RU2004613C1 - Process of nitriding parts manufactured from alloyed steel - Google Patents

Abstract

The essence of the invention: parts of alloy steels are subjected to volume hardening, surface hardening by laser with a radiation power of 2 to 5 kW, a beam diameter of 3 to 5 mm, a beam travel speed of 40 to 80 mm / s, high tempering, machining and nitriding to a depth of not more than depth of surface hardened layer. 1 tab

Description

The invention relates to chemical-thermal treatment and can be used for nitriding parts of alloy steels operating under high contact and bending loads, in particular for gears of gears.

A known method of low-temperature (up to 600 ° C) nitriding of alloy steels, which consists in hardening, high tempering, machining of parts and nitriding

The disadvantage of this nitriding method is the formation of iron nitrides and alloying elements in the nitrided layer along the boundaries of the former austenitic grains, the so-called nitride weaving. Grain boundaries, consisting of iron nitrides and alloying elements, are highly brittle in comparison with a more plastic grain body, which leads to the generation of fatigue cracks in the nitride network during the operation of zorated parts working under contact and bending loads. A fatigue crack with high activity also occurs along the boundaries of former austenmite grains and, as a result, leads to the destruction of the nitrided layer and the failure of parts.

A known method of hardening the surface of machine parts from alloy steels by laser treatment is to build local surface areas by subjecting an intense laser beam to dark. temperatures above critical and subsequent cooling at high speed, leading to the formation of martensigue structures to a depth of one millimeter.

The disadvantage of this method of surface heat hardening of parts is their low contact durability and wear resistance compared to nitrided parts.

The closest technical solution, selected as a prototype, is a method of nitriding machine parts from alloy steels, which consists in volume hardening, surface hardening with high frequency currents, high tempering, machining and nitriding to a depth of not more than the depth of the surface hardened LAYER,

It is known that under contact and bending loads, the development of fatigue cracks of nitrided machine parts occurs along a nitride network, which is the release of iron nitrides and alloying elements along the boundaries of former austenitic grains. Studies have shown that the thickness of the nitride network depends on the size of the former austenitic grain: the larger the grain, the more intense

The nitride network is formed during nitriding and its thickness increases.

In the known nitriding method, grinding of austenitic grain is achieved

using surface hardening by high frequency currents.

However, surface quenching by high-frequency currents allows grain to be crushed only up to 11-12 points, which leads to

5 to the formation of a nitride network during nitriding, although of small thickness.

The aim of the invention is to increase the contact and flexural durability of nitrided parts by completely reducing the nitride network.

This goal is achieved by the fact that the proposed nitriding method performs volume hardening, surface hardening by laser, high tempering,

5 machining and nitriding to a depth of not more than the depth of the surface hardened layer,

It is known that an increase in the rate of heating during quenching contributes to

0 about steel of finer grain. The heating rate during hardening of the HDTV is 102-104 deg / s, and during laser hardening up to 106 deg / s. When laser hardened, it is possible to grind grains in alloy steel up to 14 points.

5 Change in grain to 14 points leads to a significant increase in the length of high angle (intergranular) and small angle (interblock.) Boundaries. In addition, in contrast to the hardening of the HDTV after laser filling, in the obtained fine grains, as a result of phase hardening, an increase in intragranular defects (vacancies, dislocations, stacking faults, etc.) is observed hundreds of times in comparison with surface hardening of the HDTV. Thus, a sharp increase in the number of defects in the crystalline structure (an increase in the order of the length of high-angle and small-angle boundaries, an increase in the number of intragranular defects by several orders of magnitude), which are additional ways of nitrogen diffusion, leads to a new quality of the structure of the nitrided layer; complete suppression of education

5 iron nitrides along the boundaries of former austenitic grains (nitride network) and allows to increase the bending and contact durability of nitrided parts,

The described structure of the nitrided layer can only be formed after laser irradiation in certain modes.

Laser treatment modes should provide:

temperature-time gradient leading to phase transformations and grain refinement in steel:

grinding grain at a distance from the surface of at least 0.5-0.6 mm;

lack of surface melting.

The laser exposure regimes providing the specified parameters of the treated layer were selected taking into account the theoretical assumptions and adjusted according to the experimental results.

The main parameters of laser hardening by cw lasers are: spot diameter d (mm), radiation power P (kW) and speed of movement of the part (beam) v (mm / s) relative to the beam (part), which determines the duration of laser exposure .

The spot diameter is 3-5 mm. Increasing the spot size increases the heterogeneity of the power distribution over the spot, which leads to uneven depth of the laser exposure zone. Reducing the spot diameter, as shown by experiments, reduces the stability of the laser exposure and can lead to surface fusion.

The radiation power at the specified beam diameter was chosen so that the power density of the laser radiation provided local heating of the metal to the melting temperature without noticeable evaporation of the metal. Using the recommended values of the radiation power density of 10 -10 kW / m and the experimental results, we chose an upper threshold of the radiation power that does not lead to surface melting of 5 kW, and a lower threshold of the radiation power that provides heating of the surface to a phase transformation temperature of 2 kW.

The speed of movement of the laser beam relative to the part during heat treatment should provide a duration of laser exposure of at least s, which for a given size of the spot beam is not more than 3000 mm / s. The experiments showed that an increase in the speed of the beam moving above 80 mm / s does not provide the necessary depth of the laser exposure zone, and when the speed decreases below 40 mm / s, surface melting is possible.

Thus, the following laser processing modes were selected that provide the required quality of the laser exposure zone:

Beam spot diameter 3-5 mm. Radiation power 2-5 kW. Travel speed 40-80 mm / s

The claimed technical solution differs from the prototype in that in the proposed method of nitriding, the preliminary surface heat treatment is carried out by a laser with the following modes: radiation power of 2-5 kW. beam diameter 3-5 mm; beam travel speed 40-80 mm / s. These distinctive features compared with the prototype allow us to conclude that the claimed technical solution meets the criterion of novelty,

In the proposed nitriding method, the set of features in the following sequence: volumetric hardening; laser surface hardening with the following modes: radiation power 2-5 kW, beam diameter 3-5 mm, beam moving speed 40-80 mm / s; high vacation; mechanical processing; nitriding to a depth not exceeding the depth of the surface hardened layer leads to an increase in the contact durability of nitrided parts due to the complete elimination of the nitride network by means of a sharp increase in the number of nitrogen diffusion paths over defects in the crystal structure.

It is this new property of the totality of features that allows us to conclude

5 according to the proposed technical solution, the criterion of inventive step.

The proposed nitriding process has been tested in laboratory conditions.

0 EXAMPLE According to the proposed technology, samples were processed from low-carbon low-alloy steel 10KhZG2MYuFT. According to the proposed technology, the samples were quenched in the PN-35 furnace.

5 Laser hardening was carried out on a continuous-wave laser apparatus HLTU-25. High tempering was carried out in the PN-35 furnace. After that, the samples were subjected to final processing.

By grinding to remove the oxidized layer and gas nitriding in ammonia in a US furnace, 6.5: 22.10 / 6. Samples in the form of cylinders with a length of 30 mm and a diameter of 14 mm with seats for bearings underwent volume hardening with a temperature of 850 ° C in oil. After that, the samples were processed on a laser installation in the following modes:

Diameter p tna

beam 3 mm

Radiation power

The speed of the beam relative to the part (sample rotation speed)

Coefficient of overlap (speed of sample movement along the generatrix of the cylinder) 0.2 (5 mm / s) Absorbing black gouache The hardened layer has a depth of 0.5-0.6 mm. After high tempering at a temperature of 600 ° С for 2 h, final grinding to a depth of 0.02 mm, the samples were subjected to gas nitriding according to the regime: Nitriding temperature 550 ° С Nitriding time 50 h

Ammonia dissociation 45-50%

Samples processed by the proposed technology have a depth of the surface of the laser-hardened zone of 0.5-0.6 mm, a depth of the nitrided layer of 0.5-0.6 mm. the size of the former austenitic grain is 13-14 number within the nitrided layer and 8-9 number in the core.

The results of comparative tests for contact durability of samples processed by the applied and proposed nitriding technology are shown in the table.

Thus, the use of the proposed nitriding method makes it possible to reduce the size of the former austenitic grain up to 14 numbers, completely eliminate the nitride network in the nitrided layer and increase the contact durability of nitrided parts by 30%.

(56) Copyright certificate of the USSR No. 1574649, cl. C 21 C 1/78, .1990.

Nitriding process

Applicable

Volumetric hardening of Т 850 ° С, oil is superficially quenching of TVCh, Т 850-930 ° С, shutter speed 3 + 1 s,

oil Vacation T 600 ° C, holding 2 hours, air

Chistova machining

Zoting T 550 ° C, holding for 50 hours, degree of dissociation of ammonia 45-50%

Applicable

Volumetric quenching T 850 ° C, oil

Laser surface hardening

Vacation 600 ° C, holding 2 hours, air

fine machining

Nitriding T 550 ° C, holding 50 h, degree

ammonia dissociation 45-55%

Claims (1)

The claims METHOD OF NITROGENIZING PARTS FROM ALLOYED STEELS, including volume hardening, surface hardening, high tempering, machining and nitriding to a depth not Compiled by EditorTehred M.Mo Contact life, million cycles 11.5 14.7 more than the depth of the surface hardened layer, characterized in that the surface hardening is carried out with a laser using radiation of 2-5 kW, a beam diameter of 3-5 mm, and a beam travel speed of 40 - 80 mm / s. Proofreader V. Petrash