WEI Hongming,JIANG Yehua,ZHANG Xiaozu,JIA Yuanwei,ZHOU Mojin,XUE Da
(1.Faculty of Materials Science and Engineering,Kunming University of Science and Technology,Kunming 650093,China;2.State Key Laboratory of Powder Metallurgy,Central South University,Changsha 410083,China)
Abstract: In order to solve the problem of cracking of ZTAP (Zirconia toughened alumina ceramic particles) reinforced HCCI (high chromium cast iron) matrix composites,the quenching process was optimized.ZTAP reinforced HCCI matrix composites were prepared by infiltration method with gravity sand casting.The thermal expansion curves of HCCI and the composites were measured at different cooling rates by Glebble-3500.The microstructure of the HCCI matrix and the composites were characterized by X-ray diffraction,light microscopy,SEM,ESD,and EPMA.The tested mechanical properties include Rockwell hardness and impact toughness.The deformation differences of HCCI and the composite at different cooling rates were obtained according to the test results of thermal expansion coefficient curve and changes in microstructure and mechanical properties,and air cooling was the most favorable for the composites to have good hardness and not easy to crack.The cooling rate during air cooling is approximately equal to 21 ℃/s in this work.When the quenching process was air cooling,the impact toughness and hardness of the composites are 3.7 J/cm2 and 61.8 HRC,respectively,and the deformation difference between the composites and HCCI was 20 μm at 300 ℃.
Key words: composites;ZTAP;quenching cooling rate;microstructure;thermal expansion
ZTAP(zirconia toughened alumina ceramic particles) reinforced HCCI (high chromium cast iron)honeycomb structure composites make full use of the metallic properties of HCCI and the high hardness of ZTAP,show excellent wear resistance,are currently the most economically valuable material for preparing roller wear parts,and have been widely used in electric power,mining,cement and other industries[1-3].
The composites were prepared by gravity sand casting,and the reaction mechanism of the composites is infiltration-reaction synthesis.Specifically,the ZTAPis uniformly coated by HCCI.In the composites,ZTAPmainly plays the role of anti-wear,and HCCI mainly plays a protective role to prevent ZTAPfrom falling off[4-6].It is a mutual protection relationship between them,showing excellent wear resistance in this synergy.
The hardness of as-cast HCCI is only about 56HRC.It is far from enough to ensure that ZTAPis protected well.One effective way to solve this problem is to obtain a martensite structure with higher hardness by quenching[7,8].This can effectively avoid premature exposure of ZTAPduring the service process due to the HCCI being quickly worn away[4,6,7].
Although some works on the heat treatment process of ZTAPreinforced HCCI honeycomb structure composites have been performed,fundamental knowledge of the relationship between the structure,crack tendency and quenching process of the composites is lacking[9-11].This work was intended to reveal the effect of quenching cooling rate on the structure and cracking tendency of the composites,and achieve the optimization of the quenching process.
2.1 Specimen preparation
Raw materials applied in this work were ZTAP(60.0% Al2O3,39.0% ZrO2,0.15% TiO2,0.10% SiO2,0.15% Fe2O3and others),Al2O3powder (purity of 99.9%,particle size of 300 mesh),HCCI (26%Cr,3.1%C,1%Mn,0.5%Si,0.6%Mo,0.3%Ni,and 0.1%Cu).The composites castings were prepared by combining HCCI metal liquid and ZTAPpreform with gravity sand casting,and the pouring temperature was 1550 ℃.The preparation of ZTAPpreform was a composition consisting of Al2O3powder,ZTAPand sodium silicate after homogeneous mixing with a mixer,then molded using a mold with a honeycomb configuration.
2.2 Control cooling rate using Glebble-3500
The test specimens with a size of 10 mm × 10 mm× 80 mm were cut out from the composites castings using the method of metallographic sampling.The deformation difference between ZTAPand HCCI during quenching was analyzed and measured by a thermal simulation machine (Glebble-3500) according to the thermal expansion curve of the composites and HCCI.The samples were heated by Glebble-3500 in the form of current,and the temperature rising rate and cooling rate of the samples can be precisely controlled.The quenching process was heated to 1020 ℃ at a heating rate of 0.5 ℃/s,kept warm for 10 minutes,and then cooled to room temperature at different cooling rates.
2.3 Phase characterization using X-ray diffraction (XRD)
Phase analysis of the composites and HCCI specimens was performed by means of XRD using a Siemens D500 diffractometer (Bragg-Brentano configuration),equipped with a Cu tube and a graphite monochromator in the diffracted beam(Cu Kα radiation: k=1.54056 A˚).The diffraction angle,2 ℃,in the range of 10°-90°was scanned with a step size of 5°/min.Phase identification was performed comparing the position of the measured peaks with the data derived from the software Jade 6.
2.4 Other microanalysis
The morphology was investigated with a Leica EZ4D optical microscope and ZEISS EVO18-21-57 scanning electron microscopy.Electron probe microanalysis (EPMA,JXA-8230F) was used to examine element distribution in the samples.
2.5 Mechanical property
The hardness of the composites was measured using a Rockwell hardness tester (HR-150).The impact tester (JBT-50) was used to measure the impact toughness of the composites,and the sample size was 10 mm × 10 mm × 55 mm without a gap.
3.1 Microstructure
The microstructure of the composites is shown in Figs.1(a) and 1(b),it can be found that ZTAPare embedded in HCCI matrix with a good combination,and no obvious defects are found.The microstructure of the as-cast HCCI matrix is shown in Fig.1(c),mainly consisting of primary carbide,eutectic carbide and austenite,besides,a small amount of martensite is also found because the casting cooling rate was not slow enough.
Fig.1 The microstructures of the composites and HCCI: (a) Metallographic photograph of the composites;(b) SEM of the composites;(c)SEM of HCCI
The microstructure of the HCCI matrix quenched at different cooling rates is shown in Fig.2.In Fig.2(a),it can be found that the microstructure of the HCCI matrix is composed of lamellar pearlite,fine dispersed secondary carbide particles and primary austenite when the cooling rate of heat treatment was 1 ℃/s.Bainite is found in the microstructure of the HCCI matrix when the cooling rate was increased to 3 ℃/s,moreover,its content increases clearly and a small amount of martensite is also found when the cooling rate was 5 ℃/s,as shown in Figs.2(b) and 2(c).Furthermore,as the cooling rate further increases,the martensite content in the microstructure of the HCCI matrix increases significantly,as shown in Figs.2(d)-2(f).The low-temperature products of the HCCI matrix are basically continuous martensite when the cooling rate was up to 12 ℃/s,as shown in Fig.2(f).
Fig.2 The microstructures of the HCCI matrix at different cooling rates: (a) 1,(b) 3,(c) 5,(d) 8,(e) 10,and (f) 12 ℃/s
The XRD patterns of the HCCI matrix quenched at different cooling rates are shown in Fig.3.The compositions of the HCCI matrix are mainly M7C3,γ-Fe (Austenite) and α-Fe (Ferrite/Martensite).By comparing the results of the XRD diffraction peaks(43°),it can be found that the characteristic of the austenite peak in the as-cast state is more obvious,but the strength of the α-Fe peak is weaker than that of after quenching.This result is consistent with the microstructure of the as-cast HCCI matrix mainly composed of austenite and carbide.Pearlite transformation occurs during the cooling of the HCCI matrix with a cooling rate of 3 ℃/s,and its main peak(44°) is stronger than that of the as-cast HCCI matrix,and shifts to the left.Besides,it can be found that the intensity of the main peak (44°) gradually increases and a slight left shift occurs with the cooling rate further increases,as shown in the XRD diffraction pattern of the HCCI matrix quenching at 8,12 ℃/s,and air cooling.The reason for this phenomenon of the main peak (44°) is that the martensite content of low temperature transformation products increases with the cooling rates.In addition,when the cooling rate is faster,the diffusion capacity of carbon is weakened,resulting in a decrease in the amount of precipitation of carbides,and higher carbon content in the martensite during the transformation of the low-temperature product[19].
Fig.3 XRD patterns of the HCCI matrix quenched at different cooling rates
Fig.4 shows the XRD diffraction results of ZTAP.There is no obvious change in the phase composition of ZTAPbefore and after heat treatment,main phase is α-Al2O3,and the secondary phase of ZrO2includes Tetragonal ZrO2(T-ZrO2) and Monoclinic ZrO2(M-ZrO2).Generally,ZrO2is stable in tetragonal form at room temperature,and reversible phase transition as shown in formula 1 will occur when the temperature changes[12].However,the existence of T-ZrO2in the ZrO2phase in the composites at room temperature is mainly due to the transformation of the crystal structure of ZrO2from M to T at high temperature,and the transition from T to M can not occur due to the restriction of the HCCI matrix during the cooling process.
Fig.4 XRD patterns of ZTAP
Fig.5 shows the EMPA point energy spectrum analysis diagram of the composites interface.The content of each test point element is shown in Table 1.Fe and Cr elements in the HCCI matrix and Zr elements in the ZTAPhave a slight diffusion in the transition layer depending on the content of the elements of Zr,Fe,and Cr.
Fig.5 The composites interface (1000 × EMPA diagram)
Table 1 EDS spectrum analysis of each point of the composites/wt%
Fig.6 shows the surface energy spectrum of the electron microprobe after heat treatment of the composites.Fe and Cr elements of the HCCI matrix diffused into the transition layer of the composites,but they did not spread to the ZTAP.Besides,Al and Zr elements of the ZTAPalso did not diffuse into the HCCI matrix.It shows that ZTAPand the HCCI matrix have good chemical stability,and they do not react with each other during heat treatment[13].
Fig.6 EPMA surface energy spectra of the composites: (b) SEM topography of the composites;(a) Al;(c) Cr;(d) Fe;(e) O;(f) Zr
3.2 Thermal expansion difference
It is difficult for the composites and HCCI to cool at a fast cooling rate when the temperature is low[14-15].Therefore,we mainly observe the thermal expansion curves of HCCI and the composites at different cooling rates in the process of heating up to 1020 ℃ for 10 minutes and then cooling down to 300 ℃ at different cooling rates.As shown in Fig.7,the expansion and contraction curves of HCCI and the composites under the conditions of the cooling rate of 3,8,12,18 ℃/s,air cooling,and argon cooling with a flow rate of 3 L/min.The difference in the amount of expansion of HCCI and the composites when cooled to 300 ℃changes significantly with the cooling rate,as shown in Table 2.In the cooling rate measured in the experiment,the difference in the amount of expansion is small when the cooling rate is 3 ℃/s and air cooling.In the heat preservation stage (t=1200 ℃),the HCCI continues to expand,while the composites shrink.In addition,the shape variable of the composites is negative when they are cooled to 300 ℃.This phenomenon is attributed to ZrO2in ZTAPchanging from M to T when the temperature is higher than 1000 ℃,and its volume is reduction,but reversible phase transformation cannot occur during cooling due to the influence of compressive stress of the HCCI matrix.
Table 3 Impact properties of the composites at different cooling rates
Fig.7 Thermal expansion curves of HCCI and the composites at different cooling rates: (a) 3 °C/s,(b) 8 °C/s,(c) 12 °C/s,(d) 18 °C/s,(e) Air cooling,and (f) Argon cooling
Table 2 Differences in expansion of HCCI and the composites at different cooling rates
3.3 Mechanical properties
The hardness of ZTAPbasically maintained a Vickers hardness of 19 GPa for 50 g load before and after heat treatment.The Macro-hardness of the composites mainly depends on the hardness of the HCCI matrix.It is considered that the hardness of the HCCI matrix is affected by primary carbide,secondary carbide,martensite content and carbon content in martensite[16-18].Fig.8 shows the hardness changes of the HCCI matrix at different quenching rates (setting ascast state=0 ℃/s,air cooling=21 ℃/s,argon cooling=26 ℃/s),and the hardness of the as-cast HCCI matrix is 56HRC.When the cooling rate is within the range of 3 to 12 ℃/s,the hardness of HCCI increases with the cooling rate,which is attributed to the martensite content in HCCI structure increasing with the cooling rates.Carbon content of martensite and secondary carbide precipitation are the main factors affecting the hardness of HCCI when the cooling rate is high enough to make the low temperature transformation products of HCCI almost all continuous martensite structures.This is why there is no obvious change in the hardness of HCCI when the cooling rate is greater than 12 ℃/s,and has a maximum when the cooling rate is 18 ℃/s.The increase in cooling rate is beneficial for obtaining high carbon martensite but will limit the precipitation of secondary carbides[19].
Fig.8 The Hardness of HCCI at different cooling rates
Table 3 shows the results of the impact toughness of the composites.The impact toughness of all the composites samples measured after quenching was slightly reduced due to the change of microstructure in the HCCI matrix,and the toughness of martensite was worse than that of austenite[20].The impact toughness of the composites quenched at a cooling rate of 12 ℃/s is only 3.0 J/cm2.However,it behaves well when the cooling rate is air cooling,and its value is 3.7 J/cm2.In order to analyze the reason for the difference in impact toughness between the composites at these two cooling rates,the fracture surface of the samples was observed by means of SEM,and the morphology is shown in Fig.9.The fracture morphology of the test sample is a typical cleavage feature of a brittle fracture.After quenching at 12 ℃/s,the fracture morphology of the sample shows obvious cracks between HCCI and ZTAP,and there are pits on the HCCI matrix caused by ZTAPshedding,and no fracture is observed on the surface of ZTAP.On the contrary,there is no gap in the fracture morphology of the sample between HCCI and ZTAP,and the surface of ZTAPshows an obvious fracture phenomenon,and there is no pit left due to ZTAPshedding on the HCCI matrix when air cooling is adopted.Under the impact of external force,the fracture mode of the samples quenched at 12 ℃/s is that the cracks pass through ZTAPand penetrate into the HCCI matrix.While quenched by air cooling,the fracture mode of the sample is that the cracks pass through ZTAPand then transfer to the HCCI matrix,the impact property of the composites is the result of the interaction of the HCCI matrix and ZTAPwhen they are quenched by air cooling.These two different fracture modes prove that the inconsistent deformation between ZTAPand the HCCI matrix during the quenching process of the composites is likely to lead to cracks between the HCCI matrix and ZTAP.
Fig.9 The composites impact port morphologies: (a) Cooling rate of 12 ℃/s,200×;(b) Cooling rate of 12 ℃/s,1000×;(c) Cooling rate is air cooling,200×;(d) Cooling rate is air cooling,1000×
The effect of quenching cooling rate on the microstructure and properties of ZTAPreinforced HCCI composite was studied,and the conclusions are as follows:
a) The Martensite appears in the microstructure of HCCI at a cooling rate of 5 ℃/s,and its content increases with the cooling rate.Further,almost all cryogenic phase transition products become continuous martensite when the cooling rate is above 12 ℃/s.
b) The ZrO2phase in ZTAPunder quenching treatment will undergo irreversible M to T phase transition,accompanied by volume reduction.
c) The macro hardness of the composites is best when the cooling rate is 18 ℃/s.Whereas,the difference in thermal expansion between HCCI and the composites is minimum when the cooling rate is air cooling,and the impact toughness of the composites cooled with air cooling is a good value of 3.7 J/cm2.
d) According to the results of microstructure,mechanical properties and thermal expansion difference,the suitable quenching rate for the composites is air cooling (cooling rate was 21 ℃/s in this work).
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