GB2234266A - Chemical-thermal treatment of parts - Google Patents

Abstract

A method of chemical-thermal treatment comprises thermal treatment of the parts in a nitrogen-containing atmosphere at a temperature of no more than 680 DEG C thereby forming a carbonitride or oxynitride phase having a thickness of at least 5 mu m, forming on the parts a porous structure, exposing the parts to an oxidizing atmosphere at a temperature equal to that of thermal treatment, and subsequently filling the pores of the porous structure with oil containing 0.5-10 mass % of sulphur. The diffusion coating on parts made of aluminium-based alloys consists of crystalline structures of Al2O3 and of oxynitride phase of the (A1Me)2-3 (NO) type, where Me is metal. The diffusion coating on parts made of iron-based alloys consists of three structural components the first from the surface component has a fine-porous structure consisting, mainly, of Fe3O4 with pores filled with oil containing 0.5 - 10 mass % of sulphur, the second component has a nonporous structure of a carbonitride phase of the Fe2-3, (NC) type, and the third component has the bainite structure. An installation for realizing the method illustrated in the Fig. comprises the following units communicated with one another through a transport system:- a furnace 6 for thermal treatment of parts connected hermetically with a chamber 9 for isothermal holding of the parts which, in its turn, is connected hermetically with an oxidizing chamber 12 in the direction of moving the parts, a cooling chamber 15, and a chamber 21 for tempering the parts. <IMAGE>

Description

1 1 1 1 METHOD OF CHEMICAL-THERUAL TREAWNT OF PARTS9 DIFFUSION COATINdS

OBTAINED BY THE PROPOSEb MTHOD9 AND INSTALLITION FOR REAZIZIIG TH=P The present invention relates to metallurgy and more particularly to a method of chemical-thermal treatment of parts, diffusion coatings obtained by the proposed methodg and an installation for realizing thereof, The invention can be used in engineering for appli cation of wear- and corrosioh-resistant coatings to parts made of Iron- and aluminium-based alloys, for instance, In automobile industry for application to engine and gear-box parts operating under conditions of wear and corrosive-wear failure.

At present the processes of surface saturation of the parts with carbon and nitrogen from gaseous atmosphere are most widely used since they are more economical than saturation of the parts from salt baths containing cyanic compounds. These processes ensure the formation on the parts of diffusion coatings of a carbonitride phase characterix.ed by high wear resistance but insufficient corrosion resistance.

Known in the art is a method of chemical-thermal treatment of parts (GB Ay 2136028) including thermal treatment of the parts made of iron-based alloys in 2 - the atmosphere of catalytic gas (exogas or endogas, which are hydrocarbon- containing gases useful for saturating the surfaces of steel parts, nitrogen) and ammonia at 55G.-8000 0 up to the f ormation of a carbonitride phase structure on the surface Of the parts and repeated shoxt-t:Lme thermal treatment of tile parts in a water-oil emulsion in an oxidizing atmosphere (up to 60 s). As a rtsult of repeated fast heating and coolings dissociation of carbonitride from the carbonitride structure up to 1 um thick and oxidation of iron to Fe 3 0 4 take place. To realize the known methods an installation is proposed, comprising a furnace for thermal treatment of the parts, oxidizing and reducing chambers which are located in a technological sequence and interconnected through the transport system. On the surface of the parts treated as described above a diffusion coating is formed consisting of two structu ral components one of whichs namely that closest to the surfaces represents a nonporous Pe 3 0 4 structure and the second component is at iron eaxbonitride structure of the Fe 2-3 (NC) type, the mass ratio of the structure being 1:24, respectively, Such a composition of the coating does not ensu- re high wear- and corxosion-wesistance of the parts because of a small thickness of the f irst structural couponent. The control over the thickness of this components is u.ot possible under the conditions of the described process which, in addition, is characterized by a low productivity.

Also known in the art Is a method of chemical-ther- 1 mal treatment of the parts (DE, B, 3225686) made of iron-based alloysy including thermal treatment of the parts in a nitrogen-containing atmosphere at a temperature of no more than 680009 pmedominantly at 600- 680 0 c polishing, and exposure of the parts in a water steam atmosphere at a temperature of 400-6600C which ensures g direct transformation of nitrous austenite into bainite. As a nitrogen-containing atmosphere use is made of a mixture of gases containing 40 - 70 mass of ammonia. The diffusion coating formed on the parts consists of three structural components: the first componenty namely, the closest to the surface, represents a nonporous Pe 3 0 4 structure, the second component Is a structure of the carbonitride phase of the Fe 2-3 CINC) typel and the third one is tile bainite structure. The polishing process increases wear resistance of the obtained coating but corrosion resistance remains low because of a small thickness of the first component. Besidesq repeated heating causes deformation of the parts and, henceg decreases fatigue strength The main object of the invention is to provide a method of chemical- thermal treatment of the parts. characterized by high technological paraae"ko,o.-.s and minimum labour and energy consumption, to obtain with the aid of the proposed method structuraIly novel diffusion coatings on Che parts made of iron- or C3 aluminiumbased alloys ensuring high physical and mecha- 4 - nical properties of the parts and to develop an installation for realizing the proposed method which will possess high productivity and reliability under operation conditions.

Said object is accomplished by that a method of chemioal-thermal treatment of parts is proposed, includ ing thermal treatment ofparts in a nitrogen-oontaining atmosphere at a temperature not exceeding 6800C. keep ing said parts in a oxidizing atmosphere at above temperaturel and subsequent cooling, wherein, according to the inventiong thermal treatment is carried out up to the formation of the parts of carbonitride or oxynitride phase having a thickness of at least 5)UM, after whichg prior to keeping the parts in the oxidiz- ing atmosphereg the formation of a finelyporous structure on the parts is performed, said structure beinp filled after cooling with an oil con-z;aining 0.5 - 10 mass wo of sulphur.

The proposed method makes it possible to obtain on the parts diffusion coatings having a structure different from the known one and ensuring high wear resistance, corrosion resistance and fati-ue strength. Besides, the method is highly technological and has low labour and energy consumption due to elimination of repeated heating of the parts, which usually results in deformation of the yarts and requires intermediate mechanical operations such as finishing and polishing.

1 It is prop osed to p erf orm the f ormation of the porous structure on the parts by means of isothermal exposure at temperatures 400-5000 C in a protecting atmosphere such as nitrogen or nitrogen and ammonia at a mass ratio of 90.0 - 99.9: 0.1 - 10.0. Under these conditions the porosity and thickness of the porous structure being obtained can be easily controlled.

To obtain the structural component from a carbonitride phase with an optimum thickness, it is expedient to accomplish thermal treatment at 570 - 680 0 0 for 2 - 5 hours.

In the case ofpreparing the structural component from an oxynitride phase, it is recommendable to maintain the temperature of taermal treatment within the range from 450 to 570 0 0 for 2 - 3 hours. To ensure a maximum rate of the formation of carbonitride or oxynitride phase, it is expedient top- erform thermal treatment in a nitrogen-containing atmosphere consisting of endogas and ammonia at a mass ratio thereof equal to 30 50 - 70, respectively.

To carry out an oxidation process under optimum conditions, it Is desirable to keep the parts in an oxidizir4 atmosphere containing oxygen, nitroSen and ammonia at tile following ratio(mass,7;).. 0.2 - 22.0 oxygen, 0.05 - 3.0 ammonlap up to 100 nitro6en.

TO fill effectively the pores of the porous stipucture formed on the parts with an oil containitS 0.5 - 10 mass7a of sulphurg. the parts are tempered in said oil at 120 - 140 OC for 20 - 40 minutes. To accelerate the thermal treatmentq it is desirable to subject the parts to preliminary exposure at 350 - 400 00 which ensures a partial surface oxidation of the parts.

SEdd object is also accomplished by that two diffusion coatings are proposed. The diffusion coatings deposited on theparts from aluminiumbased alloys consist of crystalline structures of Al 2 0 3 and of an oxynitride phase of the (AlMe) 2-3 (NO) type, where Me is metal.

Along the interface the structures of these phases are filled with oil containing 0.5 - 10 mass% of. sulphur.

The diffusion coatings deposited on the parts from iron-based alloys consist of three structural cokoonents, the second of which has nonporous structure of the carbonitride phase of the Fe 2-3 (171C) type, and the triird component represents the bainite structure. According to the invention, the first component (the closest to the surface) has a finely porous structure consisting, mainly, of Fe 304 with pores filled with oil containing 0.5 - 10 mass % of sulphur, the ratio of the thickness of the first "and second structural components being within the range from 1:2 10.

-The diffusion coatings of the above described structure and composition ensure:

- a rise in the endurance limit by 50,Fo and, hence, z a decrease in the weight of the parts and saving in f errous metals; - a considerable increase of the corrosion resistance parameter which exceeds a similarparameter for galvanio coatings (chemical nickel-plating, solid chromium-plating) and a possibility of manufacturing some parts vAthIthe use of low-carbon steel instead of stainless steel; - a rise in wear resistance parameter with elimina- tion of using expensive bronze and brass bushings in plain bearings.

To solve the problem an installation is also proposed for realizing the proposed method, said installation comprisinE a furnace for thermal treatment of theparts, and oxidizing and cooling chambers located in a technological sequence and communicated with one another by a transport systems wherein, according to the invention, a chamber for isothermal exposure of the parts is located between the fur- nace and_oxidiz:Lng chamber along the movement of the partsp said chamber being hermetically connected with the above furnace and oxidizing chamber, and after the cooling chamber a chamber for tempering the parts is mounted. Such an installation ensures a closed technological cycle and stable properties of the diffusion coatings obtained on the parts being processed, the eff ioiency of the installation being up to 96%.

8 To enhance the productivity, a chamber for thermal holding of the parts is placed ahead of the furnace for thermal treatment.

A specif ic example of realizing the proposed method is given hereinbelow with the attached drawing whichl according to the invention, is a schematic plan view with a section of ttie installation for realizing the method of chemical-thermal processing of parts.

The installation for realizing the proposed method, according to the invention, (see the drawing) comprises the following units located in the technological sequence: a mechanism I for charging the parts into bottom plates 2t a device 3 for washing the parts, a chamber 4 for preliminary thermal holding of the parts, an intermediate chamber 51 a furnace 6 for thermal treatment of the parts fitted with a window 7 for discharging the partsq said window being equippea with a damper 8, a chamber 9 for isothermal holding of the parts, the input of the chamber 9 being connected hermetically with the above window..After the chamber 99 an oxidizing chamber 12 islocated in a housing 10 common to chambers 9 and 129 the chamber 12 being separated from the chamber 9 with the aid of a damper 11 oLnd having a damper 13 at the outlet. The furnace 6 and chambers 49 9 and 12 have a lining 14, are sealed (except the chamber 4) and fitted with heaters (not shown in the drawing). A cooling chamber 15 is 9 mounted after the chamber 12. The bottom plates axe moved with the aid of moving devices, namely# pushers 16 and a belt conveyer 17. The f urnace 6 and tile chambers 9 and 12 are equipped with branch pipes 189 19 and 20 for delivering a nitrogen-containing, protecting and oxidizing atmospheres. A chamber 21 for tempering the parts and a device 22 for discharging the bottom plate 2 are mounted in series after the chamber 15. The installation has a control system and all the required electromechanical and power equipment (not shown in the drawing). The principle of operation of the installation,, according to the invention, and specific features of tile proposed method are given hereinbelow.

The parts subjected to chemical-thermal processing axe put on the bottom plates 2 with the aid of the device 1 and delivered to the device 3 for washing with the aid of tile oonveyer 17. Then the conveyer 17 supplies the parts into the chamber 4 for prelira!Lary -eL heating and oxidation in an air atMOSIDA6--r9 at a sp er at,= -- of f r om 3.50 t c) l+00 oG to prepare the surface of the parts for further saturation. This operation is optional since it is only aimed at an increase of the productivity of the technological process. After leaving the chamber 4 the bottom plates 2 with ther parts are delivered into the intermediate chamber 5 with the aid of the conveyer 17 and then Into the furnace 6 with the aid of the pusher 16 for thermal treatment wherein through the branch pipe IS a nitrogen-containing at;osphere is fed consisting, preferably, of endogas andammonia at a mass ratio thereof equal to 30-50: 50-709 respeotivelyl although exogas and nitrogen can also be used. Thermal treatment of the parts made of iron-based alloys is performed at 570-680 0 0 for 2 - 5 hours via-the formation on the surface of two structural oomponentsl namelyq a nonporous structure of carbonitride phases of the Fe 2-3 (110) t;ype and a structure of nitrous austenite. The process is carried out till the formation of the carbonitride phase structural component at least m thick and predominantly from 10 jum thick. The parts from aluminium-based alloys are thermally treated predominantly at 450 - 570 0 C for 2 - 5 hours with the formation of a structural component consisting mainly of oxynitride phases and havinE thickness of at least 5 V"Ime After leaving the furnace 6 the bottom plate 2 with the Darts is delivered wita the aid of the pusher 16 and the conveyer 17 through the window 7, the damper 8 being open, into the chamber 9 for isothermal holding. The completely sealed chamber 9 with closed dampers 8 and 11 is filled with a protecting medium consisting, predominantlys of either nitrogen and ammonia at a mass ratio thereof equal to 90.0-99.9:0.1-10.09 respectively or nitrogen. Isotaer- 11 mal exposure of the parts Is performed at a temperature of from 400 to 50000 up to the formation of a finely porous structure. The formation of this structure on the surface of the parts made of iron-based alloys differs from that for the parts made of aluminiumbased alloys. In the first case dissociation of carbonitridesq the formation of molecular nitrogeng and removal thereof from the phase take place. The structural component (the first one) is being formed over the second structural component (consisting of carbonitride phases) which has a porous structure with a uniform depth distribution of pores. The ratio of the first and second structural components is 1:2 - 10. The porosity of the first structural com- ponent can be varied by changing the composition of - the protecting atmosphere. A different degree of porosity is required for attaining optimum properties of the surface at different operation conditions (for Instanceg more developed porosity improves tribologi- cal properties of the surface and less developed porosity increases corrosion resistance). The presence of less than 10 mass ra of ammonia in the protecting atmosphere makes It possible to vary the porosity of the first structural component. In the case of proces- sing the parts from aluminium-based alloys the thickness of the oxynitride structure increases-and non dense crystalline structure of the aluminium oxide 12 t;ype is formed. At the interface in the structures of these phases the- defects representing small pores are formed. After the completion of isothermal holdingg the bottom plates 2 with the parts are moved by the pusher 16 and the conveyer 17, the damper 1.1 being open, into the oxidizing chamber 12. The dampers 11 and 13 close the inlet and outlett respectivelyg of the chamber 12 and an oxidizing atmosphere is fed through the branch pipe 20, said atmosphere 'represent ing a mixture of nitrogen, oxygeng and ammonia at the following ratio thereof (mass 7o): 0.2 22.0 oxygen. 0.05 - 3.0 ammonia, up to 100 nitrogen.

The oxidation process la performed at a temperature corresponding to that of thermal treatment. In the case of processing the parts of iron-based alloys the temperature is maintained vilthin the range from 570 to 6800C at which the iron in the finely diapersed structural component is oxidized to Fe 3 0,,,., i.e. as if the surface of the pores is being clad with an oxide film characterized by low inner tensions and uncapable of cracking. This enhances considerably the corrosion resistance.

The parts from aluminium-based alloys are oxidized at 450 - 57000 with a complete formation of the diffusion coating consisting of crystalline structures of A1 2 0 3 and oxynitride phase of tile (AiMe) 2-3 ( NO) t5Pe, where Me is metal. After the completion of the oxida- 13 tion process, the damper 13 is opened and the bottom plates 2 are moved with the aid of the pusher 16 1hto the oxidizing chamber 15 where the parts are rapidly cooled in water or in a water-air mixture. On the parts from Iron-based alloys the austenite 'structure is transformed Into balnite in the third structural component. Then the parts are delivered by the conveyer 17 into the chamber 21 for tempering the parts perform,. ed, preferablyg at a temperature of from 120 to 140o for 20 - 40 min in an oil containing 0.5---10 mass % of sulphur.

A.t this stage the diffusion coatings are completely formed on the parts made Cj iron- or aluminiumbased alloys. The pores become filled witil said oil and the sulphur deposited on the surface of, the pores enhances considerably the wear resistance of the parts under conditions of dry and half-dry friction, After leaving the chamber 21, the bottom plates 2 with the parts are delivered by the conveyer 17 into the device 22 for discharging the parts. The cycle of chemicalthermal treatment is repeated.

The diffusion coatings obVained on the parts made of aluminium-based alloys consist of crystalline structures of A1 2 0 3 and of an oxynitride phase of the (AlUe) 0 type, where Me Is metal and along 2- 3 ( 1".

the Interface the structures of these phases are filled with oil containing 0.5 - 10 mass % of sulphur.

14 The diffusion coatings obtained on the parts made of Iron-based alloys consist of three structural components the first of which (the closest to the surface) has a finely porous structures mainly, of Pe 3 0 4 with pores filled with oil containing 0.5 10 mass % ofsulphur. the second component has a nonporous structure of the carbonitride phasel predominantly, of the Fe 2-3 ( NC) type, and the third component has the bainite structure.

The diffusion coatings were tested for wear resis tances lapping, corrosion resistanceg and fatigue strength.

The wear-resistance tests were carried out by the raethod of evaluating the attrition ability of the sur15 face under sliding friction and 5 kg loading.

The duration of one test (tile formation of one hollow) was equal to the time interval in the course of which the engine disk made 3000 revolutions. The disk was 30 mm in diameter and 2.5 mm thick. The ro20 tation rate of the disk upon testing amounted to C2 860 r.p.m. A solution (0.5%) of K 2 Gro 3 in distilled water was used as a cooling liquid delivered at a flow rate of 0.8 1/min. Five hollows were formed by attrition on each specimen.

The corrosion-resistance tests were carried out in a salt fog chamber with regular spraying a salt solution (3%) for one minute every 14 minutes.

1 1 A jet of the solution from a sprayer was directed at an angle of 45 0 to the surface of the parts located at a distance of 200 am from the nozzle, the direct contact of the solution with the parts under testing being ruled out.

The tests were performed at 24 - 260C and corrosion resistance was determined by tile time of the ap-Dearanee of corrosion sites when examining the parts with an unaided eye under natural illumination.

The fatigue strength and lapping tests were conducted by following the known procedures.

For a better understanding of the present invention specific examples of realizing thereof under different temperature-time conditions and the description of the diffusion coatings obtained by the proposed method on parts made of aluminium- and iron-based alloys are given hereinbelow by way of illustration.

Examp 1 e 1 A piston of a V-shaped engine made of A1-30 aluminium alloy was degreased in the device 3 with an alkali solution for 10 minutes and subjected to thermal treatment in the furnace 6 in a nitrogen-containing atmosphere consisting of 50 mass % of ammonia and 50 mass % of endogas at 570 0 0 for4.5 hours up to the formation of an oxynitride phase structure 10 jum thick. The treated piston was placed into the chamber 9 and kept under isothermal conditions in a protect- 16 ing atmosphere consisting of 0.1 mass % of ammonia and 99.9 mass % of nitrogen at 5000C for 0.5 hr. As a result a n6ndense crystalline structure of the aluminum oxide type was formed, the oxynitride structure thick- ness was increased, and defects in the form of small pores were formed in the structuresof these phases along the interface. Then the piston was placed into the chamber 12 and kept at 5700C in an oxidizing atmosphere consisting of 10.5 mass A of oxygen, 1.0 mass % of ammonia, and up to 88.5 mass 76 of nitrogen.

After that the piston was sharply cooled with hot water at a temperature of 95 00 by spraying in the chaffber 15 and subjected to tempering performed in the chamber 21 at 1200C for 30 minutes in oil containing 2.0 mass % of sulphur.

The diffusion coating on the piston obtained by the described method consists of the crystalline struc- tures of Al 0 and oxynitride phase of the (AlMe) '0) 2 3 2-3('T type and along the interface the structures of these phases are filled with oil containing 2 mass;7o of sulphur. The thickness of the coating is 12 'J" 0 The results ofphysico-mechanical tests of the piston with the described diffusion coating are as follows:

wear resistanceg 1T3 mm 102 lappingg b-r 0.25.

l? Exakple 2 A gasket for an engine made of an aluminium,alloy was detreased in the device 3 with an alkali solution for 10 minutes and thermally treated at 450 0 C in the furnace 6 in a nitrogen-containing atmosphere consist- ing of 70 mass % of ammonia and 30 mass % of endogas for 2 hrs up to the formation of the oxynitride phase structure 5 thick. The treated gasket was placed into the chamber 9 and kept under isothermal conditions in a protecting atmosphere consisting of ammonia (10 mass %) and nitrogen (90 mass %) at 400 0 0 for 20 minutes. As a result a nondense crystalline structure of the aluminium oxide type was formed and the thickness of the oxynitride structure was increased. the defects In the form of small pores being formed along the interface in the structures of these Phases. The treated gasket was then placed into the chamber 12 and kept in an oxydizing atmosphere consisting of oxygen (0. 2 mass ammonia (0.05 mass A), and nitro6en (99.75 mass at 4500C. The cooling was performed by follow ing the procedure described in Exakple 1 and temper ing of the gasket was carried out at 130 0 0 for 20 mi nutes in oil containing 3 mass % of sulphur.

The diffusion coating on the gasket consists of crastalline structures of A1 2 0 3 and oxynitride phase of the (Allde) 2-3 (NO) type and along the interface the structures'of these phases are filled with oil con- 18 tainins 3 mass % of sulphur. The thickness of the coating is 61m.

The results of physico-mechanical tests of the gasket with the described diffusion coating are as follows: wear resistanceg 10-3 mm 1 app ing, hr 162 0.2.

Example 3

A easing of a water pump made of an alluminium alioy was subjected to cheMical-thermal treatment by following the procedure described In Exawple 21 with the exception that the duration of thermal treatment was 5 hoursq an oxidizing atmosphere consisted of oxygen (2 mass 7.2), ammonia (2 mass %), and nitrogen (96 mass:75) and the pump wai tempered at 14.0 OC for 40 qLin in oil containing 8 mass % of sulphur. The diffusion coating on the pump consisted of crystalline structures of A120 3 and of oxynitride johase of the (A111e) 2-3 (NO) type andalorig the interface the structures of these phases were filled with oil containing 8 mass 7 of sulphur. The thickness of the coating was 9.0.pm and that of the oxynitride structure formed In the course of thermal treatment was 8.5 V1-,Me The results of physico-mechanical tests of the water pump with the described diffusion coating are as follows. wear resistance, 10-3 mm lapping. hr 141 0.2.

19 Example 4

Nozzle needlesq rods# and studs made of aluminium were subjected to chemical-thermal treatment as described in Example 1 with the exception that an Isother- mal exposure was carried out at 400 0 0 for 40 min and thermal tre4tment was performed for 2 hxs with the use of an oxidizing atmosphere consisting of oxygen (0.75 mass %)l ammonia (0.5 mass %), and nitrogen (9b.75 mass %). The tempering was accomplished at 13000 for 20min in oil containing 5 mass.% of sul phur. The thickness of the oxynitride structure formed in the course of thermal treatment was 8.0tum. The diffusion coating on the parts consisted of crystal line structures of Al 2 0 3 and of oxynitxide phase of the (AlUe) 2-3 (ITO) type and along the interface the structures of these phases were filled with oil containing 5 mass % of sulphur. The thickness of the coating was 9.5 lme The results of physico-mechanical tests of the parts with the described diffusion coating are as follows: wear - reslstancel 1C)-3 mm lappingg hr Example 5

127 0.25.

Brake cylinders and compression rings made of V, low-carbon steel were deogreased ia the device 3 --sith an alkali solution for 15 min and delivered into the chamber 4 f or preliminary heating and oxidation in an air atmosphere at 350 0 0. Then the parts were delivered into the furnace 6 where they were thermally treated in a nitrogen-containing atmosphere consisting of ammonia (50 mass %) and endogas (50 mass YS) at 650 0 a f Or 5 hours up to 'the formation of a carbonitride phase 25 tm thick and the structure of nitrous austenite. Then the parts 'were subjected to isothermal holding in the chamber 9 as described in Example 1. As a result, a finely porous structure was formed located above the carbonitride phase. The following procedure was similar to that described In Example 1 but exposure in the oxidizing atmosphere Vias performed at 650 0 0. After oxidation the porous structure consisted mainly of Pe 304. The parts were cooled in the chambe- 15 and tempered in the chamber 22.1 by following the procedures described in Exarkple 1.

The diffusion coating obtained on the above parts consisted of three structural components: a porous structure consistingg mainly, of Fe 304 with the pores filled with oil containing 2 mass % of sulphur a nonporous structure of the carbonitride phase of the Pe 2-3 (NO) type, and the bainite structure formed from the austenite phase. The diffusion coating was 400 "tm thick, the first structural component being 5 jim thick and the second 20 im thick. The results of physico-mechanical tests of the treat- 1 ed parts with the obtained diffusion coating are as f ollows:

wear resistance, 10-3 mm 52 corrosion resistance, hr 108 lappingg hr 0.60 fatigue limit, 14Pa 1110.

Example 6 Lead screws and carriages made of steel were degreased in the device 3 with an alkali solution for 10 min and subjected in the chamber to preliminary heating and oxidation in an air atmosphere at 400 0 0. Then the parts were delivered into the furnace-6 for tJae.rmal treatment in a nitroSen-containing atmoSDhere consisting of ammonia (70 mass 16) and endogas (30 mass /,"o)_at 570 0 C for 2 hxs up to the formation of a carbonitride phase 6 ium thick and the structure of nitrous austenite. The further 1.1he prooez;sing of ' parts was carried out as described in Example 2, but the exposure in an oxidizing atmosphere was performed at 57000. As a result of the isothermal eK,)Osuxe a porous structure was formed located over a carloonitri de structural compone nt; after oxidation this structu re consisted mainly o; Fe 3 04. The diffusion coating on the parts consisted of the structures similar to those described in Example 5 with the e-xception that the pores of the first structural component were filled with oil containing 3 mass % of sul?hur, The t 22 diffusion coating was 60 jum thick, the first structu ril component being 2 1,111 thick and the second 6 m thick. The results of physico-mechanical tests of the processed parts with the

described diffusion coating were as follows:

wear resistance, 10-3 mm corrosion resistance, hr 1 app ing, hr fatigue limit. MPa -Example 7

87 72 0.5 915.

Valve rockers and snock-absorber rods made of steel were degreased in the device 3 with an alkali solution fox 10 min and delivered into the furnace 6 for thermal treatment in a nitrogen-containing atmosphere consisting of ammonia (60 mass a) and endogas (40 mass 7o) at 60000 for 5 hrs up to the formation of a carbonitride phase 28 jum thick and the structure of nitrous sustenite. The further process was similar to that described in Examples I and 3 but the exq, posure in an oxidizing atmosphere was performed at 600 0 C. As a result of isothermal e4osure a porous structure was formed located Over the carbonitride structure. After oxidation this.sructure consists, mainly,, of Fe 3 04 The diffusion coating on the parts consisted of the structures similar to those described in Example 5 but the pores of the f irst i structural component viere f illed with oil containing 3 mass % of sulphur. The diffusion coating was 142 klm thick, the first structural component being 3pm thick and the second 28 pm thick. The results of physico-mechanical tests of the treated parts with the described diffusion coating were as follows:

wear resistance, 10-3 mm 78 corrosion resistance, hr lapping, hr fatigue limit, ILIPa Example 8

0.5 990.

Oa.-Lburettcr parts made of steel were de;reased in the device 3 with an alkali solution for 15 min and delivered into the furnace 6 Ifor thermal treatment in a nitroGen-containing atmosphere consisting of ammonia (50 mass %) and endogas (50 mass Yo) at 'oSOc)C for 2 hours up to the formation of a carbonitride phase 10 um thick and the structure of nitrous austenite. Then the process was carried out by following the procedures described in Examples 1 and 4. As a result of the isothermal exposureq a porous structure was formed consistingg mainly, after oxidation of Fe 3040 The diffusion coating on the parts consisted of the structures similar to those described in Example 5 but the pores of the first structural components were filled with oil containing 5 mass jo of sulphur. The diffusion coating was 240,um thick, the first 24 Struc. tural component being 2 jam thick and the second 10 jam thick. The results of physico-mechanical tests of the treated parts with the described diffusion coating are given below: wear resistances 10-3 mm corrosion resistance$ hr lappingg hr fatigue limit, LIPa &ample 9 64 95 0.55 975.

Valve rockers and shock-absorber rods made of steel were degreased in the device 3 with an alkali solution for 10 min and delivered into the chamber 4 for preliminary heating and oxidation in an air at- mosphere at 400 OC. Then the parts were taermally treated in the furnace 6 in a nitrogen-containing atmosphere consisting of ammonia (50 mass -/a) and ex--dogas (50 mass lo) at 600 0 C for 4'hrs up to the forma- tion of a carbonitride phase 2--1 tum thick and the j structure of nitrous austenite. The processed parts were placed into the chainber 9 and kept under isothermal conditions in a protecting atmosphere consisting of nitrogen at 480 0 0 fox I hr. A porous structure was formed located under the oarbonitride phase. Then the parts were delivered into the chamber 12 and kept in an oxidizing atmosphere consisting of oxygen (0.2 mass;)q ammonia.(3 mass %), and nitrogen 1 (96.8 mass-%) at 600 0 0. Af ter that the parts were. cooled in te chamber 15 and tempered in the chamber 21 at 140 0 0 for 40 min in oil containing 10 mass % of sulphur. The diffusion coating on the parts consisted of the structures similar to those described in Example 5 but the pores Of the first structural component were filled with oil containing 10 mass 26' of sulphur. The diffusion coating was 106 jum thick, the first structural component beinG 3 _Pm thick and the second IS IUM The results of physic o-mechaniGal tests of the processedparts with the described diffusion coating are as follows:

wear resistance, 10-3 jam corrosion resistance, hr l aiP ing, hr fatigue limits MPa 59 95 0.55 1070.

i k 26

Claims (17)

HE CLAIM:

1. A method of chemical thermal treatment of partsq residing in that thermal treatment of the parts is carried out in a nitrogen-containing atmosphere at a temperature of no more than 68000 up to the formation on said parts of a structure of oarbonitride or oxynitride phase having a thickness of at least 5 p M, this being followed by formation on the parts of a porous structure, and by exposure in an oxidizing atmosphere at a temperature equal to thgt of thermal treatment, cooling of the parts, and subsequent filling of the pores of the above porous structure with an oil containing 0.5 - 10 mass 7S of sulphur.

2. A metaod as claimed in Claim 1, wherein the formation of a porous struoture on the parts is perf ormed by keeping said parts under isothermal conditions at a temperature of from 400 to 500 00 in a protecting atmosphere.

3. A method as claimed in Claim 2, wherein as a protecting atmosphere use is made of nitrogen and ammonia at a mass ratio thereof equal to 90.0 99.9 0.1 - 10.

4. A method as claimed in Claim 2, wherein as a protecting atmosphere use is made of nitrogen. 25.

5. A method as claimed in any of the above Claims wherein thermal treatment is carried out at a temperatuxe of from 450 to 57000 for 2- 5 hrs.

27

6. A method as claimed in Claim 1 - 4, wherein thermal treatment is performed at a temperature of from 570 to 68000 for 2 - 5 hrs.

-

7. A method as claimed in Claims 59 6, wherein 5 thermal treatment Is carried out in a nitrogen-containing-atmospliere consisting of endogas and ammonia at a mass ratio thereof equal to 30 - 50 50 - 70.

8. A method as claimed in Claim 11 wherein the parts are kept In an oxidizing atmosphere consisting lo of oxygen. nitrogeng and ammonia at a mass ratio thereof 0.2 - 22.0: 0.05 - 3.0 75 - 99.75. respectively.

9. A method as claimed in Claim It wherein the porous structure of the parts Is filled with the above oil by tempering said parts in the oil at a temperature of from 120 140 0 0 for 20 - 40 min.

10. A method as claiLed in Claim 19 wherein, prior to thermal treatment, theDarts are subjected to preliminary holding at a temperature of from 350 to 4000C.

11. A diffusion coating on parts made of a lumi- niun-based alloys obtained by the proposed method according to any of the above Claimse consisting of crystalline structures of Ai 2 0 3 and an oxynitride phase of the (AlMe) 2-3 (NO) type, where Me is metal, the interface of the structures of the above phases being filled with an oil containing 0.5 10 mass % 28 of sulphur.

12. A diffusion coating on parts made of Iron-based alloys obtained by the proposed method according to Claims 1 - 10 consisting of three structural components of which that closest to the surface has a fine-porous structure consisting mainly of Pe 3 0.. the pores being filled with an oil containing 0.5 - 10 mass ro of sulphur, the second component has a nonporous structure of the carbonitride phase of the Fe 2-3 (NO) type2 and the third component has the bainite structure, the ratio of the thicknessesof the first and second structural components being within the range from 1 2 to 1 10.

13. A diffusion coating as claimed in Claim 122 wherein the first structural cokponent is at least 2 im thick.

14. An installation for realizing the method as claimed in Claim 19 comprising the following units located in a technological sequence and communicated with one another through a transport system: a furnace for thermal treatment of parts connected hermetically with a chamber for isothermal holding of the parts which, in its turn, is connected hermetically with an oxidizing chamber in the direction of moving the parts, a cooling chambert and a chamber for tempering the parts.

15. An installation as dlaimed in Claim 14t wherein the chamber for isothermal holding the Parts 1 9 29 is mounted ahead of the furnace for thermal treatment,

16. A method as claimed in Claim s 1 - 10, substantially as described in the disclosure and Examples 1 through 9. 5

17. Coatings as claimed in Claims 1 - 10, substanti ally as described in the disclosure and Examples 1 through 9. 1d. An installation as claimed in Claims 14. 15 substantially as described in the disclosure and shown in the drawing Published 1991 at The Patent O(Rce. State House, 66/71 High Holborn, London WCIR47?. Further copies Tn2y be obtained from Sales Branch, Unit 6. Nine Mile Point. Cwrnfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent.