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.     

Analysis of powder metallurgy process parameters for mechanical properties of sintered Fe–Cr–Mo alloy steel

The present work involves an investigation to find out the best combination of process parameters for a Fe–Cr–Mo alloy with the help of Design of Experiments (DOE) tool. The Fe–Cr–Mo alloy containing 0, 0.4 and 0.8?wt.% carbon is compacted at 650?MPa pressure and sintered at 1120°C and 1200°C temperature, respectively, with 3.5 or 6°C/minute cooling rate. Quality characteristics like hardness and tensile strength are analyzed for various combinations of graphite weight %, sintering temperature, and cooling rate. The conducted experimental trials are based on the design matrix obtained from general factorial design. Significant regression models are developed from the above mentioned process parameters to predict the quality characteristics using DOE tool. The developed mathematical model during the course of research helped in investigating best combination of process parameters for powder processing. The desirability test showed its usefulness in finding out the number of optimization strategies to achieve the optimum values of hardness and tensile strength. The observed results are correlated with the microstructure. Diffusion of carbon during sintering decides the optimum amount of carbon. Higher carbon addition results in residual graphite which weakens the sintered alloy.     

Origins of tertiary creep in microalloyed 25Cr–35Ni centrifugally cast alloy tubes

Abstract

Constant stress tensile creep tests have been undertaken in air on as cast 25Cr35Ni austenitic steel at temperatures of 880, 900 and 950°C. The creep curves were found to consist of a relatively short primary stage, a minimum strain rate and an extended tertiary stage. There was no convincing evidence of a period of steady-state creep but the minimum creep rate was found to be highly dependent on applied stress through a power law with a stress exponent of ～10. The early occurrence of tertiary creep, after strains of only ～0·01, was shown not to be caused by a loss of section associated with necking, internal void development nor with the formation of surface cracks. It is concluded that the most likely cause of the early onset of tertiary creep in this alloy is the coarsening of strengthening precipitates.     

Damage analysis in Fe–Cr–Ni centrifugally cast alloy tubes for reforming furnaces

Reformer furnaces tubes work under high temperature and pressure for a long time, which are very critical conditions for creep deformation and life of most common materials. Cast austenitic Fe–Cr–Ni alloys in the widely know HP grades are used for reformer tubes to allow a good service at temperatures that can be close to 1000 °C. This paper reports a study devoted to the damage analysis of reformer furnace tubes after more than 100,000 h of service. Tubes, made of a HP grade modified with Nb and Ti additions, were inspected in situ by a laser optic system to measure their internal diameter and evaluate creep deformation. With the aim of developing a criterion for deciding the substitution of components, samples of as cast material and samples, cut from the most deformed tubes put out of service, were considered to check changes of mechanical properties and metallurgical characteristics. Tensile and creep tests were carried out; moreover the metallographic observations included optical and scanning electron microscopy and energy dispersive X-ray microanalysis in order to measure locally the chemical composition.     

X-ray analysis of nano-carbide transformations for actual life assessment of service-exposed 2.25Cr–1Mo boiler tubes in a thermal power station

Abstract

Quantification of carbide phase changes by X-ray analysis, with an objective of actual life assessment of service exposed 2.25Cr – 1Mo boiler tubes is described. Carbide transformations and their compositional variations in specimens subjected to extended service (~5 – 18 years) from a power plant were investigated by X-ray diffraction and X-ray fluorescence analysis of extracted carbide precipitates. Quantitative estimation of equilibrium carbide phase M₂₃C₆ by Rietveld’s refinement technique showed a systematic increase with service. Carbide composition analysis revealed a decrease in Mo wt% with ageing. Based on this result, two life assessment methods were proposed and applied to few unknown specimens for its validation. The computed life was in good agreement with plant operating data.     

Hot ductility of a Fe–Ni–Co alloy in cast and wrought conditions

The hot ductility of Fe–29Ni–17Co alloy was studied in both cast and wrought conditions by hot tensile tests over temperature range of 900–1250 °C and at strain rates of 0.001–1 s⁻¹. Over the studied temperature range, the wrought alloy represented higher elongation and reduction in area as compared to the cast alloy. Dynamic recrystallization was found responsible for the higher hot ductility of the wrought alloy and the improvement of hot ductility of the cast alloy at high temperatures. At temperature range of 1000–1150 °C the wrought alloy exhibited a hot ductility drop while a similar trough was not observed in case of the cast alloy. It was also found that at temperatures of 1150–1250 °C the best hot ductility is achieved in both cases of cast and wrought alloy. The experimental data of flow stress were constitutively analyzed and the apparent activation energy of deformation was estimated to be 344 kJ/mol.     

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     

Hydrogen and temper embrittlement interactions in fatigue of 2·25Cr–1Mo steel

Abstract

The influence of strength, precipitate microstructure, temper embrittlement, and environment on fatigue crack growth in 2·25Cr–1 Mo steel has been investigated. Particular attention was paid to the interaction between hydrogen embrittlement and temper embrittlement in fatigue. A range of tempered and aged conditions was examined in air, vacuum, and gaseous hydrogen environments at growth rates between 10^?10 and 10^?5 m/cycle. In this paper, discussion focuses on effects observed in hydrogen. Gaseous hydrogen was found to encourage crack growth by promoting intergranular fracture, which peaked at intermediate growth rates, and by reducing the general plasticity associated with transgranular fracture at high growth rates. Mechanisms underlying these effects, which involve stress-driven hydrogen segregation and the facilitation of crack-tip dislocation emission, are considered in detail. Reversible temper embrittlement encouraged crack growth at near-threshold and intermediate rates in hydrogen by increasing susceptibility to intergranular fracture. The magnitude of this effect was directly related to the degree of intergranular phosphorus enrichment, thus clearly demonstrating synergy between hydrogen embrittlement and temper embrittlement in fatigue. In contrast, one-step temper embrittlement encouraged transgranular crack growth in hydrogen only at high growth rates. This is considered to result from a concentration of slip on glide planes intersecting the crack tip under the combined influences of hydrogen and an increasingly dense precipitate microstructure.

MST/583     

Grain boundary segregation of phosphorus and molybdenum in 2·25Cr–1Mo steel

Abstract

Segregation of phosphorus and molybdenum to grain boundaries in a commercial grade of 2·25Cr–1Mo steel subjected to different heat treatments has been examined using a field emission gun scanning transmission electron microscope with energy dispersive X-ray microanalysis. The results indicate that P and Mo concentrations at prior austenite grain boundaries increase with aging time. This follows the prediction of McLean's equilibrium segregation model, when modified to take account of the interaction energy between phosphorus and molybdenum.     

Degradation in tensile and creep properties of 2.25Cr–1Mo steel by long term service in plant

Abstract

Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 10⁵ h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in total elongation.     

Structure and mechanical properties of Ti–5Cr based alloy with Mo addition

The effects of molybdenum (Mo) on the structure and mechanical properties of a Ti–5Cr-based alloy were studied with an emphasis on improving its strength/modulus ratio. Commercially pure titanium (c.p. Ti) was used as a control. As-cast Ti–5Cr and a series of Ti–5Cr–xMo (x = 1, 3, 5, 7, 9 and 11 wt.%) alloys were prepared by using a commercial arc-melting vacuum-pressure casting system, and investigated with X-ray diffraction (XRD) for phase analysis. Three-point bending tests were performed to evaluate the mechanical properties of all specimens and their fractured surfaces were observed by using scanning electron microscopy (SEM). The experimental results indicated that Ti–5Cr–7Mo, Ti–5Cr–9Mo and Ti–5Cr–11Mo alloys exhibited ductile properties, and the β-phase Ti–5Cr–9Mo alloy exhibited the lowest bending modulus. However, the Ti–5Cr–3Mo and Ti–5Cr–5Mo alloys had much higher bending moduli due to the formation of the ω phase during quenching. It is noteworthy that the Ti–5Cr–9Mo alloy exhibited the highest bending strength/modulus ratios at 26.0, which is significantly higher than those of c.p. Ti (8.5) and Ti–5Cr (13.3). Furthermore, the elastically recoverable angle of the Ti–5Cr–9Mo alloy (30°) was greater than that of c.p. Ti (2.7°). The reasonably high strength (or high strength/modulus ratio) β-phase Ti–5Cr–9Mo alloy exhibited a low modulus, ductile property, and excellent elastic recovery capability, which qualifies it as a novel implant materials.     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.     

Effect of aging temperature on the heterogeneous microstructure and mechanical properties of a 12Cr–10Ni–Mo–Ti maraging steel for cryogenic applications

Journal of Materials Science - The evolution of heterogeneous microstructure and mechanical properties of a 12Cr–10Ni–Mo–Ti maraging steel was investigated at different aging...     

Microstructure characterization of nanometer carbides heterogeneous precipitation in Ti–Nb and Ti–Nb–Mo steel

The influences of micro-alloying elements and hot deformation on the precipitation morphology of Ti–Nb and Ti–Nb–Mo steels were investigated. The nanometer sized carbide particles randomly dispersed in the ferrite matrix are attributed mainly to severe deformation at high temperature and low isothermal holding temperature. Of the two steels with different combinations of the micro-alloying elements, Ti–Nb and Ti–Nb–Mo, the steel with Ti–Nb–Mo was more effective in precipitating hardening due to its slower carbide coarsening rate. Based on observations of micrographs, the nano-sized TiMoC and TiNbC precipitated in polygonal ferrite grains when the Ti–Nb–Mo and Ti–Nb steels were isothermally treated at 650 °C for 3 min and 180 min. The smaller of the two carbides, TiMoC, precipitated in the ferrite grain, and the hardness of Ti–Nb–Mo steel was higher than that of Ti–Nb steel. Moreover, the tiny ferrite grains and high dislocation density in the Ti–Nb–Mo steel were found to provide an attractive combination of strength and toughness.     

Method for improving parametric creep rupture life of 2·25Cr–1Mo steel using artificial neural networks

Abstract

A number of well known parametric models and a multilayer neural network are subjected to an ‘acid test’ of extrapolation to determine whether the latter can produce improved long term rupture life predictions for 2·25Cr–1Mo steel. Linear and non- linear least squares analysis was used to estimate the parametric models and genetic algorithms were used to identify and train the network. All the parametric models produced lifetime predictions for stresses below 60 MPa that were in error by some 20–40% on average. This reflected their tendency to overfit the data sets used for their estimation. Standard statistical measures of model inadequacy were of little use in overcoming this problem and the more non-linear models (e.g. Manson–Haferd) produced implausible extrapolations. In contrast, the optimised neural network was able to identify general patterns in the training data that were useful for extrapolation purposes and this was reflected in an average error of some 4–5%.     

Hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC) of 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener

This paper brings a failure case study of high strength 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener, which failed due to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). 0.35C–3.5Ni–1.5Cr–0.5Mo steel in hardened and tempered condition, meeting the specified axial tensile stress rating of 1250 MPa is widely used as fasteners in space programmes.In the course of assembly of the structural parts of a satellite launch vehicle, 10 nos of fasteners developed cracks on tightening using a torque wrench set to 6 N m torque surprisingly.Also some fasteners, which were under assembly load of more than 6 months in the same vehicle assembly, were found to be cracked.The failure was attributed to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). The details of the analysis and mechanism involved in the HI-IGSCC are presented in detail.Detailed metallurgical analyses of the cracked fasteners support the successive steps of the corrosion enhanced plasticity model, which is based on a local softening in the SCC crack region. The mechanism of a dislocation pileup ahead of a crack under corrosion and stress due to diffusing hydrogen promotes stress concentration against micro-obstacle and caused failure.     

Identification of processing parameters for Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel using processing maps

Abstract

The processing parameters for hot working of Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel (alloy D9) are identified using processing maps developed on the basis of the dynamic materials model and hot compression data in the temperature range 850–1250°C and strain rate range 0.001–100 s^-1. The efficiency of power dissipation increased with increase in temperature and decrease in strain rate. Dynamically recrystallised microstructures resulted when the efficiency of power dissipation was in the range 27–37%, i.e. in the temperature range 1050–1250°C and strain rate range 0.001–0.5 s^-1. Flow localisation occurred in the regions of instability at temperatures lower than 1000°C and at higher strain rates. The dynamic recrystallisation regime observed in this alloy is compared with other austenitic stainless steels, namely, AISI type 304L and 316L.     

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600 °C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600 °C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50 μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600 °C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.     

Quinary icosahedral quasicrystalline Ti–V–Ni–Mn–Cr alloy: A novel anode material for Ni-MH rechargeable batteries

The substitution of manganese and chromium for 6 at.% nickel in Ti_1.6V_0.4Ni leads the rapid quenching synthesis of quinary icosahedral phase (i-phase) evidenced by the observations of 2-, 3- and 5-fold symmetries. As negative electrode in Ni-MH battery, the quinary Ti–V–Ni–Mn–Cr i-phase can deliver a maximum discharge capacity of 278 mAh g^?1 at 30 mA g^?1, larger than that of Ti_1.6V_0.4Ni master alloy anode owing to Mn and/or Cr doping. After a preliminary test of 30 consecutive cycles the cycling capacity retention rate (CR%) is 80%. The strong chemisorption of hydrogen shown in cyclic voltammetric (CV) response indicates that the electrocatalytic activity improvement for the i-phase negative electrode is highly demanded.     

Critical conditions for the occurrence of quench cracking in an Al–Zn–Mg–Cu alloy

The occurrence of quench cracking in small cuboidal samples of aluminium alloy AA7150 was determined to be related to the maximum temperature difference (?T _max) between various locations within samples during quenching. When ?T _max between different locations is between 96 and 124 °C, there is some risk of quench cracking under various quenching conditions. When the ?T _max value is higher than 124 °C, quench cracks cannot be avoided. Quench cracks preferentially occur at sample corners and edges and are preferentially propagating in the short transverse direction–long transverse direction plane. Finite element modelling results indirectly indicate that the quench cracking should occur at the very early stages of the quenching process. Microscopy reveals that the quench cracking mode is intergranular, and cracks preferentially occur at high-angle grain boundaries with an average misorientation angle of ~42°. Moreover, quench cracks can penetrate through the whole thickness of a sample quenched from 495 into 20 °C water. Fractography reveals that no constituent particles exist in the quench fracture region, indicating that, unlike impact fracture, the occurrence of quench cracks is not dependent on the presence of coarse particles.