Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Isothermal transformations in advanced high strength steels below martensite start temperature

Abstract

The transformation products in advanced high strength steels have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Rapid cooling to various temperatures below martensite start was carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite, followed by isothermal holding. The transformation kinetics was monitored by means of dilatometry and microstructural characterisation by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below M_S are mainly composed of different fractions of tempered martensite, isothermal bainite with carbide precipitation and retained austenite.     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

Simple model for austenite grain growth in microalloyed steels

Abstract

Previous models for grain growth are usually based on Beck's formula, which are inadequate for quantitative prediction of austenite grain growth during reheating of as cast microstructures in microalloyed steels. The applications of these empirical grain growth models are limited to some particular categories of steels, such as Nb, Nb–Ti and Ti–V microalloyed steels, etc. In this study, a metallurgically based model has been developed to predict the austenite grain growth kinetics in microalloyed steels. This model accounts for the pinning force of second phase particles on grain boundary migration, in which the mean particle size with time and temperature is calculated on the basis of the Lifshitz–Slyozov–Wagner (LSW) particle coarsening theory. The volume fraction of precipitates is obtained according to the thermodynamic model. The reliability of the model is validated by the agreement between theoretical predictions and experimental measurements in the literature.     

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.     

Design of novel high strength bainitic steels: Part 1

Abstract

Mixed microstructures consisting of fine plates of upper bainitic ferrite separated by thin films of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series of bainitic alloys, given a set of industrial constraints. Part 2 of the study concerns the experimental verification of the design process.     

Effects of carbon content on mechanical properties of 5%Mn steels exhibiting transformation induced plasticity

Abstract

A series of highly ductile, high strength steels exhibiting transformation induced plasticity due to retained austenite was developed by varying the carbon content in the range 0·01–0·4 wt-% in 5 wt-%Mn based steel. For up to 0·l%C the mechanical properties are insensitive to cooling rate after intercritical heating, but afurther increase in carbon content causes a large sensitivity to the cooling rate, owing to carbide precipitation occurring during slow cooling. By suppressing this carbide precipitation with water quenching after the intercritical holding, an excellent combination of tensile strength (1580 MN m^?2) and uniform elongation (21%) was attained at 0·3%C in this series.

MST/1964     

Tensile deformation of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

Two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~400–250°C) have been deformed in tension. The bainitic microstructures in these steels contain appreciable amounts of retained austenite (instead of interlath cementite), and the behaviour of this phase during tensile deformation, and its apparent influence on the mechanical properties, has been examined. In particular, it is shown that provided the retained austenite exists in an interlath, thin-film morphology it has appreciable mechanical stability. Larger volumes of retained austenite have less mechanical and thermal stability, forming plate martensite structures and also undergoing deformation twinning. The effects of these variations on tensile strength and ductility are discussed.

MST/527     

Effect of thermomechanical processing on mechanical behavior and microstructure evolution of C–Mn multiphase high strength cold rolled steel

Multiphase (MP) steels have complex microstructures containing polygonal ferrite, martensite, bainite, carbide, and small amounts of retained austenite. This mixture of phases and constituents is responsible for a good combination of strength and ductility in this class of steels. The present work shows how different annealing parameters can be used to create the suitable microstructure to improve mechanical properties of MP steels. Samples were first heated to 740, 760, or 780 °C, held for 300 s, and then quickly cooled to 600 or 500 °C. They were then soaked for another 300 s and finally accelerated cooled in the range of 10–30 °C s⁻¹. The microstructures were examined at the end of each processing route using optical, scanning, and transmission electron microscopy. Hardness values were determined for all conditions. Analysis of the available data allowed to establish the simple and yet useful quantitative relationship between the microstructural parameters, cooling rates, and hardness of the steel.     

Relative importance of transformation temperatures and sulphur content on hot ductility of steels

Abstract

Low (0·3%) and high manganese (1·4%) plain C – Mn steels with varying sulphur levels have had their hot ductility determined over the temperature range 700 – 1000°C, both after 'solution treatment' at 1330°C and directly after casting. It has been established that the width, depth and position of the hot ductility curves after solution treatment is more related to the transformation behaviour than either the sulphur in solution or the sulphide volume fraction or distribution. The growth of deformation induced ferrite at the austenite boundaries seems to be mainly diffusion controlled, and the higher is the transformation temperature for the γ – α phase change, the faster is the growth. Large amounts of ferrite can then form, giving good ductility. Thus, high transformation temperatures Ae ₃ or Ar ₃ are required to produce narrow ductility troughs. It is believed that any detrimental influence of the sulphides on these 'solution treated' steels is swamped by the rapid increase in ferrite volume fraction. For the as cast state, as more sulphides are able to precipitate at the interdendritic boundaries and austenite grain boundaries than in the solution treated condition, increasing the sulphur level causes a small deterioration in ductility at the high temperature end of the trough. In the present work, only narrow troughs have been found. This is in contrast to previous work on as cast C – Mn – Nb – Al steels, which exhibited wide troughs in the ductility curves, where it was shown that higher total sulphur levels lead to considerably worse ductility and that sulphur can be as detrimental to the ductility as niobium. It is recommended that, to avoid transverse cracking during continuous casting, in addition to keeping the sulphur level low, the carbon and manganese should also be as low as possible.     

Fracture toughness of steels with nickel content in respect of carbide morphology

Abstract

This paper is focused on the influence of Ni addition on the microstructure and fracture toughness of structural steels after tempering. Nickel is known to increase the resistance to cleavage fracture of steel and decrease a ductile–brittle transition temperature. The medium carbon, low alloy martensitic steels attain the best combination of properties in low tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. In the present research, four model alloys of different Ni contents (from 0·35 to 4·00%) were used. All samples were in as quenched and tempered condition. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200°C for 2?h. An increase in nickel content in the investigated model structural steels causes a decrease in ε carbide volume fraction in their microstructure. Cementite nucleates independently in the boundaries of martensite laths and in the twin boundaries in the areas where the ε carbide has been dissolved. It was stated that stress intensity factor K_Ic significantly decreases in the case of the presence of dispersive elongated cementite precipitations at the boundaries of the prior austenite grains.     

Differential scanning calorimetry study of diffusional and martensitic phase transformations in some 9 wt-%Cr low carbon ferritic steels

Abstract

The results of a comprehensive characterisation study of different phase transformations that take place upon heating and cooling in some low carbon, 9 wt-%Cr steels with varying concentrations of microalloying additions are presented in this paper. The steels investigated include: standard 9Cr–1Mo grade, V and Nb added modified 9Cr variety, controlled silicon added versions of plain 9Cr variety, (Ni+Mn) content controlled modified 9Cr welding consumables and one composition of W, Ta added reduced activation steel. The various on-heating diffusional phase changes up to the melting range and subsequent rapid cooling induced martensitic transformations are investigated in a controlled manner using differential scanning calorimetry under different heating and cooling rates, in the range 1–100 K min^?1. In addition to the accurate determination of Ac₁, Ac₃, M₂₃C₆, MX carbide dissolution and δ-ferrite formation temperatures upon heating, the melting range and the associated fusion enthalpy have also been established for these steels. The effect of prolonged thermal aging at temperatures of 823–873 K on austenite formation characteristics has also been investigated for standard and modified 9Cr–1Mo steels. The critical cooling rate for the formation of martensite on cooling from single phase austenite region is estimated to be about 4–5 K min^?1 for all 9Cr steels investigated in this study. The effect of holding at 1273 K in the austenite region on martensite start temperature M_s, has also been evaluated as a part of this study. The experimental results are discussed in the light of the prevailing understanding of the physical metallurgy of high chromium low carbon steels.     

Corrosion fatigue characterisation of sintered multiphase stainless steels

Abstract

Multiphase stainless steels are produced for their attractive properties of mechanical strength and corrosion resistance relative to their austenite–ferrite structure. The manufacture of these steel by powder metallurgy technology presents some advantages in terms of low cost and formability of complex shapes. Mechanical and corrosion resistances are not at the level of the wrought steels due to their porous nature. In this work the fatigue and corrosion fatigue behaviour of some sintered steels obtained by sintering from 316L and 434L base powders has been studied for characterisation and comparison. The sintered steels were fatigue tested in two different environments: air and NaCl aqueous solution. The tests performed indicate that the chemical and microstructral composition has no great influence on fatigue behaviour in comparison with the manufacturing technology (sintering). This is most evident in the more aggressive environment, like seawater, in which these steels could be advantageously used. The analysis of fracture surfaces using SEM microscopy shows a peculiar crack propagation characterised by cleavage, stress intensification due to porosity, and features of localised ductility on sintering necks and base powder particles.     

Sliding wear and low cycle fatigue properties of new carbide free bainitic rail steel

Abstract

A new carbide free bainitic rail steel was prepared, whose comprehensive mechanical properties are equal or superior to current premium pearlitic and bainitic rail steels. The new bainitic rail steel possesses better low cycle fatigue properties and approximate resistance to wear compared with current pearlitic rail steel. The carbon enriched film-like austenite between the ferrite of the new bainitic rail steel can delay crack initiation and propagation in fatigue processes, resulting in a relatively high low cycle fatigue life (about two times) compared to the pearlitic rail steel. Finally, a wear model during sliding wear and deformation model during the low cycle fatigue of pearlitic and bainitic rail steels were established. As a result, the bainitic rail steel with a relatively reasonable combination of wear and fatigue properties compared to pearlitic rail steel is obtained, in which the initiation and propagation of cracks may be partially or entirely removed during the wear process.     

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.     

Evaluation of rate constants in tempering reaction of hardened steels from dilatometric data on continuous heating at controlled rates

Abstract

A new method of analysis has been suggested whereby the characteristic constants of the rate equation in the second and third stages of the tempering reaction of hardened steels may be evaluated rapidly from the dilatometric data obtained during continuous heating at a controlled rate. Using the new method, the kinetic order, activation energy, and frequency factor of the second and third stages in the tempering reaction can be obtained from a plot of the length fraction of the reactant remaining on continuous heating versus the temperature. The tempering kinetics can thereby be investigated continuously over an entire temperature range. In the new method only one pair of test specimens containing different but known quantities of retained austenite is required; decomposition to a time scale is not required. Thus, the method can serve as means of comparing rapidly the thermal stability of the decomposition of martensite and retained austenite in various hardened steels using fewer test specimens.

MST/554     

A Study of Microstructures and Properties of High‐Carbon Si‐Cr Cast Steel Containing Rare Earth and Titanium

The microstructure, tensile and impact behaviour of high‐carbon Si‐Cr cast steel containing rare earth (RE) and titanium have been determined after austempering. The additions of RE and titanium refined the primary austenite grain size resulting in improving toughness. The addition of silicon handicapped the formation of carbide and carbide‐free bainitic ferrite and carbon enriched retained austenite could be obtained in the austempering structures of high‐carbon Si‐Cr cast steel, which had excellent mechanical properties and abrasion resistance. Moreover, the basic tendency of the mechanical properties of high‐carbon Si‐Cr cast steel influenced by the austempering temperature was that the hardness and tensile strength reduced and the impact toughness and fracture toughness increased with increasing temperature. The comprehensive properties were the best while austempering at 330^oC.     

Study on carbide-bearing and carbide-free bainitic steels and their wear resistance

Carbide-free and carbide-bearing bainitic steels have been obtained. The relationship between the bainitic microstructure and wear resistance has been studied. Results show that carbide-free upper and lower bainitic microstructures obtained in the steel with Si?+?Al mainly consist of bainitic ferrite and retained austenite. Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si?+?Al consist of bainitic ferrite, carbide and trace amounts of retained austenite. The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite, especially the toughness. Under lower wear loading, carbide-bearing lower bainite (LB) exhibits higher wear resistance. Under higher wear loading, carbide-free LB exhibits higher wear resistance, which results from the improved surface hardness due to strain-induced martensitic transformation from the retained austenite.     

Evaluation of changes in X-ray elastic constants and residual stress as a function of cold rolling of austenitic steels

Abstract

Austenitic steels rapidly attain high mechanical strength when subjected to cold working. The heterogeneous plastic deformation produced in cross section of the specimen, development of preferred orientation and martensitic transformation contribute to the occurrence of residual stress in cold worked steels. AISI 304 and 316 steels were cold rolled at room temperature from 10% up to 70% deformations in steps of 10%. The formation and sigmoidal growth of martensite caused by cold rolling (CR) 304 steel was studied by X-ray diffraction. The residual stresses generated were evaluated in both the austenite and martensite phases using sin² ψ technique. The accurate determination of residual stress by X-ray diffraction requires experimental determination of X-ray elastic constants for both the austenite and martensite phases. The changes in X-ray elastic constants as a function of CR of 304 and 316 steels were measured and their effect on residual stress values was established. The results show that tensile stress was generated initially on cold working in the austenite phase in both steels and in the dominant martensite phase in 304 steel, which decreases, passes through zero and becomes compressive at higher deformations. X-ray elastic constants were found to decrease in all cases and a maximum reduction of 15% was found.     

Intensive Interstitial Strengthening of Stainless Steels

The high chromium content of stainless steel impairs the interstitial solubility of carbon in austenite at solution annealing or hardening temperature. Replacing carbon by nitrogen improves the interstitial solubility which is raised most, if carbon and nitrogen are added jointly. Respective thermodynamic equilibrium calculations have led to a new group of austenitic and martensitic steels as well as to a thermochemical surface treatment, that make use of the C + N concept to intensively strengthen stainless steels with about 0.5 to 1 mass% of these interstitials. Pressurized electro slag remelting (PESR) is required to raise the N content in the melt and to avoid degassing during solidification of martensitic stainless steels with C + N known as CRONIDUR®. For austenitic CrMn steels the C + N concept affords to dissolve about 1 mass% of these elements in the melt at atmospheric pressure and keep them in solid solution during solidification of the new CARNIT® steels. Case hardening of stainless martensitic steels with nitrogen instead of carbon, called SolNit®, combines the effect of C in the steel and N dissolved in a surface zone via heat treatment in N₂ gas of controlled pressure. The key properties and respective microstructures of these three versions of the C + N concept are discussed along with some applications.