Effect of heating rate on reaustenitisation of low carbon niobium microalloyed steel

Abstract

Austenite formation during a continuous heating in a low carbon niobium microalloyed steel with a pearlite and ferrite initial microstructure has been studied. Characteristic transformation temperatures, Ac ₁, Ac _θ and Ac ₃ and the evolution of austenite formation have been determined by combining dilatometry and metallography in a range of heating rates from 0˙05 to 10 K s^–1. It has been observed that nucleation and growth of austenite depends highly on the applied heating rate. At low heating rates (0˙05 K s^–1) nucleation of austenite takes place both at pearlite nodules and at ferrite grain boundaries, while for higher heating rates (≥0˙5 K s^–1), nucleation at grain boundaries is barely present compared to the nucleation at pearlite nodules. The heating rate also affects the austenite growth path and morphology and, thus, the distribution of martensite in the dual phase microstructure obtained at room temperature.     

Effects of chemical composition and thermomechanical treatment on austenite to ferrite transformation in 12 wt-%Cr steels

Abstract

The kinetics of austenite to ferrite transformation was studied in 11–12 wt-%Cr steels having an essentially austenitic microstructure at hot rolling temperatures (750–1050°C). The effects of chemical composition, high temperature γ/δ phase balance, and deformation before the transformation were assessed. The phase transformation was monitored using dilatometry, metallography, and hardness measurements. Small variations in chemical composition, particularly in the nickel and manganese content, resulted in significant differences in the kinetics of the transformation. These are a result of changes in the Ac₁ temperature, pre-existing δ ferrite content at high temperature, and probably the solute drag effect. Deformation at low temperatures of 850–750°C accelerated the transformation. The magnitude of this effect wasfound to depend on the degree of deformation and the cooling rate above the transformation temperature. Using a reduction of 30%, the cooling rate that resulted in a specific fraction of ferrite in the final structure was increased threefold. The results suggest that if the steel composition, particularly the nickel and manganese content, can be adjusted within narrow limits, controlled rolling together with controlled, retarded cooling can be applied to produce 11–12 wt-%Cr steels with adequate mechanical properties and excellent weldability, without the need for tempering.     

Transformation kinetics of a superhardenability treated steel

Abstract

The influence of high-temperature austenitization on the isothermal transformation kinetics of a superhardenability treated steel has been studied by comparing the isothermal transformation diagrams. It is shown that the superhardenability effect is more pronounced in bainite than in ferrite–pearlite transformations. The critical transformation temperatures M_s, M_f, Ac₁ and Ac₃ are unchanged. The superhardenability effect is completely suppressed by austenitization above 1200°C and recovered by reaustenitization at typical (880°C) temperatures. It is also shown that this effect is related to a low oxygen content. From comparison of the transformation kinetics of a superhardenability treated steel and boron containing steels after high-temperature and grain refining treatments it is suggested that the superhardenability effect is possibly related to the boron effect by which boron increases the hardenability of low- and medium-carbon alloy steels.

MST/397     

Microstructure and tensile properties of modified 9Cr–1Mo steel (grade 91)

Abstract

The influence of different soaking temperatures in the range 973–1623 K (below Ac ₁ to above Ac ₄) before oil quenching and tempering, on the microstructure, hardness, grain size, and tensile properties of modified 9Cr–1Mo steel has been studied. This was done in an effort to assess the tensile behaviour of the different microstructures likely to be encountered in the heat affected zone of a fusion welded joint of the steel. The steel developed predominantly martensitic structure after quenching. Soaking of steel in the intercritical temperature range (between Ac ₁ and Ac ₃) reduced the prior austenitic grain size and hardness. Soaking temperatures above Ac ₃ increased the grain size and hardness of the steel until the formation of δ ferrite at temperatures above Ac ₄. The δ ferrite formation at soaking temperatures above Ac ₄ reduced the grain size and hardness of the steel. The tensile strength of the steel exhibited a minimum for soaking in the intercritical temperature range where the ductility was highest. Strength increased and ductility decreased with further increases in soaking temperatures above Ac ₃. The formation of δ ferrite at soaking temperatures above Ac ₄ improved the ductility. The tensile properties have been correlated with the microstructures.     

Microstructure and mechanical properties of medium-carbon ferrite–pearlite steel microalloyed with vanadium

Abstract

Microstructural analysis and mechanical testing have been carried out on medium-carbon steels to which additions of vanadium in the range 0·075–0·6 wt-% were made. The steels were either continuously cooled or isothermally heat treated after austenitization. Vanadium carbide precipitation in the proeutectoid ferrite regions of the microstructure and, more unusually, also in the pearlitic ferrite lamellae, were identified by transmission electron microscopy. Moreover, in both ferrite phases the precipitates are aligned in rows, indicative of interphase precipitation at the austenite/ferrite transformation interface. These observations are discussed in terms of the various mechanisms that have been proposed for the interphase precipitation reaction. In the alloys studied the vanadium additions were found to increase the strength of the steels by up to 100%, but to reduce the ductility and notched impact resistance. The most useful combination of increased strength with reasonable ductility and impact toughness was achieved with an addition of 0·15 wt-% V. The vanadium additions contributed to a number of variations in microstructure and therefore in strengthening mechanisms, but the largest effect was the interphase precipitation strengthening of the ferritic phases. The highest strength levels were achieved in fully pearlitic microstructures with the pearlitic ferrite lamellae strengthened by interphase precipitation of the vanadium carbide.

MST/536     

Effects of Mn and Ti on microstructure and inclusions in weld metal of high strength low alloy steel

Abstract

The influences of alloying elements on chemical composition of non-metallic inclusions, impact toughness and microstructure in weld metals of high strength low alloy steels have been studied. Results indicated that microstructure had changed from a mixture of acicular ferrite, proeutectoid ferrite, ferrite side plates and microphases to a mixture of acicular ferrite, bainite and microphases due to the addition of Mn and Ti. The impact toughness of weld metal was improved correspondingly. The volume fraction and composition of inclusions both influenced the proportion of acicular ferrite. Mn and Si based oxide globular inclusions located at the boundary of acicular ferrite plates in the weld metal produced using C–Mn–Si–Cu wire. When Mn and Ti were added to welding wires, the inclusions within acicular ferrite plates permitted fewer primary acicular ferrite plates to grow into relatively larger dimensions. Secondary acicular ferrites nucleating on pre-existing ferrite plates refined microstructure effectively.     

Evolution of microstructure in laser clad Fe–Cr–Mn–C alloy

Abstract

The laser surface cladding technique was used to form in situ Fe–Cr–Mn–C alloys on AISI 1016 steel substrate. In this process, mixed powders containing Cr, Mn, and C in the weight ratio 10: 1 : 1 were delivered using a screw feed, gravity flow, carrier gas aided system into the melt pool generated by a 10 kW CO₂ laser. This technique produced an ultrafine microstructure in the clad alloy layer. The microstructure of the laser surface clad region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic transformation or retained austenite phase.

MST/356     

Effects of vanadium addition on nucleation and growth of pearlite in high carbon steel

Abstract

The influence of vanadium addition on the microstructure of high carbon steels has been investigated. A careful examination of the initial stages of austenite decomposi~ion has been made, using a range of high resolution metallographic techniques. It has been confirmed that vanadium addition results in the formation of grain boundary ferrite films, even in the eutectoid composition range. It is argued that this ferrite is the product of eutectoid transformation, and is not proeutectoid ferrite. This is because the first event is the nucleation of carbide particles along the grain boundaries. These carbides have been identified mainly as cementite. The presence of vanadium appears to change the morphology and distribution of the grain boundary cementite, so that rather than forming a grain boundary network, the cementite occurs in the form of a high density of small discrete particles along the boundaries. It is proposed that this occurs because vanadium increases the driving force for cementite nucleation. The formation of the grain boundary cementite depletes the surrounding region of carbon and encourages the formation of ferrite, but because of their discrete and fine dispersion, the cementite particles are engulfed by the more voluminous ferrite phase. In such regions, the onset of afully cooperative growth regime is delayed. Pearliteforms later at the ferrite/austenite interfaces.

MST/1923     

Microstructure evolution during thermomechanical processing in 3Mn-0.1C medium-Mn steel

ABSTRACT

Medium-Mn steels are energetically investigated as a candidate of the third generation advanced high strength steels (AHSSs). However, their phase transformation and microstructaure evolution during various heat treatments and thermomechanical processing are still unclear. The present study first confirmed the kinetics of static phase transformation behaviour in a 3Mn-0.1C medium-Mn steel. Hot compression tests were also carried out to investigate the influence of high-temperature deformation of austenite on subsequent microstructure evolution. It was found that static ferrite transformation was quite slow in this steel, but ferrite transformation was greatly accelerated by the hot deformation in austenite and ferrite two-phase regions. Characteristic dual-phase microstructures composed of martensite and fine-grained ferrite were obtained, which exhibited superior mechanical properties.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Factors influencing ferrite/pearlite banding and origin of large pearlite nodules in a hypoeutectoid plate steel

Abstract

The microstructure and distribution of alloying elements in a hot rolled, low alloy plate steel containing (wt-%) 0·15%C, 0·26%Si, l·49%Mn, and 0·03%Al were examined using light microscopy and electron probe microanalysis. Microstructural banding was caused by microchemical banding of manganese, where alternate bands of proeutectoid ferrite and pearlite were located in solute lean and solute rich regions, respectively. Bands were well defined for a cooling rate of 0·1 K s^?1, but banding was much less intense after cooling at 1 K s^?1. At a cooling rate of 0·1 K s^?1 and for austenite grains smaller than the microchemical band spacing, austenite decomposition occurred via the formation of ‘slabs’ of proeutectoid ferrite in manganese lean regions resulting in the growth of ferrite grains across austenite grain boundaries. Abnormally large austenite grains result in the formation of large, irregularly etching pearlite nodules which traversed several bands. In specimens cooled at 1 K s^?1, ferrite/pearlite banding did not exist in regions where austenite grains were two or more times larger than the microchemical band spacing.

MST/1397     

Application of Koistinen and Marburger’s athermal equation for volume fraction of martensite to diffusional transformations obtained on continuous cooling 0·13%C high strength low alloy steel

Abstract

Koistinen and Marburger’s (KM) equation for the variation of volume fraction of athermal martensite y with temperature Tbelow the M _s has been applied to continuous cooling diffusional data. The data consisted of dilatometer curves obtained on continuous cooling of a 0·13%C high strength low alloy steel. The KM equation takes the form, ln(1 - y) = α(M _s - T ). Plots of -ln (1 - y) against temperature for what are thought to be grain boundary ferrite, intragranular ferrite, proeutectoid ferrite, and pearlite give a series of straight lines of increasing slope α. Intersections of these lines give the temperature of transformation points in good agreement with those on the dilatometry curves. Values of α obtained for each transformation are compared with those previously obtained for martensite in plain carbon and alloy steels and ferrite in Fe–9%Ni.     

Formation of ultrafine grained ferrite during thermomechanical treatment in microalloyed steel

Abstract

In the present work, the formation of ultrafine grained ferrite has been studied by applying suitable thermomechanical treatment. A high amount of deformation (～80%) at varying strain rates (0·01–10 s^?1) was applied in the temperature range of Ar₃ to Ac₃ followed by water quenching. This treatment resulted in a two-phase ferrite–martensite microstructure as compared to fully martensite structure after quenching without deformation. The formation of ultrafine ferrite (?3 μm) during deformation was favourable at a lower temperature and a slower strain rate. A maximum ～50% ferrite formed during deformation at 780°C with a strain rate of 0·01 s^?1. Experimental rolling with a high strain (～1·3) with finish rolling temperature just above Ar₃ (～750°C) resulted in fine ferrite–pearlite of ?3 μm, and the properties showed a high value of strength as compared to steels rolled in a conventional way. Dual phase microstructure (ferrite and martensite) was produced after partial austenisation to 780°C followed by quenching in water, and this resulted in an excellent combination of properties (high ultimate tensile strength, low yield strength/ultimate tensile strength, high elongation and high n values).     

Influential microstructural changes on rolling contact fatigue crack initiation in pearlitic rail steels

Abstract

Rail life is controlled by the balance between wear and fatigue damage due to in service loading. To model and optimise rail life, knowledge of the fatigue crack initiation mechanism is required. This paper reports the effect of in service loading on microstructural changes in the subsurface layer of pearlitic rail steels and observations of early stage (10–50 μm length) fatigue crack formation. Micro and nanohardness measurements are reported, along with microstructural observations, showing differential work hardening in the proeutectoid ferrite and pearlite phases. It is proposed that the differential straining results in ductility exhaustion in the proeutectoid ferrite and therefore fatigue crack initiation and initial growth in the proeutectoid ferrite phase. Observations of short (<50 μm) cracks in rails taken out of service containing significant amounts of proeutectoid ferrite (≈20%) confirm the proposed mechanism.     

Pressure induced martensite transformation in plain carbon steel

Abstract

In the present study, plain low carbon steel with 0·033 wt-% carbon content was subjected to severe pressure during continuous cooling from austenite region. The pressure increased gradually and then suddenly released by the breakdown of ram under pressure. As a result, a microstructure composed of 80% lath martensite and 20% ferrite was produced. Results showed that the martensite formation is not due to the effect of cooling rate but the effect of hydrostatic pressure on the austenite to ferrite transformation start temperature Ar₃.     

Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 1 – Strip processing

Abstract

Simulations of industrial thermomechanical processing and on line accelerated cooling of a low carbon microalloyed strip steel were carried out using a quench deformation dilatometer. Effects of processing parameters, such as accelerated cooling rate T and accelerated cooling interrupt temperature T_I on the critical transformation temperatures and final microstructure were determined. The most important on line accelerated cooling (OLAC) processing parameter is the accelerated cooling interrupt temperature, which controls whether the transformed microstructure is predominantly ferrite or bainite. A variety of (Ti, Nb, Fe) carbide, nitride, and carbonitride precipitates are present in the OLAC processed samples. The final precipitate distribution is developed at three stages of processing, namely: dissolution and coarsening of pre-existing precipitates at the reheat temperature, precipitation in deformed austenite during the deformation schedule, and precipitation in ferrite after the interruption of accelerated cooling. Maximum precipitation strengthening occurs for T_I=700–640°C.

MST/3424     

Alloy optimisation of bainitic steel for large plastic mould

Based on the phase transformation theories, especially the T₀ concept of bainite transformation, alloy optimisation of bainitic steel with carbides has been carried out aiming at the produce of plastic mould with large cross-section. The effect of manganese and silicon on proeutectoid ferrite and bainite transformation is explored by dilatometric analysis, XRD and different microscopy techniques. The results show that after the alloy optimisation, the transformation of proeutectoid ferrite is suppressed and when the cooling rate is lower than 0·1°C?s^??1, the new lower bainite transformation appears by decreasing carbon capacity of austenite and promoting carbide precipitation. Industrial production proves that the optimised alloy SDP1 can meet the demand for the plastic mould with the thickness of 1050?mm.     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Acicular ferrite formation during hot plate rolling for pipeline steels

Abstract

The transformation of supercooled austenite in a commercial pipeline steel was investigated by means of continuous cooling transformation (CCT) and hot simulation experiments. Based on the obtained results, an improved thermomechanical control process (TMCP) was proposed, which could produce a mixed microstructure dominated by acicular ferrite. Results indicated that an increase in the cooling rate could improve the percentage of acicular ferrite in the final microstructure under the present experimental conditions. Furthermore, the acicular ferrite dominated microstructure could be obtained by a two stage controlled rolling in the austenite recrystallisation region plus the non-recrystallisation region and controlled cooling at a cooling rate of 30 K s^-1.     

Microstructural evolution during thermomechanical processing of microalloyed medium carbon steels

Abstract

A laboratory study was carried out to determine the characteristics of austenite grain growth and recrystallisation, strain induced precipitation, and continuous cooling transformation kinetics for two microalloyed medium carbon steels (1541 + Ti,V and 1541 + Nb). Austenite grain refinement is achieved by a combination of undissolved carbonitride precipitates at the reheat temperature, deformation recrystallisation at temperatures above T _NR and strain induced carbonitride precipitation. Deformation below T _NR promotes transformation to grain boundary ferrite (GBF), intragranular ferrite (IGF), and pearlite (P) at the expense of bainite (B) in both steels. This is attributed to increased density of nucleation sites for ferrite and pearlite at austenite grain boundaries, twin boundaries, and deformation bands. The results suggest that thermomechanical forging schedules could be designed to produce refined F + P microstructure, and hence, to realise improved strength, toughness, and machinability in the forging.     

Chemical driving force,shear moduli of ferrite and metastable austenite,and strain energy for γ→α transformation in pure iron

Abstract

The chemical driving force/unit volume ?G_v for the γ→α.transformation. in pure iron is computed as a function of undercooling. Similarly, the strain δ accompanying the transformation at different temperatures is also calculated from lattice parameter measurements. Shear moduli μ_α of ferrite and metastable austenite μ_γ are derived from various sources. At temperatures down to the Curie temperature (768°C), μ_α and μ_γ are very similar, and enable the strain energy/unit volume W to be calculated from the expression W=2 (μ_α+ μ_γ)δ². The data give the metastable equilibrium temperature T;amp;#x2032;₀ for the massive ferrite transformation as 1156 K. The distinction between equiaxed ferrite, massive ferrite, and Widmanstätten ferrite is discussed.

MST/3138