GB1569506A

GB1569506A - Method of coating the inner cylindrical surface of a tubular body

Info

Publication number: GB1569506A
Application number: GB724/78A
Authority: GB
Original assignee: COUSSINETS STE INDLE
Current assignee: COUSSINETS STE INDLE
Priority date: 1977-01-10
Filing date: 1978-01-09
Publication date: 1980-06-18
Also published as: DE2800758B2; FR2376975B1; IT7819107A0; DE2800758C3; DE2800758A1; IT1091835B; FR2376975A1

Description

(54) A METHOD OF COATING THE INNER CYLINDRICAL SURFACE OF A TUBULAR BODY (71) We, SOCIETE INDUSTRIELLE DES COUSSINETS, a French body corporate, of 12, rue du General Foy, 75008 Paris, France, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a process for coating the inner surface of an ordinary or alloyed steel tubular body with an anti-friction metal alloy.

In the manufacture of motors and pumps in hydraulic units where pistons slide in cylinders, it is necessary for the internal bore of the cylinders to have good contact characteristics so as to facilitate the alternating sliding movement of the piston in the cylinder. Therefore the cylinder must be made from a solid material having good anti-friction properties and a satisfactory mechanical strength, for example a copper alloy. However, this solution results from a compromise between two different and often contradictory requirements leading to a relatively unsatisfactory result. It is possible to use a steel body coated by casting an anti-friction alloy but this generally leads to a high price. It is also possible to force-fit a cylindrical ring having anti-friction properties within the steel cylinder. To prevent play between the two members the deformations which could result therefrom and leakage of pressurised liquid between the ring and its housing, it is necessary for the shrinkage pressure of the ring in the cylinder to be higher than the discharge pressure. However, the stresses to which said ring is subject due to the very considerable shrinkage may lead to weak areas in the ring which could cause its fracture.

Therefore in place of a simple mechanical connection, it has been proposed to provide a metallurgical connection between the cylinder and the ring. Thus, French Patent 1,457,620 granted on 26.9.1966 describes an assembly process in which, after careful cleaning and polishing of the surfaces to be joined, a ring made from an anti-friction alloy with at least two constituents is fitted into the steel cylinder and the assembly is heated to a temperature between the melting temperatures of the two constituents in such a way as to cause a diffusion of the low melting point constituent into the steel of the cylinder. In the case of a ring made from lead-bronze (80% Cu, 10% Sn and 10%Pb) and a shrinkage pressure of approximately 140 kg/cm2, heating takes place for at least 14 hours at between 843 and 885"C in such a way as to cause a diffusion of the copper in a partly molten state in the vicinity of the interface to a depth of several microns into the steel of the cylinder.

However, this process has a certain number of disadvantages for cylinders of high pressure hydraulic pumps. Firstly, it is always necessary for the purpose of bringing about copper diffusion to have a shrinkage pressure between the ring and the cylinder with the inherent risks of deformation and fracture. Furthermore, due to the partial melting of the copper in the area adjacent to the interface, the ring has a tendency to deform and become porous which may impair its mechanical strength. Finally, this process does not function satisfactorily with phosphur-bronze (P > 0.05 So) because a very brittle iron-phosphide compound is formed at the interface making it necessary to choose more expensive special bronzes.

The present invention provides a method of coating the inner cylindrical surface of an ordinary or alloyed steel tubular body with an anti-friction metallic alloy, that comprises depositing a thin and compact connecting layer constituted by a copper alloy that is substantially free of phosphorus (as hereinafter defined) and contains 5 to 20% of lead on the outer surface of a cylindrical ring having an external diameter slightly smaller than the internal diameter (d) of the tubular body and at least the inner surface made from anti-friction metallic alloy; placing the coated ring inside the tubular body with a clearance below 0.005 d; and heating the assembly at a temperature in the range 750 to 8400C under a hydrogen-rich atmosphere for at least 15 minutes.

By proceeding in accordance with the present invention, diffusion takes place between the steel cylinder and the ring of anti-friction metal material and various standard anti-friction materials can be used in such a way that the friction and mechanical strength characteristics of the ring are not impaired. It thus becomes possible to use an unwelded rolled ring, if desired.

The cylindrical ring can be made from a random anti-friction metallic material able to support the deposit of the connecting coating and withstand without damage passage through the furnace at the treatment temperature. Examples of such metals are bronzes with or without lead, including bronzes containing a certain amount of phosphorus. The ring can be in multi-layer form, for example, steel with an inner coating of anti-friction material. This ring can be obtained by centrifugal or continuous casting, with machining of the internal and external diameters. However, it is also possible to use rings with contiguous edges obtained by rolling in the press a blank cut from a sheet or strip. This method reduces manufacturing costs and prevents losses of primary material, whilst ensuring excellent dimensional precision.

The connecting layer can be deposited by an appropriate means for example by metallisation with a spray gun or electrolytically.

In the case of a rolled ring with contiguous edges, it is possible to deposit the connecting layer on the strip and manufacture the ring from said coated strip, whereby the coating is on the outer surface. A convenient process for coating the strip comprises depositing a metallic powder of a copper alloy thereon and then sintering the layer.

The alloy used for the connecting layer is a copper alloy that is substantially free of phosphorus, by which term is meant herein "with a maximum phosphorus content of below 0.05%", so as to prevent the formation of fragile iron phosphide at the interface with the steel cylinder during diffusion. The lead content must be at least 5% to permit diffusion to take place and must not exceed 20% because beyond the latter value the mechanical strength of the connecting zone may be inadequate. The deposited layer must be compact and is desirably above 90 % of the theoretical density in order to prevent it from being porous, which would weaken the connection.

The external diameter of the coated ring must be slightly less than the internal diameter of the steel cylinder so that there is a clearance of a few hundredths of a millimetre. This clearance must permit of the introduction of the coated ring into the cylinder but must remain small, i.e. below 0.005 d, in order to facilitate the diffusion operation.

The ring is introduced into the cylindrical tubular body whose inner surface has been previously precision-machined and carefully cleaned. The assembly is heated in a furnace at a temperature in the range 750 to 8400C, preferably 800"C to 8200C, for at least 15 minutes under a hydrogen atmosphere or an atmosphere that contains a significant proportion of hydrogen. The function of the hydrogen is to aid the wettability of the steel surface by the lead in the molten state.

In the case where, for example, the connecting layer comprises lead-bronze containing approximately 80% copper, 10% tin and 10% lead, a molten lead phase forms from 320"C.

However, by keeping the temperature below 840"C the copper remains in the solid state and there is no formation of the so-called a + molten phase as is the case in the process described in French Patent 1,457,620. For example, at 8200C 4% of the copper is soluble in the molten lead. Due to the action of capillary forces the liquid rises between the surfaces of the steel cylinder and the connecting layer, permitting a diffusion of the copper dissolved in the molten lead to a depth of several microns on the surface of the steel cylinder. However, lead is not soluble in copper and the quantity of molten lead remains constant throughout the thermal treatment. After spending about 30 minutes in the furnace, the copper diffuses into the steel to a sufficient extent to ensure an appropriate metallurgical connection between the coated ring and the cylinder.

In the following illustrative example, all parts and percentages are by weight unless otherwise stated: EXAMPLE Coating takes place of the inner surface of a cylinder made from CC 35 steel according to Afnor A 35-551 Standard containing the following materials in addition to iron: C = 0.35 Mn = 0.60 Si = 0.25 P < 0.04 S < 0.04 whose bore has a diameter of 20 + 0.005 mm.

A rolled cylindrical ring with contiguous edges is made from a two-layered strip having a bronze support and a connecting layer deposited by sintering.

The support is of UE 9P bronze according to Standard AFNOR A 53-012 of the following composition: Cu = 91% Sn = 8.8% P = 0.2% The connecting layer has the following composition: Sn = 10.2% Pb = 10% P 0.03% the remainder being copper.

The average external diameter of the coated ring is 19.96+ 0.03 mm, so that the clearance is from 0.01 to 0.08 mm, including out-of-true faults of the ring. The thickness of the connecting layer is 0.15 to 0.20 mm.

The coated ring is placed inside the cylinder and the assembly is heated at 8200C for 30 minutes under a cracked ammonia atmosphere (75 % Hz and 25 N2 by volume) with a flow rate of 5 m3/hour. Cooling takes place at a rate of 100 degree per hour to 400"C.

Microscopic examination reveals a diffusion zone with interpenetration of the constituents by a thickness of 2 microns.

WHAT WE CLAIM IS: 1. A method of coating the inner cylindrical surface of an ordinary or alloyed steel tubular body with an anti-friction metallic alloy, that comprises depositing a thin and compact connecting layer constituted by a copper alloy that is substantially free of phsphorus (as hereinbefore defined) and contains 5 to 20%of lead on the outer surface of a cylindrical ring having an external diameter slightly smaller than the internal diameter (d) of the tubular body and at least the inner surface made from antifriction metallic alloy; placing the coated ring inside the tubular body with a clearance below 0.005 d; and heating the assembly at a temperature in the range 750 to 8400C under a hydrogen-rich atmosphere for at least 15 minutes.

2. A method as claimed in claim 1, in which the anti-friction ring is rolled with contiguous edges from a blank cut from a sheet or strip.

3. A method as claimed in claim 1 or 2, in which the connecting layer is deposited on the ring by metallisation with a spray gun.

4. A method as claimed in claim 1 or 2, in which the connecting layer is deposited electrolytically on the ring.

5. A method as claimed in claim 1 or 2, in which the connecting layer is deposited on the sheet or strip by sintering a metal powder.

6. A method as claimed in any one of claims 1 to 5, in which the ring is made fron bronze.

7. A method as claimed in any one of claims 1 to 5, in which the ring is made from steel internally coated with an anti-friction metal layer.

8. A method as claimed in any one of Claims 1 to 7, in which the connecting layer is of lead-bronze.

9. A method as claimed in Claim 9, in which the lead-bronze has the approximate composition 80% copper, 10% tin and 10% lead by weight.

10. A method as claimed in Claim 1 substantially as hereinbefore described in the Example.

11. A coated body made by the method as claimed in any one of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. bronze support and a connecting layer deposited by sintering. The support is of UE 9P bronze according to Standard AFNOR A 53-012 of the following composition: Cu = 91% Sn = 8.8% P = 0.2% The connecting layer has the following composition: Sn = 10.2% Pb = 10% P 0.03% the remainder being copper. The average external diameter of the coated ring is 19.96+ 0.03 mm, so that the clearance is from 0.01 to 0.08 mm, including out-of-true faults of the ring. The thickness of the connecting layer is 0.15 to 0.20 mm. The coated ring is placed inside the cylinder and the assembly is heated at 8200C for 30 minutes under a cracked ammonia atmosphere (75 % Hz and 25 N2 by volume) with a flow rate of 5 m3/hour. Cooling takes place at a rate of 100 degree per hour to 400"C. Microscopic examination reveals a diffusion zone with interpenetration of the constituents by a thickness of 2 microns. WHAT WE CLAIM IS:

1. A method of coating the inner cylindrical surface of an ordinary or alloyed steel tubular body with an anti-friction metallic alloy, that comprises depositing a thin and compact connecting layer constituted by a copper alloy that is substantially free of phsphorus (as hereinbefore defined) and contains 5 to 20%of lead on the outer surface of a cylindrical ring having an external diameter slightly smaller than the internal diameter (d) of the tubular body and at least the inner surface made from antifriction metallic alloy; placing the coated ring inside the tubular body with a clearance below 0.005 d; and heating the assembly at a temperature in the range 750 to 8400C under a hydrogen-rich atmosphere for at least 15 minutes.