EP0822265B1

EP0822265B1 - Nitrogen-containing sintered hard alloy

Info

Publication number: EP0822265B1
Application number: EP97115279A
Authority: EP
Inventors: Keiichi Tsuda; Kazutaka Isobe; Akihiko Ikegaya; Nobuyuki Kitagawa
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1994-05-19
Filing date: 1995-05-18
Publication date: 2001-10-17
Anticipated expiration: 2015-05-18
Also published as: DE69513086D1; KR950032671A; DE69523342D1; EP0687744A3; KR0180522B1; DE69513086T2; DE69523342T2; EP0822265A3; EP0687744A2; EP0687744B1; EP0822265A2

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a nitrogen-containing sintered hard alloy, and more particularly, it relates to a nitrogen-containing sintered hard alloy which is improved in thermal shock resistance, wear resistance and strength for serving as a material for a cutting tool and enabling application to wet cutting.

Description of the Background Art

A nitrogen-containing sintered hard alloy having a hard phase of a carbo-nitride mainly composed of Ti, which is bonded by a metal containing Ni and Co, has already been put into practice as a cutting tool. This nitrogen-containing sintered hard alloy is widely applied to a cutting tool similarly to the so-called cemented carbide which is mainly composed of WC, since the hard phase is extremely fined as compared with a conventional sintered hard alloy which is free from nitrogen to remarkably improve high-temperature creep resistance as the result.
In this nitrogen-containing sintered hard alloy, however, resistance against a thermal shock is reduced for the following reasons:
(i) The thermal conductivity of this nitrogen-containing sintered hard alloy is about half that of the cemented carbide since the thermal conductivity of Ti which is the main component of the carbo-nitride is extremely smaller than that of WC which is the main component of the cemented carbide, and
(ii) the thermal expansion coefficient of the nitrogen-containing sintered hard alloy is about 1.3 times that of the cemented carbide, since this coefficient also depends on the characteristic value of the main component similarly to the thermal conductivity.
In order to solve such problems of the conventional nitrogen-containing sintered hard alloy, various improvements have been attempted as follows: For example, Japanese Patent Laying-Open No. 2-15139 (1990) proposes means of improving surface roughness of a material containing at least 50 percent by weight of Ti in terms of a carbide or the like and less than 40 percent by weight of an element belonging to the group 6A (the group VIB in the CAS version) in terms of a carbide and having an atomic ratio N/(C + N) of 0.4 to 0.6 with a high nitrogen content by controlling the sintering atmosphere, for forming a modified part having high toughness and hardness in a surface layer part. On the other hand, Japanese Patent Laying-Open No. 5-9646 (1993) discloses a cermet which is prepared by sintering a material, which is mainly composed of Ti, containing less than 40 percent by weight of W, Mo and Cr in total in terms of a carbide, and thereafter controlling a cooling step for providing a surface part with a region having a smaller amount of binder phase as compared with the interior, to leave compressive stress on the surface.
However, each of the cermets disclosed in the aforementioned gazettes is insufficient in chipping resistance as compared with the coated cemented carbide, although wear resistance and toughness are improved. Further, the cermet is so inferior in thermal shock resistance that sudden chipping is easily.caused by occurrence of thermal cracking or crack extension resulting from both thermal and mechanical shocks in particular, and sufficient reliability cannot be attained. Although the manufacturing cost for such prior art is reduced due to omission of a coating step, the performance cannot be sufficiently improved. This suggests that improvement in strength against chipping is naturally limited in the category of the so-called cermet which is prepared on the premise that the same contains Ti in excess of a certain degree of amount.
The inventors have made deep study on analysis of cutting phenomenons such as temperature distributions in various cutting operations and arrangements of material components in tools, to obtain the following recognition:
During cutting, a cutting portion is partially exposed to high-temperature environment in a surface part of an insert which is in contact with a workpiece, a part of a rake face which is fretted by chips, and the like. Comparing the cermet with the cemented carbide, the thermal conductivity of the former is about half that of the latter as hereinabove described, and hence heat which is generated on the surface of the cermet is so hardly diffused into the interior that the temperature is abruptly reduced in the interior although the surface is at a high temperature. Once cracking is caused in such a state, the cermet is extremely easily chipped. When the cermet is rapidly quenched with water-soluble cutting oil from a high temperature state or cooled with cutting in lost motion, further, only an extremely small part of its surface is quenched.
Comparing the cermet with the cemented carbide, further, the thermal expansion coefficient of the former is about 1.3 times that of the latter as hereinabove described, and hence tensile stress is caused on a surface layer part to-extremely easily cause thermal cracking. In relation to either characteristic, the cermet is inferior in thermal shock resistance to the cemented carbide.
Comparing the cermet and the cemented carbide having the same grain sizes and the same amounts of binder phases, further, the fracture toughness of the former is reduced by about 30 to 50 % as compared with the latter, and hence crack extension resistance is also reduced in the interior of the alloy.
In the conventional nitrogen-containing sintered hard alloy, as hereinabove described, there are limits to improvement of thermal conductivity, reduction of the thermal expansion coefficient and improvement of crack extension resistance with a large content of Ti which can bring an excellent machined surface and is advantageous in view of the resource.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nitrogen-containing sintered hard alloy which can be employed as a cutting tool in high reliability with no surface coating also in a working region under conditions bringing a strong thermal shock with no requirement for the high-priced coated cemented carbide which has been employed in general.
The problem is solved by the features of claim 1.
The nitrogen-containing sintered hard alloy according to the present invention is provided in its interior with a larger amount of WC as compared with the conventional nitrogen-containing sintered hard alloy in structure, to be improved in resistance against crack extension. When a large amount of WC is blended, WC particles toward the alloy surface appear in the conventional nitrogen-containing sintered hard alloy to provide a tool material called a P-type material, while this tool material is inferior in smoothness of the machined surface. Therefore, this material is also remarkably inferior in abrasive wear resistance to the so-called cermet or coated cemented carbide.
However, it has been proved possible to eliminate WC particles from a soft layer which is present in the outermost surface of the tool, i.e., a surface part up to a specific depth from a portion immediately under the so-called exudation layer, deciding smoothness of the machined surface. Thus, abrasive wear resistance and crater wear resistance can be remarkably improved, while the amount of a binder phase is reduced in the vicinity of the surface layer and a group 6A metal such as W is solidly solved in hard phase particles at the same time when cooling is carried out in a decarburizing atmosphere such as a vacuum. Further, the alloy surface is hardened and toughness can be improved by such an effect that compressive stress against the surface part is caused by difference in thermal expansion coefficient due to a gradient in the amount of the binder phase, whereby wear resistance and thermal shock resistance can be remarkably improved.
Thermal cracking is caused by temperature difference between the surface part and the interior of the alloy. In order to prevent such thermal cracking, the thermal conductivity of the nitrogen-containing sintered hard alloy itself may be improved, while the improvement of the thermal conductivity of the nitrogen-containing sintered hard alloy is naturally limited. As a result of study, however, it has been clarified that heat which is generated during cutting is conducted to the overall alloy to attain a heat divergence (fin) effect when a layer having high thermal conductivity which is rich in WC with a rest of a metal binder phase mainly composed of Co and Ni is arranged on a surface part of a nitrogen-containing sintered hard alloy.
Accordingly, a nitrogen-containing sintered hard alloy according to the present invention, which has been proposed on the basis of the aforementioned result of the study, includes a hard phase containing WC serving as an essential element and a carbide, a nitride or a carbo-nitride of at least one transition metal selected from the groups 4A, 5A and 6A of the periodic table or a composite carbo-nitride thereof, and a binder phase containing Ni, Co and unavoidable impurities, and has the following structure and composition:
An exudation layer 1 containing a metal binder phase, mainly composed of Ni and Co, and WC is present on an alloy surface part (see Figs. 1 to 3), and this layer 1 is internally divided into three layers including an outermost layer containing at least 0 percent by volume and not more than 30 percent by volume (preferably 0 to 5 percent by volume) of WC with a rest formed by a metal binder phase which is mainly composed of Co and Ni, an intermediate layer containing at least 50 percent by volume and not more than 100 percent by volume (preferably 80 to 100 percent by volume) of WC with a rest formed by a metal binder phase which is mainly composed of Co and Ni, and a lowermost layer containing at least 0 percent by volume and not more than 30 percent by volume (preferably 0 to 5 percent by volume) of WC with a rest formed by a metal binder phase which is mainly composed of Co and Ni.
The outermost and lowermost layers are at least 0.1 µm and not more than 10 µm (preferably 0.1 to 0.5 µm) in thickness, while the intermediate layer is at least 0.5 µm and not more than 10 µm (preferably 0.5 to 5 µm) in thickness.
In the nitrogen-containing sintered hard alloy having the aforementioned structure, thermal shock resistance is remarkably improved. While the outermost and lowermost layers are substantially rich in the metal binder phase mainly composed of Ni and Co, these layers are inevitably formed in the manufacturing steps, and no problem is caused in performance when the thicknesses thereof are in the aforementioned range.
In the numeric limitation of the aforementioned structure, the intermediate layer contains at least 50 percent by volume and not more than 100 percent by volume of WC since desired thermal conductivity cannot be attained and the layer cannot serve as a thermal divergence layer if the WC content is not more than 50 percent by volume with a rest of the metal binder phase mainly composed of Co and Ni. The thickness of this intermediate layer is set in the range of at least 0.5 µm and not more than 10 µm since desired thermal conductivity cannot be attained if the thickness is less than 0.5 µm while wear resistance is remarkably deteriorated if the thickness exceeds 10 µm.
Each of the outermost and lowermost layers, which are necessarily formed for obtaining the most important intermediate layer, must have a thickness of 0.1 µm, while the same may cause welding with a main component of a workpiece and iron in cutting leading to chipping if the thickness exceeds 10 µm. It has been proved by a result of study that no influence is exerted on cutting performance if the outermost and lowermost layers are not more than 10 µm in thickness.
In a preferred embodiment, the inventive nitrogen-containing sintered hard alloy of the aforementioned structure has a region containing absolutely no or not more than 2 percent by volume of a metal binder phase in its surface part immediately under the exudation layer 1 containing the metal binder phase, which is mainly composed of Ni and Co, and WC, and this region has a thickness of at least 2 µm and not more than 100 µm (preferably 2 to 50 µm) from the portion immediately under the exudation layer 1 toward the interior. According to this structure, the region immediately under the exudation layer 1 has extremely high hardness, whereby both of wear resistance and thermal shock resistance can be compatibly attained.
In the aforementioned structure, the surface part of the alloy contains not more than 2 percent by volume of the metal binder phase which is mainly composed of Co and Ni since no remarkable improvement of wear resistance is recognized if the metal binder phase is present in a higher ratio. The thickness of the region located immediately under the exudation layer 1 is set in the range of at least 2 µm and not more than 100 µm since no improvement of wear resistance is recognized if the thickness of the region is less than 2 µm while the region is rendered too hard and fragile to deteriorate chipping resistance if the thickness exceeds 100 µm.
In a more preferred embodiment of the inventive nitrogen-containing sintered hard alloy having the aforementioned structure, the region containing absolutely no or not more than 2 percent by volume of WC located immediately under the exudation layer 1 has a thickness of at least 1 µm and not more than 500 µm (preferably 20 to 100 µm) toward the interior of the alloy. Under such conditions, further, the abundance of WC is preferably gradually increased from the aforementioned region located immediately under the exudation layer 1 toward the interior so that the volume percentage of WC reaches the average WC volume percentage of the overall alloy at a depth within 1 mm (preferably 0.3 to 0.7 mm) from the portion immediately under the exudation layer 1. According to this structure, the Young's modulus of the overall alloy is increased due to the presence of WC, whereby mechanical strength is remarkably improved. Further, both of thermal shock resistance and chipping resistance can be compatibly attained by providing WC only in the interior with no presence on the surface part of the alloy.
In the aforementioned structure, the thickness of the region, located immediately under the exudation layer 1, containing absolutely no or not more than 2 percent by volume of WC toward the internal direction is set in the range of at least 1 µm and not more than 500 µm since wear resistance is deteriorated due to influence by reduction in hardness caused by WC if the thickness is less than 1 µm while the effect of improving toughness of the alloy itself by WC cannot be attained if the thickness exceeds 500 µm.
The aforementioned structure of the inventive alloy can be obtained by setting a sintering temperature in the range of 1350 to 1700°C in a specified composition and controlling a sintering atmosphere and a cooling rate. The thicknesses of the three layers forming the exudation layer 1 can be adjusted by controlling the sintering temperature and the cooling rate.
The volume percentage of WC is measured by the following method: A section of a WC-Co cemented carbide member having a known WC content is lapped to take a SEM photograph of 4800 magnifications. An area occupied by WC in this photograph is calculated by an image analyzer, to draw a calibration curve on the area occupied by WC. As to the inventive alloy, a section of a portion to be observed is lapped and an area occupied by WC is calculated from an SEM photograph of 4800 magnifications by an image analyzer, for obtaining the volume percentage of WC from a calibration curve.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a microphotograph (SEM photograph) of an alloy structure indicating an exudation layer which is divided into three layers with presence of Co and Ni binder layers in outermost and lowermost layers and a WC layer in an intermediate layer; and
Figs. 2 and 3 are microphotographs (EDX analysis) indicating distributions of Co and Ni elements in the structure respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Concrete Examples of the present invention are now described.

Example 1

TiCN powder, WC powder, TaC powder, NbC powder, Mo₂C powder, VC powder, (Ti_0.5W_0.3Ta_0.1Nb_0.1)C_0.5N_0.5 powder, Co powder and Ni powder of 1.5 µm in mean particle size were blended into a composition shown at A in Table 1, mixed with each other in a wet attriter for 12 hours, thereafter worked into green compacts of a CNMG432 shape under a pressure of 1.5 ton/cm², and the green compacts were honed to thereafter prepare sintered hard alloys having structures shown in Tables 3 to 5 under sintering conditions shown in Table 2. Referring to Tables 3 to 5, the columns "structure from portion immediately under exudation layer toward interior" show composition rates of hard phases and binder phases varied with depths toward interiors of alloys with reference to portions immediately under exudation layers which are set at 0. In a sample a-7, for example, the WC content is identical to the alloy-average WC volume percentage from the portion immediately under the exudation layer toward the interior, while the binder phase content is 1.8 percent by volume up to 2.5 µm, gradually increased from 2.5 µm up to 60 µm, and identical to the alloy-average binder phase volume percentage in an internal portion beyond 60 µm. The content of the hard phase forming the rest is expressed in 100 - (alloy-average binder phase volume percentage) - (alloy-average WC volume percentage) in each depth.

Blending Composition (wt.)
	Hard Phase Component	Binder Phase Component
A	TiCN 46 % WC 40 %	Co 7 % Ni 7 %
B	TiCn 41 % WC 30 % TaC 5 % NbC 5 % Mo₂ C3 % VC 2 %	Co 7 % Ni 7 %
C	(Ti_0.5, W_0.3, Ta_0.1, Nb_0.1) (C_0.5, N_0.5) 86 %	Co 7 % Ni 7 %
D	TiCN 66 % WC 16 %	Co 9 % Ni 9 %

Sintering No.	Sintering Condition
	Sintering Temperature (°C)	Sintering Atmosphere (Torr)	Cooling Rate (°C/min)	Cooling Atmosphere (Torr)
1	1530	Nitrogen : 5	8	Nitrogen : 3
2	1520	Nitrogen : 50	2	Nitrogen : 4
3	1400	Nitrogen : 3	4	Nitrogen : 4
4	1460	Nitrogen : 6	2	Nitrogen : 5
5	1460	Nitrogen : 10	2	Nitrogen : 10
6	1420	Nitrogen : 5	1	Nitrogen : 12
7	1435	Nitrogen : 6	4	Vacuum
8	1530	Nitrogen : 5	8	Vacuum
9	1520	Nitrogen : 2	2	Methane : 2
10	1400	Nitrogen : 50	4	Methane : 1
11	1460	Nitrogen : 6	2	Methane : 2
12	1420	Nitrogen : 5	1	Argon : 2
13	1435	Nitrogen : 6	4	Argon : 5
14	1530	Nitrogen : 5	8	Vacuum
15	1420	Nitrogen : 10	1	Vacuum

The samples a-1 to a-15 were subjected to a thermal shock resistance test and a wear resistance test under conditions (A) and (B) respectively. Table 6 shows the results.
(A)
Workpiece: SCM435 (HB: 250) with four flutes
Cutting Speed: 100 (m/min.)
Depth of Cut: 1.5 (mm)
Feed Rate: 0.20 (mm/rev.)
Cutting Time: 30 sec.
Wet Type
(B)
Workpiece: SCM435 (HB: 250) with four flutes
Cutting Speed: 180 (m/min.)
Depth of Cut: 1.5 (mm)
Feed Rate: 0.30 (mm/rev.)
Cutting Time: 20 min.
Wet Type

Sample
a-1	38 Inserts	0.29 mm
a-2	16 Inserts	0.19 mm
a-3	36 Inserts	0.30 mm
a-4	37 Inserts	0.31 mm
a-5	38 Inserts	0.29 mm
a-6	16 Inserts	0.25 mm
a-7	10 Inserts	0.10 mm
a-8	10 Inserts	0.08 mm
a-9	11 Inserts	0.18 mm
a-10	19 Inserts	0.08 mm
a-11	10 Inserts	0.19 mm
a-12	10 Inserts	0.18 mm
a-13	12 Inserts	0.23 mm
a-14	17 Inserts	0.18 mm
a-15	5 Inserts	0.07 mm

It is understood that thermal shock resistance which is superior to that of the prior art can be attained when a sintered hard alloy having a hard phase consisting of TiCN and WC is provided with an exudation layer as specified. It is also understood that wear resistance and thermal shock resistance are improved respectively when binder phase and WC distributions as specified are provided.

Example 2

Raw powder materials which were identical to those of Example 1 were blended into a composition shown at B in Table 1, worked into green compacts by a method identical to that in Example 1, and the green compacts were honed to prepare sintered hard alloys having structures shown in Tables 7 to 9 under the sintering conditions shown in Table 2. Samples b-1 to b-15 were subjected to a thermal shock resistance test and a wear resistance test under conditions (C) and (D) respectively. Table 10 shows the results.
(C)
Workpiece: SCM435 (HB: 300) with four flutes
Cutting Speed: 120 (m/min.)
Depth of Cut: 1.5 (mm)
Feed Rate: 0.20 (mm/rev.)
Cutting Time: 30 sec.
Wet Type
(D)
Workpiece: SCM435 (HB: 300) with four flutes
Cutting Speed: 200 (m/min.)
Depth of Cut: 1.5 (mm)
Feed Rate: 0.30 (mm/rev.)
Cutting Time: 20 min.
Wet Type

Sample
b-1	39 Inserts	0.31 mm
b-2	15 Inserts	0.17 mm
b-3	37 Inserts	0.32 mm
b-4	38 Inserts	0.33 mm
b-5	39 Inserts	0.31 mm
b-6	15 Inserts	0.23 mm
b-7	9 Inserts	0.08 mm
b-8	9 Inserts	0.06 mm
b-9	10 Inserts	0.15 mm
b-10	18 Inserts	0.05 mm
b-11	9 Inserts	0.16 mm
b-12	9 Inserts	0.15 mm
b-13	11 Inserts	0.20 mm
b-14	16 Inserts	0.15 mm
b-15	4 Inserts	0.04 mm

It is understood that thermal shock resistance which is superior to that of the prior art can be attained when a sintered hard alloy having a hard phase consisting of an element belonging to the group 4A, 5A or 6A is provided with an exudation layer as specified. It is also understood that wear resistance and thermal shock resistance are improved respectively when binder phase and WC distributions as specified are provided.

Example 3

Raw powder materials which were identical to those of Example 1 were blended into a composition shown at C in Table 1, worked into green compacts by a method identical to that in Example 1, and the green compacts were honed to prepare sintered hard alloys having structures shown in Tables 11 to 13 under the sintering conditions shown in Table 2. Samples c-1 to c-15 were subjected to a thermal shock resistance test and a wear resistance test under conditions (E) and (F) respectively. Table 14 shows the results.
(E)
Workpiece: SCM435 (HB: 280) with four flutes
Cutting Speed: 120 (m/min.)
Depth of Cut: 1.5 (mm)
Feed Rate: 0.20 (mm/rev.)
Cutting Time: 30 sec.
Wet Type
(F)
Workpiece: SCM435 (HB: 280)
Cutting Speed: 200 (m/min.)
Depth of Cut: 1.5 (mm)
Feed Rate: 0.30 (mm/rev.)
Cutting Time: 20 min.
Wet Type

Sample
c-1	39 Inserts	0.32 mm
c-2	16 Inserts	0.16 mm
c-3	37 Inserts	0.33 mm
c-4	38 Inserts	0.34 mm
c-5	39 Inserts	0.32 mm
c-6	17 Inserts	0.22 mm
c-7	10 Inserts	0.07 mm
c-8	10 Inserts	0.05 mm
c-9	11 Inserts	0.14 mm
c-10	19 Inserts	0.04 mm
c-11	10 Inserts	0.15 mm
c-12	10 Inserts	0.14 mm
c-13	12 Inserts	0.19 mm
c-14	17 Inserts	0.14 mm
c-15	Inserts	0.03 mm

It is understood that thermal shock resistance which is superior to that of the prior art can be attained when a sintered hard alloy having a solid solution hard phase consisting of an element belonging to the group 4A, 5A or 6A is provided with an exudation layer as specified. It is also understood that wear resistance and thermal shock resistance are improved respectively when binder phase and WC distributions as specified are provided.

Example 4

The samples a-1 and a-2 shown in Table 3 and the sample a-1 shown in Table 13 were subjected to a thermal shock resistance test under conditions (G). Table 15 shows the results.
(G)
Workpiece: SCM435 (HB: 280) with four flutes
Cutting Speed: 100 (m/min.)
Depth of Cut: 1.5 (mm)
Feed Rate: 0.20 (mm/rev.)
Cutting Time: 30 sec.
Wet Type

	Sample	(G)
	a-1	15 Inserts
	a-2	32 Inserts
	a-3	36 Inserts

It is understood that no improvement of thermal shock resistance is recognized despite an exudation layer, if no layer which is mainly composed of WC is present.
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only.

Claims

A nitrogen-containing sintered hard alloy including a hard phase containing WC serving as an essential element and a carbide, a nitride or a carbo-nitride of at least one transition metal being selected from the groups 4A, 5A and 6A of the periodic table or a composite carbo-nitride thereof, and a binder phase containing Ni, Co and unavoidable impurities, an exudation layer containing a metal binder phase, mainly composed of Ni and Co, and WC being present on an alloy surface part, said exudation layer being internally divided into three layers in order of an outermost layer, an intermediate layer and a lowermost layer, said outermost layer containing at least 0 percent by volume and not more than 30 percent by volume of WC with a rest being formed by a metal binder phase mainly composed of Co and Ni, said intermediate layer containing at least 50 percent by volume and not more than 100 percent by volume of WC with a rest being formed by a metal binder phase mainly composed of Co and Ni, said lowermost layer containing at least 0 percent by volume and not more than 30 percent by volume of WC with a rest being formed by a metal binder phase mainly composed of Co and Ni, said outermost and lowermost layers being at least 0.1 µm and not more than 10 µm in thickness, said intermediate layer being at least 0.5 µm and not more than 10 µm in thickness.
The nitrogen-containing sintered hard alloy in accordance with claim 1, being provided with a region containing absolutely no or not more than 2 percent by volume of said metal binder phase mainly composed of Co and Ni in a portion immediately under said exudation layer, said region having a thickness of at least 2 µm and not more than 100 µm from said portion immediately under said exudation layer toward the interior.
The nitrogen-containing sintered hard alloy in accordance with claim 1, being provided with a region containing absolutely no or not more than 2 percent by volume of WC in a portion immediately under said exudation layer, said region having a thickness of at least 1 µm and not more than 500 µm from said portion immediately under said exudation layer toward the interior.
The nitrogen-containing sintered hard alloy in accordance with claim 3, wherein the abundance of WC is gradually increased from a portion immediately under said region containing absolutely no or not more than 2 percent by volume of WC toward the interior so that the volume percentage of WC reaches the average WC volume percentage of overall said alloy at a depth within 1 mm from a portion immediately under said exudation layer.
The nitrogen-containing sintered hard alloy in accordance with claim 2, wherein an exudation layer containing a metal binder phase, mainly composed of Ni and Co, and WC is present on an alloy surface part, and a region containing absolutely no or not more than 2 percent by volume of WC is provided in a portion immediately under said exudation layer, said region having a thickness of at least 1 µm and not more than 500 µm from said portion immediately under said exudation layer toward the interior.
The nitrogen-containing sintered hard alloy in accordance with claim 5, wherein the abundance of WC is gradually increased from a portion immediately under said region containing absolutely no or not more than 2 percent by volume of WC toward the interior so that the volume percentage of WC reaches the average WC volume percentage of overall said alloy at a depth within 1 mm from a portion immediatly under said exudation layer.