Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pet) steels, with and without the addition of 0.039Nb (in wt pet), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.

Effect of alloying elements on the microstructure property relationship in thermomechanically processed C-Mn-Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe-C-Mn-Si TRIP steel on the final microstructure and mechanical properties after simulated  thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X-ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The  results have shown that the final microstructures of thermomechanically processed TRIP steels comprise 50 % of polygonal ferrite, 7 - 12 % of retained austenite, non-carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure-property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C-Si-Mn-Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb-Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C-Si-Mn-Nb steel leads to a good combination of strength (∼ 940 MPa) and elongation (∼ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ∼7 - 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb-Mo-Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb-Mo-Al steel appears to be more granular than in the Nb-Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.

Effect of alloying elements on the microstructure-property relationship in thermomechanically processed C-Mn-Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe-C-Mn-Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X-ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise 50 % of polygonal ferrite, 7 - 12 % of retained austenite, non-carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure-property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C-Si-Mn-Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb-Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C-Si-Mn-Nb steel leads to a good combination of strength (∼ 940 MPa) and elongation (∼ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ∼7 - 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb-Mo-Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb-Mo-Al steel appears to be more granular than in the Nb-Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.

A study of the strengthening mechanism in the thermomechanically processed TRIP/TWIP steel

The strengthening mechanism responsible for the unique combination of ultimate tensile strength and elongation in a multiphase Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel was studied. The microstructures with different volume fraction of polygonal fenite, bainite and retained austenite were simulated by controlled thermomechanical processing. The interupted tensile test was used to study the bainitic ferrite, retained austenite and polygonal ferrite behavior as a function of plastic strain. X-ray analysis was used to characterize the volume fraction and carbon content of retained austenite. TEM and heat-tinting were utilized to analyze the effect of bainitic fenite morphology on the strain induced transformation of retained austenite and retained austenite twinning as a function of strain in the bulk material. The study has shown that the austenite twinning mechanism is more preferable than the transformation induced plasticity mechanism during the early stages of deformation for a microstructure containing I5% polygonal ferrite, while the transformation induced plasticity effect is the main mechanism in when there is 50% of polygonal ferrite in the microstructure. The baillitic fenite morphology affects the deformation mode of retained austenite during straining. The polygonal fenite behavior during straining depends on dislocation substructure tonned due to the deformation and the additional mobile dislocations caused by the TRIP effect. TRIP and TWIP effects depend not only on the chemical and mechanical stability of retained austenite, but also on the interaction of the phases during straining.

Experiments

Projected on two screens, with two separate soundtracks, the always exceptional, and occasionally brilliant, photographic images are enhanced by Dirk de Bruyn's rigorous control over a wide variety of experimental techniques.

Without overindulging in any of them, de Bruyn uses animation, optical illusions, time lapse, solarization, hand tinting, flash frames, refilming and flicker effects, accompanied by a dense atmosphere of word puns, dialogue, primal screams, music and even recycled and letraseted soundtracks.

Current trends in machinability research

The manufacturing index of a country relies on the quality of manufacturing research outputs. Theemergence of new materials such as composites and multi component alloy has replaced traditionalmaterials in certain design application. Materials with properties like high strength to weight ratio,fatigue strength, wear resistance, thermal stability and damping capacity are a popular choice for adesign engineer. Contrary, the manufacturing engineer is novice to the science of machining thesematerials. This paper is an attempt to focus on the current trends in machinability research and anaddition to the existing information on machining. The paper consist of information on machiningAustempered Ductile Iron (ADI), Duplex Stainless Steel and Nano-Structured Bainitic Steel. Thevarious techniques used to judge the machinability of these materials is described in this paper.Studying the chip formation process in duplex steel using a quick stop device, metallographic analysisusing heat tinting of ADI and cutting force analysis of Nano-structured bainitic steel is discussed.