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THE STUDIES OF COERCIVE FORCE OF ANNEALED AND HARDENED STEEL
OF TYPE 2% C AND ABOUT 12% Cr WITH ADDITION OF W AND V AFTER
HEATING AT THE RANGE OF 835 ºC TO 1150 ºC
Tadeusz Nykiel, Tadeusz Hryniewicz
TECHNICAL UNIVERSITY OF KOSZALIN
Raclawicka 15-17, 75-620 Koszalin, Poland, E-mail: [email protected]
Abstract
The main purpose of the investigation was to establish practical usefulness of coerciometer of
1.094 type to the evaluation of the structural changes and relationship/correlation between the
coercive force and the hardness of the NCWV/D3 steel quenched after austenitizing in the
temperature range from 835 ºC to 1150 ºC during up to 90 minutes.
As the result of studies carried out it was found that NCWV/D3 steel under annealed state, is
characterized with a small value of coercive force, equalled about 11 Oe. The coercive force
of the same steel under the quenched state after heating at 835 ºC, that is of about 10 ºC
higher than AC1F, increased up to about 34 Oe. It was stated in the studies that a substantial
increase in coercive force occurs in the steel samples heated during first 180 s.
The studies concerned with the effect of austenitizing temperature and time on the magnitude
of the coercive force in quenched NCWV/D3 steel samples indicates that its value is
increasing with the increase of temperature and time up to 30 minutes, from about 56 Oe at
900 ºC up to about 118 Oe, at 1150º C. The structure of matrix of the studied steel, quenched
after austenitizing in the temperature range of 900 to 1150 ºC, changes from martensitic to
austenitic, at the presence of about 22.5 to 13 vol % of carbides, respectively. A maximum
value of coercive force, equalling 122 Oe, was obtained for samples quenched after
austenitizing at 1150 ºC for 10 minutes. It was also found that extending of austenitizing time
from 30 to 90 minutes at 900, 950, and 1050 ºC did not affect any essential changes in
coercive force.
1. INTRODUCTION
It is well known that iron belongs to soft magnetic materials and is characterized with a small
value of self-restrained intensity (coercive force) which in turn is affected by multiple factors.
According to Reinboth [1] the iron under cast state reveals the coercive force of about 3 Oe,
and cheaper sorts of industrial use, about 1.5 Oe. Armco iron reveals coercive force about 1
Oe. The coercive force may be decreased even down to about 0.04 Oe by applying
appropriate technology of heat treatment. Carbon steels containing about 1 to 1.5% C under
quenched state are characteristic with a considerable magnitude of coercive force equalling
from 25 to 35 Oe [1]. Apart from carbon, other non-metallic elements such as N, S, P, and O2
highly affect the coercive force value with nitrogen causing the biggest effect on that value.
Concerning metallic elements, the tungsten addition with the amount of about 6% W allows to
obtain the coercive force value of about 65 to 70 Oe. The addition of chromium to steel also
enables to obtain the increase in the coercive force [1]. Experiments concerned with the effect
of annealing temperature on the coercive force of high-speed steels were carried out by Häfke
et al. [2] and they were carried out for DMo5 and EMo5Co5 steels. In the course of the
studies they found that DMo5 steel in the quenched state revealed coercive force on the level
of about 6500 A/m (81.66 Oe), and for EMo5Co5 steel, 7300 A/m (91.71 Oe).
Investigations of changes in the coercive force in Fe-Cr-N and Fe-Cr-C alloys during their
annealing were carried out by Moskalenko [3]. This author in his work claims that steel
containing, besides Fe, 1% Cr and 0.93% N, under quenched state is characteristic with the
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coercive force equalling about 60 Oe, whereas steel containing, besides Fe, 1% Cr and 0.79%
C, about 39 Oe.
The coercive force changes were studied also by Vajnsztejn and Livszic [4]. They
investigated the effect of bearing steel (SzX15 composed of 0.95-1.1% C, 1.3-1.6% Cr, 0.20.4% Mn,0.15-0.35% Si, and 0.25% Ni) structure on the changes of coercive force. They
found [4] that the value of coercive force of this steel depended in a high degree on the
content of retained austenite.
Apart from the steel chemical composition, the stresses arising in material during heat
treatment and machining, significantly affect the coercive force value.
The purpose of the present studies, with the results presented in this paper, was to establish to
what extent the inverstigations of changes in coercive force, performed by means of
coerciometer of 1.094 type, make it possible to use this apparatus to analyse the structure and
change in hardness as the function of austenitizing temperature and time in quenched steel
containing about 2% C and 12% Cr.
2. MATERIAL AND STUDY METHOD
The studies were carried out for steel of chemical composition given in Table 1.
Table 1. Chemical composition of the studied NCWV/D3 steel, in wt%
C
1.95
Cr
11.56
W
1.32
V
0.31
Mo
0.05
Ni
0.122
Mn
0.44
Si
Cu
P
0.27 0.073 0.024
S
N
0.022 0.016
The samples for the study were prepared from bars of diameter 13 mm, forged and soft
annealed, coming from one the same heat/melt.
Austenitizing of samples in the temperature range of 900 to 1150 ºC was carried out in the
furnace with controlled/protective nitrogen atmosphere. The temperature control accuracy
was ± 2 ºC. The austenitizing of samples at 835 ºC was carried out in a salt furnace using SH
630 (BaCl2+NaCl+SiO2+Al2O3) salt with the temperature control accuracy of ± 5 ºC. After
austenitizing, the samples were cooled down in the hardening oil.
2.1 Experimental procedure
Hardness testing of the steel after heating at 835 ºC was carried out using Vickers method
with the load of 294 N (30 kG/mm2 ). Hardness of the steel quenched after austenitizing in the
temperature range of 900 to 1150 ºC was measured using Rockwell method with C scale.
Coercive force measurements were performed by means of precision coerciometer of 1.94
type. Magnetizing of samples was carried out in two directions: L – left (HCL), and R – right
(HCR). The samples used fulfilled the proportion l/d=10 and l/d=5.5.
The structural investigation was carried out by means of microscope of Epityp 2.
Microsections of the samples quenched were prepared mechanically and afterwards they were
etched using reagent: 1 g picric acid + 5 cm3 HCl + 100 cm3 C2H5OH.
3. RESULTS OF THE STUDY AND DISCUSSION
In the course of investigations carried out it was found that NCWV/D3 steel in the annealed
state is characteristic with the coercive force value equalling about 11 Oe, whereas in the steel
quenched after annealing at the temperature about 10 ºC higher than AC1F, that is 835 ºC, the
coercive force increased up to about 34 Oe with the essential growth occurring after heating in
time up to 180 s (Fig. 1).
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600
Coercive force, Oe
40
500
30
400
Heating temperature 835 °C
20
300
10
200
835
Hardness, HV30
50
0
0
60
180
300
420
540
600
Heating time, s
Fig. 1. Effect of heating time at 835 ºC on hardness and coercive force value in the quenched
NCWV/D3 steel
Qualitatively very similar is the effect of heating time on the steel hardness (Fig. 1). A small
value of coercive force of the annealed steel results from its ferritic-carbides structure,
whereas the fact that the value of coercive force is much higher than that of Armco iron
results from the occurrence of high amount of carbides in the steel. Besides in ferrite of the
discussed steel, chromium (0.94%), tungsten (0.25%), vanadium (0.034%), with some
additive elements like manganese (0.38%), molybdenium (about 0.04%) and others
exist/occur. They surely result in the increase of coercive force in relation to that in iron.
The effect of austenitizing
temperature and time on
the coercive force value in
the quenched NCWV/D3
steel after austenitizing in
the temperature range of
900 to 1150 ºC for 10, 30,
and 90 minutes, is
presented in Fig. 2.
140
Coercive force, Oe
120
1150ºC
100
80
1050ºC
60
950ºC
900ºC
40
20
11.4
0
0
10
30
60
Austenitizing time, min
90
Fig.
2.
Effect
of
austenitizing temperature
and time on the coercive
force and hardness of the
quenched
NCWV/D3
steel
It results from Fig. 2 that the value of coercive force in the quenched NCWV/D3 steel
increases with the increase of austenitizing temperature and time in practice until 30 minutes
with the most intensive growth occurring after 10-minute austenitizing. Maximum value of
the coercive force, equalling 122 Oe, was obtained for samples austenitized at 1150 ºC during
the period of 10 minutes. It results also from Fig. 2 that, for NCWV/D3 steel, extending the
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austenitizing time from 30 to 90 minutes, for the assumed in the experiment temperatures,
practically does not influence a change in the coercive force value. The obtained results of
changes in the coercive force are well correlated with the steel structure, presented in Fig. 3.
Fig. 3. Effect of austenitizing temperature and time on structure of the quenched NCWV/D3
steel: (a, b) at 900 ºC, (a) for 10 min, (b) for 90 min; (c, d) at 1050 ºC, (c) for 10 min, (d) for
90 min; (e, f) at 1150 ºC, (e) for 10 min, (f) for 90 min
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120
120
100
100
80
80
60
60
40
40
Coercive force, Oe
Hardness, HRC
Contents of retained austenite, vol%
Carbide contents, vol%
In the studied NCWV/D3 steel quenched after austenitizing at 900 ºC, besides carbides,
martensite is a basic structural component with some insignificant contents of residual
austenite, that is on the level of about 3%. With the austenitizing temperature increase the
contents of residual austenite increases so that in the quenched steel after austenitizing at 1150
ºC and time above 10 minutes, the steel matrix in practice consists of austenite with about
12.5 vol% of carbides occurring in it.
To sum up the content of the paper, in Fig. 4 there is a graphical presentation of the course
change of the investigated properties in NCWV/D3 steel as the function of austenitizing
temperature for constant time equalling 30 minutes.
20
CARBIDES
0
900
950
1000
1050
1100
Austenitizing temperature, °C
1150
Fig. 4. Effect of austenitizing temperature at a constant time of 30 minutes on the following
properties of quenched NCWV/D3 steel: - coercive force,
- hardness, - contents of
residual austenite,
- contents of carbides in vol% (determined based on the measurements
of carbide grains’ intercept lengths [5])
It results from Fig. 4 that there is a distinct agreement/dependence between the hardness and
the value of coercive force in the quenched NCWV/D3 steel after austenitizing in the
temperature range of 900 ºC to about 1000 ºC. The obtained growth in the coercive force
value of samples in this temperature range results from the martensite formation and the
increasing contents of carbon and Cr, W, V. It is also known that high internal stresses arise
during quenching. From Fig. 4 it also results that the course of changes in coercive force and
hardness of samples quenched after austenitizing in the temperature range of 1000 ºC to 1150
ºC is opposite in its trend, that is the value of coercive force increases with the temperature
growth, whereas the hardness decreases. It should be stressed that the increase in coercive
force is more intensive than the decrease in hardness.
The increase in coercive force in samples of NCWV/D3 steel quenched after austenitizing in
the temperature above 1000 ºC results from the presence of the retained austenite, increase in
it the contents of carbon and Cr, W, V. Besides, the morphological features of the martensite
obtained in the temperature range of 1000 to 1150 ºC and defected structure of retained
austenite during cooling steel may affect the changes. In order to clarify these phenomena
univocally an extended investigation should be carried out.
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The stresses arising in the steel during its cooling lead also to defected structure of crystals of
carbides occurring in steel. Examles of defects existing in carbides of NCWV/D3 steel
quenched after austenitizing at 1150 ºC for 90 minutes, found during observation of thin foils
by means of a BS 613 transmission electron microscope, are presented in Fig. 5.
Fig. 5. Examples of defects occurring in M7C3 carbides of NCWV/D3 steel quenched after
austenitizing at 1150 ºC for 90 minutes, with austenite as the steel matrix. Thin foil,
photographs made on a transmission electron microscope: magnif. (a) 20000×, (b.c) 30000×
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5. CONCLUSIONS
The paper may be concluded with the following statements:
(1) The soft annealed NCWV/D3 steel is characteristic with a small magnetic/coercive
force equalling about 11 Oe. The value of the coercive force systematically increases
up to 120 Oe with the increase of temperature for the quenched steel after
austenitizing at 1150 ºC.
(2) The coercive force and hardness of the studied NCWV/D3 steel increases with the
increase of austenitizing temperature up to 1000 ºC and time of 30 minutes. Further
increase of temperature up to 1150 ºC results in a reverse course of these properties as
the function of temperature growth, that is the coercive force increases, and hardness
decreases, with more intensive coercive force than the decrease in hardness.
(3) It results from the investigation that the coerciometer of 1.094 type proved to be
helpful in the evaluation of the structural changes and the analysis of hardness
changes of quenched tool steels.
BIBLIOGRAPHY
1. REINBOTH H., Technologia i zastosowanie materiałów magnetycznych, Warszawa,
1964 (transl. from Technologie und Anwendung magnetischer Verkstoffe, Copyright
1958, VEB Verlag Technik, Berlin).
2. HÄFKE U., et al., Magnetische und elektrische Eigenschaften von Schnellarbeitsstahl
nach dem Anlassen, Neue Hütte, 1974, 12, 741-745.
3. MOSKALENKO Yu.N., Izmenenie koercitivnoj sily na pervoj stadii otpuska
zakalionnych splavov sistemy Fe-Cr-N i Fe-Cr-C, Cernaja Metallurgia, 1973, 8, 109111.
4. VAJNSZTEJN B.K, and LIVSZIC B.G., Zurnal techniceskoj fiziki, in Instrumentalnye
stali, by Yu.A. Geller, 1955, p. 230.
5. NYKIEL, T., HRYNIEWICZ, T., The Analysis of Carbide Phase Distribution in the
Annealed and Hardened Steel of about 2% C and 12% Cr Type with the Addition of
Tungsten and Vanadium, Paper pres. at this Conference, Hradec nad Moravici, 20-23
May, 2003.
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