Method of surface hardening sintered bodies by using vibrations

Abstract

The present invention relates to a method of surface hardening a plurality of sintered bodies having a hard phase and a binder phase. The method includes the steps of placing the bodies in a container, and forming a system including the container and the bodies therein, and causing the bodies to move and collide with each other and with inside walls of the container. The container is vibrating utilizing a mechanical resonance frequency of the system.

Claims

1. A method of surface hardening a plurality of sintered bodies comprising a hard phase and a binder phase, said method comprising the steps of: placing the bodies in a container and forming a system including the container and the bodies therein, each of the bodies having a surface zone and a bulk zone extending below the surface zone; and hardening the surface zone of each of the bodies by vibrating the container at a resonance frequency within 0.05 Hz of the resonance frequency of the system to cause the bodies to move and collide with each other and with inside walls of the container, wherein the hardened surface zone has a hardness that is greater than a hardness of the bulk zone of the bodies.

2. A method according to claim 1, wherein said hard phase is WC and said binder phase is Co and/or Ni.

3. A method according to claim 1, wherein the container is vibrated with uniaxial vibrations.

4. A method according to claims 3, wherein the movements of the bodies originate from the vibrations.

5. A method according to claim 1, wherein the vibrations are acoustic vibrations.

6. A method according to claim 1, wherein the container is vibrating with vibrations having a frequency of 20-80 Hz.

7. A method according to claim 1, wherein the volume of one body is more than 100 mm.sup.3.

8. A method according to claim 1, wherein the weight of one body is more than 0.01 kg.

9. A method according to claim 1, wherein the bodies are sintered cemented carbide bodies for oil, gas or mining applications or drill bit inserts.

10. A method according to claim 1, wherein the container is vibrating with vibrations having an acceleration of 30-100 G, where 1 G=9.81 m/s.sup.2.

11. A new method according to claim 1, wherein the surface zone includes a first surface zone that extends from a surface of the body to 1 mm below the surface into the body, wherein the bulk zone extends from 5 mm below the surface and into the body, and wherein, inside the first surface zone, a hardness of the first surface zone is more than 4% higher than the hardness of the bulk zone.

12. A new method according to claim 11, wherein the surface zone includes a second surface zone, and wherein a hardness of the second surface zone is more than 1.5% higher than the hardness of the bulk zone.

13. A new method according to claim 11, wherein the surface zone includes a second surface zone that extends to 5 mm below the surface into the body, and wherein, inside said second surface zone, a hardness of the second surface zone is more than 1.5% higher than the hardness of the bulk zone.

14. A method of surface hardening a plurality of sintered bodies comprising a hard phase and a binder phase, said method comprising the steps of: placing the bodies in a container and forming a system including the container and the bodies therein, each of the bodies having a surface zone and a bulk zone extending below the surface zone; and hardening the surface zone of each of the bodies by vibrating the container at a frequency at or close to the resonance frequency of the system to cause the bodies to move and collide with each other and with inside walls of the container; wherein the container is vibrating with vibrations having an acceleration of 30-100 G, where 1 G=9.81 m/s.sup.2, and wherein the hardened surface zone has a hardness that is greater than a hardness of the bulk zone of the bodies.

15. A method of surface hardening a plurality of sintered bodies comprising a hard phase and a binder phase, said method comprising the steps of: placing the bodies in a container and forming a system including the container and the bodies therein, each of the bodies having a surface zone and a bulk zone extending below the surface zone; causing the bodies to move and collide with each other and with inside walls of the container by vibrating the container at a frequency within plus or minus 0.05 Hz of the resonance frequency of the system and wherein the container is vibrating with vibrations having an acceleration of 30-100 G, where 1 G=9.81 m/s.sup.2; and hardening the surface zone of each of the bodies, the hardened surface zone having a hardness that is greater than a hardness of the bulk zone of the bodies.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Embodiments of the invention will now be described with reference to the accompanying drawings, wherein:

(2) FIG. 1 is a drawing of a drill bit insert,

(3) FIG. 2 is a graph of hardness as a function of depth in accordance with Example 5.

DETAILED DESCRIPTION

(4) In the following, Example 1 discloses samples before any surface hardening treatment, Example 2 describes one example of a method in accordance with one embodiment of the invention and Example 3 and 4 describes a tumbling and high energy tumbling treatments known in the art. Example 5 discloses the results from hardness tests as a function of depths for the samples treated according to the invention as compared to prior art treatment and Example 6 discloses the results from toughness tests. Example 7 discloses a crush test performed on samples treated according to the invention as compared to samples treated according to prior art treatments. Example 8 discloses the change in Coersivity due to treatments according to the present invention.

EXAMPLE 1

Prior Art

(5) Samples of cemented carbide comprising the hard phase WC and the binder phase Co were manufactured. Powders of WC and Co were wet milled, spray dried and pressed to bodies of the shape of drill bits. The pressed bodies were GPS sintered at vacuum at a temperature of 1410 C. to dense samples of cemented carbide. Each body was in the form of a bit 1 as shown in FIG. 1, with a cylindrical body with one spherical end 2 and one flat end 3. The size of one body is 15 mm in height and 12 mm in diameter or width. The weight of one sample is about 25 g. The samples were centre less ground using a centre less grinding equipment of Lidkping type.

(6) The samples are characterized and the compositions and properties are shown in Table 1.

(7) The grain size is measured at a polished through cut with mean intercept method in accordance with ISO 4499 and the values presented in Table 1 are mean values.

(8) The hardness is measured with a Vickers indenter at a polished surface in accordance with ISO 3878 using a load of 30 kg.

(9) The porosity is measured in accordance with ISO 4505, which is a method based on studies in light microscope of polished through cuts of the samples. Good levels of porosity are equal to or below A02maxB00C00 using ISO4505 scale.

(10) TABLE-US-00001 TABLE 1 Composition and properties of samples tested. Type A B C Co (wt %) 11 10 6 WC Balance Balance Balance WC grain size 2 3 3 (m) Hardness 1250 1150 1270 (HV30) Porosity A02maxB00C00 A02maxB00C00 A02maxB00C00

EXAMPLE 2

Invention

(11) The samples of type A, B and C were treated by a method in accordance with one embodiment of the invention. The samples were treated in an equipment which is aimed for mixing of liquids, powders or slurries, called Resodyn LabRam. This machine is constructed for a load of maximum 500 g. The container aimed for the powder or liquid was loaded with 10 bodies of 25 g each. An Auto function was used to reach the resonance frequency within the interval of 58-68 Hz, landing on a frequency of about 60 Hz. The time of treatment was varied as disclosed below. The energy was adjusted such that a maximum acceleration of 20 G, 40 G or 60 G was achieved, wherein 1 G=9.82 m/s.sup.2.

EXAMPLE 3

Tumbling

(12) Samples of type A were tumbled in a standard vibration tumbling machine. The tumbling machine is a vibrating machine comprising a bowl that mounts on top of a vibration generator. The tumbling machine is a Sweco model X FMD-3-LR which can be loaded with maximum 70 kg. The number of treated bodies during this example was about 2000 bodies. The frequency was 25 Hz, the acceleration 2 G and the time of tumbling was 2 hours.

EXAMPLE 4

High Energy Tumbling

(13) Samples of type A were treated in a high energy tumbling machine of type Vibro Benz. This is a modified tumbling machine in which the samples are vibrated and move in a spiral motion. This method can also be called cascading. The machine can be loaded with maximum 70 kg. The number of treated bodies during this example was about 2000 bodies. The frequency was 26 Hz, the acceleration 4 G and the time of high energy tumbling was 2 hours.

(14) High-energy tumbling involves placing parts in a barrel. The barrel, which is sealed with a lid, is then rotated on a carousel holding four barrels. While the carousel spins one way, the barrels go the other direction. This creates a powerful centrifugal force that results in a surface treatment of the parts.

EXAMPLE 5

Hardness vs Depth

(15) The surface hardening method according to the present invention was compared to the well known surface hardening method tumbling, and to untreated samples with regards to hardness increase and depth of hardness increase.

(16) Samples of type A were treated with the tumbling disclosed above and in accordance with the invention with 40 G, and compared with an untreated sample. The samples were through cut and polished and the hardness was measured as a function of depth from treated surface with Vickers Hardness tests with a load of 3 kg. The results are presented in Table 2 and shown in FIG. 2.

(17) TABLE-US-00002 TABLE 2 Hardness (HV3) as a function of treatment and depth. Distance from surface 0.5 mm 1 mm 2 mm 3 mm 4 mm 5 mm 6 mm Untreated 1313 1313 1313 1313 1313 1313 1313 Tumbling 1339 1326 1317 1314 1313 1313 1313 (prior art) 2 G, 2 hours Invention 1398 1372 1351 1352 1350 1335 1326 40G, 75 min

(18) As shown in Table 2, the samples treated with a surface hardening according to one embodiment of the present invention shows both a higher level of hardness and a larger depth of the hardness incensement. It is notable that the time of treatment is 2 hours for the tumbling treatment compared to 1 hour in accordance with the method according to the invention.

EXAMPLE 6

Toughness

(19) The surface hardening method according to the present invention was compared to the well known surface hardening method tumbling, and to untreated samples with regards to toughness increase. The sample type A was surface hardening treated and the toughness was measured. The toughness was studied based on crack lengths at the corners of Vickers indents made with a load of 100 kg, a so called Mean Palmquist crack length, and the results are presented in Table 3.

(20) In the surface zone no cracks were detected in a light optical microscope at 500, whereas in the core zone, at 500, the crack length were typically 77 m in a material not subjected to a surface treatment.

(21) TABLE-US-00003 TABLE 3 Palmquist crack length (m) Palmquist crack Palmquist crack length (m) length (m) Surface treatment close to treated surface at core Untreated surface, as ground 77 77 Tumbling (prior art) 33 77 Invention 40 G, 75 minutes 0 77

EXAMPLE 7

Crush Test

(22) A so called crush test was performed by taking a sample placing it between two anvils and applying a continuously increasing load until breakage. The load at failure is then recorded as the maximum compressive strength that the sample can withstand before failure. The tests were done on samples of type A with the geometry as disclosed above, and the results are presented as compressive strength as shown in Table 4.

(23) TABLE-US-00004 TABLE 4 Load at breakage Surface treatment Compressive strength (kN) Untreated surface, as ground 83.32 Tumbling (prior art) 115.38 High energy tumbling (prior art) 124.26 Invention 40 G, 75 minutes 134.72 Invention 60 G, 75 minutes 141.55

EXAMPLE 8

Coercivity

(24) The Coercivity (Hc) is measured with a Foerster equipment suitably calibrated using cemented carbide reference samples. The coercivity is increased by the surface treatment according to the invention, as shown in Table 5.

(25) TABLE-US-00005 TABLE 5 Coercivity, (kA/m) of body which is untreated and on body treated according to invention. Type A B C Untreated 8.2 6.0 6.8 Invention 40 G, 75 minutes 9.2 7.0 7.6

(26) As shown in Table 6, the Coercivity level is increasing with treatment time and with acceleration during the treatment.

(27) TABLE-US-00006 TABLE 6 Coercivity (kA/m) as a function of treatment time and acceleration for sample of type A. Time treated 15 min 30 min 45 min 60 min 75 min Invention 20 G 8.44 8.55 8.63 8.66 8.68 Invention 40 G 8.66 8.84 8.97 9.09 9.21 Invention 60 G 9.00 9.32 9.39 9.48 9.56

(28) An advantage with Coercivity measurements is that they can be performed on bodies without the need of any destroying step, as compared to hardness measurements, which requires a through cut. Coercivity measurements can thus be performed as a quantitative step during for example a production line to check that the surface hardening treatment has been sufficient.

(29) While the invention has been described in connection with various exemplary embodiments, it is to be understood that the invention is not to be limited to the disclosed exemplary embodiments, on the contrary, it is intended to cover various modifications and equivalent arrangements within the appended claims.