Interaction of Cyclic Damage Mechanisms at Elevated Temperatures

This work investigates the interaction of fatigue and creep-type damage components at 540 °C and 300 °C for OFHC copper and at 540 °C for commercial purity copper. Round specimens were subjected to a series of fatigue and creep-type damage components by sequential strain cycling at high and low frequency respectively. Maximum damage interaction occurred when a sequence of one low frequency cycle followed by one to six high frequency cycles was repeated to failure. Under these conditions an intercrystalline fracture mechanism was dominant. Little damage interaction was observed when large numbers of high and low frequency cycles were sequentially applied. It is also shown that for high temperature life prediction, the linear damage summation rule must be used carefully since it depends upon the amount of mixing as well as the order of damage for a given material and temperature. For OFHC copper at 540 °C, linear summation to failure may vary from less than 0.4 for maximum interaction to greater than unity for little, or no interaction of the damage mechanisms. Formerly Research Engineer, University of Waterloo     

Metallurgical Studies of Austenitic Stainless Steel 304 under Warm Deep Drawing

Austenitic stainless steel 304 was deep drawn with different blank diameters under warm conditions using 20 t hydraulic press. A number of deep drawing experiments both at room temperature and at 150 ℃ were conducted to study the metallography. Also, tensile test experiments were conducted on a universal testing machine up to 700 ℃ and the broken specimens were used to study the fractography of the material using scanning electron microscopy in various regions. The microstructure changes were observed at limiting draw ratio （LDR） when the cup is drawn at different temperatures. In austenitic stainless steel, martensite formation takes place that is not only affected by temperature, hut also influenced by the rate at which the material is deformed. In austenitic stainless steel 304, dynamic strain regime appears above 300 ℃ and it decreases the formability of material due to brittle fracture as studied in its fractography. From the metallographic studies, the maximum LDR of the material is observed at 150 ℃ before dynamic strain regime. It is also observed that at 150 ℃, grains are coarse in the drawn cups at LDR.     

Flow Behavior of SUS304 Stainless Steel Under a Wide Range of Forming Temperatures and Strain Rates

To determine the flow behavior of SUS304 stainless steel under different conditions, axisymmetric compression tests were conducted over a wide range of forming temperatures (25 °C to 400 °C) and strain rates (10⁻³ to 10 s⁻¹). Flow curves were obtained for different forming conditions to study the influence of the forming temperature and strain rate on the flow behavior. Moreover, electron backscatter diffraction analysis, X-ray diffraction analysis, transmission electron microscopy, and Feritscope were used to study the microstructure evolution of SUS304 stainless steel under different conditions for determining the underlying reasons for the variations in flow behavior. The experimental results indicated that the flow stress decreased with increasing the forming temperature. With increasing strain rate at 25 °C to 200 °C, the flow stress first increased and then decreased; however, the strain rate had little effect on the flow stress at 300 °C and 400 °C. By analyzing the variation in the phase transformation inside compressed SUS304 stainless steel samples under different forming conditions, the key factors affecting the flow behavior of stainless steel were identified. Finally, by examining the variation in the martensite content and the dislocation density, the dominant deformation mechanism under different forming conditions was determined.

     

Bulk strain and grain strain in axisymmetric deformation

Bulk strain, defined in terms of original and final dimensions of a specimen, correlates well with the logarithm of the average number of intercepts per unit length on test lines parallel to axes of symmetry after uniaxial deformation of an axisymmetric specimen. This suggests a definition of principal grain strain based on the mean linear intercepts before and after deformation, measured along principal directions. Results of type 304 stainless steel strained to varying amounts between necking and fracture support the hypothesis that the axial gradient of the total grain boundary area projected onto any cylindrical surface coaxial with the axis of deformation remains unchanged as deformation progresses. Although the behavior of the grain boundary network leads to simple correspondences between grain and bulk strains, they are not numerically equal for this type of deformation.     

Failure behavior of 304l stainless steel in torsion

The workability of 304L austenitic stainless steel has been investigated using torsion testing at temperatures from 20 °C to 1200 °C and strain rates of 0.01 and 10.0s ^-1 For the lower strain rate, temperature changes due to deformation heating were minimal, and failure was found to be fracture controlled at all temperatures. As for many other metals, the 304L exhibited a ductility minimum at warm-working temperatures. For the higher strain rate, failure was controlled by flow-localization processes at 20 °C and 200 °C. At these temperatures, flow softening resulting from deformation heating was deduced to be the principal cause of flow localization. A model to predict the strain at the onset of localization was developed and applied successfully to the 304L results. For high strain-rate torsion tests at 400 °C and above, failure was fracture controlled as in the low strain-rate tests, and the ductilities were shown to be correlated to those at the lower strain rate through the Zener-Hollomon parameter by employing an activation energy derived from flow-stress data.     

Effect of strain wave shape on high temperature fatigue life of a type 316 steel and application of the strain range partitioning method

Effects of strain rate and strain wave shape on low-cycle fatigue life of a Type 316 stainless steel were investigated at 600 and 700 °C. A great reduction in the fatigue life corresponded with a variation in the fracture mode. Especially extensive grain boundary microcracks such as wedge- and cavity-type cracks were observed in slow-fast sawtooth wave shape tests and in tension hold time tests, respectively. The test results were analyzed by the strain range partitioning method proposed by Manson, Halford and Hirschberg. In applying the method, a new technique of partitioning the inelastic strain range was proposed and used. Four component strain range vs life relationships were dependent on the testing temperature between 600 and 700 °C. However, the difference between 600 and 700 °C was eliminated by using normalized component strain range by dividing by ductility of creep rupture test and tensile test.     

The influence of polycrystal grain size upon the creep ductility of copper

A study of the relationship between grain size and creep ductility has been made on OFHC copper when the fracture process involves intergranular cavitation. Tests have been carried out at a constant nominal strain-rate at 10^?2 hr^?1 at temperatures of 350°, 425°, and 500°C. At all temperatures a peak in the grain size vs ductility plots is obtained at (approximately) 30 μm for 350°C, 60 μm for 435°C, and 150μm for 500°C. The work involved a metallographic study of the fracture process and it is deduced that the final stage of fracture (cavity linkage) controls ductility, rather than either the rate of nucleation or individual growth of cavities. At coarse grain sizes and low temperature the crack length seems to be critical and increasing grain size decreases ductility. At fine grain sizes and high temperature, failure is by a ductile void-sheet process and the volume fraction of cavities is controlling so that decreasing grain size decreases ductility.     

Tribological Investigation of Effect of Grain Size in 304 Austenitic Stainless Steel

Stainless steels are widely used in a variety of engineering applications such as food appliances, surgical instruments, nuclear reactors and cryogenic applications. The properties of stainless steel are greatly affected by the grain size. The present study investigates the effect of grain size on sliding wear behavior of AISI 304 stainless steel. The sliding wear properties are measured using a Pin-on-Disc machine. Annealing heat treatment process varies the grain size of steel at 1100 °C. The wear test is performed on different grain sizes of AISI 304 steel at various sliding speeds under dry condition. The wear rate of the steels at different sliding distances is plotted as a function of grain size. The maximum wear rate is obtained at an intermediate grain size. It is noted that frictional force and temperature initially increases and then reaches the saturation plateau. The results are used to establish a correlation between the grain size and sliding wear properties of stainless steel. The present study is useful in enhancing the life of various components made of the stainless steel.     

Chromium depletion and martensite formation at grain boundaries in sensitised austenitic stainless steel

A microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted. Grain boundary depletion of chromium has been quantified in terms of sensitisation time, temperature and boundary type by energy dispersive X-ray microanalysis in the transmission electron microscope. Chromium depleted profiles measured in grain boundary vicinities are sometimes asymmetrical and correlate with the expected profiles generated by growth of semicoherent and incoherent carbide interfaces. The depletion of chromium promotes martensite formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C. Transformation occurs at the most severely depleted boundaries and initiation is favoured at slip band-boundary intersection points and along grain boundaries whose plane orientation matches that of the martensite habit plane. The preferential formation of grain boundary martensite could be an important factor in the stress corrosion and environment sensitive failure of this material.     

The Recrystallization Behavior of an Austenitic Stainless Steel Ingot Structure Due to Hot Deformation

The kinetics of recrystallization were determined metallographically for an ingot casting of AISI type 304 stainless steel deformed over a range of strains at temperatures of 1600 to 2250°F (860 to 1232°C) at several strain rates, and annealed at temperatures of 1900 to 2250°F (1037 to 1232°C). As with recrystallization following room temperature cold work, the time for recrystallization was reduced for increasing deformation and annealing temperature and for increasing strains. Decreasing the deformation temperature resulted in a reduction of time for recrystallization at a given strain and annealing temperature. Increasing strain rate resulted in a reduction of recrystallization time for a given deformation and annealing temperature. The dependence of recrystallization time upon strain rate and deformation temperature is related to the change in deformation stress encountered for the various deformation conditions.     

Hydrogen-Induced subcritical crack growth of a 12 Cr-1 Mo ferritic stainless steel

Subcritical crack growth and tensile ductility measurements have been made on a 12 Cr-1 Mo ferritic stainless steel at cathodic potentials in a 1 N H₂SO₄ solution at 25 °C. The tensile ductility was found to be a minimum at −600 mV (SCE) and both the subcritical crack growth behavior and tensile ductility were similar for material in the tempered (760 °C/2.5 h) or tempered-plus-segregated (540 °C/240 h) condition. A rising-load crack growth threshold of 20 MPa √m was measured and a rising-load fracture toughness of 110 MPa √m was determined from extrapolation of the stage III crack growth curve. A K-independent stage II was observed and a stage II crack growth rate of about 1 × 10⁻⁵ mm/s was measured. The fracture mode was a mixture of intergranular and quasi-cleavage for both heat treatments and for subcritical and tensile fracture tests. Impact fracture properties were independent of heat treatment and grain boundary composition with the fracture mode predominantly transgranular. The difference in the fracture mode for hydrogen-induced crack growth and dynamic crack growth was explained by a difference in the relationship between their stress profiles and the maximum grain boundary segregation distribution.     

Influence of grain size on the low-cycle fatigue lives of austenitic stainless steels at high temperatures

The influence of grain size on the fatigue lives was investigated for eight kinds of austenitic stainless steels with the grain size numbers from 9 to 1. Fatigue tests were carried out at 600 and 700 °C under triangular wave shapes at strain rates of 6.7 × 10^-3/s and 6.7 × 10^-5/s, respectively, and under truncated wave shape with 30 m;n hold-time at tension side. When a strain rate was 6.7 × 10^-3/s at both 600 and 700 °C, the fracture modes were always transgranular, and the fatigue lives scarcely depended on the type of steels or the grain size. When a strain rate was 6.7 × 10^-5/s at 600 °C, the fracture modes changed from a dominantly transgranular mode to a completely intergranular one and the fatigue lives decreased with decreasing the grain size number. When a strain rate was 6.7 X 10^-5/sVs at 700 °C, grain size dependence of the fatigue lives was divided into two groups of the steels depending on the type of steel. The fracture modes of some types of the steel were completely intergranular, and others mixed. In hold-time tests, the grain size dependence of the fatigue lives was similar to that in the tests of triangular wave shape at a strain rate of 6.7 × 10^-5/s.     

Mechanical properties of new stainless steels based on the system Fe‐30Mn‐5A1‐XCr‐0.5C

New stainless steels based on the system Fe‐30Mn‐5AI‐XCr‐0.5C (Cr mass contents of ≤ 9 %) were developed and evaluated as a replacement of conventional AISI 304 steel. The alloys were produced by induction melting and thermomechanically processed to obtain a fine equiaxed microstructure. A typical thermomechanical processing for AISI 300 austenitic stainless steels was used and included forging at 1200°C, rolling at 850 °C and final recrystallization at 1050 °C. A final fully austenitic microstructure with grains of about 150 μm in size was obtained in all the steels. Tensile tests at temperatures ranging from ‐196 to 400 °C showed similar results for the various alloys tested. In accordance with the values for the elongation to fracture, this temperature range was subdivided into three regions. In the temperature range of ‐196 °C to room temperature, elongation to fracture increases with decreasing temperature. At temperatures ranging from 100 to 300 °C, elongation to fracture increases with testing temperature and serrations on the stress‐strain curve were observed. Finally, higher testing temperatures were accompanied by a decrease in ductility. Examination of the microstructures after deformation led to the conclusion that mechanical twinning was the dominant mechanism of deformation at the tested temperatures.     

Recrystallization following hot-working of a high-strength low-alloy (HSLA) steel and a 304 stainless steel at the temperature of deformation

The recovery and recrystallization behavior of two commercial quality steels, a Cb(Nb) strengthened high-strength low-alloy (HSLA) steel and a 304 stainless steel, was studied following hot-working. Specimens were deformed in tension at a constant head velocity of 2 in.Js to reductions-in-area of 30 to 50 pct at temperatures in the austenite range from 1600° to 1900°F. The subsequent annealing behavior was observed at the temperature of deformation. Decreasing recrystallization rates with decreasing temperature andJor deformation were observed. It is suggested that CbC precipitation occurred during annealing of the HSLA steel and accounted for an arrest in the softening behavior. For the 304 stainless steel it is concluded that dynamic recrystallization took place during deformation, that thermal microtwinning was an active recovery mechanism during annealing, and that there was a preference for grain boundaries as nucleation sites for recrystallized grains. These conclusions regarding the annealing behavior of 304 stainless steel were supported by metallographic analysis of specimens water quenched from the temperature of deformation. Formerly with Rensselaer Polytechnic Institute,Troy, New York. Formerly Professor, Rensselaer Polytechnic Institute. This paper is based upon a thesis submitted by T. L. CAPELETTI in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Rensselaer Polytechnic Institute.     

Optimization of cold and warm workability in 304 stainless steel using instability maps

The deformation characteristics of stainless steel type AISI 304 under compression in the temperature range 20 °C to 600 °C and strain-rate range 0.001 to 100 s^-1 have been studied with a view to characterizing the flow instabilities occurring in the microstructure. At strain rates less than 5 s^-1, 304 stainless steel exhibits flow localization, whereas dynamic strain aging occurs at intermediate temperatures and below 0.5 s^-1. At room temperatures and strain rates less than 10 s^-1, martensite formation is observed. To avoid the preceding microstructural instabilities, cold and warm working should be carried out at strain rates greater than 5 s^-1. The continuum criterion, developed on the basis of the principles of maximum rate of entropy production and separability of the dissipation function, predicts accurately all the preceding instability features. S. VENUGOPAL, Scientific Officer, on leave from the Materials Development Division, Indira Gandhi Centre for Atomic Research     

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.     

Effect of Temperature on the Fracture Toughness of Hot Isostatically Pressed 304L Stainless Steel

Herein, we have performed J-Resistance multi-specimen fracture toughness testing of hot isostatically pressed (HIP’d) and forged 304L austenitic stainless steel, tested at elevated (300 °C) and cryogenic (? 140 °C) temperatures. The work highlights that although both materials fail in a pure ductile fashion, stainless steel manufactured by HIP displays a marked reduction in fracture toughness, defined using J_0.2BL, when compared to equivalently graded forged 304L, which is relatively constant across the tested temperature range.     

A new approach to the prediction of low-cycle fatigue data

Low-cycle fatigue data for annealed AISI Type 304, 316, and 348 stainless steel to 816°C have been analyzed and a new relationship has been identified associating fatigue data with shortterm tensile behavior. A logarithmic plot of plastic strain range vs time to fracture was shown to be linear with a slope of minus unity. This line was also found to originate at the point corresponding to the tensile ductility. A similar analysis for elastic strain range data revealed a relationship with the strain-hardening exponent measured in a short-term tensile test at the same temperature and strain rate. These correlations have been combined to provide an equation for predicting the relationship between total strain range and cycles to fracture. This approach has been termed the Method of Characteristic Slopes.     

含4.35%铜抗菌不锈钢的热变形行为

含铜奥氏体不锈钢具有优异的抗菌性能而广泛应用在食品加工、医疗等领域,然而铜的加入会显著影响不锈钢的加工性能。用Gleeble-3800热模拟试验机对含铜4.35%奥氏体抗菌不锈钢进行了单道次等温热压缩试验,研究了不锈钢在变形温度为900～1 150℃、应变速率为0.01～10 s^-1和变形量为50%下的高温变形行为,构建了反映其材料特性的本构方程,使用金相显微镜观察了热变形后的微观组织,分析了各变形工艺下的微观组织演化规律,为含铜不锈钢的加工成型工艺及组织优化提供了理论参考。结果表明,4.35%Cu-304L钢的流动应力对变形工艺是敏感的,应力随着变形温度的升高和应变速率的降低而减小。采用得到的应力应变曲线建立了一种基于Arrhenius的5阶多项式拟合的应变补偿本构模型,根据此模型计算了相关系数R和平均相对误差AARE分别为0.972和9.03%,这表明所构建模型可以准确地反映含铜不锈钢的流动行为。结合微观组织发现较高的温度和较快的应变速率有利于再结晶的发生,由于0.01 s^-1低应变速率提供的变形能低,在变形温度为1 100℃、应变速率为...     

Effect of strain wave shape on low-cycle fatigue crack propagation of SUS 304 stainless steel at elevated temperatures

Effect of strain wave shape on strain-controlled low-cycle fatigue crack propagation of SUS 304 stainless steel was investigated at 600 and 700 °C. It was found that the rate of crack propagation in a cycle-dependent region was successfully correlated with the range of cyclicJ-integral, ΔJf, regardless of the strain wave shape, frequency, and test temperature. It was also shown that the rate of crack propagation gradually increased from cycle-dependent curve to time-dependent one with decreasing frequency and slow-fast strain wave shape, and that one of the factors governing the rate of crack propagation in such a region was the ratio of the range of creepJ-integral to that of totalJ-integral, ΔJ _c/ΔJT. Based on the results thus obtained, an interaction damage rule proposed semi-empirically was interpreted, with regard to crack propagation. Furthermore, fatigue crack initiation mechanism in slow-fast strain wave shape was studied, and it was shown that grain boundary sliding took an important role in it.