Localized hardening of metallic surfaces

Abstract

The present invention relates to a method and system for treatment of a surface of a metallic material component, the method comprising the steps: electro-spark treating the surface of the metallic component by means of an electro-spark electrode, wherein the metallic material is a basically ferritic, perlitic and/or austenitic steel and the method creates a thin layer with martensitic microstructures at the surface of the metallic material component. Serpentines and quartz can be incorporated by an additional step as well as the surface randomly structured by this.

Claims

1. A method for treating a surface of a metallic workpiece for transforming a thin layer at the surface of the workpiece into martensitic microstructures, the method comprising the steps of: electro-spark treating the surface of the workpiece by means of an electro-spark electrode, wherein the electrode performs a rotation around its longitudinal axis with a rotational speed between 10 rpm to 1500 rpm, and wherein no electrolyte is provided between the electro-spark electrode and the workpiece; and doping the electro-spark treated surface of the workpiece with mineral particles, wherein a randomly distributed pattern of cavities or dimples or indents is created on the surface of the workpiece.

2. The method according to claim 1, wherein the electro-spark electrode is made from a wear-resistant alloy.

3. The method according to claim 1, wherein the initial surface roughness Ra of the workpiece prior to treatment is 0.01 to 1.6 m.

4. The method according to claim 1, wherein a relative movement between the workpiece and the electrode is provided during treatment.

5. The method according to claim 1, wherein a tip point of the electrode which gets in contact with the workpiece moves along its longitudinal axis back and forth during treatment.

6. The method according to claim 1, wherein electro-spark treating the surface transforms the thin layer at the surface of the workpiece into martensitic microstructures without depositing material from the electro-spark electrode at the surface.

7. The method according to claim 1, wherein subsequent to the electro-spark treatment the surface of the workpiece is densified by means of a tool selected from the group consisting of a roll, a metallic sphere and a working tool of an ultrasonic device.

8. The method according to claim 7, wherein the hardness of the tool selected from the group consisting of a roll, a metallic sphere; and a working tool of an ultrasonic device is >60 HRC.

9. The method according to claim 7, wherein the workpiece is rotated during the densifying treatment.

10. The method according to claim 1, wherein the mineral particles are provided in a suspension, and wherein the workpiece is immersed in the suspension or wetted by the suspension before or during treatment.

11. The method according to claim 1, wherein the mineral particles are indented on the surface of the workpiece by means of an indenter to fill pores or irregularities in the surface.

12. The method according to claim 1, wherein the minerals comprise at least one of the following components: serpentines and quartz.

13. The method according to claim 1, wherein the minerals comprise at least one of the group consisting of: optical quartz, SiO.sub.2+Fe, burned serpentine, calcined, Mg.sub.6[Si.sub.4O.sub.10](OH).sub.8, Eudialyt, Na.sub.12Ca6Zr.sub.3[Si.sub.3O.sub.9] [Si.sub.9O.sub.24(OH).sub.3].sub.2, baddeleyite, monoclinic ZrO.sub.2, zirconia, fused zirconia, crushed, sieved and/or milled to micrometer or nanometer size, zirconia further stabilized by MgO and/or CaO and/or Y.sub.2O.sub.3, titanomagnetite.

14. The method according to claim 2, wherein the wear-resistant alloy is WCCo cemented carbide tools or steel.

15. The method according to claim 3, wherein the initial surface roughness Ra of the workpiece prior to treatment is between of 0.1 to 1.6 m.

16. The method according to claim 4, wherein at least the workpiece is rotated during treatment.

17. The method according to claim 16, wherein the workpiece is rotated with a rotational speed of 0.1 m/min to 1 m/min.

18. The method according to claim 1, wherein a tip point of the electrode moves along its longitudinal axis back and forth with a frequency between 40 Hz to 50000 Hz.

19. The method according to claim 7, wherein the surface of the workpiece is densified by rolling or press rolling.

20. The method according to claim 9, wherein the workpiece is rotated with a rotational speed of 3 m/min to 300 m/min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various features of the embodiments can be more fully appreciated, as the same become better understood with reference to the following detailed description of the embodiments when considered in connection with the accompanying figures, in which:

(2) FIG. 1 shows schematically a cross section of a treated X12Cr13 surface by means of focussed ion beam (FIB) cutting;

(3) FIG. 2 shows schematically an enlargement of the cross section of the treated X12Cr13 surface shown in FIG. 1 by means of transmission electron microscopy (TEM); and

(4) FIG. 3 shows a SEM micrograph as enlargement of a cavity/dimple containing Serpentine

(5) FIG. 4 shows SEM micrographs of sample PT507 (electro-spark treatment) with mapping of the elements silicon, magnesium and oxygen [left column: magnification of 1000; right column: magnification of 500)]

(6) FIG. 5 shows a roll as used as instrument N1;

(7) FIG. 6 shows an example for instrument N2 with a metallic sphere;

(8) FIG. 7 SEM micrograph of the serpentine powder used for the disclosed embodiments; and

(9) FIG. 8 shows an example of an instrument N3.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) A X12Cr13 (20Cr13) steel was electro-spark treated in accordance with the present invention (sample PT501) and the cross sections are shown in the FIG. 1 and FIG. 2.

(11) As shown in FIG. 1 and depending on the operating parameters, a more or less deep transformation zone 100 is created. The transformation zone in FIG. 1 is approximately 4 m.

(12) In contrast to hardening by laser, the microstructure is micro- or nano-crystalline with an average grain size smaller than the untreated material below. The affected zone or penetration depth in FIG. 1 is 3-4 m and shows martensite with inclusion of FeCr.sub.2O.sub.4. The grain sizes of the transformed layer were refined, but still remained crystalline. The formation of FeCr.sub.2O.sub.4 indicates locally high temperatures during the treatment, even for short periods. The ferritic X12Cr13 (1.4006; 12X13-III in Russian, closely to X20Cr13) substrate treated in FIG. 1 showed M23C6 carbides on the grain boundaries.

(13) The FIG. 2 highlights the oxidic inclusions of FeCr.sub.2O.sub.4, which are marked by reference sign 200. The formed martensite is a lath martensite with twinnings. FIG. 2 clearly shows that the electro-spark treatment has not created an amorphous microstructure. The average universal hardness and the average plastic hardness determined by nanoindentation (using Fischerscope H100) of the untreated X12Cr13 surface were: Universal hardness: 2260 MPa70 MPa and Plastic hardness: 2530 MPa80 MPa.

(14) The treatment shown in FIG. 1 resulted in an increase of hardness, which is coherent with the findings in FIG. 1: Universal hardness: 4300 MPa700 MPa and Plastic hardness: 6300 MPa1000 MPa.

(15) The treatment according to the present invention may be repeated in order to achieve the desired depth of affected zone. Two passes are preferred in order to avoid annealing of the martensite. Depending on the metallurgy and annealing temperature of the metal/steel in question, repeated treatments may apply.

(16) As a consequence from the localized treatment according to the present invention, the part as a whole will not be heated. This assures, inter alia, the dimensional stability of the part during and after treatment as well as allows to avoid further machining and finishing operations. In consequence, it is preferred that only the tribologically stressed sections of a part/component will be treated. This widens the freedom in selecting of metallic materials for tribological applications, because no thermochemical treatments, like nitriding or carburizing, are necessary.

(17) FIG. 4 illustrates the results of a treatment according to an electro-spark treatment. The scanning electron microscope (SEM) pictures were taken using a SEM Supra 40 from ZEISS equipped with an EDX-Detector X-Flash from BRUKER and the software Quantax 4000. The pictures in the left column are taken at a magnification of 1000 (x=times). The corresponding pictures in the right column are taken at a magnification of 5000.

(18) The (SEM) picture in the top row (1.sup.st row) show the network of cavities, indents or dimples. In the SEM pictures in the 2.sup.st to 4.sup.th row show one element mapping overlayed to the SEM pictures of the top row. The elements were silicon (Si; 2.sup.nd row), magnesium (Mg; 3.sup.rd row) and oxygen (O; 4.sup.th row), because Serpentines are composed by these elements and not the X12Cr13 steel. All three predominant elements of Serpentines are in the cavities, proving that the X12Cr13 surfaces were doped with serpentines by the indenter.

(19) The FIG. 4 also shows that nearly all the cavities were filled with Serpentine.

(20) Details for preparing the example 1.2-10 T 1.2 PT501, shown in FIG. 4 are summarized in the following Table 1, 3.sup.rd column. Preferred parameter ranges are summarized in the 2.sup.nd column of Table 1.

(21) TABLE-US-00001 TABLE 1 Preferred range of operating conditions according to the PT501 (1.2) present invention 1.2-10 IIIT Material of part Metals and alloys X12Cr13 (1.4006; 12X13-III) Step 1 (electrospark hardening) Initial surface roughness Ra = 3.2 m to Ra = 0.1 m Ra = 1.6 m to Ra = 2.2 m Electrode material VK8, T5K10, VK10OM, or VK8 steels 95X18 Shape of electrode tip not relevant Flat ended rod Electrode diameter 1 mm to 10 mm 3 mm Auto-rotational speed of 10 rpm to 1500 rpm 800 rpm electrode Travelling speed over surface of 0.01-2 0.8 part, mm/rotation Axial load on electrode 50 gr. to 3000 gr. 200 gr. Electrode axial vibration 100 Hz to 50 000 Hz 400 Hz frequency Electrode axial vibration 0.01 mm to 0.5 mm 0.20 mm amplitude Voltage amplitude fed to first run: 20-40 V first run 30 V, electrode, [V] subsequent runs: >40 V subsequent runs 50 V Voltage in mains 220 V 220 V Frequency of voltage 50 Hz 50 Hz Step 2 (densification) Material type of tool N1 or N2, or N3 steel 40X nitrided Geometry of tool roller Rotational speed of part 3 m/min to 300 m/min 30 m/min Traverse speed of tool, mm per 0.01 mm to 10 mm 0.2 mm per rotation rotation Load on tool 0.5 kg/mm.sup.2 to 100 kg/mm.sup.2 20 kg/mm.sup.2 Suspension (type of mineral, particle size, concentration) Number of double strokes up to 10 2 Step 3 (mineral doping) Material type of tool N1 or N2, or N3 steel 40X nitrided Geometry of tool roll Rotational speed of part 3 m/min to 300 m/min 30 m/min Traverse speed of tool, mm per 0.01 to 10 mm per rotation 0.2 mm per rotation rotation Load on tool 5 kg to 100 kg 20 kg/mm.sup.2 Number of double strokes up to 10 1

(22) FIG. 5 shows an example of a tool N1 (see ref. Sign 10). The roller 10 preferably comprises a shaft or mandrel 12 for mounting the tool to an appropriate mounting structure such that tool 10 can be forced against the workpiece to be treated. It is preferred that at least the roll 11 of tool 10 is made from a hard alloy, e.g., based on tungsten carbide or titanium carbide bonded with nickel, nickel/molybdenum or cobalt. The roller 10 can be used for the densification step (pressing step 2). In particular, according to the present invention it is preferred to use the shown roller as an instrument N1.

(23) FIG. 6 shows a further example of an instrument 20 for the densification step with a spherical tip 21 that may rotate (also labeled tool N2). The tool 20 preferably comprises a shaft or mandrel 22 for mounting the tool to an appropriate mounting structure such that tool 20 can be forced against the workpiece to be treated. It is further preferred that at least the spherical ball/tip 21 of tool 20 is made from a hard alloy, e.g., based on tungsten carbide or titanium carbide bonded with nickel, nickel/molybdenum or cobalt.

(24) FIG. 8 shows a further example of an instrument 30, which may be used as tool N3. The shown tool 30 is an ultrasonic indentor with the indentor 1 an ultrasonic device 20 which can be used to generate dimples/cavities/indents at the surface of the workpiece. In particular, the shown ultrasonic indentor allows that the tip point 1 can be actuated the along the longitudinal axis of the tool 30. This actuation (longitudinal or axial movement) can be achieved by a pneumatic mechanism. The axial actuation of the tip 1 may be performed with the following parameters. The actuation frequency is preferably between 40 Hz to 50 000 Hz. Preferably, the frequency is between 50 Hz and 1000 Hz. However, according to the present invention ultrasonic actuation of the electrode is also possible, i.e., actuation with a frequency >20 000 Hz. The axial (vibrational) amplitude is preferably in the range between 0.005 to 0.5 mm. The axially indenting load is preferably in the range from 10 g to 3000 g (0.1 N to 30 N).

(25) While the invention has been described with reference to the exemplary embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments without departing from the true scope of the invention. The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. In particular, although the methods and devices has been described by examples, the steps of the method may be performed in a different order than illustrated or simultaneously. Those skilled in the art will recognize that these and other variations are possible within the scope as defined in the claims and their equivalents.