Transformation Behaviour of Austenite in Steel under Condition of Stress-strain and Its Application

Austenite can be retained at ambient temperature in steels by alloying and processing control. The transformation from austenite to martensite occurs under a certain conditions : thermal or deformation. Stress-strain induced martensitic transformation is important to improve the plasticity of steels which is called transformation induced plasticity (TRIP). Strength-ductility balance of the steels is greatly superior to that of other high strength steels due to the TRIP effect. A new type of steels-TRIP steel is developed     

Designing steel to resist hydrogen embrittlement: Part 1 – trapping capacity

A novel steel has been designed for use in the oil and gas industry, displaying properties comparable with the currently available F22 grade and possessing the additional quality of excellent hydrogen trapping capacity. Its high strength is derived from a martensitic microstructure containing a dispersion of fine vanadium-molybdenum carbides that evolve during thermal treatments. If the tempering cycle is controlled such that the precipitates maintain a degree of coherency with the matrix, then they act as hydrogen trapping sites, due to the associated strain fields, thus mitigating the problem of diffusible hydrogen. Using material modelling programmes and small-scale sample alloys this work describes the process of the new steel design and demonstrates its superior trapping capacity through thermal desorption analysis.     

Hydrogen embrittlement property of a 1700-MPa-class ultrahigh-strength tempered martensitic steel

The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels.     

Control of hydrogen cracking in the welded steel using microstructural traps

Hydrogen diffusion into steel can embrittle the material in H₂S environments, but this effect can be offset by suitable hydrogen trapping sites in the microstructure. Fine Ti(C,N) inclusions have been studied as the trapping sites in high strength low alloy (API X-70) welds, with Ti additions ranging from 0.004 to 0.16?wt.%. The trapping sites were investigated by electron microscopy and thermal desorption spectroscopy. Manganese sulphide particles were the main initiation sites for hydrogen induced cracking as expected. The optimum Ti addition was around 0.02% with no evidence of cracking in the weld. The estimated values of trapping activation energy for dislocations, microvoids, MnS and Ti(C, N) were approximately 25.9, 34.6, 65.1 and 87.6?kJ?mol^?1, respectively.     

Hydrogen embrittlement property of a 1700-MPa-class ultrahigh-strength tempered martensitic steel

Abstract

The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels.     

Evaluation of automobile service performance using laboratory testing

A methodology is presented to evaluate martensitic advanced high-strength steels in auto service. There was essentially no influence of hydrogen for (i) linearly increasing stress tests of specimens with hydrogen contents much greater than for automobile service, and (ii) for tensile tests with simultaneous hydrogen charging and with a substantial hydrogen content. These results allow evaluation of the hydrogen influence for the tested steels for auto service. In contrast, electrochemically hydrogen charged martensitic advanced high-strength steels showed an influence of hydrogen on mechanical properties, manifest as (i) a decrease in yield strength, by hydrogen enhanced macroscopic ductility, and (ii) a change of the micro-fracture mode, by hydrogen-assisted micro-fracture.     

Dislocation engineering and its effect on the oxidation behaviour

Abstract

Shot-peening of the surface of steel prior to oxidation can have a beneficial effect. Shot-peening can improve the oxidation resistance by introducing a localised plastic deformation in the near surface region resulting in an increase of the dislocation density. These dislocations can act in Cr-containing steels as fast diffusion paths for Cr promoting the formation of protective Cr-oxides. However, the effect of shot-peening has some limitations such as working temperature and microstructure. It has different effects on austenitic steels and ferritic martensitic steels. The effect of shot-peening can become futile due to recovery and recrystallisation of the alloy when subjected to higher temperatures for longer periods. In the present work, the main emphasis is put on the type of dislocation arrangement promoting the positive effect on the oxidation behaviour. Dislocation engineering was applied on shot-peened samples by means of some pre-annealing procedures resulting in a recovery process. During the process, dislocations were assumed to rearrange and form certain combinations nearer to the alloy grain boundaries. These arrays of dislocations can result in different oxidation behaviour. In the present study, 18 wt% Cr and 12 wt% Cr steels were shot-peened and vacuum annealed at 750°C for 1 h, 2 h, 3 h, 5 h and 15 h. Subsequently these steels were oxidised at 750°C. The mass gain in all cases is different for both steels, and in the case of both 12 wt% Cr and 18 wt% Cr steels the best oxidation resistance was achieved for the shot-peened +1 h pre-annealed sample.     

The Effect of Hydrogen on Fatigue Properties of Metals used for Fuel Cell System

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and two annealed medium-carbon steels (0.47 and 0.45%C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behaviour of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the case of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little degradation due to hydrogen. A marked increase in the amount of martensitic transformation occurred in the hydrogen-charged SUS304 specimens compared to specimens without hydrogen charge. In case of SUS316L, little martensitic transformation occurred in either specimens with and without hydrogen charge. The results of S-N testing showed that in the case of the 0.7C–13Cr stainless steel and the Cr–Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steels hydrogen did not cause a reduction in fatigue behaviour. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. Thus, it is presumed that a synergetic effect of hydrogen and martensitic structure enhances degradation of fatigue crack resistance.     

Die Werkstoffdämpfung von Stählen bei hohen Dehnungsamplituden

Damping of Steels at High Strain Amplitudes The amplitude dependent damping has been investigated in two martensitic chromium steels (X 22 CrMoV 12 1, X 20 Cr 13) and a cold worked austenitic steel (X 12 CrNiWTi 16 13) at room temperature. Due to the magneto-elastic effects, the martensitic steels show a strong amplitude dependent damping. It cound be shown that the damping is reduced remarkably by a reduction of the annealing temperature. The austenitic material shows a very high damping in the cold worked condition. Tempering at relatively low temperatures (above 100 °C) reduces damping to the low values usually expected for austenitic materials. This effect was attributed to the pinning of dislocations during tempering.     

The mechanism of hydrogen embrittlement: the stress interaction between a crack, a hydrogen cluster, and moving dislocations

Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models.     

The mechanism of hydrogen embrittlement: the stress interaction between a crack, a hydrogen cluster, and moving dislocations

Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models.     

Proposed Theory for the Hydrogen Embrittlement Resistance of Martensitic Precipitation Age-Hardening Stainless Steels such as Custom 455

Over the years, the author has had experience on various programs and vehicle platforms in which martensitic precipitation age-hardening, corrosion-resistant stainless steels such as Custom 455 have demonstrated hydrogen embrittlement resistance. Custom 455 is a double vacuum-melted, martensitic precipitation age-hardening stainless steel, which is reported to be susceptible to hydrogen embrittlement. This paper proposes a plausible rationale for the resistance to hydrogen embrittlement of this type of stainless steel.     

Hydrogen Permeability of Proton-Irradiated Reactor Steels with Oxide Films

The influence of oxidation and proton irradiation on the hydrogen permeability of 12Kh18N10T, ÉP-838, and 10Kh9VFA reactor steels is investigated. The critical temperatures of thermal desorption of implanted hydrogen are determined. It is shown that, unlike hydrogen penetration, the proton irradiation of oxidized specimens increases their thermal stability.     

Diffusion properties of point defects in barium strontium titanate thin films

The relationship between the diffusion behavior of hydrogen and the electrical properties of (Ba, Sr)TiO3 (BST) thin-film capacitors was investigated using thermal desorption spectroscopy and secondary ion mass spectroscopy analyses. It has been clearly shown that the frequency dependence of the complex impedance profile of the BST thin-film capacitors could be successfully represented by two parallel resistor-capacitor (RC) electrical equivalent networks in series correlated with the distribution of the hydrogen, namely, the Pt/BST interface region with the influence of hydrogen and the BST bulk region without the influence of hydrogen. However, the I-V properties of the BST thin-film capacitors could be determined almost from the hydrogen atoms existing at the Pt/BST interface.     

Recent advances in creep-resistant steels for power plant applications

The higher steam temperatures and pressures required to achieve increase in thermal efficiency of fossil fuel-fired power-generation plants necessitate the use of steels with improved creep rupture strength. The 9% chromium steels developed during the last three decades are of great interest in such applications. In this report, the development of steels P91, P92 and E911 is described. It is shown that the martensitic transformation in these three steels produces high dislocation density that confers significant transient hardening. However, the dislocation density decreases during exposure at service temperatures due to recovery effects and for long-term creep strength the sub-grain structure produced under different conditions is most important. The changes in the microstructure mean that great care is needed in the extrapolation of experimental data to obtain design values. Only data from tests with rupture times above 3,000 h provide reasonable extrapolated values. It is further shown that for the 9% chromium steels, oxidation resistance in steam is not sufficiently high for their use as thin-walled components at temperatures of 600°C and above. The potential for the development of steels of higher chromium contents (above 11%) to give an improvement in steam oxidation resistance whilst maintaining creep resistance to the 9% chromium steels is discussed.     

Thermoelastic Phase Analysis (TPA): a new method for fatigue behaviour analysis of steels

In literature, there are already well‐established thermal methods which allow for the estimation of fatigue limit, in particular for metallic materials such as austenitic steels. These methods are based on heat source generation analysis or on surface temperature evaluation of material subjected to different types of cyclic loading. General application of methodology found limitation in those cases in which temperature changes on material related to fatigue damage were very low and, furthermore, thermal methods require high‐performance equipment and a difficult setup. This is the case, for instance, with brittle materials (such as martensitic steels), welded joints and aluminium alloys. In this work, a new thermal method named Thermoelastic Phase Analysis is used to evaluate the fatigue limit of martensitic steels. This thermal method is based on an empirical approach. The main idea is that phase of thermoelastic response of the material subjected to fatigue loading is influenced by the presence of a heat source due to dissipative phenomena related to damage. Monitoring of the phase parameter provides a more stable setup and an independent means of identifying the fatigue limit of material. The method has also proven to be potentially one order of magnitude faster than traditional thermal methods.     

Structure and properties of ion-nitrided stainless steels

The structure and properties of ion-nitrided layers on several stainless steels, 410 martensitic stainless steel, 430 ferritic stainless steel and 321 austenitic stainless steel, has been studied under varying process conditions with microhardness-depth correlations, optical microscopy and transmission electron microscopy. The process variables studied include time (2 to 10 h) and temperature (400 to 600° C). The highest case depth values and hardness levels were observed in martensitic stainless steels. The lowest case depths were observed in austenitic stainless steel. In general, the behaviour of matensitic and ferritic stainless steels were similar. All three steels showed increasing case depths and decreasing surface hardnesses with increasing ion-nitriding temperatures and times. Nitriding depth was found to be parabolic with ion nitriding time in all three steels at all ion-nitriding temperatures investigated, the nitriding reaction being faster in martensitic stainless steel than the others. Electron microscopy showed that almost no structural difference arises in the core of ferritic and austenitic stainless steels whereas recrystallization of the martensitic structure was observed in the core of martensitic steel following ion nitriding. Electron microscopy results also showed that ion nitriding produces platelets or disc-shaped precipitates on {001} matrix planes, coherent with the matrix. These platelets showed a striated morphology which is thought to be the result of the elastic strain in the matrix.     

The design of advanced performance high strength low-carbon martensitic armour steels: Microstructural considerations

Neither a higher hardness nor higher mechanical properties (yield strength, ultimate tensile strength, impact energy, and %elongation) appear to be exclusive or even reliable criteria for predicting the ballistic performance of martensitic armour steels, as shown in our previous work [K. Maweja, W.E. Stumpf, Mater. Sci. Eng. A (February), submitted for publication]. An alternative design methodology for tempered martensitic armour steels is, therefore, proposed which is based on the effect of retained austenite on the ratio of the yield to ultimate tensile strength (YS/UTS), the microstructure of the tempered martensite and its martensite start temperature M_s. This approach was developed using 6 mm thick armour plates and later was successfully applied to the design of eight experimental armour steels with plate thicknesses ranging from 4.7 to 5.2 mm and tested by the standard R4 (5.56 mm rounds) ballistic test.     

扩散连接技术在核聚变反应堆包层模块制造中的应用

国际受控热核聚变实验堆计划是全球规模最大、影响最深远的国际科研合作项目之一,有望彻底解决能源危机。核聚变反应堆关键部件——包层模块的结构复杂、体积庞大,且服役环境恶劣,焊接接头成为影响反应堆安全运行的薄弱环节。以扩散连接为代表的固相焊接技术对接头性能及组织影响较小,已逐渐取代熔化焊应用于包层模块复杂构件制造。在简要介绍扩散连接及其原理的基础上,对包层模块构件扩散连接的研究进展进行了阐述,包括低活化铁素体/马氏体钢及氧化物弥散强化钢构件的扩散连接,Be,W,Si C等其他先进高温材料的扩散连接等。     

Influence of Hydrogen on the Equilibrium of a Dislocation Submicrocrack in α-Iron

Numerical methods are used to analyze the influence of hydrogen on the fracture of steels with bcc lattice by the mechanism of microcleavage. The behavior of submicrocracks at the head of a dislocation pileup is studied under uniaxial tension. In the case of delivery of hydrogen by merged dislocations of the pileup into the submicrocrack, it loses stability due to the adsorption decrease in the specific surface energy earlier than it becomes possible under the action of the pressure of hydrogen in the crack. We deduce the dependences of the fracture stress on the specific surface energy and the amount of hydrogen delivered by dislocations into the submicrocrack. The influence of hydrogen condensed on dislocations on the formation of submicrocracks is significant. We establish the relationship between the amount of hydrogen condensed on dislocations, the concentration of diffusing hydrogen in the metal, and rapid degradation of mechanical properties of steel even for small concentrations of dissolved hydrogen.