Review Of Related Literature On Quenched And Tempered Steel In Construction Industries
The current Construction Industries in the Philippines are hesitant to embrace changes in the traditional way of execution of projects. The Department of Trade and Industry (DTI) gave light to a new variable to reconsider in the realization of project. The Quenched and Tempered Steel (QT) is a cost-effective reinforcement steel bar if deemed suitable and safe alternative. The review contains researches, literatures, and readings about the key concepts regarding QT that propose ideas that strengthen the other frameworks of this research.
Properties of Quenched and Tempered Steel BarsThermo-mechanical treatment (TMT) process is where red hot reinforcement steel bars are quenched by a series of water jets causing the outer layer hardened (martensite structure) surrounding softer core (ferrite-pearlite structure). The resulting steel bars obtain higher yield strength and characterized with definite superior ductility, weldability and bendability. This bar is also known as Quenched and Tempered Steel Bars, simply QT Bars. One of the requirements of construction project is its safety and serviceability wherein the strength and ductility of the infrastructure is very important. QT Bars having superior ductility makes it top-notch material for further improvement of the efficiency of the project. Quenched and Tempered Steel Bars offer several advantages revolving around the high strength to weight ratio. These steels are characterized by high strength and hardness. Additionally, they also possess good ductility and toughness. Welding Quenched and Tempered Steel should be given extra precaution as it is prone to Hydrogen induced cracking and formation of soft zone on heat affected zone. The interrupted quenching provided a greater impact toughness value which is attributed to the auto- tempering effects and increased amount of retained austenite. The direct quenched variants showed low toughness even at room temperature. In contrast, the surface hardness was lower for the QFT variants.
A steel is developed and called as yield-ratio-control-steel (YRCS) which the yield strength can be controlled although the same tensile strength. By the advantage of the lower yield strength, forging load is lower relatively and tool life is enhanced on cold forging. Moreover, cold forged part can be used directly without post-heat-treatment (quenching and tempering) because the strength increased by work hardening after cold forging.
The most significant properties affecting the bendability are the yield strength and the purity close to the plate surface. The toughness of the material can be increased but it does not affect the bendability. Other properties such as hardness, ultimate tensile strength and elongation have no major impact on the bendability for this specific composition. Mechanical properties of quenched steel directly depend on the degree of quenched steel hardening. Fracture toughness and fatigue limit depend on microstructural constituents, and distribution of the usual intermetallic particles and non-metallic inclusions. Fatigue resistance of quenched and tempered steel is achieved by eliminating coarse alloy carbides present in steel. According to Sec. Ramon Lopez of Department of Industry, QT rebars are microstructure of a ferrite core and a tempered martens tile layer, he clarified in the steel forum that though the microstructure is as described due to quenching and tempering processes, the mechanical properties of the rebar as the same throughout its length as it is not a composite material. The table 3 contains the tensile and impact test result. The strengths of RQT are lower than those steel with the same chemical composition processed by direct quenching. The variation in impact toughness was insignificant. Quenched and Tempered Steel are low-carbon and weldable material which are used in application such as pressure vessels, buildings columns, mining plant, road tankers and submarine hulls. In the weld thermal cycle, the regions of heat affected zone (HAZ) that experience temperature above Ac3 (about 875°C) needs to be full re-austenitised and can subsequently transform into martensite and bianite with a minimum of 690MPa yield strength.
Quenched and Tempered (Q&T) steels are widely used in ship building industry due to high hardness, high strength and excellent toughness. However Q&T steels are prone to cold crack which is induced by the diffusion hydrigen. During welding of Q&T steels, to avoid the crack it should be to control the heat input, decrease the stress concentration of the joints and restrict the diffusible hydrogen content. In addition, the austenitic stainless steel and low hydrogen ferritic steel consumables were utilized for welding Q&T steels to avoid crack.
Direct Quenching and Re-heat Quenching
There are two types of Quenched and Tempered processes. The process for Direct- Quenching and Tempering (DQT) involves the elimination of a heat treatment step for rolled bars by the installation of on-line water-quenching units permitting the immediate quenching of this rebars after the hot deformation. In the conventional off-line quenched and tempered steel referred as Reheat Quenching and Tempering (RQT), the austenite composition and grain size is controlled by the austenitizing temperature. Direct quenching (DQ), which is the quenching of products immediately after hot deformation, has been considered as an effective way to improve the strength and reduce the cost of the steel in the late 20th century. The process of direct quenching and tempering (DQ-T) has been developed as a part of thermo-mechanically controlled process (TMCP), and has been used to produce high strength steels in order to replace the traditional quenching and tempering process (reheat quenching and tempering or RQ-T).
There are numbers of benefits in direct quenching. Firstly, it reduces production cost by the elimination of reheating process. Secondly, hardenability obtained through direct quenching eliminates the use of costly alloying elements to achieve the required mechanical properties and weldability. Tensile properties and Charpy impact toughness of RQT and DQT were compared. The tensile strength of DQT was higher than of the RQT even after tempering. In contrast, the DQT was inferior to RQT in terms of impact toughness.
The DQ-T process has several advantages over the traditional reheat quenching and tempering (RQ-T) process. First, the strength/toughness balance and the weldability can be enhanced in that the microstructures and the precipitation behavior upon heat-treatment can be diversified. Second, it reduces the manufacturing cost by eliminating the reheating and quenching steps. Third, it saves costly alloying elements by satisfying the mechanical properties and weldability requirements through control of alloy chemistry.
The difference between DQ-T and RQ-T processes is the state of austenite In typical DQ-T process, the repeated recrystallization of austenite brings about the grain refinement by setting the finish rolling temperature at the austenite range, thereby making a fine quenched structure and introducing a considerable amount of dislocations in it. The rolling parameters affecting the DQ-T microstructure are direct quenching temperature, rolling ratio, and cooling rate. A variety of microstructures can be achieved by varying these parameters. In the RQ-T process, on the contrary, the possibility to achieve microstructural modification by controlling the austenite structure is quite limited, because the austenite structure is completely recrystallized during austenitization.
The tensile strength of DQ-T (quenched at 850 ℃ and tempered at 600 ℃) specimen was 975 MPa, which was about 6% higher than that of RQ-T (reaustenitized at 900 ℃ and tempered at 600 ℃) specimen showing 920 MPa. The yield strength of DQ-T specimen was 925 MPa, which was also about 6% higher than that of RQ-T specimen showing 871 MPa. DQ process could easily enhance effective hardenability in comparison with the case of RQ. This effect is mainly due to higher reheating temperature in DQ than in RQ, where in DQ both more uniform solution of alloying elements in austenite and coarser austenite grain enhance effective hardenability. More dislocation in DQ than in RQ, which has inherited from the deformed austenite in DQ process, is another reason.
Effects of Quenching Process on Mechanical Properties and Microstructure of High Strength Steel
This study focuses on the strengthening mechanism of QT Steel tempered on low temperature and the correlation of microstructure and mechanical properties of DQT and RQT. Fig. 1 is a schematic illustration of the two thermo-mechanical processes. As for the RQT process, the ingot is rolling on 12mm thick plate with a 20% rolling ratio in each pass, the plate was air-cooled from the finish rolling temperature to room temperature and was reaustenitized at 1193K for 1 hour and quenched to a room temperature. And for the DQT, the ingot was rolled on 12mm thick plate with rolling ratio of 20% on each passes. The finish rolling temperature was 1123K for controlled-rolled direct quenched (CR-DQT) and 1173K for recrystallization-controlled-rolled direct-quenched (RCR-DQT). The plate was quenched at approximately 303K/s after finish rolling the steel was tempered at 573K for 3 hours.
Thermo-Mechanical Control Process
The benefits of thermo-mechanical treatment (TMT) are well known for achieving desired microstructure meeting specific mechanical properties of steel. Adopting TMT techniques of controlled rolling and accelerated cooling, procuring a fine-grained microstructure with excellent combination of high strength and toughness is possible. The process of on-line direct quenching has been developed as a part of thermo-mechanical control process (TMCP) is being applied to produce high strength steel plates requiring good toughness and weldability.
In a typical on-line direct quenching and tempering process, the repeated recrystallization of austenite leads to refined grain by setting the finish rolling temperature at the austenite range making a fine quenched structure.
Thermo-mechanical control process strengthens and toughens steel essentially by the refinement of transformed microstructure. TMCP also reduces the need for alloying materials, and therefore realizes other merits such as improved weldability. In the TMCP the transformed steel is refined by suitable combination of accelerated cooling and controlled rolling (CR). In cooling stage, CR is used to refine the grains and strains the austenite with purpose to increase the nucleation site of ferrite. After controlled rolling, the transformed steel is further refined by accelerated cooling wherein the diffusion of atoms is limited while the large driving force of transformation is applied. This way, the fine microstructure of the processed steel helps realized the mechanical properties of high strength and excellent toughness.
Thermo-Mechanical processing (TMP) is the most effective means to improve mechanical properties through microstructural control in various metals and alloys such as steels, aluminum alloys or titanium alloys. TMP in steels is basically different from classical Thermo-Mechanical Treatment (TMT) in the following points. TMT had been technology for improvement of strength and toughness by providing hot or warm working during the processing of heat treatment, and thus, its application was restricted to heat treated steels. On the other hand, TMP aims to improve various properties in hot rolled steel products through the optimization of microstructure due to the strict and ingenious control of various parameters in hot rolling and subsequent cooling processes. Therefore, TMP allows a much wider application to a variety of shape and grade of steel products.
NSCP Restrictions
The National Building Code of the Philippines (2015) provided restrictions in quenched and tempered reinforcement. The restrictions in the code which prohibits the use of quenched and tempered reinforcing bars are stated in section 420. 7. 7. 1 to structures located in Seismic Zone 4: (1) There is preheating greater than 275 degrees Celsius, (2) bending of reinforcing bars requiring preheating, (3) splicing of reinforcing bars that will require welding or lap or butt welded joints, (4) threading of reinforcing bar ends for use of mechanical couplers, (5) tack welding for grounding wires will be required. In the year 2010 a study of a structural engineer named Emilio Morales, also a member of National Structural Code, QT poses a high risk when used in high-rise buildings and can submit to failure when subjected in the event of a high-intensity earthquake. QT bars may be used only for low-to-medium rise buildings. A study conducted by the Department of Science and Technology – Metal Industry Research and Development Center (DOST-MIRDC) entitled, “Characterization of Locally-Manufactured Quenched and Self-Tempered Reinforcing Steel Bars, ” the rebar process through quench-tempering and thermo-mechanical treatment passed all the chemical, physical, and mechanical requirements.
