Kinetics of austenite transformation during thermomechanical processes

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Canadian Metallur`ical Quarterly\ Vol[ 26\ No[ 1\ pp[ 64Ð78\ 0887 Þ 0887 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd Printed in Great Britain[ All rights reserved 9997Ð3322:87 ,08[99¦9[99

Pergamon

PII ] S9997Ð3322"86#99929ÐX

KINETICS OF AUSTENITE TRANSFORMATION DURING THERMOMECHANICAL PROCESSES V[ M[ KHLESTOV\ E[ V[ KONOPLEVA$ and H[ J[ MCQUEEN% Priazovsky State Technical University\ Mariupol\ Ukraine $Institute for Solid State Physics\ RAS\ Chernogolovka\ Moscow\ Russia %Mechanical Engineering Department\ Concordia University\ Montreal H2G 0M7\ Canada "Received 16 February 0886 ^ in revised form 04 July 0886# Abstract*One of the most important fundamental problems of thermomechanical processing "TMP# of steel is the transformation kinetics of deformed austenite\ which has a great in~uence on the _nal microstructure and mechanical properties[ In the present work\ the results of more than 19 year|s systematic study of deformation e}ects on the ferrite!pearlite and bainite transformations conducted by the authors and their colleagues are discussed[ The progress of isothermal austenite decomposition was measured by magnetometry\ while dilatometry was used to monitor transformation under continuous cooling[ From a research base of about 39 steel grades\ general conclusions of the characteristic features of austenite transformation kinetics during TMP are formulated and the factors responsible for changes in the kinetics are discussed[ The acceleration e}ects of deformation on the ferrite!pearlite transformation and the much more complicated variations in austenite decomposition rates in the bainite range are considered depending on deformation parameters "temperature "T#\ strain\ post! deformation delays#\ transformation conditions\ and carbon and alloying content of the steels[ Þ 0887 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd[ All rights reserved[ Resume*L|un des problemes fondamentaux les plus importants du traitment thermomecanique "TMP# de l|acier est la cinetique de transformation de l|austenite deformee\ qui a une grande in~uence sur la micro! structure _nale et sur les proprietes mecaniques[ Dans le present travail\ on discute des resultats de plus de 19 ans d|etudes systematiques par les auteurs et leurs collegues\ des e}ets de la deformation sur la trans! formation en ferrite!perlite et en bainite[ On a mesure\ par magnetometrie\ le progres de la decomposition isotherme de l|austenite\ alors qu|on a utilise la dilatometrie pour suivre la transformation en refroidissement continu[ A partir d|une base de recherche d|environ 39 types d|acier\ on formule des conclusions generales des points caracteristiques de la cinetique de transformation de l|austenite lors de traitement TMP et l|on discute des facteurs responsables des changements de cinetique[ On considere les e}ets d|acceleration de la deformation sur la transformation en ferrite!perlite ainsi que les variations beaucoup plus compliquees destaux de decomposition de l|austenite dans le domaine de la bainite[ Ces considerations incluent les parametres de deformation "temperature "T#\ deformation\ delais d|apres!deformation#\ les conditions de transformation ainsi que le contenu en carbone et en ł elements d|alliage des aciers[ Þ 0887 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd[ All rights reserved[

teristics of the transformation kinetics of deformed austenite\ which have not been accounted for adequately although some experimental results have started to appear in journals since the 0869s ð01Ð07Ł[ If the change in austenite stability caused by deformation is not considered\ it is di.cult ] "i# to choose e}ec! tively the steel grade for the product ^ "ii# to develop control ranges of austenitization\ deformation and cooling ^ "iii# to evaluate the structure produced and its mechanical properties[ In this connection\ it seems reasonable to determine the fun! damental e}ects of deformation on the kinetics of austenite transformation for a wide range of steels and also to employ these results in designing TMP practice[ The experiments were carried out in a laboratory equipped with a small rolling mill and a specially designed balance mag! netometer ð08Ł\ as well as a dilatometer ð19Ł[ Rolling has the advantage that industrial schedules can be simulated on lab! oratory mills[ This installation allowed the specimens to be deformed in di}erent TMP modes and the austenite decompo! sition to be measured under both isothermal and continuous cooling conditions[ Over 19 years\ the authors and colleagues

INTRODUCTION Extraordinary e}ects of thermomechanical processing developed in the mid!0849s ð0\ 1Ł have attracted considerable interest from metal engineering scientists\ and new hardening treatments have been devised[ As a result\ many of the fun! damental aspects of TMP have already been investigated[ How! ever\ the achievements in practical use are much more limited due to several reasons\ the principal one being the absence of an adequate data base[ Of the many potential types of TMP\ only controlled rolling or forging are widely applied in industry ð2Ð00Ł[ The information presented can provide valuable insights into control of cooling rates and selection of coiling tem! peratures at the end of hot rolling[ The industrial TMP known as ausforming\ i[e[ the deformation of unstable austenite and its transformation to martensite\ will not be discussed[ A very important theoretical problem of TMP is the charac!

 Authors to whom correspondence should be addressed[ 64

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V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

"R[ I[ Entin\ G[ Ya[ Betin\ A[ A[ Gotsulyak\ G[ K[ Dorozhko and Z[ V[ Frolova# ð02\ 10Ð27Ł carried out examinations of approximately 39 steel grades ] HSLA\ structural\ spring and tool\ thus di}ering in carbon content and also in the type and quantity of alloying[ The extensive experimental data obtained made it possible to formulate general conclusions on the charac! teristic features of the austenite transformation kinetics during TMP and to suggest some practical recommendations[ In the next section\ compositions of the steels studied and experimental techniques for integrating measurement of trans! formation kinetics with deformation modes are presented[ Then the e}ects of various parameters on the deformation acceleration of austenite isothermal transformation to ferrite "Section 0# and to pearlite "Section 1# are described along with microscopic indi! cation of a high initial nucleation rate[ Next it is shown that the e}ects of straining on bainite formation are more complex "Section 2#[ Finally\ the behaviour in continuous cooling is com! pared with that in isothermal processing "Section 3#[

EXPERIMENTAL TECHNIQUE Low!\ medium! and high!carbon steels alloyed with Cr\ Mn\ Ni\ Si\ Mo\ V\ Nb and others in di}erent combinations have been studied "Table 0#[ The steels have been designated in the Russian manner ] the _rst 1 or 2 digits indicate C content ^ the signi_cant elements are listed with the rounded weight percent following if it exceeds 0[4)[ In order to accurately measure the e}ect of deformation on isothermal transformation kinetics\ a balance magnetometer with a non!uniform magnetic _eld of 5999Ð6999 Oersted has been constructed at the Mariupol Metallurgical Institute "Ukraine# ð08Ł[ Through the ferro!magnetic transition\ the vol! ume fraction of a!phase in a specimen of constant shape is determined by measuring the increase of mechanical force exerted by the magnetic _eld using a strain gauge load cell[ The main distinguishing feature of the magnetometer ð24\ 25Ł is its simple operation ^ the previously heated specimen after any pre! treatment is transferred quickly "within 0[4Ð1 s# into a tin! bath within the magnetometer and attached for recording the isothermal transformation[ To investigate the e}ect of hot deformation on austenite decomposition\ a laboratory mill was located near the magnetometer in order to facilitate rapid trans! fer of the specimens to the test _xture thus minimizing the uncontrolled cooling[ The mill rolls of 59 mm diameter had a speed of 9[4 m s−0 resulting in o  09Ð14 s−0 depending on the reduction[ Since the _nal specimen dimensions must always be the same\ the initial dimensions must be varied to suit the speci_c reduction in height and to take care of the spread[ There were two air furnaces at both the entry and exit sides of the mill in order to provide the required temperatures for either single passes of various strains or a series of passes with speci_ed intervals[ For measuring the transformation during continuous cooling or isothermal transformation at T × 699>C\ a special dila! tometer ð19Ł which is capable of taking specimens that could di}er in length or section was employed[ After deformation\ the specimen was transferred very quickly "0[4Ð1 s# and installed between the holders of the dilatometer[ The experimental procedure for the investigation of the e}ect of hot deformation on austenite transformation is described as

follows[ After the specimen was austenitized at 899Ð0099>C\ it was cooled in the furnace to the deformation temperature rang! ing from 0999Ð799>C and then rolled[ Immediately after defor! mation\ the specimen was transferred to the tin!bath in the magnetometer or to the dilatometer for measuring the progress of transformation[ In the experiments with warm deformation\ the austenitized specimen was placed in another furnace pre! viously heated to 499>C\ deformed and allowed to undergo the isothermal bainite transformation inside the furnace of the magnetometer[ For comparison\ the transformation of unde! formed austenite was studied under every condition[ 0[ Polymorphic transformation In most cases\ the steels subjected to TMP are low! or med! ium!carbon hypoeutectoid grades[ In consequence\ the trans! formation of austenite starts with the formation of proeutectoid ferrite[ Therefore\ it would be reasonable to start with the analy! sis of the polymorphic transformation[ The experiments were carried out on the low!alloy ferrite!pearlite group of steels "09CrSiNiCu\ 09Mn1Si\ 98Mn1VNb\ 06Mn1Si\ 19Mn1NbV and others#\ medium!alloy bainitic group "Cr1Mn1B with the carbon content 9[96\ 9[00 and 9[10)# and high!alloy marten! sitic group "Cr3Mn1 with the carbon content 9[95\ 9[02 and 9[19)\ and Cr6Ni1 with the carbon content 9[997\ 9[915\ 9[939\ 9[954\ 9[97 and 9[006)# "Table 0#[ The specimens were heated at 849Ð0999>C for 09Ð04 min and at 0049>C for those micro! alloyed with V and Nb[ Then they were rolled in one or several passes to 19Ð59) reduction at the temperatures between 0999 and 799>C[ After rolling or a post!deformation pause\ the specimens were held isothermally in the pearlite range[ Analysis of the numerous experimental results ð15\ 17\ 18\ 21Ł made it possible to conclude that the deformation e}ects on austenite stability are qualitatively similar for all the steels[ Figure 0 presents kinetics curves showing typical deformation e}ects on the g : a transformation in low!carbon steels\ stron! gly di}ering one from another by their chemical composition[ The _rst and most important e}ect is that the hot deformation accelerates transformation of austenite to ferrite\ the accel! eration being greater for higher strain[ The same deformation e}ect on the g :a transformation was observed by many others ð01\ 07\ 28Ð30Ł[ The second e}ect is also typical for all the steels studied ] deformation accelerates austenite transformation to ferrite most markedly at the initial stage of the reaction[ As the trans! formation proceeds\ the e}ect of deformation diminishes gradu! ally[ The more deformation decreases the austenite stability\ the greater is this e}ect[ A comparison of the data given in Figs 1 and 2 allows more quantitative estimation[ For instance\ if the deformation reduced the time for 4) ferrite formation by a factor of 2Ð5\ then for 49) it was by a factor of 1[1Ð2[4 ^ thus at the stage of half decomposition\ the deformation e}ect was less by a factor of 0[4 than at the initial stage of the g : a transformation[ In a case where deformation decreased the austenite stability more radically "Fig[ 2#\ the acceleration diminished considerably as transformation was developing ] at the stage of half!decomposition\ it was less by a factor of 1[4Ð 2 than at the initial stage[ By the end of transformation "89) ferrite# the acceleration decreased by a factor of 4Ð5[ Figures 1 and 2 show the third e}ect of deformation on the g : a transformation kinetics which is inherent in all the steels

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

66

Table 0[ Designation and composition "wt)# of the steels Designation 09CrSiNiCu 09Mn1Si 98Mn1VNb 06Mn1Si 19Mn1VN 96Cr1Mn1B 00Cr1Mn1B 10Cr1Mn1B 95Cr3Mn1 02Cr3Mn1 19Cr3Mn1 997Cr6Ni1 915Cr6Ni1 93Cr6Ni1 95Cr6Ni1 97Cr6Ni1 01Cr6Ni1 79C 69Ni1 49CrW1Si 19Cr02 24Cr 39CrNi 39CrNiMo 29CrSiMnNi1 24CrNi4Si 34Cr2Mo 59Si1 59CrSi1 44CrMnSi1 39Cr1Mn1B 49Mn1MoB 099C 89CrWMn 69Cr2 24Cr2 49CrNiMo 34Ni4

C

Si

Mn

Cr

Ni

Mo

9[00 9[09 9[98 9[06 9[19 9[96 9[00 9[10 9[95 9[02 9[19 9[997 9[915 9[93 9[954 9[97 9[006 9[70 9[69 9[41 9[11 9[25 9[26 9[27 9[17 9[24 9[32 9[47 9[46 9[44 9[31 9[41 9[87 9[80 9[62 9[23 9[44 9[33

9[86 0[96 9[16 9[50 9[38 9[26 9[21 9[39 9[13 9[13 9[13 9[24 9[24 9[24 9[24 9[24 9[24 9[07 9[27 9[85 9[18 9[13 9[10 9[18 9[69 0[93 9[29 0[69 0[65 0[53 9[39 9[29 9[16 9[16 9[12 9[17 9[13 9[11

9[69 0[35 0[33 0[37 0[35 1[12 1[07 1[96 1[06 1[06 1[06 9[93 9[93 9[93 9[93 9[93 9[93 9[13 9[55 9[22 9[32 9[66 9[42 9[60 0[97 9[92 9[14 9[63 9[69 0[19 1[02 0[63 9[17 0[93 9[24 9[38 9[43 9[33

9[80 * * * * 1[29 1[13 1[12 3[08 3[08 3[08 6[03 6[03 6[03 6[03 6[03 6[03 9[09 * 0[19 02[3 9[87 9[52 9[81 0[02 0[02 1[84 * 9[81 9[86 1[12 * * 0[97 2[49 1[86 9[69 *

9[61 * * * * * * * * * * 1[16 1[16 1[16 1[16 1[16 1[16 9[03 1[19 * * * 0[11 0[34 0[82 4[09 * * * * * * * * * * 0[34 3[79

* * 9[97V * 9[97V * * * * * * * * * * * * * * * * * * 9[06 * * 9[22 * * * * 9[48 * * * * 9[06 *

Others 9[32 Cu * 9[920 Nb * 9[91 N 9[9918 B 9[9911 B 9[9920 B * * * * * * * * * * * 1[09 W * * * * * * * * * * 9[992 B 9[992 B * 0[47 W * * * *

References $ 15 18\ 21\ 23 17\ 29 16 21

22 11\ 12\ 22 22 10\ 12\ 13 22

16 02\ 16 16

 C content altered by cementation[ $ The alloys without references were studied in Priazovsky State Technical University and the results were published in di}erent Russian journals[

studied[ The destabilizing e}ect of deformation is greater\ the higher is the temperature of transformation\ that is the less the austenite is undercooled[ For instance\ as the transformation temperature is raised from 599Ð519 to 579Ð699>C\ the destabil! izing e}ect of hot deformation increases by a factor of 0[4Ð2[ The experimental data in Figs 0Ð2 show that all observed e}ects of austenite destabilization\ and the main e}ect in particular\ are very much dependent on the chemical composition of the steel[ For instance\ deformation of 14) at 719>C caused a decrease in austenite stability "time for 4) transformation# at 579Ð699>C in 06Mn1Si steel by a factor of 3\ in 98Mn1VNb by 5\ and in 95Cr3Mn1 by 19[ A more detailed study of the e}ect of chemical composition has been made in experiments with specially selected steels\ in which the type and amount of alloying\ and also the carbon content were studied[ The main results of this series of exper! iments are summarized in Fig[ 3[ Surprisingly\ the alloying elements did not a}ect directly the degree of the austenite desta! bilization by deformation\ whereas carbon produced a strong in~uence\ but only within a narrow concentration range of

9[91Ð9[95)[ With increasing carbon concentration above 9[01Ð 9[02)\ it did not signi_cantly in~uence the austenite desta! bilization[ The absence of in~uence of alloying elements on destabilization may be supposed to result from the conditions of the experiment[ A relatively low temperature of deformation "719>C# and immediate undercooling of the specimens appar! ently caused a similar density of structural defects irrespective of metallic alloying\ and thus an equal degree of g : a trans! formation acceleration[ When the temperature of rolling was increased to 849Ð0999>C\ or the cooling start was delayed by 2Ð4 s\ the e}ects were essentially di}erent for di}erently alloyed steels[ For instance\ the rolling of 09Mn1Si steel at 0999>C destabilized austenite less by a factor of 1Ð1[4 than rolling at 719>C ^ however\ 98Mn1VNb steel exhibited almost no di}er! ences in the e}ects of a similar TMP change[ As it is well! known\ hot deformation leads to a signi_cant increase in dis! location and vacancy density resulting in the formation of sub! structure much more recovered than a cold!worked structure ð31Ð35Ł[ However\ at the deformation temperature such defect structure is very unstable and static recrystallization reducing

67

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

Fig[ 0[ Transformation kinetics curves at 549>C for deformed austenite of 09CrSiNiCu\ 6Cr1Mn1B and 95Cr3Mn1 steels "TA  849Ð0999>C# ð21Ł[

Fig[ 1[ Relative acceleration of 4) "a# and 49) "b# ferrite formation in deformed austenite of 06MnSi\ 09CrSiNiCu "from Fig[ 0a# and 98Mn1VNb steels ð24Ł[

the density of defects in austenite may occur during a brief pause ð2\ 3\ 6\ 36Ð49Ł[ The higher the deformation temperature and the less a steel is alloyed with the elements retarding recrys! tallization\ the faster the defect density decreases and as a result the degree of acceleration of austenite transformation is reduced[ Therefore\ the indirect in~uence of alloying elements "by a}ecting the evolution rate of the austenite dislocation structure ð31\ 38\ 40Ł# on the destabilization of deformed austenite may be very signi_cant and it should be considered and used in practice[ In our opinion\ the special role of carbon could be related to the density of vacancies in deformed austenite and to interaction of carbon atoms with vacancies ð41Ł[ In cases where the con! centration of carbon atoms in austenite is not su.cient for e}ective locking of vacancies\ the latter enhance the transit of

atoms through the g!a boundaries ð42Ł and highly accelerate the transformation ð28Ł[ The content of about 9[91Ð9[93) of carbon in the g!solution seems to be su.cient to lock the vac! ancies in undeformed austenite\ but insu.cient for deformed austenite since deformation causes a great increase in vacancy concentration[ As a result\ the remaining free vacancies together with other activating factors lead to the remarkably high desta! bilization of deformed austenite[ In our experiments\ if after deformation at 719>C\ cooling was delayed by 2Ð3 s "to remove excess vacancies#\ the acceleration in steels with 9[93Ð9[95) and 9[00Ð9[02) carbon became practically the same[ It should be noted that the steels under direct comparison di}ered in the carbon content only\ since they were produced by means of short time cementation of a single base steel so that the e}ects of diverse melting factors could be ruled out[ A sharp decrease

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

68

Fig[ 2[ Relative acceleration of 4) "a# and 49) "b# ferrite formation in deformed austenite of 5Cr3Mn1 steel "from Fig[ 0c# ð24Ł[

Fig[ 3[ E}ect of deformation at 719>C "TA  849Ð0999>C# on relative acceleration of 4) ferrite formation at 549>C in the steels of di}erent composition[

in the deformation e}ect with increasing carbon content above 9[94) "Fig[ 3# is likely to be associated with an increase in locking of vacancies by carbon atoms[ The 9[01Ð9[02) carbon in g!solution turns out to be su.cient for e}ective vacancy blocking in a deformed austenite\ and further increase in the concentration of this element in steels does not a}ect the degree of the g : a transformation acceleration[ Destabilization of deformed austenite is mainly de_ned by strong acceleration "by 2Ð3 orders of magnitude# of the ferrite nucleation rate "Fig[ 4a\b# while its growth rate "Fig[ 4c\d# increases only slightly[ The increased nucleation rate of ferrite in deformed austenite leading to enhanced transformation g : a has been observed and discussed by others ð4\ 07\ 43Ð45Ł[ It should be noted that the acceleration of nucleation caused by rolling proceeds most e}ectively at the initial stage of trans! formation and diminishes rapidly as the transformation is developing "Fig[ 4a\b#[ By about 4) ferrite formation\ the

nucleation rates for deformed and underformed specimens are almost at the same level[ Numerous studies of the process of ferrite formation in aus! tenite makes it possible to conclude that a sharp drop in the rate of nucleation in deformed specimens is due to rapid exhaus! tion of the most favorable nucleation sites[ As a rule\ by 4) ferrite formation\ almost a solid chain of ferrite grains appears along the boundaries of the austenite grains in deformed speci! mens "Fig[ 5#[ Even after a high strain at temperatures in the intercritical range\ ferrite nucleates mainly at the boundaries of the austenite grains and much less along twin boundaries and deformation bands "Fig[ 6a#[ The question of ferrite nucleation sites in deformed austenite has been under discussion for many years ð4\ 07\ 43\ 46\ 47Ł[ Deformation produces additional sites for nuclei such as deformation bands\ subboundaries and other inhomogenities of the structure[ So\ if in non!deformed austenite\ ferrite mainly nucleates at grain boundaries\ after deformation\ nucleation may occur at both austenite grain boundaries and structural defects[ But the contribution of inter! and intra! granular nucleation is estimated contradictorily by di}erent researchers[ In ð39\ 48Ł\ it is assumed that polygonal ferrite nucleates principally at the austenite grain boundaries[ Many authors ð07\ 43\ 44\ 46\ 47Ł speculated that both acceleration of nucleation at grain boundaries and at structural defects are responsible for enhancing transformation after deformation[ Others pointed out that the favoured nucleation sites depended on transformation temperature ð45Ł and amount of deformation ð4Ł[ Our experiments do not con_rm the concept that\ inside the deformed austenite grains\ the process of nucleation on structural defects proceeds e}ectively[ Moreover\ it seems that\ at the later stages of transformation\ the nucleation inside the deformed austenite grains proceeds rather sluggishly\ as was shown in Figs 5 and 6"a#[ Had it been more active\ a homo! geneous and _ne!grain ferrite!pearlite structure would have formed\ and not the observed non!uniform grain size "Fig[ 6b#[ The danger of producing a non!uniform grain size\ which sometimes happens under controlled rolling\ is well!known ð3\

79

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

Fig[ 4[ E}ects of deformation on the ferrite nucleation "a\ b# and growth "c\ d# rates at 699>C in 09CrSiNiCu "Figs 0a and 1# and at 549>C in 95Cr3Mn1 "Figs 0c and 2# steels[

Fig[ 5[ Nucleation of ferrite grains in austenite of 96Cr1Mn1B steel deformed at 799>C ð24Ł[ "a# o  9\ 09) ferrite ^ "b# o  49)\ 9[4) ferrite ^ "c# o  49)\ 4) ferrite[

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

70

Fig[ 6[ Ferrite nucleation at austenite grain boundaries of 98Mn1VNb steel "Fig[ 1# during rolling at 639>C followed by quenching "a# or cooling at 9[7>C:s "b# Austenitization at 0049>C\ rough rolling at 0999>C in three passes and _nish rolling at 639>C in _ve passes of 19) reduction in each[

6\ 46\ 47Ł[ Consideration of the above discussion indicates that non!uniformity in grain size can result from the grain!boundary nucleation in a strongly deformed austenite as well[ In con! trolled rolling\ even when ~attened grains are produced\ the growth of the ferrite crystals from the boundary completely through the austenite grains may lead to formation of large grains[ In practice this can be avoided by providing a _ne austenite grain structure in the billet before the _nish rolling[ Analysis of the experimental results made it possible to con! clude that the e}ects of deformation on the kinetics of austenite transformation to ferrite could be described mainly in ther! modynamic terms[ In local austenite regions near and at the grain boundaries\ the energy level increases due to deformation ^ this favors the nucleation process[ As it has been calculated\ if the level of the austenite boundary energy F increases by 09) as a result of deformation\ the nucleation rate may increase by 1Ð2 orders of magnitude\ and the austenite stability decreases by a factor of 3Ð5[ In these calculations F is referred to the free energy of the grain boundaries[ Estimation of possible acceleration of ferrite nucleation in deformed austenite has been made on the basis of classical theory of heterogeneous nucleation ð59Ł[ 1[ Pearlite transformation The e}ect of hot deformation on the pearlite transformation was studied on approximately 19 di}erent steels\ among them were ] eutectoid "79C\ 69Ni1\ 49CrW1Si and 19Cr02#\ hypo! eutectoid "24Cr\ 39CrNi\ 39CrNiMo\ 29CrMnSiNi1\ 24CrNi4Si\ 34Cr2Mo\ 59Si1\ 59CrSi1\ 44CrMnSi1\ 39Cr1Mn1B and 49Mn1MoB# and hypereutectoid "099C\ 89CrWMn and 69Cr2#[ The qualitative and quantitative e}ects of deformation at 799Ð759>C on the austenite transformation kinetics ð02\ 10\ 18Ł were almost the same for all steels if the transformation occurred under similar undercooling[ Typical examples of deformed austenite transformation in hypoeutectoid 29CrMnSiNi1 and eutectoid 49CrW1Si steels are presented in Figs 7 and 8[ Comparison of kinetics curves for pearlite and polymorphic "Fig[ 0# transformation shows that there is no critical di}erence in the deformation e}ects[

Thus for the austenite transformation to pearlite the major in~uences of deformation are the same as for the polymorphic transformation ] "0# Deformation accelerates the pearlite transformation\ the e}ect is greater for higher strain and lower deformation temperature ð10Ł[ "1# An especially high acceleration is achieved at the initial stage of decomposition but as the eutectoid reaction is developing\ the enhancing e}ect of deformation diminishes gradually[ "2# For higher transformation temperature "lower austenite undercooling#\ there is higher acceleration of deformed aus! tenite decomposition[ The experimental data obtained by studying di}erently alloyed steels with the carbon content from 9[1Ð0[9) are pre! sented in a generalized form in Fig[ 09[ They show that neither the alloying elements nor a change in the carbon content a}ect directly the magnitude of the acceleration caused by defor! mation at 799Ð759>C[ Some quantitative di}erence in the aus! tenite destabilization e}ect observed for di}erent steels\ is likely to be associated with experimental errors\ since the scatter is of a random character[ For instance\ the magnitude of the deformed austenite destabilization is almost the same for both the high alloy steel 19Cr02\ in which austenite transformation starts with the alloy carbides\ and in non!alloyed eutectoid steel 79C[ However\ results\ which do not _t the above comparatively narrow range of experimental points\ were obtained only for steels containing 1Ð4) Ni[ This is likely to be connected with the fact that Ni lowers the transformation points A2 and A0[ Therefore at equal temperatures of transformation\ the austenite in Ni steels turns out to be less undercooled than in other steels\ consequently austenite transformation is more accelerated due to deformation[ Besides this alloying e}ect on the variation in the A2 and A0 temperatures\ other e}ects related to chemical composition of the steels were revealed in the experiments[ As in the case of polymorphic transformation\ the alloying elements retarding

71

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

Fig[ 7[ Kinetics curves "a\ b# and pearlite range of TTT!diagram "c# for deformed austenite of 29CrMnSiNi1 steel "TA  849>C#[

Fig[ 8[ Kinetics curves "a\ b# and pearlite range of TTT!diagram "c# for deformed austenite of 49CrW1Si steel "TA  0999>C#[

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

72

Fig[ 00[ Schematic of the hot deformation e}ect on the pearlite range of TTT!diagram in steels with di}erent types of alloying ] a*alloying with the elements increasing Tnose "Mo\ W\ Cr\ Si# ^ b*alloying with the elements decreasing Tnose "Ni\ Mn# ^ c*combined alloying with elements which increase or decrease Tnose[ 0*beginning undeformed austenite transformation ^ 1*beginning the transformation of austenite deformed at 799Ð899>C[

reasons of acceleration of both these transformations are the same[ 2[ Bainite transformation

Fig[ 09[ E}ect of 14) reduction at 799Ð759>C on the relative accel! eration of austenite transformation in the pearlite range in steels of di}erent compositions[ Band of acceleration for 4) "0# and 49) "1# transformation[

the softening "recrystallization# of the deformed austenite favor preserving the e}ects of transformation acceleration with increasing temperature of rolling up to 849Ð0999>C and during short!time post!deformation pauses ð14Ł[ The e}ect of alloying elements on the nose temperature of the austenite transformation Tnose might be very important in practice[ Figure 00 presents a graph based on experimental data which leads to the conclusion that steels with a high Tnose "Fig[ 8# exhibit a much greater decrease in hardenability after TMP than those with a low Tnose "Fig[ 7#[ So\ it is more advisable to use the steels with Tnose situated well below the A0\ for the products manufactured by a TMP in which martensite is the objective[ And in this case\ the decrease in hardenability under TMP would be minimal[ A number of steels were employed for studying hot defor! mation e}ects on the nucleation and growth of pearlite[ The results obtained did not di}er essentially from those for the polymorphic transformation[ Nucleation in both undeformed and deformed austenite proceeds mainly at grain boundaries[ The rate of nucleation for deformed samples is higher by a factor of 1 or 2 than that for undeformed ones[ At the same time the growth rate is very little a}ected by deformation[ These data and also other facts of analogous deformation e}ects on polymorphic and pearlite transformations show that the

Bainite "B# transformation occurs in a di}erent way from di}usional austenite transformation in the pearlite range[ The g : B transition proceeds by a shear mechanism similar to martensite transformation ð50Ł[ But as distinct from the latter\ the bainite reaction is accompanied by carbon di}usion and formation of carbides during the transformation[ Because of the complex mechanism\ the response of the bainite trans! formation to straining is variable depending on the loading and transformation conditions ð03\ 04\ 13\ 16Ł[ This is why the results obtained by di}erent authors sometimes look con! tradictory[ In Russian literature\ there is a systematic study of bainite formation in deformed austenite and a number of papers have been published[ The main e}ects determined reliably by the authors ð02\ 04\ 13\ 15\ 16\ 29Ł and con_rmed by others ð03\ 05\ 51Ł are discussed in the present work[ In the Western literature\ transformation in deformed austenite was not studied so thoroughly[ Some results reporting the behavior of deformed austenite relative to the bainite transformation are summarized by Bhadeshia ð52Ł[ Both retardation "mechanical stabilization# ð06Ł and acceleration ð53Ł of the reaction were observed for di}erent steels and various deformation conditions[ Davenport ð06Ł established that both re_nement and deforming of austenite grains lowered the bainite transformation temperature range by 29Ð49>C during subsequent air!cooling[ In ð54Ł\ as!hot rolled austenite transformed to bainite faster at the temperature of the nose in a medium carbon alloy steel[ Bainite start tem! peratures were increased by 099>C due to thermomechanically working the austenite of low!carbon steel ð01Ł[ In contrast\ deformation was shown to have little e}ect on the formation of lathlike bainite although it promotes the nucleation of acicular ferrite ð55Ł[ In our experiments\ the e}ect of hot "0999Ð799>C# and warm "499>C# deformation on the bainite transformation have been studied on approximately 29 steels di}ering in carbon content and in amount and type of alloying ð02\ 10Ð14\ 16Ð20\ 22\ 26Ł[

73

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

Fig[ 01[ Kinetics curves "a\ b\ c# and bainite range of TTT!diagram "d# for deformed austenite of 49CrNiMo steel "TA  849>C# ð16Ł[

Di}erent materials were used for analysis because the e}ect of deformation on the bainite transformation depends in a complicated way on the experimental conditions and the chemi! cal composition of the steel[ It should be emphasized that the e}ect is ambivalent ] the deformed austenite may be either sta! bilized or destablized with respect to the bainite transformation[ Of particular interest is a stabilizing deformation e}ect which has been _rst found in 0855 ð56Ł and described in our papers ð10\ 02Ł in the early 69s[ A typical example of hot deformation e}ects on the kinetics of bainite transformation is presented in Fig[ 01[ In the 49CrNiMo and other steels "24Cr\ 39CrNi\ 49Mn1MoB\ 59CrSi1 and 09 other grades# with the upper boundary of the bainite range above 399Ð349>C\ retardation of the transfor! mation in the lower part "149Ð249>C# and acceleration in the upper part of the bainite range "399Ð499>C# were observed[ Conversion of the stabilizing e}ect of deformation into a destabilizing one proceeds gradually with increase in the trans! formation temperature[ First acceleration is observed at earlier stages of transformation "at½399>C# and retardation at later stages\ and then during the whole period of transformation "379>C#[ If the composition of the steel is such that the upper boundary of the bainite range does not exceed 399>C "24CrNi4Si\ 44CrMnSi1\ 39Cr1Mn1B#\ the hot deformation has a stabilizing e}ect only "Fig[ 02# ð02\ 10\ 18Ł[ Stabilizing and destabilizing e}ects of hot deformation on austenite transformation also give rise to changes in com! pleteness of the bainite transformation[ Figure 01 shows that\ when deformation retards kinetics "at 249Ð399>C#\ it sim! ultaneously diminishes the overall bainite fraction attained[ At

the same time\ if deformation accelerates the transformation rate not only at the earlier stages but during the entire reaction "at 379>C#\ it provides an increase in the completeness of the bainite reaction[ The magnitude and direction of the deformation e}ect on the bainite transformation depend in a complicated way on deformation parameters[ First of all\ it is true for a stabilizing e}ect of deformation[ In the very lower part of bainite range "149Ð299>C#\ an increase in the strain up to 49Ð59) promotes normally stronger retardation of the bainite transformation "Fig[ 02#[ However at 249>C\ an increase in the strain up to 01Ð 04) enhanced stabilization of austenite transformation while at higher strain the austenite stabilizing e}ect was less[ As transformation temperature was raised above 249>C\ the accel! eration of bainite reaction was _rst observed in the specimens deformed to high reductions[ Above 399>C the acceleration e}ect changed in a simpler way ] it was greater for higher strain and lower rolling temperature "Fig[ 01c\d#[ Decreasing the deformation temperature within the stable austenite range had the same e}ect on the bainite kinetics as increasing the strain ð24Ł[ Analysis of extensive experimental data from di}erent steels allows one to conclude that carbon and alloying elements have a direct and indirect e}ect on the stability of austenite subjected to hot deformation[ Carbon exhibits an ambivalent in~uence[ In the lower part of the bainite range at about 149Ð299>C\ it increases the e}ect of austenite stabilization caused by hot deformation[ However\ an increase in carbon content leads to a decrease in the transitional temperature where the stabilizing e}ect of deformation turns to a destabilizing one[ In the upper

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

74

Fig[ 03[ Generalized scheme of the hot deformation e}ect on the bainite range of the TTT!diagram[ 0\ 1\ 2*the lines show the beginning of austenite transformation for the steels with the compositions which predetermine the position of Bs approximately at 499\ 349 and 399>C respectively[ Dashed lines*undeformed austenite transformation ^ solid lines*transformation of austenite deformed to 49)[

Fig[ 02[ Kinetics curves at 169 and 249>C "a# and bainite range of TTT! diagram "b# for deformed austenite of 24CrNi4Si steel "TA  0999>C# ð10Ł[

part of the bainite range\ carbon enhances the destabilizing e}ect of hot deformation[ Carbide!forming elements Cr\ Mo\ and W and also Si enhance retardation of the transformation in the lower part of bainite range and increase the temperature at which the direc! tion of the deformation e}ect is changed[ That is\ they promote directly the stabilizing e}ect of deformation[ Nickel is the only element which has an opposite e}ect for a mono!alloyed steel\ i[e[\ it enhances the destabilization of deformed austenite with respect to bainite transformation[ An indirect e}ect of alloying elements arises from the fact that they change the position of the bainite range in the temperature domain[ When alloying elements increase the upper boundary of bainite range "Bs#\ the promoting e}ect of hot deformation on the transformation "Fig[ 03# becomes stronger and\ in contrast\ if they lower the range BsÐMs the e}ect of deformed austenite stabilization is enhanced[ The e}ect of warm deformation at 499>C and post!defor! mation pauses is shown in Fig[ 04[ In this case deformation causes a great acceleration of transformation\ reducing aus! tenite stability in the bainite range[ The post!deformation pauses much diminish the in~uence of deformation[ It should be noted that only static recovery takes place during these pauses[ If Figs 01 and 04 are compared\ a certain analogy in the e}ects of hot and warm deformation can be revealed[ In both cases the austenite destabilization can be observed most of all

Fig[ 04[ E}ect of deformation at 499>C "TA  0999>C# and post!defor! mation pauses on the bainite range of the TTT!diagram for 24CrNi4Si "a# "Fig[ 02# and 44CrMnSi1 "b# steels ð12Ł[

at the upper part of the bainite range[ However\ in the case of rolling at 499>C\ the in~uence produced on austenite stability is multiply stronger ] the incubation period "the time for 4)

75

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

transformation# at 279Ð399>C is reduced by a factor of approxi! mately 099\ and the temperature of the bainite nose and the Bs temperature are increased by 69Ð099>C[ At low temperatures "159Ð299>C# a destabilizing e}ect of the strain under con! sideration is rather low and in some cases after the post!defor! mation pause of 299 s\ even a slight stabilization of austenite was observed[ Thus\ the e}ect of hot and warm "499>C# defor! mation on the bainite transformation in the upper part of bain! ite range di}ers signi_cantly from that observed in the lower part[ Numerous experimental data show that in deformed aus! tenite during the bainite transformation\ opposite factors act and compete[ Some of them activate the deformed austenite with respect to the bainite transformation\ others stabilize it[ In order to reveal these factors electron!microscopic and x!ray studies were carried out on the retained austenite after incom! plete bainite transformation[ Nucleation and growth of a!crys! tals of bainite and also the e}ect of post!deformation pauses at di}erent temperatures and times were examined[ The e}ects of factors such as austenite grain size and chemical homogeneity have been varied by thermal cycling and step treatments to clarify their role ð26Ł[ Analysis of all the experimental data lead to the conclusion that the factors destabilizing deformed austenite relative to bainite transformation are ] "i# an increase in inhomogenity of carbon distribution in deformed austenite ð31Ł ^ "ii# stimulation of carbide formation in austenite ð31Ł ^ "iii# limited restoration in the deformed austenite during the bainite transformation ð57Ł[ The formation of localized segregations of carbon occurs in hot!deformed austenite as a result of interraction of carbon atoms with structural defects produced by deformation[ This promotes carbide formation during bainite transformation and also stimulates the nucleation of a!phase which requires for! mation of areas with decreased carbon concentration ð50Ł[ These areas in austenite form near segregations of carbide atoms and creates the additional sites for nuclei of bainite crystals[ Inhomogeneities of the austenite play an increased part in the formation of upper bainite because of small ther! modynamic stimulus for the transformation[ The factors stabilizing deformed austenite are ] "i# enhanced resistance to shear of a deformed grain!re_ned austenite ð26\ 31\ 58\ 69Ł ^ "ii# retardation of carbon di}usion ð60\ 61Ł ^ "iii# the greater redistribution of carbon between the phases needed for bainite transformation in deformed austenite "x!ray exam! ination of deformed retained austenite showed that bainite transformation occurred in austenite areas with lower carbon concentration ð20Ł#[ Formation of a high density of defects in the austenite structure is supposed to suppress the g : a tran! sition proceeding by a shear mechanism\ especially when a subgrain structure forms in austenite as a result of hot defor! mation ð50Ł[ Decrease of martensite point by 14Ð29>C in hot! worked austenite ð50Ł and slower rate of bainite transformation in the _ner austenite structure ð26Ł also indicate this[ There is also some evidence that hot deformation retards carbon di}usion at the temperatures of bainite range ð60\ 61Ł[ It has been found that the rate of carbon di}usion was 0[2Ð1[7 times less than in non!deformed austenite[ Possibly\ this factor is mainly responsible for the decrease in growth rate of bainite crystals in both lower! and upper!parts of bainite range ð16Ł[ The e}ectiveness of destabilizing factors in the hot!deformed

austenite strongly depends on the bainite transformation tem! perature[ At relatively low temperatures "149Ð299>C# of iso! thermal holding of hot!worked austenite\ the conditions for the action of destabilizing factors are not favorable\ and a change in the kinetics of bainite transformation is mainly determined by stabilizing factors[ An increase in the temperature of isothermal holding favors development of destabilizing factors[ Therefore\ as the transformation temperature reaches 399Ð349>C\ their e}ect on the kinetics of transformation becomes predominant[ Estimation of nucleation and growth rates of bainite crystals in a number of alloy steels ð16Ł showed that acceleration of formation of upper bainite is attributed to a drastic increase in nucleation rate while growth rate was slowed down[ In the lower!part of bainite range\ deformation decreased both nucleation and growth rates of a!phase[ In the case of warm deformation the e}ect of destabilizing factors becomes stronger because of the more developed defect structure ð31Ł and thus even at the lowest bainite transformation temperatures the destabilization e}ect dominates[ It should be noted in conclusion that the factors a}ecting the deformed austenite stability in the bainite range were analyzed on the basis of the concept of a martensitic type shear mech! anism of g : a transformation developed _rst by G[ Kurdumov as early as 0825 ð62Ł[ The experimental data prove unam! biguously the validity of this concept\ since the e}ect of defor! mation on the kinetics of bainite and martensite transformations has many common features[ Whereas\ retar! dation of transformation in deformed austenite has never been observed when the g : a transition occurs by a di}usive mech! anism ^ this is a.rmed by the experiments on austenite trans! formation to ferrite and pearlite described in previous sections as well as in many other papers[ But shear transformation of austenite to martensite can be retarded due to deformation under speci_c conditions ð31\ 69Ł[ 3[ Transformation under continuous coolin` For practical purposes it is most important to know how deformation a}ects the kinetics of ferrite!pearlite and bainite transformations under continuous cooling[ Continuous cooling transformation diagrams "CCT# provide a complete picture of the austenite transformation kinetics[ Superposition of trans! formation diagrams of undeformed and deformed austenite on one and the same coordinate grid makes it possible to draw valid conclusions about deformation in~uences on transformation under continuous cooling[ The _rst thorough data on defor! mation e}ects on CCT!diagrams were obtained by Smith and Siebert ð01Ł for low!carbon molybdenum!boron steels deformed by compression[ Increased deformation raised the limiting coo! ling rate[ Formation of polygonal ferrite in deformed austenite started and _nished at higher temperature[ Acceleration of fer! rite transformation due to deformation under continuous coo! ling was also reported ð30Ł[ Jonas and do Nascimento ð39Ł found that ferrite transformation in deformed dual!phase steels occurred at higher temperatures and many times faster\ result! ing in higher ferrite fraction than in non!deformed austenite[ Our experiments were made on di}erently alloyed steels with low! and medium!carbon contents ð15\ 17\ 29\ 22\ 24\ 25\ 27Ł[ A typical example of hot deformation e}ects on the CCT! diagram for a low!carbon low!alloy steel is shown in Fig[ 05 ^ the e}ects are qualitatively the same for ferrite and pearlite

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

Fig[ 05[ E}ect of deformation at 799>C "TA  0999>C# on the CCT! diagram for 09CrSiNiCu steel "Figs 0a and 1# ð15Ł[

transformations under both isothermal conditions and con! tinuous cooling[ A decrease in the austenite stability manifests itself in the shift of ferrite and pearlite ranges upwards to the left on the CCT!diagram[ Since under continuous cooling there is always post!deformation delays at high temperatures "period of time from the moment of deformation completion until the beginning of austenite transformation#\ the degree of accel! eration depends on the cooling rate[ The faster the cooling rate\ thus the shorter post!deformation delay\ the greater is the transformation acceleration e}ect for deformed austenite[ At low cooling rates\ particularly in the case of deformation com! pletion at 0999Ð899>C\ not only static recrystallization but grain growth can take place ð23Ł[ As a result\ under these conditions\ the acceleration of the ferrite!pearlite transformation for a deformed austenite is almost negligible or does not take place at all[ For instance\ in 29CrMnSiNi1 and 39CrNiMo ð22Ł steels the pearlite transformation is observed only under rather slow cooling due to the high stability of austenite[ In the specimens of these steels deformed at 899>C\ austenite recrystallizes during

76

such cooling so that its stability is restored completely "Fig[ 06[ As a result\ the pearlite ranges for such deformed and undeformed steels coincide on CCT!diagrams[ Under continuous cooling\ the bainite transformation of a hot!deformed austenite is either retarded or is not accelerated due to post!deformation delays or to the action of ferrite formed during cooling before bainite transformation "Figs 05 and 06#[ Thus\ under continuous cooling the hot deformation e}ect may di}er considerably from that which could be expected from the results of isothermal transformation[ The magnitude and direction of the deformation e}ects on the kinetics depend essentially on the parameters of deformation\ transformation conditions and chemical composition of the steel\ so they cannot be predicted exactly[ When more accurate knowledge of the transformation of deformed austenite is required for certain products manufactured from some particular steel\ exper! imental studies should be carried out for the speci_c conditions of industrial treatment[

CONCLUSIONS "0# Deformation of austenite accelerates the transformation to ferrite or pearlite\ more markedly when the undercooling is less and in the earlier stages of transformation[ The e}ect is increased when the deformation is greater and at lower temperature[ The creation of defects in the austenite increases the rate of nucleation along the grain boundaries at the beginning of the decomposition[ The deformation e}ect is most intense in the carbon range 9[91Ð9[95 and is relatively una}ected by the alloy additions which alter the hardenability[ "1# As distinct from the pearlite transformation\ hot defor! mation a}ects the bainite reaction in a more complex way[ At temperatures in the lower range " ³ 399>C#\ deformed austenite transforms more slowly than undeformed aus! tenite and with less overall bainite fraction[ In the upper part of bainite range " × 399>C# hot deformation accel! erates transformation[ The transition from retardation to

Fig[ 06[ CCT*"solid lines# and TTT*"dashed lines# diagrams for 29CrMnSiNi1 "a# "Fig[ 7# and 39CrNiMo "b# steels[ 0*undeformed austenite ^ 1*austenite deformed to 14) at 899>C ð22Ł[

77

V[ M[ KHLESTOV et al[ ] KINETICS OF AUSTENITE TRANSFORMATION

acceleration proceeds gradually with rising transformation temperature _rst in the early stages and then in all stages of the reaction followed by increase in the completeness of transformation[ Both e}ects are intensi_ed by an increase in strain or a decrease in deformation temperature[ "2# Carbon enhances both the e}ect of stabilization in deformed austenite in the lower bainite range and the desta! bilization e}ect in the upper region accompanied by a decrease in the transitional temperature at which the direc! tion of the deformation e}ect is changed[ Cr\ Mo\ W and Si promote the stabilizing e}ect of deformation and raise the transitional temperature[ The complicated way in which deformation a}ects the bainite transformation is due to the action of several factors\ some of them retard transformation and the others stimulate it[ Their combined action depends on the defor! mation parameters and transformation temperature and results in di}erent changes in the bainite kinetics[ "3# Warm deformation at 499>C causes a great acceleration of the bainite transformation which is stronger in the upper bainite range[ It also increases signi_cantly the temperature of the nose of bainite transformation and the Bs tempera! ture[ "4# Under continuous cooling\ the deformation e}ects can di}er considerably from those under isothermal trans! formation[ The ferrite!pearlite range in a hot!deformed aus! tenite is shifted to higher temperatures and shorter times on the CCT!diagrams[ At slower cooling rates\ the enhancing e}ect of deformation is reduced because of greater static restoration during the longer post!deformation delays at high temperatures[ The bainite transformation is either retarded or is not a}ected[

01[ 02[ 03[ 04[ 05[ 06[ 07[ 08[ 19[ 10[ 11[ 12[ 13[ 14[ 15[ 16[ 17[

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