Quenched and Tempered Reinforcement bars
Quenched and Tempered Reinforcement bars
The reinforcement steel bars (rebar) are produced these days by the application of the technology of quenching and tempering of the bars during its production in the rolling mill. The rebars produced by this technology are popularly known TMT (thermo-mechanically treated) reinforcement bars.
TMT rebars are basically made from plain low carbon steels and are specified for yield strength, ductility, carbon or carbon equivalent and yield to tensile ratio. The maximum and minimum specified carbon content intends to ensure weldability and hardenability. With too low carbon content hardenability of the steel is not likely to be sufficient and thus more severe quenching is needed affecting rolling mill design, such as, speed of rolling mill, as well as the length and efficiency of cooling chamber. Carbon steel with carbon content in the range of 0.13 % – 0.24 % and the carbon equivalent (CE) of less than 0.48 % has been proved to be the best balance to satisfy the above considerations.
TMT rebar is an appropriate material for reinforcing concrete structures, because the material’s thermal expansion is similar to concrete structures. Moreover, the material is compatible when it is bonded with concrete. The rebar has also the capacity to bear the maximum tensile stress acting on the structure. Apart from being a key product for the construction industry, TMT rebars are also high quality materials which can meet the consumers’ concerns about the standard mechanical properties for this application. TMT rebars are also useful in the general fabrication works, where bending, machining and welding is needed.
The production process for the TMT rebars is an economical method in producing high strength rebars. Compared to cold twisted deformed (CTD) rebars, production process for TMT rebars has obvious advantages in saving the cost of the mechanical twisting treatment which is expensive especially for small diameter rebars. A further advantage comes from reduced alloying element requirements, off-grade heat, off-grade products, stock piling expenses and some other minor steelmaking factors. The only factor which increases cost is the rolling operation related to quenching installation and operation.
There are two main processes for the production of TMT rebars which are popular. These are (i) Tempcore process, and (ii) Thermex process. The Tempcore process for the manufacture of reinforcement bars has been developed in the 1970s by Centre de Rechaerche Metallurgiques (CRM) Belgium in order to manufacture high yield strength weldable concrete reinforcement bars from mild steel billets. The Thermex process was developed and branded by Germany engineering firm Hennigsdorfer Stahl Engineering (HSE) also in the 1970s.
The production process of TMT rebars is based on the thermo-mechanical processing. Thermo-mechanical process is a metallurgical process which combines plastic deformation process with the thermal processes such as the heat-treatment, water quenching, heating, and cooling at various rates into a single process. The process imparts high strength to the rebars by the technique of thermo mechanical treatment as against mechanical working by cold twisting which is used for the manufacture of Torsteel reinforcement bars. The strength of the rebar is due to the tempered martensite layer while the ductility of the rebar is due to the ferrite- pearlite layer.
The thermo mechanical treatment converts the rebar surface to a hardened structure (martensite) and subsequently the phase evolves by cooling at ambient temperature to allow the hot core to temper the surface through thermal exchange. This results in a unique composite microstructure comprised of tempered martensite in the peripheral zone/case, transition zone of pearlite and bainite just after the martensite periphery and a fine grain ferrite-pearlite at the central zone/core (Fig 1). Due to the quenching and self-tempering production process, rebars produced are also called ‘quenched and self-tempered (QST) rebars’.
Fig 1 Microstructure of TMT reinforcement bars
Both the Tempcore and Thermex processes are similar in nature and are based on the above thermo-mechanical treatment principle. Tempcore process has the quenching box in which water flows from one end to the other end while in case of Thermex process the water is sprayed on the hot rolled bar in the quenching box.
The schematics of the process on a continuous cooling transformation (CCT) diagram are given in Fig 2.
Fig 2 Schematics of the process on a continuous cooling transformation (CCT) diagram
Thermo-mechanical treatment of the rebars is carried out after the heated steel billets are rolled in the rolling mill to the final size and shape of the reinforcement bar. The operational parameters which affect this process are the temperature at the end of the rolling, the cooling rate, time during rapid cooling, and the chemical composition of the steel. The process flow is shown in Fig 2 and is carried out in three successive stages as described below.
The first stage begins as soon as the rebar leaves the final mill stand. It constitutes fast water-cooling of the hot-rolled product. The rebar is rapidly and energetically cooled (quenched) with water through a cooling installation, where it undergoes the surface hardening. Effectiveness of the cooling installation is to be high enough to achieve the cooling rate of the surface of the rebar higher than the critical speed of martensite formation. At the end of this operation the rebar has a microstructure consisting of austenitic structure which is surrounded by a layer of the martensite-austenite mixture. Obtaining the martensitic layer of the required thickness is the purpose of this stage of the process.
The second stage begins as soon as the rebar leaves the water-cooling zone (quenching box) and moves towards the air-cooling one. At this point the temperature of the core is higher than the temperature of the surface. Due to this temperature gradient, heat starts flowing from core to the surface thus tempering the martensite layer formed in the first stage. Because of this, the surface martensitic layer is tempered by utilizing the residual heat left in the core of the rebar (self tempering of the martensitic layer). Tempcore process has derived its name from this step (The name Tempcore illustrates the fact that the martensitic layer is ‘TEMPered’ by the heat left in the ‘CORE’ at the end of the quenching stage). The core remains still in the austenitic phase at this stage. The tempering process ensures the proper ductility of material, simultaneously preserving its high yield strength.
The third stage occurs during a free cooling of the rebars on the cooling bed and consists of the austenite transformation into ductile ferrite and pearlite structure in the bar core. Hence, a TMT steel rebar is essentially a composite material consisting of concentrically disposed hard outer layer and soft core with an intermediate and intermediately hardened layer.
The final structure consists of strong tempered martensite structure in the outer layer at the surface and a ductile ferrite – pearlite structure in the core. This gives the reinforcement bars a unique property of strength in combination with ductility. Morphology of structural components depends on the steel chemical composition, bar diameter, time and eﬀectiveness of the cooling.
A typical quenching water flow diagram for the production of TMT rebars is given at Fig 3.
Fig 3 Typical quenching water flow diagram
Properties of the TMT rebar
TMT rebars produced by the above process have excellent properties which are superior to the properties shown the CTD rebars. The properties of the TMT rebars are given below.
- The rebars have high strength due to the tempered martensite layer at the periphery of the rod. The strength of the rebar can be varied by controlling the thickness of this layer.
- The rebars have high ductility due to ferrite-pearlite structure in the core. Because of this property the rebars can be bent easily at construction site. The rebars have ability to be bent and rebent, galvanized and straightened without cracking or loss of tensile properties. The severe bending capability of the rebars is shown in Fig 4.
- The rebars have good bond strength and hence are ideal for use in the concrete structures.
- The rebars have high ductility and toughness at low temperatures even when damaged mechanically or by welding arc strikes.
- The rebars show absence of significant strain age embrittlement after bending and galvanizing.
- The rebars are fully weldable. The rebars are produced from steels having low ‘carbon equivalent’ (CE).
- The rebars are compatible with all mechanical reinforcing rebar splices normally available to join rebars for both compression and tension loading.
- The rebars resist loss of strength at elevated temperatures. This property is very important in case of fires. Practical results have shown that TMT rebars retain more than 70 % of its yield strength in case of rebars having yield strength of 415 newtons per square millimeters (N/sq mm) and 40 % in case of rebars having yield strength of 500 N/sq mm.
- The rebars meet the fatigue strength requirements as per European standards.
- For most of the steels, shear strengths fall within the range of 60 % to 80 % of the tensile strengths. The rebars are having a shear strength which is towards the top of the range.
- The rebars unlike CTD rebars leaves no torsional stresses. This results into better corrosion resistant properties of the rebars. Further since the rebars are not subjected to mechanical working, hence the blue secondary scale is retained on the surface of the rebars. This results into protection of rebars from atmospheric corrosion
- These rebars has superior seismic resistant properties. Tests conducted with these rebars have shown that their performance under repeated reverse loading with inelastic strains (normally encountered during an earth quake) is better since the energy dissipation is almost the same for each cycle and uniform ductility is maintained till failure.
- Other properties of the TMT rebars include good low temperature toughness, and less sensitivity to surface damage.
Fig 4 Severe bending capability of the TMT rebars
Typical etched cross section of the TMT rebars shows three metallurgical regions. Tempered martensite in the form of packets of thin plates with martensitic morphology characterizes the hardened layer. A mixture of bainite and polygonal ferrite is in the intermediate hardened layer and the region is the polygonal ferrite and pearlite develops in the core. The microstructure is usually fine due to a relative fast cooling in the core and because of the thermo-mechanical treatment involved in the process.
If the martensite layer is thicker, the retained heat is less during the quenching of the rebar then the tempering is modest and the rebar shows higher yield strength and lower elongation. The process parameters and steel compositions play part in the final properties. Longer quenching time, lower finishing temperature and higher intensity of quenching result in thicker martensitic layer and lower tempering temperature. Higher carbon and manganese content increases the hardenability of the steel, and hence more martensite is formed. Additionally, the strength of tempered martensite increases as the carbon content increases.
The range of typical yield strength of TMT rebars is between 415 N/sq mm to 550 N/sq mm and elongation on a 5d gauge length is 30 % down to 25 % in the same order. The ratio of yield stress to tensile strength is around 0.85.
Reinforcement bars produced by quenching and tempering process have several advantages which include (i) the rebars has got consistent quality since they are produced by an on line process, (ii) the combination of high strength with high ductility in these rebars imparts safety to the structures made from these rebars, (iii) fabrication activities with these rebars are simple and easy, (iv) the high strength of the rebars results into saving of steel.