EP1222316B1

EP1222316B1 - Coated cemented carbide insert

Info

Publication number: EP1222316B1
Application number: EP00961293A
Authority: EP
Inventors: Mikael Lindholm; Anders Lenander; Per Lindskog
Original assignee: Sandvik AB
Current assignee: Sandvik AB
Priority date: 1999-09-06
Filing date: 2000-09-05
Publication date: 2004-06-16
Anticipated expiration: 2020-09-05
Also published as: IL147781A; IL147781A0; EP1222316A1; JP2003508637A; DE60011653T2; DE60011653D1; ATE269424T1

Description

The present invention describes coated cemented carbide cutting tool inserts for the turning of stainless steels, particularly in applications with high demands of toughness behaviour. It could be for turning of stainless steels with different composition and microstructure 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.

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 such as chemical and abrasive wear but except for cutting data such as cutting speed, depth of cut, cutting feed rate also external cutting conditions such as use of coolant, off-centred work piece, cast skin on work piece etc. require a number of different properties of the cutting edge. The tool edge may for example fracture under a heavy intermittent cutting load resulting in so-called edge chipping. The chipping is due to lack of edge toughness. Furthermore, when turning a stainless steel, still another wear mechanism is active called adhesive wear, which is caused by the adhesive force between the stainless steel chip 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 utilising 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. 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 are usually optimised 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 of a tough cemented carbide substrate coated with a wear resistant and an adherently bonded coating e.g. cemented carbide tools suitable for the turning of stainless steels SE 9602413-8 (WO-A-9720082) and milling of stainless steel SE 9901149-6 (EP-A-1038989).

Multilayer coating of the type Ti_xAl_1-xN or TiN-T_1-x AlxN are already known in the art as disclosed in EP-A-701982 or in WO-A-9848072.

It has now been found that excellent cutting performance in the turning of stainless steels with high toughness demands at the same time as high resistance to plastic deformation especially in heavy intermittent cutting conditions can be obtained by 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/Al ratio along the normal to the substrate/coating interface, and an outermost layer of TiN.

The cobalt binder phase is alloyed with W. The content of W in the binder phase can be expressed as the CW-ratio= M_S / (wt% Co * 0.0161), where M_S is the measured saturation magnetisation of the cemented carbide substrate in kA/m 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 0.82-0.92.

According to the present invention defined in claim 1 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 cemented carbide may contain small amounts, <1 volume %, of η-phase (M₆C), without any detrimental effect. 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 layer of TiN
a second layer comprising a multilayered structure of sublayers of the composition (Ti_xAl_1-x)N in which x varies repeatedly between the two ranges 0.45<x<0.55 and 0.70<x<0.80. The first sublayer of (Ti_xAl_1-x)N adjacent to the TiN bonding layer having an x-value in the range 0.45<x<0.55, the second sublayer of (Ti_xAl_1-x)N having an x-value in the range 0.70<x<0.80 and the third sublayer having 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_xAl_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 each 0.05-0.2 µm.
a third thin at least 0.2, preferably 0.4-0.8 µm layer of (Ti_xAl_1-x)N having an x-value in the range 0.45<x<0.55.
a fourth outermost thin 0.1-0.2 µm layer of TiN.

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.

The present invention according to claim 9 also relates to a method of making the above mentioned coated cutting tool insert comprising a WC-Co based cemented carbide body including an amount lower than 2.0 wt% of cubic carbides and with a composition of WC/Co in the range of 9-12 wt% Co, preferably 10-11 wt% Co and most preferably 10.2-10.8 wt% Co, and an amount of cubic carbides in the range of 0.2-2.0 wt%, and the balance is made up by WC. The average WC grain size is found in the range of 1.5-2.0 µm, preferably 1.6-1.8 µm.

The hard and wear resistant refractory coating is deposited onto the cemented carbide substrate by applying conventional PVD (Physical Vapor Deposition) methods and according to the present invention said coating 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_xAl_1-x)N in which x varies repeatedly between the two ranges 0.45<x<0.55 and 0.70<x<0.80. The first sublayer of (Ti_xAl_1-x)N adjacent to the TiN bonding layer having an x-value in the range 0.45<x<0.55, the second sublayer of (Ti_xAl_1-x)N having an x-value in the range 0.70<x<0.80 and the third sublayer having 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_xAl_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 each.
a third thin at least 0.2, preferably 0.4-0.8 µm layer of (Ti_xAl_1-x)N having an x-value in the range 0.45<x<0.55.
a fourth (outermost) thin 0.1-0.2 µm layer of TiN.

Example 1.

A. Cemented carbide turning inserts according to the invention with the composition 10.5 wt% Co, 1.16 wt% Ta, 0.28 wt% Nb and balance made up by WC and with an average WC grain size of 1.7 µm, with a binder phase alloyed with W corresponding to a CW-ratio of 0.90, were coated with a total coating thickness of 4.5 µm by applying conventional PVD cathodic arc technique. The coating comprised a first (innermost) 0.2 µm layer of TiN followed by a 3.7 µm thick second layer comprising 23 alternating sublayers of (Ti_xAl_1-x)N, where x alternatively varied between 0.5 and 0.75, and a third 0.4 µm (Ti_xAl_1-x)N, where x= 0.5, and, finally, an outermost 0.2 µm layer of TiN.

B. A commercial cemented carbide turning insert with the composition 8.0 wt% Co, and balance made up by WC with an average WC grain size of 3.0 µm, with a binder phase low alloyed with W corresponding to a CW-ratio of 0.94, were coated with a 2.8 µm thick conventional multilayered CVD-coating according to the sequence TiN/TiCN/TiCN/TiN. The sublayers had approximately the same thickness.

Operation	Alternating axial and facial turning of an 80 mm diameter bar.
Work-piece material	Austenitic stainless steel (AISI316L)
Cutting speed	160 m/min
Feed rate	0.4 mm/rev
Depth of cut	2 mm
Insert-style	CNMG120408-MM
Results		Tool life (cycles):
Inserts A	(invention)	5
Inserts B	(prior art)	2
Comment: The main wear mechanism was plastic deformation of the cutting edge line resulting in a flank wear. Tool life criterion was flank wear >0.3 mm.

Example 2.

Inserts from A and B were tested in a turning operation.

Operation	Facing of a 130 mm bar with two flat sides
Work-piece material	Austenitic stainless steel (SS2343)
Cutting speed	190 m/min
Feed rate	0.3 mm
Depth of cut	2 mm
Insert-style	CNMG120408-MM
Results		Tool-life (cycles)
Inserts A	(invention)	14
Inserts B	(prior art)	6
Comment: The performance criterion was bad cutting action. The insert B shows more chipping along the cutting edge caused by more adhesion to the chips during the operation.

Example 3.

Inserts from A and B were tested in a turning operation.

Operation	External turning of a ring with 18 mm thickness.
Work-piece material	Austenitic stainless steel (AISI316L)
Cutting speed	140 m/min
Feed rate	0.35 mm
Depth of cut	4 mm
Insert-style	CNMG190616-MR
Results		Tool-life (cycles)
Inserts A	(invention)	40
Inserts B	(prior art)	29
Comment: The tool-life criterion was spalling on work piece caused by edge chipping due to lack of edge toughness.

Example 4.

Inserts from A and B were tested in a turning operation.

Operation	Longitudinal turning with Interrupted cuts.
Work-piece material	Steel (SS1312)
Cutting speed	80 m/min
Feed rate	0.4 mm
Depth of cut	2 mm
Insert-style	CNMG120408-MM
Results		Tool-life (minutes)
Inserts A	(invention)	10
Inserts B	(prior art)	5
Comment: The tool-life criterion was edge breakage. Insert A ran a full cycle. Since the difference is very big you can consider insert A to have a better toughness behaviour.

Example 5.

Inserts from A and B were tested in a turning operation.

Operation	Alternating facing and turning of a bar 70 mm in diameter.
Work-piece material	Ferriteaustenitic steel (SAF2205)
Cutting speed	100 m/min
Feed rate	0.3 mm
Depth of cut	2 mm
Insert-style	CNMG120408-MM
Results		Tool-life (cycles)
Inserts A	(invention)	11
Inserts B	(prior art)	4
Comment: The tool-life criterion was bad cutting action due to edge frittering, which is due to lack of edge toughness.

Example 6.

Inserts from A and B were tested in a turning operation.

Operation	External turning of a bar 120 mm in diameter.
Work-piece material	Steel (SS2541)
Cutting speed	110 m/min
Feed rate	0.6 mm
Depth of cut	2 mm
Insert-style	CNMG120408-MM
Results		Edge depression (mm)
Inserts A	(invention)	0.035
Inserts B	(prior art)	0.085
Comment: Tool-life criterion was predetermined time in cut (0.5 min). The depression was caused by lack of resistance to plastic deformation.

Claims

A coated cemented carbide cutting tool (indexable insert) for wet or dry machining, particularly for applications with high toughness demands, of stainless steels of different composition and microstructure, and of low and medium alloyed non-stainless steels, comprising a WC-Co based cemented carbide body and on said body a hard and wear resistant coating is deposited, characterised in that said cemented carbide body comprises a composition of 9-12 wt% Co in form of W-alloyed binder, 0.2-2.0 wt% cubic carbides from elements from group IVa, Va or VIa of the periodic table and balance WC with an average grain size of the WC is 1.5-2 µm. Further a W-alloyed binder phase with a CW-ratio in the range of 0.77-0.95 whereby CW = ratio = M_s/(wt% Co × 0,0161), where M_s is the measured saturation magnetization of the cemented carbide substrate in kA/m, and that said coating comprises
a first (innermost) thin layer of TiN
a second layer comprising a multilayered structure of each 0.05-0.2 µm thick sublayers of the composition (Ti_xAl_1-x)N in which x varies repeatedly between the two ranges 0.45<x<0.55 and 0.70<x<0.80, the first sublayer of (Ti_xAl_1-x)N adjacent to the TiN bonding layer having an x-value in the range 0.45<x<0.55, the second sublayer of (Ti_xAl_1-x)N having an x-value in the range 0.70<x<0.80 and the third sublayer having x in the range 0.45<x<0.55 and so forth repeated until 8-30 sublayers are being built up
a third at least 0.2 µm thick layer of (Ti_xAl_1-x)N, where x is found in the range 0.45<x<0.55
a fourth (outermost) thin layer of TiN making the total coating thickness vary in the range of 2-9 µm and where the thickness of the second layer constitutes 75-95% of the total coating thickness.
A cutting tool insert according to claim 1 characterised in that the Co content is within 10-11 wt%.
A cutting tool insert according to any of claim 1-2 characterised in containing 1.2-1.8 wt% cubic carbides of the metals Ta, Nb and Ti.
A cutting tool insert according to any of claim 1-3 characterised in that the content of Ta is preferably over 0.8 wt%.
A cutting tool insert according to any of claim 1-4 characterised in that the CW-ratio is 0.82-0.92.
A cutting tool insert according to any of claim 1-5 characterised in that the coating comprises of a first (innermost) thin 0.1-0.5 µm layer of TiN, a second main layer has a multilayered structure of 22-24 sublayers of the composition (Ti_xAl_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 preferably 0.4-0.8 µm layer of (Ti_xAl_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.
A cutting tool insert according to any of claim 1-6 characterised in that coating has a total thickness of 3.5-7 µm.
A cutting tool insert according to any of claim 1-7 characterised in that the average WC grain size is between 1.6 and 1.8 µm.
A method of making a cutting insert comprising a cemented carbide substrate with refractory coating according to any of the above claims characterised in forming a powder mixture containing WC, Co and cubic carbides
mixing said powders with pressing agent and possibly W metal such that the desired CW-ratio is obtained.
milling and spray drying the mixture to a powder material with the desired properties
pressing and sintering the powder material at a temperature of 1300-1500°C, in a controlled atmosphere of about 50 mbar followed by cooling
applying conventional post sintering treatments including edge rounding and
applying a hard, wear resistant coating by PVD-technique comprising
a first (innermost) thin layer of TiN
a second layer comprising a multilayered structure of each 0.05-0.2 µm thick sublayers of the composition (Ti_xAl_1-x)N in which x varies repeatedly between the two ranges 0.45<x<0.55 and 0.70<x<0.80, the first sublayer of (Ti_xAl_1-x)N adjacent to the TiN bonding layer having an x-value in the range 0.45<x<0.55, the second sublayer of (Ti_xAl_1-x)N having an x-value in the range 0.70<x<0.80 and the third sublayer having x in the range 0.45<x<0.55 and so forth repeated until 8-30 sublayers are being built up
a third at least 0.2 µm thick layer of (Ti_xAl_1-x)N, where x is found in the range 0.45<x<0.55
a fourth (outermost) thin layer of TiN making the total coating thickness vary in the range of 2-9 µm and where the thickness of the second layer constitutes 75-95% of the total coating thickness.