Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

Visual analysis of solidification and δ–γ transformations in steels

Abstract

The theory of solidification of steels and the kinetics of austenite to α-ferrite phase transformation were extensively studied; however, comparatively, little information is available concerning the kinetics of the δ-ferrite to austenite transformation due to the difficulty of making in situ observations. In the present study, a laser scanning confocal microscopy with an infrared image furnace was implemented with which the in situ observations at the high temperature of the dynamic behaviour of the δ/γ grain nucleation and growth and interphase boundaries of the steels are made possible. The solidification mode of the carbon steel and the austenitic stainless steel during welding can be directly observed, and the definitive sequence of phase transformation that led to the final microstructure was detected in real time. Finally, new experimental results will be presented and compared with previous studies.     

Strain–life curves of steels and methods for determining the curve parameters. Part 1. Conventional methods

The available publications give much consideration to strain–life curves which are usually described by the Basquin–Manson–Coffin equation. The parameters of this equation are related to those of the Ramberg–Osgood equation that represents the cyclic stress–strain diagram. Many different methods have been put forward for the assessment of parameters of these equations on the basis of static strength and plasticity characteristics. Most of the methods rely on a fairly small body of experimental evidence. Using the experimental data on static and cyclic strength and plasticity characteristics of about 200 various steels from the well-known publications, a statistical analysis of parameters of Basquin–Manson–Coffin and Ramberg–Osgood equations has been performed by each of the assessment methods, revealing their advantages and disadvantages.     

Effects of co-addition of titanium and boron on microstructure of superplastic Cr–Mo steels

Abstract

The effects of titanium and boron on the microstructure of a low alloyed Cr–Mo steel with 0·6 wt-%C have been investigated by comparison with a steel containing only titanium and a steel free from both titanium and boron. Each of the steels was subjected to thermomechanical treatment and annealed at 700°C, resulting in small grains of size a few micrometres. The steel containing both titanium and boron possessed the smallest ferrite grains and M₃C carbides of the three examined. This is attributed to a fine dispersion of borides (TiB₂ ) and borocarbides (Ti(C,B)) of size 10 nm in the ferrite matrix through the pinning effect. At the grain boundaries small carbide particles were present which were effective in inhibiting grain boundary migration. The extremely fine borides and/or borocarbides were useful in suppressing intragranular deformation of ferrite grains due to precipitation hardening. This may have assisted in promoting grain boundary sliding, resulting in superior superplastic elongation.     

Effect of carbon content on hydrogen occlusivity and embrittlement of ferrite–pearlite steels

Abstract

Experiments on a series of pure Fe–C alloys consisting of ferrite and pearlite only have shown that the ferrite/pearlite and pearlite/pearlite interfaces are effective hydrogen trapping sites. The ferrite/cementite interfaces within the pearlite colonies, however, have little effect on the hydrogen occlusivity. With an increase in carbon content, more ferrite/pearlite interfaces are created and these increase the hydrogen occlusivity. Although the ferrite/cementite lamella interface has little effect on the hydrogen occlusivity, it does appear that the lamellae interfere with the hydrogen diffusion path across the pearlite colonies. Thus, the higher-carbon alloys in the pearlitic condition have a lower apparent hydrogen diffusivity. Hydrogen has little effect on the tensile strength, but significantly reduces the ductility. After hydrogen charging, high-carbon alloys suffer a lower ductility loss. However, in terms of absolute values, the low-carbon specimens are always more ductile than the high-carbon alloys when saturated with hydrogen.

MST/433     

On the theory ofα-γ-transformation in iron and steels

The fluctuation model of --transformation in iron and steels is developed to phenomenologically describe the phase transformation mechanism in terms of the principles of stability loss of one of the phases due to contact interaction. The interphase boundary is treated as a stable highly excited state, in which a low-temperature -phase becomes unstable and turns into a high-temperature -phase during contact. A phase transition is considered to be a collective process. Low degrees of transformation are discussed when lattice rearrangement is a limiting factor. The theory is used for numerical calculations of kinetics of the --transformation in iron and U8 steel under rapid heating. The obtained results show fair agreement with experimental data.Branch of P. N. Lebedev Institute of Physics, Russian Academy of Sciences, Samara. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 1, pp. 95–101, July, 1992.     

Influence of titanium and nitrogen on hot ductility of C–Mn–Nb–Al steels

Abstract

The influence of small additions of titanium on the hot ductility of C–Mn–Nb–Al steels has been examined. Titanium and nitrogen levels varied in the ranges 0·014–0·045 and 0·004–0·011 wt-%, respectively, so that a wide range of Ti/N ratios could be studied. The tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min^-1 to test temperatures in the range 1100–800°C and strained to failure at a strain rate of 2 × 10^-3 s^-1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the Ti/N ratio above the stoichiometric ratio for TiN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ～0·005 wt-%N ductility was very poor at the stoichiometric Ti/N ratio of 3·4 : 1. Ductility was better at the higher Ti/N ratios but only two of the titanium containing niobium steels gave better ductility than the titanium free niobium containing steels and then only at temperatures below about 950–900°C. One of these steels had the lowest titanium addition (0·014 wt-%), thus limiting the volume fraction of fine Ti containing particles and the other had the highest Ti/N ratio of 8 : 1. However, even for these two steels ductility was worse than for the titanium free steels in the higher temperature range. The commercial implications of these results are discussed.     

Evolution of microstructure and strength during the ultra-fast tempering of Fe–Mn–C martensitic steels

Ultra-fast tempering cycles, combining a rapid heating rate (R _H = 300 °C/s) from room temperature to a peak temperature within the range 400–700 °C and subsequent rapid cooling, were performed on a Fe–Mn–C martensitic steel. The influence of the peak temperature reached during the cycle was determined on both the tensile properties of the steel and on its microstructure. The mechanisms controlling the microstructural evolution occurring during rapid tempering were studied by combining both TEM observations and 3D reconstructions by FIB/SEM. A theoretical analysis coupled with the acquired experimental data was then proposed to explain the evolution of mechanical properties. The results obtained support the assumption that carbide precipitation during fast tempering plays a key role in the evolution of mechanical properties.     

Relationship between hardness and microstructure of continuously cooled C–Mn–V steels

Abstract

The relationships between hardness and the volume fraction of martensite for continuously cooled steels containing 0·1–0·3%C, 1·6%Mn, 0–0·3% V (all wt-%), which are used to measure the hardenability expressed by the ideal critical diameter, have been established. It has been shown that for steels transformed to a fine mixture of lath martensite and lath bainite it is not possible to measure the volume fraction of martensite by means of quantitative optical metallography and therefore the volume fraction of martensite was determined from dilatometric records of continuously cooled specimens. The continuous cooling transformation diagrams were determined and microstructures of the dilatometric specimens were examined by optical and transmission electron microscopy. The values of the 50% martensite hardness established for the steels containing vanadium were found to be outside the range given by the widely used Hodge and Orehoski relationship. The relevance of the results to the assessment of hardenability by means of the ideal critical diameter concept is discussed.

MST/484     

Elevated temperature low-cycle fatigue of the austenitic stainless steels type 316 and 253MA— influence of microstructure and damage mechanisms

Low-cycle fatigue tests have been performed at 750°C for type 316 (17Cr13Ni2Mo) and 253MA (21CrllNiO.2N) REM and at 900°C for 253MA. The investigation at 750°C also included hold-time fatigue with 5 minute holds at the maximum tensile strain. At this temperature 316 exhibits a higher resistance to fatigue than 253MA. Five minute holds decreased the life significantly for both alloys but did not change the relative fatigue resistances. Transmission electron microscopy (TEM) studies revealed that the dislocation structure consisted of a substructure at 750°C. This contrasts with the previous observations at 600°C for 253MA, where a planar dislocation structure was found during continuous fatigue tests.

A model for creep damage has also been developed based on creep rupture data. The model can quantitatively predict the influence of hold time on the number of cycles to crack initiation at 750°C and 900°C.     

TIG–MIG hybrid welding of ferritic stainless steels and magnesium alloys with Cu interlayer of different thickness

The joining of ferritic stainless steels and magnesium alloys is light and economic for weight reduction of automobiles. Unlike previous conventional welding method, a novel TIG–MIG hybrid welding is applied for the joint successfully in this study. The melted Mg weld metal wets the ferritic stainless steels surface to form a brazed Mg–Cu to steel connection when the interlayer thickness is 0.02 mm. When the interlayer thickness is 0.1 mm, the intermetallic compounds transition layer determined the tensile-shear strength of joints. Intermetallic compounds transition layer has been found in the 0.1 mm thick interlayer joints and no particle has been found in the 0.02 mm thick interlayer joints. Based on the analysis of microstructure and properties, joining and strengthen mechanisms of the joint were discovered. As the thickness of the Cu interlayer increases, the joining mechanism changed. The joining and strengthen mechanisms are mainly determined by the thickness of the interlayer. The tensile-shear strength of 0.1 mm thickness Cu interlayer joints is improved by 47% compared to 0.02 mm Cu.     

Nucleus geometry and mechanical properties of resistance spot welded coated–uncoated DP automotive steels

In this study, mechanical properties of resistance spot welding of DP450 and DP600, galvanized and ungalvanized automotive sheets have been investigated. The specimens have been joined by resistance spot welding at different weld currents and times. Welded specimens have been examined for their mechanical, macrostructure and microstructure properties. Depending on the weld current and time, effects of zinc coating on tensile properties, microhardness values as well as microstructure nugget geometry and nucleus size ratio have been investigated. X-ray diffraction analysis has been used to investigate the phase that formed at the joint interface. Result of the experiment show that nugget diameter, indentation depth and tensile load-bearing capacity are affected by weld parameters. Coating prevents full joining at low parameters. Microhardness increased in heat-affected zone and weld metal.     

Alloying effects on the microstructure and phase stability of Fe–Cr–Mn steels

Austenitic Fe–Cr–Mn stainless steels interstitially alloyed with nitrogen have received considerable interest lately, due to their many property improvements over conventional Fe–Cr–Ni alloys. The addition of nitrogen to Fe–Cr–Mn stabilizes the fcc structure and increases the carbon solubility. The benefits of increased interstitial nitrogen and carbon content include: enhanced strength, hardness, and wear resistance. This study examines the effect of carbon, silicon, molybdenum, and nickel additions on the phase stability and tensile behavior of nitrogen-containing Fe–Cr–Mn alloys. Nitrogen and carbon concentrations exceeding 2.0 wt.% were added to the base Fe–18Cr–18Mn composition without the formation of nitride or carbide precipitates. Minor additions of molybdenum, silicon, and nickel did not affect nitrogen interstitial solubility, but did reduce carbon solubility resulting in the formation of M₂₃C₆ (M=Cr, Fe, Mo) carbides. Increasing the interstitial content increases the lattice distortion strain, which is directly correlated with an increase in yield stress.     

Development of structure and effect of processing parameters on Strength–structure relationships for two ferritic stainless steels

Abstract

The development of substructure as material passes through the roll gap has been examined for a ferritie stainless steel and it is shown that the final structure evolves close to the roll gap exit. Temperature variations as the material was rolled were monitored and the subsequent effect on substructure investigated. The variation of substructure with processing for two ferritic steels is discussed and the effect of this variation on room temperature properties is presented. It is shown that the strength–structure relationship is highly dependent upon retained martensite which degrades the tensile strength.

MST/380     

Review of stress corrosion cracking of pipeline steels in “low” and “high” pH solutions

This paper reviews the current understanding of the mechanisms of stress corrosion cracking of pipeline steels. The similarities, the differences and the influencing factors are considered for the high pH stress corrosion cracking caused by a concentrated bicarbonate-carbonate solution, and for the low pH stress corrosion cracking due to a diluter solution. For high pH stress corrosion cracking, it is well accepted that the mechanism involves anodic dissolution for crack initiation and propagation. In contrast, it has been suggested that the low pH stress corrosion cracking is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen into the pipeline steel. But the precise influence of hydrogen on the mechanism needs to be further studied.     

Intergranular cavity damage and creep fracture of 1Cr–0·5Mo steels

Abstract

The factors controlling the intergranular fracture of three 1Cr–0·5Mo steels, tested at 550°C, have been examined. Failure results from the nucleation and growth of grain-boundary cavities. It is shown that creep life is dependent on the maximum principal stress, and that variations in the rupture properties of the steels are controlled by their susceptibility to nucleate intergranular cavities. Increasing the metalloid element content and, in particular, increasing the austenitizing temperature from 930 to 1300°C resulted in an increase in the cavity nucleation rate and a concomitant decrease in the rupture life. The cavity nucleation rate was found to be dependent on the maximum principal stress and when this dependence is used in conjunction with a simple cavity diffusion growth model the stress-state dependence of rupture life and the effect of residuals and austenitizing temperature on fracture properties could be predicted. These results are discussed in terms of the material and fabrication factors and service conditions that designers and operators of high-temperature plants must consider so that the plant may be operated safely and efficiently.

MST/81     

Cyclic operation of aero gas turbines – materials and component life implications

Abstract

Historically, the issues connected with the lifing of power generation gas turbine components have been very different from those associated with aero engines. Specifically, component lives in the power generation application have been dictated by creep and high cycle fatigue, whereas low cycle fatigue has been the driver for aero engines. However, developments in the design and usage of gas turbines within the respective industries have resulted in this distinction becoming increasingly blurred. This paper highlights recent advances in the materials technology, stress analysis and lifing of aero engine components, which are potentially relevant to industrial gas turbines. In particular, the development of complex constitutive equations for modelling plasticity and anisotropic creep are discussed, with particular reference to the behaviour of single crystal turbine blades. Moreover, developments in the methodologies used to estimate safe service lives for the components are considered. Specifically, a new lifing procedure, capable of accurately predicting component lives from plain specimen data alone, is discussed.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.