Multi-phase microstructure design of a novel high strength TRIP steel through experimental methodology

The multi-phase structure of a novel low-alloy transformation induced plasticity (TRIP) steel was designed through experimental analysis. The evolutions of both microstructure and mechanical properties during the two-stage heat treatment were analyzed. The phase transformations during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry. It was shown that two types of C diffusion were detected during intercritical annealing and a complex microstructure was formed after heat treatment. The processing parameters were selected in such a way to obtain microstructures with systematically different volume fractions of ferrite, bainite and retained austenite. The volume fractions of ferrite and retained austenite were found to be two main factors controlling the ductility. Furthermore, a high volume fraction of C-rich retained austenite, which was stabilized at room temperature, was the origin of a TRIP effect. The resulting material demonstrates a significant improvement in the ultimate tensile strength (1077. MPa) with good uniform elongation (22.5%), as compared to conventional TRIP steels. © 2014 Elsevier B.V.

Recent advances in machining of Austempered Ductile Iron to avoid machining induced microstructural phase transformation reaction

Austempered Ductile Iron (ADI) is a type of nodular, ductile cast iron subjected to heat treatments-austenitising and austempering. Whilst machining is conducted prior to heat treatment and offers no significant difficulty, machining post heat treatment is demanding and often avoided. Phase transformation of retained austenite to martensite leading to poor machinability characteristics is a common problem experienced during machining. Study of phase transformations is an investigative study on the factors-plastic strain (εp) and thermal energy (Q) which effect phase transformations during machining. The experimental design consists of face milling grade 1200 at variable Depth of Cut (DoC) range from 1 to 4 mm, coolant on/off, at constant speed, 1992 rpm and feed rate, 0.1 mm/tooth. Plastic strain (εp) and martensite content (M) at fracture point for each grade was evaluated by tensile testing. The effect of thermal energy (Q) on phase transformations was also verified through temperature measurements at DoC 3 and 1 mm using thermocouples embedded into the workpiece. Finally, the amount of plastic strain (εp) and thermal energy (Q) responsible for a given martensite increase (M) during milling was related and calculated using a mathematical function, M=f (εp, Q). The future work of the thesis involves an in-depth study on the new link discovered through this research: mathematical model relating the role of plastic strain and thermal energy in martensite formation.

Thermo-mechanical processing of TRIP-aided steels

The effects of the partial replacement of Si with Al and the addition of P on the microstructure and mechanical properties of experimental TRIP-aided steels subjected to different thermo-mechanical cycles were studied. Based on the available literature and thermodynamics-based calculations, three steels with different compositions were designed to obtain optimum results from a relatively low number of experiments. Different combinations of microstructure were developed through three different kinds of thermo-mechanical-controlled processing (TMCP) routes, and the corresponding tensile properties were evaluated. The results indicated that partial replacement of Si with Al improved the strength-ductility balance along with providing an improved variation in the incremental change in the strain-hardening exponent. However, the impact of the P addition was found to depend more on the final microstructure obtained by the different TMCP cycles. It has also been shown that an increase in the volume fraction of the retained austenite ($$ V_{{\gamma_{\text{ret}} }} $$Vγret) or its carbon content ($$ C_{{\gamma_{\text{ret}} }} $$Cγret) resulted in an improved strength-ductility balance, which can be attributed to better exploitation of the TRIP effect.

Effect of single and double austenitization treatments on the microstructure and hardness of AISI D2 tool steel

This investigation is concerned with the study of effect of Double Austenitization (DA) and Single Austenitization (SA) heat treatment processes on microstructure and mechanical property of AISI D2type cold worked tool steel. To maximize hardness, tool steels are used in a quenched and tempered condition. This involves heating the material to the austenitizing temperature (~850-1100. °C), quenching at an appropriate rate to form martensite, and tempering to reduce the retained austenite content and induce toughness. The merits of DA treatment isto promote dissolution of carbides at the same time proscribe grain coarsening significantly was attempted in D2 tool steel. The study has found that DA treatment has induced high hardness with insignificant growth in grains. The increase in hardness is attributed to increase in carbon content in matrix due to dissolution of carbides; whereas finer grains due to role of inclusions.

Optimization of the mechanical properties of low-carbon steels by formation of a multiphase microstructure

In this work five methods of heat treatments are investigated in order to obtained convenient volume fractions of ferrite, bainite, martensite and retained austenite, starting with a low carbon steel and seeking the distinction of the phases, through optical microscopy. Specific chemical etching is improved. The results in tensile and fatigue tests were accomplished and the results were related with the microstructural parameters. The results show that the mechanical properties are closely related with the phases, grains size and the phases morphology. Copyright © 2001 Society of Automotive Engineers, Inc.

Otimização das propriedades mecânicas por meio de modificações microestruturais em aços de baixo carbono

This paper discusses the formation of microstructures with different volume fractions, as an outcome of a specific heat treatment, with the following phases: ferrite, martensite, bainite and retained austenite. For the microstructure characterization it is developed a chemical etching that allows to distinguish the phases by optical microscopy. The evaluation of the mechanical properties is done based on the results of tensile and fatigue tests. The experimental results show that appropriate heat treatments can contribute to a significant improvement in the mechanical properties of the steel. In this process it is essential to control the fraction volume, morphology of the phases, and grain size.

Microstructural effects on fatigue crack growth behavior of a microalloyed steel

Thermal transformations on microalloyed steels can produce multiphase microstructures with different amounts of ferrite, martensite, bainite and retained austenite. These different phases, with distinct morphologies, are determinant of the mechanical behavior of the steel and can, for instance, affect the crack path or promote crack shielding, thus resulting in changes on its propagation rate under cyclic loading. The aim of the present work is to evaluate the effects of microstructure on the tensile strength and fatigue crack growth (FCG) behaviour of a 0.08%C-1,5%Mn (wt. pct.) microalloyed steel, recently developed by a Brazilian steel maker under the designation of RD480. This steel is being considered as a promising alternative to replace low carbon steel in wheel components for the automotive industry. Various microstructural conditions were obtained by means of heat treatments followed by water quench, in which the material samples were kept at the temperatures of 800, 950 and 1200 °C. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The results allowed correlating the tensile properties and crack growth resistance to the microstructural features. It is also shown that the Region II FCG curves of the dual and multiphase microstructural conditions present crack growth transitions that are better modeled by dividing them in two parts. The fracture surfaces of the fatigued samples were observed via scanning electron microscopy in order to reveal the fracture mechanisms presented by the various material conditions. © 2010 Published by Elsevier Ltd.

Caracterização metalográfica de um aço de fases complexas (CP)

Pós-graduação em Engenharia Mecânica - FEG

A influência da deformação plástica sobre a estabilidade mecânica da Austenita retida em aços 300M

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Metodologia para caracterização metalográfica de um aço TRIP por microscopia óptica

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Caracterização microestrutural de um aço médio carbono e baixa liga (com estrutura bainítica/martensítica) via microscopia óptica

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Efeitos das microestruturas bainíticas e multifásicas nas propriedades mecânicas de um aço AISI 4340

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Caracterização mecânica e microestrutural de um aço de baixo carbono

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Estudo da transformação da austenita retida em martensita induzida por deformação plástica em aços multifásicos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Caracterização mecânica e microestrutural de um aço 300M com microestrutura multifásica

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)