Method and device for hardening the inner surface of holes, in mechanical pieces of cast iron of predominantly ferritic matrix

Abstract

A method and a device for hardening at least one portion of the inner surface (12) of a hole (11) made in a mechanical piece (10) of cast iron of predominantly ferritic matrix, comprising the step of subjecting said portion of the inner surface (12) to the action of a laser beam (100), until it causes the melting of its surface layer. In particular, said method comprises the steps of: projecting the laser beam (100) in axial direction inside the hole of the piece (10); deviating the laser beam (100) through a deviator means (4), so that the same is projected onto the inner surface of the hole itself, causing the melting of a surface layer; rotating the deviator means (4) or the piece (10) with respect to each other, and sliding in axial direction the deviator means (4) or the piece (10) with respect to each other, until the desired portion of the inner surface (12) is subjected to the laser beam (100).

Description

[0001] The present invention generally refers to a method and device for the hardening of the inner surface of holes made in mechanical pieces in iron casting, and mechanically worked. It is known that for manufacturing these pieces, an easily workable cast iron is employed, such as the cast iron of predominantly ferritic matrix. More in particular, the invention regards a method and device intended to be employed in the process of manufacturing pump bodies of normal fluid pumps, to harden the surfaces of the cylindrical cavities adapted to define the reception and guide seats of members in movement, such as sliding pistons or rotors, so to avoid seizure.

[0002] As is known, the body pumps are realised by means of a forming process, which foresees pouring a metallic alloy inside appropriate dies in order to obtain a rough casting. This is then subsequently subjected to mechanical working, which confers the necessary precision to predetermined surfaces and in particular to the above mentioned surfaces intended to be in contact with the members in movement.

[0003] For these reasons, the metallic alloy utilised in the manufacture of the pump bodies is, as said, a cast iron of predominantly ferritic matrix, easily workable in the machine tool, but little adapted to being subjected to a thermal hardening treatment.

[0004] The use of the pump at low ambient temperatures may lead to drawbacks. This occurs when the inner surfaces of the "cold" pump are struck with hot oil. The difference in mass between the pump body and the inner members prevents a homogenous dilation of the system elements, favouring seizure phenomena, which often irreparably damages the pump itself. Presently, to overcome this drawback, an anti-seizure surface treatment is used on the surfaces intended to be in contact, for example a phosphate treatment.

[0005] Nevertheless, the equipping of a phosphate plant is quite difficult, and moreover leads to a considerable environmental impact.

[0006] Object of the present invention is, in general, that of superficially hardening the surfaces of holes made in mechanical pieces of cast iron of predominantly ferritic matrix and, more in particular, hardening the surfaces of the cylindrical cavities of the pump bodies in contact with the members in movement, so to effectively avoid the onset of seizure phenomena. Further object of the invention is to obtain a lasting, reliable hardening which may be realised in a simple, quick and economic manner on a cast iron which is highly workable by the tool.

[0007] Such objects are achieved, according to the finding, with a laser treatment of the relevant surfaces, until it causes the melting of their surface layer. The treatment may be conveniently localised, i.e. limited to certain surface portions.

[0008] More precisely, the treatment foresees the steps of:

[0009] a) projecting a laser beam in axial direction inside the hole of the piece in cast iron of predominantly ferritic matrix;

[0010] b) deviating the laser beam through a deviator means which is inserted in the hole, so that the same is projected on the inner surface of the hole itself, causing the surface melting;

[0011] c) rotating the deviator means or the piece with respect to each other; and

[0012] d) making the deviator means or the piece slide in axial direction with respect to each other, so to subject at least one desired portion of said inner surface of the hole;

[0013] In this manner, a thermal treatment of surface melting of the cast iron is carried out, obtaining a layer with homogenous and particularly hard structure.

[0014] Indeed, the melting, which involves a very small thickness of the material, less than 0.5 mm, and the subsequent quick and spontaneous cooling, due to the great mass of material surrounding the melted zone, causes a change of phase in the metallic structure of the cast iron.

[0015] In particular, the carbon present in the form of graphite is largely dissolved in the melted metal matrix, which is enriched with carbon and solidifies with predominantly ledeburitic structure, characterised by high hardness.

[0016] According to the invention, in particular, the treatment may be interrupted at the surface portions having edges with specific roles, for example sealing functions in association with other components.

[0017] The treatment, in fact, may cause a slight local deformation of said edges which therefore would not be fit for carrying out their function.

[0018] According to the invention, moreover, the aforesaid deviator comprises a faceted body which is realised with a material of high thermal conductivity, such as copper, whose appropriately prepared surface possesses a high reflecting power.

[0019] In particular, said surface is made specular by means of a surface finishing treatment, preferably by means of polishing; furthermore, during the treatment, it is properly cooled.

[0020] Further and advantageous characteristics of the invention will be clear from the reading of the following description, provided as merely exemplifying and not limiting, with the air of the figures reproduced in the attached drawing tables, in which:

[0021] FIG. 1 is a side schematic view of a device adapted to carry out the hardening treatment, in a manner in accordance with the invention;

[0022] FIG. 2 is a detail of the trace section II-II indicated in FIG. 1, enlarged and shown during the execution of the treatment;

[0023] FIG. 3 is the trace section III-III indicated in FIG. 2.

[0024] From the mentioned figures, a device 1 is noted which is adapted to realise, by means of a laser, a hardening treatment of the inner surface 12 of a hole 11, realised in a mechanical piece 10 of cast iron of predominantly ferritic matrix (see also FIG. 3).

[0025] Such device 1 comprises a laser source 2, which is provided with an emission head 20 which projects a laser beam 100 (indicated with dashed line in FIGS. 2 and 3) along a predetermined direction.

[0026] The emission head 20 is firmly associated with a support 3, which supports a faceted copper cylinder 4 arranged along the emission direction of the laser beam 100 and adapted to deviate the latter by substantially 90.degree..

[0027] As is visible in FIG. 2, said faceted cylinder 4 has a first section axially inserted within a containment sleeve 30 of the frame 3, and a second section which projects from said containment sleeve 30 from the part of the emission head 20.

[0028] In this manner, the axis of the faceted cylinder 4 lies substantially in the emission direction of the laser beam 100 (see also FIG. 3), and the bottom face 40 of the cylinder 4 itself, which is made specular by means of polishing, acts as a reflecting surface.

[0029] To avoid that, during the treatment, the faceted surface 4 is melted or damaged when the reflecting surface 40 is struck by the laser beam 100, means (not shown) are foreseen which are adapted to cool it; said means may comprise, for example, channelling made inside the cylinder 4 itself, in which a refrigerating liquid circulates.

[0030] Furthermore, as illustrated in FIGS. 1 and 2, the device 1 comprises a supply nozzle 6 which is adapted to direct, during the treatment, a jet of protective gas on the reflecting surface 40 of the cylinder 4 and on the treated zone, in order to avoid oxidation; said protective gas being contained in an appropriate pressure cylinder (not shown), to which the nozzle 6 is connected.

[0031] As is shown in FIG. 2, the reflecting surface 40 is sloped 450, and is hit by the laser beam 100 so to reflect it with an angle of deviation equal to 900. In case it is necessary to reach particular positions, however, the slope of the aforesaid reflecting surface may also be different from 450.

[0032] The mechanical piece 10 to be treated is mounted on a support and movement device 5, which comprises a motorised carrier 50, sliding with reciprocating motion on a rectilinear guide 51.

[0033] In particular, said motorised carrier 50 is provided with a rotating mandrel 52 adapted to support the cast iron piece 10 of predominantly ferritic matrix, so that the axis of the hole 11 lies on the rotation axis of the mandrel 52 itself.

[0034] As is illustrated in FIGS. 1 and 3, the light source 2 and the movement device 5 are mutually positioned so that the rotation axis of the mandrel 52, and therefore the hole axis 11, is parallel to the direction of the laser beam 100 leaving the emission head 20, and the mouth of the hole 11 is turned towards the laser source 2.

[0035] With reference to FIG. 2, at the beginning of the treatment, the carrier 50 is advanced towards the laser source 2, and the faceted cylinder 4 axially penetrates inside the hole 11, until it is positioned so to be capable of deviated the laser beam 100 on a first edge 12' of the cylindrical surface 12 to harden (see also FIG. 3).

[0036] At this point, the source 2 is activated to generate the laser beam 100 and, at the same time, the mandrel 52 is placed in rotation at a predetermined speed, and the carrier 50 is also driven to slide at a predetermined speed, until the laser beam 100 reaches the second edge 12'' of the cylindrical surface to harden.

[0037] In this manner, the laser beam 100 eventually hits all the points of surface 12 of the hole 12, causing the melting of a layer of cast iron of predominantly ferritic matrix having a very small thickness, about 0.5 mm.

[0038] Consequently, the metallic structure of said melted layer, cooling rapidly due to the proximity of a great quantity of non-melted material, undergoes-a change of state. The carbon in the form of graphite, present in the cast iron of predominantly ferritic matrix, is dissolved in the metallic matrix, which is enriched with carbon and assumes a ledeburitic structure to confer a very high hardness to the surface layer.

[0039] In order to obtain a quick and effective melting of the surface 12 of the hole 11, however, the treatment must be conducted respecting several optimal process parameters.

[0040] In particular, the thickness of the melted layer substantially depends on the maximum temperature reached by the surface 12 of the hole 11 during the treatment, which in turn depends both on the energy of the laser beam 100 which is absorbed in the time period by the cast iron of predominantly ferritic matrix of the piece 10 and, of course, by the time of exposure of the same surface 12 to said laser beam 100.

[0041] Regarding the energy absorbed by the cast iron of predominantly ferritic matrix, this is determined in particular by the physical properties of the laser beam 100, and by its geometrical characteristics.

[0042] Physical properties include: the wavelength of the laser beam 100, on which the absorption coefficient of the cast iron of predominantly ferritic matrix depends, that is the percentage of energy absorbed and utilised in an active manner for the treatment, with respect to that reflected by the surface 12 to be treated; and the power of the laser ray 100 itself.

[0043] Geometrical characteristics, on the other hand, include the focal distance of the laser beam 100 leaving the source 2, and the dimensions of the "spot", that is the luminous point projected on the surface 12.

[0044] Regarding the time of exposure to the laser beam 100, finally, this is a function of the speed with which the luminous "spot" moves with respect to the surface 12 to be treated, which depends of course on the rotation speed and translation speed of the piece 10, with respect to the faceted cylinder 4.

[0045] It should therefore be underlined that, regarding energetic parameters, optimal results were obtained with a laser beam 100 of wavelength comprised between 800 nm and 950 nm, and power comprised between 1500 W and 2200 W.

[0046] This along with a focal distance of the laser beam 100 comprised between 66 mm and 500 mm, and a rectangular luminous "spot" with sides comprised between 4 mm and 9 mm.

[0047] Regarding the exposure time, optimal results were demonstrated with a speed of the luminous "spot", with respect to the surface 12, comprised between 20 mm/s and 40 mm/s.

[0048] Lastly, as merely exemplifying, the process parameters which permitted obtaining optimal results were reported, in terms of surface hardness and treatment time, on the inner surface of a cylindrical cavity of a pump body in cast iron of predominantly ferritic matrix GS400.

[0049] These parameters, which were compiled in the table placed at the end of the present description, were obtained during a series of experiments conducted by the Applicant, in which there were used: a diode laser source 2 adapted to emit a continuous laser beam 100; a faceted copper cylinder 4 with 20 mm diameter; a reflecting surface 40 sloped at 45.degree. and superficially polished to have a roughness Ra<0.08; and a supply nozzle 6 of the protective gas which, connected to a pressure cylinder, directs the jet directly onto a reflecting surface 40, and onto the surface 12 struck by the laser beam 100. TABLE-US-00001 Parameter Value LASER SOURCE ROFIM DL022 LASER WAVELENGTH 808-940 nm ABSORBED POWER 6680 WATTS POWER SUPPLIED .about.2,000 WATTS FOCAL LENGTH 136 mm "SPOT" SIZE 4 .times. 9 mm "SPOT" SPEED 25 mm/s PROTECTIVE GAS NITROGREN

Claims

1. Method for hardening at least one portion of the inner surface (12) of a hole (11) made in a mechanical piece (10) of cast iron of predominantly ferritic matrix, characterised in that it comprises the step of subjecting said portion of the inner surface (12) to the action of a laser beam (100), until it causes the melting of its surface layer.

2. Method according to claim 1, characterised in that it comprises the steps of: a) projecting a laser beam (100) in axial direction inside the hole of the piece (10); b) deviating the laser beam through a deviator means (4) so that the same is projected onto the inner surface (12) of the hole itself, causing the melting of a surface layer; c) rotating the deviator means (4) or the piece (10) with respect to each other; and d) making the deviator means (4) or the piece (10) slide in axial direction with respect to each other, until said portion to be treated of the inner surface (12) is entirely subjected to the laser beam (100).

3. Method according to claim 1, characterised in that the thickness of the melted layer of cast iron of predominantly ferritic matrix is less than 0.5 mm.

4. Method according to claim 1, characterised in that the wavelength of the laser beam (100) is comprised between 800 nm and 950 nm.

5. Method according to claim 1, characterised in that the power of the laser beam (100) is comprised between 1500 W and 2200 W.

6. Method according to claim 1, characterised in that the focal distance of the laser beam (100) is comprised between 66 mm and 500 mm.

7. Method according to claim 1, characterised in that the laser beam (100) generates on the inner surface (12) a rectangular luminous spot, with sides comprised between 4 mm and 9 mm.

8. Method according to claim 1, characterised in that the laser beam (100) generates on the inner surface (12) a luminous spot which moves, relative to said surface (12), at a speed comprised between 20 mm/s and 40 mm/s.

9. Device for hardening, by means of laser, at least one portion of the inner surface (12) of a hole (11) made in a mechanical piece (10) of cast iron of predominantly ferritic matrix, characterised in that it comprises: a laser source (2) adapted to generate a laser beam (100), and project it in axial direction inside the hole (11) of the piece (10); a deviator means (4) of the laser beam (100) which, associated with said laser source (2), is adapted to be inserted in the hole (11) of the piece (10), and to deviate the beam (100), projecting it onto the inner surface (12); and a support device (5) of the cast iron piece (10); said support device (5) and deviator means (4) being adapted to rotate with respect to each other, and to slide in axial direction with respect to each other, until said portion to be treated of the inner surface (12) is entirely subjected to the laser beam (100).

10. Device according to claim 9, characterised in that the support element (5) comprises a rotating mandrel (52) adapted to hold the piece (10) so that the axis of the hole (11) coincides with the rotation axis of the mandrel (52) itself, and to set it in rotation at a predetermined speed.

11. Device according to claim 9, characterised in that the support element (5) comprises a motorised carrier (50) adapted to slide with reciprocating motion along a rectilinear direction, so to engage the piece (10) to slide in the direction defined by the axis of the hole (11).

12. Device according to claim 9, characterised in that the deviator means comprises a faceted body (4), provided with a reflecting surface (40) which is sloped with respect to the direction of the laser beam (100) leaving the laser source (2).

13. Device according to claim 12, characterised in that the faceted body (4) is realised in copper.

14. Device according to claim 12, characterised in that the reflecting surface (40) is made specular by means of polishing.

15. Device according to claim 9, characterised in that it comprises means adapted to cool the deviator means (4).

16. Device according to claim 9, characterised in that it comprises means adapted to convey a flow of protective gas which, leaving a supply nozzle (6), is adapted to hit the deviator means (4) and the surface (12).