Showing papers on "Tempering published in 2000"

The glass transition temperature concept in rice drying and tempering : Effect on milling quality

Abstract: Research on rice drying and tempering has shown that high drying temperatures (up to 60 ³ C) and high moisture removal rates (up to 6 percentage points moisture content) can be used without reducing milling quality as long as sufficient tempering at a temperature above the glass transition temperature (Tg) is allowed between drying passes. Using drying air temperatures above the Tg of the rice significantly reduces drying and tempering durations since kernel moisture diffusivity is much higher above Tg. Understanding the effects of glass transition is important in optimizing the drying and tempering processes in terms of overall required drying durations to achieve given moisture removals. The objective of this study was to investigate the effect of Tg on drying rates when using drying air temperatures above and below Tg. Both medium–grain and long–grain rice was harvested during 2000 and dried under various air conditions above and below the Tg of the rice. Results showed that rice dried significantly faster above Tg than below Tg. In addition, high temperature/low relative humidity drying air conditions, which result in a low equilibrium moisture content, apparently caused the surface of the kernel to transition from a rubbery to a glassy state and reduced the drying rate.

Toughness and microstructure of 13Cr4NiMo high-strength steel welds

Abstract: The microstructures and tensile, Charpy, and crack tip opening displacement (CTOD) properties of 13Cr4NiMo soft martensitic stainless steel flux cored are welding process (FCAW) weld metals have been studied through different applied postweld heat treatments (PWHT). Phases and microstructural characteristics have been analyzed by scanning electron microscopy (SEM) and x-ray diffraction. The effect of the tempering and double tempering, with and without previous solution annealing, on the impact and fracture toughness has been studied. The role of the retained austenite resulting from tempering has been recognized, and it is suggested that the austenite particles improve the toughness of the welds through their transformation by the transformation-induced plasticity (TRIP) mechanism.

Microstructural evolution during tempering of arc-evaporated Cr–N coatings

Abstract: Cr-N coatings were arc-deposited at 50 and 300 V. The changes in the coating microstructure and phase content during tempering were monitored. As a result, the phase stability and activation energi ...

Effects of alloy additions and tempering temperature on the sag resistance of Si–Cr spring steels

Abstract: The effects of Mo, W, and V additions and tempering temperatures on the sag resistance in relation to the microstructural evolution during tempering of Si–Cr spring steels were investigated by performing TEM examination, torsional Bauschinger tests, hardness tests and tensile tests. The hysteresis loop area measured in torsional Bauschinger tests, which is closely related with the sag resistance, is directly influenced by the distribution of precipitates. It increases and then decreases with tempering temperature, after reaching its maximum value at 350°C. The additions of Mo and/or W result in a finer distribution of tempered carbide particles due to the decrease in the rate of the spheroidization and of the coarsening of carbide particles above 400°C. However, they do not affect the behavior of tempered carbides at low tempering temperatures. Accordingly, the additions of Mo and/or W increase the hysteresis loop area at tempering temperature above 400°C. The V addition increases the hysteresis loop area as well as hardness due to the precipitation of vanadium carbonitrides, regardless of tempering temperature. In considering industrial application of spring steels, a new parameter, the ratio of the hysteresis loop area in the torsional Bauschinger test to hardness, is suggested.

Toughness and austenite stability of modified 9Cr–1Mo welds after tempering

Abstract: The influence of tempering treatments on toughness and austenite stability was conducted on modified 9Cr–1Mo welds. Deteriorated impact toughness was noticed for the welds tempered between 450 and 610°C for 1 h, in which the peak hardness and the lowest impact energy occurred at 540°C. The impact toughness of the welds increased rapidly at tempering temperatures above 680°C, and the associated fracture surface exhibited mainly dimple fractures for 750°C/1 h tempered welds, in contrast to quasi-cleavage for 540°C/1 h welds. Retained austenite at martensite interlath boundaries was found to be mechanically stabilized by transformation stresses in the welds tempered at lower temperatures. It could also be transformed to untempered martensite upon cooling if the stresses were relieved to a certain degree by proper selection of temperature and time combinations in tempering. For welds tempered at higher temperatures, e.g. 750°C, the elimination of transformation stresses and the isothermal transformation of retained austenite to ferrite proceeded simultaneously. As a result, no martensitic transformation upon cooling could be found for such welds. The transformation of austenite to untempered martensite would certainly contribute to the minimum toughness of the weld after being tempered at 540°C/1 h.

Influence of heat treatments on microstructural parameters and mechanical properties of P92 steel

Abstract: The microstructural parameters (dislocation density, martensite lath width, precipitate diameters, and volume fractions) have been measured for the 9%Cr steel P92 (NF616) after different heat treatments. The austenitising temperatures were 970, 1070, and 1145°C and the tempering temperatures 715, 775, and 835°C. Increasing the austenitising temperature led to an increase in the austenite grain size and in the martensite lath width, but no significant effect on the tensile properties at 20, 600, and 650°C was observed. The creep strength was, however, reduced by tempering at 835°C due to rapid recovery of the martensitic structure with a sharp decrease in dislocation density. The lowest creep strength was found for the P92 steel subjected to a heat treatment that produced a fully ferritic microstructure; the secondary creep rate was four orders of magnitude higher than that of the steel in the usual martensitic condition.

Correlation of the microstructure and fracture toughness of the heat-affected zones of an SA 508 steel

Abstract: In this study, microstructures of a heat-affected zone (HAZ) of an SA 508 steel were identified by Mossbauer spectroscopy in conjunction with microscopic observations, and were correlated with fracture toughness. Specimens with the peak temperature raised to 1350 °C showed mostly martensite. With the peak temperature raised to 900 °C, the martensite fraction was reduced, while bainite or martensite islands were formed because of the slow cooling from the lower austenite region and the increase in the prior austenite grain size. As the martensite fraction present inside the HAZ increased, hardness and strength tended to increase, whereas fracture toughness decreased. The microstructures were not changed much from the base metal because of the minor tempering effect when it was raised to 650 °C or 700 °C. However, fracture toughness of the subcritical HAZ with the peak temperature raised to 650 °C to 700 °C was seriously reduced after postweld heat treatment (PWHT) because carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting fracture toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.

Quenching and tempering-induced molybdenum segregation to grain boundaries in a 2.25Cr–1Mo steel

Abstract: Molybdenum is one of the major alloying elements in low-alloy structural steels and is beneficial for mitigating temper embrittlement. Enhancement of the grain boundary cohesion by molybdenum segregation is one of the mechanisms for molybdenum to alleviate the temper embrittlement. To understand clearly the segregation behaviour of molybdenum in low-alloy structural steels, examination of molybdenum grain boundary segregation during quenching and tempering in a 2.25Cr–1Mo steel is conducted experimentally by virtue of field emission gun scanning transmission electron microscopy. There is some molybdenum segregation during quenching. Supersaturated vacancies created by quenching play a certain role in the segregation. Combined equilibrium and non-equilibrium segregation of molybdenum is the nature of segregation in the tempered steels.

Method and apparatus for heating glass

Abstract: A method and apparatus for heating glass, in which glass (4) is heated upon rollers (3) in a tempering furnace (1) from above and below of the glass (4). At least the upper surface of the glass (4) is heated with hot air jets directed substantially perpendicularly relative to the surface of the glass (4). The air jets have been provided by sucking air mainly from the inside of the furnace and by pressurizing the air taken from the inside of the tempering furnace (1) to an overpressure of over 0.1 bar relative to the pressure in the tempering furnace (1).

Effect of austempering time on mechanical properties of a low manganese austempered ductile iron

Abstract: An investigation was carried out to examine the influence of austempering time on the resultant microstructure and the room-temperature mechanical properties of an unalloyed and low manganese ductile cast iron with initially ferritic as-cast structure. The effect of austempering time on the plane strain fracture toughness of this material was also studied. Compact tension and round cylindrical tensile specimens were prepared from unalloyed ductile cast iron with low manganese content and with a ferritic as-cast (solidified) structure. These specimens were then austempered in the upper (371 °C) and lower (260 °C) bainitic temperature ranges for different time periods, ranging from 30 min. to 4 h. Microstructural features such as type of bainite and the volume fraction of ferrite and austenite and its carbon content were evaluated by X-ray diffraction to examine the influence of microstructure on the mechanical properties and fracture toughness of this material.

Tensile Properties of 11Cr-0.5Mo-2W, V, Nb Stainless Steel in LMFBR Environment

Abstract: The tensile strength of ferritic-martensitic HCr-0.5Mo-2W, Nb, V stainless steel (PNC-FMS), which had been developed for core component applications in LMFBR by Japan Nuclear Cycle Development Institute, was evaluated for the effects of thermal aging, sodium exposure, and neutron irradiation. The tensile strength of thermal aged specimens (~1,023K, ~12,000h) decreased at aging conditions above the initial tempering parameter, and the aging effect was considerably enhanced for the wrapper tubes tempered at lower temperatures. The tensile strength of sodium exposed specimens (~973K, ~10,000h) decreased more than aged specimens due to decarburization, and the effect of decarburization was greater in thin wall cladding tubes. Evaluation of the contribution of both thermal aging and decarburization effects on the tensile strength of cladding tubes irradiated in JOYO (~1,013K, ~6,030h, ~29dpa) suggested that the radiation showed smaller effect on tensile properties than thermal aging and decarburization. By usi...

Insight into the microstructural characterization of ferritic steels using micromagnetic parameters

Abstract: The influence of tempering-induced microstructural changes on the micromagnetic parameters such as magnetic Barkhausen emission (MBE), coercive force (Hc), residual induction (Br), and maximum induction (Bmax) has been studied in 0.2 pct carbon steel, 2.25Cr-1Mo steel, and 9Cr-1Mo steel. It is observed that, after short tempering, the micromagnetic parameters show more or less linear correlation with hardness, which is attributed to the reduction in dislocation density, but long-term tempering produces nonlinear behavior. The variation in each of these parameters with tempering time has been explained based on the changes in the size and distribution of ferrite laths/grains and precipitates. It has been shown that the individual variation in the microstructural features such as size and distribution of laths/grains and precipitates during tempering can be clearly identified by the MBE parameters, which is not possible from the hysteresis loop parameters (Hc and Br). It is also shown that the MBE parameters can not only be used to identify different stages of tempering but also to quantify the average size of laths/grains and second-phase precipitates.

Tempering effects on the tensile response and fatigue life behavior of a sinter-hardened steel

Abstract: The mechanical behavior of a sinter-hardened steel has been evaluated for different tempering conditions. Microstructural changes, as a consequence of the post-sinter-hardening treatments, were found to significantly affect the hardness, tensile stress–strain response and high-cycle fatigue resistance of the material. Optimum tempering parameters were found to depend strongly on the service conditions: monotonic or cyclic loading. The material behavior is discussed and rationalized considering: (1) sintering necks as the load bearing units of the sintered material, (2) the role of pore tips as micronotches on these connecting regions, and (3) notch effects (strength, ductility and fatigue sensitivity) at the local level of sintering necks as critical for controlling the macroscopic mechanical properties.

Effect of stress and heat treatment on magnetic barkhausen emission in case carburized steels

Abstract: There is currently much interest in the use of magnetic barkhausen emission for monitoring the surface integrity of hardened steel components finished by grinding. However, the magnetic barkhausen emission response could vary with composition, and there is a need to calibrate magnetic barkhausen emission response for changes in residual stress and tempering. Magnetic barkhausen emission was investigated in three commercial mechanically equivalent case carburized steels with a range of compositions. Specimens were tested to determine the effect of tempering and macroscopic stress. Increased tempering produced increased emission, but this was found to be greatest when a tensile stress was acting. Although the three steels have closely similar mechanical properties, the sensitivity of magnetic barkhausen emission to tempering was found to vary substantially with composition and was greatest in the steel with the highest nickel content. The effect of glass bead peening after heat treatment was also examined. After allowing for the effect of the residual compressive stress, it was found that the plastic deformation produced by peening had a significant effect on the magnetic barkhausen emission stress relation.

Systems design of high performance stainless steels II. Prototype characterization

Abstract: Within the framework of a systems approach, the design of a high performance stainless steel integrated processing/structure/property/performance relations with mechanistic computational models. Using multicomponent thermodynamic and diffusion software platforms, the models were integrated to design a carburizable, secondary-hardening, martensitic stainless steel for advanced gear and bearing applications. Prototype evaluation confirmed the predicted martensitic transformation temperature and the desired carburizing and tempering responses, achieving a case hardness of R c 64 in the secondary-hardened condition without case primary carbides. Comparison with a commercial carburizing stainless steel demonstrated the advantage of avoiding primary carbides to resist quench cracking associated with a martensitic start temperature gradient reversal. Based on anodic polarization measurements and salt-spray testing, the prototype composition exhibited superior corrosion resistance in comparison to the 440C stainless bearing steel, which has a significantly higher alloy Cr concentration.

Residual stresses at the surface of automotive suspension springs

Abstract: This study concentrates on the origins of unfavourable stresses at the surface of silicon-manganese automotive suspension springs. The residual stresses have been investigated at the various stages of the spring manufacturing--quenching, tempering and shot peening. Residual stresses from quenching depend in a complex fashion on the microstructural state of the surface and on the variation of the thermal gradient into the quenched spring wire. Contrary to expectations, oil-quenching of decarburised spring wire results in tensile residual stresses at the surface, while water quenching results in compressive residual stresses. The residual stresses do not disappear after tempering. Moreover the shot peening after quenching and tempering, if not conducted properly, may result in small compressive or even tensile residual stresses at the surface, which severely diminishes the fatigue resistance of the suspension springs.

Constant speed universal coupling

Abstract: PROBLEM TO BE SOLVED: To provide a contact speed universal coupling in which improvement in endurability and strength has been attained. SOLUTION: In the subsurface of a tripod member, a layer (carburized layer) is formed by high density carburizing and quenching/tempering and the layer has the structure in which carbides are dispersed in martensitic matrix. And, in the subsurface of a supporting ring 32, a roller 34 and a needle roller 36, which constitute a roller assembly, layers (carburized layer) are also formed by high density carburizing and quenching/tempering and the layers have the structure in which carbides are dispersed in martensitic matrix.

Quenching apparatus and method for hardening steel parts

Abstract: A quenching apparatus and a method for hardening steel parts is disclosed. The method optimizes cooling conditions preventing the occurrence of film boiling and the “self-regulated thermal process” while optimizing the depth of the hardened layer and providing increased strength for the steel part. The optimum depth of hardening is considered to be when the surface compressive stresses on the part are at their maximum value and depth. In addition, “maximum” steel strength is achieved when the cooling rate of the part is above a certain minimum level. However, additional strengthening of the part occurs when the rate of cooling avoids both film boiling and subsequent nucleate boiling, and cooling of the part proceeds directly into the convection cooling process. At the end of the quenching process, some steel parts may benefit from isothermal cooling in air, i.e., self tempering. The method causes additional strengthening of the steel part and the achievement of maximum surface compressive stresses resulting in increased service life for the part.

Strength differential effect in four commercial steels

Abstract: The difference between compressive and tensile flow stress of a material at a given strain termed as strength differential (S-D) effect, has been evaluated in case of four commercial steels via a series of heat treated conditions. The results have unequivocally established that the magnitude of S-D was maximum in the as quenched condition and tempering of the quenched structure led to a decrease in S-D. Spheroidised and/or annealed structures exhibited the lowest value of S-D. A linear relationship of S-D value with hardness and mean stress for each case has been established. Attempts have been made to explain the observed S-D effect in terms of models based on atomic mechanism and of continuum mechanics.