Time-Temperature-Transformation Diagram within the Bainitic Temperature Range in a Medium Carbon Steel

Maldonado

et al. 2019

ICBI

ResumenLos aceros de medio C son ampliamente utilizados en la fabricación de piezas y componentes mecánicos tales como engranajes, ejes, pernos, acoplamientos, husillos, ruedas dentadas, bielas y cigüeñales, debido a su buena resistencia mecánica, tenacidad y resistencia al desgaste. Muchas investigaciones para este tipo de aceros se han enfocado en estudiar el comportamiento al desgaste, deformación y análisis de propiedades mecánicas a temperaturas elevadas. Sin embargo, existen pocos estudios que se han enfocado en evaluar su comportamiento mecánico en función del tamaño de grano austenítico (TGA). En esta investigación, se presentan los resultados experimentales y mediante modelado numérico, del efecto del TGA antes del temple en dos aceros de medio C, AISI 1045 y 4140, con la finalidad de determinar las condiciones óptimas de procesamiento para obtener las propiedades mecánicas deseadas. El tamaño de grano fue evaluando entre 5 y 110 µm. Se utilizaron técnicas experimentales para determinar la microestructura, resistencia a la cedencia, resistencia a la tensión, la dureza y pruebas de dilatometría de temple para determinar su deformación. Los resultados simulados fueron obtenidos mediante el software JMatPro. De estos resultados se pueden mencionar que el TGA juega un papel importante en la evolución de las propiedades mecánicas para los aceros estudiados. Cuando el TGA fue superior a 15 y 45 µm para los aceros AISI 4140 y 1045 respectivamente, las propiedades mecánicas estuvieron en rangos más elevados. Los resultados mediante JMatPro estuvieron muy cercanos a los obtenidos experimentalmente, validando la simulación numérica en la predicción de las propiedades mecánicas estudiadas. Palabras Clave:Simulación, Tratamiento Térmico, Tamaño de Grano Austenítico, Propiedades Mecánicas. AbstractMedium-carbon steels are widely used in the making of mechanical pieces and components, such as gears, shafts, bolts, couplings, spindles, sprockets, connecting rods and crankshafts, because of their good mechanical resistance, toughness, and wear resistance. Many research has been focused on studying the behavior of wearing off, deformation and analysis of mechanical properties at high temperatures. However, little has been studied about evaluating its mechanical behavior related to the austenitic grain size (AGS). In this research, experimental results of the effect of AGS before quenched of two medium-C steels AISI 1045 y 4140, are presented through the numerical modeling, in order to determine the optimal processing conditions to obtain the desired mechanical properties. The grain size was evaluated between 5 y 110 µm. Experimental techniques were used to determine the microstructure, tensile strength, yield strength, hardness, and dilatometry to determine its deformation. The simulated results were obtained through the JMatPro. From the results, it can be mentioned that the AGS has an important role in the evolution of mechanical properties for the studied steels. When the AGS was higher than 15 and 45 µm for the steel...

Section: Procedimiento Experimentalunclassified

Efecto del Tamaño de Grano Austenítico en el Comportamiento Mecánico para los Aceros AISI 1045 y 4140 Mediante Experimentación y Modelado

Maldonado

et al. 2019

ICBI

“…[192,231]. In present case, the decrease rate of Ms temperature reduces when the C concentration in the steel increases.…”

Section: Influence Of Carbon On M S Temperaturementioning

confidence: 56%

“…The temperature corresponding to the upper part of the shear transformation C curve is often called the bainite-start temperature Bs [108,231] …”

Section: Bainitic Transformation Start Temperature (B S )mentioning

confidence: 99%

“…It was known already that martensite first phase forms at a large under-cooling, below the T 0 temperature, at which ferrite and austenite of identical composition have equal free energy [242]. The microstructure, (dislocation, vacancies, grain, twin, inter-phase boundaries, and precipitates), external stress and plastic deformation, may sometimes play an important role, but the chemical composition of a steel is a main factor in affecting its M S [192,231].The basis for the calculation of the martensite-start (M S ) temperature was reviewed recently by Kaufman and Cohen [243]. However its experimentally estimated values of 320 °C for Ti-V-N microalloyed steel and 330 °C for V-N steel emphasize the small difference.…”

Section: Austenite Decompositionmentioning

confidence: 99%

“…However its experimentally estimated values of 320 °C for Ti-V-N microalloyed steel and 330 °C for V-N steel emphasize the small difference. It has been shown recently [231,244] that both Ms and Mf temperatures depend on carbon content and the austenite grain size. As reported by [245,246] the number of plates per unit volume needed in order to obtain a detectable fraction of martensite increases as the austenite grain volume (Vγ) decreases.…”

Section: Austenite Decompositionmentioning

confidence: 99%

See 2 more Smart Citations

Austenite decomposition in medium carbon microalloyed steels: Mechanism, structure and properties

Fadel¹

The aim of this work was to determine TTT diagrams of two mediumcarbon V microalloyed steels V-N (0.256%C, 0.0235%N, 0%Ti) and Ti-V-N (0.309%C, 0.221%N, 0.011%Ti ). The isothermal treatment was carried out at 350, 400, 450, 500, 550 and 600 ο C. These treatments were interrupted at different times in order to analyze the evolution of the microstructure.Isothermal decomposition of medium carbon vanadium microalloyed austenite was evaluated by optical and SEM metallography.In the first step, austenite grain size is establish in temperature range 850-1150 °C. Ti bearing steel exibits lower grain size at high temperatures. This effect is attributed to pinning effect of TiN particless. Nevertheless, temperature of 1100°C provided grain size very simillar to 60μm, value suggested as optimal for intragranular nucleation and formation of acicular ferrite.Isothermal treatment enabled plotting the TTT diagrams. In both diagrams, equal transformations are observed. Influence of Ti addition is not very clear; it is assumed that increased level on carbon has covered expected influence on intragranular nucleation. Four curves are found to be relevant for austenite decomposition in medium carbon Vmicroalloyed steels:(1) Grain boundary ferrite is the first phase to be generated at all temepratures. In the lower temperature range the widmanstetten ferrite is formed, while on higher temperatures grain boundary allotriomorphs are produced. This difference is attributed to displacive nature of transformation at lower and diffusional transformation at higher temperatures.(2) Second curve is related to nucleation of Intragranular ferrite (IGF). In the lower temperature range (350-400°C ) acicular ferrite plates are grouped in sheaves; at intermediate temperatures (450-500°C), a more interlocked microstructure of acicular ferrite was clearly observed, while microstructure generated at high temperatures (550-600°C) is characterized by polygonal idiomorphic ferrite.(3) Third curve is related to onset of pearlite. It occurs at temperatures ≥ 500°C, followed by an incomplete reaction phenomenon.vi (4) The transition between an acicular ferrite sheaf morphology and interlocked microstructure is observed to take place at 400/450°C. However the bainitic sheaves are frequently observed when the isothermal transformation time is increased at 400°C and temperature diminishes to 350°C.Finish of transformation was clearly observed at temperatures below 500°C. However at 550and 600°C, incomplete reaction phenomenon occurs. This behaviour is attributed to carbon enrichment in austenite and decrease of driving force for austenite decomposition.Martensite start temperature is established to be 320°C and 330°C for Ti-V-N and V-N steels respectively. Slightly lower Ms temperature of Ti-V-N steel is attributed to higher content of carbon and Ti, elements that increase hardenability.The lower yield stress for Ti-free steel (V-N) compared with titanium -containig steels (Ti-V-N) is attributed to lower carbon content compared with titanium-containing steel...

A Method to Estimate the Microstructures of Low‐Alloy Cold‐Rolled TRIP Steels

Sun

Wangyue

et al. 2017

steel research int.

A method is implemented in the present study to estimate the microstructures of low-alloy cold-rolled TRIP steels. This method includes simulations of intercritical annealing (IA) process and calculations of isothermal bainitic transformation (IBT) process. The evolutions of volume fractions and elements concentration profiles with temperatures and holding times for ferrite phase and austenite phase could be acquired assisted by Thermo-Calc and DICTRA during IA process. Bainitic transformation from austenite, with chemical compositions determined at the end of IA, takes place during the subsequent IBT process. By taking into account the martensitic transformation during cooling from IA to IBT and/or from IBT to room temperature (RT) and avoiding the complex discussions of bainitic transformation kinetics, the volume fractions of bainite, martensite and retained austenite could be approximately determined after IBT and subsequent water quench. At the same time, the experimental validation of this method is conducted with well agreements. This implemented method is expected to relate chemical compositions and heat treatment parameters to the final microstructures of low-alloy cold-rolled TRIP steels and be beneficial for optimizing TRIP microstructures without amounts of experiments.