It is well-recognized that the value of transverse rupture strength, TRS, can exceed that of ulti... more It is well-recognized that the value of transverse rupture strength, TRS, can exceed that of ultimate tensile strength, UTS, of the same PM material, identically processed, by a factor up to ~2, although both these parameters appear relate to the fracture stress in tension by the same mechanism. Except for completely brittle materials failing after only elastic deformation, these parameters are not true fracture stresses and the plasticity correction for simple tension from UTS to the true tensile fracture strength, σ[T]max, is well-established. To take account of plasticity in bending, the Euler-Bernouilli analysis is extended for linear work hardening and the relationship between UTS and the true maximum stress in bending, derived. Generally stillσ[B]max > σ[T]max. Taking note of the stress distribution in a bend specimen being different to the uniformly stressed tensile specimen, Weibull statistical approach is used to calculate normalised values for the maximum tensile stress...
Fe-1.4C-0.65Si-0.85Mo ultrahigh carbon steel was produced from Hőganas Astaloy 85 Mo HP base iron... more Fe-1.4C-0.65Si-0.85Mo ultrahigh carbon steel was produced from Hőganas Astaloy 85 Mo HP base iron, fine graphite and silicon carbide powders mixed with polypropylene glycol. Liquid phase sintering in 10%H2-90%N2 at 1300oC resulted in density increase from ~ 6.8 g·cm -3 to ~7.7 g·cm -3 and a microstructure comprising fine pearlite and grain boundary cementite networks. Austenitisation, isothermal quench/autotemper at M(10%) temperature followed by cooling to room temperature produced a crack-free martensitic microstructure. To obtain ferrite plus fine spheroidised carbides, the material was annealed for 3 hours at 750oC. To attain full density and well-distributed submicron carbides, specimens were warm forged at 700-750oC. Microstructures and hardness values are presented for all stages of processing.
In sintered steels there is evidence for static, as well as dynamic, loading that microcracks are... more In sintered steels there is evidence for static, as well as dynamic, loading that microcracks are nucleated, grow and coalesce, before attaining catastrophic size (Stage III in fatigue) for which fracture mechanics holds. The mechanisms are step-wise, thus Paris type analysis, applied e.g. to artificial cracks, does not apply. These processes were studied by combining fractography with surface replication of the most highly stressed region of specimens undergoing three-point bending, as progressively the tensile stress or the number of fatigue cycles was increased. Typically 10-20 microcracks were nucleated, some were arrested, others grew and coalesced until the final coalescence resulted in the catastrophic crack. Assuming semi-elliptical shape for the surface microcracks, the local stress intensity factor, Ka was calculated for each microcrack using Irwin’s formula. Initial values of Ka were 1-4 MPa.m 1/2 and the eventual values correspond well to K1C, independently determined.
Liquid phase sintered and spheroidised Fe-1.4C-0.65Si-0.85Mo specimens were warm forged to discs ... more Liquid phase sintered and spheroidised Fe-1.4C-0.65Si-0.85Mo specimens were warm forged to discs at 750 and 700°C. The former experiments were conducted on a screw press and the latter, using a Gleeble instrumented tester, at strain rates of 10 -3 , 10 -2 , 10 -1 and 1 s -1 to ~ 1.15 natural strain. Ferrite grain size of the spheroidised PM steel, ~30 μm, diminished as a result of the forging to 6-7 μm, with a fine distribution of sub-micron carbides. The discs were tested in diametral compression and a procedure is presented for the determination, in this testing geometry, of the (compressive) yield strength. These values, above 740 MPa, compare favourably with 350-410 MPa, determined directly in tension, for the as-spheroidised material.
ABSTRACT Thermo-Calc modelling was employed to predict liquid phase amounts for Fe?0·85Mo?(0·4?0·... more ABSTRACT Thermo-Calc modelling was employed to predict liquid phase amounts for Fe?0·85Mo?(0·4?0·6)Si?(1·2?1·4)C in the temperature range of 1285?1300°C and such powder mixes were pressed and liquid phase sintered. In high C steels, carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat treated. To assist the break-up of these continuous carbide networks, 0·4?0·6% silicon, in the form of silicon carbide, was added. After solution of processing problems associated with the formation of CO gas in the early part of the sintering cycle, and hence large porosity, densities in excess of 7·75 g cc?1 were attained. A spheroidising treatment resulted in microstructures having the potential of producing components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains up to 16% were attained.
The sintering behaviour of Y-TZP ceramics, their resulting microstructures and properties are inf... more The sintering behaviour of Y-TZP ceramics, their resulting microstructures and properties are influenced not only by the characteristics of the raw materials but also were found to be dependent on the thermal history during the fabrication process. It is generally understood that fracture toughness increases as grain size increases up to a certain limit but in the present investigation, the
Fatigue properties of PM Fe-1.5Cr-0.2Mo-0.7C steel were determined in bending at 24Hz with R=-1 a... more Fatigue properties of PM Fe-1.5Cr-0.2Mo-0.7C steel were determined in bending at 24Hz with R=-1 and the fatigue strength found to be ~ 240 MPa, at which stress level no microcracks were detected in static loading. Microcracks, whose sizes ranged from <5 to ~20 microns, could nucleate below 100 cycles. Stress amplitudes, S, of 260 MPa were selected to investigate the progressing failure mechanism; specimens fatigued for 1000, 5000, 15000, 24000 and 79300 [failure] cycles were examined in detail by replica microscopy in the regions where maximum stress was applied. Subsequently, fractographic examination identified the failureoriginating site, which could be associated with the crack system[s] observed on the “last” pre-failure replica micrograph. Detailed examination showed microcrack nucleation, subcritical growth and coalescence with continuing cycling to ~ 0.5 mm size. This subcritical crack extended conventionally to the critical size of ~ 3 mm, when the then estimated stress ...
Fe-3Mn-0.6C was processed from ferro-manganese, graphite and Hoganas sponge or water atomised iro... more Fe-3Mn-0.6C was processed from ferro-manganese, graphite and Hoganas sponge or water atomised iron powders by pressing at 660 MPa and sintering in semi-closed containers for 1 hour in dry nitrogen or hydrogen at 1120 or 1250°C. Neither densification nor swelling took place, whatever the powder mix and sintering conditions. Yield strengths were higher for the atomised ABC 100.30 base alloy and for 1250°C and nitrogen sintering. Tensile and bend strengths, however, were by ~10% higher for specimens from sponge NC 100.24, resulting in ~740 MPa tensile strength for sintering in hydrogen at 1250°C, to be compared to ABC 100.30 base at ~650 MPa. The higher, >3%, plasticity of the sponge-based steels is associated with the increased surface area available to the Mn vapour for alloying, maintaining a much better Mn-C balance for forming pearlite at sinter necks.
Low carbon ferro-manganese and graphite powders were admixed to Hoganas sponge, NC 100.24, and wa... more Low carbon ferro-manganese and graphite powders were admixed to Hoganas sponge, NC 100.24, and water atomised, ABC 100.30 and ASC 100.29, iron powders - to produce three variants of Fe-3Mn-0.8C. These were pressed into tensile and bend specimens at 660 MPa, sintered in semi-closed containers for 1 hour in dry nitrogen or hydrogen at 1120 or 1250°C and cooled at 64°C/min. Neither detectable densification nor swelling took place, whatever the powder and sintering conditions. Green and sintered densities were expectedly highest for ABC 100.30 base at ~7.1g.cm -3 , >0.2 g.cm -3 larger than for the sponge-based alloy. Similarly, Young's modulus in the former material attained ~130 GPa, being ~117GPa in the latter. Yield strengths were higher for the atomised iron base alloys and for 1250°C and nitrogen sintering, from 370 - 440 MPa (1120°C – NC 100.24) to ~ 520 MPa (1250°C - ABC 100.30). Tensile and bend strengths were somewhat higher for nitrogen sintering and, generally by ~10%,...
It is well-recognized that the value of transverse rupture strength, TRS, can exceed that of ulti... more It is well-recognized that the value of transverse rupture strength, TRS, can exceed that of ultimate tensile strength, UTS, of the same PM material, identically processed, by a factor up to ~2, although both these parameters appear relate to the fracture stress in tension by the same mechanism. Except for completely brittle materials failing after only elastic deformation, these parameters are not true fracture stresses and the plasticity correction for simple tension from UTS to the true tensile fracture strength, σ[T]max, is well-established. To take account of plasticity in bending, the Euler-Bernouilli analysis is extended for linear work hardening and the relationship between UTS and the true maximum stress in bending, derived. Generally stillσ[B]max > σ[T]max. Taking note of the stress distribution in a bend specimen being different to the uniformly stressed tensile specimen, Weibull statistical approach is used to calculate normalised values for the maximum tensile stress...
Fe-1.4C-0.65Si-0.85Mo ultrahigh carbon steel was produced from Hőganas Astaloy 85 Mo HP base iron... more Fe-1.4C-0.65Si-0.85Mo ultrahigh carbon steel was produced from Hőganas Astaloy 85 Mo HP base iron, fine graphite and silicon carbide powders mixed with polypropylene glycol. Liquid phase sintering in 10%H2-90%N2 at 1300oC resulted in density increase from ~ 6.8 g·cm -3 to ~7.7 g·cm -3 and a microstructure comprising fine pearlite and grain boundary cementite networks. Austenitisation, isothermal quench/autotemper at M(10%) temperature followed by cooling to room temperature produced a crack-free martensitic microstructure. To obtain ferrite plus fine spheroidised carbides, the material was annealed for 3 hours at 750oC. To attain full density and well-distributed submicron carbides, specimens were warm forged at 700-750oC. Microstructures and hardness values are presented for all stages of processing.
In sintered steels there is evidence for static, as well as dynamic, loading that microcracks are... more In sintered steels there is evidence for static, as well as dynamic, loading that microcracks are nucleated, grow and coalesce, before attaining catastrophic size (Stage III in fatigue) for which fracture mechanics holds. The mechanisms are step-wise, thus Paris type analysis, applied e.g. to artificial cracks, does not apply. These processes were studied by combining fractography with surface replication of the most highly stressed region of specimens undergoing three-point bending, as progressively the tensile stress or the number of fatigue cycles was increased. Typically 10-20 microcracks were nucleated, some were arrested, others grew and coalesced until the final coalescence resulted in the catastrophic crack. Assuming semi-elliptical shape for the surface microcracks, the local stress intensity factor, Ka was calculated for each microcrack using Irwin’s formula. Initial values of Ka were 1-4 MPa.m 1/2 and the eventual values correspond well to K1C, independently determined.
Liquid phase sintered and spheroidised Fe-1.4C-0.65Si-0.85Mo specimens were warm forged to discs ... more Liquid phase sintered and spheroidised Fe-1.4C-0.65Si-0.85Mo specimens were warm forged to discs at 750 and 700°C. The former experiments were conducted on a screw press and the latter, using a Gleeble instrumented tester, at strain rates of 10 -3 , 10 -2 , 10 -1 and 1 s -1 to ~ 1.15 natural strain. Ferrite grain size of the spheroidised PM steel, ~30 μm, diminished as a result of the forging to 6-7 μm, with a fine distribution of sub-micron carbides. The discs were tested in diametral compression and a procedure is presented for the determination, in this testing geometry, of the (compressive) yield strength. These values, above 740 MPa, compare favourably with 350-410 MPa, determined directly in tension, for the as-spheroidised material.
ABSTRACT Thermo-Calc modelling was employed to predict liquid phase amounts for Fe?0·85Mo?(0·4?0·... more ABSTRACT Thermo-Calc modelling was employed to predict liquid phase amounts for Fe?0·85Mo?(0·4?0·6)Si?(1·2?1·4)C in the temperature range of 1285?1300°C and such powder mixes were pressed and liquid phase sintered. In high C steels, carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat treated. To assist the break-up of these continuous carbide networks, 0·4?0·6% silicon, in the form of silicon carbide, was added. After solution of processing problems associated with the formation of CO gas in the early part of the sintering cycle, and hence large porosity, densities in excess of 7·75 g cc?1 were attained. A spheroidising treatment resulted in microstructures having the potential of producing components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains up to 16% were attained.
The sintering behaviour of Y-TZP ceramics, their resulting microstructures and properties are inf... more The sintering behaviour of Y-TZP ceramics, their resulting microstructures and properties are influenced not only by the characteristics of the raw materials but also were found to be dependent on the thermal history during the fabrication process. It is generally understood that fracture toughness increases as grain size increases up to a certain limit but in the present investigation, the
Fatigue properties of PM Fe-1.5Cr-0.2Mo-0.7C steel were determined in bending at 24Hz with R=-1 a... more Fatigue properties of PM Fe-1.5Cr-0.2Mo-0.7C steel were determined in bending at 24Hz with R=-1 and the fatigue strength found to be ~ 240 MPa, at which stress level no microcracks were detected in static loading. Microcracks, whose sizes ranged from <5 to ~20 microns, could nucleate below 100 cycles. Stress amplitudes, S, of 260 MPa were selected to investigate the progressing failure mechanism; specimens fatigued for 1000, 5000, 15000, 24000 and 79300 [failure] cycles were examined in detail by replica microscopy in the regions where maximum stress was applied. Subsequently, fractographic examination identified the failureoriginating site, which could be associated with the crack system[s] observed on the “last” pre-failure replica micrograph. Detailed examination showed microcrack nucleation, subcritical growth and coalescence with continuing cycling to ~ 0.5 mm size. This subcritical crack extended conventionally to the critical size of ~ 3 mm, when the then estimated stress ...
Fe-3Mn-0.6C was processed from ferro-manganese, graphite and Hoganas sponge or water atomised iro... more Fe-3Mn-0.6C was processed from ferro-manganese, graphite and Hoganas sponge or water atomised iron powders by pressing at 660 MPa and sintering in semi-closed containers for 1 hour in dry nitrogen or hydrogen at 1120 or 1250°C. Neither densification nor swelling took place, whatever the powder mix and sintering conditions. Yield strengths were higher for the atomised ABC 100.30 base alloy and for 1250°C and nitrogen sintering. Tensile and bend strengths, however, were by ~10% higher for specimens from sponge NC 100.24, resulting in ~740 MPa tensile strength for sintering in hydrogen at 1250°C, to be compared to ABC 100.30 base at ~650 MPa. The higher, >3%, plasticity of the sponge-based steels is associated with the increased surface area available to the Mn vapour for alloying, maintaining a much better Mn-C balance for forming pearlite at sinter necks.
Low carbon ferro-manganese and graphite powders were admixed to Hoganas sponge, NC 100.24, and wa... more Low carbon ferro-manganese and graphite powders were admixed to Hoganas sponge, NC 100.24, and water atomised, ABC 100.30 and ASC 100.29, iron powders - to produce three variants of Fe-3Mn-0.8C. These were pressed into tensile and bend specimens at 660 MPa, sintered in semi-closed containers for 1 hour in dry nitrogen or hydrogen at 1120 or 1250°C and cooled at 64°C/min. Neither detectable densification nor swelling took place, whatever the powder and sintering conditions. Green and sintered densities were expectedly highest for ABC 100.30 base at ~7.1g.cm -3 , >0.2 g.cm -3 larger than for the sponge-based alloy. Similarly, Young's modulus in the former material attained ~130 GPa, being ~117GPa in the latter. Yield strengths were higher for the atomised iron base alloys and for 1250°C and nitrogen sintering, from 370 - 440 MPa (1120°C – NC 100.24) to ~ 520 MPa (1250°C - ABC 100.30). Tensile and bend strengths were somewhat higher for nitrogen sintering and, generally by ~10%,...
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