Carburized Gears Research Articles

A practical estimation of the residual stress of carburized spur gears.

The residual stress of carburized spur gears is calculated being based on the assumption that the main cause of residual stress is the volume difference between case and core due to the martensitic transformation in cooling. The relation between hardness and carbon content is determined for SCM420H test gears by the measurement, and is used to estimate the specific volumes of the structures from the distribution of hardness. The two-dimensional FEM is used for the calculation The reliability of the method is examined by comparison with stresses measured by the X-ray diffraction method. The present method is then applied to the estimation of residual stress of carburized gears whose hardness and retained austenite are recommended in the AGMA standard. A formula is proposed to estimate the residual stress from hardness and the amount of retained austenite, and it is applied to the estimation of the fatigue strength of carburized spur gear teeth.

SAE 4820

Abstract SAE 4820 is a low-carbon (nominally 0.20%) carburizing steel containing nominally 3.5% nickel and 0.25% molybdenum. It is deal for used that require a strong, tough core along with a highly wear-resistant carburized case. In addition, it offers a good combination of strength and low-temperature toughness for non-carburized applications. It is recommended for a wide range of moderately heavy-duty applications. Among its many uses are carburized gears, carburized aerospace components and non-carburized machinery parts. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SA-409. Producer or source: Alloy steel mills and foundries.

Failure analysis and wear mechanism study of a heavily loaded gear

Failure analysis and a study of the wear mechanism were performed on a heavily loaded carburized gear with some unusual features. The analysis showed that the different failure forms on the tooth surface of the gear were dependent on the sub-surface shearing stress, the lubrication conditions and the surface roughness. Delaminations formed on the addendum surface were the main reason for the gear failure. A suggestion for improvement has been tried, and a desirable result has been obtained.

Capacity Limits of the Largest Pairs of Carburized Gears to be Produced With Available Equipment

Independent from the application and from the expected probability of failure, each pair of gear wheels has an inherent power transmission capacity. Particularly the large-sized, carburized, and flank-ground gears are subjected to very different opinions in regard of their capacity. Recently published calculation methods, however, allow for a detailed determination of the ratings. The comparison of their relative results with actual applications thus seems of great interest in order to check the factors to be applied.

On the soft layer in carburized gears

A frequently observed feature in gear carburizing is the formation on the part surface of a shallow layer showing traces of grain boundary oxidation. This layer and that immediately underlying it often do not transform completely to martensite upon quenching, do not attain full hardness, and adversely affect the fatique behavior of the gear.

Case depth requirements in carburized gears

Summary Spalling fatigue in carburized gears occurs when the applied alternating subsurface shear stress exceeds the fatigue strength of the material. An analytical procedure is introduced which calculates, from the applied load and gear data, the applied subsurface shear stress gradient at the point of contact of the mating teeth. The hardness gradient required to accomodate the applied load is then determined from the shear stress gradient by applying a torsional fatigue strength-hardness relationship and a suitable design safety factor.

Study on Load-Carrying Capacities of Carburized Gears of Leaded Free-Cutting Steel

Study on Load-Carrying Capacities of Carburized Gears of Leaded Free-Cutting Steel

Residual stress and microstructure in quenched and tempered and hot oil quenched carburized gears

The residual stress and microstructure in carburized cases developed by a conventional quench and temper and a hot oil quench were determined for two automotive differential gears. A Ithough the results for the two gears could not be directly compared because of the different geometries, the changes in residual stress produced by hot oil quenching was found to depend on the hardenability of the steels. Hot oil quenched, carburized SAE 4130 drive pinion gears had higher compressive residual stresses than quenched and tempered gears, although the microstructures were similar. The higher compressive stresses are believed caused by 1) the initiation of austenite transformation nearer the core because of the lower thermal gradient, and 2) the elimination of tempering. In contrast, hot oil quenched, carburized SAE 1526 ring gears had lower compressive residual stresses than quenched and tempered gears. The microstructure of the carburized case of quenched and tempered gears was mainly martensitic while bainite was the primary microconstituent in hot oil quenched gears. The lower residual stress in the hot oil quenched ring gears may result from the lower transformation strain associated with the austenite → bainite transformation.

Progress in plasma carburizing

Experiments on high-rate carburizing in a dc glow-discharge plasma were extended to cover a broader range of temperature, carburizing gases, and steel compositions. Results show that the enhanced carburizing rate observed with methane is also obtained with its heavier homologues over the temperature range explored. The efficiency at which the carburizing gas is utilized during the carburizing stage was measured for both pure methane atmospheres and methane/nitrogen mixtures. AISI 4615 steel differential gears were plasma carburized at 1040 °C for performance tests to determine whether the more rapid carburization at the higher temperature would have any deleterious effect on impact fatigue life. In accelerated fatigue tests in an automobile, the performance of the plasma carburized gears equalled that of gears that had been carburized in the conventional way.

An analytical approach for establishing case depth requirements in carburized gears

An analytical approach for determining case depth requirements in carburized gears is presented. It is indicated that the minimum case depth requirement in carburized gears is dictated primarily by spoiling fatigue which occurs when the applied alternating shear stress exceeds the allowable fatigue strength in the vicinity of the casecore boundary. Based on these concepts, the following procedure for establishing case depth is suggested: 1) From design load and gear geometry data, compute the applied shear stress gradient at the most severe location along the path of contact. 2) Transform applied shear stress gradient into a critical shear stress gradient using suitable design factors. 3) Convert the critical shear stress gradient into a hardness gradient by applying a torsional fatigue strength = UTS = hardness relationship. The validity of this approach has been substantiated by conducting constant torque four square dynamometer tests on several transmission gears carburized to varying case depths. By analyzing present production data and by applying basic hardenability principles, the stress/strength intersection concepts have been further used to demonstrate that current case depth requirements established on the basis of commonly used empirical rules may be over-specified.

Fatigue Strength of RDK401 Nitrocarburized at Low Temperature

In order to examine the application limit of gears made of RDK401 which has a deep case depth by a low temperature gas nitrocarburizing treatment, the Ono-type rotating beam fatigue test, the Nishihara-type rolling contact fatigue test and gear test were carried out, then the underwritten results were obtained.(1) The rotating beam fatigue strength is 42kgf/mm2, when the stress concentration factor is 1.84.(2) The rolling contact fatigue strength is 180kgf/mm2, when the effective case depth is 0.19mm.(3) According to the gear tests, the allowable tooth root bending stress is 41kgf/mm2 and its value is near to that of the carburized gears. Then the allowable tooth surface stress is over 145kgf/mm2 in Hertz pressure.

BASIC STUDIES ON SCORING OF SPUR GEARS : 2ND REPORT, EXPERIMENTAL RESULTS USING POWER CIRCULATING GEAR TEST RIG

The authors previously cleared from the experimental results which are derived from the four-ball test and the roller test that the condition or scoring could be anticipated by the surface temperature rise, and discussed how the thermal conductivity in the surface layer of material affects the surface temperature rise. In this report, the effects of the following factors on scoring are discussed ; the material and the hardness of gear, the viscosity of lubricant and the surface roughness of gear tooth. The results for scoring test are summarized as follows ; on both the normalized gears and the hardened gears, the progress of wear is especially remarkable in the zone of single pair of contact and the tendency of the increase of wear depth on the tooth surface well coincides with that of the weight loss due to wear. When the scoring failure of the hardened gear is compared with that of the normalized gear, the scar of the hardened gear tooth looks like a number of the micro fragments of the welding material gathered, and the size of the wear particles of the hardened gear is smaller than that of the normalized gear. As the thermal conductivity fall, the scoring limit decreases. Though the effect of the hardness on the scoring limit is larger than that of the thermal conductivity, the influence of the hardness of gear is inseparable from the scoring limit because the thermal conductivity and the hardness interact each other. Lastly, as the viscosity of lubricant grows higher, the scoring limit increases. In the lubricating condition of 140-Turbine oil, the index values of the flash surface temperature in AGMA Standard which were calculated by the measurement value of the surface roughness after running in, are within 161172°C on the carburized gear which consists of SCM22H material, and the mean value of the flash temperature is about 168°C.

Bending Fatique Strength of Gear Teeth under Random Loading : Part 1, About the Gear Tooth Pulsator and the Results of Fatique Tests under Uniformly Distributed 16 Steps Program Loading

Power transmission gears are usually used under fluctuating loads. But in the case of strength design of such gears we cannot get sufficient fatigue data and therefore the effectiveness of Miner's rule on fatigue strength is uncertain. This report deals with a new gear tooth pulsator for program and random fatigue tests and with the results of a test which was carried out under a programmed 16 steps loading. The frequencies of loads are distributed uniformly and the variation of mean load is like a saw tooth wave with positive and negative slopes. Normalized S38C and carburized SCM21 gears were tested. Every test gear had 27 teeth, 4mm module and 8mm width. Under the above condition the effects of loading sequence, changing rate and mean level of program load, gear material and heat treatment were investigated. And, though Hardrath has shown a program loading sequence effect for 7075-T6 aluminium alloy, the author's experiment indicates that its effect is not significant at the 5% level for both S38C and SCM21 gears. By arranging the cumulative damage statistically, it is concluded that Miner's theorem is not valid in the strict sense and further investigation is necessary to conclude whether Miner's rule is applicable to the fatigue design of gears.

Nitrocementation of chromium ? nickel steels

1. After nitrocementation, steel 20KhGSNT has a high wear resistance and fatigue strength considerably higher than that of steel 20KhN3A after carburizing and, especially, after nitrocementation. The replacement of carburized gears of steel 20KhN3A with nitrocementated gears of steel 20KhGSNT increases the working life of the gears and reduces the cost of the parts due to the technological advantage of the nitrocementation process and the substantial saving of nickel. 2. Nitrocementation with direct quenching of steel 20KhN3A substantially lowers the fatigue strength by comparison with carburizing and cannot be recommended for critical gears.

Mechanical properties of steel 20KhNM after quenching and carburizing

To increase the strength and working life of quenched carburized gears of steel 20KhNM it is recommended that they be reheated by high-frequency current and quenched in a high-speed oil spray. This also permits the gears to be manufactured from steel with a lower concentration of alloying elements.