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Plasma heat treatment of steel: microstructure, properties and applications
474
Materials Science and Engineering, A 140 ( 1991 ) 474-478
Plasma heat treatment of steel: microstructure, properties and applications Z. Nitkiewicz and L. Jeziorski Department of Metallurgy, Technical University Czfstochowa, AI.Zawadzkiego 19, 42-200 Czcstochowa (Poland)
Abstract The surface plasma heat treatment of three different steel grades 55, NC6 and NC10 (according to Polish Standards) was carried out in our research. The thickness of the hardened surface layer was related to the amount of the supplied energy. A hardening of the subsequent layers resulted in a martensite tempering in previous layers. In the case of the carbon steel (grade 55) it caused a significant decrease in the hardness. The alloy steels NC6 and NC10 after hardening contained a large amount of the residual austenite (about 50-65%), which reduced the tempering effect. The distance between hardened layers influences the surface hardness distribution and the resistance to abrasive wear. The research results obtained were used to determine the optimal distances between hardened layers during the plasma hardening of large areas.
1. Introduction
2. Parameters of the plasma treatment
T h e application of an arc plasma for surface heat treatment of steels by impulse heating and subsequent self-hardening which results in rapid heat penetration through the unheated part of a material has been described in many papers [1-6]. Recent work on this topic has been concerned with optimization of the treatment parameters for different steel grades, enrichment of the plasma gas in the hardening elements (e.g. carbon and nitrogen), and hardening with the surface melting [3-6]. In the case of multilayer hardening, the tempering of the previous layer by heat from the subsequent layer can occur. This can influence the hardness and the wear resistance of the surface. In this paper, investigations of the effect of the tempering p h e n o m e n o n during the multilayer hardening of three steel grades (medium carbon steel and low and high chromium alloy steels) were carried out. T h e results of these investigations show the significant importance of the arcplasma-hardening process for large areas.
For three steel grades 55, NC6, N C 1 0 (for chemical compositions see Table 1), plasma arc parameters were chosen so as to harden the surface layer. T h e relation between the thickness of the hardened layer and the amount of the supplied energy is shown in Fig. 1. In the preliminary research, two different nozzles with diameters of 2 and 4 m m were used for the plasma burner. For subsequent research the nozzle with the larger diameter was chosen. With such a nozzle diameter and for a distance from the burner to the
0921-5093/91/$3.50
TABLE 1 Chemical compositions of the steels used in the plasma surface treatment Steel grade 55 NC6 NC10
Amount (wt.%) C
Mn
Si
Cr
V
0.56 1.38 1.65
0.65 0.60 0.35
0.27 0.25 0.35
-1.45 12.00
-0.17 --
© Elsevier Sequoia/Printed in The Netherlands
475 TABLE 2
500 -~. " ~ ~00
" 55111
~
~/"Z
lvc6 "
55
Results of the hardness measurement of the plasmahardened layers in relation to the distance to the subsequent layers
,/VCO
NC¢O
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300
•
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Distance between layers (mm)
Vickers microhardness (HV 10) for the following steel grades 55
NC6
NC 10
3 5 6 9 15
402 670 79(1 858 860
681 1100 1022 960 964
776 1081 1048 974 982
200
(6
~'8
210
2,2
214
2,6
, ~o ~ ~,[.7.~ -~] Fig. 1. Dependence of the amount of energy supplied by the plasma stream on the thickness of the hardened layer.
cating movement was carried out at a load of 10 N; sintered carbide was used as a counterspecimen. 4. Discussion of research results
Fig. 2. Macrostructure of a steel 55 specimen after plasma hardening.
hardened surface of 5 mm the width of the hardened layer was about 5 mm (Fig. 2). In the plasma arc, argon was used and in the preliminary research a few per cent of propane and nitrogen were added to the argon. These additions resulted in an enrichment of the carbon and the nitrogen contents of the surface layer to a depth of 10-20 Mm. A detailed description of the research and the precise choice of the process parameters have been published elsewhere [5, 6]. 3. Research results
In the surface plasma treatment results an increase in the hardness of the treated layers was obtained (Table 2). The variations in the structures are shown in Fig. 3. The diffraction profiles obtained by X-ray investigations are shown in Figs. 4 and 5, and the range of residual austenite determined in the alloy steels in Fig. 6. A comparison of the wear of steels which had been volume hardened with those which had been plasma hardened is shown in Fig. 7. A wear test on kinetic pairs in recipro-
In steel 55, after plasma hardening, a surface layer of homogeneous martensite is formed which transforms subsequently into inhomogeneous martensite (light + dark), via a martensitic-ferritic transient structure from the initial structure (pearlitic-ferritic). In steels NC6 and NC10 the surface layer contains a large amount (up to 65%) of austenite. The hardening of the next few layers depending on the distance between layers results in different degrees of tempering of the previous layers. In steel 55 it causes decreases in the hardness and in the wear resistance. The intensity of the tempering process for that steel is illustrated in the form of the variation in the (110) diffraction peak of a-Fe (see Fig. 4). The decrease in distances between the hardened layers in alloy steels NC6 and NC 10 results in a decrease in the amount of the austenite and finally increases in the hardness and in the wear resistance (see Figs. 5 and 7). However, in all cases the wear resistance of these steels after plasma treatment was smaller than after conventional treatment. In the case of steel 55, on the contrary, the wear resistance after plasma hardening is higher than after conventional treatment. The results of the laboratory investigations were confirmed by industrial tests. The sliding parts of the shear of steel 55 after plasma hardening achieved a higher exploitation durability than after volume hardening.
476
Fig. 3. (a), (c) Optical micrographs of cross-sections of plasma-hardened layers and (b), (d) scanning electron micrographs of fractures: (a), (b) steel 55; (c), (d) steel NCI 0.
477
In the case of steel NC 10 the result contrasted with that for the hardening of forms for the production of ceramic materials.
In medium carbon steels (55), decreases in hardness and wear resistance were observed as a result of the tempering process. In the case of this steel, one can expect hardened zones not to overlap each other. In alloy steels (NC6 and NC10) the tempering effect can be utilized to decrease the amount of austenite; in this case the partial overlapping of the hardened zones is advantageous. The results obtained in the laboratory investigations were verified by industrial tests in the case of plasma hardening of areas up to 1 m: for parts machined from steels 55 and N C I 0 working under wear abrasive conditions. The plasma
5. Summary Multilayer hardening by means of the arc plasma results in tempering of the previous layer.'
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Fig. 4. (110) peak profile variations of a-Fe in a steel 55 specimen after plasma hardening. T h e distances between subsequent layers are the same: (a) 15 ram; (b) 6 ram; (c) 5 ram; (d) 3 ram.
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Fig. 6. Variations in the amount of austenite in steels NC6 and N C I 0 in relation to the distances between the plasmahardened
08
52
layers.
54
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Fig. 5. ( 111 ) peak intensity variations of 7-Fe and (110) peak intensity variations of ~z-Fe in relation to the distances between subsequent layers (steel NCI 0): (a) 5 ram; (b) 6 ram; (c) 15 ram.
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hardening of a medium carbon steel (55) allows one to obtain higher hardnesses and higher wear resistances than after volume hardening. In alloy steels NC6 and NC 10 a large amount of austenite limits the possibility of improving such properties.
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Fig. 7. Comparative results of the wear tests of specimens of steels 55 and NC6: o, volume hardening; e, plasma hardening of one layer; ®, multilayer plasma hardening.
References 1 R. Roggen, Trait. Therm., 136(1979)90-95. 2 W. A. Linnik, A. K. Oniegina and A. J. Andriejew, Metalloved. Term. Obrab. Mater., 4(1983)2-5. 3 V.N. Gurarie, Met. Forum, 7( 1 ) (1984) 13-21. 4 N.W. Popowa, W. E Baschew and E. G. Popow, Fiz. Khim. Obrab. Mater., 4 (1986) 98-105. 5 Z. Nitkiewicz, L. Jeziorski and M. Kubara, Wiad. Hum., 7-8 (1989) 205-207. 6 Z. Nitkiewicz, Proc. 14th Conf. on Applied Crystallography, Cieszyn, August 5-8, 1990, Silesian University, Katowice, 1990, pp. 229-235.
Materials Science and Engineering, A 140 ( 1991 ) 474-478
Plasma heat treatment of steel: microstructure, properties and applications Z. Nitkiewicz and L. Jeziorski Department of Metallurgy, Technical University Czfstochowa, AI.Zawadzkiego 19, 42-200 Czcstochowa (Poland)
Abstract The surface plasma heat treatment of three different steel grades 55, NC6 and NC10 (according to Polish Standards) was carried out in our research. The thickness of the hardened surface layer was related to the amount of the supplied energy. A hardening of the subsequent layers resulted in a martensite tempering in previous layers. In the case of the carbon steel (grade 55) it caused a significant decrease in the hardness. The alloy steels NC6 and NC10 after hardening contained a large amount of the residual austenite (about 50-65%), which reduced the tempering effect. The distance between hardened layers influences the surface hardness distribution and the resistance to abrasive wear. The research results obtained were used to determine the optimal distances between hardened layers during the plasma hardening of large areas.
1. Introduction
2. Parameters of the plasma treatment
T h e application of an arc plasma for surface heat treatment of steels by impulse heating and subsequent self-hardening which results in rapid heat penetration through the unheated part of a material has been described in many papers [1-6]. Recent work on this topic has been concerned with optimization of the treatment parameters for different steel grades, enrichment of the plasma gas in the hardening elements (e.g. carbon and nitrogen), and hardening with the surface melting [3-6]. In the case of multilayer hardening, the tempering of the previous layer by heat from the subsequent layer can occur. This can influence the hardness and the wear resistance of the surface. In this paper, investigations of the effect of the tempering p h e n o m e n o n during the multilayer hardening of three steel grades (medium carbon steel and low and high chromium alloy steels) were carried out. T h e results of these investigations show the significant importance of the arcplasma-hardening process for large areas.
For three steel grades 55, NC6, N C 1 0 (for chemical compositions see Table 1), plasma arc parameters were chosen so as to harden the surface layer. T h e relation between the thickness of the hardened layer and the amount of the supplied energy is shown in Fig. 1. In the preliminary research, two different nozzles with diameters of 2 and 4 m m were used for the plasma burner. For subsequent research the nozzle with the larger diameter was chosen. With such a nozzle diameter and for a distance from the burner to the
0921-5093/91/$3.50
TABLE 1 Chemical compositions of the steels used in the plasma surface treatment Steel grade 55 NC6 NC10
Amount (wt.%) C
Mn
Si
Cr
V
0.56 1.38 1.65
0.65 0.60 0.35
0.27 0.25 0.35
-1.45 12.00
-0.17 --
© Elsevier Sequoia/Printed in The Netherlands
475 TABLE 2
500 -~. " ~ ~00
" 55111
~
~/"Z
lvc6 "
55
Results of the hardness measurement of the plasmahardened layers in relation to the distance to the subsequent layers
,/VCO
NC¢O
'
300
•
'
'
IVC40
Distance between layers (mm)
Vickers microhardness (HV 10) for the following steel grades 55
NC6
NC 10
3 5 6 9 15
402 670 79(1 858 860
681 1100 1022 960 964
776 1081 1048 974 982
200
(6
~'8
210
2,2
214
2,6
, ~o ~ ~,[.7.~ -~] Fig. 1. Dependence of the amount of energy supplied by the plasma stream on the thickness of the hardened layer.
cating movement was carried out at a load of 10 N; sintered carbide was used as a counterspecimen. 4. Discussion of research results
Fig. 2. Macrostructure of a steel 55 specimen after plasma hardening.
hardened surface of 5 mm the width of the hardened layer was about 5 mm (Fig. 2). In the plasma arc, argon was used and in the preliminary research a few per cent of propane and nitrogen were added to the argon. These additions resulted in an enrichment of the carbon and the nitrogen contents of the surface layer to a depth of 10-20 Mm. A detailed description of the research and the precise choice of the process parameters have been published elsewhere [5, 6]. 3. Research results
In the surface plasma treatment results an increase in the hardness of the treated layers was obtained (Table 2). The variations in the structures are shown in Fig. 3. The diffraction profiles obtained by X-ray investigations are shown in Figs. 4 and 5, and the range of residual austenite determined in the alloy steels in Fig. 6. A comparison of the wear of steels which had been volume hardened with those which had been plasma hardened is shown in Fig. 7. A wear test on kinetic pairs in recipro-
In steel 55, after plasma hardening, a surface layer of homogeneous martensite is formed which transforms subsequently into inhomogeneous martensite (light + dark), via a martensitic-ferritic transient structure from the initial structure (pearlitic-ferritic). In steels NC6 and NC10 the surface layer contains a large amount (up to 65%) of austenite. The hardening of the next few layers depending on the distance between layers results in different degrees of tempering of the previous layers. In steel 55 it causes decreases in the hardness and in the wear resistance. The intensity of the tempering process for that steel is illustrated in the form of the variation in the (110) diffraction peak of a-Fe (see Fig. 4). The decrease in distances between the hardened layers in alloy steels NC6 and NC 10 results in a decrease in the amount of the austenite and finally increases in the hardness and in the wear resistance (see Figs. 5 and 7). However, in all cases the wear resistance of these steels after plasma treatment was smaller than after conventional treatment. In the case of steel 55, on the contrary, the wear resistance after plasma hardening is higher than after conventional treatment. The results of the laboratory investigations were confirmed by industrial tests. The sliding parts of the shear of steel 55 after plasma hardening achieved a higher exploitation durability than after volume hardening.
476
Fig. 3. (a), (c) Optical micrographs of cross-sections of plasma-hardened layers and (b), (d) scanning electron micrographs of fractures: (a), (b) steel 55; (c), (d) steel NCI 0.
477
In the case of steel NC 10 the result contrasted with that for the hardening of forms for the production of ceramic materials.
In medium carbon steels (55), decreases in hardness and wear resistance were observed as a result of the tempering process. In the case of this steel, one can expect hardened zones not to overlap each other. In alloy steels (NC6 and NC10) the tempering effect can be utilized to decrease the amount of austenite; in this case the partial overlapping of the hardened zones is advantageous. The results obtained in the laboratory investigations were verified by industrial tests in the case of plasma hardening of areas up to 1 m: for parts machined from steels 55 and N C I 0 working under wear abrasive conditions. The plasma
5. Summary Multilayer hardening by means of the arc plasma results in tempering of the previous layer.'
I
I'
i
]
< f--H
÷~
-
T--
-
80
--
,
ro
b
~,
Nc ~o
i~~ 5O
B
i
5',~
0/, °' ~
i,~i,
53
52
5~
L
53
t t
52
51 53
52
5~
53
52
5~
5
Fig. 4. (110) peak profile variations of a-Fe in a steel 55 specimen after plasma hardening. T h e distances between subsequent layers are the same: (a) 15 ram; (b) 6 ram; (c) 5 ram; (d) 3 ram.
63
52
6~
50
53
52
54
50
~5
'
i
{0
t25
I
. , . ~ r o ~ spe'elmen
I
15
[75
;
20
,<[,,~m]
22.5
Fig. 6. Variations in the amount of austenite in steels NC6 and N C I 0 in relation to the distances between the plasmahardened
08
52
layers.
54
2G
° -
Fig. 5. ( 111 ) peak intensity variations of 7-Fe and (110) peak intensity variations of ~z-Fe in relation to the distances between subsequent layers (steel NCI 0): (a) 5 ram; (b) 6 ram; (c) 15 ram.
478 I
,-~8
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o
o
5---
.........
#
.
.
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+
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¢0
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r
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!
,
.
.
_,m~
i
.
_
.
-
~!"
r
20
)
~
@
l
3O
_
I
40
hardening of a medium carbon steel (55) allows one to obtain higher hardnesses and higher wear resistances than after volume hardening. In alloy steels NC6 and NC 10 a large amount of austenite limits the possibility of improving such properties.
"
.
,
I _q
_
,
-
_
__
i
50 x
60 L~l~]
Fig. 7. Comparative results of the wear tests of specimens of steels 55 and NC6: o, volume hardening; e, plasma hardening of one layer; ®, multilayer plasma hardening.
References 1 R. Roggen, Trait. Therm., 136(1979)90-95. 2 W. A. Linnik, A. K. Oniegina and A. J. Andriejew, Metalloved. Term. Obrab. Mater., 4(1983)2-5. 3 V.N. Gurarie, Met. Forum, 7( 1 ) (1984) 13-21. 4 N.W. Popowa, W. E Baschew and E. G. Popow, Fiz. Khim. Obrab. Mater., 4 (1986) 98-105. 5 Z. Nitkiewicz, L. Jeziorski and M. Kubara, Wiad. Hum., 7-8 (1989) 205-207. 6 Z. Nitkiewicz, Proc. 14th Conf. on Applied Crystallography, Cieszyn, August 5-8, 1990, Silesian University, Katowice, 1990, pp. 229-235.