Abstract
A series of iron based hardfacing alloys with varying tungsten contents were fabricated using slag free self-shielded flux cored wire, and the effects of tungsten addition on microstructure and wear performance were investigated. The experimental results showed that the iron base tungsten free hardfacing alloy has a typical hypereutectic microstructure, which consists of primary hexagonal M7(C, B)3 carbides and eutectic long bar-like M3(C, B) carbides in the austenite and martensite matrix. The addition of tungsten increases the size of primary hexagonal M7(C, B)3 carbides and promotes the formation of martensite, as well as changes the morphology of eutectic carbides. It was found that the eutectic carbides change from the long bar-like shape to the scattered web shape with the increase in tungsten content to 9 wt-, and subsequently change to the plate shape with further increasing the tungsten content to 12 wt-. For the all tungsten containing hardfacing alloys, tungsten exists in both the carbides and matrix uniformly. Results also showed that the wear loss of the sample with 9 wt- tungsten was the smallest among all the samples owing to the higher hardness and reinforced microstructure.
Keywords
Introduction
Hardfacing alloys are commonly used for engineering components with required surface properties, particularly resistance to wear.1–7 Among the hardfacing alloys, the hypereutectic high chromium iron base alloys are adopted as a preferential material to withstand wear conditions due to the low cost and excellent wear resistance.3,5,7 Their typical microstructures consist of primary chromium rich carbides and Fe–Cr matrix. The type and distribution of the chromium rich carbides in the microstructure determine the hardness, which influences the wear performance of hardfacing alloys. The matrix also influences the wear resistance; for instance, the austenite, martensite and ferritic matrix of the Fe–32 wt-Cr–4·5 wt-C hardfacing alloys exhibits various wear resistance behaviours. 8
According to the high melting point and hardness of MC type carbide, researchers9–12 investigated the effect of strong carbide forming elements, such as niobium, titanium and vanadium on iron base hardfacing alloy. These elements can react with carbon to form MC carbides, which further improve the wear resistance. Tungsten, as a strong carbide forming element similar to chromium, niobium and vanadium, was combined with titanium to add into alloys with the formation of MC carbides. 13 However, there is little reference concerning the influence of single tungsten on the microstructure and wear resistance and its optimal content in iron base hardfacing alloy.
Several welding methods such as shielded metal arc welding, gas metal arc welding, plasma transferred arc welding, gas tungsten arc welding, submerged arc welding and self-shielded flux cored arc welding can be implemented for hardfacing.3,14–16 Among these techniques, self-shielded flux cored arc welding has the highest welding efficiency, which is particularly important for regeneration of heavy working surfaces. In our earlier studies, 11 a new slag free type of iron based self-shielded flux cored wires was developed. Here, the effects of tungsten on the microstructure and wear performance of iron based self-shielded flux cored hardfacing alloys are investigated.
Experimental
The self-shielded metal cored wire consists of a steel sheath with a core. The steel sheath was filled with a powdery metal core (filling rate, 55 wt-) and then drawn to a weld wire with a diameter of 2·8 mm. The chemical composition of steel sheath and metal core is shown in Tables 1 and 2 respectively. In order to investigate the effects of tungsten addition on microstructure and wear performance of iron base self-shielded flux cored hardfacing alloys, the mass fraction of tungsten powder added into core wire is 0, 3, 6, 9 and 12 wt- respectively.
Composition of steel sheath
Composition of flux core
Mild steel with a size of 150×75×50 mm was selected as substrate material. In order to obtain the homogeneous specimen, hardfacing alloys with five layers were prepared by means of metal cored self-shielded arc welding without pre- and postheat. The welding parameters are presented in Table 3.
Welding parameters
The samples were cleaned in an ultrasonic cleaning machine for 5 min before and after the test. Chemical phases of the top surface of the hardfacing were detected by X-ray diffraction (XRD) with Cu Kα radiation. A step of 0·02° was used to scan 2θ from 30 to 100°. The macrohardness was taken on the top surface of the hardfacing alloys by an HR-150A Rockwell hardness tester. The etching agent was composed of 15 mL of 38 hydrochloric acid solution, 50 mL H2O, 3 mL of 68 nitric acid solution and 3 g ferric chloride. The microstructure was observed by optical microscope and scanning electron microscope (SEM).
Wear resistance tests were performed using an HT-500 pin on disc tribometer with the test material in direct contact against a ball. The ball material was AISI T2 high speed tool steel, quenched and tempered to a hardness of 62 HRC. The parameter of the wear test was summarised in Table 4 for each hardfacing test. The corresponding friction coefficient was obtained from the ratio of the friction force to the applied load. The wear loss, V, was determined using the following equation
11
Sliding wear test conditions
Results and discussion
Effect of tungsten addition on microstructure
Figure 1 shows the XRD spectra of the hardfacing alloys. The phase in the hardfacing alloy without tungsten addition is mainly composed of M7(C, B)3, M3(C, B), martensite and residual austenite. This is in conformity with other works, 17 which identify M7(C, B)3, M3(C, B) and martensite in the presence of Fe–Cr–B–C alloys. The addition of different tungsten does not change the phase compositions, except some tungsten solutes into M7(C, B)3 and M3(C, B) carbides. The figure also shows that the peak intensity of austenite in tungsten containing alloy becomes weaker than that in tungsten free alloy. On the contrary, the peak intensity of martensite enhances with the increase in tungsten in the hardfacing alloy. This phenomenon might result from fraction decrease in the austenite phase and increase in martensite phase, attributed to higher tungsten content.
X-ray diffraction of hardfacing alloy with different tungsten addition
The microstructures of hardfacing alloys with different tungsten (0, 6, 12, 9 and 12 wt-) are shown in Fig. 2. Here, the tungsten free alloy is composed of primary M7(C, B)3 carbides surrounded by the eutectic colony, which contains M3(C, B), martensite and residual austenite. Moreover, the size of primary M7(C, B)3 carbides increased as the tungsten content increased from 0 to 12 wt- (Fig. 2b–e). The addition of tungsten could increase the driving force for the formation of carbides. The changes of average diameter of primary M7(C, B)3 carbides with different tungsten contents were measured by a quantitative metallographic method, and the results are displayed in Fig. 3. The average diameter of primary M7(C, B)3 carbides in the tungsten free hardfacing alloy is 5·5 μm, which is increased to 16·2 μm in the alloy with 9 wt- tungsten. With further increase in tungsten content, the primary M7(C, B)3 carbides continued to coarsen and thus cluster together. The average carbide size in the alloy with 12 wt- tungsten addition is 20·3 μm.
Microstructures of hardfacing alloys with different tungsten addition
Average diameter of primary M7(C, B)3 carbide with different tungsten additions
Figure 4 shows the SEM images of hardfacing alloys with various tungsten contents. From Fig. 4a and b, there is a typical hypereutectic structure in the tungsten free alloy. During solidification process, the primary hexagonal M7(C, B)3 carbides form in the high temperature liquids, followed by the eutectic reaction. The eutectic colonies consist of long bar-like eutectic carbides plus martensite and residual austenite matrix. Owing to the addition of chromium and manganese, which consumed carbon from the austenite during hypereutectic solidification, the narrow area in the austenite lacks carbon. The lack of carbon at the austenite region raises the martensite starting point, and thus, the austenite transforms to martensite during rapid cooling process. 18 In alloys with 9 wt- tungsten, the eutectic carbides are the scattered web form (Fig. 4c and d), whereas in the alloy with 12 wt- tungsten, the eutectic carbides are in the plate shaped form (Fig. 4e and f).
Images (SEM) of hardfacing alloys
According to Li and Smith, 19 the segregation of elements is towards the periphery of the proeutectic dendrites. The solid/liquid interface appears a solute accumulation during solidification owing to the low partition coefficient of these elements in both phases. Thus, the concentration of segregating elements will be higher in front of the solid/liquid interface. A high concentration of solute leads to an acceleration in the eutectic solidification process. Therefore, an increase in the nucleation of eutectic carbides should cause a higher precipitation in the final microstructure. However, tungsten exists in both the carbides and matrix as shown in Fig. 5. This figure also indicates that the morphological variety of carbides does not affect the distribution of tungsten. The size of tungsten atom is bigger than that of chromium and iron with a higher melting point. During solidification process, carbides may nucleate as the single particles, and then grow as long bars along the <0001> direction. 20 With the increase in tungsten content, the nucleating points are increased. When adding 12 wt- tungsten into the hardfacing alloy, carbide particles may join together during growing process and forming plates instead of bars because one carbide particle nucleates close to the others.
Electron probe microanalysis for sample 4 (9 wt- tungsten) and sample 5 (12 wt- tungsten)
Effect of tungsten addition on harness and wear resistance
Figure 6 shows the effect of tungsten on the hardness and wear resistance of hardfacing alloys. It can be seen that the hardness is increased steadily with the increase in tungsten content. The hardness of tungsten free alloy is 59·5 HRC and increased to 65·3 HRC with the addition of 12 wt- tungsten. As reported by Zhang et al., 13 hardness is increased rapidly when titanium and tungsten were added into cast irons owing to the formation of MC carbides inside grains and at grain boundaries dispersedly. However, seldom MC carbides exist in the alloy with single tungsten addition in the present study. Solid solution of tungsten in the carbides and matrix enhances hardness. With increasing tungsten content, the transformation from austenite to martensite also leads to the increase in bulk hardness. In addition, the volume fraction of eutectic hard carbide phases including tungsten is increased due to the addition of tungsten, resulting in a positive influence on the bulk hardness.
Hardness and wear loss of hardfacing alloys with different tungsten addition
Figure 7 shows the friction coefficient as the function of sliding distance for the hardfacing alloys with different tungsten contents. Friction coefficient shows a high value in the first 2 min during wear process and then decreases continuously until a final steady state value after ∼8 min. It starts with a high value owing to the direct metal–metal contact, which causes adhesion. As the asperities adhere during the wear process, the moving parts stick, leading to the high friction values. 12 In this way, new adhesions cause a greater transfer of material and in the generation of new wear debris, which are plastically deformed and compacted owing to the rubbing effect between the hardfacing and the ball. Thus, the oxide layer forms on the worn surface in subsequent wear process. The formation of oxide layer leads to a decrease in friction coefficient. 21 It is shown that the friction coefficient of hardfacing alloy with 9 wt- tungsten addition (0·51) is less than that of tungsten free alloy (0·83). This is because the high hardness of the tungsten containing alloy causes lower real area of contact, which requires less energy to be sheared during sliding than that of tungsten free alloy. Additionally, tungsten rich carbides in the tungsten containing alloy reduce the contact area of the matrix with the counterface, thus reducing the smearing effect on the counterface surface. 6
Friction coefficient versus sliding time for tungsten free and containing (9 wt-) hardfacing alloy
Figure 8 shows the worn surface of the hardfacing alloys without tungsten and with 9 wt- tungsten. The surface of the alloy without tungsten is mainly rough with some detaching wear debris and deep ploughing grooves, which indicates severe adhesive wear. The alloy surface is easily plastically deformed and ploughed owing to the low hardness; therefore, large chips and detaching wear debris can be observed, as shown in Fig. 8a. On the contrary, the worn surface of the hardfacing alloy with 9 wt- tungsten becomes obviously smooth with slight scratches (Fig. 8b). A major effect of tungsten is to enhance matrix and carbides by solid solution, which enhances the wear resistance. According to Lampke et al., 22 the wear resistance is also restricted by the volume fraction of the carbides in the iron based alloy. When the tungsten content is 9 wt-, the presence of higher volume fraction of eutectic M3(C, B) carbides is of benefit to the wear resistance than that of tungsten free alloy. The tungsten also arouses the transformation of martensite, which is harder than austenite. Figure 9a shows that fine checking cracks appear on the alloy (containing 9 wt- tungsten). The stress relief cracking only exists in the surface of the alloy, and the crack tip can be blunted due to stress relief. The internal stress relaxation of hardfacing alloy contributes to the improved wear properties. The effect of stress relief cracking on wear resistance was similar to other researchers’ work. 23 However, when the tungsten content reaches 12 wt-, the volume fraction of matrix in the microstructure is too low to form a bond between carbides, leading to a decrease in toughness of the alloy. In fact, wear test is in progress to confirm this point. In the beginning of the sliding wear process, the fracture penetrates the whole interface that stops the testing proceeding (Fig. 9b). Therefore, the wear loss of the sample with 9 wt- tungsten was the smallest among all the samples owing to the higher hardness and reinforced microstructure.
Worn surface of both hardfacing alloys with different tungsten addition
Macromorphology of worn surface of hardfacing alloys with different tungsten additions
Conclusions
1. The iron base tungsten free hardfacing alloy shows a typical hypereutectic microstructure, which consists of primary hexagonal M7(C, B)3 carbides and eutectic long bar-like M3(C, B) carbides in the austenite and martensite matrix. The addition of tungsten increases the size of primary hexagonal M7(C, B)3 carbides and promotes the formation of martensite as well as changes the morphology of eutectic carbides. It was found that the eutectic carbides change from the long bar-like shape to the scattered web shape with the increase in tungsten content to 9 wt- and subsequently change to the plate shape with further increase in tungsten content to 12 wt-.
2. For the all tungsten containing hardfacing alloys, tungsten exists in both the carbides and matrix uniformly.
3. The wear loss of the sample with 9 wt- tungsten was the smallest among all the samples owing to the higher hardness and reinforced microstructure.