Reheat Quenching And Tempering Research Articles

Influence of Upset Pressure on Microstructure and Mechanical Properties of Friction-Welded AISI 4140 Low Alloy Steel Pipes

In the present study, low alloy steel AISI 4140 pipe in reheated, quenched and tempered (RQT) condition was friction-welded using 6.4 MPa upset pressure followed by post-weld heat treatment as per the manufacturing practice prevailing in the drill pipe industry and compared the same with friction welds made at different upset pressures in the RQT condition and in the as-received condition to evaluate bond quality. The weld region consisted of a mixture of tempered martensite and ferrite in case of RQT condition and mixture of lath martensite/upper bainite and ferrite in the as-received condition. As the upset pressure was increased, the martensite lath size decreased in both as-received and RQT conditions. In general, as the upset pressure was increased, the average microhardness increased. The average microhardness of the material adjacent to partially deformed zone showed lower hardness due to tempering effect because of weld heat. The upset pressure 6.4 MPa showed better notch tensile strength ratio confirming better ductility of the weld zone. The impact strength decreased as the upset pressure increased in both as-received and RQT conditions. But as-received condition showed significantly lower impact strength due to higher carbide content at the weld interface.

Impact of welding on the strength of high performance steel T-stub joints

In this study, experiments were carried out to investigate the influence of welding on the strength of the reheated, quenched and tempered (RQT) steel S690 and the thermo-mechanically controlled processed (TMCP) steel S385. Firstly, a specially designed welding procedure was applied to produce RQT and TMCP steel plates that were fully affected by the welding heat input for the whole plate thickness. These fully weld-affected plates were then fabricated into specimens for tensile test and metallurgical examination including microstructure observation and Vickers hardness measurement. The results showed that welding softened the heat affected zone of the RQT-S690 steel significantly but its effect on the TMCP-S385 steel was insignificant. Secondly, T-stub joints were fabricated by using the RQT and TMCP steel plates and tested in tension until failure. The design plastic resistances of the joints are then compared with the prediction by EC3 equations. It is found that the EC3 equations could produce unsafe prediction for the RQT joint. Based on the test results, a reduction factor is proposed to modify the EC3 equations to produce reasonable conservative prediction of the RQT joint design plastic resistance.

Effects of Welding on the Tensile Performance of High Strength Steel T-stub Joints

In this study, the effects of welding on the tensile performance of the Reheated, Quenched and Tempered (RQT) S690 high strength steel T-stub joints are investigated by both experimental and numerical methods. Firstly, six RQT-S690 T-stub joints as well as two Thermal-Mechanical Controlled Processed (TMCP) S385 T-stub joints are fabricated and tested. The results are validated against the design plastic resistance equations provided by EC3. It is found out that EC3 predicts the plastic resistance of the TMCP joints conservatively, but tends to overestimate that of the RQT-S690 joints although the latter is much superior. It is deducted that the problem may come from the compromised properties of the heat affected zone (HAZ) at the weld toe. Further, finite element analysis is carried out to investigate the effects of property alteration in the HAZ on the tensile performance of the RQT-S690 T-stub joints. It is shown that the models with welding simulation agree well with the test results, while the models without considering the welding effects predict the load carrying capacity unconservatively when the displacement increases.

Tensile behavior of high performance structural steel T-stub joints

In this study, experiments were conducted to investigate the tensile behavior of ten T-stub joints made of different types of high performance structural steel including the Reheated, Quenched and Tempered (RQT) grade S690 high strength steel, the Thermal-Mechanical Controlled Processed (TMCP) grade S385 and S440 structural steels. The results were further validated against the analytical yield line method and the design resistance equations provided by the EC3. It is found that the behavior of the T-stub joints made of S385 and S440 steels reasonably agreed with analytical approach and the predictions by the EC3. However, although the S690 specimens showed higher design resistance than the S385 and S440 specimens with the same thickness, the results were significantly lower than both the predictions by the analytical approach and the EC3. In addition, the ductility of the S690 specimens is also found to be significantly lower than that of the S440 and S385 specimens and failed in more brittle failure modes.

Post weld heat treatment for high strength steel welded connections

In this study, experiments were conducted to investigate the effect of post-weld heat treatment (PWHT) on the reheated, quenched and tempered (RQT) grade S690 high strength steel welded connections. Firstly, the effect of PWHT on the mechanical properties after welding is investigated. It is found that the loss of both strength and ductility after welding could be serious but PWHT could be able to improve the ductility of the affected specimens at the expense of strength. Secondly, four Y-shape plate-to-plate (Y-PtP) and nine T-stub RQT-S690 joints are fabricated to study the effect of PWHT on the residual stress level near the weld toe and the tensile behavior of the joints, respectively. The hole drilling tests employed to study the residual stress reveal that PWHT is able to decrease the residual stress level near the weld toe significantly. The tensile test results show that proper PWHT could improve both the ductility and the maximum resistance while the reduction of plastic resistance can be kept in a negligible level. However, it is found that if the specimens are overheated, although the ductility could still be increased, the reduction of load carrying capacity was severe.

Improvement of Microstructure and Mechanical Properties of a Low Alloy Cast Steel Processed by Direct Quenching-Partitioning-Tempering Technique

A novel direct quenching–partitioning–tempering (DQ–P–T) process was developed to produce a low-alloy Mn–Si–Cr type high performance cast steel for automobiles without using expensive alloying elements Ni, Mo, or Nb as commonly used in some rolled Q–P–T steels. The cast steel treated by DQ–P–T process exhibits a better combination of strength and ductility as well as higher impact toughness than that treated by the conventional reheat quenching and tempering (RQT) process, and shows the improved microstructure consisting of the dislocation-type lath martensite (or plus a little bainite) and nano-sized flake-like retained austenite between the martensite laths. The DQ–P–T process has shown to be more conducive to enhancing the overall mechanical properties of the cast steel compared with traditional RQT process, and is a promising technique for industrial production.

Mechanical properties of heat-treated high strength steel under fire/post-fire conditions

Heat treated high strength steel (HSS) has attracted more and more applications in both on-shore and off-shore structures. According to the classical theories of metallurgy, the hardening effects in the microstructures that grant HSS its high strength may be weakened or eliminated by heat. Structures built using such materials may behave differently after they are heated and cooled down (e.g. during and after a fire situation). This experimental study investigated the mechanical properties of reheated, quenched and tempered (RQT) HSS plate manufactured to grade S690 at elevated temperatures as well as its residual strength after heating and cooling down. It is found out that while RQT-S690 HSS showed good resistance to low temperature (below 400°C) heating, serious deterioration in strength would occur at higher temperatures and its post-fire mechanical properties are different from those of traditional hot-rolled normal strength steels.

Impact Toughness of an NM400 Wear-Resistant Steel

An NM400 wear-resistant steel was hot rolled and then the plates were heat-treated by direct quenching and tempering (DQT) and reheat quenching and tempering (RQT) techniques, respectively. The Charpy impact test was carried out with an instrumented Charpy impact tester. The microstructure and fracture surface were investigated by a combination of optical microscopy, transmission electron microscopy and scanning electron microscopy methods. It was found that the impact toughness of DQT specimen was much higher than that of RQT specimen. The microstructure of both DQT and RQT specimens was characterized by a mixture of tempered lath martensite and lower bainite. The lower bainite in DQT specimen extended into prior austenite grains and the content was higher than that in RQT specimen. The lower bainite in DQT specimen improved the impact toughness by increasing the proportion of large-angle boundaries and relieving the stress concentration at the crack tip. A number of fine and dispersed carbides in DQT specimen also contributed to the improvement of the impact toughness.

Effects of quenching process on mechanical properties and microstructure of high strength steel

The effects of direct quenching and tempering (DQ-T) process and conventional reheat quenching and tempering (RQ-T) processes on mechanical properties and microstructure of high strength steel were investigated. The DQ process was found to enhance the hardenability of steel effectively. The tensile strength and yield strength of DQ specimen was 975 MPa and 925 MPa respectively, which were higher than those of RQ specimen’s of 920 MPa and 871 MPa. In contrast, low temperature toughness (−40 °C, AKV) of DQ-T specimen (124 J) was generally inferior to that of RQ-T specimen (156 J). The direct quenching temperature was one of the potential process parameters to determine strength/toughness balance of steel manufactured by DQ process. The experimental results showed that excellent strength/toughness balance was obtained when the specimens was quenched at temperature in the range of 850–910 °C. The yield strength and impact energy (−40 °C) of DQ steel decreased significantly with increasing of quenching temperature, although the tensile strength was nearly stable.

Studies on influence of reheating temperature and cooling rate on structure–property behaviour of lean chemistry HSLA-100 steel

Simulation studies on the influence of reheating temperature on austenite grain coarsening in lean chemistry high strength low alloy (HSLA)-100 steel were carried out to establish optimum soaking temperature before hot rolling. Experiments carried out in ‘Gleeble-3500’ dynamic thermomechanical simulator revealed that prior austenite grain sizes varied between 26 and 98 and 34 and 126 μm after soaking at 1150, 1200 and 1250°C for 1 and 5 min respectively; a soaking temperature of 1200°C was found to be optimum. Simulation experiments on the influence of cooling rate on microstructural changes and dilatometric studies indicated lowering of transformation temperature with faster cooling. Microstructural examination of dilatometric samples confirmed martensitic transformation at faster cooling rate. The martensite structure is desirable to achieve better strength and toughness. The findings of simulation studies were subsequently used for standardising thermomechanical treatments of Nb–Cu bearing lean chemistry HSLA-100 steels. One laboratory heat of Cu bearing HSLA steel containing 0·028%Nb was made. This heat was hot rolled into 12·5 mm thick plate by varying finish rolling temperature in the range of 800–1000°C. The soaking temperature was maintained at 1200°C. The rolled plates were heat treated by both conventional reheat quenching and tempering (RQT) as well as direct quenching and tempering (DQT) techniques. Evaluation of mechanical properties revealed that plates processed through DQT route were superior to those processed through RQT route. Transmission electron microscopy revealed that martensite structure and finer interlath spacing in DQT plates resulted in superior strength and impact toughness properties as compared to RQT steels.

Comparision of Direct Quenching and Reheat Quenching Process on Microstructure and Mechanical Properties of a High Strength Steel

The effects of direct quenching and tempering (DQ-T) process and conventional reheat quenching and tempering (RQ-T) processes on the microstructure and mechanical properties of a high strength low alloy steel were investigated. In the as-quenched DQ steel, prior austenite grains are elongated parallel to the rolling direction, whereas the as-quenched RQ steel mainly consists of equiaxed grains; The DQ process was found to enhance the hardenability of steel effectively. The tensile strength and yield strength of DQ specimen, were higher than that of RQ specimen. In contrast, low temperature toughness of DQ-T specimen was generally inferior to that of RQ-T specimen.

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

Abstract The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb–Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb–Cu–B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.

Microstructure and mechanical properties of 780 MPa high strength steels produced by direct-quenching and tempering process

Microstructure and mechanical properties of 780 MPa grade steel plate manufactured by conventional reheat-quenching and tempering (RQ-T) and direct-quenching and tempering (DQ-T) processes were investigated. The DQ process was found to enhance the hardenability of steel effectively so that tensile strengths of a range from 780 to 860 MPa have been achieved using DQ-T process, while tensile strength of about 770 MPa has been obtained from the RQ-T sample. In contrast, low temperature toughness of DQ-T samples was generally inferior to that of RQ-T sample, unless hot rolling and cooling processes were optimized in a controlled manner. For example, fracture appearance transition temperature (FATT) of DQ-T samples was varied in a range from −50°C to −120°C, while RQ-T specimens exhibited nearly constant FATT of about −80°C. The finish-rolling temperature (FRT) was one of potential process parameters to determine strength/toughness balance of the steel manufactured by DQ process, while the effect of FRT was closely associated with the cooling rate applied in the process. It has been demonstrated that, for the specimens quenched with a cooling rate higher than 20°C/sec, it may seem to be appropriate to adjust the FRT as low as possible in the non-recrystallization region. In contrast, for the specimens quenched with a low cooling rate of less than 10°C/sec, it may seem to be proper to apply higher FRT to obtain excellent strength/toughness balance of the steel.