The data from mechanical testing togethe!:"with the pear- lite volume fraction and the true 'ferrite grain size of the steels are presented in Tables 8-12. Using the values of yield strength and d-l, a Hall-Peteh relationship is plotted in
Figure 8.
The steels started with a COmmOnclUstenite grain size, but their subsequent ferrib-, 'Jril~nsIze under the same cooling ,"onditions WilS not ttl", same. 'Ihis indicated that the precipita- tion kinetics and effectiveness in ferrite grain control of NbC, NbIC,N) and 1I1N were all (,.liferent.
4.5 Impact Test
The impact energy Clbso:r.bedat different temperatures
ilre presented in Tables 13 leo 17 and plotted in Figure 9 to 13.
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The impact trilIlsition to~peratures of the steels at each cooling rate was tClken from the curves curresponding to 20 ft. lb. impact
energy and listed in Table 18.
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DISCUSSION
'l'hecompositions of th", steels (Table 1) indicate that the second phase particles in steels 2,3 and 5 aJ;e essentially NbC,high nitrogen Nb(C,Ni and AIN respectively and that those in steel 4 are a mixture of AIN and Nb(C,N).
5.1 Prior-Austenite Grain Size
5.1.1 Isothermal TransfoJ;miltion Technique
This technique is based on the principle that ferrite or cementite pJ;ecipitates lJreferentially at the austenite grain boundaries.40 Steels from aust",nitizing temperature when held
, ,
, I I:
I' 1
for sometime at intercJ;itical temperature, will rej~t ferrite or cementite at the pJ;ior austenite grain boundaries. The' volume of this ferrite Or cementite is a function of inter-critical
temperature and time at this inter-critical temperature'.
The inteJ;-critical temperature at which ferrite rejection begins was obtained for e"ch steel by trial and error. Holding time at the intercritical,temperature to get a thin ferrite net work ilround tit",pJ;ior ilustenite grain was also obtained in the same manlier though it was very time consuming.
I
This technique worked well for steel 1, plain carbon steel at all temperdtures. The 'effectiveness of this process in reVea- ling the prior austenite grain boundaries is shown from Figure 1.
Problems were encountered with the determination of the inter-eric-lcal temperature and holding time for steels 2,],4 and 5, as the Fe-Fe3c phas8 diagram is altered by the alloy additions and the extent of: depression of the transformation region was not known. The experiment had to be repeated many times to optimize both the intercritial tmnperature and time at intercritical temperature for getting reasonable ferrite net- work, around the prior aURtenite grains.
5.1.2 Effect of Precipitates on the Prior Austenite Grain Size The base steel 1, being a pluin curbon steel, does not
. ,
conta;Ln second phase particle to inhibit grain growth. From Figure 2 it is evident that the austenite gruin size of steel 1 increuses rupidly and linearly with increasing temperature
over the temperature range investiguted. This is due to the free migration of grain boundaries to attain the equilibrium grain
size when the temperutll:r:eis raised. Second phase particles
in the other steals pin the i:lustenitegrain boundaries, callsing the steels to retain a finC' ']rcnn size which is almost unchanged during heating i:ltsuccessive. high temperatures. However, as SOOn uS the grain g:r:owthin!,ibitors begin to be ineffective at some elevated temperature as a result of solution or coalescence, the grain cOi:lrsening rate is then groater than that of steeL 1.
(\ .
This is due to the fact that ,,"henthe precipitates are no longer effective, the restricted grains want to achieve the equilibrium grain size as quickly as possible and thus cause an abrupt
growth in the grain size.
l\mong the precipitates of NbC, Nb(C,N) and AIN, it is accepted that tlw order of increasing solution temperature is NbC, Nb(C,N) and A1N. Calculations of solubility products to determine the solution temperatures for the precipitates
in
steels 2,3,4 and 5 were made using the solubility relationships
suggested by Irvine et al.7 These calculations show that the
i
temperatures for the precipitvtes in steels 2,3,4 and 5 to dissolve completely are 1135, 1162, 1272 cllld148SoC respectively. TheI"efore, it is expected that grain growth I"estI"iction in steel 3 will
persist to a higher tempervture than in steel 2 and in steel 5 to the highest temperature. This Was ~n Eact the case, as can be seen from Figure 2. Ilo,","ever,1.n the loweI" tempeI"atuI"e range of 950 to 1050~where the precipitates in the three steels were still effective as grain growth inhibitors, steel 3 produced the finest grains while steel 5 had a smaller grain size than steel 2. The significant difference ~n grain size between steels 2 and 3 strongly suggests that Nb(C,N) is a much better grain growth controller than NbC. Niobium carbonitride is also more effective initially than !lIN, but the effect decreases with increasing temperature and the r"verse
above ~bout l050oC. The greater eff8ct
~s true at temperature , of Nb(C,N) is thought to be due to the fact tilat its precipitates are present in a
finer dispersion than NbC und AIN, and that it does not dissolve or coalesce to the Same extent during reheating as NbC or AlN.
Shamsls reported that NbC and NbIC,N) show similar grain coarse- nlng churacteristics on uustenitizing. Thus this isadisagreement with the results of present work. However Sham's results are rather unexpected.
Steel 4 is basically steel 3 with aluminium added to it and the grain coarsening characteristic is almost exactly that of steel 3 up to lOsOoC. lJoweve,;,the grain coarsening in steel 4 starts at the slightly higher temperature of l07soC cOmpa~ed
to steels 2 and 3. This may be due to the less coalescence
of the precipitates in steel 4 because of their higher solution temporature by the presence of aluminium and nitrogen and also due to the higher volume fraction of its precipitates !Nb(C,Nj and AlNJ compared to those of steels 2 and 3. Consequently, the austenite grain size is smaller at any given temperature.
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However, the grain coursening rctte is similar to that found for steels 2 and 3.
5.2 Effect of precipitate on Ferrite Grain Size
The mOan ferrito gruin size of the steels at four different cooling rates employed in the present experimental work l.S
shown in Figure 7. lt is evident from this figure that, although the steels had originally a common austenite grain size, the subsequent ferrite grain Sl.Ze at the Silme cooling rate was not
the Same. 'l'hisindicates that the precipitates in each steel
o
influence grain refinement to differing degrees. It should be pointed out th~~ the steels ~t the he~t-treatment temperature contained some undissolved particles, the amount of which varied from steel to steel. It ~s possible that these particles may act as nucleation sites for ferrite and as obst~cles in ferrite grain boundary migration, thus possibly significantly affecting the final ferrite grain size.
It is evident from Figure 7 th~t steel 3 produced coarser fGrrite grain th~n steels 2 and 4 while steel I produced coarser ferrite grain than steel 3. Steel 2 produced the finest ferrite grains ~t all cooling rates except at 3.6oC per min. At the slow cooling of 3.6oC per min steel 2 produced the co~rsest ferrite gr~~ns. The behaviour of these steels should be contrasted with that of steel 5 which contains ~n ~ppreciable amount of undissol- ved AIN at the ~usteniti7.ing temperature. It can be seen from Figure 7 that ferrite grain sizGS produced in this steel are almost identical to those for the plain carbon steel 1. Conse- quently, it would appear that undissolved particles do not exert
any influence on ferrite grain size and the observed effects in steels 2,3 and 4 require anothcr explanation.
Niltaw~ch8 ~nd Bepari9,11 reported that the undissolved particles had no direct cnntl:"olover the ferrite grain size and that the ferrite grain refinement carne from the particles preci- pitated- during cooling. 'l'herefore, the mass fraction of preci- pitates available and the particle size or distribution in
ferrite must have an important role in ferrite gr~in refinement.
It is fairly certilj.n that the precipitation in steel 2 1.S essentially NbC, and tho>;e in steels 3 and 5 "re essentially reliltively high nitrogen Nb(C,N) ilnd 1IlN respectively and that those in steel 4 are a mixture of Nb(C,N) and AIN as precipita- ting phase. Figure 7 clearly indicates that while the repreci- pitated AIN is a poor ferrite grain refiner, Nb(C,N) particularly NbCare excellent for ferrite grain refinement. NbC is a much more effective ferrite grain controller than Nb(C,N). This is
thought to be due to the f~ct that the precipitating NbCIpresents, ,
in a finer dispersion of particles than Nb(C,N) and thus 'exerts more effect than Nb(C,N). The coarsest ferrite grain of steel
2 at the slowest cooling rate is thought to be caused by the rapid ovorageing of tllG NbCprecipitates.
It hils been mentioned earlier that steel 4 with niobium, aluminium and nitJ:ogen prod";ced finer feJ:rite grain size !than steel 3 with niobium and nitr.ogen. This indicates clearly ! that in presence of alUminiUmNb(C,N) is more 'effective in ferrite grain refinement than NbIC,N) in absence of aluminium. This is again thought to be the effect of aluminiUm which alters the precipitation kinetics i.c. its presence enhances to produce
finer Nb(C,N) precipitates.
The ineffectiveness of l\IN as ferrite grain J:efiner is a result of its slow nuclC'ution rate and its rapid coalescence after it hil.s precipitilte<1. 9,11
l
5.3 The Hall-Petch Plot of the Steels
Using the values of yield strength and d-l in Tables 8-12, a Hall-Petch Plot of the steels is shown in Figure 8. Being
plain carbon, steel 1 is anticipated to follow a straight line which was in fact observed in the experimental work. The relation-
ship of yield strength and
d-l
was found to deviate from linearityfor the other steels. The linearity of steel 1 should be accura- tely determined because steel 1 is used as the base steel with which the strength of the others is compared. The straight line through the data of steel 1 was drawn using the intercepts and slopes supplied by the standard method of least squares.
It has generally been accepted that when A1N has gone into solution it will not precipitate on cooling to give any precipitation strengthening. On this basis, it can be reasoned
that the Hall-Petch Plot of steel 5 should be a straight line wi,th the Same slope as steel 1 and that this line should lie
above that of steel 1 with an increment due to the amount of soluble nitrogen lwhich was not determined) ~n steel 5. It is evident from Figure 8 that the strength of steel 5 is lower than steel 1 at the slowest cooling rate, but it increases sharply when the cooling rate is acc"Jerated and oventually become stronger than plain carbon steel 1. It has been reported8,12 that aluminium nitride does not produce any precipitation stre- ngthening effect due to its slu'Jgish precipitation kinetics during cooling. In fact Cit tho slow cooling rate the formation of aluminium nitride retClrds the strengthening process as it
depletes the amount of solublo nitrogen which is itself a good strengthener. SteelS is therefore weaker than steel 1 at the slowest cooling rate. As the cooling rate is increased, the preci- pitation kinetics of aluminium nitride becomes mOre unfavourClble and this is accompanied by a corresponding increase in the residual soluble nitrogen. SteelS is thus strengthened sharply.
Steelsl,3,4 and 5 under the cooling condition of 1100C/min contain a few Widmanstatten ferritG~peilrlite structure. By neglec- ting thG Widmanstatten ferrite-pearlite, it is possible to assess the ferrite grain size of the steel at this condition by taking only the fields showing polygonal ferrite-pearlite structure. This procedure may give SOme small error in the measurement of ferrite grain size at this cooling rate.
5.4 yield Strength Increment From Precipitation Strengthening The yield strength increment !I YS, is defined as 1;:heyield strength contributed by precipitation hardening. The yield strength increment of the steels at each cooling rate was determined and listed in Table 19. A plot of ~YS versus cooling rate is shown in Figure 14. 'I'heresults arc obtained by subtracting the yield strength of the base steel at the Same grain size from the observed yield strength of any other stf'el at each cooling rate. The results include any effects due to variation in soluble nitrogen whiCh
has not been determined. It has been reported that the contribu- tion of soluble nitrogen present after cooling in vanadium steels
"LO tle~r,. strengt h was Oll.LYsnw-,--,-.' ,,8,9,12 For nl,O,-,~Um.•.. S ee-,-st' these are also considered to be pmall "nd hence neglected.
It is evident from Figure 14 that !J.YS of steel 2 increases
"s the cooling rate decreDses and this trend continues upto the cooling rate of 120C per min beyond which !J.YS decreases with
further decrease in cooling rate. From Figure 14 it is also seen that !J.YSof steels 3 and 4 decreases with the decrease of coo~ing rates. The above dissimilarities in the variation of !J.YS with cooling rates of thes'" steel,. may be explained as follows.
The strength increment due to precipitation increases with the reduction in precipitate size and the increase in precipitate fr"ction. The cooling rateR also control the degree of prec~p~- tation. A r"pid cooling r"te suppresses precipit"tion while a slow cooling rate allows the precipitate to overage. In between, there is an intermediate cooling rate which optimises the precipita- tion strengthening. It iR evident from Figure 14 that cooling rate of 1200C, 1200C and 120C per min are the cooling rates which opti- mize precipitation strengthening in steels 4, ) and 2 respectively.
It is also evident from }'igure 14 that steel 3 produced
lower !J.YS than steel 2 "nd higher !J.YS than steel 4 at all cool~ng ratas excapt at the slol-mst cooling rate of 3.60C/min. At this
slowest cooling rate steel 4 produced higher !J.YSthan steel,)
and lower !J.YSthan steel 2. The greater effect of NbCIn strength increment is thought to be due to its fin~dispersion and
the increased volume fraction than Nb(C,Nj". Steel 4 with Nb, A1 and N produced lowest ~ YS of all the three steels 2,3 and 4 at all cooling rates except at the slowest cooling rate. It is thought
that in steel 4 aluminium sluws down the rate of precipitation
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to a sufficient extent to roduce volume fraction resulting lower \ f
!J. YS. At slowest cooling rates steel 3 shows lower AYS than
\J
steel 4. This ffiilY be due to the mOre rapid over ageing of the Nb(C,N) particles lTI absence of aluminum in steel 3 at
cooling rate.
Figure 14 shows that '" YS of steel 5 cooling rates except at the slowest cooling
Chi,
"owep
15 positive at all
,
,o'
rate of 3.6 C pel: minute~,
" Steel 5 is actually steel 1 with aluminum and nitrogen added to it.
It is established8,9,12 that AIN is ineffective in increasing
strength by precipitation. The ineffectiveness of AIN as precipita- tion strengthener is due its sluggish precipitation kinetics and its rapid coalescence after it has precipitated. ~ YS shown in Table 19 is the combined effect of solid solution strengthening
due to soluble nitrogen ~though 'its contribut'ion to strength incre- ment is considered negligible) and precipitation strengthening
due to second phase particles. The faster the cooling rate, 'the more the nitrogen will be in solution and hence mare will be the contribution towards yield Rtrength increment. Now if the contri- bution of AIN towards 6 YS is considered zero, 6 YS of steelS shown in 'l'"ble19 at the cooling rates of 120, 36 and l2oC/min.
may be assumed solely because of the solid solution strengthening due to the soluble nitrogen and descending order of !J YS is due to the decre"se of soluble nitrogen with decreasing cooling rates.
But the negutive value of ~YS at the cooling r"te of 3.6oC/min~
is not in agreement with the above statement and further work is required to h"ve " useful ""l'lilTwtion.
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5.5 Impact Transition Temperature
The impact transitiun te'llperature (ITT) is a measure of
the impact properties of a material. The lower the impact transition temperature, the greater the toughness, therefore the better is the steel.
Figures 9-13 represents the impact energy curves and Table 18 shows impact transition telTlperntu't:esat 2() ft-lb of the steels.
It is evident from the Table 18 that for the plain carbon steel 1, the impact transition temperature d.ecre"ses as the cooling rate decreases. As the cooling rate decreases ferrite grain s~ze
increases (Figure 7) and thus impact transition temperature increases.
On tho othGr hand it has been reported that as the cooling rate decreases free nitrogen content of the steel decreases8,9 and this reduction ~n free nitrogen content decreases the ITT. The higher ITT of steel I at the fast cooling rate of 12(}oC per min than that at the slow cooling rate of 120C per m~n indicates that the bene- ficial effect of fine ferrite grains has been overcome by the
detriment"l effect of the increased amount of free nitrogen content presont at this fast cooling rato.
Table 18 shows that '"ith the decrease in cooling rate the impact transition temperatures of steels 2 and 3 increase. Steel 2
is basically steel 1 with niobium added to it and steel 3 with
niobium "nd nitrogen added to it. Since the amount of soluble nitro- gen present in the steels at the slow cooling rate is less than
thilt ilt the filst cooling rilte, its effect on ITT is small. It has also been reported8,9,12 th"t as the cooling rate decreases parti~
cles coalescence occu~s to ~educe ~ YS and hence lowe~ ITT. But as evident f~om Figu~e 14 and Table 19 ~ YS of steel 2 inc~eases up to the cooling rate of 12oC/min. Hence the higherITT of steel 2 at the slow cooling ~at() of 120C is due to this higher
In addition to the effect of coarser grains. Since .the ilYS of steel 3 at the slower cooling rate of 12°C per min is lower than that of the filRt cooling rate (}'igure 14 and Table 19). the higher ITT of this steel at this slow cooling rate is thus due to the effect of coarser ferrite grillns.
The ITT of steel 4 cit the slow cooling rate of 120C per min is lower than that of fast cooling rate (Table 18). It has already been reported that as the cooling rate decreases nitrogen content decreases. Figures 7 and 14 show that as the cooling rate decreilses ferrite grain size increases and iIYS decreases. The deterimentill effect of coarse ferrite grain size has been overcome by the combined effect of lower soluble nitrogen and lower ~ YS causing lower ITT at this sIal'..cooling rate of 120C per min.
Table 18 also shows that impact t~ansition temperilture of steel 5 decreases with the inereasC! in the cooling rate. Steel 5 is basically steel I with aluminum and nitrogen added to it. As already been discussed IT'f dec~eases with the decreasl"!of ferrite grain size and with the reduction of free nitrogen content. At the
slower cooling rate nitrogen favo\lynbly combil"lewith aluminium as i\lN but at the Easter cooling rate precipitation kinetics of AIN is very unfavourable 8.9,12 ,...hich causes a corresponding increase in the resid\lal soluble nitrogen. ~herefore. the much lower ITT