Fatigue crack propagation in intercritically tempered Fe-9Ni-0.1C and Fe-4Mn-0.15C

Fatigue crack propagation was studied for two intercritically tempered cryogenic steels, Fe-9Ni-0.1C and Fe-4Mn-0.15C, at both intermediate (stage II) and low (stage I, near threshold) stress intensity ranges. Propagation rates were determined for varying intercritical tempering times corresponding to varying amounts of retained austenite and untempered martensite. The results show that the heat treatments that optimize impact fracture properties in the nickel steel are also beneficial with respect to the fatigue crack propagation rate in stage I, while no beneficial effect beyond that attributable to carbon redistribution was observed for stage II. For the manganese steel, heat treatments leading to increased concentrations of retained austenite also increased the threshold stress even though no improvement in fracture toughness was observed. To clarify the origin of this improved behavior, the fracture surface was analyzed by Mössbauer Spectroscopy and Auger Electron Microprobe. The Mössbauer results indicated that the retained austenite in the crack path is transformed to martensite as was earlier shown in this laboratory for Charpy specimens. Auger composition analysis suggested a tendency for a stage I crack tip to avoid the mechanically induced brittle untempered martensite in the Fe-Mn steel, while no such preference was observed for stage II.     

Hot tearing susceptibility of Fe-20.96 Cr-2.13 Ni-0.15 N-4.76 Mn-0.01 Mo duplex stainless steel

The hot tearing susceptibility of a Fe-20.96Cr-2.13Ni-0.15N-4.76Mn-0.01 Mo duplex stainless steel was investigated using method of constrained solidification shrinkage in one dimension.An apparatus for realtime measuring the contraction stress and temperature during solidification was developed,which can achieve the in-situ observation of melting and solidification and avoid the large temperature gradient of casting under the condition of pouring.The results show that the contraction stress increases significantly when the core temperature of casting reaches the liquidus temperature.The contraction stress is released when the core temperature of casting reaches 1456°C.At this temperature,the hot tearing susceptibility of duplex stainless steel is the largest.With decreasing the core temperature to 1363°C,the slope of contraction stress increases,which is related to the ferrite-to-austenite transformation.     

Growth Kinetics of Proeutectoid Ferrite in Fe-0.1C-1.5Mn-1Si Quaternary and Fe-0.1C-1.5Mn-1Si-0.2Al Quinary Alloys

The growth kinetics of proeutectoid ferrite in the early stages of transformation were studied in Fe-0.1C-1.5Mn-1Si (mass pct) quaternary and Fe-0.1C-1.5Mn-1Si-0.2Al quinary alloys. The observed kinetic transition temperatures from partitioned slow growth to unpartitioned fast growth of ferrite in both alloys are in good agreement with predictions using a local equilibrium model for multicomponent systems. The measured parabolic growth rate constants were smaller than those calculated assuming paraequilibrium in the unpartitioned growth region, but the difference between the measured and the calculated growth rate constants gradually diminished with a decreasing reaction temperature. The dissipation of driving force, derived from the diffusion of the substitutional solute within the transformation interface, possibly constitutes a major part of the discrepancy between the measured and the calculated growth kinetics.     

Creep and rupture properties of an austenitic Fe-30Mn-9Al-1C alloy

The creep deformation behavior and rupture properties of as-quenched austenitic Fe-30Mn-9Al-1C alloy have been studied at 923, 948, and 973 K under applied stresses ranging from 50 to 350 MPa. The creep curves of the alloy exhibited an extended tertiary stage prior to failure. The stress and temperature dependencies of the minimum creep rate indicated two regimes of creep deformation as well as a transition from creep to power-law breakdown. These two regimes of creep deformation were identified as a low-stress creep regime having an activation energy of 140 kJ/mol and a stress exponent of about 1, and a power-law creep regime having an activation energy of 350 kJ/mol and a stress exponent of about 6. Transmission electron microscope (TEM) observations of the deformed specimens revealed that a low density of dislocations, coarse dislocation networks, and profuse slip bands were developed in the low stress, power law, and power-law breakdown regimes, respectively. Optical microscope and scanning electron microscope (SEM) observations of the ruptured specimens showed that creep cavitation shifted from round-type in the low-stress creep regime to wedge-type in the power-law breakdown regime. The observed creep and rupture characteristics of the alloy are interpreted in terms of creep mechanisms, which involve the Coble creep and dislocation climb creep.     

Crack Propagation in Flexural Fatigue of Concrete

In this paper the behavior of concrete subjected to flexural fatigue loading is studied. Notched concrete beams were tested in a three-point bending configuration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static loading response. Low cycle, high amplitude fatigue tests were performed to failure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagation caused by quasi-static and fatigue loads is described in terms of fracture mechanics. It is shown that the crack propagation in the postpeak part of the quasi-static load response is predicted using the critical value of the mode I stress intensity factor (KIC). The ultimate deformation of the specimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration stage when there is a decrease in crack growth rate with increasing crack length, followed by an acceleration stage where the rate of crack growth increases at a steady rate. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the peak load of the quasi-static response. Analytical expressions for crack growth in the deceleration and acceleration stages are developed, wherein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the corresponding load in the postpeak part of the quasi-static response. Finally, a fracture-based fatigue failure criterion is proposed.     

Molecular dynamics simulation of atomic displacement cascades in Fe-9 at % Cr and Fe-9 at % Cr-0.1 at % C alloys

The atomic displacement cascades in Fe-9 at % Cr and Fe-9 at % Cr-0.1 at % C alloys are studied by molecular dynamics simulation at an initial temperature of 600 K. The average fractions of defects that “survive” in a displacement cascade are calculated for primary knocked-on atom (PKA) energies of 0.1–20 keV, and the number and size of the vacancy and interstitial-atom clusters that form in a displacement cascade are determined. Carbon at the concentration under study is found not to affect the number of survived defects and the cascade efficiency. At PKA energies of 15 and 20 keV, the presence of carbon slightly decreases the fraction of point defects forming clusters.     

Microstructure Evolution and Age-Hardening Behavior of Microalloyed Austenitic Fe-30Mn-9Al-0.9C Light-Weight Steels

The aging behavior and mechanical properties of microalloyed austenitic Fe-30Mn-9Al-0.9C light-weight steels were investigated through transmission electron microscopy analysis, electron backscatter diffraction analysis, tensile tests, and Vickers hardness tests. The base steels were aged at 823 K (550 °C) for up to 10,000 minutes. The true stress–strain responses of solution-treated samples before aging showed that the addition of Nb and/or V improved the strength by grain refinement and precipitation hardening. During the process of tensile deformation, the strain-hardening rate of Fe-30Mn-9Al-0.9C steel steadily increased due to the microband-induced plasticity (MBIP) from the onset of plastic deformation to ε = 25 pct, while such behavior was weakened and not observed in Nb- and/or V-added steels despite MBIP. In the early stage of aging, the Vickers hardness gradually increased with an increase in the aging time due to the precipitation of κ-carbide of (Fe,Mn)₃AlC and remained stagnant between the aging times of 1000 and 3000 minutes. The hardness increased again after 3000 minutes due to the formation of ferrite and brittle β-Mn.     

Tensile Fracture Modes in Fe-22Mn-0.6C and Fe-30Mn-3Si-3Al Twinning-Induced Plasticity (TWIP) Steels

Tensile tests were carried out to investigate the differences in fracture mechanisms between Fe-22Mn-0.6C and Fe-30Mn-3Si-3Al (wt pct) twinning-induced plasticity steels. Although both steels possess a strong twinning capability during tensile deformation, they display different tensile fracture modes of shear and necking. The Portevin–le Chatelier band is proposed as the key factor influencing the different fracture mechanisms.     

Fe-5Mn-2Al-0.15C中锰钢连铸凝固偏析及粒子析出行为

为研究Fe-Mn-Al-C系中锰钢连铸凝固偏析及粒子析出特性,采用光学显微镜观察Fe-5Mn-2Al-0.15C中锰钢的显微组织,并通过Thermo-Calc热力学软件研究了其凝固模式、溶质元素偏析及粒子析出行为。结果表明,该中锰钢铸锭的显微组织主要为板条状马氏体,且含有少量铁素体;其凝固模式为L→L+δ→L+δ+γ→δ+γ→γ;Al元素的平衡分配系数大于1,发生负偏析,偏聚到δ-铁素体枝晶内部;而Mn、Nb、V、S等溶质元素发生正偏析,均偏聚到枝晶间。AlN主要在枝晶内析出,其析出温度为1 448 ℃;MnS、Nb和V的富集物主要在枝晶间析出,且MnS和Nb的富集物均在1 400 ℃以上开始析出,而V的富集物的析出温度为760 ℃。     

Analytical Prediction of Fatigue Crack Propagation in Metals

An analytical model for fatigue crack propagation of long cracks in metals and metal alloys is presented. The key features of the model are an extension of Griffith’s theory of fracture to include fatigue, a dislocation model for the crack tip opening displacement, and cyclic plasticity-induced closure. The net cyclic stretch of the process zone at the crack tip plays a major role in the fatigue crack propagation under cyclic loading. Only constant amplitude loading is considered in this paper. The model predictions utilize only the readily available material properties, such as Young’s modulus, yield strength, threshold stress intensity factor, and the fracture toughness. There are no empirical fitting constants. The model predictions are validated by an extensive amount of published fatigue crack growth studies. The agreement between the model predictions and the experimental data is good.     

Hot corrosion of Fe-10Cr-18Ni-5Mn-5Al-Si-N steels in molten sulfate deposits

The effect of Na₂SO₄ coating on the corrosion behaviour of 5 steel alloys as well as two commercial alloys - type 304 stainless steel and Inconel 690 - were tested in air at 900 °C under atmospheric pressure. Corroded samples were analysed by XRD, SEM and EDX. A newly developed alloy (No. 5) having the composition Fe-10.4Cr-15.43Ni-5.52Mn-5.52AI-1.5Si showed almost comparable corrosion resistance with Inconel 690. Stainless steel 304 failed catastrophically. Other newly developed alloys (no. 1-4) showed intermediate resistance. The results have been interpreted in the light of the nature of scales formed and on the premise that Na₂SO₄ decomposes to give S^2- ions which delay the formation of an internal protective film.     

Phase transformation in an Fe-10.1Al-28.6Mn-0.46C alloy

The microstructure of an (α + γ) duplex Fe-10.1Al-28.6Mn-0.46C alloy has been investigated by means of optical microscopy and transmission electron microscopy (TEM). In the as-quenched condition, extremely fine D0₃ particles could be observed within the ferrite phase. During the early stage of isothermal aging at 550 °C, the D0₃ particles grew rapidly, especially the D0₃ particles in the vicinity of the α/γ grain boundary. After prolonged aging at 550 °C, coarse K’-phase (Fe, Mn)₃AlC precipitates began to appear at the regions contiguous to the D0₃ particles, and —Mn precipitates occurred on the α/γ and α/α grain boundaries. Subsequently, the grain boundary β-Mn precipitates grew into the adjacent austenite grains accompanied by a γ→ α + β-Mn transition. When the alloy was aged at 650 °C for short times, coarse. K-phase precipitates were formed on the α/γ grain boundary. With increasing the aging time, the α/γ grain boundary migrated into the adjacent austenite grain, owing to the heterogeneous precipitation of the Mn-enrichedK phase on the grain boundary. However, the α/γ grain boundary migrated into the adjacent ferrite grain, even though coarse K-phase precipitates were also formed on the α/γ grain boundary in the specimen aged at 750 °C.     

Hydride-Phase Formation and its Influence on Fatigue Crack Propagation Behavior in a Zircaloy-4 Alloy

The hydride-phase formation and its influence on the fatigue behavior of a Zircaloy-4 alloy charged with hydrogen gas are investigated. First, the microstructure and fatigue crack propagation rate of the alloy in the as-received condition are studied. Second, the formation and homogeneous distribution of the delta zirconium hydride in the bulk and its effect on the fatigue crack propagation rate are presented. The results show that in the presence of hydrides, the zirconium alloy exhibits reduced toughness and enhanced crack growth rates. Finally, the influence of a preexisting fatigue crack in the specimen and the subsequent hydride formation are examined. The residual lattice strain profile around the fatigue crack tip is measured using neutron diffraction. It is observed that the combined effects of residual strains and hydride precipitation on the fatigue behavior are more severe leading to propagation of the crack under near threshold loading.     

Effects of Pre-tempering on Intercritical Annealing in Fe-2Mn-0.3C Alloy

As-quenched martensite was pre-tempered at 623 K and 923 K (350 °C and 650 °C), and then it reverted to austenite by intercritical annealing at 998 K (725 °C) in a Fe-2Mn-0.3C alloy. Pre-tempering at 623 K (350 °C) accelerates austenite formation, while pre-tempering at 923 K (650 °C) significantly retards it. It is proposed that austenite nucleation is accelerated by increasing the number density and particle size of cementite during tempering, whereas austenite growth is retarded by Mn enrichment in cementite during tempering at high temperature, leading to opposite effects of pre-tempering on reversion kinetics.     

Fe-0.5Mn-0.5C预混合钢粉性能及粘结剂作用机制

设计4种不同有机粘结剂,分别对合金元素Mn、石墨进行粘结处理,制备Fe-0.5Mn-0.5C预混合钢粉。研究4种粘结剂对预混合钢粉合金元素与石墨的粘结率、预混合钢粉的流动性、松装密度、压坯密度的影响。并通过对粉末表面基团的表征,研究高分子粘结剂与铁基体间的相互作用方式。结果表明:以丙烯酸类树脂制备的预混合钢粉工艺性能最好,其流动速率为24.3 s/50 g松装密度为24.3 s/50 g,在600 MPa压力下的压坯密度为3.03~3.23 g/cm3粘结剂中的极性基团与铁基粉末通过氢键作用相吸附。     

Fe-22Mn-0.7CTWIP钢的热塑性与断裂机制

基于Gleeble-1500热力模拟试验机测定了Fe-22Mn-0.7C TWIP钢和Q235钢700~1300℃范围内的静态拉伸行为.采用光学显微镜、扫描电子显微镜、能谱仪、电子探针微区分析等技术表征两钢种不同温度下的变形特征和断口形貌.通过分析基体化学成分、相体积分数、晶粒尺寸、凝固缺陷等因素探讨TWIP钢铸态热塑性的变化规律及其影响机制.研究结果表明,Fe-22Mn-0.7C TWIP钢700~1250℃范围内的铸态抗拉强度高于Q235,而其断面收缩率低于40%,且断口均以沿枝晶间断裂方式为主.晶粒细化和控制溶质显微偏析有利于提高TWIP钢热塑性,与基体均质性改善有关.此外,增加应变速率TWIP钢拉伸强度和断面收缩率同时增大.     

The Tensile Properties of an Fe-4 Pct Mo-0.2 Pct C Martensite Tempered under Applied Stress

In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure occurred in tensile test at 600 °C to 700 °C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P, S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries. Formerly with Kyoto University Formerly with Kyoto University This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.