Research on microstructure and high-temperature friction and wear properties of

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Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: The hot extrusion die is a key tool for determining the surface quality and dimensional accuracy of extruded products.Because its service process is subject to high temperature,high pressure,and wear,it must be resistant to these conditions.In this paper,the high-temperature friction and wear properties of a cobalt (Co)-based alloy were investigated and compared with those of a titanium carbide (TiC) cemented material.The results show that the high-temperature wear performance of the Co-based alloy is better than that of the TiC cemented material,and that Co-based materials have the potential for replacing TiC cemented materials as hot-extrusion-die materials.Due to the high density and good combination of the matrix and carbide,the carbides do not easily peel off from the matrix during the wear process.Due to the higher impact toughness of the Co-based alloys,microcracks that can cause worn-surface peeling are not easily generated.As a result,the high-temperature wear performance of Co-based alloys is found to be better than that of TiC cemented materials.

Key words: hot extrusion die; cobalt-based alloy; microstructure; high-temperature friction and wear properties

1 Introduction

In the pipe production lines,extrusion and drawing processes are often used and the hot extrusion die is a key tool for determining the surface quality and dim-ensional accuracy of the final products.During ser-vice,this die is subject to high temperature,high pres-sure,wear,thermal fatigue,and metal flushing,which can result in cracks,sticking of the steel,and other defects.Due to the continuously improving mech-anical properties of extruded materials,the working conditions of the hot extrusion die have become even worse,which means that the die materials must have better properties with respect to their high-temperature strength,wear resistance,oxidation resistance,thermal shock resistance,and high stability in relation to extruded metals and their oxides.

Powder-metallurgy-produced titanium carbide(TiC) has low tensile strength but high-temperature compressive resistance,corrosion resistance,and wear resistance[1],and is often used for the extru-sion processing of alloy rods at temperatures above 800 ℃,with a service life about 20 times longer than that of the hot-work-die steel.However,in the current use situation,cracks are often generated on the TiC cemented material during the hot extrusion process,which affect the quality of the finial products.

In this paper,the development of a cobalt (Co)-based alloy is presented with respect to the microstructure and mechanical properties of the hot extrusion die and its working conditions.This alloy is intended to replace the TiC cemented material as a hot-extrusion-die material.The microstructure and mechanical properties of the Co-based alloy and TiC cemented material,especially the high-temperature friction and wear properties,were compared.

2 Experimental method

With reference to the microstructure and mechanical properties of the hot extrusion die and its working conditions,a Co-based alloy powder ratio was designed and provided by Höganäs (China) Co.,Ltd..Then,it was melted in a 500 kg vacuum induction furnace,cast into an alloy ingot ofφ80,and machined to the desired sample sizes.The TiC cemented hot extrusion die was prepared by powder metallurgy.The room-temperature and high-temperature hardness values were measured using an AHR-150P automatic touch screen digital Rock-well hardness tester and a DAKOMASTER 300 high temperature hardness tester,respectively.The microstructures of the samples were observed using a Zeiss LSM800 confocal microscope (OM) and a scanning electron microscopy (SEM).High-temperature impact tests were conducted using an instrumented impact method.A Rtec MFT-5000 friction tester was used in a ball-disk-type high-temperature friction and wear test,using an alumina ball as the upper sample,and the sample studied in this paper as the lower sample.The test temperature was set at 600 ℃ with a load of 30 N,a rotation speed of 3 000 rpm,a rotation radius of 15 mm,and test time of 15 min.

3 Results and discussion

The composition,microstructure,and mechanical properties of the prepared Co-based alloy were tested and compared with those of the TiC cemented alloy.The wear mechanism of the Co-based alloy was investigated,and then it was used instead of the TiC cemented material as a hot extrusion die for the investigation of the possibility of using the Co-based alloy instead of the TiC cemented material as a hot extrusion die.

3.1 Composition

Table 1 shows the composition of the Co-based alloy.

Table 1 Composition of the Co-based alloy %

As shown in Table 1,the main elements of the Co-based alloy were Co,Mo,and Cr,along with some amounts of Si,W,and Ni.

Table 2 shows the composition of the TiC cemented material.

Table 2 Composition of the TiC cemented material %

As shown in Table 2,the main elements of the TiC cemented material were Ti,Ni,and Cr,along with some amounts of W and Co.

3.2 Microstructure

Fig.1 is an SEM image of the microstructure of the Co-based alloy that shows the Co-based alloy has a dense structure and uniform crystal grains.Fig.2 shows energy dispersive spectroscopy(EDS)images and qualitative analysis of the Co-based alloy,which indicate that the microstructure area with a light color in the matrix is a carbide of Cr,that with a gray color is a carbide of Mo,and that with a black color is the Co element.

Fig.3 shows the microstructure of the TiC cemented material and Fig.4 shows EDS images and the qualitative analysis of the TiC cemented material.It can be seen from the microstructure characteristics that the matrix consists of TiC particles,Ni and Cr metal binders,and a small number of W carbides.In addition,there are a few pores evident in the matrix.

3.3 Room-temperature and high-temperature hardness values

Table 3 shows the room-temperature and high-temperature hardness values (HRC) of the Co-based alloy and TiC cemented material.

Table 3 Room-temperature and high-temperature hardness values (HRC) of the Co-based alloy and TiC cemented material

Temperature/℃The Co-based alloyThe TiC cemented material2051.262.140040.648.645039.647.250036.545.7

It can be seen from Table 3 that the hardness of the Co-based alloy was 51.2 at room temperature,which then decreased with increases in temperature.When the temperature increased to 500 ℃,the hardness decreased to 36.5.

The hardness of the TiC cemented material was 62.1 at room temperature,which decreased with increases in temperature.When the temperature was 450 ℃,the hardness decreased to 47.2,which was 19.2% higher than that of the Co-based alloy (39.6).At 500 ℃,the hardness decreased to 45.7,which was 25.2% higher than that of the Co-based alloy (36.5).This reveals that the reduction in the high-temperature hardness of the Co-based alloy was greater than that of the TiC cemented material and that the high-temperature hardness retention ability of the TiC cemented material was better than that of the Co-based alloy.

3.4 High-temperature impact test

The Co-based alloy and TiC cemented material were heated to 600 ℃ in a high-temperature impact test,and Table 4 shows the high-temperature impact properties determined by the test.It can be seen from Table 4 that the impact energies at 600 ℃ for the Co-based alloy and TiC cemented material were 2.93 and 2.71 J,respectively.This indicates that the impact properties of the Co-based alloy were basically equal to those of the TiC cemented material,although the impact properties of the TiC cemented material were slightly lower.

Table 4 Impact properties of the Co-based alloy and TiC cemented material at 600 ℃ J

3.5 High-temperature friction and wear test

Table 5 shows the high-temperature friction and wear properties of the Co-based alloy and TiC cemented material.

Table 5 High-temperature friction and wear properties of the Co-based alloy and TiC cemented material

Temperature/℃Load/NRotating rate/rpmRadius of rotation/mmWearTime/min600303 0001515

Fig.5 shows two-dimensional (2D) diagrams of the wear morphologies of these two materials.Specifically,Figs.5(a) and (b) show the high-temperature friction and wear morphologies of the Co-based alloy and TiC cemented material,respectively,in which it can be observed that the width of the wear scar on the Co-based alloy was significantly narrower than that on the TiC cemented material.

Figs.6(a) and (b) show three-dimensional (3D) diagrams of the high-temperature friction and wear scars of the Co-based alloy and TiC cemented material,respectively.Specifically,Fig.6 shows the depth and morphologies of the wear scars.From the macroscopic morphology diagrams of the two materials,it is obvious that the wear scar width and depth of the Co-based alloy were both significantly smaller than those of the TiC cemented material.

Table 6 shows the quantitative wear scar measure-ment results for the Co-based alloy and TiC cemented material.

As shown in Table 6,the wear scar width,depth,and volume of the Co-based alloy were 855.372 μm,5.742 μm,and 8.55×10-3mm3,respectively,and those of the TiC cemented material were 1 835.562 μm,15.062 μm,and 6.35×10-2mm3,respectively.It is obvious that the wear scars of the TiC cemented material were larger than those of the Co-based alloy;that is,the wear resistance was poorer than that of the Co-based alloy.

Table 6 Quantitative measurement results for wear scars

Fig.7 shows the friction coefficient curve of the Co-based alloy obtained during the high-temperature friction and wear test,in which it can be seen that the stable friction coefficient fluctuated between 0.4 and 0.8,and the friction coefficient increased slightly with increases in the wear time.

Fig.8 shows the high-temperature wear curve of the Co-based alloy.

Fig.9 shows the friction coefficient curve of the TiC cemented material during the high-temperature friction and wear test,in which it can be seen that the stable friction coefficient fluctuated between 0.2 and 0.8,and the friction coefficient slightly increased with increases in the wear time.

Fig.10 shows the high-temperature wear curve of the TiC cemented material.

Fig.11 shows a comparison of the friction coeffi-cients of the Co-based alloy and TiC cemented material,which indicates that the variation trends of the friction coefficients were the same for the Co-based alloy and TiC cemented material.Fig.12 shows a comparison of the wear scar curves of the Co-based alloy and TiC cemented material,in which it can be seen that the width and depth of the wear scar of the TiC cemented material were larger than those of the Co-based alloy.This is consistent with the results shown in Table 6,which demonstrates that the wear resistance of the Co-based alloy was better than that of the TiC cemented material,and that the potential for the Co-based alloy to replace the TiC cemented material as a hot-extrusion-die material.

Fig.11ComparisonofthefrictioncoefficientsoftheCo-basedalloyandTiCcementedmaterial

Fig.12ComparisonofthewearscarsoftheCo-basedalloyandTiCcementedmaterial

3.6 Discussion of mechanism of the friction and wear

Fig.13 shows the high-temperature wear surface morphology of the Co-based alloy and Fig.14 shows the cross-sectional morphology of its wear scar.It can be seen from Fig.13 that the main effects on the morphology include micro-cutting,pear groove,flaky spalling,and compacted rolling of the spalling.There are also many spherical oxidative particles on the wear surface.It can be observed in Fig.14 that some slight plastic deform-ation occurred during the wear process,and fatigue microcracks had been generated on the wear sur-face,which then propagated downward into the subsurface.

In the high-temperature friction and wear test of the Co-based alloy,an alumina ceramic ball was used as the upper sample.Therefore,the friction pair was metal-ceramic.

Fig.13High-temperaturewearsurfacemorphologyoftheCo-basedalloy

Fig.14Cross-sectionalmorphologyofthewearscaroftheCo-basedalloy

During the high-temperature friction and wear process,the elements and components on the surface of the Co-based alloy and alumina ceramic ball samples were exchanged,so that the surface of the alumina ceramic ball became adhesively worn during the initial stage of wear,and the surface of the Co-based alloy was abrasively worn by a small number of alumina ceramic particles.

With the development of the wear process,the surface of the Co-based alloy was partially oxidized at high temperature,but due to the thin oxide layer and cyclic contact stress,contact-fatigue micro-cracks were generated on the surface and then extended into the subsurface of the sample,resulting in contact-fatigue failure and the formation of a blocky surface and oxide spalling.As the wear test progressed,part of the metal exfoliation block was rolled to the surface of the sample again,which resulted in the formation of microabrasive particles that contributed to the subsequent abrasive wear process.

During the high-temperature wear process,chromium,molybdenum,and tungsten carbides were present in the Co alloy matrix as wear resistant hard phases,which pinched the matrix and hindered the wear.As a result,the friction process required greater friction and the degree of wear was greatly reduced under the same load conditions.Due to the presence of these wear resistant hard particles in the matrix,the wear marks are shallow at the hard particles.As the wear test progressed,the hard carbide particles were exposed to the friction surface,which led to micro-cutting,deformation,and displacement of the friction pairs,resulting in the deposition of metal around the carbides and a wear-resistant platform or hillock that formed many trenches on the surface of the wear scar.

Generally,the high-temperature friction and wear mechanisms of the Co-based alloys mainly took the form of fatigue wear,abrasive wear,and oxidative wear.

In the high-temperature friction and wear test of the TiC cemented material,an alumina ceramic ball was used as the upper sample.Therefore,the friction pair was ceramic-ceramic.

Fig.15 shows the high-temperature wear surface morphology of the TiC cemented material;Fig.16 shows the cross-sectional morphology of its wear scar.As can be seen in Fig.15,the wear process resulted in micro-cutting,flaking,microcracking,and a large number of abrasive grains on the high-temperature wear surface of the TiC cemented material.Compared with the surface morphology of the Co-based alloy wear scars,more wear debris and abrasive particles and deeper microcracks can be observed on the high-temperature wear surface of the TiC cemented material.It can be observed in Fig.16 that there was almost no plastic deformation on the high-temperature wear surface of the TiC cemented material,and more fatigue microcracks had extended from the wear surface into the subsurface.The micropores in the TiC cemented matrix provided ready resources for the develop-ment of microcracks;the propagation of micro-cracks during the high-temperature friction and wear process was the main cause of blocky spalling.

Fig.15High-temperaturewearsurfacemorphologyoftheTiCcementedmaterial

Fig.16Cross-sectionalmorphologyofthewearscaroftheTiCcementedmaterial

Studies have shown that the friction coefficients of ceramic-ceramic and ceramic-metal pairs are generally less than 1.0,and that ceramic has a higher wear rate due to its brittleness[2-5].Because of its high hardness and brittleness,the friction coefficient of the TiC cemented material ranges between 0.2 and 0.8,and its main wear mech-anisms are surface fractures and abrasive wear caused by hard particles.That is,brittle surface microcracks were generated and extended into the subsurface under the wear load,and the resulting surface fracture caused the expansion of the cracks parallel and perpendicular to the wear surface,causing the ceramic to peel off.The exfoliated large flakes were then rolled into smaller flakes during the wear process,which caused abrasive wear on the worn surface by continuous friction,resulting in a rough surface and serious wear.As the wear progressed,the surface of the sample was also oxidized and an oxide film formed at high tem-perature,which would improve the friction per-formance during the later stage of the friction and wear test.

Therefore,the high-temperature friction and wear test of the TiC cemented material and alumina ball primarily featured dry friction,and the cracks and microregion fractures in the depth range of 20-30 μm under the friction surface were the main causes of wear.In summary,the friction coefficient of the TiC cemented material ranges between 0.2 and 0.8,as shown in Fig.9,and the main wear mechanisms are abrasive wear and fatigue wear.

3.7 Discussion of high-temperature friction and wear performance

From the above test results,it can be seen that under the same test conditions,the high-temperature friction and wear properties of the Co-based alloy were better than those of the TiC cemented material,as determined by their composition,microstructure,and mechanical properties.

The composition of the material was one of the factors affecting its wear resistance.From Tables 1 and 2,it can be seen that in the Co-based alloy,Co was the matrix and the main alloy elements were Mo and Cr.For the TiC cemented material,Ni was the binder and Ti and Cr were hard particles.The alloy elements in the matrix of the material were responsible for the grain refinement,thereby improving the strength and hardness of the matrix,and contributing to the improvement in the wear resistance.Although the amount of the alloy element in the TiC cemented material was higher than that of the Co-based alloy,its wear resistance was lower than that of the Co-based alloy because the wear resistance of a material is related not only to its composition but also to its microstructure and bonding strength.

The microstructure,density,and bonding strength of the phases have a great influence on the wear resistance.From the microstructures of the Co-based alloy and TiC cemented material shown in Figs.1 and 3,it can be seen that the Co-based alloy matrix was densified,with carbide particles homogeneously distributed throughout the matrix,which were tightly wrapped by the Co matrix alloy.That is,the matrix alloy and the carbide were tightly integrated.In the TiC cemented material,Ni served as the binder,and its content was less than that of the Co binder in the Co-based alloy.At the same time,it can be seen that the distribution of Ni was not completely uniform and there were a few pores in the matrix.With respect to the microfracture wear mechanism of the cemented material[6],the empirical formula for the elastic modulus (E) and porosity (p) is as follows:

E/E0= 1_Kp

(1)

where,E0is an elastic modulus in the absence of any pores;andKis a constant.Due to the characteristics of the cemented material,there was little microregion plastic deformation,which means thatEwas the main parameter hindering the expansion of the microcracks.It can be seen from Equation (1) thatEof the TiC cemented material decreased linearly with increases inp.In addition,the energy of dislocations and resistance to dislocation motion decreased with decreases inE.Therefore,microcracks and failure were more prone to be generated in the cemented material with a higher porosity and lower density,resulting in a great amount of wear.This was also one of the reasons that the wear resistance of the TiC cemented material was lower than that of the Co-based alloy.

The difference in density between the Co-based alloy and TiC cemented material is related to their manufacturing methods.Since the Co-based alloy was prepared by the casting method,its density was higher,whereas the TiC cemented material was prepared by the powder metallurgy method,whereby the distribution of the binder depends on the powder grinding and mixing processes,and the density is closely related to the hot-press sintering temperature and pressure.It can be stated that the higher the density of the TiC cemented material matrix,the better the bond between the matrix and carbides,and thus the better the wear resistance.

Mechanical properties also have a great impact on the high-temperature friction and wear properties.The wear rate is generally considered to be inversely proportional to hardness,especially high-temperature hardness.The working conditions of the hot extrusion die used in the production of seamless steel tubes are hot extrusion and impact.Therefore,the high-temperature hardness and impact properties of the Co-based alloy and TiC cemented material at different temperatures were measured,as shown in Tables 3 and 4.

At room temperature,the hardness of the TiC cemented material was greater than that of the Co-based alloy.With increases in temperature,the high-temperature hardness of both materials decreased.When the temperature was 500 ℃,the hardness of the TiC cemented material was still greater than that of the Co-based alloy,although the impact energy of the TiC cemented material was lower than that of the Co-based alloy at 600 ℃.According to the test results,the degree of high-temperature wear of the TiC cemented material at 600 ℃ was higher than that of the Co-based alloy.This is because the high hardness value of the TiC cemented material was not determined by its composition,but by con-trolling the heat treatment process.Studies have shown that the hardness of the cemented material does not decrease with increased porosity,but in fact increases[7].At the same time,the high-tem-perature impact toughness of the cemented material has been reported to be low and its brittleness high.During the high-temperature friction and wear process,the continuous change in the local stress gen-erated on the friction surface was found to con-centrate and induce microcracks.For the polycrys-talline cemented material,the further expansion of cracks is reportedly blocked by the grain boundaries and the cracks aggregate to form a core,which induces brittle fracture in the microregion[8].Crack-free zones cause fatigue wear due to their repeated deformation.

Although the TiC cemented material exhibited high-temperature hardness,the hard-phase TiC par-ticles were easily detached and separated,and then the detached particles acted as abrasive grains,which caused more serious wear on the sample that resulted in an increase in the wear rate.It can be seen that materials with high-temperature hardness,a thermally stable microstructure,high bonding strength between the matrix and hard particles,and high-temperature impact toughness will have high wear resistance properties.

4 Conclusions

(1) The high-temperature wear performance of the Co-based alloy was better than that of the TiC cemented material,which demonstrates its potential for replacing the TiC cemented material as a hot-extrusion-die material.

(2) The results showed that the microstructure had great influence on high-temperature friction and wear performance.When the amounts of the alloy elements were basically equivalent,the high-temperature friction and wear performance of the materials depended on the bonding strength between their matrix and hard particles such as carbides.

(3) Mechanical properties such as high-tempera-ture hardness and impact were found to be import-ant factors for high-temperature friction and wear performance,but these were not the most decisive factors.

(4) The high-temperature wear performance of the Co-based alloy was better than that of the TiC cemented material.This is because the Co-based alloy was characterized by high density,a good bonding strength between the matrix and carbide,and the fact that the carbides could not be easily peeled off from the matrix during the wear process.In addition,due to its better impact toughness,microcracks and wear surface peeling were not prone to occur.