Surface hardening of cemented carbide body

Abstract

A cemented carbide body includes WC in a metallic binder phase. The cemented carbide body has a bulk portion and a surface portion. The grain size of the WC in the surface portion is smaller than the grain size in the bulk portion of the body and this gives an increased surface hardness and an increased wear resistance. The median grain thickness, tg, of WC in the surface portion is 20-300 nm and the average grain size in the bulk portion is 0.5-8 μm. A method of surface hardening a cemented carbide body is also provided.

Claims

1. A method of making a surface hardening of a cemented carbide body comprising the steps of: providing a cemented carbide body of sintered cemented carbide having WC and a metallic binder phase, wherein said cemented carbide body is a bulk portion surrounded by a surface; providing a first local heat treatment of a portion of the surface of the cemented carbide body at a temperature that is higher than or equal to a melting temperature of the WC, thereby forming a surface portion of the cemented carbide body; and providing a second heat treatment of the cemented carbide body at a temperature at or below a melting temperature of the metallic binder phase, in a neutral of carburizing atmosphere, such that the WC recrystallizes, thereby forming the cemented carbide body provided with the surface portion, wherein an average grain thickness, tg, which is a smallest dimension through the grain of WC is 20-300 nm, a metallic binder phase content in the surface portion is 50-120 vol % of a metallic binder phase content in the bulk portion, and the average grain size of WC in the bulk portion is 0.5-8 μm.

2. The method in accordance with claim 1, wherein said first local heat treatment is made with laser.

3. The method in accordance with claim 1, wherein said first local heat treatment is made with Electrical Discharge Machining (EDM).

4. The method in accordance with claim 1, wherein the second heat treatment includes a step of heating the body at 1050-1150° C.

5. The method according to claim 4, wherein the temperature 1050-1150° C. is applied for 50-70 minutes.

6. The method in accordance with claim 1, wherein the second heat treatment occurs in a vacuum or in an atmosphere including CO, CH.sub.4 and/or Ar.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) In the drawings:

(2) FIG. 1. A Scanning Electron Microscope (SEM) image of a top view of a surface portion of the sample D+P3+ T(soak)1100. The WC grains are light in colour.

(3) FIG. 2. A Scanning Electron Microscope (SEM) image of a trough cut (cross section) of a surface portion of the sample D+P3+ T(soak)1100. The larger WC grains of the bulk portion (2) is visible below the smaller WC grains of the surface portion (1).

(4) FIG. 3. A Scanning Electron Microscope (SEM) image of a trough cut (cross section) of a surface portion of the sample E+P3+ T(soak)1100. The larger WC grains of the bulk portion (2) is visible below the smaller WC grains of the surface portion (1).

(5) FIG. 4. A schematic drawing of a grain showing a WC grain with a grain length, lg, and a grain thickness, tg.

(6) FIG. 5. A Scanning Electron Microscope (SEM) image of a fractured cross section of a surface portion of the sample D+P3+ T(soak)1100.

(7) FIG. 6. A Scanning Electron Microscope (SEM) image of a fractured cross section of an initiated surface portion before the second heat treatment has been applied, i.e. directly after the first local heat treatment, of the sample D+P3.

DETAILED DESCRIPTION OF THE INVENTION

(8) Embodiments of the present invention will be disclosed in more detail in connection with the following examples. The examples are to be considered as illustrative and not limiting embodiments.

EXAMPLES

Example 1

(9) Cemented carbide inserts of SNUN 120408 geometry with the compositions shown in Table 1 were manufactured. The cemented carbide inserts were produced using standard powder processes including powder milling and mixing, spray drying, pressing and sintering. The composition shown in Table 1 is based on the weight of the powders as added in the initial milling and mixing step. The grain size of the WC was measured with linear intercept method on a through cut of the sintered insert. The hardness is measured as HV30 in a Vickers indenter on the rake face of the sintered insert.

(10) TABLE-US-00001 TABLE 1 Cemented carbides bodies Body Average Composition grain size Coercivity Co Ti TaNb Cr WC of WC Hardness HCj ID [wt %] [wt %] [wt %] [wt %] [wt %] [μm] [HV30] [kAm−1] A 6 —  0.16 — 93.84 0.9 1600 16.5 B 7 — — 0.3 92.7 0.65 1700 23 C 10 — 0.5 0.4 89.1 0.6 1610 20.5 D 10.2 — 1.5 — 88.3 1.1 1320 12.3 E 25.5 — — — 74.5 4 900 5

(11) The cemented carbide bodies A, B, C, D and E were laser treated to initiate the formation of the local hardened surface area, here also called the surface portion. A first local heat treatment, here a laser heat treatment, was performed with a Medicom LD50s Nd:YAG laser with a wave length of 1064 nm and a maximum power of 50 W. An area of about 2 mm.sup.2 was treated at each cemented carbide body. The used settings of the laser at the different treatments T1, T2, T3, T4, T5 and P3 are shown in Table 2.

(12) TABLE-US-00002 TABLE 2 Laser settings Laser settings T1 T2 T3 T4 T5 P3 Pulse length 120 120 120 120 120 120 (ns) Spot diameter 0.1 0.1 0.1 0.1 0.1 0.1 (mm) Power (W) 23.4 19.5 15.55 11.63 7.7 38.5 Frequency (Hz) 3000 3000 3000 3000 3000 15000 Speed (mm/s) 270 270 270 270 270 600 Pulse energy 7.1 6.5 5.18 3.88 2.6 2.6 (mJ) Overlap (%) 10 10 10 10 10 60 Passes (No) 1 1 1 1 1 4 Environment Air Air Air Air Air Air (Room (Room (Room (Room (Room (Room temp) temp) temp) temp) temp) temp)

(13) The settings of the laser are selected to be high enough to give a melting of the WC grains, while low enough not introduce increased surface roughness or cracks to the surface. If the overlap is too high, the crack density will be unacceptable high.

(14) The laser treated samples were subsequently heat treated in a second heat treatment. The purpose of the second heat treatment is to let the WC grains re-nucleate and grow to a suitable grain size, and to let the metallic binder phase diffuse into the laser treated portion to increase its toughness. The shape of the re-nucleated WC grains may be plate-like and/or needle-like.

(15) The second heat treatment was performed by placing the laser treated samples D+P3 in a PVA-Tepla COV 231R Machine. In a first step the temperature was increased to 350° C. in H.sub.2 atmosphere. In a subsequent second step the temperature was increased by 10° C. per minute up to the temperature T(soak) in Ar atmosphere and 90 mBar. In a subsequent third step the temperature was held at the temperature T(soak) in Ar protective atmosphere in 90 mbar for 1 hour. In a fourth and final step the temperature was lowered and the samples were cooled down to room temperature. A second heat treatment was also done in 1 atm H.sub.2 at 800° C. and 900° C., but in this case no WC grains were visible.

(16) Table 3 shows an evaluation of temperature T(soak) for second heat treatment. The temperature T(soak) was applies for 1 hour. It was found that 1100° C. was a suitable temperature to use based on the size of the WC grains recrystallized and on the hardness. The micro hardness of the surface layer was measured on samples with a micro hardness indenter Fischerscope with the load 15 mN/20 seconds with the method 15014577. Calibration was made with a WC/Co standard sample with HUpI (300 mN/60 s) of 25521 N/mm.sup.2.

(17) TABLE-US-00003 TABLE 3 Hardness of surface portion after second heat treatment Micro hard- Temper- ness surface ature portion, T(soak) Grain size of WC in HUpl Sample (° C.) surface portion (N/mm2) D + P3 + T(soak)600 600 No WC grains visible 25590 D + P3 + T(soak)700 700 No WC grains visible 24710 D + P3 + T(soak)800 800 Ultrafine WC grains, 28320 difficult to measure D + P3 + T(soak)900 900 Very fine WC grains, 26690 difficult to measure D + P3 + T(soak)1000 1000 Very fine WC grains, 29070 difficult to measure D + P3 + T(soak)1100 1100 Fine WC grains 35050 D + P3 + T(soak)1200 1200 Fine WC grains 29090 D + P3 + T(soak)800 800 No WC grains visible — (H.sub.2) D + P3 + T(soak)900 900 No WC grains visible — (H.sub.2)

(18) The thickness of the surface portion was measured at cross section image, after a second heat treatment of T(soak) 1100° C. The results are shown in Table 4.

(19) TABLE-US-00004 TABLE 4 Thickness of surface portion Laser parameter/Average thickness of surface portion [μm] SAMPLE T1 T2 T3 T4 T5 P3 A + T (soak) 1100 0.89 0.94 1.11 1.07 0.8 — B + T (soak) 1100 1.51 1.35 0.78 0.89 1.06 — C + T (soak) 1100 1.12 1.02 1.15 1.4 1.055 — D + T (soak) 1100 1.07 0.77 0.9 0.87 0.81 1.35 E + T (soak) 1100 0.91 1.08 1.12 0.88 0.89 —

(20) The surface portion for the samples A-E heat treated with T5 and sample D+P3 was analyzed with regards to WC grain size. The WC grain length, lg, and thickness, tg, of the surface portion were measured with the following method: Four SEM micrographs of 50.000× magnification of a surface of the surface portion was provided for the same sample. A straight line was drawn in the micrograph that represented a distance at the surface of the sample of 7 μm. The thickness, tg, and the length, lg, of each WC grain that was hit by the line was measured. The procedure was repeated on the second micrograph of the same sample. An median thickness, tg, and an median length, lg, for this specific surface portion were calculated. The ratio of lg/tg was calculated for each sample. The results are shown in Table 5.

(21) TABLE-US-00005 TABLE 5 WC grain size of surface portion and bulk portion Median Median WC grain WC grain Average thickness, length, WC grain tg, surface lg, surface size bulk portion portion Ratio portion Sample [nm] [nm] lg/tg [nm] A + T5 + T(soak) 1100 35 165 4.7 900 B + T5 + T(soak) 1100 33 173 5.2 650 C + T5 + T(soak) 1100 36 183 5.1 600 D + T5 + T(soak) 1100 32 162 5.1 1100 D + P3 + T(soak) 1100 43 256 6.0 1100 E + T5 + T(soak) 1100 33 214 6.5 4000

Example 2

(22) Sample D+P3+T(soak)1100 of Example 1, but with a milling geometry SPKN 1203EDER, were PVD coated with a coating comprising an inner layer of 0.3 μm thick TiN, an intermediate layer of 4 μm thick nano layered TiAlN and an outermost layer of 0.5 μm TiN, all deposited with arc evaporation.

(23) The PVD coated samples were evaluated in two separate dry milling cutting test in a work piece material of C60. The cutting parameters are shown in Table 6. A subsequent study of the cutting edges after the tests showed that no coating flaking occurred during the cutting.

(24) TABLE-US-00006 TABLE 6 Cutting parameters Cutting parameter Milling test 1 Milling test 2 Cutting speed, Vc (m/min) 280 320 Feed, Fz (mm/tooth) 0.25 0.15 Depth of cut, ap (mm) 2.5 2.5 Cutting fluid No No

Example 3

(25) Sample D+P3+T(soak)1100 of Example 1, but with turning geometry CNMG 120408, were CVD coated with CVD coating comprising an inner layer of 0.3 μm TiN, a layer of 3.5 μm MT-TiCN, a bonding layer of 0.2 μm TiCNO and an outermost layer of 3.5 μm Al.sub.2O.sub.3. The CVD coated samples were successfully blasted in a wet blasting process with alumina. The CVD coated samples were evaluated in a turning cutting test against a work piece material of AISI 316L. No cutting fluid was used. The cutting parameters were: Cutting speed: 200 m/min, Feed: 0.25 mm/tooth and Depth of cut: 1.5 mm. No coating flaking was observed. The life time of the coated insert comprising a surface portion in accordance with the present invention was comparable to the lifetime of the references, i.e. the coated inserts that was not provided with a surface portion.

(26) 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. Furthermore, it should be recognized that any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the appended claims appended hereto.