Effect of Intercritical Heat Treatment on Mechanical Properties of Reinforcing Steel Bars

Intercritical heat treatments attempts were made to enhance the mechanical properties of reinforcing steel bars milled from scrap metal. For this, two grades of steel bars were obtained from different steel mills and their mechanical properties that include hardness, ultimate tensile strength, and percent elongation before and after intercritical heat treatment were determined. Results indicated that 25.5 and 17.6%, improvements in UTS (Ultimate Tensile Strength) and 18.8 and 14.3% improvement in percent elongation in two grades of reinforcing steel samples containing 0.17 and 0.24% carbon respectively was achieved while heating at 750C for 2h. Appreciable improvement in the mechanical properties was noted due to birth of sufficient quantity of martensite along with ferrite.


INTRODUCTION
T he most common steels used in the automobile and structural applications are low-carbon steels having low strength level and good formability.
Despite their forming and cost advantages over high strength steels, the ultimate strength level of these steels remains at relatively low levels which is compensated with increasing the cross-sectional area of the material.
Traditional strengthening methods are alloying and work hardening which are not cost effective [1]. Production of steels with improved mechanical properties at lowest possible cost is therefore a matter of practical significance [2]. To achieve this goal dual phase steels were developed [2][3]. features, dual phase steels shows continuous yielding behavior, low yield to tensile strength ratio (YS/TS), high formability and good fatigue resistance [4,5]. These properties are due to the combined effect of two phases present in dual phase steel in which martensite controls the strength of the steel while ferrite is responsible for formability properties [5,9].
Reinforcing steel bar is very important construction material in this modern world, the skyscrapers such as

Material Collection
The reinforcing steel bars studied in present work were manufactured by Magna Steel Mills and Amreli Steel Mills Pakistan. These Mills are manufacturing the reinforce steel bars from steel scrap.

Chemical Composition
The chemical composition of as received bars given in Table 1 was determined using optical spark emission spectrometer (Bruker-Q2 ion).

Sample Preparation
From each grade of reinforced steel bars six tensile test samples of the geometry shown in Fig. 1 were prepared.
During cutting water was used to avoid the localized heating effect. Thereafter samples were polished using 500 mesh emery paper to minimize the surface stress raisers. From each grade of reinforcing bars samples were selected for heat treatment and for tensile testing.

Heat Treatment
The tensile test samples prepared from reinforcing steel bars were subjected to five different heat treatment conditions. Description of heat treatment conditions along with samples code (ID) is given in Table 2 and schematically represented in Fig. 2. Fig. 2

Mechanical Testing
Tensile tests of as received and heat treated samples were performed using Zwick/Roell (Model: A65272) UTM (Universal Tensile Testing Machine). Tensile testing speed was set 3cm/min. Vicker hardness testing machine was used to measure hardness of the samples at 10 kg load.

Metallography
The microstructures of as received and heat treated samples were examined using optical microscope.

Hardness
Vicker hardness result of Grade-1 and Grade-2 of as received and heat treated reinforcing steel bar specimens aregraphically represented in Fig. 3(a-b). Hardness profile, shown in Fig. 3(a-b)    are also shown in Fig. 3(a-b) respectively. It can be seen from micrographs given in Fig. 3(a-b)

Tensile Properties
Tensile test result of as received and heat treated reinforcing steel bar specimens are graphically shown in Fig. 4(a-b) which represents the UTS and percent elongation verses heat treatment conditions respectively.
It can be understood from the graphs that tensile strength, yield strength and percent elongation of all dualphase steels is higher than those of as-received specimens.
When dual phase induced specimens of both gradesare compared with each other, it is observed that the maximum tensile strength and yield strength, have been attained for the specimens subjected to heat treatment condition C (heated at 750 o C for 2 hours). This can be attributed due to maximum martensite volume fraction obtained when specimens are subjected to heat treatment condition C.
Moreover Fig. 4(b) indicates that maximum increase in percent elongation is obtained when specimens were subjected to heat treatment condition "E" (heated at 850 o C for 2 hours). This can be attributed with presence higher volume fraction of ferrite and coarser grain size of pearlite and ferrite. Contrary specimens subjected to heat treatment condition C as compared to other heat treated specimens indicated the minimum percent elongation value. This can be described by considering the higher volume fraction of martensite network produced in these specimens compared to others.

Comparison of Results
Comparing the UTS and percent elongation values of reinforcing steel bar specimens of Grade-2 are comparatively better than Grade-1.
To validate the significance of inter-critical heat treatment the improvements in the UTS and percent elongation of locally manufactured reinforced bars were compared with the improvements reported for reinforced bars .
Oguzhanet. al. [2] and Alanemeet. al. [11] have attempted the inter-critical heat treatment to improve the properties of reinforced bars of Eregli Iron and Steel Turkey and Ispat Steel Nigeria. The results shown in Figs. 5-6 validate that substantial improvement achieved in the UTS and percent elongation respectively by employing the intercritical heat treatment on locally manufactured reinforced bars are in consistent with other international reinforced bars of SAE 1010 steel grade. The general trend of the variation in the UTS and percent elongation results shown in Figs. 5-6 clearly demonstrate that in case of inter-critical heat treatment condition soaking time is a critical factor that played a significant role in the modification of the properties.

CONCLUSIONS (i)
The results indicate that the strength of the specimens having ferrite-martensite dual phase microstructure has been significantly improved which were intercritical heat treated as compared to as conventionally heat treated specimen.
(ii) It has been found from the result that in case of inter-critical heat treatment soaking time is a critical factor that played a significant role in the modification the microstructure and mechanical properties of dual phase steel specimens.The volume fraction of martensite has increased and the average grain size decreased with increasing the soaking time. The strength of the DP steel specimens is function of hardness and volume fraction of the martensite phase.