Hardened Case Properties and Tensile Behaviours of TMT Steel Bars

Abstract

Nowadays, high strength steels are getting very popular all over the world for various applications such as in auto body making, machine parts manufacturing, steel structure buildings, concrete reinforcement, etc. In Bangladesh, use of high strength TMT steels bars has got a momentum in the construction of flyovers, bridges and high rise buildings because of its good combination of the required mechanical properties. In this research work, locally produced high strength TMT reinforcing steel bars (20 mm dia.) of two different companies have been investigated. As a part of this research work, at first, details microstructural morphologies of hardened case, transition zone and core of both steels were investigated by optical microscope and they were then photographed. After that, Rockwell hardness was measured on different zones. Tensile tests were also carried by a Universal Testing Machine at room temperature. After tensile tests, details of the fracture modes of the steel bars were studied carefully. The fracture modes were then discussed in terms of chemical compositions and carbon equivalents, resulted microstructures, case and core hardness, depths of hardened case and transition zone of the steel bars.

Influence of the microstructure of TMT reinforcing bars on their corrosion behavior in concrete with chlorides

“…TMT bars, often known as "TempCore" bars, are manufactured by passing the red heated steel -just after the rolling process-through a chamber with a control water flow that quenches the outer surface of the bars, while their core remains hot as austenite [6]. A martensite case is formed in the outer part of the bar.…”

Effect of Water flow Rate on the Yield Strength of a Reinforced bar

“…Casting is performed in billet casters and the billets are sent to the roughing mill and hot rolled for initial austenite decomposition. Though the strength of rebar can be increased by the addition of various alloying elements, there is a risk of reducing the ductility, weldability and the bendability of the finished product if this strengthening route is not controlled [3,4].…”

“…However, there are possible problems encountered in TMT of rebars. These would either be that: the quenching rate is relatively low or the rolled rebar was in the water cooling chamber for a relatively longer period of time [3]. In all these scenarios, the transition zone of steel is highly affected and this usually results in larger area of transition zone of steel.…”

“…The case area will be either very soft or hard. For soft case, the steel will not provide optimum design strength and for very hard case the steel will not provide optimum ductility which might cause brittle type of failure [3].…”

“…As a standard practice, the surface temperature should be cooled to below 200 ℃ at the cooling bed and this should be done within few seconds [3]. In the absence of online temperature tracking, a pyrometer or an Infrared Thermometer (IR) can be used to monitor this temperature at the cooling bed.…”