1 INTRODUCTIONZackey et al<1>found that high austenitizingtemperatures can suppress plate martensite forma-tion,and canincrease the amount of lath martens-ite in structural steels ,and further suggested thatthe toughness of steels may bei mproved by raisingthe quenching temperature . Up to now, many re-searchers have studied the influence of hardeningtechnologies on martensitic morphologies andproperties , and have drawn the same conclusionthat the amount of lath martensite may be in-creased<2 ,3>and the plasticity and toughness of as-quenched steels may be enhanced by raising aus-tenitizing temperature , and they hope that hightemperature hardening will become a means of ex-ploiting the latent power of materials<4 ,5>. How-ever ,so far they have not succeeded.It is commonly found that the microstructuresquenched at lower temperatures are plate martens-ite and lath martensite<6>or all plate martensite ,and those quenched at high austenitizing tempera-tures are all lath martensites<7 ,8>. Particularly theentire lath martensite may be obtainedin high car-bon steels by high temperature austenitizing<9 12>.The change sequence of as-hardened microstructurefor steel 5Cr MnMo ( 0 .5 %C, 1 %Cr , 1 %Mn,0 .3 %Mo) with an enhancement in austenitizingtemperature is :acicular martensite →acicular mar-tensite + lath martensite →lath martensite +mass-like residual austenite →lath martensite withlath-like residual austenite .The types of martensite have been al mostidentified by using an optical metallographicalmethod in the whole investigation of hardeningtechnologies<6 ,13>,i .e .the packet quenched micro-structure under optical microscope is of lath mar-tensite .Tan et al<14 17>pointed out that packet mar-tensite should be dividedintotwo categories :pack-et thin sheet martensite and packet plate martens-ite . The former is generally found in as-quenchedlowcarbon steels ,whose natureis of lath martens-ite ,and the latter usually occurs in medium andhigh carbon steels , whose nature is of plate mar-tensite .In this paper ,the effect of austenitizing tem-perature on actual morphologies of as-quenched mi-crostructuresin mediumand high steels was studied.2 EXPERI MENTALThe chemical compositions of steels studiedare listedin Table 1 .Speci mens ofd(10 20) mm×5 mm werequenched at 860 1 300 ℃,and holding ti me was10 60 min. All the speci mens bore center holewith diameter of 3 mm.Steel 40 was quenchedinto cold water ,and allother steels were quenchedinto oil .The speci mens of steel 18Cr MnTi ,20Cr2Ni4 ,20Cr2 MnMoV, Cr WMn etc . were carbonized insolid carburizer at 1 200 ℃for 5 10 h and cooledTable 1 Chemical compositions ofsteels(mass fraction, %)Steel C Mn Si Cr Ni Ti Mo V18Cr MnTi 0 .23 1 .03 0 .32 1 .20 - 0 .095 - -20Cr2Ni4 0 .20 0 .42 0 .19 1 .68 3 .70 - - -20Cr2 MnMoV 0 .22 0 .98 0 .27 1 .87 - - 0 .64 0 .2340 0 .38 0 .650 .21 - - - - -40Cr 0 .43 0 .66 0 .35 0 .95 - - - -45CrNi 0 .47 0 .67 0 .35 0 .98 1 .25 - - -T9 0 .86 0 .47 0 .19 - - - - -T10 1 .04 0 .28 0 .21 - - - - -T11 1 .12 0 .25 0 .10 - - - - -9CrSi 0 .91 0 .43 1 .35 1 .15 - - - -GCr15 1 .09 0 .33 0 .28 - - - - -Cr WMn 1 .03 1 .04 0 .27 - 1 .48 - - -Cr4 Mn2 1 .24 1 .92 4 .27 - - - - -in furnace ,then homogenizing annealed at 1 200℃for 10 h, and cooled in oil . Finally , the carboncontent of speci mens was analyzed again. For ex-ample , 100Cr2 MnMoV steel represents a meancarbon content of 1 .00 %.Steel Cr4 Mn2 was pre-pared usinginduction melting.The speci mens were etched with 4 % Nital ,the martensitic morphologies and substructures ofthe speci mens were examined by using optical mi-croscope ,scanning electron microscope and trans-mission electron microscope .3 RESULTS AND ANALYSES3 .1 High temperature quenched microstructures inmediumcarbon steelsOptical microstructures quenched at 1 100 ℃in the medium carbon steel 40 are all single con-trast packet martensites without block structures ,as shown in Fig.1(a) . This packet martensiteFig .1 Optical micrograph (a) ,scanning electron
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