RU2631436C2 - Method for restoration of stregthened layer partially removed from steel parts - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: provide electroerosion alloying by the graphite electrode (EEA) with the discharge energy at which the thermal effect area does not exceed the thickness of steel part surface layer residue, strengthened by the mentioned ion nitriding. In particular cases of carrying out the invention after EEA, the non-abrasive ultrasonic finishing processing (NAFP) is performed. EEA is implemented in stages with the discharge energy decreasing at each subsequent stage. The steel part is restored as the protective sleeve of the rotor end seal.

EFFECT: steel parts surface quality is provided, in which during the manufacturing process or after assembly, the hardened surface layer is partially removed without dismantling the assembly.

4 cl, 22 dwg, 2 tbl

Description

The technical solution relates to electrophysical and electrochemical methods of machining parts, in particular to electroerosive alloying with a graphite electrode and ion nitriding of the surfaces of steel parts.

The main reason for the failure of parts during operation, as a rule, are processes occurring in the surface layer: stress concentration, development of microcracks, burn-out of alloying elements, softening, wear, oxidation, redistribution of residual stresses, etc. Therefore, technical solutions aimed at developing new technologies that improve the quality parameters of the surface layers of parts are relevant and important.

Modern resource-saving technologies that significantly improve the physico-mechanical characteristics of the surfaces of parts (wear resistance, fatigue resistance, corrosion resistance, etc.), being promising and innovative, use a variety of methods and techniques for metal processing.

The analysis of scientific and technical literature showed that there is no uniquely defined method for improving the quality of the surface layer of machine parts that would be applicable under any given conditions

The main objective of the methods used is to increase the quality parameters of the surface layer of parts: increase hardness, decrease the roughness parameter, increase wear resistance and restore worn surface areas, change the magnitude and sign of residual stresses, increase fatigue strength, etc. [Machine quality: reference book. In 2 vol. T. 2 / A.G. Suslov, Yu.V. Gulyaev, A.M. Dalsky [et al.]. - M .: Engineering, 1995. - 430 p.].

One of the simplest technological methods for creating protective coatings is surface electroerosive alloying (EEL). Its advantages are: locality of influence, low energy consumption, lack of volumetric heating of the material, ease of automation and “integration” into the technological process of manufacturing parts, as well as the possibility of combining operations.

Using EEL, you can either increase the hardness of the metal surface by applying a material of higher hardness to it or by diffusion introducing the necessary chemical elements into the surface layer from the environment or from the anode material, or lower the surface hardness by applying softer materials to it [N. Lazarenko. Electrospark alloying of metal surfaces. - M.: Mechanical Engineering, 1976. - 45 p.].

However, EEL of heat-treated parts subjected to high specific loads under operating conditions, for example, parts of dies, shafts of rolling mills and other similar parts, does not always lead to the desired result. The reason for the failure of some of them is that under the layer of increased hardness after EEL a tempering zone appears, that is, a zone of reduced hardness. This leads to the so-called punching of the hardened layer and, as a consequence, to the rapid wear of the part. EEL in this case will be harmful, especially if the permissible wear of the alloyed surface exceeds the thickness of the layer of high hardness [Lazarenko N.I. Electrospark alloying of metal surfaces. - M.: Mechanical Engineering, 1976. - 45 p.].

According to the source of the prior art [Andreev V.I. Improving the operational characteristics of the working surfaces of parts // Herald of mechanical engineering. - 1978. - No. 7. - S. 71-72], the "failure" of hardness in the heat-affected zone can be eliminated by applying additional processing after EEL to create hardening by the method of surface plastic deformation. However, in this case, a general increase in hardness in the transition zone is not observed.

на винахiд №103701, 23Н 5/00. Cпociб змiцнення поверхонь сталевих деталей, пiдданих термiчнiй обробцi / B.C. Марцинковский, В.Б. Тарельник / Опубл. 11.11.2013, бюл. №21. Прототип], проведение ионного азотирования (ИА) до или после ЭЭЛ позволяет устранить зоны пониженной твердости при использовании электродов из чистых твердых износостойких металлов. Кроме того, наблюдается плавное изменение твердости упрочненного слоя и увеличение общей глубины зоны повышенной твердости.According to the source [Patent

on wines No. 103 701, 23H 5/00. How can I see the surface of steel parts, add a thermal sample / BC Martsinkovsky, VB Tarelnik / Publ. 11/11/2013, bull. No. 21. Prototype], conducting ion nitriding (IA) before or after EEL allows you to eliminate areas of low hardness when using electrodes made of pure solid wear-resistant metals. In addition, there is a smooth change in the hardness of the hardened layer and an increase in the total depth of the zone of increased hardness.

на винахiд №82948, 23С 8/00. Cпociб

сталевих деталей електроерозiйним легуванням / B.C. Марцинковський, В.Б. Тарельник, А.В. Белоус / Опубл. 25.03.2008, бюл. №10. Прототип], когда при ЭЭЛ в качестве электрода используют графит (углерод). Способ ЦЭЭЛ имеет ряд достоинств, основными из которых являются:A known method of cementing steel parts by electroerosive alloying (CEEL) [Patent

on wines No. 82948, 23C 8/00. Thank you

steel parts for electrical erosion / BC Martsinkovsky, VB Tarelnik, A.V. Belous / Publ. 03/25/2008, bull. No. 10. Prototype], when using EEL, graphite (carbon) is used as an electrode. The CEEL method has several advantages, the main of which are:

- achievement of 100% continuity of hardening of the surface layer;

- increase the hardness of the surface layer of the part due to diffusion-hardening processes;

- alloying can be carried out in strictly specified places without protecting the rest of the surface of the part;

- the lack of volumetric heating of the part, and, consequently, the leash and warpage.

на винахiд №101715, 23Н 9/00. Cпociб

сталевих деталей електроерозiйним легуванням / B.C. Марцинковський, В.Б. Тарельник, М.П. Братущак / Опубл. 25.01.2013, бюл. №8].In order to reduce the surface roughness of machine parts while maintaining the quality of the surface layer (the absence of microcracks, the presence of a layer of increased hardness, 100% continuity, etc.) and, therefore, expanding the scope of their application, it was suggested that CEEL be carried out in stages, reducing the discharge energy at each stage , [Patent

on wines No. 101715, 23H 9/00. Thank you

steel parts for electrical erosion / BC Martsinkovsky, VB Tarelnik, M.P. Bratushchak / Publ. 01/25/2013, bull. No. 8].

Compared to cementation and hardening, the nitriding process proceeds at a lower temperature. The nitrided surface has a higher hardness, wear and corrosion resistance, better polishability; the properties of the nitrided surface practically do not change upon repeated heating up to 500-600 ° C, while when the cemented and hardened surfaces are heated to 225 275 ° C, its hardness decreases.

Given this property, in a pre-nitrided surface, one should not expect a decrease in hardness in the heat-affected zone after CEEL.

When the CEEL of a nitrided steel surface occurs, a process similar to nitrocarburization occurs, only in this case, the surface is saturated with nitrogen and carbon alternately, and in traditional nitrocarburization - simultaneously.

To increase the performance characteristics of machine parts such as wear resistance and fatigue strength, their surface layer is usually subjected to hardening. In this case, the core remains softer and more plastic.

Compared with conventional liquid (carbonitration) and gas nitriding, the advantages of IA are the possibility of targeted control of the structure of the obtained surface layer, the use of relatively low temperatures (up to 500 ° C), the absence of leashes and warping, the elimination of hydrogen bothering and the prevention of temper brittleness processes mainly metal, the safety and environmental safety of the process, reducing the processing time. The duration of the IA varies from 0.5-36 hours depending on the required depth of the hardened layer [Polevoy S.N., Evdokimov V.D. Hardening of metals. Directory. - M.: Mechanical Engineering, 1986. - S. 135].

After surface hardening (cementation, gas nitriding, carbonitration, ion nitriding, etc.), in order to eliminate the deviation of parts from the correct geometric shape, it is often necessary to remove part of the surface, the hardest, since surface hardening decreases as the hardness decreases recesses.

Often, the hardened surface layer of the part is removed after assembly with other parts, for example, after pressing on the shaft. In this case, from 0.05 to 0.15 mm can be removed. It is practically impossible to restore the surface layer of such a part by the above technologies without the necessary preliminary disassembly, both because of the large sizes of the units and because of the need to expose the mating parts to which the above hardening methods were applied to undesirable effects.

A known method of restoring partially worn surfaces of composite backup rolls, including disassembling the node with subsequent operations, ensuring the restoration of its performance. [V.P. Prikhodko, Yu.A. Officers, V.V. Garkavy, A.T. Chepelev, Yu.A. Grushko, I.A. Bobukh, S.A. Novachuk, V.T. Swan, E.I. Traiger, V.M. Sukhanov, V.D. Morozov and A.E. Rudnev. A.S. 1696023, B21B 28/02; A1. The way to restore the health of the composite backup rolls. Publ. 12/07/91, bull. No. 45]. The main disadvantage of this method is the high cost of disassembling the assembly, followed by the frequent complete replacement of some parts.

The technical problem to which this technical solution is directed is to ensure the appropriate quality of the surface of steel parts for which the hardened surface layer is partially removed during manufacturing or after assembly without dismantling the assembly. To solve this problem, a method of restoring a partially removed hardened layer of steel parts obtained by ion nitriding followed by electroerosive alloying, including the operation of electroerosive alloying with a graphite electrode (CEEL), in which CEEL, in accordance with the claimed technical solution, is carried out with a discharge energy, in which the heat affected zone during alloying does not exceed the thickness of the remainder of the surface layer hardened by ion nitriding. In this case, after CEEL can be applied the method of non-abrasive ultrasonic finishing (BFFS). In addition, CEEL can be implemented in stages, reducing at each subsequent stage the discharge energy.

It should be noted that in some cases, although it is extremely rare, when, after the first stage of CEEL, the surface roughness of the part is within the necessary limits of the technical specification, i.e. satisfies the requirements of the drawing, you can limit yourself to one stage CEEL.

The inventive method can be applied to repair individual parts of the parts without dismantling, in particular, to restore the hardened surface layer of the protective sleeve of the end seal of the rotor.

Given the above, it is of scientific and practical interest to carry out metallographic and durometric studies of steel surfaces after their IA, removal of part of the surface layer and subsequent CEEL.

The following is the methodology and results of the research, as well as an example of the application of the claimed technical solution for the repair of a particular product with links to illustrative material, where

- in FIG. 1 shows a sample for IA and CEEL;

- in FIG. 2 is a photograph illustrating the processing of CEEL samples of 40X steel on a lathe;

- in FIG. 3 presents thin sections made from samples hardened in accordance with the claimed method;

- in FIG. 4 and FIG. 5 respectively represent: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after IA;

- in FIG. 6 and FIG. 7 respectively presents: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after CEEL;

- in FIG. 8 and FIG. 9 respectively presents: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after CEEL + IA;

- in FIG. 10 and FIG. 11 respectively presents: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after IA + CEEL;

- in FIG. 12 and FIG. 13 respectively presents: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after IA + CEEL, in which after IA the hardened surface layer is partially removed to a depth of 0.05 mm;

- in FIG. 14 and FIG. 15 respectively presents: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after IA + CEEL, in which after IA the hardened surface layer is partially removed to a depth of 0.10 mm;

- in FIG. 16 and FIG. 17 respectively presents: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after IA + CEEL, in which after IA the hardened surface layer is partially removed to a depth of 0.15 mm;

- in FIG. 18 and FIG. 19 respectively presents: the microstructure and microhardness distribution of the surface layer of samples of steel 40X after IA + CEEL, in which after IA the hardened surface layer is partially removed to a depth of 0.20 mm;

- in FIG. 20 shows the appearance of the protective sleeve of the end seal of the rotor after hardening by carbonitration and cleaning;

- in FIG. 21 shows the appearance of the sleeve after CEEL;

- in FIG. 22 shows the appearance after processing BUFO.

For IA and CEEL, special samples of 40X steel were used, heat-treated for a hardness of 3000-3100 MPa. The samples were made in the form of a coil consisting of two disks with a diameter of 50 mm and a width of 10 mm, interconnected by a spacer with a diameter of 15 mm and having two technological sections of the same diameter (Fig. 1). The surface of the disks was ground to Ra = 0.5 μm.

The CEEL process was carried out automatically using the installation of the EIL-8A model. The samples were fixed in the lathe chuck, after which a step-by-step CEEL was made by subsequent alloying with a graphite electrode of the EG-4 grade (OST 229-83) with a discharge energy of 0.42 J (1st stage) and 0.1 J (2nd stage) with a productivity of 2 and 5 min / cm ² , respectively (Fig. 2).

Ion nitriding of the samples was carried out at a temperature of 520 ° C for 12 h on an NGV-6.6 / 6-I1 installation.

Hardening of the samples was carried out in a different sequence: IA; CEEL; CEEL + IA; IA + CEEL (Fig. 4 - Fig. 11).

In addition, some samples were subjected to IA, then grinding to various depths (0.05; 0.10; 0.15 and 0.20 mm), (Fig. 12 - Fig. 19), and then CEEL was performed. Moreover, the samples in which the layer was removed to a depth of 0.05 and 0.10 mm were subjected to CEEL with a discharge energy of 0.42 J (stage 1) and 0.1 J (stage 2), and to a depth of 0, 15 and 0.20 mm - 0.60 J (1st stage) and 0.1 J (2nd stage). In order to reduce surface roughness after CEEL, BUFO was used.

From the hardened samples, segments were cut out from which thin sections were made (Fig. 3).

Sections were examined using a Neofot-2 optical microscope, where the quality of the layer, its continuity, thickness and structure of the sublayer zones — the diffusion zone and the heat-affected zone — were evaluated. At the same time, a durometric analysis was carried out on the distribution of microhardness in the surface layer and along the depth of the thin section from the surface. The microhardness was measured on a PMT-3 microhardness meter by indentation of the diamond pyramid under a load of 0.05 N.

At all stages of processing, surface roughness was measured on a profilograph device — a mode profilometer. 201 factory "Caliber".

In the table. Figure 1 shows the distribution of microhardness over the depth of the surface layer and the surface roughness for various types of IA and CEEL.

All microphotographs clearly show the “white” layer that is not etched by conventional reagents. Its microhardness, depending on the type of hardening, ranges from 7010 MPa for IA and CEEL to 8250 and 11190 MPa for IA + CEEL and CEEL + IA, respectively.

Below is a transitional, diffusion zone, with a smoothly decreasing microhardness, passing into the microhardness of the base (3000-3100 MPa).

The depth of the zone of increased hardness is at CEEL, IA, CEEL + IA, IA + CEEL, respectively, 60-70, up to 190, 220 and 250 microns.

As can be seen from the table. 2 and FIG. 4 - FIG. 11, the largest thickness (250 μm) and hardness of the hardened layer (11190 MPa) belong to the integrated method of hardening IA + CEEL (Fig. 11). Moreover, the surface roughness Ra, is 0.8 μm, which is lower than when using metal electrodes in EEL.

Thus, the most preferred way to increase the hardness of the surface layer of steel parts in which the hardened surface layer is partially or completely removed is CEEL.

As already noted, as a preliminary hardening, it is most expedient to use an IA having a number of advantages over other methods.

The results of the study of the microstructure and microhardness of the surface layer of samples of steel 40X, hardened by IA and subject to grinding and CEEL are summarized in table. 2 and shown in FIG. 12 - FIG. 19.

The analysis of FIG. 12 - FIG. 19 and tab. 2 shows that with increasing depth of the removed layer, the thickness of the hardened layer and its microhardness after CEEL decreases.

In 2013, TRIZ LLC in Sumy completed the reconstruction of a centrifugal compressor of natural gas 22TSKO pos. 252-3 methanol production workshop of OJSC NAC Azot.

In the manufacture of rotors for compressors, a problem arose in the manufacture of protective sleeves of the rotor end seals mounted on the rotor shaft in a hot seat (Fig. 20).

Traditionally, bushings were made of monel metal (base) of the NMZhMts brand 28-2.5-1.5 with application of wear-resistant powders to the outer surface of the sleeve, followed by melting of the coating in a vacuum furnace.

After manufacturing, the protective sleeves are mounted on the shaft with an interference fit, processed with elbor and hexanite cutters and polished with elbor sandpaper. The thickness of the applied layer is ≈2 mm, and after grooving on the shaft - 1 mm, with a total sleeve thickness of 3 mm.

As an alternative, to replace protective monel-metal bushes with cheaper ones, but not inferior in their operational characteristics to monel-metal, TRIZ specialists developed two other methods for their manufacture.

The first is the manufacture of bushings made of steel 38Kh2MYuA, followed by hardening of the surface layer by carbonitration in a molten salt (Fig. 12). The depth of the hardened layer was 0.55-0.6 mm, and the surface hardness after carbonitration HRC 55-62. The hardening temperature by carbonitration is in the range of 540-600 ° C, which does not exceed the tempering temperature of steel 38Kh2MYuA, which is 640 ° C.

The difficulty in developing the technology for manufacturing bushings using hardening of the surface layer by carbonitration was to ensure the hardness of the surface layer after hot landing on the rotor shaft and grinding, since the hardness of the hardened surface layer, maximum on the surface, decreases as the surface deepens, and the final hardness of the surface layer after grinding depends on the depth of the removed layer.

The second method was first applied during a stop repair in March 2014 at the Azot PJSC in Cherkasy during the repair of the CPV compressor rotor pos. 103J. It consisted in the fact that after landing on the shaft and polishing the bushings subject to IA, they were surface hardened by the CEEL method followed by the BUFO method (Fig. 21 - Fig. 22).

The repair was completed in a short time within 24 hours. At the same time, the seal operation parameters have not changed.

Claims (4)

1. A method of recovering a partially removed layer of a steel part hardened by ion nitriding, characterized in that the electroerosive alloying is carried out with a graphite electrode (CEEL) with a discharge energy in which the heat-affected zone during alloying does not exceed the thickness of the remainder of the surface layer of the steel part hardened by the said ion nitriding. 2. The method according to p. 1, characterized in that after CEEL conduct non-abrasive ultrasonic finishing (BFFS). 3. The method according to p. 1, characterized in that the CEEL is carried out in stages with a decrease in each subsequent stage of the discharge energy. 4. The method according to p. 1, characterized in that restore the steel part in the form of a protective sleeve of the end seal of the rotor.

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