- Email: [email protected]
An investigation of the properties and wear behaviour of plasma-nitrided hot-working steel (H13)
331
Wear, I50 (1991) 331-342
An investigation of the properties and wear behaviour of plasma-nitrided hot-working steel (H13) M. B. Karamg Department
(Received
of Mechanical
February
Engineering,
Engineering
Faculty,
Erciyes
27, 1991; revised May 13, 1991; accepted
University,
Kayseri
(Turkey)
May 24, 1991)
Abstract The microstructural properties and wear behaviour of AN H13 steel which had been plasma nitrided at 530 and 550 “C for times between 4 and 100 h have been investigated. The effect of treatment temperature and time on the microstructure have been examined. The wear behaviour of material treated for 4 and 100 h has also been observed. It was seen that a total case depth of 0.55 mm with a hardness of 1000 HV can be achieved in 100 h. However, the white layer thickness is increased to 17 pm while the core hardness is reduced to 480 HV at 550 “C. The wear rate of the sample treated at 550 “C for 100 h is higher than that of the sample treated at 550 “C for 4 h.
1. Introduction Hot-working tool steels are widely used as dies in the metal-working industry for applications such as die casting, extrusion and hot forging. AISI H13 is a steel in this group which needs relatively high surface hardness and toughness after hardening, because it is subject to mechanical and severe thermal shock during hot metal working. The friction occurring between die and component leads to wear of the die surfaces, besides which the metal oxide layer formed on the component surface causes damage of the stressed tool surface since it provides a source of hard particulate material which leads to abrasive wear in the friction process. Since the wear resistance of a material is closely related to its hardness, it is necessary to apply a surface-hardening process [l] to hot-metal-working dies, such as nitrocarburzation, plasma nitriding, spark discharge or cladding [2], surfaced by welding [3], coating or chemical vapour deposition [4]. AISI H13 is often nitrided prior to use in the above type of application [S]. Although traditional gas nitriding can produce a surface hardness of 950-1200 HV 161, the compound layer produced on the surface often leads to spalling during working. Therefore this compound layer is removed by a finishing operation before the component is put into service. However, this operation reduces the adhesive resistance of the component. The abrasive and adhesive wear resistance of a hot-working die steel can be improved by means of a plasma-nitriding process. The plasma-nitrided die can be put into service without any finishing operation. Thus it has a reduced finishing cost and more importantly a reduction in the tendency to pick up. Besides, there are less downtime and repolishing costs [S]. The presence of these nitrided layers with a high tensile strength and very high resistance to wear enables the working life of the dies
0043-1648/91/$3.50
0 1991 -
Elsevier
Sequoia,
Lausanne
332
to be much extended. Plasma nitriding gives a working life four or five times longer than that of non-nitrided specimens and twice as long as that of pieces nitrided in a salt bath
[7].
2. Experimental 2.1. Surface
details
treatments
Hot-working tool steel (AISI H13 -x40 CrMoV 51) was used in this investigation. Its chemical composition analysed on an E 1000 Polyvac optical emission spectrometer is shown in Table 1. Two kinds of samples of different shape and dimensions were prepared from this material. One type were round bars turned to 25 mm diameter and 40 mm length in order to determine the results of plasma nitriding; the other type were wear discs. The wear couple consisted of an upper disc, a counterface and a lower (test) disc. The wear couple was machined to the dimensions shown in Fig. 1. All the samples were ground before vacuum hardening. They were preheated to 840 “C for 45 min, then heated rapidly to 1000 “C and oil quenched. Finally they were tempered at 550 “C for 90 min to give a hardness of 48-50 HRC. The hardened samples were prepared for plasma nitriding by polishing and degreasing. A 20 kW plasma-nitriding unit manufactured by Klockner Ionon GmbH was used for treatment. The wear discs and bar samples were plasma nitrided together in cracked TABLE
1
Compositions
of tested materials
Bar sample Wear disc
TABLE
(weight per cent)
C
Si
Mn
Ni
Cr
MO
V
cu
0.433 0.410
0.789 0.833
0.410 0.420
0.143 0.200
4.660 4.540
1.510 1.520
0.805 0.813
0.097 0.098
2.
Plasma-nitriding Treatment duration (h)
conditions
and results
Treatment temperature
(“C)
Compound layer and thickness (pm)
Total case depth (mm)
Maximum surface hardness (HV 0.3)
5.9 5.5
0.14 0.145
1274 1261
10.5 11.5
0.215 0.215
1300 1215
y’+e
13.5 14.8
0.330 0.395
1261 1204
y’+c
14.1 16.8
0.465 0.545
1192 1015
4
530 550
y’+e
16
530 550
y’ + E
49
530 550
100
530 550
333
Fig. 1. Test configuration of samples.
ammonia (pressure 2.5 mbar) at temperatures of 530 and 550 “C and for treatment times up to 100 h. The details of the plasma-nitriding processes are listed in Table 2. The samples were heated to the required temperature in about 2 h. After the chamber had been charged and the pressure in it had been reduced to 0.1 mbar, the power was switched on. The cracked ammonia was introduced slowly over a period of 2 h until the pressure was 2.5 mbar. At the end of each treatment the power was switched off automatically and the samples were cooled below 150 “C in the chamber in dissociated ammonia before discharging. Some of the hardened wear discs were not nitrided, so their wear was compared with that of plasma-nitrided discs. The treated bar samples were sectioned, mounted and polished for metallographic examination. Case depths were determined from microhardness measurements performed on a Leitz miniload hardness tester employing a 300 g load. The case depth at a hardness value 10% above the core hardness was taken as an accurate measure of the total case depth [8]. The microstructure of the plasma-nitrided cases was examined by optical microscopy. The phase structures in the compound layer produced on the samples and in the wear debris were determined by X-ray diffraction using Cr KY radiatioin. Scanning electron microscopy (SEM) was used to study the worn surface topography.
334 2.2. Wear experiments The wear tests were carried out as dry sliding wear. The upper discs were plasma nitrided at 510 “C for 25 h after hardening to give them a slightly higher surface hardness than the lower discs. Wear tests were carried out on a general purpose wear machine. Various rotation speeds, loadings and sample configurations are available with this machine, but a rotating test disc loaded &by the stationary counterface disc was chosen (Fig. 1). The upper disc was capable of being indexed in 12 positions, enabling 12 results to be obtained from each upper disc. Loads up to 25 kg dead-weight can be applied by the lever loading arm arrangement, giving an effective maximum sample load of 75 kg. The wear tests were carried out with effective loads of 15, 30, 45 and 60 kg applied. The rotational speed of the machine was 173.7 rev min-‘, giving a sliding speed 0.503 m s-‘. It was seen that under load this speed was maintained constant by the machine. The amount of wear was determined by means of an amdyticat balance accurate to 0.1 mg. The frictional force was not measured continuously in order to avoid destruction of the lever with the transducer by the high frictional force. After certain running times (certain sliding distances) the runs were interrupted and the test discs were taken out of the machine to determine the weight lost. Thus the effect of the test time on wear was determined. Wear debris generated during testing was carefully collected after completion of the wear tests and analysed by X-ray diffraction. The wear discs were cleaned in methanol before and after the tests. 3. Results
and discussion
3.1. Compound layer The structure of the layer depends on the plasma-nitriding conditions such as carbon content, pressure and mixture of the treatment gas, treatment temperature and time. A number of treatments carried out show that the compound layer thickness on H13 steel is increased by increasing the treatment time (Fig. 2). Increasing the treatment time from 4 to 100 h carried an increase in compound layer thickness from 5.5 to 16.8 pm at 550 “C. However, the increase in compound layer thickness at 530 “C is less than that at 550 “C. The compound layer thickness obtained from H13 steel treated at 510 “C for 25 h is 6.3 pm. For the same treatment time at temperatures of 530 and 550 “C the compound layer thickness is too thick, i.e. 11.8 and 12.8 /*rn respectively. On the other hand, a compound-free diffusion layer can be produced on H13 material by limiting the supply of nitrogen [9]. If the nitrogen content in the treatment atmosphere is extremely low, the compound layer is completely suppressed and only the diffusion zone remains. This structure is restricted to plasma nitriding and is very suitable for certain types of tool [lo]. It was determined by X-ray diffraction using Cr Kcr radiation that the compound layer on the plasma-nitrided samples at 530 “C consists of E-Fez--3N and y’-Fe4N phases. However, the compound layer produced at 550 “C also included CrN phases in addition to the above phases (Table 2). Figure 3 shows how the white layer thickness changes with the total case depth. It should be pointed out that the variation of white layer thickness vs. total case depth is similar to its variation with the square root of time at both treatment temperatures. The deeper the total case, the thicker is the white layer; i.e. the longer the treatment time, the deeper is the total case and the thicker is the white layer.
335
16 -
20 0
2
4
8
6
12
IO
Square rOot of Time, \jh Fig.
2. Compound
layer
thickness
vs. square
root
of treatment
time.
16 -
E
14-
g
12-
1
5 -2
10
b b
8
3 ma Q) .3
6-
B
4: 20 0.0
I
I
0.1
0.2
0.3
,
I
0.4
0.5
0.6
Total Case Depth ,mm Fig.
3. Compound
3.2. Properties
layer
thickness
of the di@sion
vs. total
case
depth.
zone
Typical hardness distributions after plasma nitriding for various treatment times at 510, 530 and 550 “C are shown in Fig. 4. Figure 4(a) shows the hardness distribution of H13 steel after treatment at 510 “C for 25 h. It can be seen that the maximum hardness for this treatment condition is 1375 HV, with a total case depth of 0.185 mm. After treatment for 4 h at 530 “C the maximum hardness of the diffusion zone is slightly above 1250 HV. At the same treatment temperature the maximum surface hardness is slightly changed for treatment times of 16, 49 and 100 h such that it is about 1300, 1260 and 1192 HV respectively (Table 2).
0.1
“0
(a)
0.2
0.4
03
Distance from surface, mm
14al ,
1200
loo0
800
6lm
400
200
0.1
@I
0.2
0.3
0.4
0.5
0.6
0.7
Distance from Surface. mm
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.X
Distance from surface, mm
Fig. 4. Microhardness gradient of plasma-nitrided H13 steel: (a) stationary “C for 25 h; (b), (c) bar samples treated at 530 and 550 “C respectively.
disc treated
at 510
337
When the treatment time is increased from 4 to 100 h, the depth of nitride hardening is increased from 0.14 to 0.465 mm (Fig. 4(b)). On increasing the temperature from 530 to 550 “C, the maximum hardness of the diffusion zone is reduced by only 5@-80 HV. However, the total case depth produced at 550 “C is slightly deeper than that at 530 “C. If the plasma-nitriding temperature is increased from 510 to 590 “C, a significant loss of surface hardness is observed (81. The material core hardness decreased from 598 to 560 HV during elevated temperature nitriding at 550 “C for 16 h because the material continued to temper ahead of the nitriding front. Besides, this temperature is above the tempering temperature of the material. When the treatment time was increased to 49 and 100 h, the core hardness also decreased, to 520 and 480 HV respectively. This hardness value gives it a good combination of strength and toughness. In contrast, it was not altered during treatment at the same temperature for 4, 16 and 49 h at 530 “C since 530 “C is below the tempering temperature of the material, although after 100 h the core hardness had decreased from 598 to 480 HV. On the other hand, the tempering and treatment temperatures used are above the normal operating temperature, which reduces (but does not eliminate) thermal softening in service, because this material is usually operated at temperatures up to 400-500 “C [5, 111. The treatments carried out at 550 “C for 4 and 100 h produced total case depths from 0.145 to 0.545 mm, but the maximum surface hardness decreased considerably (Fig. 4(c)). At both 530 and 550 “C the total case depth increases with the square root of the treatment time up to 100 h (Fig. 5). It can be seen that the total case depths obtained from the treatment at 550 “C for different times are larger than those at 530 “C. Although in gas and bath nitriding, precipitations have an embrittling effect at the grain boundaries near the edges of the diffusion zone, these precipitations of nitrides and carbonitrides at the grain boundaries can be suppressed by plasma-nitriding treatment in ammonia. Figure 6 shows the precipitation (seen dark in the longitudinal direction) along the grain boundaries after etching in boiling sodium picrate of a sample treated at 550 “C for 100 h. The suppressed grain boundary precipitations are
0.6 c 0.5 ’-
E r”
[
0.4 -
Z B t
0.3 -
J 7 s r-
0.2
0.0
-
’ 0
1
2
4
6
I
I
8
10
Square root of time, dh
Fig. 5. Total
case
depth
vs. square
root
of time.
12
Fig. 6. Carbonitride precipitations for 100 h and etched in boiling especially
retention
advantageous of hardness,
in grain boundaries sodium picrate.
for thermal low adhesion
of H13
steel
plasma
nitrided
at 550 “C
stability, ductility, resistance and resistance to hot cracking
to hot [lo].
abrasion,
Dty sliding wear results The amount of wear has been widely determined by various methods. Therefore the comparison of wear results presents some difficulties. Nevertheless, the results obtained from different wear tests should be compared with each other; at least the results must be stated in the same wear unit. The determination of wear amount as weight loss is widely used and gives results exactly. If desired, weight loss can be altered to wear volume, e.g. where the change of dimensions is more important in mechanical systems [l]. The relationship between weight loss during the wear test and wear time for hot-working tool steel (H13) subjected to the plasma-nitriding process is given in Fig. 7. It can be seen that for short-time plasma-nitrided specimens the relationship is linear (Fig. 7(a)). On the other hand, the relation for long-time-treated specimens follows the classic form in that an initially high rate of wear is followed by a steady state wear rate (Fig. 7(b)). However, there is a considerable difference between the wear rates of the two treatment processes. Normally, the wear rate is highest at heavy loads and lowest at light loads for both treatment conditions. The wear rate of H13 steel plasma nitrided for 100 h is higher than that following treatment for 4 h owing to the thicker compound layer which has been produced on it, because thicker compound layers include some pores and are more brittle. Such compound layers can be easily broken off. Figure 8 shows the variation of weight loss US. test load. It can be clearly seen that the wear rates obtained from both wear tests are uniform and linear. Again, the higher wear rate is observed on the sample following long-time treatment. 3.3.
339
II
.
4M41.45
N
UntreatedO47.15 UntreatedD94.5
”
0
N N
10
5
15
20
25
30
35
Test time, min
6) 80
70
q
lOOh/147.15 N
.
lOOh/294.5 N
q
lOOhM41.45 N
.
60
lOOh/588.6 N UntreatedO47.15
II
N
Untreated1294.5 N Untreatedl441.45
N
30
20
0
5
10
Fig. 7. Weight h.
20
15
25
30
loads:
(a) treated
55
Test time, min
@)
loss VS. wear
time
under
various
for 4 h; (b) treated
for 100
The frictional forces during running were much higher. They were not recorded because the transducer lever for the measurement of frictional forces was not strong enough for high frictional forces. The phase structure of wear debris found using X-ray diffraction analysis consists of l-Fe2_3N and cr-FezOs. After the wear tests the thicknesses of the compound layer is reduced in both treatments. This reduction in compound layer thickness is 36.36% and 43.75% for treatments of 4 and 100 h respectively. Figure 9 shows the microstructures of the cross-section of wear discs plasma nitrided for 100 h (H13 steel) before and after wear testing under dry sliding conditions. It can be seen that the disc surface
“oFig.
8.
loo
200
300
400
500
600
Test load, N Sliding wear rate US. load for plasma nitrided wear disc.
treated for 100 h shows surface damage and failure. This is associated with the lower surface hardness and thicker compound layer of the disc treated for 100 h at 550 “C. However, the area below the compound layer of samples treated for 4 h was affected by frictional heat (Fig. 9(c)). The surface topographies following the plasma-nitriding treatment after wear testing are shown in Fig. 10. The surface damage on a disc treated for 100 h after running is much more extensive than that of a disc treatedfor 4 h, because the former has thicker compound layer which is porous and brittle. The mechanism is a mixture of adhesion and abrasion. The shearing of wear plates can be seen from the pictures. It can also be seen that there are some darker-coloured oxides and many shallow grooves parallel to the rolling direction. There are also many oxide abrasive particles adhering loosely to the metal surface. Consequently, the wear rate of H13 steel plasma nitrided for a long time, i.e. 100 h, but under the same conditions is much more than that of samples treated for 4 h owing to the fact that the former has a thicker compound layer and lower surface hardness.
4. Conclusions Plasma-nitriding treatments applied to hot-working tool steels for a short time can also be extended to long-time treatment. A considerable improvement in the case depth of AISI H13 is achieved with the aid of the long-time plasma-nitriding process. Long-time plasma nitriding makes it possible to achieve a total case depth in H13 of up to 0.5 mm with a cole~ hardness of 5fMl HY_ Althor@ a 245 KV loss of maximum surface hardness is observed, the total case depth is increased by about 3.7 times when the treatment time is raised from 4 to 100 h at 550 “C. In addition, the risks of distortion and surface finish operations are absent. Thus a significant increase in die life and reduction in cost can be achieved. The properties achieved by the plasmanitriding treatment are quite adequate for the actual requirements of tools made from
Fig. 9. Microstructures of surface layer of H13 steel treated for 4 and 100 h at 550 “C before and after wear test under 60 kg load: (a), (b) after treatment for 100 h; (c) heat-affected zone after treatment for 4 h. H13. The wide range of time and temperatures available with plasma nitriding can be used to obtain the required material condition. The wear behaviour of plasma-nitrided H13 steel depends on the thickness of the compound layer, because when the compound layer thickness is increased, the wear rate is increased by spalling of this layer. A compound-free diffusion layer formation can be readily achieved with plasma nitriding by simply choosing a treatment gas with an extremely low nitrogen content. When a thin monophase (y’-Fe,N), extremely ductile and wear resistant, which is never ground off, is desired, the only adjustment to be made is an increase in the nitrogen content of the treatment gas. Although ammonia was used as the treatment atmosphere in the present work, a change in nitrogen content of the treatment gas can be achieved by using a mixture of nitrogen-hydrogen or nitrogen-hydrogen-hydrocarbon as the treatment gas [S, 121.
(4 Fig. 10. Topographies of worn surfaces under load: (a) 4 h treatment; (b) 100 h treatment. Grinding of the compound layer (y’ + e) can also be done to reduce wear, thus decreasing the thickness of the compound layer.
the amount
of
Acknowledgments The author wishes to thank Professor T. Bell and Dr. A. M. Staines (Wolfson Institute for Surface Engineering, School of Metallurgy and Materials, University of Birmingham, U.K.) for their invaluable help with and comments on the work. References 1 M. B. Karamtg,
2 3 4 5 6
The analyses of the wear behaviours of tillage tool steels, Ph.D. Thesis, Erciyes University, Kayseri, 1985. Q. F. Peng, Improving abrasion wear by surface treatment, Wear, 129 (1989) 195-203. J. Kohopal, H. Hakonen and S. Kivivuori, Wear Resistance of hot forging tools surfaced by welding, Wear, 130 (1989) 103-112. J. K. Dennis and E. A. A. G. Mahmoud, Wear resistance of surface treated hot working dies, T&of. Inf., 20(l) (1987) 10-17. A. M. Staines and T. Bell, Plasma nitriding of high alloy steels, Proc. Con$ on Heat Treatment - Methods and Media, Institution of Metallurgists, London, 1979, pp. 58-69. T. Bell, Plasma heat treatment of tooling for the plastic industry, Plust. Rubber Process., I(4) (1976)
161-166.
7 B. Edenhofer,
Part 2. Industrial applications of the process (conclusion), Trait. Therm., 73 (in French). M. B. Karamrg and A. M. Staines, An evaluation of the response of 722M24 material to high temperature plasma nitriding treatments, Heat Treut. Met., 3 (1989) 79-82. B. Edenhofer, The ionitriding process thermochemical treatment of steel and cast iron materials, Metd Muter. TechnoL, 8(8) (1976) 421-426. B. Edenhofer, Improving tool surface quality by ionitriding, We&start und Betrieb, IO9(5) (1976) 289-293 (in French). D. L. Cocks, Longer life from H13 die casting dies by the practical application of recent research results, H&t %wt. Met., 2 (1988) 39-43. B. Edenhofer, Applications and advantages of nitriding treatments outside the normal range of temperatnres. Part 2. Treatments at high temperatures (above 580 “C), Hurf. Tech. Mitt& 30(4) (1975) 204-208 (in German). (1973)
8 9 10 11 12
31-43
Wear, I50 (1991) 331-342
An investigation of the properties and wear behaviour of plasma-nitrided hot-working steel (H13) M. B. Karamg Department
(Received
of Mechanical
February
Engineering,
Engineering
Faculty,
Erciyes
27, 1991; revised May 13, 1991; accepted
University,
Kayseri
(Turkey)
May 24, 1991)
Abstract The microstructural properties and wear behaviour of AN H13 steel which had been plasma nitrided at 530 and 550 “C for times between 4 and 100 h have been investigated. The effect of treatment temperature and time on the microstructure have been examined. The wear behaviour of material treated for 4 and 100 h has also been observed. It was seen that a total case depth of 0.55 mm with a hardness of 1000 HV can be achieved in 100 h. However, the white layer thickness is increased to 17 pm while the core hardness is reduced to 480 HV at 550 “C. The wear rate of the sample treated at 550 “C for 100 h is higher than that of the sample treated at 550 “C for 4 h.
1. Introduction Hot-working tool steels are widely used as dies in the metal-working industry for applications such as die casting, extrusion and hot forging. AISI H13 is a steel in this group which needs relatively high surface hardness and toughness after hardening, because it is subject to mechanical and severe thermal shock during hot metal working. The friction occurring between die and component leads to wear of the die surfaces, besides which the metal oxide layer formed on the component surface causes damage of the stressed tool surface since it provides a source of hard particulate material which leads to abrasive wear in the friction process. Since the wear resistance of a material is closely related to its hardness, it is necessary to apply a surface-hardening process [l] to hot-metal-working dies, such as nitrocarburzation, plasma nitriding, spark discharge or cladding [2], surfaced by welding [3], coating or chemical vapour deposition [4]. AISI H13 is often nitrided prior to use in the above type of application [S]. Although traditional gas nitriding can produce a surface hardness of 950-1200 HV 161, the compound layer produced on the surface often leads to spalling during working. Therefore this compound layer is removed by a finishing operation before the component is put into service. However, this operation reduces the adhesive resistance of the component. The abrasive and adhesive wear resistance of a hot-working die steel can be improved by means of a plasma-nitriding process. The plasma-nitrided die can be put into service without any finishing operation. Thus it has a reduced finishing cost and more importantly a reduction in the tendency to pick up. Besides, there are less downtime and repolishing costs [S]. The presence of these nitrided layers with a high tensile strength and very high resistance to wear enables the working life of the dies
0043-1648/91/$3.50
0 1991 -
Elsevier
Sequoia,
Lausanne
332
to be much extended. Plasma nitriding gives a working life four or five times longer than that of non-nitrided specimens and twice as long as that of pieces nitrided in a salt bath
[7].
2. Experimental 2.1. Surface
details
treatments
Hot-working tool steel (AISI H13 -x40 CrMoV 51) was used in this investigation. Its chemical composition analysed on an E 1000 Polyvac optical emission spectrometer is shown in Table 1. Two kinds of samples of different shape and dimensions were prepared from this material. One type were round bars turned to 25 mm diameter and 40 mm length in order to determine the results of plasma nitriding; the other type were wear discs. The wear couple consisted of an upper disc, a counterface and a lower (test) disc. The wear couple was machined to the dimensions shown in Fig. 1. All the samples were ground before vacuum hardening. They were preheated to 840 “C for 45 min, then heated rapidly to 1000 “C and oil quenched. Finally they were tempered at 550 “C for 90 min to give a hardness of 48-50 HRC. The hardened samples were prepared for plasma nitriding by polishing and degreasing. A 20 kW plasma-nitriding unit manufactured by Klockner Ionon GmbH was used for treatment. The wear discs and bar samples were plasma nitrided together in cracked TABLE
1
Compositions
of tested materials
Bar sample Wear disc
TABLE
(weight per cent)
C
Si
Mn
Ni
Cr
MO
V
cu
0.433 0.410
0.789 0.833
0.410 0.420
0.143 0.200
4.660 4.540
1.510 1.520
0.805 0.813
0.097 0.098
2.
Plasma-nitriding Treatment duration (h)
conditions
and results
Treatment temperature
(“C)
Compound layer and thickness (pm)
Total case depth (mm)
Maximum surface hardness (HV 0.3)
5.9 5.5
0.14 0.145
1274 1261
10.5 11.5
0.215 0.215
1300 1215
y’+e
13.5 14.8
0.330 0.395
1261 1204
y’+c
14.1 16.8
0.465 0.545
1192 1015
4
530 550
y’+e
16
530 550
y’ + E
49
530 550
100
530 550
333
Fig. 1. Test configuration of samples.
ammonia (pressure 2.5 mbar) at temperatures of 530 and 550 “C and for treatment times up to 100 h. The details of the plasma-nitriding processes are listed in Table 2. The samples were heated to the required temperature in about 2 h. After the chamber had been charged and the pressure in it had been reduced to 0.1 mbar, the power was switched on. The cracked ammonia was introduced slowly over a period of 2 h until the pressure was 2.5 mbar. At the end of each treatment the power was switched off automatically and the samples were cooled below 150 “C in the chamber in dissociated ammonia before discharging. Some of the hardened wear discs were not nitrided, so their wear was compared with that of plasma-nitrided discs. The treated bar samples were sectioned, mounted and polished for metallographic examination. Case depths were determined from microhardness measurements performed on a Leitz miniload hardness tester employing a 300 g load. The case depth at a hardness value 10% above the core hardness was taken as an accurate measure of the total case depth [8]. The microstructure of the plasma-nitrided cases was examined by optical microscopy. The phase structures in the compound layer produced on the samples and in the wear debris were determined by X-ray diffraction using Cr KY radiatioin. Scanning electron microscopy (SEM) was used to study the worn surface topography.
334 2.2. Wear experiments The wear tests were carried out as dry sliding wear. The upper discs were plasma nitrided at 510 “C for 25 h after hardening to give them a slightly higher surface hardness than the lower discs. Wear tests were carried out on a general purpose wear machine. Various rotation speeds, loadings and sample configurations are available with this machine, but a rotating test disc loaded &by the stationary counterface disc was chosen (Fig. 1). The upper disc was capable of being indexed in 12 positions, enabling 12 results to be obtained from each upper disc. Loads up to 25 kg dead-weight can be applied by the lever loading arm arrangement, giving an effective maximum sample load of 75 kg. The wear tests were carried out with effective loads of 15, 30, 45 and 60 kg applied. The rotational speed of the machine was 173.7 rev min-‘, giving a sliding speed 0.503 m s-‘. It was seen that under load this speed was maintained constant by the machine. The amount of wear was determined by means of an amdyticat balance accurate to 0.1 mg. The frictional force was not measured continuously in order to avoid destruction of the lever with the transducer by the high frictional force. After certain running times (certain sliding distances) the runs were interrupted and the test discs were taken out of the machine to determine the weight lost. Thus the effect of the test time on wear was determined. Wear debris generated during testing was carefully collected after completion of the wear tests and analysed by X-ray diffraction. The wear discs were cleaned in methanol before and after the tests. 3. Results
and discussion
3.1. Compound layer The structure of the layer depends on the plasma-nitriding conditions such as carbon content, pressure and mixture of the treatment gas, treatment temperature and time. A number of treatments carried out show that the compound layer thickness on H13 steel is increased by increasing the treatment time (Fig. 2). Increasing the treatment time from 4 to 100 h carried an increase in compound layer thickness from 5.5 to 16.8 pm at 550 “C. However, the increase in compound layer thickness at 530 “C is less than that at 550 “C. The compound layer thickness obtained from H13 steel treated at 510 “C for 25 h is 6.3 pm. For the same treatment time at temperatures of 530 and 550 “C the compound layer thickness is too thick, i.e. 11.8 and 12.8 /*rn respectively. On the other hand, a compound-free diffusion layer can be produced on H13 material by limiting the supply of nitrogen [9]. If the nitrogen content in the treatment atmosphere is extremely low, the compound layer is completely suppressed and only the diffusion zone remains. This structure is restricted to plasma nitriding and is very suitable for certain types of tool [lo]. It was determined by X-ray diffraction using Cr Kcr radiation that the compound layer on the plasma-nitrided samples at 530 “C consists of E-Fez--3N and y’-Fe4N phases. However, the compound layer produced at 550 “C also included CrN phases in addition to the above phases (Table 2). Figure 3 shows how the white layer thickness changes with the total case depth. It should be pointed out that the variation of white layer thickness vs. total case depth is similar to its variation with the square root of time at both treatment temperatures. The deeper the total case, the thicker is the white layer; i.e. the longer the treatment time, the deeper is the total case and the thicker is the white layer.
335
16 -
20 0
2
4
8
6
12
IO
Square rOot of Time, \jh Fig.
2. Compound
layer
thickness
vs. square
root
of treatment
time.
16 -
E
14-
g
12-
1
5 -2
10
b b
8
3 ma Q) .3
6-
B
4: 20 0.0
I
I
0.1
0.2
0.3
,
I
0.4
0.5
0.6
Total Case Depth ,mm Fig.
3. Compound
3.2. Properties
layer
thickness
of the di@sion
vs. total
case
depth.
zone
Typical hardness distributions after plasma nitriding for various treatment times at 510, 530 and 550 “C are shown in Fig. 4. Figure 4(a) shows the hardness distribution of H13 steel after treatment at 510 “C for 25 h. It can be seen that the maximum hardness for this treatment condition is 1375 HV, with a total case depth of 0.185 mm. After treatment for 4 h at 530 “C the maximum hardness of the diffusion zone is slightly above 1250 HV. At the same treatment temperature the maximum surface hardness is slightly changed for treatment times of 16, 49 and 100 h such that it is about 1300, 1260 and 1192 HV respectively (Table 2).
0.1
“0
(a)
0.2
0.4
03
Distance from surface, mm
14al ,
1200
loo0
800
6lm
400
200
0.1
@I
0.2
0.3
0.4
0.5
0.6
0.7
Distance from Surface. mm
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.X
Distance from surface, mm
Fig. 4. Microhardness gradient of plasma-nitrided H13 steel: (a) stationary “C for 25 h; (b), (c) bar samples treated at 530 and 550 “C respectively.
disc treated
at 510
337
When the treatment time is increased from 4 to 100 h, the depth of nitride hardening is increased from 0.14 to 0.465 mm (Fig. 4(b)). On increasing the temperature from 530 to 550 “C, the maximum hardness of the diffusion zone is reduced by only 5@-80 HV. However, the total case depth produced at 550 “C is slightly deeper than that at 530 “C. If the plasma-nitriding temperature is increased from 510 to 590 “C, a significant loss of surface hardness is observed (81. The material core hardness decreased from 598 to 560 HV during elevated temperature nitriding at 550 “C for 16 h because the material continued to temper ahead of the nitriding front. Besides, this temperature is above the tempering temperature of the material. When the treatment time was increased to 49 and 100 h, the core hardness also decreased, to 520 and 480 HV respectively. This hardness value gives it a good combination of strength and toughness. In contrast, it was not altered during treatment at the same temperature for 4, 16 and 49 h at 530 “C since 530 “C is below the tempering temperature of the material, although after 100 h the core hardness had decreased from 598 to 480 HV. On the other hand, the tempering and treatment temperatures used are above the normal operating temperature, which reduces (but does not eliminate) thermal softening in service, because this material is usually operated at temperatures up to 400-500 “C [5, 111. The treatments carried out at 550 “C for 4 and 100 h produced total case depths from 0.145 to 0.545 mm, but the maximum surface hardness decreased considerably (Fig. 4(c)). At both 530 and 550 “C the total case depth increases with the square root of the treatment time up to 100 h (Fig. 5). It can be seen that the total case depths obtained from the treatment at 550 “C for different times are larger than those at 530 “C. Although in gas and bath nitriding, precipitations have an embrittling effect at the grain boundaries near the edges of the diffusion zone, these precipitations of nitrides and carbonitrides at the grain boundaries can be suppressed by plasma-nitriding treatment in ammonia. Figure 6 shows the precipitation (seen dark in the longitudinal direction) along the grain boundaries after etching in boiling sodium picrate of a sample treated at 550 “C for 100 h. The suppressed grain boundary precipitations are
0.6 c 0.5 ’-
E r”
[
0.4 -
Z B t
0.3 -
J 7 s r-
0.2
0.0
-
’ 0
1
2
4
6
I
I
8
10
Square root of time, dh
Fig. 5. Total
case
depth
vs. square
root
of time.
12
Fig. 6. Carbonitride precipitations for 100 h and etched in boiling especially
retention
advantageous of hardness,
in grain boundaries sodium picrate.
for thermal low adhesion
of H13
steel
plasma
nitrided
at 550 “C
stability, ductility, resistance and resistance to hot cracking
to hot [lo].
abrasion,
Dty sliding wear results The amount of wear has been widely determined by various methods. Therefore the comparison of wear results presents some difficulties. Nevertheless, the results obtained from different wear tests should be compared with each other; at least the results must be stated in the same wear unit. The determination of wear amount as weight loss is widely used and gives results exactly. If desired, weight loss can be altered to wear volume, e.g. where the change of dimensions is more important in mechanical systems [l]. The relationship between weight loss during the wear test and wear time for hot-working tool steel (H13) subjected to the plasma-nitriding process is given in Fig. 7. It can be seen that for short-time plasma-nitrided specimens the relationship is linear (Fig. 7(a)). On the other hand, the relation for long-time-treated specimens follows the classic form in that an initially high rate of wear is followed by a steady state wear rate (Fig. 7(b)). However, there is a considerable difference between the wear rates of the two treatment processes. Normally, the wear rate is highest at heavy loads and lowest at light loads for both treatment conditions. The wear rate of H13 steel plasma nitrided for 100 h is higher than that following treatment for 4 h owing to the thicker compound layer which has been produced on it, because thicker compound layers include some pores and are more brittle. Such compound layers can be easily broken off. Figure 8 shows the variation of weight loss US. test load. It can be clearly seen that the wear rates obtained from both wear tests are uniform and linear. Again, the higher wear rate is observed on the sample following long-time treatment. 3.3.
339
II
.
4M41.45
N
UntreatedO47.15 UntreatedD94.5
”
0
N N
10
5
15
20
25
30
35
Test time, min
6) 80
70
q
lOOh/147.15 N
.
lOOh/294.5 N
q
lOOhM41.45 N
.
60
lOOh/588.6 N UntreatedO47.15
II
N
Untreated1294.5 N Untreatedl441.45
N
30
20
0
5
10
Fig. 7. Weight h.
20
15
25
30
loads:
(a) treated
55
Test time, min
@)
loss VS. wear
time
under
various
for 4 h; (b) treated
for 100
The frictional forces during running were much higher. They were not recorded because the transducer lever for the measurement of frictional forces was not strong enough for high frictional forces. The phase structure of wear debris found using X-ray diffraction analysis consists of l-Fe2_3N and cr-FezOs. After the wear tests the thicknesses of the compound layer is reduced in both treatments. This reduction in compound layer thickness is 36.36% and 43.75% for treatments of 4 and 100 h respectively. Figure 9 shows the microstructures of the cross-section of wear discs plasma nitrided for 100 h (H13 steel) before and after wear testing under dry sliding conditions. It can be seen that the disc surface
“oFig.
8.
loo
200
300
400
500
600
Test load, N Sliding wear rate US. load for plasma nitrided wear disc.
treated for 100 h shows surface damage and failure. This is associated with the lower surface hardness and thicker compound layer of the disc treated for 100 h at 550 “C. However, the area below the compound layer of samples treated for 4 h was affected by frictional heat (Fig. 9(c)). The surface topographies following the plasma-nitriding treatment after wear testing are shown in Fig. 10. The surface damage on a disc treated for 100 h after running is much more extensive than that of a disc treatedfor 4 h, because the former has thicker compound layer which is porous and brittle. The mechanism is a mixture of adhesion and abrasion. The shearing of wear plates can be seen from the pictures. It can also be seen that there are some darker-coloured oxides and many shallow grooves parallel to the rolling direction. There are also many oxide abrasive particles adhering loosely to the metal surface. Consequently, the wear rate of H13 steel plasma nitrided for a long time, i.e. 100 h, but under the same conditions is much more than that of samples treated for 4 h owing to the fact that the former has a thicker compound layer and lower surface hardness.
4. Conclusions Plasma-nitriding treatments applied to hot-working tool steels for a short time can also be extended to long-time treatment. A considerable improvement in the case depth of AISI H13 is achieved with the aid of the long-time plasma-nitriding process. Long-time plasma nitriding makes it possible to achieve a total case depth in H13 of up to 0.5 mm with a cole~ hardness of 5fMl HY_ Althor@ a 245 KV loss of maximum surface hardness is observed, the total case depth is increased by about 3.7 times when the treatment time is raised from 4 to 100 h at 550 “C. In addition, the risks of distortion and surface finish operations are absent. Thus a significant increase in die life and reduction in cost can be achieved. The properties achieved by the plasmanitriding treatment are quite adequate for the actual requirements of tools made from
Fig. 9. Microstructures of surface layer of H13 steel treated for 4 and 100 h at 550 “C before and after wear test under 60 kg load: (a), (b) after treatment for 100 h; (c) heat-affected zone after treatment for 4 h. H13. The wide range of time and temperatures available with plasma nitriding can be used to obtain the required material condition. The wear behaviour of plasma-nitrided H13 steel depends on the thickness of the compound layer, because when the compound layer thickness is increased, the wear rate is increased by spalling of this layer. A compound-free diffusion layer formation can be readily achieved with plasma nitriding by simply choosing a treatment gas with an extremely low nitrogen content. When a thin monophase (y’-Fe,N), extremely ductile and wear resistant, which is never ground off, is desired, the only adjustment to be made is an increase in the nitrogen content of the treatment gas. Although ammonia was used as the treatment atmosphere in the present work, a change in nitrogen content of the treatment gas can be achieved by using a mixture of nitrogen-hydrogen or nitrogen-hydrogen-hydrocarbon as the treatment gas [S, 121.
(4 Fig. 10. Topographies of worn surfaces under load: (a) 4 h treatment; (b) 100 h treatment. Grinding of the compound layer (y’ + e) can also be done to reduce wear, thus decreasing the thickness of the compound layer.
the amount
of
Acknowledgments The author wishes to thank Professor T. Bell and Dr. A. M. Staines (Wolfson Institute for Surface Engineering, School of Metallurgy and Materials, University of Birmingham, U.K.) for their invaluable help with and comments on the work. References 1 M. B. Karamtg,
2 3 4 5 6
The analyses of the wear behaviours of tillage tool steels, Ph.D. Thesis, Erciyes University, Kayseri, 1985. Q. F. Peng, Improving abrasion wear by surface treatment, Wear, 129 (1989) 195-203. J. Kohopal, H. Hakonen and S. Kivivuori, Wear Resistance of hot forging tools surfaced by welding, Wear, 130 (1989) 103-112. J. K. Dennis and E. A. A. G. Mahmoud, Wear resistance of surface treated hot working dies, T&of. Inf., 20(l) (1987) 10-17. A. M. Staines and T. Bell, Plasma nitriding of high alloy steels, Proc. Con$ on Heat Treatment - Methods and Media, Institution of Metallurgists, London, 1979, pp. 58-69. T. Bell, Plasma heat treatment of tooling for the plastic industry, Plust. Rubber Process., I(4) (1976)
161-166.
7 B. Edenhofer,
Part 2. Industrial applications of the process (conclusion), Trait. Therm., 73 (in French). M. B. Karamrg and A. M. Staines, An evaluation of the response of 722M24 material to high temperature plasma nitriding treatments, Heat Treut. Met., 3 (1989) 79-82. B. Edenhofer, The ionitriding process thermochemical treatment of steel and cast iron materials, Metd Muter. TechnoL, 8(8) (1976) 421-426. B. Edenhofer, Improving tool surface quality by ionitriding, We&start und Betrieb, IO9(5) (1976) 289-293 (in French). D. L. Cocks, Longer life from H13 die casting dies by the practical application of recent research results, H&t %wt. Met., 2 (1988) 39-43. B. Edenhofer, Applications and advantages of nitriding treatments outside the normal range of temperatnres. Part 2. Treatments at high temperatures (above 580 “C), Hurf. Tech. Mitt& 30(4) (1975) 204-208 (in German). (1973)
8 9 10 11 12
31-43