Repetitious-Hot-Pressing Technique in Hot-Pressing Process

486J Mater. Sci. Technol, Vol.19 No.5, 2003treated specirnens are listed in Table 4.lath martensite, bainite into lower bainite, and base metalCompared to a welded specimens, CTOD of weld andwas changed from ferrite and perlite into upper bainite andHAZ of post-weld heat-treated specimens were higher thanresidual austenite. The microstructures of diferent zones ofthat in the AW conditions. It was shown that the fracturejoints in FWPWHT condition were tempered sorbite.toughness of welded joints can be greatly improved by post-(2) The strength of welds and base metals was decreasedweld heat treatment, because of their structure improvement.and the elongation was improved by EBLPWHT and FW-However the value of fracture toughness of base metal is com-PWHT. The strength and elongation of base metals in twoparatively low because of the variation of their microstruc-joints had no evident diferences,tures(3) Except base metal, the fracture toughness of weldedThe higbest value was got at weld zone in EBLPWHTjoints canargely improved by EBLPWHT treatment be-condition. Generally, it is advantageous for the improvementcause of their structure improvement. However the value ofof ductility and toughness by refning the sbape of the marten-site structure. The welding thermal cycle and cooling process(4)。 EBLPWHT. bas application feasibility onwere changed by EBLPWHT as long as the composite strue-30CrMuSiNi2A steel although an appropriate techniquetures of lath martensite and residual austenite were formedthe EBLPWHT needs to be explored continually. Comparedin weld center zone. It is all known that the lath martensiteto FWPWHT, the advantages of convenience, speed and en-could improve the strength and toughness of welded joints.ergy saving give EBLPWHT a big potential for the futureFurthermore, when the crack expanded into the weld center,application.in which the proper quantitics of residual austenite were actedas tough pbase. For the tough phase is not easy to be cleavageAcknowledgementfractured, and more energy will be consumed in plastic defor-The project was financially aided by the National Defense Tech-mation, the crack was deterred and forced to be directed intonology Key Laboratory, No: 99JS50.3.2JW1402.where there was less resistance, less harmfulness and easy tobe extended in. Therefore, the stresses were relaxed so as theREFERENCESfracture toughness was improved. In addition, some quan-[1 ] T .Sebastiano, N,Fabio and D.M.Marello: J. Mater. Sci, Lett.,tities of finer and disperse carbides were distributed on the1996, 15(10), 827.matrix of the low-carbon lath martensite in weld of EBLP.[2] Wang Chienchun and Chang Yih: Metall. Mater. Trans. A,WHT joints. It had been proved that the hardnes of weld1996.27A(10), 3162.center zone was decreased and the matrix was softened. It} 0.C.Huang, Y.C.Pan and T.H.Chuang: J. Mater Eng, Per-would be contributed to its toughness improvement.{4] G.Cam, S.Erimm, C.Yeni and M.Kocak: Welding J, 1999,4. Conclusions78(6), 193(1) After EBLPWHT treatment, the main microstructure[5 ] BS7448(part1-Method for determination of Ktc， criticalCTOD and critical J values of metallic material, 1993of 30C:rMnSiNi2A steel weld was changed from coarse acicu-6 ] BS7448(part2)- Method for determination of Kic， criticallar martensite into lath martensite, HAZ was changed fromCTOD and critical J values of welds in metallie material, 1997.Repetitious- Hot-Pressing Technique in Hot-Pressing ProcessShixue SONG-2)t, Xing AN), Wei GAO1) and Jun ZHAOH)1) School of Mechanical Engineering, Shandong University, Jinan 250061, China2) School of Science, Jinan University, Jinan 250022, China[ Manuscripl received July 19, 2002, in revised form September 16, 2002]A new pressing method was proposed for hot-ressing process. Experimental results indicated that the porosity in Al2O3/TiC/Ni/Mo (hereafter called Al2O3/TiC composite) composite compacts decreases by 6% after adopting this new technique,compared to traditional hot-pressing technique under the same sintering temperature. The flexural strength and Vickerhardness increase from 883 MPa to 980 MPa and from 16 GPa to 21.1 GPa, respectively. A theoretical model was given toanalyze the densification mechanism of the composite in the process of repetitious-hot-pressing.KEY WORDS: Hot-pressing sintering, Repetitious- hot-pressing sintering, Composite1. Introductionistence of pores degrades mechanical properties of ceramicmaterials such as strength and hardness, and decreases theirCeramic composites have attracted great interest for theirreliability for applications. Comparer to other sintering tech-good mechanical properties such as high hardness, good re-niques, HIP technique can decrease sintering temperature andsistance to wear, chemical stability and excellent mechanicalimprove mechanical properties of coinposites. But expensiveequipment limnits its popularity and application. Hot-pressingproperties at high temperatures. Generally, ceramics are fab-technique is one of the most widely used methods to manufac-ricated through sintering of powders at high temperatures.ture ceramics components, but densification of compacts byAt present, ceramic sintering methods mainly, include botusing this technique is still a dificult problem t如be solved.pressed technique (HP), hot isostatic pressing (HIP), microwave sintering, plasma sintering, and so on. Because of in-In order to solve the densification problem, a new hot-trinsic characteristics of ceramics, it is difficult to densify cepressing technique is proposed in this paper. TWO kinds oframic powders completely. The sintering temperature is verysinteri中国煤化工t-pressing (HP) tech-high, especially for non-oxide ceramics. For example, SisN4niquenique have been usedHP S800°C and pressureto fabu-Al2O3/TiC compos-intere at temperature over 18000 ud, ex.te. :C N M H G/TiC composites ab30 MPa, a number of pores existed inevitablyt. The ex-ricated by using the two kinds of methods are compared, anda theoretical model is proposed to qualitatively analyze and↑Assoc. Prof., Ph.D， to whom correspondence should beexplain tbe mechanisms of densificat ion.addressed, E-mail: ssxue@163.com.J. Mater. Sci. Technol, Vol.19 No.5, 20034872. Experimentalof six speimens. The Vicker hardnes and fracture toughness2.1 Powder processingare average va!ue of six indentations. The microstructureHigh-purity c-Al2Ox (99.9%) with a mean particle sizeof the composite was observed using scanning electron mi-of 0.8 pumn and TiC powder with an average particle size ofcrosCoPY (SEM) after the samples were coated with gold. For0.14 um were used in this ivestigation.5v0l. pctofmixturecomparison, the mechanical propertics of AI2O3/ TiC compos-ofNiandMoweeaddabindesAhte。noutoirites fabricated using the lwo met hods were measured underwas added toi binders. A ltle amount of MgOthe same conditions.sured undero impede the abnormal growth of Al2O3 grains.2 volume fractions and pbysical properties of various con-3. Results and Discussionstituents are listed in Table 1.3.1 Mechanical propertiesTable 1 Constituents of Al2O:/TIC compositeThe mechanical propertics of Al2O3/TiC composites thatwere fabricated using two methods are listed in Tablc 2. FromConstituent Content Melting point Densitythe measurement results, it is cleur that the degree of densi-/vol. pct/°C/(g/cm3fication becomes high after adopt ing multiple pressing tech-Al2O3402050nique, the porosity decrease from 7.5% to 1.5%. The strengthTiC553400and hardness of the composite increased by 97 MPa andNi4.14558.95.1GPa, respectively. However, the fracture toughnessMo0.5262510.2creased slightly. According to the experimental formula which_MgOa ltte28003.61is proposed by Ryskewitschl2lσ = σoexp( -ap)(1)At first, the mixedwas poured into polyethy-lene barrel andal-milld inwhere p is porosity, σo is the strength when p=0, a is a con-cifc gravity of Ni and Mo are much higher than those of stant with the value 4~7. From Eq.(1), it can be seen thatAl2O3 and TiC and dfferent specific gravity will cause difer the higher the porosity, the lower the strength. The mea~ent drops in the processing of mixture drying which makes the surement results accord approximately with Eq.(1). Figuremixed power inhonogeneous. In order to solve this problem，1 is the SEM micrographs of polished surfaces of Al2O3/TiCthe mixture slurry was ultrasonically disperscd and electro-composite fabricated by the two kinds of methods. Figuredynamically stirred simultaneously until the viscosity of the1(a) indicates that a quantity of pores exist in the specimensslurry increases to such extent that all ingredients hardly sed-fabricated by using traditional HP technique. The pores areimentated. Then the slurry was dried in v、was dried in vacuum.The driedlocated: on the boundaries of Al2O3 andno grans. Roweverengaresormixed powder was sieved through a 25 pμm sieve in order to no pores are found for the specimens manufactured by usingmixed powget rid of large agglorerates.repetitious-hot-pressing technique under the same magnifica-tions shown in Fig.1(b).2.2 Cormpuct fabricationTwo methods wereadopadopted. to fabricate, compacts ofAl2O3/ TiC composite. When using ltraditional hot-pressingtechnique, the compacts were hot-the temperature of 1760°C under the pressure of 30 MPa inflowing N2 atmosphere. The holding time was 15 min. Therepetitious-pressing method is as fllows: at the temperatureof 1600°C, a pressure of 30 MPa was added and then decreasedto 0 MPa. The above procedure was repeated twice again atthe temperatures of 16509C and 1700° C, respectively. Finally,Poresthe temperature and pressure were increased to 1760°C and30 MPa, reespectively. The holding time of theand pressure，were 15 min. After that, the compacts were nat-urally cooled in the oven. The above hot-pressing processeswere all carried out in N2 atmosphere.2.3 Mechanical property tcstAll the compacts were cut, ground and polisbed to a 1 pumfinish with diamond paste. The sample density was measuredaccording to Archimedes principle with deionized water asmedium. The Vickers hardness was measured under a loadof 200 N. The indentation fracture toughness was determinedusing the lengths of indentation cracks generated by Vicker'sindent under a load of 200 N. Length of cracks was measuredin air using an optical microscope for toughness assessment.he system includes a digital pad which fciliatleda the pre-30.mcise measurement of the crack lengths. Three point bendingmethod was used to measure the fexural strength with a spanof 20 mm at a cross head speed of 0.2 mm/min. The sizes ofFig.1 SEM micrograph of polished surfaces of Al2O3/TICspecimens are of 3 mmx4 mmx30 mm in height, width andcomposite fabricated by traditional hot-pressing tech-length, respectively. T'he fexural strength is the average valuenique (a) and repetitious-hot pressing technique (b)Table 2 Mechanical properties。中国煤化工Mechanical propertiesTlexural strength i:MYHCNMH G_elative density/MPa/%Traditional method8835.0492.5Repetitious-hot-pressing technique9804.932198.5488J Mater. Sci. Technol, VoL.19 No.5, 2003Closed portepressure to the other. Under the external pressure p, the pres-sure of liquid becomes p + p”, however, the pressure of vaporSold phaseis less than p+ p" because of the exist ence of N2. In this case,Liquid phase -Liquid phase-the equilibiumn is broken and the material will be transferredfrom liquid to viapor, and the vapor concentration in the poreincreases. At the same time, under the action of externalforc, the shapes of pores are changed (shown as Fig.2(b)).a)(bThe distortion causcs the changes of curvature radii of theFig.2 Schematic diagrams of grain rearrangement withoutpores, which will result in the difference of vaporpessure inexternal pressure (a) and under unilateral pressuredifferent parts of the pores.he pressure difference makes(b)the vapor transfer from the part with larger curvature radiusto that with smaller curvature radius. The concentration of3.2 Theoretical modelvapor in the part with smaller curvature radius become higherDue to the difficulty of theoretically describing the re-and higher and finally agglomerate on the surface with smallarrangement of grains in sintering process, there is not ac-curvature radius, which will result in the reduction of radiusceptableon the densification mechanism during hot-of the pore so that a new equlibrium is reached. For example,pressing sintering process. At present, a very common modelassuming that there is the same amount of vapor and N2 infor describing sintering process is the two-sphere mode|3l. Ina pore, when adding an external pressure of p, the incrementorder to explain the superiority of repetitious hot-pressing-of the pressure of vapor is p/2 accorng to the Dalto's law.Fromtechnique over traditional hot-pressing method, a simple mod-Eq.(2)decrement of the radius of the pore isified two-sphere model was used to analyze the densificationOr≈品r3)mechanism of compacts. The following is an analysis of the .densification mechanism of repetitious-hot-pressing sintering.Equation (3) clearly indicates that greater external pressureIn the early stage of sintering, the open pores in compactswill result in greater decrernent of thdecrease due to the arrangermnent of grains, and most of theanditimpcremnent of the radius ofosed pores, ,ssible to eliminate the pores in a compact ifpores are closed. Therefore in the following of this paper, onlythere is N2 in the pores.closed pores are studied. At high temperatures, some com-According to Eq(3)， under the circumstance oponents with lower melting point such as Ni will be molten,repetitious-hot-pressing sintering, the radius of a closed poregrains of some components with higher melting points will bewill decrease by Ar, Or', Or", .... respectively, after eachenclosed by liquid phase, accordingly the sintering process ofequilibrium is reached. The radius of the pore becomesthis composite is liquid sintering in fact. Due to the action ofsmaller and smaller, even though the decrement of its radiusgas pressure in the pores, the shapes of closed pores can beprogresses gradually. If there is no Nz in a pore, and the vapordescribed by plane diagram (shown as Fig.2(a)) in the earlypressure in it becomes very high with the increment of exter-sintering process without external pressure. As the drivingnal pressure so that the vapor can be transformed into liquidforce of sintering, the surface tension of liquid phase aroundunder high pressure.The poredisappear iif the. bold-grains draws grains closer to each other, In the early stageing time is long enough. From above analyses, it is clearlyof sintering, the pores shrink quickly as the surface tension isthat the radii of the pores in the compacts fabricated throughlarger than gas pressure in the pores. With increasing tem-repetitious-hot-pressing are smaller than those in the speci-perature, the gas pressure in closed pores increases, and atmens manufactured through traditional hot-pressing methodthe same time the surface tension of liquid decreases, an equi-under the same sintering temperature.librium will be reached between the above two kinds of forcesAccording to analyses above, the mechanism of densifica-at a specific temperature, which will result in a reduction oftion of repetitious-hot-pressing sintering can be explained as :the sintering speed. From above point of view, the shrinkagethe effects of vaporization-condensation. The liquid phase va-of closed pores will cease if external force does not exist, andporizes from convex surface and condensates on the concaveit is impossible to eliminate closed pores in compacts duringsurface through vapor diffusion. After each pressing process,sintering process with external pressure. During the processa process of vaporization-condensaticn occurs. Through repe-of hot-pressing sintering; external force counteracts the gastitious vaporization-condensation processes, the pores in com-pressure in closed pores, and enhances the impetus of sin-pacts become smaller and finally disappear.tering. Under the action of external pressure, the shapes ofpores in compact will be changed. For simplicity, suppose4. Conclusionthat the transformation of pores and arrangement of grainsA new sintering technique is proposed in this paper. Whenunder unilateral pressure can be approximately ilustrated byusing this method, the porosity in compact can be signifi-Fig.2(b).cantly reduced, and consequently, t he Aexural strength andFirst of all, the mechanism of hot-pressing tecbnique willhardness increase greatly, The techaique is similar to tradi-be analyzed. Generally speaking, the gas in the closed porestional hot-pressing method,and can be used as an effectiveis the mixture of N2 and vapor of Ni because of the existencemeans to improve the degree of densification of compacts. Theof protecting atmosphere of N2, assuming that equilibriumtechnique will also play an important role in the fabricationbetween gas pressure and surface tension of liquid is reachedof ceramic materials. Theoretical analyses indicate that theat a certain temperature. According to theory of statisticalextrusion of pores results from material transformation fromphysics, the equilibrium conditions without external pressure.liquid to vapor and condensation (on the part with smallerare as follows:curvature radius.Tu=T8(2Acknowledgement; supported by the National Natural ScienceThis work waFoundation of China (No. 50105011).where a and B represent vapor and liquid phase, respectively.中国煤化工J is surface tension of liquid phase, r is the radius of gaspore.5epresent the pressure, temperature andpore,p:Tandμe phaseYHC N M H Gnuel E.Brito and Sbuzochemical potential, respectively.:quation (2) isis theKanzak: J. Am.Ceram. SoC, 1994，17(7), 1857. .transformation equilibrium conditions. If Eq.(2) is not satis- [2 ] Yu ZHOU:Ceramic Materials, The Publishing House of Harbinfied, materials will be transformed from the phase with higherInstitute of Technology, Harbin, 1995, 329. (in Chinese)chemnical potential to the other, or from the pbase with higher [3] R.W.Riee: J. Mate. Sci, 1996, 31, 1059.