Method of making a cemented carbide

The present invention relates to a cemented carbide comprising WC, particularly with submicron grain size, which is bound by means of a second phase of a metallic binder based on Co, Ni or Fe and in addition gamma phase (a cubic carbide phase) of submicron size and a method of making the same.

Cemented carbide grades for metal cutting applications generally contain WC with an average grain size in the range 1-5 µm, gamma phase (a solid solution of at least one of TiC, NbC, TaC ZrC, HfC and VC and substantial amounts of dissolved WC) and 5-15 wt-% binder phase, generally Co. Their properties are optimised by varying the WC grain size, volume fraction of the binder phase and/or the gamma phase, the composition of the gamma phase and by optimising the carbon content.

Cemented carbides with submicron WC grain size structure are today used to a great extent for machining of steels, stainless steels and heat resistant alloys in applications with high demands on both toughness and wear resistance. Another important application is in microdrills for the machining of printed circuit board, so called PCB-drills.

<patcit id="pcit0001" dnum="WO9622399A"><text>WO-A 96/22399</text></patcit> relates to a multi-phase cemented ceramic material including WC, solid solution of (Ti, Ta, W) C and Co-binder.

Submicron grades contain grain growth inhibitors. Common grain growth inhibitors include vanadium, chromium, tantalum, niobium and/or titanium or compounds involving these. When added, generally as carbides, grain growth inhibitors limit grain growth during sintering, but also have undesirable side effects, affecting the toughness behaviour in an unfavourable direction. Additions of vanadium or chromium are particularly detrimental and have to be kept on a very low level in order to limit their negative influence on the sintering behaviour. Both vanadium and chromium reduce the sintering activity often resulting in an uneven binder phase distribution and toughness, reducing defects in the sintered structure. Large additions are also known to result in precipitation of embrittling phases.

In cemented carbides for metal cutting purposes, the quality of a cemented carbide grade is dictated quite substantially by its high-temperature properties. The hardness of the cemented carbides is reduced in some cases dramatically as temperature rises. This applies particularly to submicron grades, which generally have relatively high cobalt content.

A common way of increasing the hot hardness and also the chemical wear resistance of cemented carbides is to add cubic carbides forming a suitable amount of gamma phase. However, when adding submicron cubic carbides such as NbC, TaC, TiC, ZrC and HfC or mixed carbides of the same elements to a submicron cemented carbide the gamma phase formed during sintering will have a grain size of the order of 2-4 µm. Thus, the grain size is not submicron and the beneficial effects of the submicron WC grain size will, to some extent, be lost. The gamma phase formed during sintering is growing by a dissolution and precipitation process and will dissolve substantial amounts of tungsten.

The above also relates to cemented carbide of more coarse grains size, but in this the effect is less pronounced.

It is an object of the present invention to provide a method of making cemented carbide preferably with submicron grain size containing, preferably submicron, gamma phase.

It has now surprisingly been found that alloying a submicron cubic carbide raw material with WC results in a submicron gamma phase in the sintered material.

The amount of WC dissolved in the gamma phase in equilibrium with the hexagonal WC at a temperature of 1450°C, a typical sintering temperature, for Ti, Nb and Ta based gamma phase has been experimentally determined by <nplcit id="ncit0001" npl-type="s"><text>Chatfield ("The gamma/WC solubility boundary in the quaternary TiC-NbC-TaC-WC system at 1723K", J. Mat. Sci., Vol 21 (1986), No 2, pp 577-582</text></nplcit>). The equilibrium solubility of WC in the gamma phase expressed as mol fraction, xe_WC, can with a good accuracy be expressed by the following equation:<maths id="math0001" num="(1)"><math display="block"><msub><mi>xe</mi><mi>WC</mi></msub><mo>=</mo><mfenced separators=""><mn mathvariant="normal">0.383</mn><mo>*</mo><msub><mi mathvariant="normal">x</mi><mi>TiC</mi></msub><mo>+</mo><mn mathvariant="normal">0.117</mn><mo>*</mo><msub><mi mathvariant="normal">x</mi><mi>NbC</mi></msub><mo>+</mo><mn mathvariant="normal">0.136</mn><mo>*</mo><msub><mi mathvariant="normal">x</mi><mi>TaC</mi></msub></mfenced><mo>/</mo><mfenced separators=""><msub><mi mathvariant="normal">x</mi><mi>TiC</mi></msub><mo>+</mo><msub><mi mathvariant="normal">x</mi><mi>NbC</mi></msub><mo>+</mo><msub><mi mathvariant="normal">x</mi><mi>TaC</mi></msub></mfenced></math><img file="EP1526189B1_D0001.tif" /></maths>

The amount of WC in the prealloyed cubic carbide raw material, x_WC, can be related to the equilibrium amount by the equation:<maths id="math0002" num="(2)"><math display="block"><msub><mi mathvariant="normal">x</mi><mi>WC</mi></msub><mo>=</mo><msub><mi mathvariant="normal">f</mi><mi>WC</mi></msub><mo>*</mo><msub><mi>xe</mi><mi>WC</mi></msub></math><img file="EP1526189B1_D0002.tif" /></maths>

The factor f_WC is the ratio between the WC content in the cubic carbide raw material and the WC solubility in the gamma phase and f_WC must be ≤ 1 in order to avoid decomposition of the gamma phase at the sintering temperature. A person skilled in the art can derive equations similar to equation (1) from experimental data available in the literature on the WC solubility at typical sintering temperatures for other mixed cubic carbides based on different combinations of TiC, TaC, NbC, ZrC, HfC and VC.

<figref idref="f0001">Fig. 1</figref> shows in 10000 X a Scanning electron micrograph of the microstructure of a submicron cemented carbide according to the invention.

<figref idref="f0001">Fig. 2</figref> shows in 10000 X a Scanning electron micrograph of the microstructure of a comparative submicron cemented carbide.

In <figref idref="f0001">Fig. 1 and 2</figref>    A - WC    B - gamma phase and    C - binder phase.

<figref idref="f0002">Fig. 3a, b and c and Fig. 4a, b and c</figref> show in about 10x the wear pattern of a reference insert and that of an insert made according to the invention.

According to the invention there is now provided a method for providing cemented carbide comprising WC, a binder phase based on Co, Ni or Fe and a submicron gamma phase. The binder phase content is 3-15 wt-%, preferably 6-12 wt-% and the amount of gamma phase is 3-25 vol-%, preferably 5-15 vol-% with an average grain size of &lt;1 µm, preferably &lt;0.8 µm. The ratio between the WC content in the cubic carbide raw material and the WC solubility in the gamma phase (the factor f_WC defined in equation (2)) is 0.6-1.0, preferably 0.8-1.0. Preferably the average WC grain size is &lt;1 µm, most preferably &lt;0.8 µm.

The present invention relates to a method of making a cemented carbide comprising WC, a binder phase based on Co, Ni or Fe and gamma phase by powder metallurgical methods wet milling powders forming hard constituents and binder phase, drying, pressing and sintering to bodies of desired shape and dimension. According to the invention the powders forming gamma phase, preferably with submicron grain size, are added as a cubic mixed carbide (Me,W)C, wherein Me is one or more of Ti, Ta, Nb, Zr, Hf and V, preferably wherein Me is one or more of Ti, Ta, and Nb, alloyed with an amount of WC given by the mol fraction of WC, x_WC, such that the ratio between x_WC and the equilibrium gamma phase WC content at the sintering temperature expressed as mol fraction WC, xe_WC, f_WC=x_WC/xe_WC is 0.6-1.0, preferably 0.8-1.0. The WC solubility at the sintering temperature for a (Ti,Ta,Nb,W)C cubic mixed carbide is given by the relation:<maths id="math0003" num=""><math display="block"><msub><mi>xe</mi><mi>WC</mi></msub><mo>=</mo><mfenced separators=""><mn mathvariant="normal">0.383</mn><mo>*</mo><msub><mi mathvariant="normal">x</mi><mi>TiC</mi></msub><mo>+</mo><mn mathvariant="normal">0.117</mn><mo>*</mo><msub><mi mathvariant="normal">x</mi><mi>NbC</mi></msub><mo>+</mo><mn mathvariant="normal">0.136</mn><mo>*</mo><msub><mi mathvariant="normal">x</mi><mi>TaC</mi></msub></mfenced><mo>/</mo><mfenced separators=""><msub><mi mathvariant="normal">x</mi><mi>TiC</mi></msub><mo>+</mo><msub><mi mathvariant="normal">x</mi><mi>NbC</mi></msub><mo>+</mo><msub><mi mathvariant="normal">x</mi><mi>TaC</mi></msub></mfenced><mn>.</mn></math><img file="EP1526189B1_D0003.tif" /></maths>

A person skilled in the art can derive similar equation from experimental data available in the literature on the WC solubility at typical sintering temperatures for other mixed cubic carbides.

In a preferred embodiment the WC-powder is also submicron. Cemented carbide bodies according to the invention can be provided with thin wear resistant coatings as known in the art.

Example 1

(invention)

Cutting tool inserts type N123G2-0300-0003-TF were made by wet milling of 1.75 kg WC with an FSSS grain size of 0.8 µm, 0.2 kg Co-powder and 0.04 kg of a (Ti,Ta,W)C powder with a composition expressed as mol fraction of x_TiC=0.585, x_TaC=0.119 and x_WC=0.296, corresponding to f_WC=0.867 and an FSSS grain size of 0.6 µm, drying, pressing and sintering at 1410 °C for 1 h. The microstructure is shown in <figref idref="f0001">Fig. 1</figref>. It consists of 16 vol-% Co (annotated as C), 77 vol-% submicron WC (annotated as A) and 7 vol-% gamma phase (annotated as B) with a grain size of 0.7 µm.

Example 2

(comparative)

Example 1 was repeated but the gamma phase forming elements were added as single carbides, i.e. TiC and TaC to the same composition. The corresponding microstructure is shown in <figref idref="f0001">Fig. 2</figref>, in which A indicates WC, B indicates gamma phase, and C indicates binder phase. The gamma phase B is present as large areas with a size of about 3 µm.

Example 3

Cutting inserts from examples 1 and 2 were tested in grooving of steel SS2541, Cutting speed VC= 200 m/min, feed/rev =0.2 mm and depth of cut 10 mm. As a reference cutting inserts of Sandvik Coromant grade GC1025 consisting of 0.8 µm WC and 10 wt-% Co were used. The inserts from example 1 and 2 and the reference inserts were PVD coated in the same batch with (TiAl)N+TiN according to the art.

<figref idref="f0002">Fig. 3</figref> shows the wear pattern of a reference insert and <figref idref="f0002">Fig. 4</figref> shows the wear on an insert made according to the invention. The insert from example 2 broke after 25 passes, the reference insert broke after 52 passes and the insert according to the invention after 82 passes.