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On the life of an ion-nitrided HSS cutting tool
Journal of Materials Processing Technology, 35 (1992) 113-119
113
Elsevier
On the life of an ion-nitrided HSS cutting tool M.A. B~jar and N. Vranjican University of Chile, Department of Mechanical Engineering, CasiUa2777, Santiago, Chile
(ReceivedMay 1, 1991; accepted in revised form February 3, 1992)
Industrial S u m m a r y
This paper presents the results of a study concerned with the influenceof ion-nitriding treatment on the life of a high-speedsteel (AISI-M2) cutting tool. It is shownthat such treatment can increase significantlythe life of a tool operating in both continuous- and intermittent-cutting.
1. Introduction
Ion-nitriding is a plasma process t h a t has been used to improve the fatigue, the wear a n d / o r the corrosion resistance of steels. The process is performed by placing the piece as the cathode in a glow discharge that contains nitrogen [ 14 ]. Thus, positive ions of nitrogen are accelerated towards the cathode and probably some of t h e m are implanted into it. Some other ions of nitrogen react with iron atoms forming iron nitride (FEN), which condenses on the cathode forming other nitrides (Fe2N, FesN, Fe4N), liberating nitrogen t h a t diffuses into the piece. The nitriding of steels is promoted by the presence of alloying elements with a strong affinity with nitrogen, such as aluminium and chromium. The aim of this work is to study if ion-nitriding can increase the life of a high-speed steel tool for operating in either continuous- or intermittent-cutting. It must be noted t h a t intermittent cutting is a more severe operation t h a n continuous cutting, due to the interruption of cutting t h a t occurs in the cutting cycle giving rise to a cyclic heating and cooling combined with a cyclic loading and unloading of the tool [5 ]. Consequently, the tool-life is reduced drastically in comparison with t h a t for continuous cutting. 2. Experimental details
All cutting tests were performed by the orthogonal machining on a lathe of a low-carbon steel tube (0.1% C, 0.24% Si, 0.52% Mn, 0.01% P, 0.026% S), the Correspondence to: M.A. B~jar, University of Chile, Department of MechanicalEngineering,Casilla 2777, Santiago, Chile.
0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
114
external diameter and the wall thickness of the tube being 63.5 mm and 2.9 mm, respectively. Tools of AISI-M2 high-speed steel (HSS) were used predominantly for operating in both continuous- (Fig. 1 (a)) and intermittentcutting (Fig. 1 (b)). In intermittent cutting, some sintered carbide tools (P40) were tested also. The rake and clearance angles of the tools are given in Table 1. The HSS tools were quenched in oil (1200-1240°C) and tempered for one hour at 540-560 ° C. By employing microhardness tests with a Vickers indenter under a load of 100 g, it was determined that the surface microhardness of the tools, after tempering, was about 800 Hv. Subsequently, some HSS tools were ion-nitrided (Fig. 1 (c)) for one hour in a dc glow discharge of a mixture of 2% N2 + 98% H2 at 2.2 mm Hg pressure. CHUCK
_. I
1
5
~
(b)
(a) N2+H2
t
~
VACUUMPUMP
(+)
TOOL r
'
(-)
"--7.
II (c)
Fig. 1. Experimentaldetails: (a) continuouscutting; (b) intermittent cutting; and (c) ion-nitriding chamber. TABLE 1 Tool angles Material
HSS carbide
Tool angles rake angle
clearance angle
8° 0°
8° 8°
115
The tests of orthogonal continuous cutting were run at two cutting velocities (V): 13.2 and 20 m/rain, and at a feed rate of 0.08 ram/revolution. HSS tools with and without the ion-nitriding treatment were tested. For the tests of orthogonal intermittent cutting, a rotary tool was used, this tool consisting of a single cutting tool fixed to a wheel of steel moved by an electrical motor of variable velocity. The rotary tool was mounted on the carriage of a lathe, with its axis of rotation placed perpendicularly to the bedways, as is shown in Fig. 1 (b). The depths of cut (t) were: 0.75, 1.0 and 1.5 ram, and the feeds per tooth (f) were: 0.067, 0.134 and 0.267 ram. The peripheral velocities of the tool (V), i.e., the cutting velocities were: 100, 120, 150, 200 and 250 m/rain. The life of the tools was defined as the time (in continuous cutting) or the number of cycles (in intermittent cutting) for the attaining of a flank wear of VB-- 0.3 ram. 3. Results and discussion
For the conditions of ion-nitriding used in this work, the surface microhardness of the ion-nitrided HSS tools was not much greater than that of the tools that had been simply quenched and tempered (in some cases it was lower). Further, an optical examination of polished and etched tools showed that a compound layer was not formed on the surface of the tools.
3.1. Continuous cutting For HSS tools without the ion-nitriding treatment, the life was 40 and 55 minutes for cutting velocities of 20 and 13.2 m / m i n , respectively. Figure 2 shows the values of life for HSS tools ion-nitrided at different temperatures in the range 400 and 525 ° C, and operating at a cutting velocity of 20 m/rain. From this figure, it is clear that for ion-nitriding temperatures greater 70
50
30 qO0
•
i LI25
I zlSO
i q75
i 500
525
[ON-N|TR|D|NG TEMPERATURE(*c)
Fig. 2. Influence of the ion-nitriding temperature on the life of HSS tools: continuous cutting, V-- 20 m/rain.
116
than T= 400 ° C, the life was greater than the life obtained for tools without the ion-nitriding treatment. The tool life reached its maximum value for an ionnitriding temperature of T = 500 ° C: thus, this temperature was chosen for the executing of all of the following ion-nitriding treatments for both continuousand intermittent-cutting tests. In continuous cutting at a velocity of 13.2 m / rain, the life o f a HSS tool ion-nitrided at 500 °C was 120 min. This life is more than twice the value obtained for a tool without the ion-nitriding treatment tested at the same velocity.
3.2. Intermittent cutting Figure 3 shows the values of tool life as a function of the cutting velocity, for non-ion-nitrided HSS tools operating at 0.75 m m depth of cut. In this case, it was not possible to work at velocities greater t h a n V= 150 m / m i n due to the failure of the tools by fracture at the beginning of each test. In Fig. 3, it can be seen that: (i) the life of the tools decreases with the increasing of the feed per tooth; (ii) at feeds per tooth off= 0.267 m m and f = 0.134 mm, the life decreases with the increasing of the cutting velocity; and (iii) at a feed per tooth of /= 0.067 m m and a cutting velocity of V= 120 m/rain, a maximum value exists for the tool life. By increasing the depth of cut, the life of the tools decreases, as can be seen in Fig. 4. The decreasing of the tool life with the increasing of the feed, depth of cut and cutting speed is a situation that occurs normally in metal cutting [6,7]: this is due to the increasing of the wear by the increasing of the cutting temperature. However, at low rates of metal removal, the formation and breakdown of built-up edges can become of sufficient magnitude as to reduce the tool life [8,9], which could explain the existence of the maximum value of tool life encountered for f= 0.067 m m in Fig. 3. A maximum value for the tool life has been shown also for TiN-coated HSS and carbide tools [5,6,10,11]. For carbide tools operating at 0.75 m m depth of cut and 0.067 m m feed per
fJJ'
m
M 40 I
",
)
I~ 2
0
-
~
0,267 -o
o 0
100
I
|
I
J
110
120
1T,0
lqO
I
150
CUTTING VELOCITY (=~'=¢tn)
Fig. 3. Influence of b o t h t h e cutting velocity a n d the feed per t o o t h on the life of non-ion-nitrided H S S tools: i n t e r m i t t e n t cutting, t = 0 . 7 5 ram.
117 60 A o o
o "
qO
"
~
f=0,067rnrn
"A-~,~._..__.
0.I~I
20 0
-J
0
I
0,7
0.267
-0--------_
I
0,9
I
I.I 1.3 D,:pr'Hor Cur(,,-)
1,5
Fig. 4. Influence of the depth of cut on the life of non-ion-nitrided HSS tools: intermittent cutting, V= 120 m / m i n .
80
.J'-
1¢
M
,, 20
0
|
I
150
2O0
250
CUTTING VELOCITY ( " / " n )
Fig. 5. Influence of the cutting velocity on the life of carbide tools: intermittent cutting, f= 0.067 mm, t=0.75 ram.
tooth, the tool-life values are shown in Fig. 5 as a function of cutting velocity. Here, it is seen that the life also experiences a maximum value, this value occurring at a cutting velocity of about 150 m/rain, which is a similar value to that found by other authors [ 5 ]. By increasing the depth of cut and/or the feed per tooth, the life of carbide tools decreased rapidly, not by flank wear but by fracture. For ion-nitrided HSS tools, the values of tool life are shown in Fig. 6 as a function of cutting velocity. At t= 0.75 mm and f = 0.067 mm, the life obtained for V= 120 m/rain was 73500 cycles, whilst for a non-ion-nitrided HSS and a carbide tool the life was 52700 and 47100 cycles, respectively. At V--150 m / min, the tool-life of an ion-nitrided HSS tool was 61200 cycles: this is quite similar to the life of a carbide tool and is about 80% greater than the life of a non-ion-nitrided HSS tool. Further, it is worthwhile to note that the ion-ni-
118
A
g GO
_..i
,, 20
0 120
I
1@
160
180
200
CUTTIN6 VELOCITY (mu'm.tn)
Fig. 6. Influence of the cutting velocity on the life of ion-nitrided HSS tools: intermittent cutting, T=500°C,/=0.067 mm, t=0.75 ram.
trided HSS tool can operate at a cutting velocity as great as 200 m / m i n , whilst having a life of about 16000 cycles. As the improved performance of the ion-nitrided HSS tools is made possible by the wear resistance of their surface-nitrided layer, a greater life of these tools might be secured than the values shown in this study by the ion-nitriding of the tools for a longer period than the present one hour, in order to obtain an increase in the thickness of the layer. 4. C o n c l u s i o n s
The ion-nitriding treatment can increase significantly the life of HSS tools operating in both continuous- and intermittent-cutting. In the intermittent cutting of a low-carbon steel, the life of ion-nitrided HSS tools can be longer than the life of carbide tools.
References 1 B. Edenhofer, Physical and metallurgical aspects of ionitriding, Heat Treatment Met., 1 (1974) 23-28; 2 (1974) 59-67. 2 C.V. Robino and O.T. Inal, Ion-nitriding behaviour of several low alloy steels, Mater. Sci. Eng., 59 (1983) 79-90. 3 T. Spalvins, Tribological and microstructural characteristics of ion-nitrided steels, Thin Solid Films, 108 (1983) 157-163. 4 R. Trejo-Luna, E.P. Zironi, J. Richards and G. Romero, Some features of low-temperature ion nitriding of steels, Script a Metall., 23 (1989) 1493 - 1496. 5 P.C. Pandey, S.M. Bhatia and H.S. Shah, Thermo-mechanical failure of cemented carbide tools in intermittent cutting, CIRPAnn., 28 (1) (1979) 13-17. 6 E.M. Trent, Metal Cutting, Butterworths, London, 1977. 7 N.H. Cook, Tool wear and tool life, J. Eng. Ind., Trans. ASME, (1973) 931-937.
119 8 L. Ahman, S. Hogmark, H. Wisell and H. Haag, Wear of high speed steel milling tools, Scand. J. Metall., 14 (1985) 60-68. 9 E.M. Trent, Metal cutting and the tribology of seizure, parts I, II and III, Wear, 128 (1988) 29-45; 47-64; 65-81. 10 J. Vogel and E. Bergmann, Problems encountered with the introduction of ion plating to large-scale coating of tools, J. Vac. Sci. Technol., A 4(6) (1986) 2731-2739. 11 H.Randhawa, TiN-coated high-speed steels cutting tools, J. Vac. Sci. Technol.,A4(6) (1986) 2755-2758.
113
Elsevier
On the life of an ion-nitrided HSS cutting tool M.A. B~jar and N. Vranjican University of Chile, Department of Mechanical Engineering, CasiUa2777, Santiago, Chile
(ReceivedMay 1, 1991; accepted in revised form February 3, 1992)
Industrial S u m m a r y
This paper presents the results of a study concerned with the influenceof ion-nitriding treatment on the life of a high-speedsteel (AISI-M2) cutting tool. It is shownthat such treatment can increase significantlythe life of a tool operating in both continuous- and intermittent-cutting.
1. Introduction
Ion-nitriding is a plasma process t h a t has been used to improve the fatigue, the wear a n d / o r the corrosion resistance of steels. The process is performed by placing the piece as the cathode in a glow discharge that contains nitrogen [ 14 ]. Thus, positive ions of nitrogen are accelerated towards the cathode and probably some of t h e m are implanted into it. Some other ions of nitrogen react with iron atoms forming iron nitride (FEN), which condenses on the cathode forming other nitrides (Fe2N, FesN, Fe4N), liberating nitrogen t h a t diffuses into the piece. The nitriding of steels is promoted by the presence of alloying elements with a strong affinity with nitrogen, such as aluminium and chromium. The aim of this work is to study if ion-nitriding can increase the life of a high-speed steel tool for operating in either continuous- or intermittent-cutting. It must be noted t h a t intermittent cutting is a more severe operation t h a n continuous cutting, due to the interruption of cutting t h a t occurs in the cutting cycle giving rise to a cyclic heating and cooling combined with a cyclic loading and unloading of the tool [5 ]. Consequently, the tool-life is reduced drastically in comparison with t h a t for continuous cutting. 2. Experimental details
All cutting tests were performed by the orthogonal machining on a lathe of a low-carbon steel tube (0.1% C, 0.24% Si, 0.52% Mn, 0.01% P, 0.026% S), the Correspondence to: M.A. B~jar, University of Chile, Department of MechanicalEngineering,Casilla 2777, Santiago, Chile.
0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
114
external diameter and the wall thickness of the tube being 63.5 mm and 2.9 mm, respectively. Tools of AISI-M2 high-speed steel (HSS) were used predominantly for operating in both continuous- (Fig. 1 (a)) and intermittentcutting (Fig. 1 (b)). In intermittent cutting, some sintered carbide tools (P40) were tested also. The rake and clearance angles of the tools are given in Table 1. The HSS tools were quenched in oil (1200-1240°C) and tempered for one hour at 540-560 ° C. By employing microhardness tests with a Vickers indenter under a load of 100 g, it was determined that the surface microhardness of the tools, after tempering, was about 800 Hv. Subsequently, some HSS tools were ion-nitrided (Fig. 1 (c)) for one hour in a dc glow discharge of a mixture of 2% N2 + 98% H2 at 2.2 mm Hg pressure. CHUCK
_. I
1
5
~
(b)
(a) N2+H2
t
~
VACUUMPUMP
(+)
TOOL r
'
(-)
"--7.
II (c)
Fig. 1. Experimentaldetails: (a) continuouscutting; (b) intermittent cutting; and (c) ion-nitriding chamber. TABLE 1 Tool angles Material
HSS carbide
Tool angles rake angle
clearance angle
8° 0°
8° 8°
115
The tests of orthogonal continuous cutting were run at two cutting velocities (V): 13.2 and 20 m/rain, and at a feed rate of 0.08 ram/revolution. HSS tools with and without the ion-nitriding treatment were tested. For the tests of orthogonal intermittent cutting, a rotary tool was used, this tool consisting of a single cutting tool fixed to a wheel of steel moved by an electrical motor of variable velocity. The rotary tool was mounted on the carriage of a lathe, with its axis of rotation placed perpendicularly to the bedways, as is shown in Fig. 1 (b). The depths of cut (t) were: 0.75, 1.0 and 1.5 ram, and the feeds per tooth (f) were: 0.067, 0.134 and 0.267 ram. The peripheral velocities of the tool (V), i.e., the cutting velocities were: 100, 120, 150, 200 and 250 m/rain. The life of the tools was defined as the time (in continuous cutting) or the number of cycles (in intermittent cutting) for the attaining of a flank wear of VB-- 0.3 ram. 3. Results and discussion
For the conditions of ion-nitriding used in this work, the surface microhardness of the ion-nitrided HSS tools was not much greater than that of the tools that had been simply quenched and tempered (in some cases it was lower). Further, an optical examination of polished and etched tools showed that a compound layer was not formed on the surface of the tools.
3.1. Continuous cutting For HSS tools without the ion-nitriding treatment, the life was 40 and 55 minutes for cutting velocities of 20 and 13.2 m / m i n , respectively. Figure 2 shows the values of life for HSS tools ion-nitrided at different temperatures in the range 400 and 525 ° C, and operating at a cutting velocity of 20 m/rain. From this figure, it is clear that for ion-nitriding temperatures greater 70
50
30 qO0
•
i LI25
I zlSO
i q75
i 500
525
[ON-N|TR|D|NG TEMPERATURE(*c)
Fig. 2. Influence of the ion-nitriding temperature on the life of HSS tools: continuous cutting, V-- 20 m/rain.
116
than T= 400 ° C, the life was greater than the life obtained for tools without the ion-nitriding treatment. The tool life reached its maximum value for an ionnitriding temperature of T = 500 ° C: thus, this temperature was chosen for the executing of all of the following ion-nitriding treatments for both continuousand intermittent-cutting tests. In continuous cutting at a velocity of 13.2 m / rain, the life o f a HSS tool ion-nitrided at 500 °C was 120 min. This life is more than twice the value obtained for a tool without the ion-nitriding treatment tested at the same velocity.
3.2. Intermittent cutting Figure 3 shows the values of tool life as a function of the cutting velocity, for non-ion-nitrided HSS tools operating at 0.75 m m depth of cut. In this case, it was not possible to work at velocities greater t h a n V= 150 m / m i n due to the failure of the tools by fracture at the beginning of each test. In Fig. 3, it can be seen that: (i) the life of the tools decreases with the increasing of the feed per tooth; (ii) at feeds per tooth off= 0.267 m m and f = 0.134 mm, the life decreases with the increasing of the cutting velocity; and (iii) at a feed per tooth of /= 0.067 m m and a cutting velocity of V= 120 m/rain, a maximum value exists for the tool life. By increasing the depth of cut, the life of the tools decreases, as can be seen in Fig. 4. The decreasing of the tool life with the increasing of the feed, depth of cut and cutting speed is a situation that occurs normally in metal cutting [6,7]: this is due to the increasing of the wear by the increasing of the cutting temperature. However, at low rates of metal removal, the formation and breakdown of built-up edges can become of sufficient magnitude as to reduce the tool life [8,9], which could explain the existence of the maximum value of tool life encountered for f= 0.067 m m in Fig. 3. A maximum value for the tool life has been shown also for TiN-coated HSS and carbide tools [5,6,10,11]. For carbide tools operating at 0.75 m m depth of cut and 0.067 m m feed per
fJJ'
m
M 40 I
",
)
I~ 2
0
-
~
0,267 -o
o 0
100
I
|
I
J
110
120
1T,0
lqO
I
150
CUTTING VELOCITY (=~'=¢tn)
Fig. 3. Influence of b o t h t h e cutting velocity a n d the feed per t o o t h on the life of non-ion-nitrided H S S tools: i n t e r m i t t e n t cutting, t = 0 . 7 5 ram.
117 60 A o o
o "
qO
"
~
f=0,067rnrn
"A-~,~._..__.
0.I~I
20 0
-J
0
I
0,7
0.267
-0--------_
I
0,9
I
I.I 1.3 D,:pr'Hor Cur(,,-)
1,5
Fig. 4. Influence of the depth of cut on the life of non-ion-nitrided HSS tools: intermittent cutting, V= 120 m / m i n .
80
.J'-
1¢
M
,, 20
0
|
I
150
2O0
250
CUTTING VELOCITY ( " / " n )
Fig. 5. Influence of the cutting velocity on the life of carbide tools: intermittent cutting, f= 0.067 mm, t=0.75 ram.
tooth, the tool-life values are shown in Fig. 5 as a function of cutting velocity. Here, it is seen that the life also experiences a maximum value, this value occurring at a cutting velocity of about 150 m/rain, which is a similar value to that found by other authors [ 5 ]. By increasing the depth of cut and/or the feed per tooth, the life of carbide tools decreased rapidly, not by flank wear but by fracture. For ion-nitrided HSS tools, the values of tool life are shown in Fig. 6 as a function of cutting velocity. At t= 0.75 mm and f = 0.067 mm, the life obtained for V= 120 m/rain was 73500 cycles, whilst for a non-ion-nitrided HSS and a carbide tool the life was 52700 and 47100 cycles, respectively. At V--150 m / min, the tool-life of an ion-nitrided HSS tool was 61200 cycles: this is quite similar to the life of a carbide tool and is about 80% greater than the life of a non-ion-nitrided HSS tool. Further, it is worthwhile to note that the ion-ni-
118
A
g GO
_..i
,, 20
0 120
I
1@
160
180
200
CUTTIN6 VELOCITY (mu'm.tn)
Fig. 6. Influence of the cutting velocity on the life of ion-nitrided HSS tools: intermittent cutting, T=500°C,/=0.067 mm, t=0.75 ram.
trided HSS tool can operate at a cutting velocity as great as 200 m / m i n , whilst having a life of about 16000 cycles. As the improved performance of the ion-nitrided HSS tools is made possible by the wear resistance of their surface-nitrided layer, a greater life of these tools might be secured than the values shown in this study by the ion-nitriding of the tools for a longer period than the present one hour, in order to obtain an increase in the thickness of the layer. 4. C o n c l u s i o n s
The ion-nitriding treatment can increase significantly the life of HSS tools operating in both continuous- and intermittent-cutting. In the intermittent cutting of a low-carbon steel, the life of ion-nitrided HSS tools can be longer than the life of carbide tools.
References 1 B. Edenhofer, Physical and metallurgical aspects of ionitriding, Heat Treatment Met., 1 (1974) 23-28; 2 (1974) 59-67. 2 C.V. Robino and O.T. Inal, Ion-nitriding behaviour of several low alloy steels, Mater. Sci. Eng., 59 (1983) 79-90. 3 T. Spalvins, Tribological and microstructural characteristics of ion-nitrided steels, Thin Solid Films, 108 (1983) 157-163. 4 R. Trejo-Luna, E.P. Zironi, J. Richards and G. Romero, Some features of low-temperature ion nitriding of steels, Script a Metall., 23 (1989) 1493 - 1496. 5 P.C. Pandey, S.M. Bhatia and H.S. Shah, Thermo-mechanical failure of cemented carbide tools in intermittent cutting, CIRPAnn., 28 (1) (1979) 13-17. 6 E.M. Trent, Metal Cutting, Butterworths, London, 1977. 7 N.H. Cook, Tool wear and tool life, J. Eng. Ind., Trans. ASME, (1973) 931-937.
119 8 L. Ahman, S. Hogmark, H. Wisell and H. Haag, Wear of high speed steel milling tools, Scand. J. Metall., 14 (1985) 60-68. 9 E.M. Trent, Metal cutting and the tribology of seizure, parts I, II and III, Wear, 128 (1988) 29-45; 47-64; 65-81. 10 J. Vogel and E. Bergmann, Problems encountered with the introduction of ion plating to large-scale coating of tools, J. Vac. Sci. Technol., A 4(6) (1986) 2731-2739. 11 H.Randhawa, TiN-coated high-speed steels cutting tools, J. Vac. Sci. Technol.,A4(6) (1986) 2755-2758.