Heat treatment of gears When machining the side profile of teeth, grinding and even polishing the bottom of the depressions after heat treatment results in longitudinal risks in the danger zone, i.e., reduces the load capacity of the gears. Therefore, the processing and finishing of the bottom of the cavities should be fully completed before heat treatment. Before heat treatment at the bottom of the hollows, it is necessary to completely eliminate the longitudinal processing risks by polishing.
The fillets are trimmed before heat treatment by polishing with the help of a finger circle until the longitudinal processing marks that occur after the teeth are cut completely disappear. The use of such fillets is very effective: the load capacity of the gears increases by 47-58%. Thus, the fatigue strength of gears increases.
During the period of cementation and hardening, residual stresses arise in a dangerous section of the root of the teeth; relatively large compressive stresses occur on the surface of the teeth, as well as at a depth in the zone of the cemented layer, and relatively small tensile stresses occur in the core of the tooth. The stresses from the applied load and the residual stresses arising during cementation and hardening are redistributed so that in the dangerous section of the root of the teeth the fatigue strength and load capacity of the gears increases.
When grinding teeth without a fillet (across the entire profile with gripping the bottom of cavities and transition curves), a part of the cemented layer is removed from the surface of the teeth, and as a result, the residual stresses on the ground surface of the teeth root are smaller, and if the metal is removed from the root of the teeth ( with a significant variation of the allowance at the bottom of the troughs), the residual compressive stresses on the surface can be completely eliminated.
The fatigue strength of the teeth during the heat treatment of gears from cemented chromium-nickel steels does not increase.
On the surface of the dangerous zone of the root of the teeth during grinding, residual tensile stresses arise, which reduce the fatigue strength of the gears along the bending of the teeth. The fillet protects the root from touching its surface with a grinding wheel when grinding the teeth, and the whole cemented layer is preserved on this surface.
At our factory gears are made of steel 12HNZA and 18H2N4VA with a diameter of 100-600 mm, with a tooth height of 50-218 mm, with a module 5 and 8; cementation depth 1.5-1.9 mm on gears with a module of 5 and 1.8-2.2 mm on gears with a module 8. The surface hardness was HRC 58-62, cores are less than HB 241 for steel 12HNZA and not less than HB 321 for steel 18Х2Н4ВА. Gears in shape and size of the most diverse – crowns, such as crowns, with a hub, without a hub, such as rollers, etc.
The full cycle of how the heat treatment of gears occurs is shown in fig. 1. Of great importance is the symmetrical arrangement of the holes in the part, which ensures that the quenching balances the resulting stresses. To reduce warping in some cases, it is desirable to make gears composite (prefabricated) – separately the crown and hub.
Image1. The scheme of heat treatment of gears: a – from steel 12HNZA; b – from steel 18X2H4BA.
While maintaining the geometry of the cemented gears, it is difficult to establish an excessively large depth of the cemented layer, since the elongation of cementation increases the distortion.
Excessive mechanical requirements (HRC 58-62 and 50-56) force us to resort to an increased cooling rate, which increases distortion in complex parts. The reduction of distortion is greatly affected by the removal of internal stresses: for steel 12KhNZA, normalization is applied, for steel 18Х2Н4ВА – tempering of pre-machined gears.
To obtain gears without warping, uniform heating in the furnace is necessary, depending on the position of the parts in the furnace during carburization (Fig. 2) and heating for quenching; This is especially important for gears with a diameter of 400-600 mm with a tooth height of 218 mm and for gears with a sharply variable section (crown thickness 218 mm, disc thickness 40 mm).
Image2. Device for cementation gears.
The most rational way to prevent or reduce the distortion of gears during heating is the use of salt baths. When a part is immersed in a molten salt, a thin crust of crystallized salt forms on the part, drastically reducing the heat transfer coefficient, which protects the part from thermal shock, which increases distortion.
One of the modern methods of hardening gears, providing minimal warping, is stepped hardening. Martensitic transformation during step quenching begins simultaneously over the whole section of the part, which eliminates the appearance of stresses.
The plant mastered the step hardening of gears weighing up to 10 kg, with a diameter of up to 260 mm, with a tooth height of not more than 50 mm; on average, the distortion in diameter does not exceed 0.1 and 0.15 mm. Marriage on warping such gears almost did not exist.
Studies have shown that products in austenitic state at temperatures above AC3, never regain their shape to the same extent as during step quenching, when austenite is at lower temperatures. One of the reasons for this is the sudden weakening during the martensitic transformation, accompanied by an increase in hardness with a simultaneous decrease in the bending resistance and elastic properties of the parts. The greatest plastic properties occur in the final stage of the martensitic transformation, apparently as a result of the different times of transformation at different points of the product.
To reduce the buckling of gears, the combination of stepped and fixing quenching is particularly favorable, that is, quenching in dies or on mandrels. The greatest deformation occurs by the end of the martensitic transformation, but at this time the gear wheel is fixed. As a result of softening, the part for some time becomes more plastic and takes the form of a device (stamp or mandrel). The initial design of the stamp – the transition from the crown to the disc is not at a right angle – has not justified itself.
Further improvement of the stamp and the processing of the gear profile (the transition from the rim to the disk strictly at an angle of 90 °) made it possible to perform quenching in dies on mandrels (for steel 18Х2Н4ВА) with minimal distortion.
Positive results are obtained by locking hardening on mandrels and dies, which create tension during the cooling of the gear (for example, gear size 329.8A, stamp size 330); The tension in comparison with the relative linear elongation is small: 0.2-0.3 mm for a diameter of 300-500 mm. True, this experience needs more thorough and detailed confirmation.
Mandrel shown in Fig. 3, do not allow disassembly after fixing hardening and ensure reliability and safety in operation.
Image3. The mandrel for air quenching of large crowns and gears from steel 18Х2Н4ВА: 1 – split mandrel; 2 – cone.
One of the main measures to combat buckling is the replacement of high-temperature (first) quenching by normalization – for steel 12HNZA and its exclusion (or rather, combination with cooling after a single carburizing) for steel 18Х2Н4ВА. After all, warping during hardening of cemented parts is similar to that of homogeneous steel with limited hardenability.
The use of even the most advanced methods of quenching can not lead to zero distortion. Practice has shown that working out the technology of hardening gears of complex shape with minimal distortion should be made on products, since the magnitude and nature of distortion is determined by the complexity of the geometric shape. To combat buckling, systematic observation of the buckling and deformation of parts during heat treatment using statistical control methods is necessary.
A. A. BORISOV
Kolomna Diesel Locomotive Plant
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, № 4. 1965
This article was taken from this resource.