Decomposition Of Residual Austenite Research Articles

Thermal Stability of Retained Austenite in Advanced TRIP Steel with Bainitic Ferrite Matrix for Automotive Industries

Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Effect of tempering on microstructure and mechanical properties of 3Mn-Si-Ni martensitic steel

The dependence of microstructures and mechanical properties on tempering temperature (from 180 to 650°C) in a designed 3Mn-Si-Ni martensitic steel was systematically analyzed. Microstructure was characterized using scanning and transmission electron microscopy; mechanical properties were measured using uniaxial tensile test and Charpy V-notch impact test. After tempering at different temperatures, recovery, partial recrystallization, carbides precipitation and decomposition of residual austenite were observed. After tempering at 230°C, an excellent combination of strength (1550MPa) and toughness (91.5J) was achieved, due to high dislocation density and ε-carbides precipitation. However, with an increase in tempering temperature from 320 to 550°C, tempered martensite embrittlement was observed, where impact energy was ~ 10J. It was ascribed to cementite formation instead of transition carbides and decomposition of residual austenite. With an increase in tempering temperature up to 650°C, high fracture impact toughness of 75J was obtained with deteriorated tensile strength of 850MPa due to strong recovery and partial recrystallization.

Mechanical Properties and Transformation Behaviors of Ti-Zr Killed Low-Carbon Steels with High-Temperature Hot-Rolling Process

The authors describe here the relationship between the microstructure and mechanical properties of Ti–Zr killed low-carbon steels based on hot-rolling experiments and thermo-mechanical simulation. High-temperature hot-rolling and controlled cooling are carried out. Microstructural evolution is studied in quenched steels subjected to hot-rolling simulation. The study suggests that non-metallic inclusions in steel are (Ti, Zr)-rich and are effective in promoting nucleation of acicular ferrite in the recrystallized austenite grains such that the as-rolled microstructure is significantly refined in spite of coarse austenite grain size induced by high-temperature rolling. The addition of boron and accelerated cooling also favored nucleation of acicular ferrite. The tensile strength and low-temperature impact toughness is significantly improved in comparison to the Al-killed steel. The microstructural evolution is divided into two stages including formation of interlocking acicular ferrite structure and coarsening of the ferrite plates with the decomposition of residual austenite.

Mechanism of Formation of Exfoliations on the Surface of Rolls for Cold Rolling

We studied the phase composition and the distribution of stresses (residual and contact) over the depth of the surface layer of rolls for cold rolling. The results of this investigation are used to explain the mechanism of formation of exfoliations. Exfoliations appear in the zone of maximum residual tensile stresses on the boundaries of microinclusions of phases. The evolution of exfoliations passes through four stages: initiation of microcracks at a certain depth, formation of the main crack, its stable development, and final fracture. The crack-opening displacement is promoted by stresses and the decomposition of residual austenite under the influence of cyclic loads of rolling. The probability of formation of exfoliations can be lowered by optimizing the conditions of thermal treatment of the rolls.

Characteristics of decomposition of residual austenite under the action of shock waves excited by laser pulses

After laser thermal treatment of high-alloy steels, in the quenched surface layer a significant amount of austenite remains, depending on the original structural state, the grade of steel, and the thermal cycle of the laser quenching. Generally the amount of residual austenite increases with an increase in the carbon content in the steel and with an increase (within a certain interval) in the cooling rate. There are two ways to decrease the amount of residual austenite after traditional quenching: the thermal method and the deformation method. The thermal method is most widely used to regulate the content of residual austenite in machined pieces and tool. In order to decrease the amount of residual austenite in high-alloy steels, high-temperature tempering at 400-500°C or cold treatment is used. The deformation mechanism for decomposition of the austenite is used more rarely. In the case of local laser hardening, the amount of residual austenite can be regulated by changing the laser treatment conditions. In [1], decomposition of residual austenite was observed under the action of short, intense pressure pulses excited by giant laser pulses. In this paper we have studied the quantitative characteristics of decomposition of residual austenite in quenched steel under the action of shock waves initiated by laser radiation. In particular, we have studied the change in the amount of residual austenite as a function of the amplitude of the shock wave and the number of laser pulses. The studies were done on samples of 45, U8, and R6M5 steel. Surface laser quenching was accomplished by the emission from a YAG laser operating in the free oscillation regime. The irradiation parameters were: energy, 25 J; pulse length, 5 msec; inversion ratio, 2. The mechanical characteristics and structural features of layers hardened upon laser treatment have been rather well studied (see, for example, [2]). Accordingly, in tiffs paper we limited ourselves to x-ray phase analysis, which was done on the DRON-3 apparatus in filtered cobalt Kcz radiation.

Heat treatment of high-speed steel with continuous laser

1. When preliminarily hardened high speed steel, tempered at 350–560°C, is treated by a continuous CO2-laser with energy density J=34±3 MJ/m2, a strengthened layer with maximal thickness and hardness forms. 2. Accelerated heating by laser beam to temperatures in the range between Ac3 and Tpl and practically instantaneous cooling to normal temperature at rates of more than 104°C/sec give rise to a highly disperse (in melting) and fine-grained structure recrystallized by precipitation hardening (in quenching in the solid state) and consisting of martensite, residual austenite (in increased amount), and carbides (in a small amount). The intense dissolution of ledeburitic carbides type M6C in the laser-hardened zone causes additional alloying of the solid solution, increased stability of the residual austenite, and super-sautration of the finely accular martensite. 3. The decomposition of residual austenite and the intense dispersion hardening in the process of tempering at 560–600°C 1 h increase the hardness of the laser-hardened layer of high speed steel R6M5 by 2–4 HRCe, and resistance to tempering by 40–50°C compared with conventional heat treatment. The absence of coarse carbide particles in the hardened layer reduced the probability of brittle failure by chipping in operation of the cutting tool.

Influence of plastic deforamtion by shot peening and of cyclic loading on the properties of the surface layer of 30KhGSN2A steel

1. The high effectiveness of repeated surface hardening, revealed in an increase in cyclic life of the high-strength steel, is caused by the high amount and depth of penetration of the first order residual compressive stresses (in comparison with the first peening) and by the significant changes in phase composition and fine structure of the surface layer of the material. 2. Cyclic loading of the material occurring before repeated peening intensifies the processes occurring in the surface layer in plastic deformation of it (decomposition of residual austenite and the martensite phase with precipitation of carbide particles, relaxation of local microstresses and a reduction in deformation nonuniformity of the structure, an increase in dislocation density). All of these processes promote an increase in the resistence to microplastic deformation and in fracture toughness and, consequently, lead to an increase in the resistance to the origin and development of fatigue cracks.

Isothermal phase diagram of residual austenite in tempering

1. The temperature-time conditions of intermediate high tempering after quenching at the carburization temperature can be substantiated on the basis of the results of investigations of the decomposition of residual austenite during the tempering process. For that isothermal diagrams of tempering have to be constructed. 2. The isothermal diagram of tempering of the model steel 95KhGNMFL shows that the intermediate high tempering of steel 18KhGNMFL is inexpedient. 3. The decomposition of residual austenite of the model steel 95KhGNMFL is accompanied by stabilization of the process, the same as is found with supercooled austenite. The temperature-time parameters of the stabilization of residual austenite are smaller than of supercooled austenite. 4. The thermokinetic diagram of tempering can be plotted according to the results of the investigation of the phase transformations during tempering with different heating rates.

Kinetics of decomposition of residual austenite in high-speed steel powders in tempering and cooling in liquid nitrogen

A study of the original condition of high-speed steel powders showed that tile quantity of decomposed residual austenite depends upon the degree of its aging as the result of tempering and subsequent cooling of the steels in liquid nitrogen. The least stability of the residual austenite is characteristic of R0M6F3K8 and RI0M6F1K8 powders.

Magnetometric investigation of the decomposition of residual austenite in ROMlOF3 and R2M1OF3K8 High-speed steel powders

The magnetic method of investigation makes it possible to estimate the relative amounts of the ferromagnetic phase in starting particles of various diameters and chemical compositions and provides a vivid qualitative picture of the decomposition of austenite during the heating and cooling of a powder. It has been established that in the starting condition fine powder particles contain the largest amount of the ferromagnetic phase. The stability of residual austenite depends on the character of heat treatment and on the grade of steel. Thus, the residual austenite of R2M1OF3K8 steel is the most stable on exposure to subzero temperatures and less stable during high temperature tempering.

Crystal-structure changes during tempering of hardened high-strength cast irons with nodular graphite

1. The martensite of high-strength cast iron is decomposed by a two-phase mechanism in the first stage of tempering (up to 100°C); the second stage of martensite decomposition occurs at 100–300°C and has the same character as that of high-carbon steel. 2. The decomposition of residual austenite occurs in two stages: The austenite is decomposed with the segregation of α-phase crystals and the carbon enrichment of intact austenite segments in the first stage (100–400°C); the decomposition is completed in the second stage (400–550°C) with the segregation of carbide from the carbon-enriched residual austenite. 3. Decomposition of residual austenite during tempering begins at 100°C in silicon cast iron, and at 200°C in manganese cast iron.

Kinetics of the decom position of residual austenite during heat treatment of sormite No. 1

Kinetics of the decom position of residual austenite during heat treatment of sormite No. 1

Surface hardening of case hardened steel by rolling

1. Rolling of case hardened quenched steel produces an additionally hardened layer as thick as 1 mm. At the same time, the hardness increases by 100 HV units. 2. The effect of rolling depends on the type of steel, the preliminary heat treatment, and the value of the contact stress. 3. The increase in hardness is due to the decomposition of residual austenite and the formation of fine grains of martensite. At the same time, there is an increase in the width of the martensite and austenite x-ray interference lines. 4. For 12Kh2N3MA and 20KhN3A steels, the stress producing hardening is 30,000–35,000 kg/cm2; the optimum contact stress is 45,000–50,000 kg/cm2.

Decomposition of residual austenite as the result of ultrasonic vibration

We have shown that ultrasonic vibration with an amplitude of 20 μ and 12 μ at 250°C favors the decomposition of the austenite in KhVG steel, fragments the microstructure of the steel, and increases its hardness. Ultrasonic vibration of the same amplitude does not induce any significant changes in the steel at 200°C.

ON THE FATIGUE STRENGTH AND DAMPING CAPACITY OF TIME PIECE SPRING

The authors examined the relation between the damping capacity and fatigue strength of time piece spring.The results obtained are as follows:(1) The damping capacity of those springs which are oil quenched and tempered is the lowest at the tempering temperature 300-400°C. At this tempering temperature the structure of the spring is most stable and strongest for repeated stress. This low damping capacity and high fatigue strength are due to the fine tempering structure.(2) When the spring is tempered at constant temperature, the damping capacity rapidly decreases and then slowly increases as the tempering time. This co-ordinates the relaxation of internal stress and the decomposition of residual austenite.(3) About the Austempering, the experimental results were so irregular that no conclusion was secured.