Thermally activated growth of lath martensite in Fe–Cr–Ni–Al stainless steel

The austenite to martensite transformation in a semi-austenitic stainless steel containing 17 wt-%Cr, 7 wt-%Ni and 1 wt-%Al was investigated with vibrating sample magnetometry and electron backscatter diffraction. Magnetometry demonstrated that, within experimental accuracy, martensite formation can be suppressed on fast cooling to 77 K as well as on subsequent fast heating to 373 K. Surprisingly, martensite formation was observed during moderate heating from 77 K, instead. Electron backscatter diffraction demonstrated that the morphology of martensite is lath type. The kinetics of the transformation is interpreted in terms of athermal nucleation of lath martensite followed by thermally activated growth. It is anticipated that substantial autocatalytic martensite formation occurs during thermally activated growth. The observation of a retardation of the transformation followed by a new acceleration during slow isochronal (i.e. at constant rate) cooling is interpreted in terms of the combined effect of the strain energy introduced in the system during martensite formation, which thermodynamically and/or mechanically stabilises austenite, and autocatalytic nucleation of martensite.     

Magnetic measurement of strain-induced martensite formation in Fe–30Ni steel

This investigation focuses on deformation-induced plasticity in Invar-type steel alloys. The effect is being studied in an austenitic model alloy, containing 30 wt-% of nickel. Its temperature dependent mechanical properties are being presented. Furthermore, the martensitic phase content has been determined by magnetic means in an alloy with two ferromagnetic phases for the first time. The results show that the α′-martensite formation within the austenitic phase with primarily wavy glide mechanism allows an increase in ductility of around 10% at the M_sσ temperature of ?5°C. This is the point of maximum uniform elongation. Near the M_s temperature, a microstructure of 70 vol.-% deformation-induced α′-martensite can be achieved.     

Effect of strain rate on microstructures and mechanical properties of Fe–18Cr–8Ni steel

The effect of strain rate on deformation microstructures and mechanical properties of Fe–18Cr–8Ni austenitic stainless steel was investigated at strain rates of from 10^?3 to 100?s^?1. The results indicated that the deformation mechanism of steel changes from transformation induced plasticity (TRIP) to TRIP?+?twinning induced plasticity (TWIP) effect when the strain rate is increased from 10^?3 to 100?s^?1. The yield strength of steel increases gradually with strain rate increased, while the tensile strength and elongation first decreases and then increases slowly. The changes in tensile strength and elongation are due to the change of deformation mechanism with the strain rate increased.     

Crack resistance characteristics study results for 12Cr–2Ni–Mo steel

The effects of temperature, holding time and profile, as well as of biaxiality ratio on crack propagation rate in 12Cr–2Ni–Mo steel have been studied under cyclic loading conditions. It is shown that all the results obtained fall into the tolerance interval with 90% confidence probability.     

Impurity effects in stress relief cracking of a Mn–Cr–Mo–Ni steel

Abstract

The energetic aspects of the classical (capillarity) theory of homogeneous precipitation kinetics, as proposed by Russell, have been tested using a method owing to Kashchiev and the extensive set of data on Cu—Co alloys published by LeGoues and Aaronson. It was concluded that heterogeneous nucleation occurred in at least two of the three alloys studied by the latter authors. The values of both incubation time and maximum cluster density are compared with theoretical predictions and are found to agree reasonably well. More detailed research is needed, particularly on the subject of the incubation time.     

Self-accommodation in polycrystalline 10M Ni–Mn–Ga martensite

10M Ni–Mn–Ga polycrystals show a typical self-accommodated microstructure consisting of macro and micro twins. The macro twin lamellae separate micro twins creating a so-called “twins within twins” microstructure. Such a configuration allows the distribution of martensitic variants with no net change in shape of the sample. The arrangement of variants can occur on different length scales, from a few nanometers up to a few millimeters, not only depending on grain size but also on processing condition (e.g., extrusion, torsion). Small austenite grains do not completely transform to martensite giving rise to some residual austenite. Furthermore, characteristic branching of macro and micro twins is observed due to lowering of the elastic energy at grain and macro twin boundaries, respectively.     

Dynamic recrystallisation of as cast austenite in 18–8 stainless steel

Abstract

Dynamic recrystallisation behaviour of an as cast 0Cr18Ni9Ti stainless steel during hot deformation was investigated by hot compression test at a temperature range of 950–1200°C and strain rate of 5 × 10^-3–1 × 10^-1 s^-1. Change of austenite grain size owing to dynamic recrystallisation was also studied by microstructural observation. The experimental results showed that the hot deformation conditions, such as temperature, strain, and strain rate determine the dynamic recrystallisation behaviour for the as cast stainless steel, and the dynamically recrystallised grain size is determined by the deformation conditions and is independent of the strain.     

Microstructure–property relationships in tempered low alloy Cr–Mo–3·5Ni–V steel

Abstract

The effect of varying normalising and hardening temperatures, before tempering at ~620°C, on the strength and toughness of a low alloy Cr–Mo–3·5Ni–V (wt-%) steel has been examined. Microstructural features including martensite packet and lath size, dislocation density, and precipitate size were measured and used in a Hall–Petch analysis of the strengthening components. It was found that a rms summation of the strengthening contributions to the 0·2% proof stress gave values in good agreement with experimental results. The 50% fracture appearance transition temperature could be described by a relationship involving the fracture facet size and the strengthening contributions from dislocations and precipitates.

MST/1802     

Mechanical properties of sinter-hardened Cr–Si–Ni–Mo based steel foam

Highly porous sinter-hardenable Cr–Si–Ni–Mo based steel foam for automotive applications was produced by space holder method. Steel powders were mixed with binder (polyvinylalcohol) and space holder (carbamide), and compacted. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were then sintered at temperatures between 1100 °C and 1250 °C for sintering times of 15, 30 and 45 min. In addition, the steel foams were sinter-hardened to enhance mechanical properties. Sinter-hardening combines sintering and heat treatment in one step by increasing the post-sintering cooling rate. This reduces the cost of operation and makes powder metallurgy more competitive. Effects of sinter-hardening process parameters on compressive strength, Young’s modulus, hardness and energy absorption of the steel foams were investigated.     

Deformation induced martensite characteristics in Fe–29Ni–2Mn alloy

Abstract

Deformation induced martensite characteristics in the austenite phase of Fe–29Ni–2Mn alloy were studied for different austenite grain sizes of alloy. Scanning electron microscopy, transmission electron microscopy, Mössbauer spectroscopy and also differential scanning calorimetry techniques were applied to study in order to clarify the deformation induced martensite characteristics from morphological, crystallographical, magnetical and thermal points of view. Scanning electron microscope revealed that the increasing of deformation amount also increased the amount of existed martensite. Transmission electron microscope observations showed that the crystal structure of these deformation induced martensites morphology was lenticular plates with a bcc crystal structure. Also the magnetism of both austenite and martensite phases were determined with Mössbauer spectroscopy. Mössbauer spectrometer measurements showed paramagnetic character for austenite phases and ferromagnetic character for martensite phases in all samples. According to obtained differential scanning calorimetry cooling curves, deformation induced martensite start temperature M _d was found to be higher (?128°C) for larger grained samples than for smaller grained samples (?135°C).     

Dependence of accelerated creep rate on carbide particle distribution in martensite for 0·18C–11·5Cr–0·29V steel

Abstract

The number of carbide particles stringers at boundaries and subboundaries of martensite per unit of surface decreases with the third root of tempering time at 1073 K. The accelerated creep rate at 853 K depends on the number of stringers of carbide particles per unit of surface, and below an inflection point, the creep rate increases strongly. Equations relating for creep rate are discussed in terms of experimental creep rate, number of carbide particles stringers and average particles spacing.     

Carbide stabilities in a Cr–Mo–V bolting steel

Abstract

An electron metallographic examination has been performed on samples of a 1Cr–1Mo–0·75V (Ti,B) bolting steel (Durehete 1055) as part of a long term creep programme. The apparently anomalous carbide composition results have been rationalised by the use of pseudoternary phase diagrams. This novel method of presenting the alloy carbide composition data enables the changes accompanying service exposure to be described. It is obvious that the conventional thermodynamic order of stabilities does not obtain over the entire range of temperatures experienced by this alloy.

MST/914     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 1 – Microstructural analysis

Abstract

Specimens of 16Cr - 5Ni - 1Mo stainless steel were solution treated at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy, and X-ray diffraction examinations were conducted. The microstructure of all aged specimens was found to consist of martensite with variable fractions of δ ferrite and reversed austenite. Very fine precipitates of Mo carbides were revealed in the specimens aged at 475 ° C. The specimens aged at 625 ° C showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe₂Mo Laves phase was detected by X-ray analysis. Above 625 ° C, Cr₂₃C₆ and TiC became the predominate carbides heterogeneously precipitated in the martensitic matrix. Partial transformation of reversed austenite to unaged martensite was observed at temperatures above 625 ° C.     

Relation between microstructures of martensite and prior austenite in 12 wt% Cr ferritic steel

Tempered martensitic 9–12 wt% Cr ferritic steels are used as heat resistant materials in power plant, where service under conditions of high temperature and pressure for several decades is required, and an adequate resistance to creep is one of the key requirements. In this type of steels, failure has been found to occur preferentially at prior austenite grain boundaries if the prior austenite grains are coarse. It appears that the prior austenite grain boundaries can act as a site of especial weakness in the tempered martensitic microstructure. It would therefore be useful to investigate whether the properties of prior austenite grain boundaries could be modified by some appropriate thermomechanical processing method. One approach to this is to attempt to increase the fraction of annealing twins in the austenite phase and to investigate whether this has an effect on the properties of the martensite after transformation and tempering. In this study, thermomechanical treatments involving hot-rolling have been applied and the fraction of austenite twins produced determined using electron backscatter diffraction analysis. The treatment giving the highest fraction of austenite twins was identified and the effect of the increase in twin fraction on the characteristics of the martensite was investigated. It was found that the fraction of coincidence site lattice boundaries in martensite along prior austenite grain boundaries increased with increasing fraction of prior austenite twin boundaries.     

Oxide/metal interface on 20Cr–25Ni–Nb stainless steel

Abstract

20Cr–25Ni–Nb stabilised stainless steel is used to contain the fuel in the advanced gas cooled reactor. During operation, this steel must withstand temperatures from 600 to 1073 K in CO₂ gas at 40 atm pressure. It is important that the oxide which forms on this steel is thoroughly characterised and the adherence of the oxide to the metal is understood. A technique of sputter ion plating has been used to remove the oxide from the metal without destroying either metal or oxide. This involves plating the oxide with nickel or molybdenum at a temperature of 600 K, while sputtering the surface with argon ions. On cooling, stresses set up between the oxide and the metal cause the oxide plus sputtered layer to peel off allowing both the metal and oxide sides of the interface to be examined. Results are presented from studies of the metal/oxide interface using scanning Auger microscopy. Analysis of grain centres and grain boundaries indicates that silicon and chromium play an important role in oxide/metal adhesion and, together with conventional analysis of the bulk oxide, assist in determining the oxidation mechanism.

MST/862     

Serrated yielding in 9Cr–1Mo ferritic steel

Abstract

Tensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300–873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr–1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load–elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol^-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr–1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations indicate reduced propensity to DSA as a result of increased precipitate sinks as well as a reduced carbon concentration in solid solution owing to an increased density of carbides in the thermally aged conditions. Reduced propensity to DSA resulted in a significant reduction in the strength values at intermediate temperatures.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 2 – Mechanical property characterisation

Abstract

Mechanical property characterisation has been carried out on specimens of 16Cr - 5Ni - 1Mo stainless steel, subjected to various aging cycles. The heat treatment cycles involved solution treatment at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, tensile, hardness, impact, and creep tests were conducted. Specimens aged at 475 ° C exhibited maximum values of tensile strength and hardness with minimum values of ductility and impact toughness, while specimens aged at 625 ° C had maximum values of impact toughness and ductility. The results were correlated with the microstructural data presented in Part 1 of this study. Softening of the martensitic matrix at 625 ° C occurs as a result of the elimination of internal stresses, the decrease in the dislocation density, and the high volume fraction of austenite which lead to the drop in values of tensile strength and hardness. The results of the study reveal that aging at 550 ° C for 4 h gives the optimum combination of strength, hardness, ductility and toughness for this steel.     

Wet thermo-mechanical behavior of steel–CFRP joints – An experimental study

The long term durability of CFRP strengthened steel structures is a key parameter for their safe use and effective design. Strengthened members can be subjected to different environmental conditions and loading scenarios during their service life, the effect of which on the failure mechanism of the strengthened member requires fundamental investigations. This paper presents an experimental investigation into the effects of wet thermo-mechanical loading on the bond strength and the failure mode of steel–CFRP single lap joints. A total of thirty four steel–CFRP single lap shear specimens were prepared and exposed to different combinations of wet thermal cycle ranges and sustained loads. The results show that these conditions (wet thermal cycles and sustained loads) have little impact on the bond strength of steel–CFRP lap joint when applied separately. However, when applied simultaneously, the bond strength of the joint is significantly reduced with failure observed at less than 30% of the static strength under temperatures that are well below the glass transition temperature of the adhesive.     

Using direct hot-rolling approach to obtain dual-phase weathering steel Cu–P–Cr–Ni–Mo

A weathering steel Cu–P–Cr–Ni–Mo has been developed which exhibits special continuous cooling transformation characteristics which permit the desired dual-phase (DP) microstructure to be obtained by direct hot-rolling. Hot-rolling procedures to obtain DP microstructures have been designed based on the continuous cooling transformation diagram of weathering steel Cu–P–Cr–Ni–Mo. The results show that the microstructures of DP weathering steels Cu–P–Cr–Ni–Mo are characterized by an irregular distribution of island-shaped martensite–austenite in the matrix of polygonal ferrite grains. DP weathering steel Cu–P–Cr–Ni–Mo with favorable corrosion resistant property, weldability and mechanical properties, such as, high strain hardening exponent values, a lower ratio of yield to tensile strength, and higher strengths; and is obtained successfully by direct hot-rolling.     

Transition from upper to lower bainite in Fe–C–Cr steel

Abstract

An analytical evaluation of transition temperature from upper to lower bainite in Fe – 0·38C – 0·93Cr (wt-%) steel was carried out. Calculations were based on the model constructed by Takahashi and Bhadeshia, which involves a comparison between the time t_θ needed to precipitate cementite within the bainitic ferrite plates with the time t_θ required to decarburise supersaturated ferrite plates. It was found that the distribution of lath widths, shown by histograms, of the bainitic ferrite varies with isothermal transformation temperatures and holding times. The transition between upper and lower bainite is found to occur over a narrow range of temperatures (350 – 410°C) and depends on the thickness of bainitic ferrite laths and the volume fraction of precipitated cementite. On comparing t _d and t_θ it was found that a transition temperature from upper to lower bainite reaction L _S of about 350°C could be predicted if the thickness of bainitic ferrite laths is set as w _o = 0·1 μm and the volume fraction of cementite set as ξ = 0·01. Calculated differences in the relative behaviour of t _d and t_θ revealed the occurrence of upper and lower bainite in steel Fe – 0·38C – 0·93Cr consistent with the results of transmission electron microscopy investigation.