the use of microscopic and hardness measurement methods in recrystallization studies
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Mat. Res. Bull., Vol. 18, pp. 1027-1036, 1983. Printed in the USA. 0025-5408/83 $3.00 + .00 Copyriff.ht (c) 1983 Pergamon Press Ltd.
THE USE OF MICROSCOPIC AND HARDNESS MEASUREMENT METHODS IN RECRYSTALLIZATION STUDIES
C.M. Kamma, Department of Mechanical Engineering, University of Lagos, Nigeria
and E. Hornbogen, Institut fur Werkstoffe, Ruhr-Univers i t~t, Bochum, W. Germany.
(Received May 15, 1983; Communicated by R. A. Huff.pins)
ABSTRACT The use of microscopic and hardness measurement methods as effect ive exper imental techniques for recrysta l l i zat ion studies has been invest igated. Considerat ion was given to the product ion of dif ferent start ing-point microstructures of varying Fe~C-part ic le dispersion, known to be associated with the compl icat ions of recrysta l l i zat ion mechanism of second-phase materials. Each of the invest igat ion method considered was used to study the various aspects of recrysta l l i zat ion phenomenon (recovery, in-s i tu-recry- stal l izat ion, part ic le-contro l led subgrain growth, partial recrystal l izat ion, grain growth) and the merits and l imitat ions of appl icat ion discussed.
INTRODUCTION The recrysta l l i zat ion mechanism in metal l ic mater ia ls is known
to be compl icated (1). Of special note in this respect is the con- tr ibutory inf luence of the nature of the start ing-point microstructure. This is usual ly a homogeneous, super-saturated solid solut ion or a microstructure containing a second-phase in the form of part icles. Prec ip i tat ion in the super-saturated solut ion can take place before, during or after the recrysta l l i zat ion process (2,3). During the de- formation stage, prior to the recrysta l l i zat ion annealing, the part ic les play the important role of inf luencing the distr ibut ion of d is locat ion-network in the deformed structure (4,5).
In consequence, the recrysta l l i zat ion behaviour of a normal metal l ic material is invariably associated with cases of combined reactions, whereby prec ip i tat ion and growth of part ic les overlap the recrysta l l i zat ion processes. The complete course of recrysta l l i zat ion in a given material is accordingly seen to be made up of four stages: I) Recovery stage, involving very l itt le annihi lat ion of dis locat ions; II) Cont inuous combined react ion stage, in which particle growth and
part ic le inhibited subgrain growth occur; III) Pr imary recrysta l l izat ion; IV) Further part ic le-and recrysta l l i zed grain-growth.
1028 C.M. KAMMA, et al. Vol. 18, No. 8
There exists the common practice of interpret ing recrystal l i rat ion phenomena with the help of results obtained from one part icular invest igat ion method. The information thus gained is l ikely to be unreliable, in view of the varying mechanisms associated with the different stages of the recrystal l izat ion process. In the present work, an invest igat ion of the recrysta i l izat ion mechanism of carbon steel has been undertaken with the view to assessing the suitabi l i ty or otherwise of microscopic and hard- ness measurement methods in recrystal l izat ion studies. The ex- perimental condit ions (start ing-point microstructures, %-defor- mation, anneal ing temperatures) have been so chosen to produce various types of possible compl icat ions in the course of the recrystal l izat ion process. The four stages of recrystal l izat ion have been systematical ly studied using light-, transmission electron- and x-ray micros- copy as well as hardness measurement methods.
Material and Spacemen Preparation
A carbon steel of the fol lowing composit ion (in wt.%) was used for the investigation: 0.23 C; 0.37 Si; 0.53 Mn. In a systematic thermal pre-treatment operation, four different types of starting point microstructures were prepared. The details of the specimens are given in Table 1.
Start ing-point Microstructures (Samples)
Sample Thermal Pre-treatment Particle Part icle dia (nm) shape
M 870C,40 min/H20 -
M300 870C,40 min/H20/300C, 20 platelet lh/H20
M550 870C,40min/H20/550C, 200 sphere lh/H20
M650 870C,40min/H20/650C, 2.700 sphere 980h/H20
M signif ies Martensite; M300, M550 and M650 are martensit ic structures tempered for lh at 300C, lh at 550C and g80h at 650oc respectively. The specimens were cold-rol led; the amount of deformation ranged between 30% and 90% reduction in thickness. The mechanism of recrystal l izat ion was invest igated for the three anneal ing temperatures 500oc, 550C and 600C.
Vol. 18, No. 8 RECRYSTALLIZATION STUDIES 1029
The hardness of the specimens after the recrysta l l i zat ion heat- treatment operat ion was measured with a Lei tz -micro hardness tester using a 5oop loading weight.
Microscopic Examinat ion
The specimens for the l ight -microscopic examinat ion were pre- pared by wel l -known standard meta l lographic techniques (6). Phi l ips EM300 was used for the t ransmiss ion electron microscopic invest igat ion. The specimens were prepared by the thin-foi l method (7). X-ray photographs were obtained with Phi l ips PW1130 equipment using Fe-f i l ter and Co-tube.
RESULTS AND DISCUSSION
Hardness vs Time Curve
During recrysta l ] i zat ion annealing, var iat ions in the ~e ~sity and Hi~tr ibut ion of d is locat ions take place, giving rise to changes in microstructure. The hardness of a material is an important s t ructure-sens i t ive property, which can react very strongly to var iat ions in the density and arrangement of crystal defects. Based on this fact, hardness measurement has been ef fect ively ut i l ized in recrysta l l i zat ion studies. Fig.1 shows the variat ion of hardness with the recrysta l l i zat ion treatment time for the three pre- t reatments (start ing-point micro- structures) M(a.), M300 (b.) and M550(c.). The results shown refer to the given cold-work (30%,50%) and recrysta l l i zat ion treatment temperatures (500C, 550C, 600oc). For pract ical convenience, the t ime-axis has been represented in log-scale.
It can easi ly be seen that the general shape of all the curves is of the form shown in Fig.2. The deformed stage corresponds with the point O. Usually, in the deformed stage, and shortly after the start of the recrysta l l i zat ion annealing, there is an interact ion between the part ic les and the d is locat ions (segregat ion), result ing in an increase in hardness. At the same time, internal -s t ress rel ieving react ions take place. The net effect of this combined action is that in stage I, the hardness- t ime-curve can rise or fall (Fig.la), depending on which react ion outweighs the other. The shape of the curve in Stage I can therefore be used to assess the prominence to be given to segregat ion effects in a given recrysta l l i zat ion process. Stage II has a character is t ic p lateau-shape. It represents the combined, cont inuous reaction (subgrain- and part ic le-growth) normal ly associated with the recrysta l l i zat ion of second-phase materials. The duratio,, of this stage is an indicat ion of the s luggishness of the recrysta l l i zat ion process aris ing from part ic le inhibi ted movement of subgra in-boundar ies or recrysta l l i zat ion fronts. The mechanism involved here is associated with the so-cal led in -s i tu - recrysta l l i zat ion . The transi t ion from stage II to III cqn be sharp(F ig~1b,550C) , showing the onset of primary recrysta l l izat ion, or gradual (F ig. lb,500C), indicat ive of a very sluggish onset of pr imary recrysta l l i zat ion in a predominant ly in -s i tu - recrysta l l i zed matrix. The end of pr imary recrysta l l i zat ion is taken as the point in stage III from where the curve becomes paral lel to the t ime-axis. Thereafter , the recrysta l l i zed grains and the part ic les continue to grow (stage IV).
1030 C .M. KAMMA, et al. Vol. 18, No. 8
Cold-work : 30% '] Recrysto, tlization : o-----o 5O0"C Treatment ~ 550C 600 ~ 600 C
50 0~ ~'"'~ x : - :~- ' : ' ' , RB: Time to Begin of Recrystollizotion
~c 200 RE RE ;'1 o~'~ i io i io ,oo SOB ,ooo
S rain h Log time la|
Pre - t reatment M
Cold -work : 50 %
Recrystallizolion :0--.-o500 =C Treo.tment ~ 550
600 z~----~60O Pre-Treatment : M 300
300 RB B
='=1 > o~, li) i 1'0 ~ 1'0 tb0
mln h sl b) gOB 10'00 Log time
Pre - t reatment M300
=lEE o LJ
Cold -- work ; 30 %
Recrystatlizotion :o~o 5OO'C Treot ment ~ 550
L>---,~600 Pre-Treotment : M 550
o ~o i io i I"o 10o soo idoo $ min~ h Log time
Pre - t reatment M550
Var ia t ion of hardness wi th recrys ta l l i za t ion t re tment t ime.
Vol. 18, No. 8 RECRYSTALLIZATION STUDIES 1031
If the condit ions are such that no primary recrysta l l i zat ion takes place at all owing to the inf luence of retarding forces, the h~rdness- t ime-curve shows only a cont inuous fall (F ig, lc,500oc). Tile hardness-measurement method is very effect ive in invest igat ing the inf luence of cold-work, temperature, and start ing-point micro- structures on the kinet ics of recrystal l izat ion. It can be used to
~) and end(t R ) of recrystal l i - furnish the times to the beginning( R~nowledge of ~ PRB' zation in a given specimen. With th and by making use of the d i f ferent ia l -hardness method (8), one can easily deter- mine the act ivat ion energy for recrystal l izat ion.
n deformed condit ion