Coated turning insert and method of making it

Details Coated turning insert and method of making it Coated turning insert and method of making it

1998-09-01

2001-03-13

Turner, Archene (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S336000, C428S469000, C428S472000, C428S697000, C428S698000, C428S699000, C428S701000, C428S702000, C427S255120, C427S255290, C427S255700, C051S295000, C051S307000, C051S309000

Reexamination Certificate

active

06200671

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for wet turning of toughness-demanding stainless steel components like square bars, flanges and tubes, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces.
When turning stainless steels with cemented carbide tools, the cutting edge is worn according to different wear mechanisms, such as adhesive wear, chemical wear, abrasive wear and by edge chipping caused by cracks formed along the cutting edge, the so-called comb cracks.
Different cutting conditions require different properties of the cutting insert. For example, when cutting in steels with raw surface zones, a coated cemented carbide insert must consist of a tough carbide and have very good coating adhesion. When turning in stainless steels, the adhesive wear is generally the dominating wear type.
Measures can be taken to improve the cutting performance with respect to a specific wear type. However, very often such action will have a negative effect on other wear properties.
So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented carbide grades have therefore been optimised with respect to one or few of the wear types and hence to specific application areas.
Swedish patent application 9503056-5 which corresponds to U.S. Ser. No. 08/703,965, herein incorporated by reference, discloses a coated cutting insert particularly useful for turning in hot and cold forged low alloyed steel components. The inserts is characterised by a cemented carbide substrate consisting of Co—WC and cubic carbides having a 15-35 &mgr;m thick surface zone depleted from cubic carbides, a coating including a layer of TiC
x
N
y
O
z
with columnar grains, a layer of smooth, fine grained &kgr;-Al
2
O
3
, and preferably an outer layer of TiN.
Swedish patent application 09/077,360 9504304-8 which corresponds to U.S. Ser. No. 09/077,360 filed concurrently herewith discloses a coated cutting insert particularly useful for wet and dry milling of low and medium alloyed steels. The insert is characterised by a cemented carbide substrate consisting of Co—WC and cubic carbides, a coating including a layer of TiC
x
N
y
O
z
with columnar grains, a layer of smooth, fine grained &kgr;-Al
2
O
3
and preferably an outer layer of TiN.
It has now been found that combinations of the substrates and coatings described in the above patent applications give rise to excellent cutting performance in stainless steels turning. A cemented carbide substrate with a cubic carbide depleted surface zone combined with a coating in accordance with patent application, 9503056-5, which corresponds to U.S. Ser. No. 08/703,965, has been found to be especially suitable for high speed turning in easy stainless steel, such as turning of machineability improved 304L, In more difficult work piece materials such as 316-Ti, and in operations with a high degree of thermal cycling such as turning of square bars a straight WC—Co substrate of the type described in patent application 09,077,360 9504304-8 which corresponds to U.S. Ser. No. 09/077,360 filed concurrently herewith, has been found the most suitable.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide a coated cutting tool particularly useful for wet turning of toughness-demanding stainless steel components.
In one aspect of the invention there is provided a cutting tool insert particularly for turning of steel comprising a cemented carbide body and a coating wherein said cemented carbide body contains WC, 6-15 wt-% Co and 0.2-1.8 wt-% cubic carbides of Ti, Ta and/or Nb and a highly W-alloyed binder phase with a CW-ratio of 0.78-0.93 and said coating comprises
a first (innermost) layer of TiC
x
N
y
O
z
with a thickness of <1.5 &mgr;m, and with equiaxed grains with size <0.5 &mgr;m
a second layer of TiC
x
N
y
O
z
with a thickness of 2-5 &mgr;m with columnar grains with an average diameter of <5 &mgr;m and
an outer layer of a smooth, fine-grained (0.5-2 &mgr;m) &kgr;-Al
2
O
3
with a thickness of 0.5-6 &mgr;m.
In another aspect of the invention, there is provided a method of making an insert for turning comprising a cemented carbide body and a coating wherein a WC—Co-based cemented carbide body with a highly W-alloyed binder phase with a CW-ratio of 0.78-0.93 is coated with
a first (innermost) layer of TiC
x
N
y
O
z
with x+y+z=1, with a thickness of 0.1-1.5 &mgr;m, with equiaxed grains with size <0.5 &mgr;m using known CVD-methods
a second layer of TiC
x
N
y
O
z
with x+y+z=1, with a thickness of 2-8 &mgr;m with columnar grains with a diameter of about <5 &mgr;m deposited by MTCVD-technique, using acetonitrile as the carbon and nitrogen source for forming the layer in a preferred temperature range of 850-900° C. and
a layer of a smooth &kgr;-Al
2
O
3
with a thickness of 0.5-6 &mgr;m.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
A turning tool insert according to the invention useful for turning of steel consists of a cemented carbide substrate with a highly W-alloyed binder phase and with a well-balanced chemical composition and grain size of the WC, a columnar TiC
x
N
y
O
z
-layer, a &kgr;-Al
2
O
3
-layer, a TiN-layer and optionally followed by smoothening the cutting edges by brushing the edges with, e.g., a SiC-based brush.
The cobalt binder phase is highly alloyed with W. The content of W in the binder phase can be expressed as the CW-ratio=Ms/(wt % Co·0.0161), where M
s
is the measured saturation magnetization 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 low CW-value corresponds to a high 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 0.78-0.93.
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 6-15 wt % Co, preferably 9-12 wt % Co, most preferably 10-11 wt % Co, 0.2-1.8 wt % cubic carbides, preferably 0.4-1.8 wt % cubic carbides, most preferably 0.5-1.7 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 IVb, Vb or VIb of the periodic table. The content of Ti is preferably on a level corresponding to a technical impurity. The preferred average grain size of the WC depend on the binder phase content. At the preferred composition of 10-11 wt-% Co, the preferred grain size is 1.5-2 &mgr;m, most preferably about 1.7 &mgr;m. The CW-ratio shall be 0.78-0.93, preferably 0.80-0.91, and most preferably 0.82-0.90. The cemented carbide may contain small amounts, <1 volume %, of &eegr;-phase (M
6
C), without any detrimental effect. From the CW-value it follows that no free graphite is allowed in the cemented carbide substrate according to the present embodiment.
The coating comprises
a first (innermost) layer of TiC
x
N
y
O
z
with x+y+z=1, preferably z<0.5, with equiaxed grains with size <0.5 &mgr;m and a total thickness <1.5 &mgr;m and preferably >0.1 &mgr;m.
a layer of TiC
x
N
y
O
z
with x+y+z=1, preferably with z=0 and x>0.3 and y>0.3, with a thickness of 1-15 &mgr;m, preferably 2-8 &mgr;m, with columnar grains and with an average diameter of <5 &mgr;m, preferably 0.1-2 &mgr;m. Most preferred thickness of the TiC
x
N
y
O
z
layer is 2-5 &mgr;m, particularly in extremely edgeline-toughness demanding work-piece materials such as Ti-stabilized stainless steel.
a layer of a smooth, fine-grained (grain size about 0.5-2 &mgr;m) Al
2
O
3
consisting essentially of the &kgr;-phase. However, the layer may

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