Effect of molybdenum and niobium on the wear behaviour of high chromium white cast iron
K. BOUHAMLA*; H. MAOUCHE**; A. HADJI**
* Iron and Steel Applied Research Unit, Annaba, Algeria (URASM / CSC), bkh13@yahoo.com
** Foundry laboratory research, Annaba University, Algeria
Abstract
High chromium white cast irons are excellent wearable materials. Their wear resistance is due to the presence of high volume fraction of hard carbides in microstructure. Accordingly, this material is very suitable for many applications such as mineral processing, cement production and other industrial fields. Researchers have tried different alloying elements in order to improve wear resistance of this material. An investigation was conducted to determine the effect of alloying elements on the friction and wear behaviour of high chromium cast iron with various concentrations of molybdenum and niobium. The alloys used in this work were made in industrial induction furnace. Alloying elements were added in crucibles after pouring operation. Amounts of molybdenum and niobium were varied between 0 and 3%. A molybdenum and niobium combination of an amount of 0.5% was also studied in this work.
Wear investigations were carried out on the as cast and heat treated state of a slightly hypoeutectic high chromium cast iron.
Results showed that the studied alloy is composed of netted eutectic chromium carbides embedded in an austenitic matrix. Wear looses carried out on as cast and heat treated cast irons indicates that the best results were obtained by abrasion with 3% Mo alloy. Friction tests has also given good results but less important. Manganese comparing to niobium addition has much more facilitated the abrasion than the friction resistance. These results indicate that the alloying elements do not act similarly on the two type of studied wear
Key words: high chromium cast iron, microstructure, wear, friction, and carbide.
1. Introduction
White chromium cast irons are widely used in several manufacturing fields. They constitute a family of materials that present a wide range of characteristics depending on chemical composition and heat treatment [1-2]. These wide application areas are due to their high wear resistance. In general wear resistance is improved by micro alloying, by heat treatment or by adopting a suitable cooling rate [3-5]. Manipulating these variables gives different carbide volume fraction, different carbide composition and different matrix. It is reported that the abrasion wear resistance is dependent of morphology, of the amount, and of the distribution pattern of the carbides and of the type of matrix.
Wear is an important area to be taken in consideration in tool and machine parts manufacturing. High chromium cast irons are well suited for these applications such as rock machining equipments, cemeteries and metallurgical machinery [6]. They have a high chromium content witch allow a higher toughness and wear resistance. Chromium carbide present in the structure of high chromium cast iron improves this material’s mechanical strength and wear resistance. These carbides are holded by the matrix during wear. When the matrix is soft, it can be worn away easily during service stress.
White cast iron martensitic matrix allows high performance of wear resistance and toughness.
Suitable carbides and matrix should be present in high chromium white cast iron microstructures. Two types of carbides are expected in these microstructures [7]; one of them is eutectic carbide with a discontinuous repartition and the other is secondary carbide.
In the present work, the effects of molybdenum and niobium on wear behaviour of high chromium cast iron are studied. Their amount was varied in the ranging from 0.5 – 3% and combined at 0.5%. Increasing molybdenum and niobium was undertaken to investigate their effects on microstructure and wear resistance in order to determine the molybdenum and niobium optimum content. Material studied is an industrial made 15% chromium white cast iron used in milling balls machines parts production. This material is subjected to a high stress during process.
2. Experimental
2.1. Melting and casting
Alloys used in this work are obtained from an industrial induction furnace. Molybdenum and niobium are added as ferroalloys to the basic high chromium cast iron. After melting, samples are transferred from the furnace into a preheated crucible were carbides former are added.
After this operation samples were poured into cylindrical sand moulds. The cooled metal castings having the dimensions 20 X 10 mm are sectioned before heat treatment for wear tests and metallographic observations (fig 1). The chemical composition of the basic white cast iron used in this work is listed in table1.
Table 1 Chemical composition of basic high chromium white cast iron
C Si Mn S P Cr Mo
2.31 0.87 0.77 0.08 0.031 15.01 0.02
2.2. Heat treatment
The test samples are austenitized at 980°C and air quenched to room temperature.
Subsequently, they are tempered at 250°C followed by air cooling to ambient temperature.
2.3. Metallographic examination
For metallographic examination, samples were prepared using 220-1200 mesh emery paper and polished using 3 μm diamond paste. Following the polishing operation, they were etched in 4% nital solution in order to distinguish the phase clearly. Optical metallography was performed on the studied samples in the as cast state and after heat treatment using LEIKA optical device.
Fig 1. Device for casting samples
Mould Sample
Fig 2. Device for abrasion test
Engin sample Ball Silica
sand
Sample holder
sample
Fig 3. Device for friction test
2.4. Wear tests Abrasion test
Abrasion tests wear were performed with a laboratory apparatus. The details of this experimental rig are shown in figure 2. This technique consists in putting samples in a crusher with some siliceous sand to increase the wear rate. The weigh loss is measured every 5 minutes of interval during the test.
Friction test
This test was realised in order to simulate the wear phenomena witch take place in industrial scale. Wear tests were carried out on pin-on disk of wear testing apparatus shown schematically in figure3. The tests were carried out against 120 quartz disk at 120 rev/mn at constant speed and under a load of 0.5 Kgf with a travel of 40 m. After the specimen were weighed using a digital scale to determine the weigh loss.
3. Results
Table 1 shows the chemical composition of the basic high chromium cast iron used for this study. Molybdenum and niobium content are varied between 0.5 and 3% each one, in order to produce several alloys with different amount of alloying element. The scope is to study the effect of carbide forming elements on structure characteristics and wear behaviour. The chemical composition shows that the master alloy is a slightly hypoeutectic high chromium cast iron. According to Fe-Cr-C phase’s diagram, austenite is the first phase precipitating during solidification witch is followed by eutectic transformation. Figures 4, 5 and 6 show micrographs of as cast and heat treated basic and alloyed high chromium cast irons. Basic cast iron (4a, 4b) consists of netted hard eutectic M7C3 carbides embedded in the matrix. In the as cast state, the matrix is austenitic and become martensitic after heat treatment. This is clearly represented by dark areas in micrographs (fig.4b). The eutectic is not affected by this transformation, it till the same. Some bright areas are observed in these micrographs witch obviously are retained austenite. Secondary fine scale precipitations appear homogenously distributed in the matrix witch may be Cr7C3 carbides.
Samples with 3% molybdenum are represented by micrographs 5a and 5b. Figure 5a shows micrographs of as cast high chromium cast iron containing 3% molybdenum. The matrix is austenitic because it is formed under non equilibrium cooling rate. The eutectic is well netted and it consists of austenite and (Cr,Fe,Mo)7C3 carbides. In figure 5b, the heat
a b Fig.4. Microstructures of basic white cast irons. a: as cast; b: heat treated
Austenite eutectic
Martensite
Eutectic
X 150 X 1000
treated microstructure exhibits a substantial secondary precipitation of nodular form in the matrix. Molybdenum and niobium are two strong carbides formers. According to literature [11-12] their carbides have a high melting point and a high hardness. Previous researches [8- 10; 16] indicate that the molybdenum is partitioned between eutectic Mo2C, M7C3 and the matrix, when it is ranged between 0.5 and 4%. When the Cr/C ratio (Cr/C=6) is low, the solidification of samples containing molybdenum lead to the formation of small patches of eutectic Mo2C carbides. Such carbide is commonly found joined together with the M7C3 [17].
The molybdenum dissolved in austenite is the one that really helps to improve hardenability in the alloy. Furthermore it promotes the secondary precipitation hardening of matrix by tempering.
Figure 6a and 6b shows micrographs of as cast and heat treated high chromium cast iron containing 3% niobium respectively. In the as cast state (fig.6a), the microstructure is mainly composed of an austenitic matrix and a netted eutectic. The solidification of samples containing niobium starts with precipitation of proeutectic niobium carbides.
a b
Fig.5. Microstructures of 3% Mo high chromium cast irons. a: as cast; b: heat treated
This transformation occurs at some high temperature when the alloy still liquid. Therefore, in hypoeutectic white cast irons, a proeutectic matrix reinforced with NbC carbides is expected at the end of solidification. These carbides may refine the structure because they act as nuclei of the austenite phase. During heat treatment a depletion of alloying element and Carbone occurs, that lead to secondary carbides precipitation. Heat treated sample microstructure (fig.6b) exhibits fine scale precipitations of uniform size witch appear dark and homogenously distributed in the matrix. It consists of primary and secondary carbides not well revealed by optical investigation. Niobium attracts some carbon of the melt and reduces the remained carbon. The formation of these carbides consume part of the carbon present in the alloy so that during eutectic solidification the volume of M7C3 carbide is diminished comparing to the base alloy and 3% Mo containing high chromium cast iron.
Casting with 0.5% (Nb+Mo) (fig.7a; 7b) do not exhibit a microstructural evolution.
Molybdenum and niobium may be partitioned in the matrix, because of their low amount in the alloy.
Wear tests results are represented by figures 8 and 9 which relate the as cast and heat treated states of the studied alloys. Figure 8 exhibits abrasion (8a) and friction (8b) looses for the different studied alloys in the as cast state. It shows that in the as cast sate (a, b) molybdenum and niobium act differently on high chromium cast iron abrasion and friction behaviour. It means that these two mechanisms occur differently.
Secondary precipitations Austenite
Eutectic
Martensite
Eutectic carbide
X 150 X 1000
a b
Fig. 6. Microstructures of 3% Nb high chromium cast irons. a: as cast; b: heat treated
a b
Fig.7. Microstructures of 0.5% (Nb+Mo) high chromium cast irons.
a: as cast state; b: heat treated state
Previous study indicates that wear resistance of high chromium cast iron is related to the microstructural parameters of the eutectic carbides, such as volume fraction, carbides type, carbide size and distribution and the matrix [13]. Molybdenum in the alloy partition in the eutectic M7C3 carbide and some dissolves in austenite, but a part of molybdenum also forms a type M2C eutectic carbide [8, 14, 16]. Niobium preferentially partions into NbC in the Fe-C- Cr alloy and its concentration in the matrix and M7C3 carbides is very low. It forms very hard carbide NbC [15]. Results obtained indicate that comparing to alloys containing niobium, tests carried out on alloys containing molybdenum, in the as cast state, exhibit good abrasion behavior. The best results are obtained by abrasion with the cast iron containing 3%
molybdenum and by friction with the cast iron containing 3% niobium. These results are due to the presence of proeutectic MC type carbide and M2C and M7C3 type eutectic carbides. The first carbides are embedded in the matrix; they precipitate at high temperature before eutectic
c
Austenite
Eutectic
Martensite
Eutectic
X600 X 150
Austenite
Eutectic
Martensite Eutectic carbide
Secondary precipitation precipitations
Martensite
X 150 X 1000
transformation witch promotes friction behavior. The second are Mo2C and (Cr,Fe,Mo)7C3. Chromium and iron (Fe) are readily dissolved in molybdenum rich carbide [18].
Metallographic device is not appropriated to reveal these carbides because of their fine size.
Figures (9a) and (9b) shows respectively abrasion and friction tests results after heat treatment. Affecting a change in molybdenum and niobium contents from 0.5% to 3% records an increase in wear resistance after heat treatment. The higher Mo containing sample shows improved wear resistance (abrasion and friction looses) compared to those containing niobium.
0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2
Basic alloy 0,5%
Mo 1%
Mo 1,5%
Mo 2%
Mo 2,5%
Mo 3%
Mo 0,5%
Nb 1%
Nb 1,5%
Nb 2%
Nb 2,5%
Nb 3%
Nb
0,5 % (M o+Nb)
Alloyed cast irons
Abrasion loos (%)
0 0,02 0,04 0,06 0,08 0,1 0,12 0,14
Basic alloy 0,5%
Mo 1%
Mo 1,5%
Mo 2%
Mo 2,5%
Mo 3%
Mo 0,5%
Nb 1%
Nb 1,5%
Nb 2%
Nb 2,5%
Nb 3%
Nb
0,5 % (M o+Nb)
Alloyeed cast irons
Friction loos (%)
a b
Fig. 8. As cast Weight loos of as cast alloyed cast irons. a: abrasion loos; b: Friction loos
0 0,01 0,02 0,03 0,04 0,05 0,06
basic alloy 0,5%
Mo 1%
Mo 1,5%
Mo 2%
Mo 2,5%
Mo 3%
Mo
0,5 % (M o+Nb)
0,5%
Nb 1%
Nb 1,5%
Nb 2%
Nb 2,5%
Nb 3%
Nb
alloy ed cast irons
Abrasion loos (%)
0 0,005 0,01 0,015 0,02 0,025 0,03 0,035
Basic alloy 0,5%
Mo 1%
Mo 1,5%
Mo 2%
Mo 2,5%
Mo 3%
Mo 0,5%
Nb 1%
Nb 1,5%
Nb 2%
Nb 2,5%
Nb 3%
Nb
0,5 % (M o+Nb)
Alloyed cast irons
Friction loos
a b
Fig. 9. Weight loos of alloyed cast irons after heat treatment. a: abrasion loos; b: friction loos
4. Conclusion
Characterizations carried out on high chromium cast iron have shown that addition of strong carbide forming element affects positively the microstructure. Fine scale precipitations appear homogenously distributed in the matrix. Results obtained show that in the as cast state, the microstructure is mainly composed of an austenitic matrix and a netted eutectic. After heat treatment, the matrix becomes martensitic and a change of the eutectic shape was observed.
This is due to the depletion of alloying element and carbon.
Abrasion and friction resistance may be increased by increasing the amount of carbide forming element. The highest abrasion and friction resistance are given by samples containing 3% Mo and 3% Nb, witch exhibited the smaller abrasion looses. Friction results are different from those given by abrasion. Abrasion and friction behaviour do not take place in the same way. The best wear resistance is obtained by abrasion after heat treatment with samples alloyed with 3% molybdenum.
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