Effect of Cerium and Lanthanum On The Microstructurea ND Mechanical Properties of AISID 2 Tool Steel
Effect of Cerium and Lanthanum On The Microstructurea ND Mechanical Properties of AISID 2 Tool Steel
Effect of Cerium and Lanthanum On The Microstructurea ND Mechanical Properties of AISID 2 Tool Steel
a r t i c l e i n f o abstract
Article history: AISI D2 tool steel has excellent wear resistance with high dimensional stability. This type of steel is
Received 20 November 2012 suitable for making molds. This paper describes investigations into the effect of adding Ce/La on
Received in revised form microstructure of AISI D2 type cold work tool steels obtained by means of optical microscopy, scanning
29 January 2013
electron microscopy, X-ray diffraction, energy dispersive X-ray spectrometry (EDS) and image analyzer.
Accepted 31 January 2013
Available online 14 February 2013
The results showed that after modification with Ce/La, the morphology, size and distribution of M7C3
carbides change greatly. The carbide network tends to break, and all carbides are refined and
Keywords: distributed homogeneously in the matrix, and also reduce the size of chromium carbides and increase
Cold work tool steel the dissolution of carbides during heat treatment. The results of mechanical tests show that the
Grain refinement
toughness of the alloy increased about 75% without reducing the hardness of the alloy.
Mechanical characterization
& 2013 Elsevier B.V. All rights reserved.
Fracture
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.msea.2013.01.074
194 M.A. Hamidzadeh et al. / Materials Science & Engineering A 571 (2013) 193–198
The alloy used in the present study conforms to AISI D2 tool Element C Cr Mo V Mn Si S P
steel, the chemical composition (in wt%) of which is: 1.5% C, 11.5%
(wt%) 1.45 11.45 0.58 0.88 0.37 0.18 0.01 0.025
Cr, 0.85% V, 0.6% Mo, 0.37% Mn, 0.2% Si, and Fe in balance. Firstly,
the alloy was melted in a 25 kg capacity medium frequency
induction furnace by using non-oxidation process. Once the alloy 2.2. Microstructural analysis
was melted in a furnace under argon gas atmosphere, about 0.1%
pure aluminum was added to deoxidize the melt. The microstructures of samples were characterized via optical
After dross and slag removal, the melt was cast into a dry CO2- microscopy, scanning electron microscopy (SEM), energy dispersive
silicate mold at a pouring temperature of about 1600 1C. The mold spectrometry (EDS), and X-ray diffraction (XRD). For metallographic
was formed in a Y-shaped geometry corresponding to ASTM examinations, the samples were prepared by grinding and polishing,
standard A571M [18] as shown in Fig. 1. and were etched electrochemically in a solution of 1 gr CrO3 þ
The melt was being cast at 1600 1C in two molds without 100 ml pure water with a power supply set to 10 V for about 60 s
adding a reformer (No. 1) and by adding a modifier (No. 2). The [20]. With this arrangement, the eutectic carbide volume fraction as
modifier included 65% Ce and 30% La, and was added about well as the size of grains was measured by using image analysis
‘‘0.03% wt’’ to the melt in the runner during pouring process. The software (Clemex Vision PE). The reported values for each sample
analyzed chemical compositions of set of samples produced for were taken from at least 10 measurements in different fields with a
this investigation are shown in Table 1. magnification of 200 times. The existing phases in the structure
The heat treatment of the specimens consisted of austenitizing were determined through XRD analysis using a diffractometer
at 1130 1C for 3 h with subsequent slow cooling in the furnace to (X’Pert Philips) operated at 30 kV and 25 mA with Cu Ka radiation
700 1C and holding at this temperature for 2 h [19]. Afterward, the (l Cu Ka ¼1.5405 Å). The local chemical microanalysis was also
samples were slowly cooled in the furnace to room temperature. carried out an EDS system (IFIABT-SR50 Eds) attached on SEM.
This heat treatment schedule was chosen in order to simulate the The mechanical properties reported for experimental specimens
cooling condition of steel ingots in industrial scale. are Charpy impact toughness and hardness. Room temperature
Samples for microstructural examination and mechanical testing impact tests were performed by using an Amsler Charpy impact
were taken from the same locations of castings, pointed by arrows in testing device. The size of specimens was 10 10 55 mm3 without
the scheme shown in Fig. 1, in order to guarantee exactly the same a notch according to ISO 5754 [21]. The reported impact toughness
cooling condition for samples during solidification. Because it was value for each sample is the average of three test results. The bulk
necessary to make sure, the obtained microstructures were controlled hardness measurements were carried out using Rockwell ‘‘C’’ scale
only by the modifying element but not by the cooling rate. with a load of 150 kgf applied for 30 s.
M.A. Hamidzadeh et al. / Materials Science & Engineering A 571 (2013) 193–198 195
Fig. 2. Optical micrographs show the effects of modifier on the morphology and distribution of eutectic carbides: (a) No. 1, unmodified steel and (b) No. 2, modified steel.
‘‘A’’ indicates the eutectic carbides and ‘‘B’’ indicates the matrix of alloy.
The volume fraction of austenite phase increases by decreas- These carbides form as net-like chromium carbide at dendrite
ing the melt temperature and, simultaneously the remaining boundaries. But in Fig. 2b, the eutectic structure is finer and the
liquid is saturated by carbon and alloy elements. Residual network chromium carbides are approximately broken. As indicated
melts and austenite form chromium carbides during the in this figure, the morphology of some eutectic carbides become
reaction of eutectic. isotropic as cerium and lanthanum are added.
(ii) Eutectic transformation (from 1289 1C): Table 2 shows the results of quantitative metallography samples.
L-M7 C3 þ g ð2Þ As seen, dendrite arm spacing (DAS) decreases in modified samples
and diameter of the eutectic chromium carbides decreases in
modified samples, too. The percentage of eutectic carbide phases is
A eutectic reaction occurs between carbon and alloying
low after improvement. Diameter and percentage of eutectic carbide
agents such as Mo, V, Cr in the retained melts, and ledeburite
phase decrease in all samples after heat treatment due to partial
comes into being. The ledeburite is composed of eutectic
dissolution of eutectic chromium carbide. It should be noted that the
carbides and austenite.
change in size and morphology of the eutectic structure is important
(iii) Finally, the M3C phase is formed as part of the peritectic
since it has a direct influence on the mechanical properties. On the
reaction (from 1183 1C):
other hand, adding modifier elements of Ce/La to the alloy caused a
Lþ M7 C3 -g þ M3 C ð3Þ decrement in the eutectic carbide volume fraction from 14.8 vol% in
sample No. 1 to 10.2 in sample No. 2.
This reaction completes the solidification process. Since the Fig. 3a shows the backscattered electron (BSE) micrographs of
former reaction involves solid-state diffusion, it is relatively sample No. 1 without improvement. The eutectic chromium
slow and rarely completed. Therefore, the M3C content in the carbides are as a black phase, and also there are very fine iron
final microstructure can be ignored. As observed in Fig. 2a, carbides as spherical or blade shapes. The iron carbides are
austenitic is fully developed. Because of the high speed of uniformly distributed in the matrix. In Fig. 3b, (BSE) micrographs
solidification, equilibrium phases can not form, which could of the improved sample No. 2 can be seen. The chromium carbides
be due to the possibility that some high cooling rate is are in the form of hexagonal blocks. The carbide particles are
converted from austenite to martensite. observed in the matrix of this alloy. The white particles are
observed in the center of some eutectic chromium carbides. Like
the previous sample, there are very fine iron carbides.
The quantitative EDS analysis of the areas specified is con-
3.2. Effect of Ce–La on the microstructure ducted, and the results are reported in Table 3 and Fig. 4. From
comparison of data in Table 3, it can be noticed that by adding
Fig. 2b shows the effect of Ce/La addition on the microstructure Ce/La, the percentage of chromium, vanadium and molybdenum is
of AISI D2 after heat treatment. As seen in the figure, the eutectic increased in the matrix. On the other hand, the atomic percentage
microstructure is dark. Generally, both samples showed a micro- of chromium, vanadium and molybdenum has decreased in the
structure comprising dendrites of austenite separated by interden- eutectic carbide.
drite eutectic carbides. As seen in Fig. 2a, the microstructure of steel XRD was performed on the samples, and the results are shown in
includes coarse austenite dendrites together with eutectic carbides. Fig. 5. According to the results of XRD analysis in Fig. 5, there are high
196 M.A. Hamidzadeh et al. / Materials Science & Engineering A 571 (2013) 193–198
Fig. 3. Enlarged backscattered electron images of eutectic carbides in (a) sample No. 1 and (b) sample No. 2 steel. Circles indicate needle-like and spot-like secondary
carbides.
Fig. 4. Representative EDS spectrum and quantitative composition analysis of cerium and lanthanum-contained precipitate in Fig. 3.
Fig. 6. Relationship of crystallizing planes of Ce2O2S and M7C3: (a) (1010)Ce2O2S//(1010)M7C3; and (b) (1010)Ce2O2S//(1010)M7C3. The hollow circle represents the atoms
in Ce2O2S, the solid circle and solid square represent the octahedron and tetrahedron in the M7C3 carbides, respectively [29].
Table 4
Mechanical properties of steels utilized in this study. The numbers after ‘‘ 7 ’’ indicate the standard deviation from the mean value.
For each alloy, the reported values are average of minimum 3 tests.
Fig. 7. Typical SEM fractographs showing the fracture surface of impact sample for (a) unmodified steel, No. 1 and (b) modified steel, No. 2.
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