Investigation of effects of boron additives and heat treatment on carbides and phase transition of highly alloyed duplex cast iron

Materials and Design 30 (2009) 3174–3179

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Investigation of effects of boron additives and heat treatment on carbides and phase transition of highly alloyed duplex cast iron Yahya Tasßgin a, Mehmet Kaplan a, Mehmet Yaz b,* a b

Fırat University, Technical Education Faculty, Department of Metal Education, 23119 Elazıg, Turkey Fırat University Teknik Bilimler MYO, Vocational High School, Vocational High School, 23119 Elazıg, Turkey

a r t i c l e

i n f o

Article history: Received 19 September 2008 Accepted 17 November 2008 Available online 27 November 2008 Keywords: White–gray duplex casting Cast iron Chromium Microstructure

a b s t r a c t The effect of boron additives and heat treatment on the microstructural morphology of the transition zone in a duplex cast iron, which has an outer shell of white cast iron (with a high Cr-content and containing boron additives) and an inner side composed of normal gray cast iron, has been investigated. For this purpose, two experimental materials possessing different compositions of white–gray duplex cast iron were produced. Subsequently, metallographic investigations were carried out to study the effect of heat treatment applied to the experimental materials by using the scanning electron microscopy technique, along with optical microscopy and energy dispersive X-ray spectroscopy. Moreover, the formation of various phases and carbide composites in the samples and their effects on the hardness were also investigated using X-ray diffraction techniques. The results of investigations, and hardness showed that addition of the elements Cr and B to high-alloyed white cast iron affected carbide formation significantly, while simultaneously hardening the microstructure, and consequently the carbide present in the transition area of white–gray cast iron was spread out and became thinner. However, B additives and heat treatment did not cause any damage to the transition region of high Cr-content duplex cast iron. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Highly alloyed cast irons generally contain more than 3% of alloy elements and are commercially grouped as white and gray (or ductile) cast iron [1,2]. Among them, high chromium-content white cast irons are widely used in the mine-processing industry for processes such as grinding during the production of roller mills because of their absolute abrasion resistance, high stroke press, and good operability under corrosive conditions [3–5]. High chromium-content white cast irons contain 11–30% Cr and 1.8–3% C, and they form hard carbide eutectics on an austenitic main matrix. Although the carbide that is formed increases the abrasion and resistance of cast iron, it decreases the fracture toughness [5–7]. Therefore, to control the shape, size, amount, and distribution of carbide, addition of some modifying alloy elements and heat treatment are recommended. Within this scope, many researches have been carried out previously [8–12]. However, gray cast irons still retain their attractiveness as designing materials for both big-sized and small-sized simple and complex molding parts because they possess many technical advantages [12–17]. Nowadays, gray cast iron is still used in 65% of casting parts because of its performance advantage and low cost. Lamellar graphites on gray cast irons show superior abrasion * Corresponding author. Tel.: +90 424 2370000x4401; fax: +90 424 2188947. E-mail address: myaz@firat.edu.tr (M. Yaz). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.11.015

resistance, hardness, perfect machinability, limited lubricated-friction resistance, and vibration–absorption properties. These materials are comparable with steel in terms of pressing resistance, dimensional balance, and retention of their plastic shape under tension [15–18]. The strength of gray cast iron varies according to the structure and size of its matrix, and the distribution and type of its graphite lamellae. Carbon and silicon are the main alloying elements of gray cast iron and have a significant effect on its microstructure [17]. Chromium, manganese, vanadium, boron, and sulfur rank first as promoters of iron–carbide formation and as carbon stabilizers [9,10,14,19]. A combination of white and gray cast irons may be required for applications that need the features of high-temperature resistance (as in roller mill), toughness, abrasion resistance, ductility, vibration absorption, and cheapness of cost [18–20]. Therefore, production of white–gray cast iron as ‘‘Duplex” has gained great importance. In this method, the outer shell of the molded material is prepared from either alloyed or highly alloyed cast iron and the inner part is made from nonalloyed steel or gray cast iron. Thus, this method can be considered an alternative to the hardfacing method in processes where the outer layer needs to be thicker and harder (5–20 mm). However, this method requires the usage of special molding methods and carefully chosen molten materials of two different compositions at the same time. Superior and economical impact plates, hammers, excavator claws, compression

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Fig. 1. Cross-sectional view of sand molds used in experiment materials casting: (a) Schematic mold section filled with white cast iron up to riser level, and (b) Schematic mold section that was filled with gray cast iron solution material.

Fig. 2. Duplex cast iron experiment material’s S2: (a) Macro photograph of broken surface, (b) OM micrograph of white cast iron outer shell, (c) OM micrograph of white and gray cast iron transition area, and (d) gray cast iron inner area.

Table 1 Chemical compositions of experiment materials (Wt.%). Specimens

Production method

Alloy elements and Wt.% C

Si

Mn

Ni

Mo

Cu

Cr

B

Fe

S1 S2

Gray cast iron

3.21 3.77

2.11 2.71

0.35 0.28

0.28 1.47

0.08 0.06

0.32 0.29

0.12 15.06

– 0.48

Balanced Balanced

3.73

2.68

0.09

1.59

0.01

0.27

14.39

1.03

Balanced

Duplex Casting S3

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Fig. 3. SEM micrograph of the sample S1.

well scale hardness C) in its outer shell, can be obtained by applying controlled cooling or controlled heat treatment following the solidification. During the duplex-casting method, after the liquid material for the outer shell is poured into the mold, the liquid that is intended for the inner part is also streamed into the mold, and the casting process is continued. Therefore, the liquid material in the inner part of the mold is pushed upwards, so that the second material that enters later fills its space. In the upper part of the vertically placed casting mold, at the feeder or at the riser outlet, an excess of the valuable shell alloy accumulates. Later, this part is separated from the working parts and can be used for other casting applications. The effects of mold type, cooling speed, carbide formation, alloy elements, and heat-treatment application on the mechanical properties and microstructural morphology of the transition area located between the high-alloyed outer shell and low-alloyed or unalloyed inner part are very important and constitute a new subject of interest. For this reason, the effects of boron additives and heat-treatment application on the phase composition and carbide formation in the transition zone of high Cr-content white–gray duplex cast iron have been investigated.

2. Experimental procedure

Fig. 4. XRD patterns of S1.

clamps, and forging roller can be manufactured using this method. The material of high Cr-content cast iron for use in the outer shell of the molded part can be formed from hard and brittle austenitic chromium carbide that is spread throughout the impact-resistant filler environment. A casting material, which is more ductile on the inner side and which possesses a hardness of 60–70 HRC (Rock-

The samples of gray cast iron (inner material) and white cast iron (outer material), each weighing 50 kg, were separately melted in an induction furnace. Initially, white cast iron containing high levels of Cr and B additives was poured into a mold; the cross-sectional view of the mold is shown in Fig. 1a. Before letting the solution to solidify and form 8–10 mm thick layers, gray cast iron S1 melting material was immediately poured into the mold at 7–8 s intervals, as shown in Fig. 1b, using the same path up to the feeder level and the casting process is further completed. By the duplexcasting method, the production of an outer shell from the S2 and S3 experimental samples (made of white cast iron having high Cr-content along with B additive) and an inner part from gray cast iron was achieved. Subsequently, the experimental materials were annealed at 950 °C for 4 h, cooled in open air, and then tempered at 250 °C for 2 h. The experimental materials obviously have different thermal expansion coefficients [21]. Therefore, heat treatment was applied to samples to determine its effects on the cracks occurring either between the white cast iron and gray cast iron or in the outer shell.

Fig. 5. SEM micrographs of the sample S2: (a) white–gray cast iron interval area, and (b) a matrix of pearlite and plus carbides.

Y. Tasßgin et al. / Materials and Design 30 (2009) 3174–3179 Table 2 EDS analyses results of S2 sample. Region a

Region b

Element

Wt.%

Element

Wt.%

Chromium Manganese Iron Nickel Silicon Carbon Boron

29.82 1.73 58.91 0.62 0.31 7.53 1.12

Chromium Manganese Iron Nickel Silicon Carbon Boron

2.92 1.63 82.34 5.22 3.35 3.84 0.92

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area containing the outer shell, the transition zone, and the inner zone was chosen for analysis. For carrying out scanning electron microscopy (SEM) analyses, the samples were polished, cleaned with alcohol, and only the inner part of the gray cast iron was etched using 2% Nital for 20 s. Moreover, the samples were prepared for metallographic investigation by etching them using a secondary solution (25 g FeCl3 + 15 ml HCl + 100 ml distilled water), with which only the outer shell of white cast iron can react [22–24]. SEM investigations were carried out with a JEOL JSM– 5600 scanning electron microscope, in conjunction with a probe analyzer of the energy dispersive X-ray spectrograph. Moreover, with a point-surface analysis energy dispersive X-ray spectrograph placed inside the SEM device, different contrasts on the surfaces of the samples, the percentage composition of elements present in the white–gray cast iron parts or transition zones, and the types of resulting carbide, phases, and the compounds formed in the process were determined. Vickers hardnesses under a load of 50 g were measured using an average of 10 measurements for each sample. To determine the phases, the carbide compounds formed by heat treatment, and the amounts of Cr and B in the outer shell material sample, X-ray diffraction (XRD) analysis was carried out. The XRD patterns were obtained using a Shimadzu XRD–600 instrument with Cu Ka radiation (the wavelength of Cu Ka radiation is 0.15406 nm). 3. Results

Fig. 6. XRD patterns of S2.

The experimental samples used in this study were produced from white–gray casting materials having dimensions of 60  120 mm. As observed from the cross-sectional photographs of the experimental ingot materials shown in Fig. 2, extra care should be taken to obtain an 8–10-mm-thick outer shell made of white cast iron (with two different compositions of high Cr-content and B additives) and with inner parts of alloy-free gray cast iron. Table 1 shows the chemical percentage and the symbols of the materials used. During the casting of samples for both the high Cr-content and B additive – containing white cast iron outer shell material and the gray cast iron inner filling material, 99% Cr, 99% Ni, and 99% graphite (in purity) and 150 mg of 97% B2O3 powder (in purity, boric anhydride) are used. As indicated in Fig. 2, experimental pieces having dimensions of 15  15 -mm were cut out for microstructural investigation. An

Fig. 2a shows a sample of the casting with a solidified layer of white cast iron (about 8–10 mm thick) in the outer shell, which contains high levels of Cr and B additives. On the contrary, the inner part of the ingot is constituted by gray cast iron, as shown in Fig. 2b. The high-alloyed white cast iron in the mold, which is still in the liquid form, passes through the upper (riser) part of the mold, aided by the pushing force of the gray cast iron poured into the mold through the same path at intervals of 7–8 s. Thus, the middle of the mold is completely filled with gray cast iron. In addition, the optical microscopy image shown in Fig. 2b proves that the outer shell of the experimental material is formed from high Cr-content white cast iron and, the inner part, from gray cast iron. The microstructural morphology of the samples was determined using SEM, in conjunction with energy dispersive X-ray spectroscopy (EDS) analyses of both the resulting phase and the carbide compounds obtained. SEM image of the S1 sample that forms the inner filling is shown in Fig. 3. The S1 sample is composed of gray cast iron, with black graphite lamellae distributed

Fig. 7. SEM micrographs of the sample S3 interval area white–gray cast iron.

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Table 3 EDS analyses results of S3 sample. Region a

Table 5 X-ray diffraction parameters of S1. Region b

Elements

Wt.%

Elements

Wt.%

Chromium Manganese Iron Nickel Silicon Carbon Boron

36.23 1.93 53.12 0.71 0.12 7.21 0.72

Chromium Manganese Iron Nickel Silicon Carbon Boron

3.93 0.32 85.51 2.44 3.72 3.43 0.41

Phase

2-Theta(h)

d(A0)

I(f)

(h k l)

Fe5C2 Fe2C FeCr Cr23C6 Ni2Si

39.855 43.759 44.949 51.386 65.185

2.2600 2.0670 2.015 1.7767 1.4300

50.0 46.0 60.0 12.0 40.0

Undefined Undefined 202 Undefined Undefined

Table 6 X-ray diffraction parameters of S2. Phase

2-Theta(h)

d(A0)

I(f)

(h k l)

B8C FeC Fe5C2 Fe2C FeCr Cr2B CrB4 Cr2B6Ni3 Ni2Si

20.785 22.463 39.855 43.759 44.949 49.537 50.885 52.228 65.185

4.270 2.016 2.2600 2.0670 2.015 1.838 1.793 1.7500 1.4300

15.0 100.0 50.0 46.0 60.0 10.0 20.0 1.0 40.0

221 Undefined Undefined Undefined 202 080 220 0 10 1 Undefined

Table 7 X-ray diffraction parameters of S3. Fig. 8. XRD patterns of S3.

in different dimensions through the main pearlite matrix. The inner areas of experimental duplex samples are also made up of gray cast iron, having similar chemical composition and microstructural characteristics. The results of the XRD analyses of the S1 sample, and the compositions of the carbide and phases formed corresponding to those patterns, are provided in Fig. 4. From the XRD patterns, the main pearlite matrix formed in the S1 sample with Fe5C2, Fe2C, Cr23C6, and Ni2Si carbide and the phase compounds inside this matrix are determined. Table 5 shows X-ray diffraction parameters of S1. In Fig. 5a, the SEM image of the S2 sample containing 97 wt.% B2O3 powder additive is shown. In this image, the outer surface of white cast iron, the transition zone with white–gray cast iron, and inner regions of gray cast iron are indicated. In Fig. 5b, the 1000-times-magnified SEM image of S2 sample is provided, the results of the EDS analyses are shown in Table 2, and, in Fig. 6, the XRD patterns of S2 are reproduced. Considering the EDS and XRD results, in the microstructural image of S2 (a), the depicted areas (b) are deduced to be formed from round-shaped FeC, Fe2C5, B8C, and Fe2C carbides, Cr2B, Cr4B, Cr2B6Ni3 boronides, and Ni2Si compounds in a pearlite matrix. Table 6 shows X-ray diffraction parameters of S2. The SEM image of the transition zone in the S3 sample is depicted in Fig. 7a and b, EDS analyses results are provided in Table 3, and the XRD patterns and the corresponding phases and carbide compounds are indicated in Fig. 8. As observed from the SEM image, the microstructure of the inner part of 1.03 wt.% boron

Table 4 Hardness results of samples. Specimens

HV50g at 950 °C 4 h annealed

HV50g after at 250 °C 2 h tempered

S1 S2 S3

290 820 753

No treatment 822 748

Phase

2-Theta(h)

d(A0)

I(f)

(h k l)

B8C Mn5C2 Fe5C2 FeCr Cr6Ni2Si Ni31Si12 Ni2Si

20.785 26.749 39.855 44.949 49.410 49.554 65.185

4.270 3.330 2.2600 2.015 1.8430 1.838 1.4300

15.0 10.0 50.0 60.0 30.0 6.0 40.0

221 111 Undefined 202 222 302 Undefined

additive – obtaining gray cast iron S3 sample is formed of pearlite with uniformly distributed graphite and partially eutectic carbide. The white cast iron – containing outer shell of the S3 sample is composed of a pearlite main matrix with uniformly distributed carbide and other phases and compounds. In the transition zone of white cast iron, wide hexagonal particle sand small carbide particles in the pearlitic main body are distributed throughout the cross-sectional face (Fig. 7b). Considering the SEM images in Fig. 7b and the EDS analyses results provided in Table 3; the phases and compounds such as B8C, Mn5C2, BFe5C5, Cr6Ni2Si, Ni31Si12, and Ni2Si within the pearlitic structure of the S3 sample are determined. Table 7 shows X-ray diffraction parameters of S3. However, it should be stated that there was neither any damage nor any negative microstructural change caused by application of heat treatment other than the difference in hardness and increase of hardness in the casting zone of duplex casting in the case of the S2 and S3 samples containing B additives. In Table 4, the average hardness values of the transition regions in the duplex casting samples are provided in HV units (Hardness Vickers). As aforementioned, the hardness values of completely alloy-free gray cast iron S1 sample is observed to be lower than those of the S2 and S3 samples. The highest hardness increase is observed in the 0.48 wt.% B additive–S2 sample, with 820 HV. As stated in Table 4, with increasing percentage of (1.03) B-content in the S3 sample, the hardness decreases to 753 HV. After the tempering processes, significant hardness changes are not detected in the B additive – containing samples. From the XRD results, the unformed

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F2B + FeB or Fe2B iron–boride phases and the BC carbide present in the experimental samples can be inferred to cause this effect. In other words, phases and compounds such as FeC, Fe2C5, and Fe2C carbides, Cr2B, Cr4B, Cr2Ni3B6, and BFe5C5 boronides, and Mn5C2, Cr6Ni2Si, Ni31Si12, and Ni2Si formed in the S2 and S3 samples prevent the formation of iron–borides and, therefore, the hardness values did not increase as expected. 4. Conclusions In this study, experimental samples having an outer shell thickness of 8–10 mm, which contained high levels of Cr and B additive – containing white cast iron, and an inner part of gray cast iron were produced using the duplex casting method. Experimental samples were annealed at 950 °C for 4 h. and tempering was carried out at 250 °C for 2 h. Subsequently, the effects of Cr and B additives and application of heat treatment on the carbides and phase compounds of transition area in white–gray casting samples were investigated and the following results were obtained: Production of pieces of white cast iron, with increased hardness and high content of Cr and B additives, in the outer shell and a gray cast iron inner part was successfully achieved by using the duplex casting technique. In the samples produced using the duplex casting technique, three different metallographic structures were observed: an outer shell with high alloy – containing white cast iron, a transition zone of white–gray cast iron, and an inner region of gray cast iron. There is no cracking between the white cast iron and the gray cast iron or in the outer shell because of the different thermal expansion coefficients of the materials used. In the white–gray transition zone of the samples, carbide blended with the inner gray cast iron region, and similarly, the inner gray phases blended with the outer shell. This situation can be correlated with the direction of heat withdrawal from the molten materials or the direction of solidification. Addition of the elements Cr and B to the alloy and subsequent heat treatments caused the formation of carbides, phase compounds such as Fe5C2, Fe2C, Cr23C6, MnSi, Ni2Si, Mn5C2, Cr6Ni2Si, and Ni31Si12, and boronides such as CrB, Cr4B, Cr2Ni3B6, and BFe5C5, which significantly increased the hardness of the samples. A sufficient number of studies are not available on this subject in the literature. Therefore, it is hereby recommended to future investigators to study similar behaviors of other alloys by application of the same method.

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