Transformation of retained austenite in carburized 4320 steel

A systematic study of stress-induced and thermal-induced transformation of retained austenite in carburized 4320 steel with an initial retained austenite of 35 pct has been conducted. The transformation was monitored by recording the change in volume of smooth fatigue specimens. Stress-induced transformation was studied by conducting monotonic and cyclic tests at temperatures in the range from 22 °C to 150 °C. The volumetric transformation strain was as large as 0.006 at 22 °C. The anisotropy of the transformation was such that the axial transformation strain component exceeded the diametral transformation strain component by a factor of 1.4. Thermal-induced transformation was investigated with temperature stepup tests in the range from 150 °C to 255 °C at constant stress (-500 MPa, 0 MPa, and 500 MPa) and with static tests where temperature was held constant at zero load. The maximum thermal-induced volumetric transformation strain of 0.006 was independent of stress. However, the anisotropy of the transformation strain components was dependent on stress direction and magnitude. An axial tensile stress increased the axial transformation strain relative to the diametral transformation strain. The influence of low-temperature creep(T = 150 °C) on the anisotropy of strains is noted. The differences between stress-induced and thermal-induced transformation mechanisms are discussed. Thermal-induced transformation primarily occurred at temperatures between 100 °C and 200 °C, with the rate of transformation increasing with temperature, while the stress-induced transformation primarily occurred at 22 °C, with the rate of transformation decreasing with increasing temperature. There was no stress-induced transformation above 60 °C.     

Transition in Failure Mechanism Under Cyclic Creep in 316LN Austenitic Stainless Steel

Cyclic creep behavior of a type 316LN austenitic stainless steel was investigated in the temperature range from 823 K to 923 K (550 °C to 650 °C). A transition from fatigue-dominated to creep-dominated failure mode was observed with an increase in the mean stress. The threshold value of mean stress for the transition was seen to be a strong function of the test temperature. Occurrence of dynamic strain aging proved beneficial owing to a substantial reduction in the strain accumulation during cyclic loading.     

Analysis of the creep behavior of silicon carbide whisker reinforced 2124 Al(T4)

The effect of stress and temperature on the steady state creep rate of SiC_w/2124 Al (T4) has been determined. The stress exponent for steady state creep of the composite is shown to increase from a value of 8.4 at 177 °C to a value of 21 at 288 °C. The activation energy for creep was determined to be 277 kJ/mol for testing in the temperature range from 149 to 204 °C and 431 kJ/mol for testing from 274 to 302 °C. These values are much greater than that for self-diffusion in aluminum. Such a severe temperature and stress dependence of the steady state creep rate is characteristic of precipitation and oxide dispersion strengthened nickel-base superalloys, where the creep behavior is explained by the particle strengthening contribution being a significant fraction of the applied creep stress. In contrast, the estimated particle strengthening for the composite is much less than the applied creep stresses. Alternate strengthening mechanisms are proposed to account for the observed creep behavior of the composite material, including the effect of temperature on the measured values of the stress exponent and activation energy for creep.     

Effect of Impurity Tin on the Creep Properties of a P91 Heat-Resistant Steel

The creep properties of P91 steel specimens undoped and doped with 0.058 wt pct tin, which was normalized from 1328 K (1055 °C) and tempered at 1033 K (760 °C), were examined under different engineering stresses (150 to 210 MPa) and temperatures [873 K to 923 K (600 °C to 650 °C)]. The creep behavior followed the temperature-compensated power law and Monkman–Grant equations. In the temperature-compensated power law equation, the apparent activation energy and stress exponent for creep were approximately 541 kJ/mol and 12 for the undoped steel and 527 kJ/mol and 11 for the Sn-doped one, respectively. In the Monkman–Grant relation, the values of constants m and C were around 1.062 and 0.0672 for the undoped steel, and 1.012 and 0.0650 for the Sn-doped one, respectively. The 100 MPa stress creep lifetime at 873 K (600 °C) was estimated as 100641 hours for the undoped steel and 35290 hours for the Sn-doped steel, respectively. These indicated that Sn substantially deteriorated the creep properties of the steel. It was found that grain or subgrain boundary segregation of Sn could promote the nucleation of cavities or microcracks, thereby leading to the deterioration of the steel creep properties.     

Dependence of Accelerated Creep Rate at 580 °C and Room Temperature Yield Stress for Two Creep Resistant Steels

Two creep resistant steels, P91 and X20, were tempered for 17520 h at 650 °C or 8760 h at 750 °C to study the growth and redistribution of carbide precipitates in martensite. On specimens annealed for a different time, yield stress at room temperature and accelerated creep rate at 580 °C were determined. With increasing yield stress in the range from 350 to 650 MPa the accelerated creep rate decreased continuously by about 2 orders of magnitude from 8·10^?7 s^?1 to 5·10^?9 s^?1. For equal yield stress, the creep rate was slightly lower for the steel P91 than for the steel X20.     

Elevated temperature creep-fatigue crack propagation in nickel-base alloys and 1 Cr-Mo-V steel

The crack growth behavior of several high temperature nickel-base alloys, under cyclic and static loading, is studied and reviewed. In the oxide dispersion strengthened (ODS) MA 6000 and MA 754 alloys, the high temperature crack propagation exhibited orientation dependence under cyclic as well as under static loading. The creep crack growth (CCG) behavior of cast nickel-base IN-738 and IN-939* superalloys at 850 °C could be characterized by the stress intensity factor,K ₁. In the case of the alloy IN-901 at 500 °C and 600 °C,K ₁ was found to be the relevant parameter to characterize the creep crack growth behavior. The energy rate line integral,C*, may be the appropriate loading parameter to describe the creep crack growth behavior of the nickel-iron base IN-800H alloy at 800 °C. The creep crack growth data of 1 Cr-Mo-V steel, with bainitic microstructure, at 550 °C could be correlated better by C * than byK ₁. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Short-term creep of Cт3 steel

The behavior of Cт3 structural steel in brief heating (no more than 2 h) at 450–650°C is investigated, when the initial stress is below the yield point. In these conditions, the initial ferrite–pearlite structure of the steel is relatively stable, and no recrystallization is observed. At a test temperature of 450°C, when the stress is ~0.8 of its yield point at 20°C, Cт3 steel may fail within a matter of hours or days. At 650°C, when the stress is σ/σ _0.2 ²⁰ > 0.8 of the yield point at 20°C, failure sets in within minutes. The calculated activation energy of creep for steel is 310.8–387.8 kJ/mol, which is comparable with the activation energy for Cт3 the self-diffusion of α iron.     

Creep and stress rupture of a mechanically alloyed oxide dispersion and precipitation strengthened nickel-base superalloy

The creep and stress rupture behavior of a mechanically alloyed oxide dispersion strengthened (ODS) and γ′ precipitation strengthened nickel-base alloy (alloy MA 6000E) was studied at intermediate and elevated temperatures. At 760 °C, MA 6000E exhibits the high creep strength characteristic of nickel-base superalloys and at 1093 °C the creep strength is superior to other ODS nickel-base alloys. The stress dependence of the creep rate is very sharp at both test temperatures and the apparent creep activation energy measured around 760 °C is high, much larger in magnitude than the self-diffusion energy. Stress rupture in this large grain size material is transgranular and crystallographic cracking is observed. The rupture ductility is dependent on creep strain rate, but usually is low. These and accompanying microstructural results are discussed with respect to other ODS alloys and superalloys and the creep behavior is rationalized by invoking a recently-developed resisting stress model of creep in materials strengthened by second phase particles. The analysis indicates that at the intermediate temperature the creep strength is controlled by the high volume fraction of γ′ precipitates and the contribution to the creep strength from the oxide dispersion is small. At the elevated temperature, the creep strength is derived mainly from the inert oxide dispersoids. Formerly at Columbia University.     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

Stress Rupture Fracture Model and Microstructure Evolution for Waspaloy

Stress rupture behavior and microstructure evolution of nickel-based superalloy Waspaloy specimens from tenon teeth of an as-received 60,000-hour service-exposed gas turbine disk were studied between 923 K and 1088 K (650 °C and 815 °C) under initial applied stresses varying from 150 to 840 MPa. Good microstructure stability and performance were verified for this turbine disk prior to stress rupture testing. Microstructure instability, such as the coarsening and dissolution of γ′ precipitates at the varying test conditions, was observed to be increased with temperature and reduced stress. Little microstructure variation was observed at 923 K (650 °C). Only secondary γ′ instability occurred at 973 K (700 °C). Four fracture mechanisms were obtained. Transgranular creep fracture was exhibited up to 923 K (650 °C) and at high stress. A mixed mode of transgranular and intergranular creep fracture occurred with reduced stress as a transition to intergranular creep fracture (ICF) at low stress. ICF was dominated by grain boundary sliding at low temperature and by the nucleation and growth of grain boundary cavities due to microstructure instability at high temperature. The fracture mechanism map and microstructure-related fracture model were constructed. Residual lifetime was also evaluated by the Larson–Miller parameter method.     

Dynamic strain aging during creep of α-Zr

In the temperature range 723 to 823 K (450° to 550°C) annealed, crystal bar α-Zr exhibits anomalous behavior with respect to both single stress and incrementally stressed creep tests. The nature and extent of the anomalous behavior depends on temperature, stress, and impurity content. Specimens with low oxygen content exhibit: 1) normal, three-stage creep behavior during single stress tests, and 2) normal transients during incremental stress and temperature tests. Specimens with higher oxygen contents exhibit: 1) multi-stage creep curves whose shapes depend on temperature and stress, 2) inverse transients following stress and temperature increments, and 3) peaks in activation energy-tempera-ture curves. The nature of the anomalous behavior is consistent with a model for strain aging in which the possibility of localized depletion of the strain aging species exists. In the material being studied oxygen is probably responsible for the observed effects. R. D. WARDA, formerly of Department of Metallurgy, University of British Columbia     

Elevated temperature creep and fracture properties of the 62Cu-35Au-3Ni braze alloy

The Cu-Au-Ni braze alloys are used for metal/ceramic brazes in electronic assemblies because of their good wetting characteristics and low vapor pressure. We have studied the tensile creep properties of annealed 62Cu-35Au-3Ni alloy over the temperature range 250 °C to 750 °C. Two power-law equations have been developed for the minimum creep rate as a function of true stress and temperature. At the highest temperatures studied (650 °C and 750 °C), the minimum creep rate is well described with a stress exponent of 3.0, which can be rationalized in the context of Class I solid solution strengthening. The inverted shape of the creep curves observed at these temperatures is also consistent with Class I alloy behavior. At lower temperatures, power-law creep is well described with a stress exponent of 7.5, and normal three-stage creep curves are observed. Intergranular creep damage, along with minimum values of strain to fracture, is most apparent at 450 °C and 550 °C. The lower stress exponent in the Class I alloy regime helps to increase the strain to fracture at higher temperatures (650 °C and 750 °C). The minimum creep rate behavior of the 62Cu-35Au-3Ni alloy is also compared with those of the 74.2Cu-25. 8Au alloy and pure Cu. This comparison indicates that the 62Cu-35Au-3Ni has considerably higher creep strength than pure Cu. This fact suggests that the 62Cu-35Au-3Ni braze alloy can be used in low mismatch metal-to-ceramic braze joints such as Mo to metallized alumina ceramic with few problems. However, careful joint design may be essential for the use of this alloy in high thermal mismatch metal-to-ceramic braze joints.     

Creep of fine wires of W-ThO2 alloys

The increasing interest in the application of fine wires as high strength structural components,e.g., in high temperature composite materials, makes an understanding of the differences between the creep behavior of large specimens and that of fine wires desirable. In this investigation, the creep properties of fine wires of recrystallized W-1 wt pct ThO₂ were studied over the temperature range between 1800° to 2750°C. In tungsten wires in which the dispersion of fine thoria particles stabilized a fine grained structure, the stress dependence of the creep rate varies with test temperature and stress. For test temperatures below 2500°C, a stress dependence ofn ~ 5 was found, indicative of creep deformation due to dislocation climb processes, while for temperatures above 2500°C and low stresses, values ofn < 2 were obtained, indicative of deformation by grain boundary sliding and diffusional creep processes. In wires which recrystallized to a large-grained structure, having a large aspect ratio, a high stress dependence of 15 was found when tested at 1800°C.     

Creep of an AISI 310 type stainless steel and its numerical simulation using the Öström-Lagneborg creep model

The creep behaviour of an AISI 310 type stainless steel was determined under constant load (stress range 80–320 MPa) at a temperature of 700°C. The stress exponent, n, monotonically increased with the applied stress from a value of 2.8 to 16. The activation energy for creep, Q_c, measured at 170, 200 and 230 MPa over the temperature range 650–800°C, is 341 kJ/mol. An activation energy of this magnitude indicates that the alloying elements in this steel are involved in the recovery climb process of the dislocation network. A simple and practical simulation was made of the experimental creep results by using the Öström-Lagneborg creep model, and the results are compared to other independent models and experimental results. A good fit between the experimental results and the calculated strain-time curves can be obtained by adjustment of model parameters to out material. The applicability of the Öström-Lagneborg theory to the creep involving subgrain structure formation is evaluated in light of the elastic theories for subgrain boundaries and recent experimental findings. By considering only forest dislocations not incorporated in subgrain boundaries and introducing a subgrain structure function S_g(t), the Öström-Lagneborg model is able to simulate the creep behaviour where a subgrain structure is formed during the creep test. Further refinement of the theory is suggested whereby an assessment is made of the dislocation network coarsening kinetics.     

Creep behavior of Ti-25Al-10Nb-3V-1Mo

A study has been made of the role of microstracture in room-temperature tensile properties as well as elevated-temperature creep behavior of an advanced Ti₃Al-base alloy, Ti-25Al-10Nb-3V-lMo (atomic percent). Creep studies have been performed on this alloy as a function of stress and temperature between 650 °C and 870 °C, since the use of conventional titanium alloys has generally been restricted to temperatures below 600 °C. A pronounced influence of microstructure on creep resistance was found. Generally, the β solution-treated colony-type (slow-cooled or SC) microstructure showed superior creep resistance. This improved creep resistance in β/SC is accompanied by lower room-temperature tensile strength and ductility. Study of the stress dependence of steady-state creep rate indicates that increasing temperature caused a gradual decrease in the stress exponentn and a transition in creep mechanism at 870 °C, depending on applied stress level. Transmission electron microscopy observations of deformed dislocation structures developed during steady-state creep and room-temperature tensile tests, as well as the corresponding fracture modes, were used to interpret properties as a function of temperature. Finally, creep behavior of the present Ti₃Al alloy was found to be superior to that of conventional near-α titanium alloys. WONSUK CHO, formerly with Carnegie Mellon University, is Senior Research Staff Member, Kia Technical Center, Yeoeuido, P.O. Box 560, Seoul, Korea. JAMES WILLIAMS, formerly Dean of Engineering, Carnegie Mellon University.     

Time-dependent deformation in an enhanced SiC/SiC composite

Time-dependent deformation in an enhanced SiC/SiC composite has been studied under constant load at high temperatures of 1200 °C, 1300 °C, and 1400 °C. Creep damage evolution was evaluated by a Young’s-modulus change of partial unloading and microscopic observation. The addition of the glassy phase in the matrix is very effective for protecting the composite from oxidation. The transient creep is dominant in creep life at all the temperatures. An empirical equation is proposed to describe creep behavior of the composite. It is found that creep activation energy increases with creep time at stresses lower than matrix cracking stress, but the activation energy remains constant at stresses higher than the matrix cracking stress. The creep strain rate of the composite is considered to be controlled by creep of fibers based on examining the time, strain, stress, and temperature dependencies of creep strain rates.     

The influence of matrix microstructure and particle reinforcement on the creep behavior of 2219 aluminum

The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum has been examined in the temperature range of 150 °C to 250 °C. At 150 °C, reinforcement led to an improvement in creep resistance, while at 250 °C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent strengthening from particle reinforcement was observed at 150 °C and was accurately modeled by existing continuum mechanical models. The absence of reinforcement creep strengthening at 250 °C can be attributed to diffusional relaxation processes at the higher temperature.     

Effect of carbide precipitation on the creep behavior of Alloy 800HT in the Temperature Range 700 ° to 900 °

The creep behavior of alloy 800HT was studied at 700 °, 800 °, and 900 ° under stresses ranging from 30 to 170 MPa. Samples that were tested in the as-quenched condition after solution treatment exhibited longer creep life than those that were overaged before testing. This difference in creep life was found to increase at lower creep stresses at a given temperature. This phenomenon is attributed to the precipitation of M₂₃C₆carbides during the early stages of creep, which strengthen the material by exerting threshold stresses on moving dislocations and thereby reducing the creep rate. A model is developed to describe the influence of carbide precipitation during creep on the behavior of the material under different creep temperatures and stresses. Comparison with the experimental results shows that the model gives accurate predictions of the creep behavior of the material in the range of stresses and temperatures used in the present study. In addition to its predictive value, the model is useful in understanding the factors that affect the creep behavior of materials when precipitation of hard phases is taking place during creep. The strengthening effect of particle precipitation during creep, as represented by the value of the threshold stress, is shown to be a complex function of the supersaturation of the matrix, the applied creep stress, and the test temperature.