Cemented carbide insert and method of making same

The present application is a reissue of U.S. Pat. No. 6 , 326 , 093 B 1 , which claims the benefit of priority to Swedish Application No. 9903122 - 1 filed Sep. 6 , 1999 .

1. Field of the Invention

The present invention relates to a coated cemented carbide cutting tool, such as a cutting tool insert for the turning of stainless steels, particularly in applications with high demands on the toughness properties of the insert. Additionally, the insert is provided not only for the turning of stainless steels with different compositions and microstructures, such as austenitic, ferritic, ferriteaustenitic, superaustenitic and precipitation hardened stainless steels, but also for the turning of non-stainless steels, such as low carbon steels and low and medium alloyed steels.

2. State of the Art

In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.

It is well known that for cemented carbide cutting tools used in the machining of steels, the cutting edge is worn by different wear mechanisms. Examples of such mechanisms include chemical and abrasive wear. Additionally, external variables, such as cutting speed, depth of cut, and cutting feed rate, and external cutting operations, such as the use of coolant, and off-centered work piece, and a cast skin on a work piece, require a number of different properties of the cutting edge. As an examples, the tool edge may fracture under a heavy intermittent cutting load resulting in so-called edge chipping. The chipping is due to a lack of edge toughness. Furthermore, when turning stainless steel, still another wear mechanism is active called adhesive wear. This type of wear is caused by the adhesive force between the stainless steel chip of the workpiece and the cutting edge. When the adhesive force grows large enough, edge chipping on the cutting edge will occur and, hence, the tool life will be shortened. Also, when utilizing a cemented carbide cutting tool for the turning of a stainless steel part when increasing the cutting speed in a stainless steel grade, the thermal energy developed in the cutting edge is considerable and the entire tool edge may plastically deform. This type of wear mechanism is known as plastic deformation wear and is in clear conflict with the edge toughness required to prevent or minimize edge chipping. Hence, another requirement of the coated cemented carbide insert is that the selection of the carbide composition and the coating material results in a cutting edge exhibiting a high resistance to plastic deformation.

Commercial cemented carbide tools suitable for the machining of stainless steels, such as those described in SE 9602413-8 and SE 9901149-6, are usually optimized with respect to some of the required tool properties mentioned above, i.e. high resistance to mechanical, chemical, abrasive, adhesive, thermal and plastic deformation wear.

Excellent cutting performance in the turning of stainless steels with concomitant high toughness demands and high resistance to plastic deformation, as is especially encountered in heavy intermittent cutting conditions, can be obtained by a combination of a coated cemented carbide body comprising a WC based substrate with small additions of cubic carbides, W alloyed Co binder and with a specific grain size range of the WC grains, a specific composition range of WC+Co and a coating including an innermost, very thin layer of TiN, a second layer of TiAlN with a periodic variation of the Ti/A1 ratio along the normal to the substrate/coating interface, and an outermost layer of TiN.

A coated cemented carbide cutting tool and a method of making the same is provided. The cutting tool is comprised of a WC-Co based cemented carbide body and a hard and wear resistant coating. The cemented carbide body is comprised of a composition of 9-12 wt % Co, 0.2-2.0 wt % cubic carbides from elements from group IVa, Va or VIa of the periodic table, preferably 1.2-1.8 wt % cubic carbides of the metals Ta, Nb and Ti, and the balance WC with an average grain size of the WC of 1.5-2 μm and a W-alloyed binder phase with a CW-ratio in the range of 0.77-0.95. Deposited on the WC-Co cemented carbide body is a refractory multilayered coating. The first innermost layer is 0.1-0.5 μm of TiN. A second layer is comprised of a multilayered structure of 8-30 sublayers each with a thickness of 0.05-0.2 μm and of the composition (Ti x Al 1−x )N in which x varies alternately and repeatedly between the two ranges 0.45<x<0.55 and 0.70<x<0.80, respectively. After the multilayered structure, a third layer of (Ti x Al 1−x )N, where x is found in the range 0.45<x<0.5, is deposited to a thickness of at least 0.2 μm. A fourth and outermost layer of TiN is deposited to a thickness of 0.1-0.2 μm. The total coating thickness is in the range of 2-9 μm and the thickness of the second layer constitutes 75-95 % of the total coating thickness.

According to the present invention, a turning tool insert particularly useful for difficult stainless steel turning is provided with a cemented carbide substrate with a composition of 9-12 wt % Co, preferably 10-11 wt % Co, 0.2-2.0 wt % cubic carbides, preferably 1. 2-1.8 wt % cubic carbides of the metals Ta, Nb and Ti and balance WC. The cemented carbide may also contain other carbides from elements from group IVa, Va or VIa of the periodic table. The content of Ta is preferably over 0.8 wt %. The preferred average grain size of the WC at the preferred composition of 10-11 wt-% Co is 1.5-2 μm, most preferably between 1.6 and 1.8 μm. The centered carbide may contain small amounts, <1 volume %, of η-phase (M 6 C), without any detrimental effect.

The cobalt binder phase is alloyed with W. The content of W in the binder phase can be expressed as the CW-ratio:

CW-ratio=M s /(wt % Co * 0.0161)

where M s is the measured saturation magnetisation of the cemented carbide substrate in kA/m hAm 2 /kg and wt % Co is the weight percentage of Co in the cemented carbide. The CW-value is a function of the W content in the Co binder phase. A high CW-value corresponds to a low W-content in the binder phase. According to the present invention, improved cutting performance is achieved if the cemented carbide substrate has a CW-ratio of 0.77-0.95, preferably 0.82-0.921. From the CW-value it follows that no free graphite is allowed.

The hard and wear resistant refractory coating deposited on the above described cemented carbide substrate according to the present invention comprises a first (innermost) thin 0.1-0.5 μm bonding layer of TiN, a second layer comprising a multilayered structure of sublayers of the composition (Ti x Al 1−x )N in which x varies repeatedly between the two ranges 0.45<x<0.55 and 0.70<x<0.80, a third thin, at least 0.2 μm, preferably 0.4-0.8 μm, layer of (Ti x Al 1−x )N having an x-value in the range 0.45<x<0.55 and a fourth outermost thin 0.1-0.2 μm layer of TiN.

The first sublayer of (Ti x Al 1−x )N adjacent to the TiN bonding layer has an x-value in the range 0.45<x<0.55, the second sublayer of (Ti x Al 1−x )N has an x-value in the range 0.70<x<0.80 and the third sublayer has x in the range 0.45<x<0.55 and so forth repeated until 8-30 sublayers, preferably 22-24 sublayers, are formed. The thickness of this second layer comprising a multilayered structure of sublayers constitutes 75-95% of the total coating thickness. The individual sublayers of (Ti x Al 1−x )N are essentially of the same thickness but their thickness may also vary in a regular or irregular way and said sublayer thickness is found in the range of 0.05-0.2 μm.

The total thickness of the coating deposited on the cemented carbide substrate according to the present invention may vary in the range of 2-9 μm, preferably 3.5-7 μm. The layer thickness, the sublayer thickness and the coating thickness quoted above refers to measurements made close to the cutting edge, i.e. the functional part of the cutting tool.