Austenite grain coarsening in low–carbon manganese steels containing niobium and aluminium

Abstract

The effect of microadditions of niobium, aluminium, and a combination of the two on the austenite grain coarsening behaviour of 0·08C–0·25Si–1·5Mn steels whose nitrogen content was varied/from 0·002 to 0·02 wt-% has been studied. Modified methods of surface oxidation and thermal grooving were used in order to determine the precise prior austenite grain size in these steels with low interstitial elements. The optimum concentration of niobium which could be effectively used for restraining austenite grain growth when steels are austenitized at high temperatures has been determined. It is also demonstrated that steels containing. NbC or NbN particles exhibit similar austenite grain coarsening behaviour. Increasing the nitrogen content from 0·002 to 0·02 wt-% to give nitrogen–rich Nb(CN) particles does not help restrain austenite grain growth. However, when nitrogen was added to steels containing aluminium or aluminium and niobium, significant improvement in grain restraining behaviour was observed.

MST/120     

Three-dimensional morphology of grain boundary Widmanstätten ferrite in a low carbon low alloy steel

The three-dimensional morphology of austenite grain boundary Widmanstätten ferrite in a low carbon low alloy steel was revealed by means of serial sectioning in conjunction with computer-aided three-dimensional reconstruction techniques. Primary and secondary Widmanstätten sideplates are better described to be wedges instead of plates. Primary Widmanstätten sawteeth appear to be spikes or triangular cones whereas secondary Widmanstätten sawteeth have the shape of elongated spikes or triangular cones. These results indicate that the three-dimensional shape and connectivity of grain boundary Widmanstätten ferrite are different from those deduced on random planar sections by two-dimensional microscopy.     

Correlation of microstructures and low temperature toughness in low carbon Mn–Mo–Nb pipeline steel

Abstract

By adjusting thermomechanical controlled processing parameters, different microstructures were obtained in a low carbon Mn–Mo–Nb pipeline steel. The microstructural characteristic and its effect on low temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallisation region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite microstructure with coarse M/A islands. The fine effective grain size (EGS) and the high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M–A/ferrite matrix interfaces. Compared with the microstructure mainly consisting of quasi-polygonal ferrite, the microstructure dominated by AF exhibits excellent low temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.     

On the failure of low carbon steel resistance spot welds in quasi-static tensile–shear loading

Failure behavior of low carbon steel resistance spot welds in quasi-static tensile–shear test is investigated. Microstructure, hardness profile and mechanical performance of the spot welds were studied. Results showed that spot welds are failed in two distinct failure modes: double-pullout and interfacial failure modes. There is a critical fusion zone size beyond which, pullout failure mode is guaranteed. Metallographic examination showed that failure is a competitive process between shear plastic deformation of weld nugget and necking of the base metal. In pullout failure mode, only the grain pattern of the base metal changes significantly and that of the fusion zone and heat affected zone remains unchanged. Strain localization was occurred in the base metal due to its low hardness. Moreover, the experimental results showed that increasing the holding time which increases the hardness of the fusion zone did not affect the peak load. It was concluded that in the pullout failure mode, the strength of the spot welds is not affected by the fusion zone strength. Fusion zone size proved to be the most important controlling factor for the spot welds’ mechanical performance in terms of peak load and energy absorption.     

Improvement of mechanical properties in a dual-phase ferrite–martensite AISI4140 steel under tough-strong ferrite formation

This paper has been concerned to investigate in details the mechanical properties of AISI4140 heat treatable steel under ferrite–martensite dual-phase (DP) microstructures in conjunction with that of conventional quench-tempered (CQT) full martensitic condition. For this purpose, a wide variety of ferrite–martensite DP samples containing different volume fractions of ferrite and martensite microphases have been developed using step quenching heat treatment processes at 600 °C for 20–55 s holding times with the subsequent hot oil quenching after being austenitized at 860 °C for 60 min in the same situation as to the CQT condition. The finalized tempering heat treatment has been carried out at 600 °C for 30 min for both of direct quenched full martensitic and DP samples in order to optimize the strength–ductility combination. Light and electron microscopes have been used in conjunction with mechanical tests to assess the structure–property relationships in the DP and CQT heat treated samples. The experimental results indicate that the DP microstructures consisting about 7% volume fraction of fine grain boundary ferrite in the vicinity of martensite are associated with excellent mechanical properties in comparison to that of CQT condition. These observations are rationalized in terms of higher carbon concentration of the remaining metastable austenite leading to the harder martensite formation on the subsequent hot oil quenching, and so developing much harder ferrite grains as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation in the remaining austenite adjacent to the ferrite area. The higher martensite volume fraction in the vicinity of thin continuous grain boundary ferrite network has been associated with the harder ferrite formation, causing higher work hardening behavior in the short time treated DP samples. Moreover, it has been found that in order to optimize the mechanical properties of ferrite–martensite DP samples, two independently parameters should be simultaneously controlled: one is the ferrite volume fraction and the other is ferrite morphology.     

Explosive welding of stainless steel–carbon steel coaxial pipes

Bi-metallic corrosion resistant steel pipes were produced through explosive welding process. The weldability window of the stainless steel pipe (inner pipe) and the carbon steel pipe (outer pipe) was determined by the use of available semi-empirical relations. The impact velocity of the pipes as the most important collision parameter was calculated by the finite element simulation. Direct effect of the explosive mass reduction on the bonding interface of the pipes was studied. Optical microscopy study showed that a transition from a wavy interface to a smooth one occurs with decrease in explosive load.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

Brittle intergranular fracture at elevated temperatures in low–alloy steel

Abstract

Recent studies of stress-relief cracking in low-alloy steels have focused attention on a novel mode of brittle intergranular fracture which occurs at elevated temperatures (300–650°C) in hard, coarse–grained heat–affected–zone microstructures. Fracture initiates at stress concentrators such as sharp cracks or inclusions, and can propagate under static loading at rates of 10^?11?10^?5 ms^?1 to produce intergranular facets with very little associated plastic deformation. The stress-intensity parameter K has been used to characterize crack growth, and three regimes of behaviour have been observed: (i) a threshold region at growth rates of 10^?11?10^?10 m S^?1, (ii)a plateau region, in which growth rates are independent of K between 10^?10 and 10^?8 m S^?1, and (iii) a region of highly K-sensitive crack growth between 10^?8 and ^?5 m S^?1. Independent Auger electron spectroscopy analyses have demonstrated that sulphur segregates locally to the high-temperature crack tip, giving rise to the embrittlement of a limited area of grain boundary. Together with other presegregated solutes, this enables brittle fracture to occur at high temperature, and the transfer of sulphur to the crack tip controls the rate of crack growth. Two models describing crack-tip sulphur segregation are currently proposed. In the first model, a quantitative analysis demonstrates that the crack-tip stress field will drive undersize solute atoms such as sulphur to the physical crack tip. In the second, the intergranular crack is modelled as a sharp cavity. Grain-boundary sulphides which are exposed by cavity formation become unstable and dissolve, saturating the cavity surface with sulphur, which is then drawn into the tip as part of the cavity growth process.

MSTj77     

Stabilisation of ferrite in hot rolled δ-TRIP steel

Abstract

A steel has recently been designed to benefit from the deformation induced transformation of retained austenite present in association with bainitic ferrite. It has as its major microstructural component, dendrites of δ-ferrite introduced during solidification. The δ-ferrite replaces the allotriomorphic ferrite present in conventional alloys of this kind. The authors examine here the stability of this δ-ferrite during heating into a temperature range typical of hot rolling conditions. It is found that contrary to expectations from calculated phase diagrams, the steel becomes fully austenitic under these conditions and that a better balance of ferrite promoting solutes is necessary in order to stabilise the dendritic structure. New alloys are designed for this purpose and are found suitable for hot rolling in the two-phase field over the temperature range 900–1200°C.     

Stress induced transformation to bainite in Fe–Cr–Mo–C pressure vessel steel

Abstract

The kinetics of the bainitic transformation in a polycrystalline Fe–Cr–Mo–C alloy designed for applications in energy generation systems has been studied, with particular attention to the influence of mild tensile stresses on transformation behaviour. The steel was found to exhibit the incomplete reaction phenomenon, in which transformation to bainite stops well before the residual austenite acquires its paraequilibrium carbon concentration. It was found that even in the absence of an applied stress, the growth of bainitic ferrite caused anisotropic changes in specimen dimensions, consistent with the existence of crystallographic texture in its austenitic condition and, significantly, with the nature of the invariant-plane strain shape change that accompanies the growth of bainitic ferrite. Thus, transformation induced plasticity could be detected in fine grained polycrystalline samples, even in the absence of applied stress. The application of an external stress was found to alter radically the transformation behaviour, with clear evidence that the stress tends to favour the development of certain crystallographic variants of bainite, even though the stress may be well below the single phase yield strength. It is concluded that the transformation is influenced significantly by stresses as low as 45 MN m^?2, even though the effect may not be obvious in metallographic studies. The results are analysed and discussed in terms of the mechanism of the bainite transformation.

MST/1394     

Prediction of low velocity impact damage in carbon–epoxy laminates

It is well known that composite laminates are easily damaged by low velocity impact. This event causes internal delaminations that can drastically reduce the compressive strength of laminates. In this study, numerical and experimental analyses for predicting the damage in carbon–epoxy laminates, subjected to low velocity impact, were performed. Two different laminates (0₄,90₄)_s and (0₂,±45₂,90₂)_s were tested using a drop weight testing machine. Damage characterisation was carried out using X-rays radiography and the deply technique. The developed numerical model is based on a special shell finite element that guarantees interlaminar shear stresses continuity between different oriented layers, which was considered fundamental to predict delaminations. In order to predict the occurrence of matrix failure and the delaminated areas, a new failure criterion based on experimental observations and on other developed criteria, is included. A good agreement between experimental and numerical analysis for shape and orientation of delaminations was obtained. For delaminated areas, reasonable agreement was obtained.     

Kinetics and dilatometric behaviour of non-isothermal ferrite–austenite transformation

Abstract

A model that describes the ferrite–austenite transformation during continuous heating in Armco iron and three very low carbon, low manganese steels with a fully ferritic initial microstructure is presented. This model allows calculation of the volume fractions of austenite and ferrite during transformation as a function of temperature, and hence knowledge of the austenite formation kinetics under non-isothermal conditions in fully ferritic steels. Moreover, since dilatometric analysis is a technique very often used to study phase transformations in steels, a second model, which describes the dilatometric behaviour of the material and calculates the relative change in length that occurs during the ferrite–austenite transformation, has also been developed. Both kinetics and dilatometric models have been validated by comparison of theoretical and experimental dilatometric heating curves. Predicted and experimental results are in satisfactory agreement.     

Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Analysis of martensite–austenite constituent and its effect on toughness in submerged arc welded joint of low carbon bainitic steel

Martensite–austenite (M–A) constituent formed during welding is generally recognized as an important factor to decrease the toughness of welded joint. In this article, the morphology and chemical composition of M–A constituent in the low carbon bainitic steel welded joint was analysed in detail by means of optical microscope, transmission electron microscope and scanning electron microscope with electron probe microanalysis. The experimental results show that the M–A constituent formed in the different sub-zones presents different morphologies and different amounts. The maximum amount of M–A constituent occurs in the coarse grained heat affected zone (HAZ). It is evident that the carbon atoms segregate on the M–A constituent and carbon concentration on the slender M–A constituent is higher than that on the massive M–A constituent. Meanwhile, the distribution profile of silicon on the M–A constituent shows an obvious inhomogeneity. Most of M–A constituents have a twinned structure and/or a high dislocation density. According to impact testing results, the crack initiation energy in the HAZ specimens deteriorates significantly because the large M–A constituent can assist the formation of cleavage crack. On the other hand, the coarse prior austenite grain in the HAZ lowers the crack propagation energy.     

A new resource-saving,low chromium and low nickel duplex stainless steel 15Cr–xAl–2Ni–yMn

A new family of resource-saving, low Cr and low Ni duplex stainless steels, with compositions of 15Cr–xAl–2Ni–yMn (x = 1.2–2.8, y = 8–12, wt.%) has been developed by examining the effect of Al and Mn on microstructure, mechanical property and corrosion property. The results show that 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn alloys have a balanced ferrite–austenite relation and that 15Cr–2.8Al–2.0Ni–12Mn alloy has a primary ferrite phase structure. The ferrite volume fraction increases with the solution treatment temperature and Al content while decreases with Mn content. No precipitate was found after solution-treated at 750 °C for 30 min. 15Cr–1.2Al–2.0Ni–8Mn alloy has a strong strain hardening effect, and 15Cr–2.0Al–2.0Ni–10Mn alloy has a good TRIP effect. Both of the 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn alloys have excellent impact toughness at low temperature with the impact energy higher than 125 J at −40 °C. The pitting corrosions always occur in austenite phase. Among the designed alloys, 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn are found to be excellent alloys with a proper phase proportion and a better combination of superior mechanical property and good pitting corrosion resistance.     

Effect of integrated extrusion–equal channel angular pressing temperature on microstructural characteristics of low carbon steel

Abstract

In the present research, a combined forward extrusion–equal channel angular pressing was developed and executed for the deformation of a plain carbon steel. In this method, two different deformation steps, including forward extrusion and equal channel angular pressing, take place successively in a single die. The deformation process was performed at different deformation start temperatures (800, 930 and 1100°C). Three-dimensional finite element simulation was used to predict the strain and temperature variations within the samples during deformation. With microstructural observations and the results of finite element simulation, the main grain refinement mechanisms were studied at different deformation temperatures. The results show that the forward extrusion–equal channel angular pressing is effective in refining the ferrite grains from an initial size of 32 μm to a final size of ～0·9 μm. The main mechanisms of grain refinement were considered to be strain assisted transformation, dynamic strain induced transformation and continuous dynamic recrystallisation, depending on the deformation temperature.     

A JMAK-like approach for isochronal austenite–ferrite transformation kinetics in Fe–0·055 wt-%N alloy

Abstract

The kinetics of the isochronal austenite γ to ferrite α transformation of Fe–0·055 wt-%N alloy was investigated for cooling rates in the range of 5–15 K min^? by high resolution dilatometry. In accordance with thermodynamic characteristics of γ→α transformation investigated in this study and previous kinetic theory, a Johnson–Mehl–Avrami–Kologoromov (JMAK)-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (transition from interface- to diffusion-controlled growth) as well as impingement for random distribution nuclei. The JMAK-like approach has been employed to fit the experimental results, and the fitting results show that the γ→α transformation of Fe–0·055 wt-%N alloy does have two stages: a first, short interface-controlled growth stage and a second, long diffusion-controlled growth stage. In addition, soft impingement effect has been recognised to become serious in the later part of the second stage.     

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     

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.