RU2522872C2 - Nitration of machine parts with production of nanostructured surface ply and ply composition - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to metallurgy. Parts are processed by quenching at 920-940°C, subjected to negative hardening with heating to 600-650°C for 2-10 hours and removal of decarbonised ply. Then, ion-plasma nitration is performed at 500-570°C, cathode voltage of 300-320 V, and current density of 0.20-0.23 mA/cm². Ammonia with dissociation of 0-80% is used as a gas medium. Ammonia flow rate makes up to 20 dm³/h. Pressure in the chamber at cathode spraying makes 1.3-1.35 Pa and, at saturation, 5-8 GPa. This nitration is performed at cyclic temperature and ammonia dissociation variation. Note here that at the first half of described cycle temperature makes 570°C at maximum nitrogen potential. During second half, temperature decreases to 500°C while nitrogen potential is decreased owing to increase in ammonia dissociation to 40-80%. Note also that the number of said cycles should make at least 10. Nitrated part has surface ply containing diffusion ply with α-phase with nanosized incoherent alloying element nitrides that makes soft matrix. Besides, it has surface ply with hard inclusions composed by nanoparticles of ε-phase iron nitrides formed by local phase recrystallisation of iron nitride lattices. This results from cyclic temperature and ammonia dissociation variation.

EFFECT: higher wear resistance, longer life of kinetic friction parts made from above described material.

2 cl, 1 tbl, 2 dwg

Description

Technical field

The invention relates to mechanical engineering, in particular to a method for nitriding machine parts to obtain a nanostructured surface layer, to obtain a nanostructured state of its diffusion layers, used to increase the wear resistance of parts of sliding friction units from iron-based alloys.

State of the art

Known methods of chemical-thermal treatment that improve the wear resistance of steel parts and containing the operation of preliminary heat treatment and subsequent nitriding. So, the technical solution contained in (RF Patent No. 2291227, IPC C23F 17/00, C23C 8/26, C21D 1/72, publ. 10.01.2007), involves a preliminary treatment before nitriding, consisting of the following operations: normalization, tempering, hardening, tempering, machining, stabilizing tempering, and then nitriding at a temperature of 530 ° C for 1.5 to 30 hours and de-nitriding for 0.4 ... 1.5 hours. This method allows to increase the wear resistance and reduce the fragility of the surface layers steel, however, the mechanical properties of the surface are sufficiently a large spread (according to the description of the patent it is not less than 15%) and nanostructuring process in the diffusion zone does not occur. As a result, the improvement in wear resistance remains within the limits determined by the wear rate I _h , not more than I _h ≈10 ^-9 .

There is also a method of processing steel products in a gaseous medium (RF Patent No. 2367716, IPC C23C 8/34, C23C 8/26, publ. 09/20/2009), including heating the products to a saturation temperature of 450 ... 780 ° C in an atmosphere of ammonia, followed by exposure in a saturating gaseous medium, where air and ammonia are used as a saturating medium during exposure, which are supplied separately, and the products are held alternately in an atmosphere of air, and then in an ammonia atmosphere, resulting in a multilayer structure consisting of a series of interconnected layers of oxide and nitride phases of iron and the corresponding alloying elements.

However, the oxide layer has low mechanical properties and low wear resistance, therefore, its presence reduces the overall effect of increasing wear resistance.

The closest technical solution is the method of chemical-thermal treatment described in (RF Patent No. 2367715, IPC C23C 8/34, C23C 8/26, publ. 09/20/2009). The main difference of this method is the use of an air atmosphere to form a layer of oxides on the steel surface before nitriding. The sequence of operations in this method is as follows: heating in an inert atmosphere, holding at the reached temperature in an air atmosphere, holding in a saturating nitrogen-containing atmosphere to obtain a diffusion layer in the form of nanoparticles of nitrides of alloying elements.

The main disadvantage of this method is the presence of an oxide layer, which according to the authors of the analogue invention promotes the penetration of nitrogen into steel and the formation of nanoparticles from special nitrides.

In this case, nanosized particles of nitrides are obtained. However, the resulting combined oxide-nitride layer is not effective enough - not wear resistant - the increase in wear resistance occurs within insignificant limits - of the order of tens of percent.

Disclosure of invention

The objective of the invention is a significant increase in the wear resistance of the surface layers formed as a result of nitriding of parts of the sliding friction units, and a corresponding increase in the durability of the sliding friction units with such a composition of the surface layer.

The technical effect is achieved by the fact that in the method of nitriding parts of the sliding friction units to obtain a nanostructured surface layer, the part is subjected to preliminary heat treatment and subsequent nitriding. In this case, quenching at a temperature of 920 ... 940 ° C, subsequent high tempering with heating to 600 ... 650 ° C for 2 ... 10 hours and removal of the decarburized layer is used as preliminary heat treatment, and then ion-plasma nitriding is carried out with the following parameters in the range temperatures 500 ... 570 ° C: - voltage at the cathode 300 ... 320V; - current density 0.20 ... 0.23 mA / cm ² ; - the composition of the gas medium is ammonia with a degree of dissociation from zero to 80%; - ammonia consumption up to 20 dm ³ / h; - the pressure in the chamber during cathodic atomization is 1.3 ... 1.35 Pa, at saturation - 5 ... 8 GPa. Moreover, nitriding is carried out in the mode of cyclic changes in the temperature and degree of dissociation of ammonia, in the first half of the cycle the temperature is 570 ° C with a maximum nitrogen potential, and in the second half of the cycle the temperature is reduced to 500 ° C, while the nitrogen potential is reduced by increasing the degree of ammonia dissociation (40 ... 80%), while the number of such cycles should be at least 10.

A detail of a sliding friction unit with a nanostructured near-surface layer contains a diffusion zone with nanosized nitride inclusions, while the considered nanostructured near-surface layer obtained by the proposed method contains a diffusion layer with an α phase with nanoscale incoherent nitrides of alloying elements, which forms a soft matrix, and a surface layer with solid inclusions representing nanoparticles of iron nitrides ε-phases formed by phase local cross-section the crystallization of iron nitride lattices, which is provided by a cyclic change in the temperature of nitriding and the degree of dissociation of ammonia.

List of drawings

Figure 1 shows the microstructure of the ε phase in the surface of the nitrided layer of steel 38Kh2MYuA: in - bright field image with a microdiffraction pattern; d is a dark-field image of the ε phase in the (110) reflection;

figure 2 - change in the wear rate of nitrided steel 38X2MUA during sliding friction.

The implementation of the invention

The main difference of the proposed processing method is that a diffusion zone is formed with nitrides of alloying elements having an incoherent bond with the matrix and a size of 10 ... 15 nm, as well as a surface layer consisting of ε-phase nanoparticles (iron nitrides Fe _2-3 N ) 20 ... 50 nm in size.

Nitriding is carried out under conditions of a cyclic change in the temperature of nitriding and ammonia supply with a different (from 0 to 80%) degree of dissociation, which makes it possible to change the nitrogen potential of the gaseous medium during the process.

As a result of this technological technique (cycling), the proposed solution creates conditions for phase recrystallization of the nitride zone and its formation in a nanostructured state. At the first active stage of the cycle at a temperature of 570 ° C in flowing ammonia under conditions of high nitrogen potential, a sufficiently thick (20 ... 30 μm) nitride layer was created from ε and γ phases, phases of iron nitrides and alloying elements formed during direct recrystallization α↔γ′↔ε. At the second passive stage of the cycle, the temperature was lowered to 500 ° C and dissociated ammonia (degree of dissociation from 40 to 80%) was supplied for some time. A sharp decrease in the nitrogen potential caused the development of the reverse transformation ε↔γ ′. When alternating cycles consisting of active and passive stages, the structure of the nitride zone is ground and the surface layer of the ε-phase is formed on the surface of the nitrided steel in a nanostructured state.

The thickness of the ε-phase layer, the size of the crystals in it, and their hardness are significantly affected by the temperature of the preliminary (before nitriding) high tempering. Under the same nitriding regime, the thickness of the ε-phase layer increases proportionally with an increase in the tempering temperature from 500 to 650 ° С. In addition, the nanosize of nitrides decreases in this phase, their hardness increases, and, as a result, its wear resistance increases.

Ion-plasma nitriding, which makes it possible to control the parameters of the process, is carried out at the following parameters in the temperature range 500 ... 570 ° C:

- voltage at the cathode 300 ... 320 V;

- current density 0.20 ... 0.23 mA / cm ² ;

- the composition of the gaseous medium is ammonia of varying degrees of dissociation (range from 0 to 80%;

- gas mixture flow rate up to 20 dm ³ / h;

- the pressure in the chamber during cathodic atomization is 1.3 ... 1.35 Pa at saturation - 5 ... 8 GPa.

Modes of nitriding and parameters of the surface layer are shown in table 1.

Table 1 The modes of ion-plasma nitriding and the parameters of the surface layer (surface and diffusion layers) of steel 38Kh2MYuA. The degree of dissociation of ammonia (number of cycles) Nitriding mode Thickness Tempering temperature before nitriding, ° C Successor
h effective mm total mm ε-phase layer, μm 0↔60% (10) 650 thirty 0.23-0.26 0.40-0.45 26-28 0↔80% (5) 600 fifteen 0.21-0.24 0.36-0.40 6-8 0↔80% (10) thirty 0.27-0.29 0.44-0.46 8-12 0↔80% (16) 48 0.32-0.34 0.52-0.54 15-17 0↔80% (5) 650 fifteen 0.25-0.27 0.41-0.43 8-10 0↔80% (10) thirty 0.31-0.33 0.46-0.48 12-15 0↔80% (16) 48 0.36-0.38 0.61-0.65 16-18

The justification of the number of cycles is contained in the table, where it is shown that the thickness of the nanostructured layer (ε-phase layer) can be at least 10 μm with the number of cycles at least 10, which ensures the necessary wear resistance of the surface layer.

The nitrided layer has a layered and multiphase structure. It is accepted that the working surface of the part should have the structure of a nitrogen-saturated solid solution with particles of nitrides of alloying elements. The surface nitride zone of the nitrided layer in the form of an ε-phase is traditionally removed by grinding parts. Meanwhile, the results of the studies performed by the authors indicate that, under certain conditions of ion-plasma nitriding established by the authors, the ε-phase layer is formed in the form of nanocrystalline particles.

The formation of the ε phase in the nanostructured state is confirmed by X-ray diffraction and electron microscopy studies. We used a special technique of a moving x-ray beam. Shooting at a small angle to the analyzed surface provided information on the structure of the thin layer of the ε phase. According to the physical broadening of the X-ray lines, it was unequivocally established that the ε phase is nanocrystalline with a crystal size of 20 to 50 nm, which is confirmed by transmission electron microscopy (Fig. 1).

It is shown for the first time that the basis for creating a nanoscale structure is phase recrystallization: ε↔γ'↔α phases of the nitrided layer, the driving force of which is a change in the process of the nitrogen potential of the gas medium.

The recrystallization mechanism causes the formation of nuclei of crystals of a new phase inside the existing (old) phase. Solid-state local recrystallization of iron nitride lattices is based on the simultaneous nucleation and growth of five-fold "twins" on icosahedral clusters stabilized by nitrogen atoms. Under conditions of a cyclic change in the value of the nitrogen potential, multiple phase recrystallization develops and a nanostructured nitride layer with a crystal size of up to 50 nm is formed on the surface of the part.

It is important that the nanostructured layer is formed directly on the surface of the hardened part during the nitriding process. This shows a significant advantage of phase recrystallization as a method of intensive grinding of grain and increase the wear resistance of sliding friction units.

The experience of experimental studies shows that special nitrides (nitrides of alloying elements) are formed in the diffusion zone. Depending on the parameters of the technology, these nitrides have a coherent, semi-coherent, and incoherent bond with the matrix; their size has a nanometer scale. They provide a strengthening effect and, accordingly, increase wear resistance.

The authors found that the greatest effect of increasing the wear resistance of materials of sliding friction pairs is provided by nitrides that do not have a coherent or semi-coherent bond with the matrix, i.e. incoherent nitrides of alloying elements. This effect is superimposed by the effect of the surface nanocrystalline layer (ε-phase), which is an increase in wear resistance by 2 orders of magnitude (up to 100 times). In this case, the diffusion zone with incoherent particles acts as a soft substrate for the solid particles of the ε phase, which creates favorable conditions for the process of deformation during friction with a minimum level of fracture.

Long-term tests of sliding friction pairs have been carried out. It was found that the ε-nitride layer, which is in the nanocrystalline state, has the effect of practical wearlessness. The mass loss of the samples with the ε-phase layer is at the limit of sensitivity of the recording equipment. As an example, Fig. 2 shows the kinetic wear curves of the diffusion zone with incoherent nitrides of alloying elements and the surface layer consisting of the ε phase during sliding friction with an average relative sliding velocity v = 0.19 m / s and pressure in the contact p = 10 MPa (line 1 - wear diffusion zone containing special nitrides (nitrides of alloying elements), incoherent with the matrix; line 2 - the wear surface layer, lying above the diffusion zone and consisting of ε-nitride (Fe _2-3 N) in the nanocrystalline state .

It is seen that the wear of the layer with the nanocrystalline ε phase is two orders of magnitude lower. Calculations show that even the durability of only this layer is sufficient to ensure the durability of many sliding friction units, including camshafts, camshafts, plunger couples, diesel engine nozzles.

Thus, the technical result of the proposed solution is to reduce the wear rate of parts of the sliding friction units by two orders of magnitude (up to 100 times). The method can be used as part of a set of technological operations in the manufacture of machine parts involved in sliding friction and subject to wear.

Consequently, a combination of a number of well-known features, namely, conducting preliminary heat treatment (high tempering) before nitriding, and then ion-plasma nitriding with a cyclic change in the temperature and nitrogen potential of the gaseous medium, allows one to obtain a new synergistic effect consisting in the formation of a diffusion zone with incoherent and nitrides of the surface layer with particles of ε-phase (Fe _2-3 N) in the nanocrystalline state, and repeatedly (on the order of 2) increasing the wear resistance Handled s details.

Claims (2)

1. The method of nitriding parts of sliding friction units to obtain a nanostructured surface layer, including preliminary heat treatment and subsequent nitriding of parts, characterized in that as preliminary heat treatment use quenching at a temperature of 920-940 ° C, subsequent high tempering with heating up to 600-650 ° C for 2-10 hours and removing decarburized layer is then carried ion-plasma nitridation in a 500-570 ° C temperature range at the cathode voltage 300-320 B, a current density of 0.20-0.23 mA / cm ^2, etc. using ammonia as a gas environment having a degree of dissociation from zero to 80%, the ammonia flow rate to 20 dm ³ / h, the pressure in the chamber during the cathodic sputtering of 1.3-1.35 Pa, when saturated 5-8 GPa, and the nitriding is carried out in the mode of cyclic changes in temperature and degree of ammonia dissociation, while in the first half of the cycle the temperature is 570 ° C with a maximum nitrogen potential, and in the second half of the cycle the temperature is reduced to 500 ° C, while the nitrogen potential is reduced by increasing the degree of ammonia dissociation to 40-80%, while the number of cycles mentioned must be at least 10. 2. Detail of a sliding friction unit with a nanostructured near-surface layer containing a diffusion zone with nanosized nitride inclusions, characterized in that the near-surface layer is obtained by the method according to claim 1, wherein said nanostructured near-surface layer contains an α-phase diffusion layer with nanoscale incoherent alloying nitrides elements, which forms a soft matrix, and a surface layer with solid inclusions, which are nanoparticles of iron nitrides of the ε-phase, nnye by the local phase gratings recrystallization iron nitride which is provided by a cyclic change of the nitriding temperature and the degree of dissociation of ammonia.

Images