Warm deformation and annealing behaviour of iron-silicon-(carbon) steel sheets

Single pass warm rolling and compression experiments were carried out from ambient to 800°C for ultra-low carbon (ULC) steel with ∼100 ppm carbon and interstitial free (IF) steels, both with two levels of silicon. Subsequently, annealing was done in order to recrystallize the deformed specimens. The main purpose of this study was to understand the effects of rolling temperature and silicon on stress responses and textures. This study comprises two main themes: flow stress and strain rate sensitivity during compression and shear banding and textures in warm rolled specimens. The effects of deformation temperature on in-grain shear bands were different between ULC-Si and IF-Si steels. As in previous work with more conventional steels, in-grain shear bands in the IF grade had low sensitivity to rolling temperature, while those in the ULC grade depended significantly on the deformation temperature. However, the temperature profile of shear banding in the ULC grade was approximately 150°C higher than in previous work. Deformation and recrystallisation textures for both IF and ULC grades depended on their rolling temperatures. The variation of both grain size and texture after annealing can be explained by the rise and fall of in-grain shear banding activity which is related to the strain rate sensitivity.

Chip formation mechanism and machinability of wrought duplex stainless steel alloys

This paper investigates the chip formation mechanism and machinability of two-phase materials, such as, wrought duplex stainless steel alloys SAF 2205 and SAF 2507. SEM and optical microscopic details of the frozen cutting zone and chips revealed that the harder austenite phase dissipates in the advancement of the cutting tool, being effectively squeezed out of the softer ferrite phase. Microhardness profiles reveal correlation in hardness from the workpiece material transitioning to the chip. The tool wear (TiAIN + TiN coated solid carbide twist drill) and machining forces were investigated. Tool wear, was dominantly due to the adhesion process which developed from built-up edge formation, is highly detrimental to the flank face. Flute damage was also observed as a major issue in the drilling of duplex alloys leading to premature tool failure. Duplex 2507 shows higher sensitivity to cutting speed during machining and strain hardening at higher velocity and less machinability due to presence of higher percentage of Ni, Mo and Cr.

Synchrotron powder diffraction study of cements pastes

 Knowledge of the degree of hydration of cement pastes is critical for determining properties such as the durability of concrete. As part of an integrated study on the prediction of chloride ingress in reinforced concrete, synchrotron Xray powder diffraction was used to estimate the degree of hydration of cement pastes. While for the past 20 years the composition of Portland cement has been determined by Rietveld analysis of X-ray diffraction, nevertheless there are a number of factors, including the amorphous content of the cement and relative proportion of mineral polymorphs present in the initial clinker, whose impact on the analysis are still not completely understood. Analysis of the resulting diffraction patterns indicated enhanced identification of polymorphs of alite, belite, ferrite and aluminate, which are present in the initial unhydrated cement and clinker, as well as improved quantification of hydrated crystalline phases such as calcium hydroxide and ettringite, which are key phases determining the speed of the chemical reactions in cement. In this paper we describe the experience that we have gained in the determination of the degree of hydration of cement pastes. We detail the standards and precautions that we took to characterize production cements and their hydration products.

A molecular dynamics simulation of alloy carbide clusters formation

 The formation of alloy carbide cluster in ferrite was investigated via molecular dynamics simulation, which disclosed the cluster property and formation mechanism. These together provided a better fundamental understanding of the cluster formation and firm information for the evolution of cluster and precipitate in high-strength low-alloy steel.

Properties and automotive applications of advanced high-strength steels (AHSS)

Advanced high-strength steels (AHSS) are a class of steel used primarily in sheet form for automotive structures. The microstructures of the types of steel in this classification were initially multiphase, with ferrite as the dominant phase; however, grades introduced more recently have been fully martensitic or based on austenite. This chapter initially introduces the requirements of an automotive body structure, then the different classes of AHSS that have been used in the automotive industry and their typical characteristic tensile properties. The specific properties that are required for steel used in automotive body structures are subsequently described, including formability and crash behaviour. Finally, some of the current and future trends in the development of new steel grades are discussed.

Weldability and machinability of duplex stainless steel

This chapter investigates two important processing methods, such as welding and machine of duplex stainless steel. The welding process welding generally degrades the properties of these materials by redistributing the phases during melting and solidification. On the other hand, the redistribution during machining mainly take place combined effect of stress, strain rate and temperature. Mechanism of machining process and several welding methods has been analysed in details. It was found that outcomes of welding processes depend on the welding methods. Most of the cases an appropriate annealing process can be used to restore the expected properties of the weld joints though the parameters of annealing process are different in different welding methods. Nonmetallic inclusions and the low carbon content of duplex stainless steel reduce the machinability of duplex stainless steel. SEM and optical microscopic details of the frozen cutting zone and chips revealed that the harder austenite phase dissipates in the advancement of the cutting tool, being effectively squeezed out of the softer ferrite phase. Abrasion and adhesion were the most common wear modes developed on the flank and rake faces. Adhesion wear being the most prevalent on the flank face, appeared to be initiated by built-up edge formation.

Recrystallization kinetic study of copper-bearing strip cast steels

The effects of copper on as-cast structure, recrystallization and precipitation kinetics of strip cast low carbon steel were investigated. As-cast microstructure mainly consists of polygonal ferrite and Widmanstatten ferrite. Recrystallization responses were strongly dependent on initial microstructure and Cu content. Precipitation strengthening was observed in high copper content alloys.

Effects of microstructure on the variation of the unloading behavior of DP780 steels

The nonlinear unloading behavior of three different commercial dual-phase steels (DP780 grade equivalent) was examined. These steels exhibited small variations in chemical composition (0.07 to 0.10 mass percent carbon) and martensite volume fraction (0.23 to 0.28), and they demonstrated similar hardening behavior. Uniaxial loading-unloading-loading tests were conducted at room temperature and quasi-static strain rates between engineering strains of 0.5 and 8%. Steel microstructures were examined using electron backscatter diffraction and nanoindentation techniques. The microplastic component of the unloading strain exhibited no dependence on the martensite volume fraction or the ferrite grain size within the small range encountered in this investigations. Instead, the magnitude of the microplastic component of the unloading strain increased as the strength ratio between the martensite and ferrite phases increased. Correspondingly, the apparent unloading modulus, or chord modulus, exhibited a greater reduction for equivalent increments of strain hardening as the strength ratio increased. These results suggest that springback can be reduced in structures containing two ductile phases if the strength ratio between the harder and softer phases is reduced.

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.

Microstructure and hardness evolution during simulated coiling of a direct strip cast low carbon low niobium steel

This paper examines the impact of coiling temperature and duration on the phase transformation and precipitation behavior of a low carbon and low niobium direct strip cast steel. Coiling was performed at three carefully chosen temperatures: (1) in the ferrite (600°C), (2) during the austenite decomposition (700°C) and (3) in the austenite (850°C). The coiling conditions were found to strongly affect the final microstructure and hardness response, thus highlighting the necessity to judiciously design the coiling treatment. Optical microscopy, and scanning and transmission electron microscopy were used to characterize the microstructural constituents (polygonal ferrite, bainite and pearlite) and the NbC precipitates. Vickers macrohardness measurements are utilized to quantify the mechanical properties. The differences in hardening kinetics for the three different temperatures are shown to come from a complex combination of strengthening contributions.

Effect of retained austenite on wear resistance of nanostructured dual phase steels

Nanostructured super bainitic and quenching-partitioning (Q&P) martensitic steels with a significant amount of retained austenite obtained by low temperature bainitic transformation and Q&P respectively were studied to explore the effect of retained austenite on stirring wear resistance. The results suggest that the Q&P martensitic steel significantly enhanced the hardness of the worn surface (from 674 to 762 HV1) and increased the thickness of the deformed layer (,3.3 mm), compared to the nanostructured bainitic steel. The underlying reason is that the Q&P martensitic steel has a higher stability of retained austenite thereby providing a superior transformation induced plasticity effect to increase surface hardness and reduce wear rate during the wear process.

Investigation on decomposition behavior of austenite under continuous cooling in vanadium microalloyed steel (30MSV6)

In the present study, investigations are focused on microstructural evolution and the resulting hardness during continuous cooling transformation (CCT) in a commercial vanadium microalloyed steel (30MSV6). Furthermore, the effects of cooling rate and austenite grain size (AGS) on CCT behavior of the steel have been studied by employing high-resolution dilatometry. Quantitative metallography accompanied with scanning electron microscopy (SEM) has efficiently confirmed the dilatometric measurements of transformation kinetics and austenite decomposition products. A semi-empirical model has been proposed for prediction of microstructural development during austenite decomposition of the steel and the resultant hardness. The model consists of 8 sub-models including ferrite transformation start temperature, ferrite growth, pearlite start temperature, pearlite growth, bainite start temperature, bainite growth, martensite start temperature and hardness. The transformed fractions of ferrite, pearlite and bainite have been described using semi-empirical Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach in combination with Scheil's equation of additivity. The JMAK rate parameter for bainite has been formulated using a diffusion-controlled model. Predictions of the proposed model were found to be in close agreement with the experimental measurements.

Dependence of deformation behavior on grain size and strain rate in an ultrahigh strength-ductile Mn-based TRIP alloy

A novel ultra-high strength TRIP (transformation induced plasticity) steel, with ~1.5. GPa strength and good ductility of ~26% has been produced. The microstructure consists of ultrafine ferrite, and a large volume fraction of austenite. The flow stress was significantly increased by a reduction in the grain size, but the effect of strain rate on the flow stress was negligible. The formation of stress induced martensite was found to increase linearly with strain, and a reduction in the grain size correlated with an increase in the stress required to form the martensite.

The microstructure evolution and softening processes during high-temperature deformation of a 21Cr-10Ni-3Mo duplex stainless steel

The austenite and ferrite microstructure evolution and softening mechanisms have been investigated in a 21Cr-10Ni-3Mo duplex stainless steel, containing about 60% austenite, deformed in torsion at 1200°C using a strain rate of 0.7 s-1. The above experimental conditions led to the formation of a small volume fraction of new austenite grains through discontinuous dynamic recrystallization (DDRX), which could not account for the observed large softening on the flow curve. DDRX grains mainly formed through the strain-induced migration of the pre-existing austenite grain boundaries, known to dominate in single-phase austenite, complemented by subgrain growth in the interface regions with ferrite. A significant portion of austenite dynamic softening has been attributed to the large-scale subgrain coalescence, the extent of which increased with strain, which seems to have contributed substantially to the observed flow stress decrease. The above process thus appears to represent an alternative mode of austenite dynamic softening to the classical DDRX in the duplex austenite/ferrite microstructure, characterised by limited availability of the pre-existing austenite/austenite high-angle boundaries, deformed at a high temperature. The softening mechanism within ferrite has been classified as "continuous DRX", characterised by a gradual increase in misorientations between neighbouring subgrains with strain and resulting in the progressive conversion of subgrains into "crystallites" bounded partly by low-angle and partly by large-angle boundaries.

The role of thermomechanical routes on the distribution of grain boundary and interface plane orientations in transformed microstructures

In the current study, a series of thermomechanical routes were used to produce different microstructures (i.e., ferrite and martensite) in low-carbon low alloy steels. The five-parameter grain boundary character distribution was measured for all microstructures. The thermomechanical processing route altered the texture of the fully ferritic microstructure and significantly influenced the anisotropy of the grain boundary character distribution. Generally, the population of (111) planes increased with an increase in the γ-fiber texture for the ferritic microstructure, but it did not change the shape of the grain boundary plane distribution at specific misorientations. The most commonly observed boundaries in the fully ferritic structures produced through different routes were {112} symmetric tilt boundaries with the Σ3 = 60 deg/[111] misorientation; this boundary also had a low energy. However, the grain boundary plane distribution was significantly changed by the phase transformation path (i.e., ferrite vs martensite) for a given misorientation. In the martensitic steel, the most populous Σ3 boundary was the {110} symmetric tilt boundary. This results from the crystallographic constraints associated with the shear transformation (i.e., martensite) rather than the low-energy interface that dominates in the diffusional phase transformation (i.e., ferrite).