variable theory. S. P. Hannula formerly Research Associate in the Department of Materials Science and Engineering, Cornell University, Ithaca, NY M. A. Korhonen, formerly Visiting Assistant Professor in the Department of Materials Science and Engineering, Cornell University, Ithaca, NY     

The effect of helium on the stress-rupture behavior of type 316 stainless steel

Flat tensile samples of type 316 stainless steels with helium contents of 1.5 × 10⁻⁶ and 4.0 × 10⁻⁵ atom fraction, were stress-rupture tested in vacuum at 700°C. The presence of helium caused large reductions in rupture life and in elongation at failure. The amount of strain produced within the grains of helium-containing samples was a small fraction of the measured total elongation. The difference in these values is accounted for by extensive intergranular cracking.     

On microstructure-property correlation of thermally aged type 316L stainless steel weld metal

This paper deals with the microstructural changes and consequent deterioration in the room temperature tensile properties of type 316L stainless steel weld metal when exposed to elevated temperatures (773 to 973 K) for prolonged periods (up to 5000 hours). The microstructure-property correlation derived in this study is based on a variety of techniques: Magne-Gage, electrochemical extraction, X-ray diffraction, tensile testing, and both optical and electron microscopy. It has been established that the amount and morphology of the sigma phase are the key factors in determining the changes in the strength levels, total elongation, and extent of work hardening. The amount and morphology of sigma, in turn, is seen to depend on the relative kinetics of the various transformations, such as dissolution of delta-ferrite, growth of carbides,etc., shape changes in sigma, and the relative stabilities of the phases at the corresponding temperature of aging. The complicated dependence of the tensile properties on the microstrutural changes has been explained with direct quantitative evidence.     

Creep strengthening of low carbon grade type 316LN stainless steel by nitrogen

Nitrogen-alloyed 316LN stainless steel is used as a structural material for high temperature fast breeder reactor components. With a view to increase the design life of the components up to 60 years and beyond, studies are being carried out to develop nitrogen alloyed 316LN stainless steel with superior tensile, creep and low cycle fatigue properties. This paper presents the results from studies on the influence of nitrogen on the high temperature creep properties of this material. The influence of nitrogen on the creep behaviour of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt%. Creep tests were carried out at 923 K at stress levels 140, 175, 200 and 225 MPa. Creep rupture strength increased substantially with increase in nitrogen content. The variation of steady state creep rate with stress showed a power law relationship. The power law exponent varied between 6.4 and 13.7 depending upon the nitrogen content. Rupture ductility was generally above 40% at all the test conditions and for all the nitrogen contents. It was observed that the internal creep damage and surface damage decreased with increase in nitrogen content. Fracture mode was found to generally shift from intergranular failure to transgranular failure with increasing nitrogen content.     

Serrated yielding in AISI 316 stainless steel

Serrated yielding behaviour of a type AISI 316 stainless steel is investigated in the temperature range 300–923 K. Detailed observations have been made on the effect of prior ageing in the temperature range (823–1323 K) for different periods of time on the serrated yielding behaviour at 923 K. Two temperature regions with different activation energies have been identified for serrated flow. The mechanism for serrated flow in the low temperature region (∼ 523–623 K) is the diffusion of interstitial solute to dislocation while substitutional solutes like Cr is responsible for serrated yielding at high temperature region. Certain time-temperature combination of ageing is found to eliminate serrated yielding at 923 K. Results also support the role of grain boundaries as preferred sites for dynamic strain ageing in austenitic stainless steels as previously suggested.