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The local fatigue strength concept and its application to martensitic surface layers, especially laser-hardened layers
Corrosion Review of corrosion studies on aluminium metal matrix composites. Turnbull, A. National Physical Laboratory (UK) Report No DMM (A) 15 Nov. 1990 27 pp The published literature on corrosion of AI metal matrix composites based on silicon carbide, graphite, alumina and boron reinforcements is reviewed. In general, the composites are more susceptible to corrosion than the matrix alloy. Corrosion tends to be localized with galvanic couples, 'active' reaction products, increased density of intermetallic particles, microcrevices and voids being cited as possible factors for specific composites. Research on corrosion fatigue and stress corrosion cracking is insufficient to permit detailed conclusions and further research is recommended. 33 refs.
Stress corrosion cracking and corrosion fatigue of steam-turbine rotor and blade materials. Speidel, M.O., Denk, J. and Scar/in, B. Commission of the European Communities Report No EUR 13186 1991 65 pp Modern steam turbines must retain a very high reliability throughout their service Hfe of typically 200000 h. Among the reasons for failure that have been observed in steam turbines during the last 20 years, stress corrosion cracking and corrosion fatigue have been prominent. Thus, attention has been drawn to the resistance to stress corrosion and corrosion fatigue of the rotor and blade materials of steam turbines. An attempt is made to describe in quantitative terms the major parameters that influence the stress corrosion and corrosion fatigue behaviour of steam-turbine rotor and blade materials in service-related environments. Five topics are given special attention: stress corrosion crack initiation, stress corrosion crack propagation, corrosion fatigue crack initiation, corrosion fatigue crack propagation and pitting corrosion. It is shown that fracture mechanics can be used to understand the transition from crack initiation to crack growth. Graphs. 45 refs.
Analysis of service water pump s h a f t failure. Shaikh, H., Khatak, H.S. and Gnanamoorthy, J.B. Prak. Metall. J u l y 1990 27, (7), 3 6 2 - 3 6 8 (in English and German) In the service water systems of fast-breeder test reactors, three service water pumps take suction from the cooling water in the turbine building and circulate the water. After approximately seven years of operation, one service water pump failed when its shaft gave way. These centrifugal pumps operate at 22 kW, 1500 rain -; with a flow rate of 1;'5 m3/h. They are negative-suction pumps (-1.5 M) with a discharge pressure of 250 kPa. In a 12-day cycle, each pump would operate for four days with eight days off. The material of the pump shaft was specified as En8 steel. Examination of the failed centrifugal pump indicated heavy corrosion of the inside of the casing, in addition to the corrosion of the broken shaft. Stagnant water was found in the casing. No regular maintenance had been carried out in the previous three years. Data from visual and metallurgical examination indicated that corrosion attack aided by stress concentration at the thread root initiated the crack at the surface. The ratchet marks and the flatness of the fracture surface suggested that crack propagation had taken place as the result of either corrosion fatigue or mechanical fatigue. The presence of multiple initiation points and the location of the final rapid failure around the centre indicated the presence of high rotary bending stresses. Improper composition of the material coupled with irregular maintenance resulted in the reduced life of the shaft. Photomicrographs. 9 refs.
General A study of a simplified estimation scheme for semielliptical surface
cracks under creep-fatigue conditions. Yagawa, G., Yoshimura, S. and Kitajima, Y. Ann. Rep. Eng. Res. Inst. Fac. Eng. Univ. Tokyo Sept. 1990 49, 153-158 (in Japanese)
Effect of potential on the corrosion fatigue crack growth rate of F e - A I - M n alloy in 3.5% NaCl solution. Dub, J.-B., Tsai, W.-T., Lee, J.T. and Chang, H. Corrosion Dec. 1990 46, (12), 9 8 3 - 9 8 8 The corrosion fatigue crack growth (FCG) behaviour of the austenitic Fe-8.87AI-27.9Mn-l.04C alloy in 3.5wt.% NaCI solution has been investigated. Corrosion fatigue crack growth rates at different electrochemical potentials were measured using compact tension specimens (CTS) at room temperature. The potentials applied ranged from 200 to -1200 mVscE, which included the transpassive, passive, and open-circuit potentials (Eop), and cathodic potentials. Sinusoidal loads at an R-ratio of 0.1 were apptied with frequencies of 1.0, 5.0 and 10 Hz. The resuits indicated that the FCG rates depend on the frequency and applied potential. The enviromental effect became evident as the frequency of the test decreased from 10 to 1.0 Hz. The FCG rates were decreased as the applied potential fell in the cathodic region. The FCG rates could be decelerated when the potential was in the passive potential. However, the FCG rate was slower at -200 mVscE (the transpassive region) than at Eop. A possible explanation for this observation was discussed. Graphs, photomicrographs. 20 refs.
The effects of an anaerobic environment on corrosion fatigue. Ferguson, W.G., Zhang, Y., Stevens, F.J. and Assefpour-Dezfuly, M. Australasian Corrosion Association Inc., New Zealand Branch, Auckland, New Zealand, CASS 90, Paper No 52 1990 4 pp The corrosion fatigue behaviour of a C-Me structural steel and its weldments has been determined in seawater and seawater containing various concentrations of H2S. The seawater tests were done under cathodic protection and the threshold values were determined. The pressure of H2S considerably enhances the crack growth rates in the plateau region. These increases result because H2S stimulates hydrogen entry into the lattice, thus giving more H to assist the cracking process, The crack growth rate enhancement with H2S reaches a threshold at 100-150 ppm H2S, below which crack growth rates increase with increasing H2S concentration and above which crack growth rates are independent of H2S concentration. Graphs. 23 refs.
Aspects of fatigue crack propagation behaviour of duplex stainless steels, Mayaki, C.M. Diss. Abstr. Int. Nov. 1990 51, (5), 259 pp The fatigue crack growth and threshold (&KTH) behaviour of austenitic-ferritic duplex stainless steels have been examined at room temperature in air. Tests were performed at two load ratios (R) of 0.1 and 0.8 to distinguish between intrinsic material properties and the contributions to threshold from roughness-induced closure mechanisms. The effect of various aqueous environments, particularly 3.5% NaCI, on the crack propagation resistance was also examined at room temperature. Both commercially available and heat-treated samples were investigated. Particular emphasis was placed on microstructures in as-received rolled plates, in welds and a coarse-grained microstructure of the type found in heat-affected zone (HAZ) regions of welds. The susceptibility of HAZ material to corrosion fatigue is of interest as weld defects and local residual stress could promote crack initiation in such regions. Fatigue crack growth in duplex steels is compared with that of an austenitic stainless steel (316L). The duplex steels showed a marked improvement in fatigue crack growth resistance compared with austenitic steeJs.This is a result of increased strength and the duplex structure. Duplex stainless steels show good resistance to corrosion fatigue in aqueous environments. However, in weld microstructures and a structure developed after fast cooling from 1300°C (similar to a weld HAZ microstructure), more than an order of magnitude increase in crack growth rate, over that in air, was observed. The high growth rates were maintained right down to threshold. The corrosion fatigue effect is thought to be associated with hydrogen, generated by the cathodic reaction at the crack tip, Duplex stainless steels undergo a series of transformations over a range of temperatures. The mechanical properties resulting from heat treatment and cooling at different temperatures and rates are examined. The changes in these properties are controlled by the amount of ferrite present, the associated microstructure and the presence of embrittling phases such as the s-phase.
430
To perform the reliable and efficient design of an FBR piping system, the application of the LBB design is highly recommended. For this purpose, a non-linear threedimensional finite-element program based on the deformation theory of plasticity of a power-law hardening material was developed and applied to obtain socelled fully plastic solutions. The treatment of the stress-relaxation behaviour in creep-fatigue phenomena was also examined in view of the application of the fully plastic solutions to the crack behaviour in the creep deformation regime. Graphs. 7 refs.
A case of service failure: turbine cone. Welding in the World 1990 28, (1-2), 2 - 5 (in English and French) An account is given of damage affecting a mechanically welded part in the shape of a truncated cone in E26-04 structural steel (14 mm in thickness), which acted as a guide for the flow of water at the outlet of a distributor of an hydraulic turbine. Cracks caused by corrosion fatigue appeared following mechanically welded modifications that gave rise to defects and discontinuities at the weld edges.
On the behaviour of necked girders under cyclic loading. Scheer, J., Scheibe, H.-J. and Kuck, D. Bauingen. Oct. 1990 65, (10), 4 6 3 - 4 6 8 (in German) Future national and international codes will permit the design of statically loaded steel constructions employing plastification in a cross section. Only one-cycle monotonic loading is considered, although in reality the construction is repeatedly loaded up to the plastic range. However, until now neither sufficient experimental nor theoretical knowledge has existed about whether the change of the material behaviour during cyclic loading can lead to a restriction on the ultimate load that has been determined for monotonic loading. At the Institute for Steel Construction of the Technical University of Braunschweig tests with necked girders have been performed to check the unrestricted applicability of the new method to determine the dimensions for this typical element of a steel construction. The results for St523 are presented. Graphs. 9 refs.
The local fatigue strength concept and its application to martensitic surface layers, especially laser-hardened layers. Winerlich, B. Materialwissen. Werkstofftech. Oct. 1990 21, (10), 3 7 8 - 3 8 9 (in German) Starting from the known local fatigue strength concept problems with its application to martensitic surface layers, especially laser-hardenedlayers, have been discussed. A method for the calculation of the fatigue limit of surface-hardened specimens or components from quench-hardenable steels has been proposed, including the influence of surface roughness on fatigue strength. A new formula is given for the estimation of the mean and residual stress sensitivity. The limits of the concept have been shown and its practical use has been demonstrated. 118 refs.
Strengthening and bending deformability of AI-4.5 mass% Mg alloy billets produced by the heated-mold continuous casting method(OCt), Ohno, A., Motoyasu, G. and Kawai, H. J. Japan Inst. Light Met. Nov. 1990 40, (11), 817-821 (in Japanese) AI-4.5 mass% Mg alloy billets were cast by the heated-mold continuous casting (Ohno continuous casting) method. The billets were cold worked by 90° bending and cyclic bending. The relationships between the bending workability, crack initiation, mechanical properties and structures of the billets were examined and compared with those of billets produced by the conventional method. The specimens bent from conventional billets tend to exhibit crack initiation at the grain boundary and fracture easily, but the specimens from the OCC billets were free from such defects. The bending fatigue life of the OCC billets was five times greater than that of the conventional billets, and the OCC billets moved through the cyclic bending process easily while the conventional billets did not. The mechanical properties of the OCC billets such as the hardness, tensile strength, elongation end fatigue strength were superior to those of the conventional billets. The mechanical properties of the OCC billets were improved by cyclic bending. Graphs, photomicrographs. 8 refs.
Int J Fatigue September 1991
Stress corrosion cracking and corrosion fatigue of steam-turbine rotor and blade materials. Speidel, M.O., Denk, J. and Scar/in, B. Commission of the European Communities Report No EUR 13186 1991 65 pp Modern steam turbines must retain a very high reliability throughout their service Hfe of typically 200000 h. Among the reasons for failure that have been observed in steam turbines during the last 20 years, stress corrosion cracking and corrosion fatigue have been prominent. Thus, attention has been drawn to the resistance to stress corrosion and corrosion fatigue of the rotor and blade materials of steam turbines. An attempt is made to describe in quantitative terms the major parameters that influence the stress corrosion and corrosion fatigue behaviour of steam-turbine rotor and blade materials in service-related environments. Five topics are given special attention: stress corrosion crack initiation, stress corrosion crack propagation, corrosion fatigue crack initiation, corrosion fatigue crack propagation and pitting corrosion. It is shown that fracture mechanics can be used to understand the transition from crack initiation to crack growth. Graphs. 45 refs.
Analysis of service water pump s h a f t failure. Shaikh, H., Khatak, H.S. and Gnanamoorthy, J.B. Prak. Metall. J u l y 1990 27, (7), 3 6 2 - 3 6 8 (in English and German) In the service water systems of fast-breeder test reactors, three service water pumps take suction from the cooling water in the turbine building and circulate the water. After approximately seven years of operation, one service water pump failed when its shaft gave way. These centrifugal pumps operate at 22 kW, 1500 rain -; with a flow rate of 1;'5 m3/h. They are negative-suction pumps (-1.5 M) with a discharge pressure of 250 kPa. In a 12-day cycle, each pump would operate for four days with eight days off. The material of the pump shaft was specified as En8 steel. Examination of the failed centrifugal pump indicated heavy corrosion of the inside of the casing, in addition to the corrosion of the broken shaft. Stagnant water was found in the casing. No regular maintenance had been carried out in the previous three years. Data from visual and metallurgical examination indicated that corrosion attack aided by stress concentration at the thread root initiated the crack at the surface. The ratchet marks and the flatness of the fracture surface suggested that crack propagation had taken place as the result of either corrosion fatigue or mechanical fatigue. The presence of multiple initiation points and the location of the final rapid failure around the centre indicated the presence of high rotary bending stresses. Improper composition of the material coupled with irregular maintenance resulted in the reduced life of the shaft. Photomicrographs. 9 refs.
General A study of a simplified estimation scheme for semielliptical surface
cracks under creep-fatigue conditions. Yagawa, G., Yoshimura, S. and Kitajima, Y. Ann. Rep. Eng. Res. Inst. Fac. Eng. Univ. Tokyo Sept. 1990 49, 153-158 (in Japanese)
Effect of potential on the corrosion fatigue crack growth rate of F e - A I - M n alloy in 3.5% NaCl solution. Dub, J.-B., Tsai, W.-T., Lee, J.T. and Chang, H. Corrosion Dec. 1990 46, (12), 9 8 3 - 9 8 8 The corrosion fatigue crack growth (FCG) behaviour of the austenitic Fe-8.87AI-27.9Mn-l.04C alloy in 3.5wt.% NaCI solution has been investigated. Corrosion fatigue crack growth rates at different electrochemical potentials were measured using compact tension specimens (CTS) at room temperature. The potentials applied ranged from 200 to -1200 mVscE, which included the transpassive, passive, and open-circuit potentials (Eop), and cathodic potentials. Sinusoidal loads at an R-ratio of 0.1 were apptied with frequencies of 1.0, 5.0 and 10 Hz. The resuits indicated that the FCG rates depend on the frequency and applied potential. The enviromental effect became evident as the frequency of the test decreased from 10 to 1.0 Hz. The FCG rates were decreased as the applied potential fell in the cathodic region. The FCG rates could be decelerated when the potential was in the passive potential. However, the FCG rate was slower at -200 mVscE (the transpassive region) than at Eop. A possible explanation for this observation was discussed. Graphs, photomicrographs. 20 refs.
The effects of an anaerobic environment on corrosion fatigue. Ferguson, W.G., Zhang, Y., Stevens, F.J. and Assefpour-Dezfuly, M. Australasian Corrosion Association Inc., New Zealand Branch, Auckland, New Zealand, CASS 90, Paper No 52 1990 4 pp The corrosion fatigue behaviour of a C-Me structural steel and its weldments has been determined in seawater and seawater containing various concentrations of H2S. The seawater tests were done under cathodic protection and the threshold values were determined. The pressure of H2S considerably enhances the crack growth rates in the plateau region. These increases result because H2S stimulates hydrogen entry into the lattice, thus giving more H to assist the cracking process, The crack growth rate enhancement with H2S reaches a threshold at 100-150 ppm H2S, below which crack growth rates increase with increasing H2S concentration and above which crack growth rates are independent of H2S concentration. Graphs. 23 refs.
Aspects of fatigue crack propagation behaviour of duplex stainless steels, Mayaki, C.M. Diss. Abstr. Int. Nov. 1990 51, (5), 259 pp The fatigue crack growth and threshold (&KTH) behaviour of austenitic-ferritic duplex stainless steels have been examined at room temperature in air. Tests were performed at two load ratios (R) of 0.1 and 0.8 to distinguish between intrinsic material properties and the contributions to threshold from roughness-induced closure mechanisms. The effect of various aqueous environments, particularly 3.5% NaCI, on the crack propagation resistance was also examined at room temperature. Both commercially available and heat-treated samples were investigated. Particular emphasis was placed on microstructures in as-received rolled plates, in welds and a coarse-grained microstructure of the type found in heat-affected zone (HAZ) regions of welds. The susceptibility of HAZ material to corrosion fatigue is of interest as weld defects and local residual stress could promote crack initiation in such regions. Fatigue crack growth in duplex steels is compared with that of an austenitic stainless steel (316L). The duplex steels showed a marked improvement in fatigue crack growth resistance compared with austenitic steeJs.This is a result of increased strength and the duplex structure. Duplex stainless steels show good resistance to corrosion fatigue in aqueous environments. However, in weld microstructures and a structure developed after fast cooling from 1300°C (similar to a weld HAZ microstructure), more than an order of magnitude increase in crack growth rate, over that in air, was observed. The high growth rates were maintained right down to threshold. The corrosion fatigue effect is thought to be associated with hydrogen, generated by the cathodic reaction at the crack tip, Duplex stainless steels undergo a series of transformations over a range of temperatures. The mechanical properties resulting from heat treatment and cooling at different temperatures and rates are examined. The changes in these properties are controlled by the amount of ferrite present, the associated microstructure and the presence of embrittling phases such as the s-phase.
430
To perform the reliable and efficient design of an FBR piping system, the application of the LBB design is highly recommended. For this purpose, a non-linear threedimensional finite-element program based on the deformation theory of plasticity of a power-law hardening material was developed and applied to obtain socelled fully plastic solutions. The treatment of the stress-relaxation behaviour in creep-fatigue phenomena was also examined in view of the application of the fully plastic solutions to the crack behaviour in the creep deformation regime. Graphs. 7 refs.
A case of service failure: turbine cone. Welding in the World 1990 28, (1-2), 2 - 5 (in English and French) An account is given of damage affecting a mechanically welded part in the shape of a truncated cone in E26-04 structural steel (14 mm in thickness), which acted as a guide for the flow of water at the outlet of a distributor of an hydraulic turbine. Cracks caused by corrosion fatigue appeared following mechanically welded modifications that gave rise to defects and discontinuities at the weld edges.
On the behaviour of necked girders under cyclic loading. Scheer, J., Scheibe, H.-J. and Kuck, D. Bauingen. Oct. 1990 65, (10), 4 6 3 - 4 6 8 (in German) Future national and international codes will permit the design of statically loaded steel constructions employing plastification in a cross section. Only one-cycle monotonic loading is considered, although in reality the construction is repeatedly loaded up to the plastic range. However, until now neither sufficient experimental nor theoretical knowledge has existed about whether the change of the material behaviour during cyclic loading can lead to a restriction on the ultimate load that has been determined for monotonic loading. At the Institute for Steel Construction of the Technical University of Braunschweig tests with necked girders have been performed to check the unrestricted applicability of the new method to determine the dimensions for this typical element of a steel construction. The results for St523 are presented. Graphs. 9 refs.
The local fatigue strength concept and its application to martensitic surface layers, especially laser-hardened layers. Winerlich, B. Materialwissen. Werkstofftech. Oct. 1990 21, (10), 3 7 8 - 3 8 9 (in German) Starting from the known local fatigue strength concept problems with its application to martensitic surface layers, especially laser-hardenedlayers, have been discussed. A method for the calculation of the fatigue limit of surface-hardened specimens or components from quench-hardenable steels has been proposed, including the influence of surface roughness on fatigue strength. A new formula is given for the estimation of the mean and residual stress sensitivity. The limits of the concept have been shown and its practical use has been demonstrated. 118 refs.
Strengthening and bending deformability of AI-4.5 mass% Mg alloy billets produced by the heated-mold continuous casting method(OCt), Ohno, A., Motoyasu, G. and Kawai, H. J. Japan Inst. Light Met. Nov. 1990 40, (11), 817-821 (in Japanese) AI-4.5 mass% Mg alloy billets were cast by the heated-mold continuous casting (Ohno continuous casting) method. The billets were cold worked by 90° bending and cyclic bending. The relationships between the bending workability, crack initiation, mechanical properties and structures of the billets were examined and compared with those of billets produced by the conventional method. The specimens bent from conventional billets tend to exhibit crack initiation at the grain boundary and fracture easily, but the specimens from the OCC billets were free from such defects. The bending fatigue life of the OCC billets was five times greater than that of the conventional billets, and the OCC billets moved through the cyclic bending process easily while the conventional billets did not. The mechanical properties of the OCC billets such as the hardness, tensile strength, elongation end fatigue strength were superior to those of the conventional billets. The mechanical properties of the OCC billets were improved by cyclic bending. Graphs, photomicrographs. 8 refs.
Int J Fatigue September 1991