1. No category

METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
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
Tadeusz Nykiel, Tadeusz Hryniewicz
TECHNICAL UNIVERSITY OF KOSZALIN
Raclawicka 15-17, 75-620 Koszalin, Poland, E-mail: [email protected]
Abstract
It was found during the experimental studies that in the forged rods of diameter of about 13
mm soft annealed and quenched after austenitizing at the temperature range of 900 ºC to
1150 ºC the distributions of intersept lengths of carbide particles are distinctly
nonsymmetrical. In the annealed steel the most numerous fraction is formed by the carbides of
the intercept lengths of about 0.5 µm, whereas after austenitizing at 1150 ºC during 30
minutes, the fractions of lengths from about 0.75 up to 1.75 µm with distinctly less number
than in annealed steel. Distributions of the carbide particles cross-sections, on the secant
0.056 mm long, in the quenched steel after austenitizing at 900, and 950 ºC are close to the
symmetrical distribution, and at 1000, 1050, and 1150 ºC are distinctly non-symmetrical.
1. INTRODUCTION
The process of carbides dissolution in austenite during austenitizing of tool steels has
fundamental meaning as it affects the degree of austenite saturation with carbon and alloying
elements [1-8]. Moreover the non-dissolved carbides inhibit the grain growth/coarsening of
austenite [9-11]. Authors of the higher mentioned references concerned with the process of
carbides dissolution analyse and discuss changes in carbides contents determined by the
electrolytic extraction method as the function of austenitizing temperature and time in steels
of type of about 2% C and 12% Cr. The weight percentage of carbides is analysed there. It is
well known that a flat metallographic specimen may serve to determine many parameters of
the spatial structure, e.g. specific volume of carbide phase, surface share/specific surface, and
assuming sphericity of carbides, also the number of spheres in one cubic milimeter and the
average diameter of spherical carbides [12, 13]. Most of the presented in the literature
investigation results concerned with the effect of austenitizing parameters on the parameters
of spatial structure in steels of about 2% C and 12% Cr type have a fragmentary character.
Some exemplary papers discussing the problems are [14-16].
The purpose of the work was to collect possibly significant amount of experimental data
concerning the effect of austenitizing temperature on the arrangement of carbides and their
size/intercept lengths. The spatial arrangement of carbides in bars of the studied NCWV/D3
steel versus austenitizing temperature and time will be presented in another paper.
2. MATERIAL AND EXPERIMENTAL PROCEDURE
The studies were carried out for NCWV/D3 steel of the composition given in Table 1.
Table 1. Chemical composition of the studied NCWV/D3 steel, 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 heat/melt. The heat treatment was carried out in the furnace with
1
METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
controlled/protective nitrogen atmosphere. The temperature control accuracy was ± 2 ºC. This
paper deals with the experimental results obtained on samples of NCWV/D3 steel austenitized
at 900, 950, 1000, 1050, and 1150 ºC during 30 minutes. After austenitizing, the samples were
cooled down in the hardening oil of a temperature of about 20 ºC.
2.1 Metallographic microscopic studies
Metallographic specimens were prepared mechanically using abrasive papers of graininess
from 100 to 2500. Afterwards the samples were polished using aquaeous Al2O3 suspension.
To reveal the carbides, microsections of annealed steel samples and quenched samples were
etched using Murakami’s reagent (3 g potassium ferrocyanide + 100 g KOH + 100 cm3 H2O).
Metallographic photographs were done by means od Epityp 2 microscope.
2.2 Determination of intercept lengths and a number of carbide grains cut
Measurements of intercept lengths were carried out on micrographs of magnification 1250x
by random drawing of secants 70 mm long, with the use of set of micrographs taken in
different sites of 3 samples quenched after austenitizing in higher mentioned temperatures.
Number of secants drawn on each set of micrographs equalled 84 whereas the number of
measured intercepts of carbide grains/particles for samples under annealed state was 1124,
and hardened after austenitizing at 900 ºC was 957, at 1000 ºC was 670, at 1050 ºC was 556,
and at 1150 ºC was 637.
In order to determine exactly the distribution of the number of carbide particles intersected,
the number of secants for samples quenched after austenitizing in the temperature range
from 900 to 1050 ºC was increased up to 154, and at 1150 ºC up to 230.
3. EXPERIMENTAL RESULTS
In order to determine the distribution of carbide particles’ intercept lengths in the NCWV/D3
steel annealed and quenched after austenitizing in the temperature range from 900 to 1150 ºC,
the obtained intercepts were grouped in respective distributive series of constant gradation of
intercepts equalling 0.2 µm. Intercept lengths’ distributions of carbide particles of lengths up to 3.2
µm and micrographs presenting carbides after etching with the use of Murakami’s reagent are
presented in Figs. 1 to 5.
Fig. 1. Picture of carbide particles in NCWV/D3 steel: (a) annealed, (b) quenched after
austenitizing for 30 minutes at 950 ºC. Murakami’s reagent was used for etching
2
METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
Number of carbide grains chords ncc
180
TA = 900 ºC
τA = 30 min
Σki = 84
ANNEALED
163
160
Σki = 84
156
140
120
100
Up to 3.2 µm
80
60
40
20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Intercept lengths of carbide grains [x0.2 µm]
Fig. 2. Histogram of the carbide particle intercept lengths in NCWV/D3 steel:
- quenched after austenitizing at 900 ºC for 30 minutes
- annealed,
Fig. 3. Picture of carbide particles in NCWV/D3 steel quenched after austenitizing for 30
minutes at: (a) 1000 ºC, (b) 1050 ºC, and (c) 1150 ºC
It results from the studies carried out that in NCWV/D3 steel bars of diameter 13 mm forged
and soft annealed, the distribution of carbide particles’ intercept lengths is approaching a
logarithmic-normal distribution, and carbides of the intercept lengths of 0.4 µm are the most
numerous fraction. Very similar behaviour concerning distribution is observed in the quenched
samples after austenitizing for 30 minutes in the temperature range from 900 up to 1150 ºC
with the successive decrease in the number of carbides of the smallest sizes with temperature
increase. In the quenched samples after austenitizing at 900 and 1000 ºC for 30 minutes the
most numerous fraction is still the carbides of the intercept lengths of about 0.4 µm, whereas at 1150
3
METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
Number of carbide grains intercepts ncc
ºC, the fractions of the range of 0.6 up to 1.4 µm. Worth of noting is the distribution of
carbides intercept lengths obtained for the steel samples quenched after austenitizing at 1200
ºC which indicates distinctly non-uniform, fluctuating, character.
180
TA = 1000 ºC
τA = 30 min
Σki = 84
Σci = 620
160
140
TA = 1050 ºC
τA = 30 min
Σki = 84
Σci = 515
120
100
Up to 3.2 µm
80
60
40
20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Intercept lengths of carbide grains [x0.2 µm]
Fig. 4. Distribution of carbide grains’ intercept lengths in NCWV/D3 steel quenched after
austenitizing for 30 minutes at: (a) 1000 ºC, (b) 1150 ºC
TA = 1200 ºC
τA = 30 min
Σki = 248
Σci = 509
Relative total number, %
20
TA = 1150 ºC
τA = 30 min
Σki = 160
Σci = 558
15
10
5
1
2
3
4
5
6
7
8
9
10
11
12
13
Intercept lengths of carbide grains [x0.2 µm]
14
15
16
Fig. 5. Distribution of relative number of carbide grains’ intercept lengths in NCWV/D3 steel
quenched after austenitizing for 30 minutes: at 1150 ºC, and
at 1200 ºC
4
METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
Fig. 6. Micrographs of sample surface fractures taken by means of scanning electron
microscope of Jeol JSM-S1 make on NCWV/D3 steel samples quenched after austenitizing
for 30 minutes at: (a) 950 ºC (b) 1000 ºC, and (c) 1100 ºC
To find out what is the behaviour of carbides of various sizes during cracking of samples
loaded with the force of impact character, the studies of samples quenched after austenitizing
at 950, 1000, and 1100 ºC for the same period of time equalled 30 minutes, were performed.
For the cracking experiment, cylindrical samples of diameter 10 mm, 100 mm long with the
V-shape notch made in the middle part of the sample, were prepared. The surface
pictures/micrographs obtained on the broken samples are presented in Fig. 6.
It results from the observation and micrographs of the fractures( Fig. 6) that:
(a) during cracking of samples due to impact loading, single big carbides crack along
different planes, the fracture surfaces are usually smooth, and often crack
perpendicular/normal to the primary fracture occur
(b) carbides of small sizes and spheroidal shape do not undergo cracking but are rather
drew out of one part and left embedded in another part of the fracture
(c) cracking of carbides in the area of their segregation leads to the arising a number of
projections appearing with sharp edges.
3.1. Discussion of results
Based on the carbide particles’ intercepts lengths measurements carried out the changes in the
volumetric share of carbides versus austenitizing temperature were determined. These specific
problems are to be considered in another paper where a point method is to be used to compare
the results with the determination results of weight share of carbides based on the method of
electrolytic extraction. Another parameters of steel structure which may be determined by
linear method of random secants are: (i) specific surface area of carbide phase and (ii) specific
length of carbide particles’ boundaries. In this paper, in order to increase the calculation
accuracy of these two parameters, the number of secants of 70 mm long, lreal = 0.056 mm, was
increased for steel samples quenched after austenitizing for 30 minutes at the temperature
range from 900 to 1100 ºC up to 154, whereas at 1150 ºC, up to 230. Counting of the number
of carbide particles cut was done based on the same micrographs which were used to measure
the carbide particles intercepts’ lengths. Distributions of the number of carbide particles cut
by secants of the length of 70 mm (lr = 0.056 mm) are presented in Fig. 7.
5
METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
Relative occurrence frequency
0.3
e
d
0.2
c
a
b
0.1
b
e
d
c
a
0
0
2
4
6
8
10
12
14
16
Number of carbide particles intersected
Fig. 7. Changes of relative frequency vs. number of carbide particles intersected after
austenitizing at: (a) 900 °C, (b) 950 °C, (c) 1000 °C, (d) 1050 °C, (e) 1150 °C
It results from the Fig. 7 that distributions of the number of carbide particles intersected in the
NCWV/D3 steel quenched after austenitizing at 900 °C, and 950 °C for 30 minutes time are
very close to symmetric, whereas the distributions after austenitizing at 1000 °C, 1050 °C,
and 1150 °C, are non-symmetric with the tail extended into the direction of increased number
of intersected carbide grains.
With the rise in austenitizing temperature the number of carbides on 1-mm distance is
decreasing from 192 in the quenched steel after austenitizing at 900 °C down to 73 after
austenitizing at 1150 °C. The results of calculations, with the course of changes of the specific
surface area and the specific length of carbides boundaries as the function of austenitizing
temperature, are presented graphically in Fig. 8.
It should be stressed that the presented distributions of carbide particles’ intercepts lenths as
well as distributions of number of carbides intersected are performed based on measurements
and counting carried out on micrographs of sample microsections normal to the bar axis
(transverse). On these microsections the big primary carbides are of shape more or less close
to the circle whereas many of these carbides in the longitudinal section are approaching rather
rectangle and other longitudinal figures resulting from the orientation of the biggest forces
acting during plastic working of metal. One should state then that calculated and given in this
paper the spatial parameters of carbide phase of alloys based only on the traverse
microsections do not reveal fully the real structure of these steels.
6
Specific length, mm/mm2
Specific surface area, mm2/mm3
METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
τA = 30 min
1000
800
600
400
200
900
950
1000
1050
1100
Austenitizing temperature, °C
1150
Fig. 8. Effect of austenitizing temperature at constant time of 30 min on:
area of carbide phase,
specific length of carabide grains boundaries
specific surface
4. CONCLUSIONS
Based on the studies carried out the following conclusions may be formulated:
1. In bars of NCWV/D3 steel of 13 mm diameter obtained by forging method and
undergoing soft annealing the main group of carbides are those of the intercept lengths
up to about 1.2 µm with the content of carbides systematically decreasing with the
increase of austenitizing temperature from 900 to 1150 °C. The process of dissolution
of these carbides affects the amount of carbon, chromium, tungsten, and vanadium in
the matrix of quenched NCWV/D3 steel.
2. During cracking of samples due to impact loading the carbides of small dimensions
are drew out of substrate remaining in one part of the fracture surface. The big primary
carbides usually crack along the plane running across the whole carbide particle
whereas cracking the carbides in the area of their segregation leads to arising series of
protruding sharp edges.
3. Distributions of intercepts lengths of carbide particles in NCWV/D3 steel annealed
and quenched after austenitizing in the temperature range from 900 to 1150 °C are
nonsymmetrical distributions, qualitatively similar one to another. On the other hand,
the distributions of number of carbide patrticle sections/cuts in samples quenched after
austenitizing at 900, and 950 °C for 30 minutes approach the symmetrical distribution,
and after austenitizing at 1000 °C, 1050 °C, and 1150 °C are non-symmetrical.
Number of carbide particles cut/cross-sectioned on the distance of 1 mm decreases,
with the increase of austenitizing temperature, from about 192 per mm in the
quenched samples after austenitizing for 30 minutes at 900 °C, down to about 73 per
mm after austenitizing at 1150 °C.
BIBLIOGRAPHY
1. SATO T., HONDA Y., NISHIZAWA Z., Tetsu To Hagane, 42, 12, 1956, 1118-1122.
2. GŁOWACKI Z., Badania nad węglikami stali chromowych, Mater. V Zebrania
Sprawozdawczego Komitetu Hutnictwa PAN, Zakopane, 1968.
7
METAL 2003
20. - 22. 5. 2003 Hradec nad Moravicí
__________________________________________________________________________________________
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
BERNS H., Härterei-Technische Mitteilungen, 29, 1974, 4, 236-247.
KAŁUŻA K., PhD Thesis, Politechnika Poznańska, Poznań, 1979.
KAŁUŻA K., Mechanik, vol. 60, 1987, 423-424.
NYKIEL T., PhD Thesis, Politechnika Poznańska, Poznań, 1982.
NYKIEL T., HRYNIEWICZ T., Effect of Austenitizing Parameters on the Change of
Concentration of Alloying Components and Phase Structure of Tool Steel of Type 2%C
and 12%Cr with Additives of W, Mo, V, Proc. of the 11th Congress of the International
Federation for Heat Treatment and Surface Engineering, Florence, Italy, 19-21 October,
1998, 116, pp.87-96.
PATANDONUJU A.H., Czernaya Metallurgia, Mietall, 3, 1980.
KOWALSKI W., ŻMIHORSKI E., Metaloznawstwo i obróbka cieplna, 6, 1973, 26-31.
OSTROWSKI B., BUDZYNOWSKI T, LISICA A ., Inżynieria Materiałowa, 5(130),
2002, 690-693.
NYKIEL T., HRYNIEWICZ T., Effect of austenitizing temperature and time on the
matrix composition and structure in steel of about 2% C and 12% Cr type with the
additives of W, V, Proc. of the METAL 2001 10th International Metallurgical and
Materials Conference, 15-17 May, 2001, Hotel ATOM, Ostrava, Czech Republic,
Symposium C: p.48, Paper no. 191.
RYŚ J., BLICHARSKI M., Proc of the 5th Conference on Metal Science. Metalografia
ilościowa, Kraków, 1968, 48-57.
RYŚ J., Wstęp do metalografii ilościowej. Biblioteka fizyki metali, wyd. Śląsk, Katowice,
19 70.
RODIĆ J., Żelezarski zbornik, 9, 1975, 2, 89-106.
RODIĆ J., Żelezarski zbornik, 10, 1976, 3, 125-142.
KULMBURG A., Mikrochimica Acta, Wien, ed. by Springer-Verlag, Suppl. 5, 1974,
181-206.
8