摘要:

AbstractThe precipitate phases in seven casts of AISI type 321 steel aged at 600°C have been studied. Apart from titanium carbides, nitrides, carbosulphides, andσ-phase, the intermetallic G-phase was also detected in samples having a small grain size. The G-phase, which has not previously been reported in type 321 steel, is a fcc nickel-titanium-silicon phase with a lattice parameter of 1·12 nm. The chemical composition was found by energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope to be 63Ni–21Ti–12Si (wt-%). The formation of G-phase is inhibited by large grain size and low‘excess’titanium content, i.e. the amount of titanium remaining after, deducting the amounts that are combined in TiC, TiN, and Ti4C2S2. When there is no‘excess’titanium, the remaining carbon is precipitated as M23C6. The formation of G-phase causes a further reduction in the amount of‘excess’titanium in solution, and this may lead to a reduction in creep strength in long-term creep tests.MST/53

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.97

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractInterdiffusion coefficients between 1000 and 1200°C have been measured in the Co-Al and Ni-Al solid-solution phases using a modified form of the Boltzmann- Matano method. Interdiffusion in the Co-Al phase is at least half an order of magnitude lower than in the corresponding Ni-Al phase. Established equations using these coefficients were used to determine theoretical critical mole fractions of aluminium necessary to form protective Al2O3scales during oxidation. While these equations are reasonably accurate in predicting the amounts of chromium necessary to form Cr2O3scales, they give inaccurate predictions of the amounts of aluminium necessary to form Al2O3scales because the kinetics of the early stages of oxidation are not taken into account.MST/144

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.101

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractSpheroidized and prequenched 0.3C-Cr-Mn-Si steel was quenched from the intercriticalα+γtemperature range and tempered at 250°C. It was shown that intercritical quenching can appreciably improve the ductility of the steel, for the same strength level, compared to conventional quenching and tempering. Additionally, prequenching from theγregion before intercritical quenching gives even larger improvements in the ductility of the steel than merely intercritical quenching, as a result of the development of a lamellar martensitic-ferritic dual-phase structure. The tensile behaviour of the intercritically quenched steel is discussed with emphasis on the strain-hardening exponents of various dual-phase structures.MST/93

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.104

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe hot ductility of low-carbon austenite has been studied in order to reveal the hot-cracking mechanism related to sulphur segregation and sulphide precipitation during solution treatment and deformation. Ductility was greatly reduced during high-strain-rate tensile deformation in the temperature range 1073-1373 K. Fracture occurred via a typical intergranular mode, characterized by both shallow dimples caused by microvoid coalescence and rather smooth facets caused by intergranular decohesion. The former morphology is a result of the dense precipitation of sulphides, identified as Fe-rich (Fe, Mn) S, on the austenite grain boundaries. Finely dispersed sulphide particles within the grains and the existence of precipitate-free zones along grain boundaries enhanced strain localization in the vicinity of the boundaries during deformation. The intergranular decohesion arises from the decrease in grain-boundary strength produced by sulphur segregation. Hot ductility is improved on decreasing the solution-treatment temperature, on decreasing the cooling rate from high solution temperatures, and on isothermal holding at the deformation temperature before starting the tensile test. These effects can be explained in terms of MnS precipitation and particle coarsening during these processes.MST/60

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.111

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe effect of cooling rate on the embrittlement of a 12 wt-%Cr steel was investigated. It was found that when the cooling rate is not as fast as that produced by a liquid quench, the carbides have a tendency to segregate to the prior austenite grain boundaries. Carbide coarsening occurred after prolonged exposure at the operating temperature of 538°C. The carbides precipitated at the prior grain boundaries or within the acicular martensitic structure were found to act as nucleation sites for microcracks. It was also postulated that the precipitation and segregation of carbides was promoted by applied stresses at high temperatures.MST/59

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.117

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractIn order to produce extremely high strength values without impairing the impact properties, the microstructures of two low-carbon microalloy steels were modified substantially by carefully designed combinations of hot working and heat treatment. The interplay of several microstructural processes in unconventional variants of thermomechanical treatment was investigated systematically. The experiments were carried out in plane-strain compression using the computer controlled hot-deformation simulator‘WUMSI’. The results indicate that for optimizing mechanical properties the variants found most suitable were those which lead to a fine microstructure by quenching or which take advantage of grain refining caused by additional reaustenitizing. Furthermore, the controlled superposition of age hardening and subsequent deformation brought about increasing yield and tensile strengths together with some reduction in shelf energy. The beneficial effect of microstructure refinement was more pronounced in the pearlite-reduced Ti steel, whereas the straining of an age-hardened microstructure could be exploited extensively in the acicular-ferritic Nb–Mo steel.MST/55

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.121

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractStrain hardening and the loss of ductility with increasing strength have been investigated for a series of cold-rolled high-strength steels. The strain hardening exponent n decreases with increasing strengthSaccording to the relationship nSp=C, where p and C are constants for a given strengthening mechanism. The value of p (and hence the ductility) at a fixed strength level depends on the operative strengthening mode. Solid-solution hardening is the most ductile method of strengthening, followed, in order of decreasing ductility, by grain refining, precipitation hardening, partial annealing, and cold working. It is proposed that the influence of a strengthening mechanism is related to the strain at which a steady-state dislocation-cell structure is developed in the material. The lower this critical strain, the lower the ductility, since further strain must then be accommodated by microband formation. The critical strain is considered to be dependent on the rate of accumulation of geometrically necessary dislocations in the structure, which is a function of the material's geometric slip distance. Strengthening mechanisms relying on obstacles in the microstructure - e.g. precipitation hardening - will have relatively short geometric slip distances, and thus low ductilities. Solid-solution hardening is preferable, because it operates only by increasing the lattice friction resistance to dislocation motion, and so does not reduce the geometric slip distance. The cell structure in the cold-worked steels is expected to approach that of the steady state at the start of the tests, a feature which, will result in very low critical strains and ductilities.MST/92

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.128

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractSpecimens of wrought Waspaloy have been reheated to above theγ' solvus temperature and tested in plane-strain compression at constant equivalent tensile strain rates in the range 0.5–50 s−1, and at initial test temperatures in the range 960–1070°C. The majority of tests were conducted with the tools and test environment at 850°C. Dynamic recrystallization was observed under all test conditions, but was only complete by the strain limit of 2·7 in tests at the highest temperatures. Static (metadynamic) recrystallization initiated rapidly after deformation. Flow stress, either at the peak or at a constant strain, is related exponentially to strain rate, and gives an activation energy of 475 kJ mol−1when related to the instantaneous temperature, which changed rapidly during deformation. For practical purposes, the stress-strain-strain-rate-temperature relationships may be treated as equations of state.MST/96

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.136

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractIncubation time for nucleation of fatigue cavities at grain boundaries has been introduced to explain the frequency dependence of fatigue-fracture modes at elevated temperatures. It is suggested that for fatigue at relatively low frequencies, cavitation is an intrinsic response to cyclic stress, mainly occurring on grain boundaries perpendicular to the stress axis. The critical frequency below which bulk cavitation occurs is determined by the incubation time for cavity nucleation. Stress-concentration sites on a sliding grain boundary become nucleation sites in fatigue only at relatively high frequencies caused by the comparatively short time for stress relaxation. In both cases, environment and material chemistry play an important role in stabilizing the cavity embryos. Fatigue cavity nucleation schema have been constructed to depict the various dominant mechanisms in different stress-frequency domains.MST/138

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.145

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractStudies of the simultaneous creep and oxidation of an Fe-5Al alloy at constant stresses of 12 and 16 MN m−2in the temperature range 973-1073 K have shown that the steady-state creep obeys a power law. The stress exponent n was found to lie between 5 and 7. The apparent activation energy for creep was in the range 77–397 kJ mol−1and depended on the oxygen partial pressure Po2of the test environment. The oxidation strengthening (steady-state creep rateεdecreasing) was found for an applied stress of 16 MNm−2for Po2in the range 10−7to 1·013×105Pa. At 12 MN m−2, strengthening occurred only in the range 973–1023 K; weakening was found at 1073 K. The primary causes of strengthening are believed to be the mechanical constraint of strongly adherent scales and internal oxide-dislocation interactions.MST/133

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.2.152

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor