High Temp. Mater. Proc., Vol. 30 (2011), pp. 39–42 Copyright © 2011 De Gruyter. DOI 10.1515/HTMP.2011.005

Effects of Heat Treatment on the Microstructure and MechanicalProperties of Low-carbon Microalloyed Steels

Cihan Ekinci,1;� Nazim Ucar,1 Adnan Calik,2

Serdar Karakas2 and Iskender Akkurt1

1 Physics Department, Art and Science Faculty, SuleymanDemirel University, Isparta, Turkey

2 Department of Manufacturing Engineering, Facultyof Technology, Suleyman Demirel University, Isparta,Turkey

Abstract. In this study, the effect of cooling rate and heattreatment on the microstructural and mechanical character-istics such as microhardness of low-carbon microalloyedsteel was investigated by means of optical microscopy andVickers hardness tests. The cooling rates were controlled byintroducing air-cooling, furnace-cooling, water-quenchingand liquid nitrogen quenching. The obtained results showedthat the difference of cooling rates and the heat treatmentssignificantly affect the microstructure and mechanical prop-erties of steels. These tendencies were dominant in liquidnitrogen cooled for 1073 K at 4 h and 1273 K heat treatmentprocesses.

Keywords. Microalloyed steels, heat treatment, hardnesstesting, microstructure.

PACS®(2010). 62.20.-x; 62.20.Qp; 68.35.Fx.

1 Introduction

Mechanical properties of steels depend strongly not onlyon composition but also on the steel structure determinedby its cooling rate that are generally performed in order toachieve a good hardness and/or tensile strength with suf-ficient ductility [1]. Hence, the effects of applied coolingrate on microstructure and mechanical behavior have beenstudied intensively. Corresponding to this Kim et al. [2]showed that an increase of the annealing temperature toabove the ˇ-region temperature induced an increase of thetensile strength and a decrease of the elongation. In addi-tion, a decrease of the cooling rate from water-quenching toa furnace cooling revealed a decrease of the tensile strengthand an increase of the elongation. On the other hand, it hasbeen shown that after introducing different cooling rates,

Corresponding author: Cihan Ekinci, Physics Department, Art andScience Faculty, Suleyman Demirel University, Isparta,Turkey; E-mail: cihan_ekinci_1982@hotmail.com.

Received: July 22, 2010. Accepted: ???.

the material exhibits different tensile characteristics, of sandcooling (slow cooling) the tensile strength is 729.5 MPaand elongation is 3.4 %. After air cooling (moderate cool-ing rate), the strength is 62.1 MPa and elongation is 2.9 %while of water quenching (drastic cooling), the strength is559.3 MPa and the elongation is 1.4 % [3].

At the same time, it is usually considered that dependingon the cooling rate different phases precipitate which meansthat different mechanical properties can be reached. It hasbeen reported that the microstructure at conventional cool-ing rate, primarily consisted of polygonal ferrite-pearlitemicroconstituents, while at intermediate cooling rate be-sides polygonal ferrite and pearlite contained significantfraction of degenerated pearlite and lath-type ferrite. Athigher cooling rate, predominantly, lath-type (acicular) orbainitic ferrite was obtained in Nb-microalloyed steels [4].In addition, it has been found that the applied cooling ratehad a significant effect on intermetallic compound size,morphology and mechanical properties of Sn-Ag alloys. Atrelatively fast cooling rates (24 K=s), a fine distribution ofspherical particles Ag3Sn is observed [5, 6].

Low-carbon microalloyed steels are widely used in me-chanical engineering, pressure vessels and pipelines trans-porting oil and natural gas for its high strength, good tough-ness and weldability [7]. The carbon content of thesesteels are much lower than conventional low carbon fer-ritic/pearlitic steels, and the controlled cooling causes thetransformations to occur at lower temperatures [8]. So,many studies have been reported about the microstructureevolvement of the steel after deformation in continuouscooling condition. In these steels, the microstructural stud-ies suggest that the increasing in the cooling rate is relatedto the change in the microstructure from predominantlyferrite-pearlite to predominantly bainitic ferrite [6, 9]. Onthe other hand, it has been found that higher hardness val-ues were obtained in microalloyed steel by cooling rate dueto phases [10]. Although many papers about the coolingrate effect on tensile behaviours of steels have been pub-lished in steels and alloys [11–14], there are few studies onthe effects of cooling rate on the microstructure and micro-hardness [15–17]. Especially, the cooling rate effect on themicrohardness of low-carbon microalloyed steels has notbeen reported in literature. The main goal of this study is toinvestigate the effect of cooling rate on the microstructureof low-carbon microalloyed steels. Another goal is deter-mine the effect of cooling rate on the mechanical propertiessuch as microhardness of these steels.

40 C. Ekinci, N. Ucar, A. Calik, S. Karakas and I. Akkurt

Element C Si Mn P Mo Mg Cr Al Nb Fe

Mass (%) 0.044 0.335 1.345 0.011 0.023 0.010 0.010 0.030 0.031 Bal.

Table 1. Chemical compositions of the low-carbon microalloyed steels (mass %).

Annealing Temperature Annealing Time Cooling/Quenching Media

673 K 4 hours FC, AC, WQ, LN

1073 K 4 hours FC, AC, WQ, LN

1273 K Immediate FC, AC, WQ, LN

FC: Furnace Cooled; AC: Air Cooled; WQ: Water Quenched; LN: Liquid NitrogenQuenched

Table 2. The heat treatments and cooling rates applied to the low-carbon microalloyed steels.

2 Experimental Method

The chemical compositions of the test materials are listedin Table 1. The substrates were cut from a 3 � 3 � 20 mm3

steel plate. Table 2 lists the heat treatment processes usedin this study. According to the diagram, the test parame-ters and conditions, such as the annealing temperature andcooling rate process were set to verify the effects of eachheat treatment process. Then, to observe the effect on themicrostructure and microhardness of low-carbon microal-loyed steels, samples were heated to 1273 K, 1073 K and673 K held at this temperature for 4 hour and then cooledin one of four different media: furnace, air, water and liq-uid nitrogen. The microstructure was observed by opticalmicroscopy. A Vickers microhardness tester with a load of100 g was used to determine the hardness of steels. Manyindentations were made on the surfaces of steels to checkthe reproducibility of hardness data. Further experimentaldetails are described in [18].

3 Results and Discussion

A mean cooling rate is defined as the time derivative ofthe spatial mean temperature, Q.t/ D dT .x; t/=dt . Thiscooling rate is a function of time but it is considered ap-proximately constant, since the function T .x; t/ is found tobe approximately linear in the temperature range in whichmost of the segregation occurs [19,20]. In the present study,in the original structure of the as-received received low-carbon microalloyed steel reveals a two-phase microstruc-ture consisting of large ferrite and pearlite (FP) phases (Fig-ure 1). The microhardness of these steels are about 243 HV.After isothermal temperature at 673 K for 4 hours, by thecomparing optical pictures obtained from air cooled, fur-nace cooled, water quenched and liquid nitrogen quenchedsteels it was seen that the microstructure was not changedwith cooling rate. This result can be confirmed by compar-ing the microhardness values. The microhardness values atall cooling rates are the same as that of the as-received steel

Figure 1. As-received microstructure of the low-carbonmicroalloyed steels.

(Table 3). This shows that heat treatment process (heating673 K at 4 h) and cooling rates do not affect on the micros-turucture of low-carbon microalloyed steel. The pearlitepercentage in the microstructure is approximately 10 % forthe water and liquid nitrogen cooled steels. In consideringthe isothermal temperature heat treatment at 1073 K and 4h, the microstructure has martensite in addition to ferriteand pearlite (Figure 2). The pearlite take place in the softferrite structure uniformly. The martensite disperse in thestructure as lamella and thin grain. On the other hand, thepercentage of the ferrite and the percentage of martensite inthe structure increases with increasing cooling rate. In thewater and liquid nitrogen quenched steels, martensite andferrite phase are dominant and it is called as a fiber struc-ture. Meanwhile, this heat treatment affect relatively thehardness of the steels. The obtained microhardness is about158 HV and increases slowly with the increasing coolingrate. These values are relatively lower than that values ofreceived and heating 673 K steels. So, we can say thatthis temperature process acts as a annealing process. The

Effects of Heat Treatment on Low-carbon Microalloyed Steels 41

673 K for 4 h 1073 K for 4 h 1273 K

Furnace-cooled 241 158 144

Air-cooled 240 163 188

Water-cooled 242 165 213

Nitrogen-cooled 244 182 277

Table 3. Measured Vickers hardness values of the low-carbon microalloyed steels at different cooling rates. The micro-hardness value of as-received steels is about 243 HV.

Figure 2. Microstructure after isothermal temperature heattreatment at 1073 K and 4 h followed by water quenching.

potential residual stresses removes with annealing process.Therefore the microhardness decreases comparing to as re-ceived steels. In literature [21] it has been shown that asteel, consisting of about 15 % martensite in a ferrite ma-trix, has become of technological interest because at a con-stant tensile strength it exhibits greater formability than astandart steels. In these steels there are decreases in yieldstress from 550 to 380 MPa while total elongation increasesfrom about 18 to 20 %.

In addition to isothermal heat treatments, when the tem-perature reach to 1273 K, a group low-carbon microalloyedsteel was rapidly cooled in air, furnace, water and liquid ni-trogen. The microstructure were found to display relativelylarge amounts of ferrite and very little pearlite (furnacecooled), large and thin grain size of the ferrite, little pearliteand retained austenite (air cooled), large amounts of fer-rite, thin pearlite, retained austenite and percentage of 10–15 % lath martensite (water quenched), very little size fer-rite ve very thin size pearlite and percentage of 15–20 % lathmartensite (liquid nitrogen cooled) respectively (Figure 3).In order to compare the microstructure photographs, we saythat the grain size of the ferrite are decreased with the in-creasing cooling rate. In the same way the the percentageof martensite structure is seen with increasing cooling ratein steels. The fastest cooling rate produced maximum mi-crohardness due to formation of lath martensite where as in-termediate cooling rate resulted in relatively low microhard-

Figure 3. Microstructure after heat treatment at 1273 Kfollowed immediately by water quenching.

ness values owing to morphology of acicular ferrite in thestructure. Considering the results of structure of steels, themicrohardness of steels increases with the increase of cool-ing rate. The microhardness obtained from liquid nitrogen-quenched samples is increased by a factor close 2 accordingto microhardness values obtained from air. It is well knownthat the liquid nitrogen-quenched is cooled to create a su-persaturated solid solution and vacancies increase in thesesamples [22]. High hardness correlates with high resistanceto dislocation motion. In addition, the microhardness ofsteels increases drastically as the martensite percentage isincreased. This is because the martensitic steels are one ofthe most common strengthening phase in steels [23]. Tosum up, heat treatments were more dominant in the 1273 Kand 1073 K at 4 h test rather than 673 K at 4 h from thecharacteristics of the microstructures which were affectedby the heat treatments. Thus, heat treatment (heating andcooling) is used to obtain desired properties of steels suchas improving microhardness besides the toughness, ductlityor removing the residual stresses, etc.

4 Conclusions

In the present study, the effects of heat treatment on themicrostructure and the microhardness of low-carbon mi-croalloyed steel were examined. The microstructural stud-

42 C. Ekinci, N. Ucar, A. Calik, S. Karakas and I. Akkurt

ies suggest that the increase in hardness of these steels withincrease in cooling rate is related to the change in the mi-crostructure from predominantly ferrite-pearlite to predom-inantly martensite. These tendencies were dominant in liq-uid nitrogen cooled for 1073 K at 4 h and 1273 K heat treat-ment processes.

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