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 ℃, 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 ℃. The yield strength and impact energy （-40 ℃） of DQ steel decreased significantly with increasing of quenching temperature, although the tensile strength was nearly stable.

Effect of Direct Quenching on Microstructure and Mechanical Properties of a Wear-resistant Steel

A wear-resistant steel was hot rolled at the same finish temperature, and subsequently treated with conventional reheat quenching and tempering process （RQ＆T） and direct quenching and tempering process （DQ＆T）, respectively. The effect of direct quenching on the microstructure and mechanical properties was investigated in detail by using optical microscope, transmission electron microscope and scanning electron microscope equipped with electron backscattered diffraction. The results showed that the microstructures of both the RQ and DQ specimens were complex constituents of lath martensite and lower bainite. Compared with the RQ specimen, the lower bainite content in DQ specimen was much higher. Furthermore, the bainite in the DQ specimen extended into and segmented the prior austenite grains, which can decrease martensite packet size. The proportion of high-angle boundary in the DQ specimen was higher than that in the RQ specimen, which may improve the impact toughness. The carbides in DQ＆_T specimen were much finer and distributed even dispersively because direct quenching can retain substantive defects which may provide more nucleation sites for carbide precipitation in the tempering process. Besides, the mechanical properties of DQ and DQ＆T specimens were superior than those subjected to RQ and RQ＆T processes, respectively.

Effect of direct quenching on microstructure and mechanical properties of medium-carbon Nb-bearing steel

The influence of direct quenching (DQ) on microstructure and mechanical properties of 0.19C-1.7Si-1.0 Mn-0.05Nb steel was studied. The microstructure and mechanical properties of reheat quenched and tempered (RQ&T) steel plate were compared with those of direct quenched and tempered (DQ&T) steel plates which were hot rolled at different finish rolling tem-peratures (1173 K and 1123 K), i.e., recrystallization-controlled-rolled direct-quenched (RCR&DQ) and controlled-rolled direct-quenched (CR&DQ), respectively. The strengths generally increased in the following order: RQ&T<RCR&DQ&T< CR&DQ&T. Strength differences between the CR&DQ&T and RQ&T conditions as high as 14% were observed at the tempered temperature of 573 K. The optical microscopy of the CR&DQ&T steel showed deformed grains elongated along the rolling direction, while complete equiaxed grains were visible in RQ&T and RCR&DQ&T steels. Transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) of the DQ steels showed smaller block width and higher density of dislocations. Inheritance of austenite deformation substructure by the martensite and differences in martensite block width were ruled out as major causes for the strength differences between DQ and RQ steels.

Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrMo4 Steel Prior to Direct Quenching

Direct quenching and tempering （DQ-T） of hot rolled steel section has been widely used in steel mill for the sake of improvement of mechanical properties and energy saving. Temperature history and microstructural evolution during hot rolling plays a major role in the properties of direct quenched and tempered products. The mathematical and physical modeling of hot forming processes is becoming a very important tool for design and development of required products as well as predicting the microstructure and the properties of the components. These models were mostly used to predict austenite grain size （AGS）, dynamic, recta-dynamic and static recrystallization in the rods immediately after hot rolling and prior to DQ process. The hot compression tests were carried out on 42CrMo4 steel in the temperature range of 900-1 100 ℃ and the strain rate range of 0. 05-1 s^-1 in order to study the high tempera- ture softening behavior of the steel. For the exact prediction of flow stress, the effective stress-effective strain curves were obtained from experiments under various conditions. On the basis of experimental results, the dynamic recrystallization fraction （DRX）, AGS, hot deformation and activation energy behavior were investigated. It was found that the calculated results were in good agreement with the experimental flow stress and microstructure of the steel for different conditions of hot deformation.

Effect of External Forced Flow and Boiling Film on Heat Transfer of AISI 4140 Steel Horizontal Rod During Direct Quenching

The effects of rod falling and moving, external flow field, boiling film and radiation were investigated on fluid flow and heat transfer of AISI 4140 steel horizontal rod during direct quenching by mathematical modeling. The flow field and heat transfer in quenching tank were simulated by computational fluid dynamics （CFD） method considering falling and moving of rods during process. Therefore, modeling of flow field was done by a fixed-mesh method for general moving objects equations, and then, energy equation was solved with a numerical approach so that effeet of boiling film heat flux was considered as a source term in energy equation for solid-liquid boundary. Simulated results were verified by comparing with published and experimental data and there was a good agreement between them. Also, the effects of external forced flow and film boiling were investigated on heat flux output, temperature distribution and heat transfer coefficient of rod. Also simulated results determined optimum quenching time for this process.

Microstructural Evolution and Properties of a High Strength Steel with Different Direct Quenching Processes

A high strength low alloy steel with low carbon equivalent was selected for simulating online direct quench- ing and coiling （DQ-C） process. The influence of stop quenching temperature on mechanical properties and micro- structures was studied and compared with normal direct quenching and tempering （DQ-T） process. The study con- firmed that required mechanical properties were obtained for both the processes. Properties of the experimental steel with DQ-C process could reach the same level as that of DQ-T process in general. In the DQ-C process, strength de- creased with increase in stop quenching temperature. Martensite was obtained and experienced an aging process at stop quenching temperature below Mi. On fast cooling below Mi, martensite was partially transformed and carbon partitioning occurred during slow cooling. The reduction in solid solution carbon and increased amount of retained austenite led to lower strength compared with the DQ-T process. DQ-C process was more favorable for microalloy carbide precipitation. However, impact toughness under different cooling conditions was adequate because of low car- bon equivalent and refined microstructure.

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 investiga-ted 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 hainite. 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 DOT 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 dis-persed carbides in DQT specimen also contributed to the improvement of the impact toughness.

Molybdenum alloying in high-performance flat-rolled steel grades

Considerable progress in developing flat-rolled steel grades has been made by the Chinese steel industry over the recent two decades.The increasing demand for high-performance products to be used in infrastructural projects as well as in production of consumer and capital goods has been driving this development until today.The installation of state-of-the-art steel making and rolling facilities has provided the possibility of processing the most advanced steel grades.The production of high-performance steel grades relies on specific alloying elements of which molybdenum is one of the most powerful.China is nearly self-sufficient in molybdenum supplies.This paper highlights the potential and advantages of molybdenum alloying over the entire range of flat-rolled steel products.Specific aspects of steel property improvement with respect to particular applications are indicated.

Effect of modifying matrix microstructures and nanosized precipitates on strengthening mechanisms and ductile-to-brittle-transitiontemperature in a 1000 MPa Ni-Cr-Mo-Cu steel

A superior combination of yield strength(1001 MPa)and-20℃ impact toughness(166 J)was obtained in Nb-V-Timicroalloyed Ni-Cr-Mo-Cu steel treated by direct quenching and tempering route(DQT).The tested steels treated by DQT route and re-austenitization and tempering route(QT)were compared with each other in terms of mechanical properties and microstructures characterized by optical microscopy,transmission electron microscopy,X-ray diffraction,electron back-scattered diffraction method and so on.Strength and Vickers hardness of the tested steel treated by the above two routes vary with isothermal aging temperature(400-600℃),shown as under-aged state,peak-aged state and overaged state.All DQT specimens show higher strength and Vickers hardness than QT specimens with the same aging condition.Furthermore,the largest difference of yield strength between DQT and QT specimens was shown in DQT600 and QT600 specimens.DQT600 or QT600 specimens refers to direct quenched(DQ)or quenched(Q)specimens isothermally aged at 600℃.The main disparities in quenched microstructure between DQ and Q specimens are mainly in morphology of prior austenite grains,dislocation density of martensite matrix and solution amount of Nb and Mo elements dissolving in martensite matrix,which play key roles in affecting microstructure and mechanical properties of DQT and QT specimens.Higher dislocation density of matrix and finer average diameter of both MC(M is any combination of Nb,Mo and V)and Cu-rich particles were shown in DQT600 specimens than in QT600 specimens.Strengthening from dislocations and nanosized MC and Cu-rich particles mainly leads to the largest difference of yield strength between DQT600 and QT600 specimens.In addition,strong dislocation strengthening and precipitation strengthening in DQT600 specimen also elevated its ductile-to-brittle-transition-temperature,compared with QT600 specimen.