摘要 :
The Hollomon-Jaffe parameter is commonly used to characterize thermal cycles and define time-temperature equivalences during tempering, but it can only be computed for simple thermal cycles defined by a couple of values [temperatu...
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The Hollomon-Jaffe parameter is commonly used to characterize thermal cycles and define time-temperature equivalences during tempering, but it can only be computed for simple thermal cycles defined by a couple of values [temperature;;time]. Its use is not adapted for complex thermal cycles. Three tempering parameters which integrate the full thermal cycles were investigated: the Hollomon-Jaffe derivative parameter, and the Tsuchiyama and Arrhenius laws. Simple and complex thermal cycles were performed on a water-quenched carbon steel (i.e. different heating and cooling rates or two-step cycles, with temperature varying between 450 °C and 750 °C and times from 30 s to 3 days). All three tempering parameters show good correlation with the tensile properties of the material, even when complex cycles were performed, provided that the material constant is optimized. The sensitivity to the determination of this constant is low for the Tsuchiyama and Arrhenius laws: they can be considered as robust and reliable. On the contrary, the Hollomon-Jaffe derivative parameter proved unstable.
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It is well known that quenched and tempered alloy steel components with ultimate tensile strength in excess of 1400 MPa are seldom employed as mechanical components, due to their not adequate ductility, as ascertained by multiple ...
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It is well known that quenched and tempered alloy steel components with ultimate tensile strength in excess of 1400 MPa are seldom employed as mechanical components, due to their not adequate ductility, as ascertained by multiple researches performed during World War II and soon after. Nevertheless, use of low temperature tempered steels in some niche applications, as well as researches performed on surface heat treated high carbon steels and on their behavior upon tempering in the vicinity of 200°C have stemmed into renewed interest in quenched and low temperature tempered low alloy steels. Application to 36NiCrMo16 steel bars is examined here, by means of tensile and hardness tests and fractographic and metallographic examinations after quenching and tempering in the 160 to 440 °C temperature range.
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Effect of copper addition on microstructure and mechanical properties of an ultra high strength low alloy NiSiCrCoMo steel is investigated in the present study. Fe-0.38C-3Ni-2Si-1Cr- ＜ 1 (Co+Mo), is selected as base steel for the...
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Effect of copper addition on microstructure and mechanical properties of an ultra high strength low alloy NiSiCrCoMo steel is investigated in the present study. Fe-0.38C-3Ni-2Si-1Cr- ＜ 1 (Co+Mo), is selected as base steel for the present study. Copper modified base steel was further austenitised and quenched, followed by tempering in the temperature range of 200-750 ℃. The effect of tempering temperature on mechanical properties along with detailed microstructure changes at low (200 ℃) and high (750 ℃) tempering temperature has been evaluated for these steels. An increase in strength due to copper addition has been observed at both low and high tempering temperature. The increase in strength is observed to be much sharper at high tempering temperature than that at low ones. The higher strength value along with low toughness in copper modified base steel has been attributed to ordered B2 domains along with lenticular precipitates.
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The fracture of 50CrVA steel spring was analyzed by means of composition analysis, metallographic structure observation, hardness tester and scanning electron microscope (SEM). The results show that the surface quality defects and...
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The fracture of 50CrVA steel spring was analyzed by means of composition analysis, metallographic structure observation, hardness tester and scanning electron microscope (SEM). The results show that the surface quality defects and lower tempering temperature led to coarser and larger grains, lowered elastic limit and yield strength, resulting in the corrosion fatigue and the failure and fracture of the steel spring ultimately.
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Tempering of 0.2 percent C alloyed steel was tried to carry out employing induction heating method in this study. Mechanical properties of the steel tempered by induction heating and salt bath heating were compared to check the po...
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Tempering of 0.2 percent C alloyed steel was tried to carry out employing induction heating method in this study. Mechanical properties of the steel tempered by induction heating and salt bath heating were compared to check the possibility of induction heating for industrial application. Results show that yield and tensile strength decreased and reduction of area and total elongation increased with increasing tempering temperature. it was found that one needs just increased tempering temperature with the induction heating method to obtain desired properties of tempered steel for proper formability, This evident fact was revealed from the unique relationship of reduction of area and total elongation as a function of ensile strength. The increased impact strength could be obtained with induction heating tempering. Prior austenite grain size affected to yield strength and impact strength but not tensile strength, reduction of area and elongation. It was concluded that induction heating method would be one of the competitive heat treating processes for steels in industrial fields.
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H13 steel is a typical hot work die steel with good strength and toughness that is often used to manufacture high-temperature disk springs. However, disk springs occasionally fail after use in the petrochemical industry. Therefore...
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H13 steel is a typical hot work die steel with good strength and toughness that is often used to manufacture high-temperature disk springs. However, disk springs occasionally fail after use in the petrochemical industry. Therefore, the effects of the quenching and tempering temperatures on the microstructure and mechanical properties of H13 steel after quenching and tempering processes are investigated herein. The results show that the lath width (lath) controls the strength of the H13 steel. The precipitated phases mainly comprise Cr23C6, Cr7C3 and VC. The coarsening of the Cr23C6 phase reduces the hardness, while reducing the dislocation density improves the toughness of the H13 steels after quenching and tempering. When the quenching temperature is 1040 degrees C and the tempering temperature is 570 degrees C, the H13 steel after quenching and tempering has a uniform microstructure with good strength and toughness.
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Temperature and irradiance are the most important factors affecting marine benthic microalgalphotosynthetic rates in temperate intertidal areas.Two temperate benthic diatoms species, Amphoracf. coffeaeformis (C. Agardh) Kutz. and ...
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Temperature and irradiance are the most important factors affecting marine benthic microalgalphotosynthetic rates in temperate intertidal areas.Two temperate benthic diatoms species, Amphoracf. coffeaeformis (C. Agardh) Kutz. and Cocconeis cf.sublittoralis Hendey, were investigated to determinehow their photosynthesis responded to temperaturesranging from 5 deg C to 50 deg C after short-term exposure(1 h) to a range of irradiance levels (0, 500, and1,100 mu mol photons . m~(-2) . s~(-1). Significant differences were observed between the temperatureresponses of maximum relative electron transportrate (rETRmax), photoacclimation index (E_k), photosynthetic efficiency (alpha), and effective quantumyield (DELTA F/F_m') in both species. A. coffeaeformis had agreater tolerance to higher temperatures than C. sublittoralis, with nonphotochemical quenching (NPQ)activated at temperatures of 45 deg C and 50 deg C. C. sublittoralis, however, demonstrated a more rapid rateof recovery at ambient temperatures. Temperaturesbetween 108C and 208C were determined to be optimal for photosynthesis for both species. High temperatures and irradiances caused a greater decreasein DELTA F/F_m' values. These results suggest that theeffects of temperature are species specific and thatshort-term exposure to adverse temperature slowsthe recovery process, which subsequently leads tophotoinhibition.
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Abstract Investigations of simple and accurate meteorology classification systems for influenza epidemics, particularly in subtropical regions, are limited. To assist in preparing for potential upsurges in the demand on healthcare...
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Abstract Investigations of simple and accurate meteorology classification systems for influenza epidemics, particularly in subtropical regions, are limited. To assist in preparing for potential upsurges in the demand on healthcare facilities during influenza seasons, our study aims to develop a set of meteorologically-favorable zones for epidemics of influenza A and B, defined as the intervals of meteorological variables with prediction performance optimized. We collected weekly detection rates of laboratory-confirmed influenza cases from four local major hospitals in Hong Kong?between 2004 and 2019. Meteorological and air quality records for hospitals were collected from their closest monitoring stations. We employed classification?and?regression trees to identify zones that optimize the prediction performance of meteorological data in influenza epidemics, defined as a weekly rate?>?50th percentile over a year. According to the results, a combination of temperature?>?25.1℃ and relative humidity?>?79% was favorable to epidemics in hot seasons, whereas either temperature??76% was favorable to epidemics in cold seasons. The area under the receiver operating characteristic curve (AUC) in model training achieved 0.80 (95% confidence interval [CI], 0.76–0.83) and was kept at 0.71 (95%CI, 0.65–0.77) in validation. The meteorologically-favorable zones for predicting influenza A or A and B epidemics together were similar, but the AUC for predicting influenza B epidemics was comparatively lower. In conclusion, we established meteorologically-favorable zones for influenza A and B epidemics with a satisfactory prediction performance, even though the influenza seasonality in this subtropical setting was weak and type-specific.
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