An equation-of-state for methane for modeling hydrogen attack in ferritic steels

A statistical mechanical-based high temperature and high pressure equation-of-state for methane has been developed using the McQuarrie and Katz formulation based on Leonard-Jones (n, 6) intermolecular potential. Fugacity coefficients for methane have been estimated, and it is shown that for plain carbon steels during hydrogen attack the methane pressures are considerably lower than the fugacities and fall into a physically meaningful range (≤2500 MPa). Further, simple, but reasonably accurate, expressions for both the equation-of-state and fugacity coefficient have been developed for the purpose of modeling hydrogen attack kinetics in ferritic steels.     

The hydrogen attack of HSLA steels

Three high strength low alloy steels with low carbon contents and varying alloy contents were exposed to 21 MPa (3000 psi) hydrogen pressure in the temperature range 350 to 510°C. The resulting sample expansion rate was measured using a highly sensitive capacitance dilatometer. In all three HSLA steels, the early expansion rate was found to be independent of exposure time, but a function of exposure temperature and pressure. The temperature dependence of the sample expansion rate was similar for all three HSLA and the reference carbon steels, and corresponded roughly to an activation energy (Q) of 190 KJ/mole. The later accelerating expansion rate fitted aQ of 160 KJ/mole. Though the sample expansion rates exhibited the same temperature dependence, they varied by as much as two orders of magnitude among the three HSLA and the carbon steels. This marked variation in rate was proportional to the estimated carbon activity in each steel. Comparison between the Nelson curves published by the Americal Petro-leum Institute (API) for 1/2 pct Mo steel and the experimental curves constructed for HSLA steels indicated that these steels should provide comparable or better resistance to hydrogen attack than the 1/2 pct Mo steel.     

The kinetics of hydrogen attack of steels

The hydrogen attack of steel involves the formation of methane bubbles along the grain boundaries and their subsequent link-up to form fissures. This paper presents a detailed model for the kinetics of growth of such methane bubbles. The model considers two parallel processes which can control the growth—one involving the bubble growth by direct power-law creep and the other involving combinations of surface diffusion, grain boundary diffusion and matrix accommodation processes. The proposed model is more general and complete than the earlier ones and considers for the first time the possibility of bubble growth being controlled by surface diffusion and accommodation processes. The predictions of the model are shown to compare well with the experimental results obtained in our lab and with the literature data. The model also indicates the relative importance of lower carbon activity and increased creep stength of steel to its hydrogen attack resistance.     

Superplasticity in low-alloy steels

Superplasticity has been investigated in three carbon-manganese steels with small additions of Nb, V, Al and Ti. Superplasticity was observed from 800 to 1000°C at strain rates from 0.002 to 0.01 min^-1. Strain rate sensitivities above 0.7 have been observed; however elongations to fracture are quite low with a maximum value observed of 184 pct. The strain rate sensitivity did not vary with strain although it was very dependent on strain rate and temperature. Cold work prior to hot tensile testing enhanced the superplasticity in the vanadium steel. The premature failure of these steels with large strain rate sensitivities has been attributed to crack nucleation due to the fine particle network from the Nb and V precipitation.     

Superplasticity in low-alloy steels

Superplasticity has been investigated in three carbon-manganese steels with small additions of Nb, V, Al and Ti. Superplasticity was observed from 800 to 1000°C at strain rates from 0.002 to 0.01 min^-1. Strain rate sensitivities above 0.7 have been observed; however elongations to fracture are quite low with a maximum value observed of 184 pct. The strain rate sensitivity did not vary with strain although it was very dependent on strain rate and temperature. Cold work prior to hot tensile testing enhanced the superplasticity in the vanadium steel. The premature failure of these steels with large strain rate sensitivities has been attributed to crack nucleation due to the fine particle network from the Nb and V precipitation.     

Evolution of secondary phases in Cr-V low-alloy steels during aging

The influence of both bulk vanadium content and aging conditions on the evolution of secondary phases in Cr-V low-alloy steels was studied. Three 0.1C-0.9Cr-V steels with different vanadium contents (0, 0.258, and 0.512 wt pct) were aged for 100 to 5,000 hours at 773, 853, 953, and 993 K. In the investigation, a limited experimental program (transmission electron microscopy (TEM)) was combined with credible thermodynamic predictions (ThermoCalc). Going out from the good agreement between the predicted and experimental results, behavior of the iron-rich M₇C₃ carbide in time-temperature scale was characterized. The influence of bulk vanadium content was determined on appearance of the M₃C carbide in equilibrium, temperature of the M₇C₃ carbide precipitation, metal compositions of M₃C or M₇C₃ carbides, and vanadium portion in the metallic part of the MX phase.     

An experimental investigation of the internal methane pressure in hydrogen attack

An experimental investigation of the internal methane pressure that is the driving force for bubble growth in hydrogen attack (HA) was done on pure iron (204 ppm C) and on two low carbon steels of slightly different compositions. The methane contentN (c.c gas/g. material) in attacked specimens was measured by a vacuum extraction technique. The total void volumeV (c.c) was determined from density measurements before and after HA exposure. The two values,N andV, were then used in an equation of state to calculate an average methane pressureP for the attack stages beyond a density loss (d.l.) greater than 0.05 pct. It was determined thatN andP depend on hydrogen exposure conditions and the presence of traces of strong carbide forming alloying elements (in steel). They are independent of specimen size and grain size over a limited range.P varies as the bubble volume increases, showing a generally decreasing trend which brings it to values lower than calculated equilibrium pressures, although well within the same order of magnitude. Possible reasons for this behavior are discussed.     

Hardness of tempered martensite in carbon and low-alloy steels

This paper presents the results of a systematic study of the effect of carbon, manganese, phosphorus, silicon, nickel, chromium, molybdenum, and vanadium on the hardness of martensite in low to medium carbon steels tempered for one hour at 100°F (56°C) intervals in the range 400 to 1300°F (204 to 704°C). Results show that the as-quenched hardness depends solely on carbon content. On tempering, the effect of carbon on hardness decreases markedly with increasing tempering temperature. Studies of carbon-0.5 manganese steels showed that the incremental increase in hardness from 0.5 pct manganese after a given tempering treatment was independent of carbon content. Based on this result, studies of the effects of the other alloying elements were made using a 0.2 or 0.3 pct carbon, 0.3 to 0.5 pct manganese steel base composition. The hardness of the resulting tempered martensite was assumed to be due to a given alloy addition, and when two or more alloying elements were added, their effects were assumed to be additive. Each of the seven alloying elements increased the hardness of tempered martensite by varying amounts, the increase being greater as more of each element was present. Nickel and phosphorus have substantially the same effect at all tempering temperatures. Manganese has essentially the same hardening effect at any temperature in the range 700 (371°C) to 1300°F; silicon is most effective at 600°F (316°C), chromium at 800°F (427°C), molybdenum at 1000 to 1100°F (538 to 592°C), and vanadium at 1200°F (649°C). Using the data obtained, a procedure is established for calculating the hardness of tempered martensite for carbon and alloy steel compositions in the range studied and for any combination of tempering time and temperature. R. A. GRANGE was formerly with U. S. Steel Corporation (retired)     

Modelling of austenite stability in low-alloy triple-phase steels

A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, i.e. chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels.     

Stress-assisted hydrogen attack cracking in 2.25Cr-1Mo steels at elevated temperatures

Crack growth in 2.25Cr-lMo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 °C to 500 °C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure, where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels. The crack growth rate increased with material strength. Above a threshold of about K_l = 20 MPa√m (at 480 °C), the crack growth rate increased rapidly with stress intensity, increasing as roughly K_l ^6.5. Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity, and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be substantially faster than in the base metal of the welds. Analysis indicates that K_l rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel. The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of crack growth and limited crack-tip opening displacement. Formerly Graduate Student, Materials Science and Engineering Department, The Ohio State University     

X射线荧光光谱法测定低合金钢中的12个元素

用高性能飞利浦PW2403X射线荧光光谱仪，测定中低合金钢中Si、Mn,P,S,Cr、Ni、Cu、V、Ti、Mo、As等12个化学元素．给出了元素的干扰和基体效应校正系数．该方法准确、灵敏、稳定性好、速度快．     

Effect of deoxidation practice and heat treatment on the hydrogen attack of carbon steels

The hydrogen attack (HA) kinetics of an electroslag refined (ESR) and a rare earth metal (REM)-treated steel in the Q. and T. condition were investigated by a highly sensitive dilatometer. Measured activation energies for bubble growth of 108 to 203 kJ/mol and pressure exponents of 0.9 to 1.6 are rationalized in terms of surface or grain boundary self-diffusion of iron as the rate controlling mechanisms depending on the external hydrogen pressure and temperature. Comparisons of the HA susceptibility of these steels with published work show that although the HA resistance of the ESR steel is not influenced by the heat treatment, the REM steel shows a significant decrease in the rate of sample expansion. SEM observations indicate that the improvement in the HA resistance of the REM steel is related to the presence of a very low density of methane bubbles.     

Austenite stabilization from direct cementite conversion in low-alloy steels

Tr ansformation i nduced p lasticity (TRIP) effects associated with austenite dispersions in low alloy Fe-Mn-Si steels can be enhanced by austenite stabilisation. Austenite which forms during conventional intercritical annealing does not possess the required stability in order to exhibit TRIP effects. In this work, thermodynamic calculations indicated that it is feasible to form austenite by a cementite to austenite conversion which occurs under paraequilibrium conditions, i.e with partition of carbon but with no partition of substitutional alloying elements. In this way the austenite inherits the manganese content of cementite and is chemically stabilised. A treatment consisting of a two-step annealing has been examined. In the first step, soft annealing, an Mn-enriched cementite dispersion in ferrite is formed. In the second step, intercritical annealing, austenite nucleates on the cementite particles, which are consumed to form austenite. It was experimentally determined that this austenite has been enriched in manganese and carbon and, therefore, is stabilised. The conversion reaction is followed by the conventional austenite nucleation at ferrite grain boundaries. This austenite is lean in manganese and is not stable. The net effect of the two-step annealing treatment is a significant austenite stabilisation relative to simple intercritical annealing, indicating a potential for enhanced TRIP effects in this class of steels.     

Erosion corrosion of low-alloy wear-resistant steels in alkaline slurry

Erosion corrosion causes significant problems in various industrial environments through a synergistic effect which results in much greater weight loss than the sum of the weight losses in the individual processes. The erosion-corrosion behavior of three low-alloy steels was investigated in a simulated concrete slurry using the rotation method. The key influencing factors and mechanism of material degradation were analyzed. The experimental results indicate that the weight loss increases with the linear velocity according to a nearly exponential relationship (W = KVⁿ), where n is 1. 40–2. 14. This weight loss is mainly caused by erosion in the alkaline slurry, and steels with higher tensile strengths show higher erosion-corrosion resistance. The formation of many platelets and ring cracks and their removal from the sample surface during erosion corrosion in the slurry are thought to constitute the mechanism responsible for this weight loss. These platelets and ring cracks are formed by solid particles striking the sample surface. Craters are initially produced and subsequently disappear as they grow and come in contact with each other. Fewer craters were observed on the surfaces of samples that exhibited higher weight loss. The surface of the material became work-hardened because of the effect of the particles striking and scratching, and a deformed layer was produced on the surface for steels of lower strengths, leading to deeper and more abundant gouges.     

Effect of prestrain on stretch-zone formation during ductile fracture of Cu-strengthened high-strength low-alloy steels

The effects of prestrain on the ductile fracture behavior of two varieties of Cu-strengthened high-strength low-alloy (HSLA) steels have been investigated through stretch-zone geometry measurements. It is noted that the ductile fracture-initiation toughness of both the steels remained unaltered up to prestrains of ∼2 pct, beyond which the toughness decreased sharply. A methodology for estimating the stretch-zone dimensions is proposed. Fracture-toughness estimations through stretch-zone width (SZW) and stretch-zone depth (SZD) measurements revealed that the nature of the variation of ductile fracture toughness with prestrain can be better predicted through SZD rather than the SZW measurements. However, for the specimen geometries and prestrain levels that were investigated, none of these methods were found suitable for quantifying the initiation fracture toughness.     

ICP-AES法同时测定低合金钢中锆和铌

利用全谱ICP -AES(CID检测器 )分析技术 ,对试样溶解方法、元素分析谱线、共存元素干扰、背景校正、仪器分析参数 (射频发生器功率、雾化器压力和泵提升量 )等因素进行了研究 ,综合确定了最佳实验条件 ,并采用稀硫酸溶样后 ,经硫磷酸冒烟 ,直接进行试样前处理 ,建立了一种可同时测定低合金钢中Zr和Nb含量的简单、快速和实用的分析方法。结果表明 :本法测定钢中锆和铌含量的分析误差和精密度符合国标GB2 2 3 3 0 -94和GB2 2 3 3 9-94的技术要求 ,其检出限均为0.0 0 0     

Coarsening kinetics of multicomponent MC-type carbides in high-strength low-alloy steels

Morphology and coarsening kinetics of MC-type carbide (MC-carbide) precipitating during the tempering process have been investigated in V- and Nb-bearing Cr-Mo martensitic steels. Detailed transmission electron microscopy (TEM) observations show that the addition of V and Nb stabilizes the B1-type MC-carbide instead of L’3-type M₂C-carbide. The morphology of the MC-carbide is characterized as disk-like with Baker and Nutting orientation relationships with the matrix. When the specimens are fully solution treated followed by quenching, the MC-carbide precipitates as a multicomponent system with continuous solid solution of VC, NbC, and MoC. The V-, Nb-, and Mo-partitioning control the lattice parameter of MC-carbide and consequently affect the coherency between MC-carbide and the matrix. The coherent MC-carbide grows into an incoherent one with the progress of tempering. The numerical analysis on TEM observations has shown that the coarsening kinetics of MC-carbide is equated to (time)^1/5 criteria, while the coarsening kinetics of the coexisting cementite is equated to (time)^1/3 criteria. It is thus suggested that the Ostwald ripening of MC-carbide is controlled by pipe diffusion of V, Nb, and Mo along dislocations. It has been confirmed that the coarsening rate of the multicomponent MC-carbide is affected by V, Nb, and Mo content. Applying the thermodynamic solution database, the rate equation for MC-carbide coarsening can be expressed as a function of V, Nb, and Mo content, and the activation energy for pipe diffusion can be estimated as ΔQ _v: ΔQ _Nb: ΔQ _Mo=1:3.9:0.6.