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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.04.006
Open Access
Effects of cooling rate and strain rate on phase transformation, microstructure and mechanical behaviour of thermomechanically processed pearlitic steel
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Indrajit Deya, Swarup Kumar Ghosha,
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skghosh@metal.iiests.ac.in

Corresponding author.
, Rajib Sahab
a Department of Metallurgy & Materials Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India
b Product Development Group, R & D Division, Tata Steel Limited, Jamshedpur 831007, India
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Tables (8)
Table 1. Chemical composition of the investigated steels (wt.%).
Table 2. Austenite start (Ac1), Austenite finish (Ac3), Pearlite start (Ps), Pearlite finish (Pf) temperatures for thermomechanically treated samples using different cooling rates.
Table 3. Austenite start (Ac1), Austenite finish (Ac3), Pearlite start (Ps), Pearlite finish (Pf) temperatures for thermomechanically treated samples using different strain rates.
Table 4. Sample processing details and volume percent of microstructure generated.
Table 5. Values of interlamellar spacing, pearlite colony size, pearlite nodule size, cementite thickness and austenite grain size.
Table 6. Sample processing details and volume percent of microstructure generated.
Table 7. Hardness and Predicted Yield Strength values under different cooling rates.
Table 8. Hardness values under different strain rates.
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Abstract

In the present investigation, thermomechanical controlled processing of a high carbon steel and a Nb microalloyed high carbon steel have been conducted in a Gleeble 3800 simulator. Different microscopic techniques have been utilised for the characterisation of the microstructure and hardness data has been used for the evaluation of mechanical properties. In order to suppress the transformation enthalpy, experiments are performed under varying cooling rate and strain rate. The effect of niobium microalloying leads to the lowering of recalescence and suppresses austenite to pearlite transformation start and finish temperatures at every cooling rate which leads to the refinement of interlamellar spacing and thereby improve hardness and predicted yield strength values. It is evident that a higher strain rate accelerates the kinetics of pearlite transformation and elevates the pearlite start temperature. The increase of strain rate in the range of 1s–1 to 100s–1 followed by a constant cooling rate (free cooling) leads to the refinement of interlamellar spacing as well as improves mechanical properties. The true stress-true strain diagram at a lower strain rate indicates higher strain hardening with sharp yield point, whereas the same at a higher strain rate indicates the sudden occurrence of strain softening. The variation in recalescence due to the alternation of the cooling rate and strain rate has been correlated with the final microstructure and mechanical properties.

Keywords:
Microalloyed steel
Phase transformation
Recalescence
Microstructure evolution
Mechanical behaviour
Full Text
1Introduction

Microalloyed (MA) steels boost their performances due to the addition of various microalloying elements like niobium, vanadium, titanium and aluminium. These microalloying elements have been considered to efficiently restrict the austenite grain growth, thereby resulting in the refinement in the prior austenitic and ferrite grains by forming of precipitates during high-temperature thermomechanical processing. In earlier days, for developing high strength steel comprising pearlite microstructure, the common route was the addition of several alloying elements or increasing the carbon percentage. Nowadays, microalloying in addition to thermomechanical controlled processing draws researchers’ interest in developing a new series of steels with ferrite-pearlite microstructure [1,2]. It has been reported that niobium contributes to the strengthening by retarding the recrystallisation of austenite during hot rolling and due to this, refinement of ferrite during transformation occurs and precipitates formed by Nb further strengthens the resultant ferrite [3–5]. During transformation, the addition of silicon is beneficial for controlling the cementite growth and the presence of niobium results in the fragmentation of the grain boundary cementite network [6].

Pearlite, a lamellar structure comprising ferrite and cementite is dominated in high carbon steel. In steel industries, steel having carbon content nearly of the eutectoid composition is generally manufactured by rapid cooling and commonly used as wire/wire rods with high strength. The obtained pearlitic microstructure attributes to very high strength and can be used for wire drawing [7,8]. Further strengthening can be achieved by the severe drawing into fine wires. The interlamellar distance between ferrite and cementite in the pearlite microstructure, known as pearlite interlamellar spacing which is the key parameter for the strength as well as drawability of pearlitic steel, has been reported to follow an inverse proportional relationship with both strength and toughness [8–10]. However, during wire drawing, if the martensite/bainite phase is formed in the pearlitic steel, then the steel usually performs weakly [8,11–13]. Therefore, it is important to choose the correct processing variables for avoiding these detrimental phases.

Accelerated cooling of thermomechanically processed microalloyed steels leads to high strength with superior toughness and formability [14–18]. The cooling rate influences the austenite transformation into different microstructural constituents, on which the final mechanical properties are determined. Accelerated cooling leads to lowering the temperature, required for the transformation of austenite to ferrite which in turn promotes nucleation of ferrite at the austenite grain and grain boundaries. The enhanced rate of nucleation suppresses grain growth by effective pinning of intergranular and intragranular ferrite which leads to the grain refinement of ferrite. The deformation also accelerates the driving force required for transformation to take place as well as the continuous cooling transformation and due to this the pearlite transformation curve shifts to the right (i.e., longer times) [14–17].

It is well established that increasing the strain rate leads to an enhancement in the flow stress. At higher strain rates, only a small work is stored which is required for deforming the material and the rest is converted into heat, resulting in an adiabatic atmosphere, responsible for work softening. If the specimen is kept at a higher temperature, then this fact can be observed more prominently. Two opposing effects i.e., work hardening and work softening occur during deformation under adiabatic conditions. Work hardening is the dominating phenomenon at low strains whereas, work softening being dominant at higher strains. At higher strain rates extreme localisation of the deformation may take place, which leads to the adiabatic shear band (ASB) formation [19–21]. Adiabatic shear banding results in rapidly losing the stress carrying capacity in the shear bands due to intense thermal softening [22]. Plain carbon steels did not exhibit ASB formation whereas alloyed steels led to the incidence of adiabatic shear bands.

A material when naturally or freely cooled by air during rolling of various bars or wire rods, the enthalpy of transformation during the exothermic reaction is known as ‘recalescence’ [8]. The heat evolved during the transformation may affect the final microstructure. The transformation enthalpy is released as heat energy during the phase transformation of austenite into ferrite or pearlite. As the specific heat of different phases varies with temperature, the latent heat associated with different phase transformations also depends on temperature. Considerable research has been made about the above aspects for low carbon steels, but similar type of investigation is limited in high carbon microalloyed steel. In this context, the present research article examines the combined effect of cooling rate and strain rate on phase transformation, microstructure evolution as well as mechanical behaviour of a high carbon steel and a niobium microalloyed steel.

2Experimental procedure2.1Material condition and processing

A high carbon steel designated as HC0 and a Nb microalloyed steel designated as HC1 were obtained from Tata Steel, Jamshedpur, India. The detailed chemical compositions obtained by the spectroscopic analysis of the two steels (HC0 and HC1) are given in Table 1.

Table 1.

Chemical composition of the investigated steels (wt.%).

Alloy code  Mn  Si  Nb  Fe 
HC0  0.66  0.67  0.20  0.00  0.006  0.014  Bal. 
HC1  0.66  0.67  0.20  0.03  0.006  0.014  Bal. 

Gleeble 3800” simulator was utilised for complex thermomechanical treatments of the machined cylindrical samples with a diameter of 10mm and length of 15mm, obtained from the forged bar. Electrical resistance heating of the sample has been utilised in this equipment. A thermocouple welded on specimen surface midway between two ends was used for monitoring the temperature of the sample.

The samples were first homogenised at 1200°C for 3min and then cooled at 5°C/s up to 1050°C. The schedule of hot compression was designed such that it allows a one-step deformation pass (true strain=0.3) with the strain rate of 1s–1 at 1050°C followed by various cooling rates as shown in Fig. 1(a) and with the variation of strain rate followed by a constant cooling rate of 1.75°C/s as shown in Fig. 1(b). All the parameters for these thermomechanical treatments like heating/cooling rates, temperature and strain rates were selected likely to a hot rolling operation as usually adopted in the industries for manufacturing a long wire rod product. However, a few of these parameters were selected reasonably to represent the process closely or may not exactly similar to that of the industrial processing condition. It is noteworthy that free cooling can be described as nonlinear cooling rather than controlled cooling. In an industrial condition, the real-time transformation has been represented by this free cooling which was implemented in the Gleeble apparatus.

Fig. 1.

A typical representation of thermomechanical treatment (a) showing the variation of the cooling rate at a fixed strain rate of 1 s–1 and (b) showing a variation of strain rate followed by a fixed cooling rate.

(0.16MB).
2.2Micro-structural analysis

A scratch-free mirror-like polished surface has been prepared by following some standard metallographic techniques. 2% nital solution was used for etching of the specimens prior to observing the microstructure under the optical (Carl Zeiss, Axiovert 40 Mat) and the scanning electron microscope (SEM) (Hitachi). The linear intercept method was performed randomly on several optical micrographs for grain size measurements and the mean linear intercept has been considered as the average grain size [23]. Reliable image analysis software (Axiovision, version 4.8) has been used for measuring the volume fractions of ferrite/pearlite, pearlite nodule size (NS) and colony size (PS) and thereby, 10–15 numbers of SEM micrographs covering 1mm2 microstructural area were utilised. Transmission electron microscopy, operating at 200kV was performed using an HRTEM (Tecnai G2) analytical microscope for measuring the pearlite interlamellar spacing. The characteristic samples with a diameter of 3mm were punched from the mechanically thinned foil of 80μm thickness. The further thinning of the specimens was accomplished by using a 90% acetic acid and 10% perchloric acid standard solution in a twin jet electro-polisher (TenuPol-5) at 12°C to make the specimens electron transparent of about 300nm thickness. TEM analysis software (Gatan Microscopy Suite 1.8.3 under Gatan Digital Micrograph) was utilised to calculate the average interlamellar spacing and cementite thickness from 10–15 numbers of TEM micrographs.

2.3Hardness evaluation

Bulk hardness values were measured under the load of 5kg and dwell time of 20s from the universal hardness tester (Innovatest Verzus 750 CCD). Six to eight indentations were analysed to get the average hardness value for each sample. The measurement error was ≈±5% of the estimated values of Vickers hardness (HV5/20).

3Results and discussion3.1Pearlite transformation start, finish temperatures and recalescence under different cooling rates

Fig. 2 shows cooling curves for samples at a fixed strain rate of 1s–1 under different cooling rates from 1.75°C/s to 30°C/s at for both the high carbon as well as the Nb microalloyed sample. Table 2 summarises austenite start (Ac1), austenite finish (Ac3), pearlite start (Ps), pearlite finish (Pf) temperatures and temperature rise due to recalescence for all the experimental thermomechanically treated samples using 1200°C soaking temperature. The start temperature of pearlite transformation to austenite (Ac1 temperature) and austenite transformation finish temperature (Ac3 temperature) have been analysed by dilatometric analysis. Both critical temperatures have been determined by changes in the slope of a dilatation versus temperature plot. The critical temperatures have been determined from the dilatometric curve by considering the fact that Ac1 is the temperature at which the expansion starts to deviate from a linear behaviour during transformation and the sample begins to contract as austenite transformation starts and Ac3 is the temperature at which expansion starts again and the cooling curve becomes linear again once it reaches in a fully austenitic condition. It is clear from the Fig. 2 that niobium addition suppresses the austenite to pearlite transformation start (Ps) and finish temperatures (Pf). The austenite start (Ac1) and finish temperatures (Ac3) for the niobium-added sample have been found to be higher as compared to those of the without niobium-added samples. This behaviour is attributed to the fact that niobium is a ferrite stabiliser [24]. It is obvious that higher deflection in the cooling curve indicates the higher volume fraction of the transforming phase. It may be mentioned that lowering of Ps and Pf temperatures indicate the formation of finer pearlite in Nb-added samples. Fig. 2(f) indicates a faint change of slope indicating Ms temperature of 244°C at the cooling rate of 30°C/s for the Nb-added specimen.

Fig. 2.

Cooling curves for samples tested in Gleeble at different cooling rates (a) HC0G10 at ∼1.75 °C/s, (b) HC1G7 at ∼1.75 °C/s, (c) HC0G11 at 10°C/s, (d) HC1G5 at 10°C/s, (e) HC0G12 for 30°C/s and (f) HC1G11 for 30°C/s. R: Recalescence and CR: Cooling rate.

(0.46MB).
Table 2.

Austenite start (Ac1), Austenite finish (Ac3), Pearlite start (Ps), Pearlite finish (Pf) temperatures for thermomechanically treated samples using different cooling rates.

Sample code  Cooling rate (°C/s)  Ac1 (°C)  Ac3 (°C)  Ps (°C)  Pf (°C)  Temperature rise due to recalescence (°C) 
HC0G10  1.75  722  821  636  589  20 
HC1G7  1.75  726  823  626  582 
HC0G11  10  727  820  632  582  21 
HC1G5  10  728  824  611  577 
HC0G12  30  727  819  613  546  34 
HC1G11  30  730  823  608  543 

It is obvious that all the cooling curves show one hump in between Ps and Pf temperature ranges indicating the sudden rise in temperature due to the recalescence phenomenon for both the steels. It has been reported earlier that for decomposing austenite into pearlite, the enthalpy of transformation at 727°C is −77kJ/kg and for austenite-ferrite decomposition the enthalpy of transformation at 911°C is −16.3kJ/kg [8]. In rolling mills, as the steel is freely cooled, the recalescence is not suppressed which leads to the formation of 100% pearlite in most of the cases. When the similar cooling condition has not been maintained in the thermomechanical simulator, but a constant cooling rate is imposed for suppressing of the recalescence on the sample, then there may be a chance that the final microstructure would not be the same as for those mills. In the case of pearlite transformation, this problem would be enhanced as pearlite has the highest transformation enthalpy than any other phases in steel [8]. This type of heat generation leads to reglowing of the metal surface which is attributed to recalescence, commonly observed in steel during austenite to pearlite transformation on cooling. A sudden increase in temperature may be resulted due to the release of the latent heat of transformation. It has been investigated from Fig. 2 and Table 2 that the magnitude of temperature rise due to this recalescence phenomenon is quite less for Nb-microalloyed sample which can be attributed to the fact that Nb, known as a ferrite stabiliser, also suppresses the transformation enthalpy at every cooling rate resulting in refinement of the microstructure. It may be concluded from Fig. 2 and Table 2 that, as the cooling rate increases, the temperature due to recalescence rises which also indicates a higher amount of pearlite transformation from the austenite regime with the increase in cooling rate for both the steels. Thus, it can be inferred that this recalescence phenomenon in high carbon steel might be an important factor under different cooling rate which in turn can influence the resultant microstructure and thereby leads to different mechanical properties.

3.2Pearlite transformation start, finish temperatures and recalescence under different strain rates

Fig. 3 shows cooling curves for samples tested under different strain rates from 10s–1 to 100s–1. Table 3 summarises austenite start (Ac1), austenite finish (Ac3), pearlite start (Ps) and pearlite finish (Pf) temperatures and temperature rise due to recalescence for thermomechanically treated samples using different strain rates. It is evident that the strain rate influences marginally to austenite start and finish temperatures, whereas its effect in pearlite start and finish temperature are prominent. As usually expected, higher strain rate accelerates the kinetics of pearlite transformation and thereby elevates the pearlite start temperature. However, the induced strain probably acts as the diffusion barrier of carbon and hence indicates the pearlite completion at a lower temperature.

Fig. 3.

Cooling curves for samples tested in Gleeble at different strain rates (a) HC1G12 (strain rate 10/s) and (b) HC1G13 (strain rate 100 s–1). R: Recalescence and SR: Strain rate.

(0.13MB).
Table 3.

Austenite start (Ac1), Austenite finish (Ac3), Pearlite start (Ps), Pearlite finish (Pf) temperatures for thermomechanically treated samples using different strain rates.

Sample code  Strain rate (s–1Ac1 (°C)  Ac3 (°C)  Ps (°C)  Pf (°C)  Temperature rise due to recalescence (°C) 
HC1G7  726  823  626  582 
HC1G12  10  727  820  640  575 
HC1G13  100  737  818  652  566  14 

Similarly, as discussed earlier for Fig. 2, the cooling curves in Fig. 3 also show one hump in between Ps and Pf temperature ranges indicating the sudden rise of temperature to the higher level due to recalescence for Nb microalloyed high carbon steel. A comparison with Fig. 2(b) with Fig. 3(a, b) and Table 3 indicates that an increase in strain rate from 1s–1 to 100s–1 marginally increases the recalescence behaviour of Nb-added specimens which could be attributed to the formation of a higher amount of adiabatic shear bands.

3.3True stress–strain behaviour under a fixed strain rate

Fig. 4(a) and (b) represent the true stress–strain plots of the experimental steels based on the data obtained during hot compression (ɛ=0.30) performed in the Gleeble simulator. It is clear that the addition of niobium is beneficial to enhance both stress and strain. It can be observed from Figs. 4(a) and (b) that the amount of plastic deformation stored within the niobium-added sample is greater compared to without niobium sample. It clearly indicates that work hardening occurs in the niobium-added sample which results in prior austenite grain size reduction through recrystallisation [25]. It has been reported earlier that the overall level of the peak strain increases due to the addition of Nb and therefore leads to a delay in the onset of recrystallisation. As strain increases, it yields improved ductility of the sample. So, a combination of both fine prior austenite grain size and ductility is achieved through the niobium addition.

Fig. 4.

True stress–strain curves for both (a) HC0 and (b) HC1 samples at a fixed strain rate of 1s–1.

(0.12MB).

The microalloyed steels are usually alloyed with small amounts of strong carbide forming element like niobium, which retards the recrystallisation by dissolving in austenite through the effect of solute-drag or by precipitate formation (also known as precipitation pinning) and as a result a fine austenitic microstructure can be developed before the pearlite transformation. The main aim of the thermomechanical experiments is to develop a microstructure with fine prior austenite grain size which can be a factor to improve the mechanical properties and also results in the refinement of interlamellar spacing, pearlite colony and nodule. It has been reported earlier that with the increase in cooling rate, the hardening rate is also increased [26]. It has been observed from Fig. 4(a) that for the high carbon steel, the true stress value has been increased in the order of ≈6MPa (from 91MPa to 97MPa) at the expense in the true strain in order of ≈0.03 (from 0.27 to 0.24) whereas it seems very prominent from Fig. 4(b) that the true stress value has been increased significantly in the order of ≈31MPa (from 106MPa to 137MPa), as well as the true strain value also increases in the order of ≈0.66 (from 0.28 to 0.94) for the Nb microalloyed steel. Thus, it can be concluded from Figs. 4(a) and (b) that Nb addition can effectively promote the peak true strain, peak true stress and the steady state strain of steels, thus causing a delay in the occurrence of dynamic recrystallisation.

3.4True stress–strain behaviour under different strain rates

Fig. 5 represents the true stress–strain curves of the samples based on the results obtained during hot compression (ɛ=0.30) where the strain rates were varied from 1s–1 to 100s–1 at a constant cooling rate (1.75°C/s). It has been observed from Fig. 5 that with the increase in strain rate from 1s–1 to 100s–1 the true stress increases in the order of ≈136MPa (from 106.46MPa to 242.88MPa), as well as the true strain value, also increases in the order of ≈0.03 (from 0.28 to 0.31). It can be seen that with the increase in strain rate from 1s–1 to 100s–1, there are significant drops, labelled as discontinuous drops, indicating sudden occurrences of strain softening in the specimen, which can be related with the presence of localised adiabatic shearing [21]. The curves as shown in Fig. 5 having a clear yield point at lower strain rate, indicating the onset of plastic deformation whereas, at higher strain rates clear yield point is not so prominent. It has been reported that until maximum flow stress (σmax) is reached, strain hardening effect is the dominating deformation process during the plastic deformation [21]. The true strain at maximum flow stress value has been reported to be varied with the strain rates. Thermal softening plays an important role in the deformation process beyond the maximum flow stress. Consequently, if the strain is further increased, the flow stress will decrease progressively. With the increasing strain, a critical strain value (ɛcrit.) is reached and a sharp drop in stress can be noticed in the true stress–strain behaviour, indicating stress collapse, which further leads to mechanical instability and strain localisation. During rapid deformation, the stress collapses and strain localisation has been reported to be taken place in a material where the defect is most prominent [27]. In the present study, it has been observed that critical strain decreases due to shear localisation with increasing strain rate, which agrees with the earlier observation that shear localisation in steel alloys mainly depends on strain rate [28]. Subsequently, the perturbation, which has been found to be increased at higher strain, has been reported to enhance the chances of stress collapse when the adiabatic heating occurs [22].

Fig. 5.

True stress–strain curves samples at different strain rates. HC1G7 (strain rate 1s–1) HC1G12 (strain rate 10s–1) and HC1G13 (strain rate 100s–1).

(0.16MB).
3.5Microstructural analysis under different cooling rate

Fig. 6 represents the optical microstructures for thermomechanically processed steels under different cooling rates from 1.75°C/s to 30°C/s. Now, Figs. 6(a)–(e) show the pearlite of dark contrast and ferrite of bright contrast) whereas, Fig. 6(f) shows some amount of martensite. The direction of loading for the compression test has been kept perpendicular to the observing plane. With increasing the cooling rates, changes also occur in the eutectoid transformation. The so-called coarse lamellar pearlite is formed at cooling rates close to the equilibrium value. By increasing the cooling rates, the time available for the diffusion of carbides also decreases. Therefore, the plates of lamellar pearlite become progressively thinner, resulting in finer pearlitic microstructure as shown in Fig. 6. With the increase in the cooling rate, the transformation starts at an even lower temperature which is very prominent in Fig. 2(f) and austenite transforms to martensite of acicular morphology as denoted by the arrow in Fig. 6(f)).

Fig. 6.

Optical microstructures obtained for different cooling rates (a) HC0G10 at ∼1.75°C/s, (b) HC1G7 at ∼1.75°C/s, (c) HC0G11 at 10°C/s, (d) HC1G5 at 10°C/s, (e) HC0G12 at 30°C/s showing pearlite and ferrite and (f) HC1G11 at 30°C/s showing martensite. The loading direction of the compression test is perpendicular to the observation plane.

(1.34MB).

It is noteworthy that when the steel of 0.66% carbon cooled very slowly from a higher temperature, it ends up with pearlite and ferrite microstructure, only. It is seen that for niobium-added samples, when the rate of cooling is 30°C/s, the normal pearlite and ferrite microstructures are not only formed, rather some amount of martensite forms due to the faster transformation kinetics. Ferrite nucleates around the grain boundaries of austenite when steel is slowly cooled from the austenitising temperature. Gradually, these ferrite grains grow in size and as the ferrite cannot retain more than 0.002% carbon at room temperature, carbon diffusion takes place from the ferrite regions into the austenite regions. However, if the steel is cooled very rapidly, then there may be insufficient time for the shuffling or diffusion of carbon atoms and thereby ferrite and pearlite formation is suppressed. Instead, such fast cooling results in some traces of ‘martensite’ as evident in Fig. 6(f).

Table 4 summarises the processing details as well as the volume fraction of microstructural constituents of thermomechanically treated samples. For various samples, the volume percentage of ferrite and pearlite differs after the completion of transformation. Mostly unresolved pearlite has been observed in HC1 steel. In resolved pearlite, alternative ferrite and cementite lamellae have been seen clearly whereas, in unresolved pearlite, ferrite and cementite cannot be distinguished even at higher magnifications. At a lower cooling rate (1.75°C/s) total volume fraction of pearlite has been found to be more, or ferrite volume fraction was found to be less in high carbon (HC0) than Nb microalloyed (HC1) steel whereas, the total volume fraction of pearlite has been found to be less, or volume fraction of ferrite is more in case of HC0 than HC1 sample at higher cooling rate (10°C/s). As the cooling rate increases transformation temperature lowers and the diffusion time shortens at high temperature, and thus pearlite becomes more dominant and the fraction of pro-eutectoid ferrite decreases which is evident for both the sample [29]. At the highest cooling rate (30°C/s), some amount of martensite (19.5%) has also been formed which is because of the diffusionless transformation of austenite into martensite for the Nb microalloyed steel.

Table 4.

Sample processing details and volume percent of microstructure generated.

Sample code  Cooling rate (°C/s)  Volume percent (%)
    Ferrite  Pearlite  Resolved pearlite  Martensite 
HC0G10  1.75  2.56  93.33  4.11  0.00 
HC1G7  1.75  3.62  96.38  0.00  0.00 
HC0G11  10  3.39  93.48  3.13  0.00 
HC1G5  10  2.53  97.43  0.00  0.00 
HC0G12  30  0.56  99.44  0.00  0.00 
HC1G11  30  0.50  80.00  0.00  19.5 

Fig. 7 represents the SEM microstructures for samples at different cooling rates from 1.75°C/s to 30°C/s showing the pearlite nodules. Several SEM micrographs have been analysed to measure the pearlite colony size as well as the pearlite nodule size. It is evident in Fig. 7(c) that the small quantity of ferrite (dark contrast) decorates the grain boundary of austenite. The observed microstructures basically consist of compressed equiaxed grains (indicated the direction of compression axis on the micrograph) but no apparent evidence of recrystallisation has been found. At higher temperature, dynamic recrystallisation plays a major role as the structural softening mechanism, which is the main reason for the lowering of flow stress. Fig. 8(a)–(f) show the TEM bright field micrographs of resolved pearlitic microstructures with the lamellar orientation of alternating ferrite and cementite layers at different cooling rates from 1.75°C/s to 30°C/s for both the HC0 and HC1 steels. One selected area electron diffraction (SAED) pattern of lamellar pearlite (represented by a circular area) along with its schematic analysis has been shown at the inset of Fig. 8(c). This analysis gives an apparent evidence that cementite exhibits (01¯1)α//(100)Fe3C orientation relationship with the ferrite. This relation has been found to be similar to that of the ‘Bagaryatski’ orientation relationship which is commonly found in the pearlite with the lamellar configuration. The subsequent measurements of interlamellar spacing have been performed based on these micrographs.

Fig. 7.

SEM micrographs obtained at different cooling rates (a) HC0G10 at ∼1.75°C/s, (b) HC1G7 at ∼1.75°C/s, (c) HC0G11 at 10°C/s, (d) HC1G5 at 10°C/s, (e) HC0G12 at 30°C/s and (f) HC1G11 at 30°C/s showing the pearlite nodules. PAGB: Prior austenite grain boundary.

(1.36MB).
Fig. 8.

TEM micrographs showing interlamellar spacing and cementite thickness of (a) HC0G10 at ∼1.75°C/s, (b) HC1G7 at ∼1.75°C/s, (c) lamellar pearlite morphology of HC0G11 at 10°C/s with the indexed SAED at the inset, (d) HC1G5 at 10°C/s, (e) HC0G12 at 30°C/s and (f) HC1G11 at 30°C/s.

(0.97MB).

The summarised values of austenite grain sizes, pearlite colony size, pearlite nodule size, interlamellar spacing and cementite thickness are shown in Table 5. It has been observed that with increasing cooling rate from 1.75°C/s to 10°C/s the austenite grain size also decreases and there is a reduction in the colony size as well as the nodule size for both the steel. Pearlite colonies with different lamellae orientations as well as neighbouring colonies join to continue its growth during the transformation and a group of colonies propagate with a nearly spherical transformation font leading to the formation of pearlite nodule [30,31]. However, it can be seen from Table 5 that for niobium-added microalloyed steel, finer nodule size, colony size, refinement in interlamellar spacing and cementite thickness is observed which may be attributed to the reduction of PAGS because of Nb microalloying. Increasing the cooling rate increases the number of cementite nuclei which leads to the formation of pearlite at several places on the austenite boundary. On increasing the cooling rate, the time available for the diffusion of carbon atoms also decreased, thus the distance of diffusion is also decreased. In this way, the thickness of cementite plates is decreased as diffusion is prevented and the transformation whichever takes place, occurs through the refinement of distance between the cementite lamellae as shown in Fig. 8. Nb microalloying can be considered to be effective for lowering the phase transformation temperature as well as for refining pearlite colony and pearlite interlamellar spacing [32]. It has been already reported that increasing cooling rate leads to a reduction in pearlite colony size, refinement in interlamellar spacing as well as cementite thickness which in turn attributes to higher strength [33].

Table 5.

Values of interlamellar spacing, pearlite colony size, pearlite nodule size, cementite thickness and austenite grain size.

Sample code  CR (°C/s)  AGS (μm)  NS (μm)  PS (μm)  λ (nm)  CT (nm) 
HC0G10  1.75  54±10  36.16±13.5±170±10  62±
HC1G7  1.75  37±10  28±7.6±110±10  26±
HC0G11  10  48±10  21.63±11.38±160±10  66±
HC1G5  10  33±10  17.48±6.5±91±10  38±
HC0G12  30  36±10  8.9±5.75±133±10  52±
HC1G11  30  25±10  7.5±3.88±68±10  22±

CR, cooling rate; AGS, austenite grain size; NS, nodule size; PS, pearlite colony size; λ, interlamellar spacing; CT, cementite thickness.

3.6Microstructural evolution under different strain rate

Fig. 9 represents the optical microstructure of samples tested at a strain rate of 10s–1 and 100s–1. Fig. 10 represents TEM micrographs showing cementite thickness and interlamellar spacing under the variation of strain rate. Table 6 summarises volume percent of phases comprising microstructure, interlamellar spacing and cementite thickness generated under various strain rates. As the strain rate increases the volume percent of the ferrite reduces (Table 4 vis-à-vis Table 6) and the thickness of the cementite lamellae also decreases (Table 5 vis-à-vis Table 6). It is evident from the TEM examination that interlamellar spacing decreases as the strain rate is increased to 10s–1 along with the reduction in cementite thickness. At the strain rate of 100s–1, the lowest interlamellar spacing is observed. It may be mentioned that the high strength of pearlitic steel is related with the close spacing cementite lamellae considering that cementite behaves like a barrier to dislocation glide. This can be attributed to the fact that a high density of dislocation structure develops in the ferrite lamellae at a higher strain rate [34]. The enriching of these lamellae can be done by carbon in solution, particularly at higher strain rates (100s–1) at which the decomposition of cementite lamellae begins which is clearly manifested by the measured cementite thickness and interlamellar spacings [35–39].

Fig. 9.

Optical microstructure of samples at strain rate (a) HC1G12 (strain rate: 10s–1) and (b) HC1G13 (strain rate: 100s–1).

(0.51MB).
Fig. 10.

TEM micrographs showing cementite thickness and interlamellar spacing of (a) HC1G12 (strain rate: 10s–1) and (b) HC1G13 (strain rate: 100s–1) sample.

(0.28MB).
Table 6.

Sample processing details and volume percent of microstructure generated.

Sample code  Strain rate (s–1Volume percent (%) of phasesAverage λ (nm)  Average CT (nm) 
    Ferrite  Pearlite  Resolved pearlite     
HC1G12  10  1.30  98.70  0.00  81.57±10  22±
HC1G13  100  1.31  98.69  0.00  77.54±10  22.6±

λ, interlamellar spacing; CT, cementite thickness.

3.7Evaluation of mechanical properties under different cooling rates

Table 7 summarises the hardness (HV) and predicted yield strength (MPa) values under different cooling rates. The predicted yield strength values were estimated using the following equation:

where λ is known as the pearlite interlamellar spacing (μm); PS is the pearlite colony size (μm); Dγ is the prior austenite grain size (μm) and σYS is the yield strength (MPa) [30,40]. It can be observed from Tables 5 and 7 that as the cementite plate shrinks as a consequence of the increased number of cementite plates due to the acceleration of transformation rate at higher cooling rates which enhance pearlite nucleation, both the hardness and predicted yield strength values are also increased. As discussed earlier, increased cooling rates refines pearlite interlamellar spacing and both the values of hardness and yield strength vary linearly with the inverse square root of pearlite interlamellar spacing as evident from Eq. (1). It is significant that with the variation of cooling rate of steel, different microstructures are produced, and due to this different mechanical property have been obtained. In the selected range of higher cooling rate (10°C/s and 30°C/s), sufficient time is not provided for the carbon diffusion to take place and other transformation processes to get completed which in turn leads to the refinement of prior austenite grain size, cementite thickness and pearlite interlamellar spacing and thus, higher values of hardness and yield strength can be achieved.

Table 7.

Hardness and Predicted Yield Strength values under different cooling rates.

Sample code  Cooling rate (°C/s)  Hardness (HV)  Predicted yield strength (MPa) 
HC0G10  1.75  286±772±10 
HC1G7  1.75  309±962±10 
HC0G11  10  315±798±10 
HC1G5  10  348±1055±10 
HC0G12  30  355±874±10 
HC1G11  30  372±1219±10 
3.8Evaluation of mechanical properties evaluation under different strain rates

Table 8 summarises the values of hardness (HV) of the Nb microalloyed steels under different strain rates. It has been found that the hardness values increased with increasing strain rate from 1s–1 to 10s–1 which is due to the change of lamellar morphology to degenerated or distorted morphology of pearlite at higher strain rates. It can also be seen that hardness value increases marginally due to increase in the strain rate from 10s–1 to 100s–1 which is attributed to the fact that as strain rate increases in the aforesaid range, no significant reduction in the lamellae thickness has occurred.

Table 8.

Hardness values under different strain rates.

Sample code  Strain rate (s–1Hardness (HV) 
HC1G12  10  360±
HC1G13  100  369±

The ferrite in the pearlite, being a softer phase than cementite is deformed during the deformation of pearlite. The plastic deformation is mainly associated with the free dislocation movement. Ferrite and cementite are subjected to thermal residual stresses because of the large difference in their thermal expansion coefficients [40–42]. The residual stress values need to be high enough to ensure the plastic deformation of ferrite to take place and as a consequence, two plastic zones are gradually created in the ferrite regime between two cementite lamellae. At the plastically deformed zones of ferrite, a large number of dislocations are generated. These dislocations may interact with each other resulting in restriction in their own free movement and causes a hardening effect in the ferrite phase and thereby improves the mechanical properties of the steel.

4Conclusions

The major findings from the present investigation can be concluded as follows.

  • 1.

    It is observed that with the increase in cooling rate from 1.75°C/s to 30°C/s, pearlite transformation start and finish temperature decreases for both the high carbon (HC0) and Nb microalloyed steels (HC1) whereas, the austenite starts (Ac1) and finish temperature (Ac3) for HC1 steel is found to be higher as compared to those of the HC0, which is attributed to the fact that Nb is a ferrite stabiliser. The addition of Nb causes lowering the pearlite transformation start and finish temperatures which result in finer pearlite morphology for microalloyed steel. The recalescence phenomenon has been found to be more prominent for high carbon steel (HC0).

  • 2.

    The total volume fraction of pearlite has been found to be more, or ferrite volume fraction has been found to be less in case of HC0 than HC1 steels at lower cooling rate (1.75°C/s) whereas at higher cooling rate (10°C/s) the total volume percent of pearlite is found to be less, or ferrite volume percent is more in case of HC0 than HC1 steels because of the lowering of prior austenite grain size.

  • 3.

    Hardness and predicted yield strength values are found to be increased with increasing cooling rate. At the higher cooling rate of 30°C/s some amount of martensite transformation occurs, which results in an increase in hardness and predicted yield strength for the Nb microalloyed steels.

  • 4.

    With the increase in strain rate from 1s–1 to 100s–1 the pearlite transformation start temperature increases but the pearlite transformation finish temperature decreases. At higher strain rate refinement of pearlite interlamellar spacing and cementite thickness occurs. The total volume percentage of ferrite decreases and pearlite volume percent with a degenerated or distorted morphology increases with the increase in strain rate.

  • 5.

    The yield point at lower strain rates (1s–1 and 10s–1) indicating strain hardening and discontinuous yield drops at higher strain rates (100s–1) indicating strain softening are very much prominent from the true stress vs. true strain curves for the Nb microalloyed steels. Hardness increases with increasing strain rate from 1s–1 to 100s–1 as refinement/degeneration of morphology of pearlite impedes the dislocation movement.

Conflicts of interest

The authors declare that they have no conflict of interest.

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