Trans Indian Inst Met DOI 10.1007/s12666-014-0504-6

TECHNICAL PAPER

TP 2896

Effects of Molybdenum Addition on the Microstructure and Mechanical Properties of Ni-Hard White Cast Iron Mohamed Mahmoud Mourad • Shimaa El-Hadad Mervat Mohamed Ibrahim

•

Received: 24 November 2014 / Accepted: 22 December 2014 Ó The Indian Institute of Metals - IIM 2015

Abstract The grades of Ni-hard white cast iron alloys (ASTM-A 532) are known as abrasion resistant white iron. A large number of castings are produced with composition modifications for specific applications. In the current study, grade IC Ni–Cr–GB was modified by addition of different amounts of Mo. The tensile strength, impact energy, hardness and wear resistance of the cast samples were determined and related to the Mo content of the alloy. It was observed that up to 0.96 wt% addition of Mo refines carbides and thereby improves the mechanical properties and wear resistance of the alloy. However, further addition showed a detrimental effect in strength and wear resistance. Concluding, Mo addition to abrasion resistant Ni-hard alloy should be decisively optimized because even a slight difference in the addition level may play a significant role in changing the wear resistance. Keywords White cast iron  Effect of molybdenum  Mechanical properties

1 Introduction White cast irons (WCI) have been the candidate of wear protection in the mining and cement industry. The alloys in the abrasion resistant white iron, ASTM A 532 is used for M. M. Mourad  S. El-Hadad (&)  M. M. Ibrahim Central Metallurgical Research and Development Institute, P.O. Box 87, Helwan, El-Tebbin, Cairo, Egypt e-mail: [email protected]; [email protected] M. M. Mourad e-mail: [email protected] M. M. Ibrahim e-mail: [email protected]

their excellent wear resistance and good impact toughness. They are of hypo- to hypereutectic composition and consist of hard carbide phases (HP) embedded in a hardenable metal matrix. Both constituents develop from the melt and vary in crystallographic structure and hardness depending on alloying and heat treatment [1]. Of this group, grade I is used to fabricate grinding balls, mill liners, etc. as shown in Fig. 1. According to ASTM, this group is a martensitic iron known as Ni-hard (IC Ni–Hi Ni–Cr–GB) cast iron. Martensitic white irons have largely displaced pearlitic white irons for making many types of abrasion-resistant castings, with the possible exception of chilled iron rolls and grinding balls. Although martensitic white irons cost more, their much superior abrasion resistance makes martensitic alloy white irons economically attractive [2]. The Ni-hard alloys are designed to be largely martensitic as-cast; the only heat treatment commonly applied is tempering. In order to improve their service performance; some researchers worked on developing Ni-hard alloys with modified chemical compositions to increase their wear resistance without reduction in the impact toughness [3, 4]. These alloying additions such as titanium, vanadium, and niobium act as a site for heterogeneous nucleation of carbides thus finer carbides and increased hardness can be obtained [4]. Recently, Mohammadnezhad et al. [5] have observed that the addition of 2 wt% vanadium to Ni-Hard4 WCI results in a fine microstructure with dispersed VC carbide. This microstructure exhibited the highest hardness and wear resistance without any decrease in impact toughness. Though the effect of alloy additions on the properties of the Ni-Hard white iron was previously investigated, most of the studies were focusing on high Cr WCI. On the other hand, low alloy white iron is the cheapest candidate for many applications such as grinding balls and mill liners. Mo is also added to increase the as-

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Trans Indian Inst Met Fig. 1 Applications of Ni-hard alloy. a Liner castings for ball mill and b grinding balls

Table 1 The chemical compositions of different heats (IC Ni–Hi Ni–Cr–GB) No.

Chemical composition (wt%) C

Si

Mn

P

S

Cr

Ni

Mo

Cu

Fe

1

3.30

0.866

0.912

0.0143

0.0119

1.16

3.06

0.02

0.151

90.46

2

3.46

0.936

0.902

0.0123

0.0251

1.18

3.01

0.522

0.152

89.77

3

3.38

0.871

0.905

0.0142

0.0120

1.15

3.04

0.96

0.157

89.48

4

3.40

0.852

0.912

0.0142

0.0128

1.14

3.07

1.45

0.163

88.96

5

3.2

0.855

0.889

0.0139

0.0123

1.13

3.03

1.53

0.156

89.16

cast hardenability thus providing a pearlite-free austenite matrix when irons are to be used in the as-cast condition for parts such as liner plates and large slurry pump bodies. In heat treated irons, Mo increases conventional hardenability so that even thick sections can be air-hardened after destabilization to give martensitic matrix [6, 7]. Mo also increases the tempering temperature which needs to be used, increases peak hardness by minimizing carbide precipitation coarsening and by precipitation hardening with Mo carbides, and broadens the temperature range over which the peak hardness is developed [8, 9]. In the current research, the influence of adding Mo on the microstructure and mechanical properties of low Cr white iron grade IC Ni–Cr–GB was investigated.

The final chemical compositions of the alloys as obtained from the spectrometer as an average of 3 readings are listed in Table 1. The investigation was carried out on alloys (No. 1–4) while alloy No. 5 was done only to study the effect of further Mo addition on the wear property. In order to control the molten metal processing, thermal analysis was performed using the standard equipment necessarily consisting of a sand cup instrumented with a thermocouple connected to a microcomputer. The thermocouples were placed in the middle part of the cup. The obtained cooling curves were used as a guide during the casting process. Based on the data obtained, the influence of Mo addition was explained in terms of its effect on the liquidus temperature and eutectic temperature recorded for each melt. 2.2 Microstructure Investigation

2 Experiments 2.1 Casting The Ni-hard alloys were prepared in a 100 kg medium frequency induction furnace. The melting temperature was about 1,500 °C. Before pouring, the dross and slag were removed and casting was started using green sand molds. The size of the specimen was the ASTM E8 standard impact test specimens (10 9 10 9 55 mm). Tensile test samples were cast according to ASTM E8 standard size.

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The microstructures of the specimens were characterized using the Olympus optical microscope (OM) and field emission microscopy equipped with an energy dispersive X-ray spectrometer. The various phases present in a specimen in different crystal structures were identified using XRD. 2.3 Mechanical Properties Evaluation An instrumented impact test machine using un-notched samples was used. The impact strength was evaluated

Trans Indian Inst Met

according to ASTM-E23. An average of five readings was considered. The samples hardness was measured on Vickers hardness tester. Six measurements were taken across each sample to obtain the average value. The wear resistance was evaluated using a pin on ring wear test type in which the sample is fixed against a rotating stainless steel wheel (63 HRC). The test was done at a speed of 265 rpm for 20 min under variable loading conditions of 90, 180 and 260 N. The wear resistance of the samples was measured in terms of the weight loss.

3 Results and Discussion 3.1 Thermal Analysis and Microstructure Characterization The thermal analysis curves of the four alloys with different amounts of Mo were recorded. Figure 2 represents the cooling curves of the alloy with minimum Mo addition; No. 1 (0.02 %) and that of 1.45 wt% Mo; No. 4. The characteristics of the cooling curves explained as liquidus temperature; TL, eutectic start; Tes, eutectic transition temperature; Te, and solidus temperature; Ts were summarized in Table 2. It is clear from this table that the liquidus temperature of the alloy increases and eutectic temperature decreases with Mo addition. Referring to the Fe–C phase diagram and the research of Li et al. [10], this shift in the temperatures occurred because of the presence of Mo which encourages formation of carbides and hence decreases the carbon and shifts the alloy position towards the left side in the phase diagram. According to [11], there is a direct correlation between the solidification temperature interval; DTE (Tes - Ts) and the diameter of the eutectic colonies (Ed) as follow; Ed ¼ 1:68  103 DTE

ð1Þ

This means that the narrower the solidification temperature interval, the shorter the time during which the liquid and solid phases co-exist in the same crystal growth conditions and thus the finer will be the resultant microstructure. Referring to Table 2, there is a general decreasing trend of DTE from alloy No. 1 to alloys No. 2–4 indicating the microstructure refinement by Mo addition.

3.2 Microstructure Investigation Figures 3 and 4 show the microstructures of the four different alloys with varying Mo additions taken for study. The microstructure of the reference alloy; No. 1 (Fig. 3a) consists of primary dendritic tempered martensite; a9 and a eutectic mixture of austenite; c and coarse cementite; h which is known as ledeburite [12, 13]. By addition of Mo, the amount of ledburite decreased gradually from alloy No. 1 (Fig. 4a) to No. 4 (Fig. 4d). The microstructure refinement observed in alloys; No. 2–4 is owed to the presence of Mo. This can be explained in the scope of the previous studies [7–9] as follow; during the solidification process, the first precipitated compounds have the maximum melting point. The melting points for the molybdenum carbide (MoC) and chromium carbide are 2,577 and 1,710 °C, respectively. As a result, it is possible for these minute high melting point particles (MoC) to act as nuclei for heterogeneous nucleation of chromium carbide, and lead to significant refinement of this carbide and improvement of their distribution and morphology. Figure 5 is the SEM micrographs of the four alloys showing the structure modification due to Mo addition. A field emission micrograph FEM of alloy No. 2 is represented in Fig. 6 as an example. The network of Cr and Fe carbides, spectrum A, is present over the austenitic–martensitic matrix. According to Eq. 1 [11] and Figs. 3, 4, 5, 6, the addition of Mo could refine this carbide network in alloys No. 2 and 3 while coarse dendrites were frequently observed for alloy No. 4 with 1.45 % Mo. 3.3 Mechanical Properties Evaluation Figure 7 represents Vickers hardness measurements of the five tested samples. It was noted that increasing the Mo content to 0.5 % has slightly increased the hardness (alloy No. 2) then a significant increase was obtained with 0.96 % Mo addition (alloy No. 3). This improvement in the hardness was expected due to the structure refinement observed for these alloys as represented in Figs. 3b, c, 5b, c. However, with further addition of Mo, the hardness remarkably decreased (alloys No. 4 and 5). Same trend was observed for the plot of tensile strength shown in Fig. 8, where the UTS increased from 312 MPa (0.02 wt% Mo) to the

Table 2 Characteristics of the cooling curves for alloys No. 1–4 Alloy no.

TL

Tes

Te

Ts

Tes - Ts

1

1230.07

1193.75

1134.11

1087.46

106.29

2

1232.07

1194.35

1130.13

1088.64

105.71

3

1235.15

1195.71

1127.77

1096.5

99.21

4

1239.05

1190.66

1125.12

1086.67

103.99

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Trans Indian Inst Met Fig. 2 Cooling and first derivative curves of Ni-hard alloy a No. 1 with 0.02 % Mo and b No. 4 with 1.45 % Mo

maximum of 353 MPa (0.96 wt% Mo). Upon increasing Mo addition, the UTS decreased to (322 MPa) (1.45 wt% Mo-alloy No. 4). The effect of Mo on the impact toughness is shown in Fig. 9. From this figure, it is obvious that the Mo addition improved the impact toughness of the alloy continuously up to 1.45 % Mo. The enhancement of impact toughness as well as the other mechanical properties up to 0.96 wt% Mo, is essentially because of the structure refinement observed with Mo addition. However, the impact toughness, Fig. 9, did not follow the same decreasing trend similar to UTS, Fig. 8, and hardness, Fig. 7, with 1.45 wt% Mo addition. In order to explain the obtained result, XRD pattern was investigated for alloys No. 3 (maximum of the increasing trend) and No. 4 (start of decreasing trend) and represented in Fig. 10. Comparing the two XRD patterns, it is observed that the

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peaks of Mo containing compound ((Cr 0.5Mo 0.5)3Si) is more pronounced in case of alloy 4 with lower hardness. Based on it, there is possibility that some ferrite is present in the matrix. This is because Mo and Cr as carbide promoting elements [14] consume the available carbon to form carbides then the rest of these elements encourage formation and stabilization of ferrite in the matrix [15]. Though the amount of ferrite might be too small to be detected by XRD, its effect on the hardness was significant. This result is in agreement with the work of Mohammadnezhad et al. [5], where they observed a sudden decrease in the hardness of low Cr NiHard4 WCI alloy with increased vanadium addition due to presence of ferrite. Since both of Mo and V are ferrite stabilizers, same effect is expected. Figure 11 shows the weight loss of the five alloys (No. 1–5) at different loading conditions. As it was mentioned in

Trans Indian Inst Met Fig. 3 Optical micrographs of alloys a No. 1 (0.02 wt% Mo), b No. 2 (0.5 wt% Mo), c No. 3 (0.9 wt% Mo) and d No. 4 (1.45 wt% Mo)

Fig. 4 Magnified optical micrographs of the Ni-hard alloys; a No. 1, b No. 2, c No. 3 and d No. 4

(a)

(b)

(c)

(d)

Sect. 2.1, alloy No. 5 (1.5 wt% Mo) was prepared mainly to investigate the effect of more Mo addition on the wear property, which is an essential property in the application of these low-Cr Ni-hard alloys as grinding balls. It is observed that under the minimum testing load, 90 N, the weight loss decreased with Mo addition up to 0.96 wt%

Mo then increased with further addition. Generally, wear resistance is dependent on matrix microstructure, carbide types and their volume fraction, fracture toughness and the hardness of the alloys [16–19]. Since increasing the hardness is known to improve the wear resistance in such kind of alloys, the results shown in Fig. 11 is almost in

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Trans Indian Inst Met Fig. 5 Scanning electron micrographs of alloys a No. 1, b No. 2, c No. 3 and d No. 4

1050

Hardness, Hv

1000

950

Alloy

Wt % Mo

No.1

0.02

No. 2

0.5

No. 3

0.96

No. 4

1.45

No. 5

1.53

900

850 No. 1

No. 2

No. 3

No. 4

No. 5

Alloy

Fig. 7 Hardness measurements of the different Ni-hard alloys 400

UTS, MPa

380

360

Alloy

Wt % Mo

No.1

0.02

No. 2

0.5

No. 3

0.96

No. 4

1.45

340

320

300 No. 1

No. 2

No. 3

No. 4

Alloy

Fig. 6 FEM micrograph of alloy No. 2 with 0.5 % Mo

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Fig. 8 Ultimate tensile strength of the different Ni-hard alloys

Trans Indian Inst Met 0.04

Alloy

Wt % Mo

No.1

0.02

No. 2

0.5

No. 3

0.96

No. 4

1.45

10

90 N

0.035

180 N

0.03

Weight loss, g

Impact toughness, J

15

260 N

0.025 0.02 0.015 0.01 0.005 0

5 No. 1

No. 2

No. 3

No. 4

Alloy

Fig. 9 Changes in impact toughness of the Ni-hard alloys with different Mo additions

Alloy

No. 1

No. 2

No. 3

No. 4

No. 5

Mo %

0.02 %

0.5 %

0.96 %

1.45 %

1.53 %

Fig. 11 Wear test results of alloys No. 1–5

1000

– 900 800

–

Intensity, (a.u.)

700 600 500 400 No. 4

–

300 200

–

100 No. 3

0

Position (2θ°)- Cu-Kα

Fig. 10 XRD pattern of alloys No. 3 (0.9 % Mo) and No. 4 (1.45 % Mo)

agreement with the plot of hardness represented in Fig. 7. Moreover, the difference in weight loss between alloys No. 3 and alloy No. 4 increased rapidly with higher testing loads. The alloy No. 5 with 1.5 % Mo and minimum hardness, Fig. 7, showed the maximum weight loss among all the alloys. Therefore a great attention should be paid to the elemental additives such as Mo when casting the low Cr–Ni-hard alloys for applications where considerable wear resistance is required.

4 Conclusions In this investigation, the role of Mo as an alloying element in controlling the mechanical properties of low Cr–Ni-hard WCI alloys was discussed and the following points were concluded;

Addition of Mo up to 1.5 wt% could modify the carbides network and accordingly refine the microstructure Addition up to 0.9 wt% Mo showed significant improvement in the mechanical properties of the Ni-hard samples, further additions caused decrease in the tensile strength and hardness while the impact toughness continues to increase The wear resistance of the Ni-hard alloys increased with Mo addition up to 0.9 wt% then more weight loss was observed with increasing Mo content. The effect was more pronounced at higher testing loads. The level of Mo addition to Ni-hard alloys should be taken into consideration especially for those applications requiring good abrasion resistance.

Acknowledgments The authors would like to thank the people in the Casting Technology laboratory, Central Metallurgical Research and Development Institute for casting and preparation of the samples.

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