Hot workability of nimonic 115 as a function of strain rate

The hot workability of Nimonic 115 was studied by means of very high strain rate stress rupture tests in the temperature interval 1323 to 1473 K (1050 to 1200°C) at strain rates of 10⁻⁴ to 10 per s. Hot plasticity, measured as elongation and reduction of area at fracture, increased generally with decreasing strain rates. Maximum values of about 40 pct elongation and 70 pct reduction of area were obtained between 1398 to 1448 K (1125 to 1175°C) for strain rates below about 1 per s. For higher rates of strain than about 1 per s, ductility at fracture fell sharply. Ductility above 1448 K (1175°C) was poor at all strain rates and fell to a minimum at 1473 K (1200°C) regardness of strain rate. The highest ductility values are associated with intermediate temperatures and intermediate strain rates where conditions are optimum for significant recovery without encountering grain growth. The presence of excess phases leads to severe intergranular embrittlement at the highest temperatures and strain rates.     

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were performed at a strain amplitude of ±0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of γ′ and carbides); and microstructure B, in a double-aged condition with γ′ of 18-nm diameter and M₂₃C₆ carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered γ′ that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 × 10^-5 to 3 × 10^-3 s^-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.     

Hot workability of steels for forgings

Sufficient hot ductility is one of the prerequisites for the successful forming and heat treatment of steels. The influence of chemical composition (including trace elements), of soaking and deformation temperatures, strain rate and duration of deformation was to be studied in hot tensile testing. To distinguish the different embrittling mechanisms from each other, a great number of steels with systematically varied composition were examined. Test conditions were chosen so as to give maximum agreement with actual hot working operations. In the temperature range from 1 200–600°C, which was covered by this study, hot embrittlement was only found on steels containing at least one of the elements N, Nb, Pb or Bi. Embrittlement due to MnS precipitations did not occur, as the soaking temperature was limited to max. 1 315°C and the Mn/S ratio was at least 30. Nitrogen is the main cause of hot embrittlement in commercial steels. The fact that also unalloyed, aluminium-free steels embrittle with sufficiently low strain rate shows that nitrogen in solid solution may cause embrittlement even in the absence of nitride formers. The nitride formers accelerate the embrittling process, provided the nitrides are dissolved at soaking temperature. Aluminium, however, has a retarding effect in the presence of vanadium. Embrittlement is attributed to nitrogen atoms entering into multiple voids and micropores, where they recombine to form molecules which impede the slip of dislocations, thus leading to embrittlement. A sufficient length of the deformation operation and recrystallisation being impeded by precipitations are the prerequisites for this type of embrittlement. Titanium, by binding nitrogen at an early stage, prevents precipitation. Also in the case of embrittlement by lead and bismuth, the most conclusive explanation is that atoms of these elements accumulate in voids. Embrittlement by niobium, however, is attributed to deformation-induced precipitation, as it only occurs on cooling from soaking to test temperature and not on direct heating to test temperature.     

Flow stress of EK-181 steel as a function of the strain rate and the strain

Branch pipes (segments of fuel claddings of a full cross section) and ring specimens of two sizes were subjected to tensile tests. The experimental results are used to plot hardening curves, which are true stress–strain curves. The set of these curves is employed to plot the desired flow surface of EK-181 steel.     

Hot workability of a 6% Mo austenitic stainless steel

Abstract

The addition of 6% molybdenum to austenitic stainless steel, although very beneficial to localized corrosion resistance, leads to hot workability difficulties. Hot short cracking at the interdendritic boundaries during initial breakdown of the cast ingot has been observed to be the principal failure mode. A hot working schedule has been developed which takes advantage of a peak in the ductility curve of this alloy at 1850°F (1010°C). An initial moderate reduction can be accomplished at this temperature without cracking which promotes a complete recry-stallization of the ingot during subsequent heating to rolling temperatures. The resultant fine-grained structure can be further processed to sheet without hot short cracking.

Résumé

L'addition de 6% de molybdène à l'acier inoxydable austénitique augmente grandement sa résistance àla corrosion localisée mais engendre des problèmes au cours du corroyage. On a observe que le principal mode de rupture, lors des premières passes du dégrossissage des lingots, était de la fissuration aux joints interdendritiques. On a mis au point un programme de corroyage axé sur le fait qu'à 1850°F (1010°C) la courbe de ductilité presente un maximum. A cette température on peut effectuer une premiére réduction modérée sans fissuration; cette réduction favorise une recristallisation complète du lingot au cours du chauffage ultérieur jusqu'aux températures de laminage. La structure à grains fins qui en résulte permet de continuer à laminer jusqu'à de la tôle sans fissuration à chaud.     

Microstructural changes during long time service exposure of udimet 500 and nimonic 115

The effects of long term service exposure on the microstructure of Udimet 500 and Nimonic 115 turbine engine components has been studied. Sigma (σ) phase was detected in both alloys, and its formation could be predicted using critical electron vacancy concentrations computed by the revised method of Barrows and Newkirk and the experimental γ phase composition, γ′ coarsening was quite pronounced in Nimonic 115 turbine blades and varied as a strong function of the temperature distribution along the blade airfoil, σ phase precipitation had no effect on γ′ coarsening rates.     

Hot workability of M2 type high-speed steel

The hot workability of an M2 type HSS in cast, forged and rolled condition has been studied by means of a torsion test and metallography. Continuous tests were used to determine the temperature and strain-rate dependence of the flow stress and temperature dependence of the strain to fracture, while multistage tests were used to determine the extent of interpass softening. Temperature and strain dependence of the flow stress is described by a relation of the form: in which the activation energy for hot working (Q_HW) is found to be temperature dependent. The hot ductility of the cast steel is not only lower but is also little affected by temperature in comparison to worked steels, in spite of the fact that critical strain for dynamic recrystallization is smaller, and the extent of metadynamic and static recrystallization during interpass interval is more extensive in the former. This is related to a large volume fraction of carbides, which give rise to a high stored energy and enhanced recrystallization on the one hand, and to the suppression of recrystallization within the continuous network of carbides on the other hand. Carbides give rise to an easy nucleation, growth and coalescence of cracks.     

Hot workability of three grades of tool steel

Three tool steels, a cold-work air-hardening grade, a hot-work die grade, and a high-speed type, were deformed by torsion in the range of 900 to 1100 °C at rates of 0.1 to 5 s^•1. In a series of continuous deformation tests the flow stress and ductility were determined. The exponent of the flow stress was proportional to the strain rate and to the temperature in a reciprocal Arrhenius relationship. In general the flow stress for a given deformation condition, the activation energy, and the strain for the start of dynamic recrystallization increased for the steels in the order listed above; however, the ductility of the hot-work grade is superior to the other two grades. Multistage tests were carried out on each steel to determine its softening behavior during intervals between passes. Each test was carried out under isothermal conditions with constant strain rate, pass strain, and interval duration. Softening occurred by both recovery and recrystallization with the amount increasing with temperature, strain rate, pass strain, and accumulated strain. The first two steels were similar in behavior having extensive softening at 1000 °C, whereas the high-speed steel experienced considerably less softening.     

Effect of strain rate and temperature on hot workability and flow behaviour of duplex stainless steel

The flow behaviour and processing map of a duplex stainless steel were studied via hot compressive tests in a temperature range of 1223–1473?K and a strain rate range of 0.01–30?s^??1. The effect of strain rate and temperature on the hot workability, strain partitioning and dominant flow behaviour of the alloy was systematically investigated. It is found that the softening mechanism of each constituent phase differs from each other. The ferrite is softened by dynamic recovery and continuous dynamic recrystallisation (CDRX), while the austenite is softened only by the limited discontinuous dynamic recrystallisation (DDRX). At lower strain rates (0.01 and 0.1?s^??1), the strain is mainly accommodated by ferrite due to its excellent softening capability, which causes the apparent activation energy Q_p to decline continuously with the increase in true strain. In this case, plastic deformation of the austenite rarely occurs, and at this time, DDRX of austenite is not observed. When the strain rate increases, CDRX of ferrite is weakened at a relative low temperature, which prompts the strain transfer into austenite and induces the strain hardening due to its restricted softening. Accordingly, interactions between the strain hardening in austenite and weakened softening of ferrite leads to one or more platforms of Q formed at the medium stage of deformation (1–30?s^??1). The processing map shows that two flow instability regions appear at high strain rate due to the lack of sufficient response time for dynamic restoration at the early deformation stage. As the strain increases, dynamic softening mechanism is activated at a higher temperature, resulting in a gradually narrowed flow instability region. Differently, a decrease in temperature suppresses dynamic softening of the alloy with a high strain rate, which deteriorates the hot workability of the alloy and induces microcrack formation after straining of 0.8.     

Hot workability of high manganese transformation induced plasticity steel

The hot deformation behavior and microstructure evolution of high manganese transformation induced plasticity steel(Fe - 20Mn - 3Si - 3Al) were investigated by using hot compression test in a temperature range from 800℃to 1 050℃and strain rate ranging from 0.01 s~(-1) to 5.0 s~(-1).The effects of temperature,strain rate,and true strain on the flow behavior and microstructures of high manganese transformation induced plasticity steel were discussed.The results show that the dynamic recrystallization occur...     

Hot workability of nickel-containing alloys after reheating using sulfur-containing fuel

The effects of fuel sulfur on the hot workability of several nickel-containing alloys have been explored in static exposures and in hot rolling trials following reheating in S-bearing. combustion atmospheres. Using thermochemical calculations, it is shown that the locus of oxidation and sulfidation potentials of such gases over practical air-fuel ratios can be made to coincide for a variety of hydrocarbon fuels, and propane doped with H₂S has been used as a fuel to simulate environments resulting from firing distillate and residual oils. Nickel-base alloys containing chromium are not affected by the presence of sulfur species, and workability is independent of reheating conditions at fuel sulfur levels of up to 2 wt pct. Stainless steels may form solid subscale sulfides under air deficient heating, but there is no effect under lean firing conditions; billet workability is not influenced by sulfur. Nickel and nickel-copper alloys form liquid sulfides during air deficient heating, and very poor workability results. Sulfur is also picked up from the gas, and other properties may be affected. The thermochemical predictions for the occurrence of these liquid sulfides agree well with observations. An attempt has been made to identify safe reheating conditions (temperature and air-fuel ratio) for sensitive alloys on the basis of equilibrium combustion gas computations.     

Hot workability of Mg-9Y-1MM-0.6Zr alloy

The behavior evolvement of Mg-9Y-1MM-0.6Zr (WE91) alloy during hot deformation process was discussed. The flow stress behavior of magnesium alloy over the strain rate range of 0.001 to 1 s-1 and the temperature range of 653 to 773 K was studied on a Gleeble-1500D hot simulator under the maximum deformation degree of 60%. The experimental results showed that the relationship between stress and strain was obviously affected by the strain rate and deformation temperature and the flow curves observed were typical of the occurrence of dynamic recrystallization. The flow stress of WE91 magnesium alloy during high temperature deformation could be represented by the Zener-Hollomon parameter in the hyperbolic Arrhenius-type equation. The average deformation activation energy Q and strain coefficient n were 224.11 kJ/mol and 3.08 by calculation,respectively. The processing maps were calculated and analyzed according to the dynamic materials model. The map at strain of 0.916 exhibited three domains with peak efficiency of 49%,44% and 42%,respectively. It was found that the alloy could be extruded at 693 K with mechanical properties of σ0.2=240 MPa,σb=315 MPa and δ=15.5%.     

Dislocation substructure as a function of strain in a dual-phase steel

Dislocation structures in the ferrite of a C-Mn-Si dual-phase steel intercritically annealed at 810°C were characterized at various tensile strains by transmission electron microscopy At strains which corresponded to the second stage on a Jaoul-Crussard plot of strain hardening behavior, the dislocation density in the ferrite is inhomogeneous, with a higher density near the martensite. The third stage on a Jaoul-Crussard plot corresponds to the presence of a well-developed dislocation cell structure in the ferrite. The average cell size during this stage is smaller than the minimum size reported for deformed iron, and the cell size was inhomogeneous, with a smaller cell size near the martensite. Formerly Research Assistant at the Colorado School of Mines     

Hot Deformation Behavior of a New 9%Cr Heat Resistant Steel G115

The hot deformation behavior of a new heat resistant steel G115 designed for 650 °C ultra-supercritical (USC) power plants was experimentally studied. Hot compression test was carried out in the temperature range of 900 — 1200 °C and the strain rate range of 0.1 — 20 s^?1 by using Gleeble-3800 thermal-mechanical simulator, and the corresponding flow curves were obtained. Experimental results show that the flow stress increases with the decrease of deformation temperature and the increase of strain rate. The hot deformation activation energy of G115 steel was determined to be 494 kJ/mol and the constitutive equation was also obtained. For convenience of the practical application, a good approximate equation was obtained for calculating the peak stress values of G115 steel under different deformation conditions. At the strain value of 0. 9, natural logarithm of the critical Zener-Hollomon parameter Z_e of G115 steel was determined to be in the scope of 49. 67 and 50. 65, above which there will be no dynamic recrystallization (DRX). And natural logarithm of the critical Zener-Hollomon parameter Z_s of G115 steel was determined to be in the scopes of 45. 58 and 46. 27, below which full DRX may occur. Then, the status diagram of dynamic microstructures of G115 steel was established. In addition, the strain rate sensitivity of G115 steel is not constant during the test temperature range and it increases linearly from 900 to 1200 °C. Therefore, hot deformation at higher temperatures would obtain better workability.     

Strengthening effects in AC8A/Al2O3 short-fiber composites as a function of temperature and strain rate

The strengthening aspect of AC8A/Al₂O₃ short-fiber composites is examined under the framework of a modified shear lag model over the range of 298 to 723 K and 10⁻³ to 10³ s⁻¹. The strength sustained by the composite at high temperatures is much higher than for the alloy. As the strain rate rises, the portion of strength that the composite or alloy can sustain is drastically increased. Also, the composite shows a lower strain rate sensitivity, likely to be caused by the higher tendency of fiber damage and local stress concentration. As the temperature is higher, the strain rate sensitivity becomes considerably higher. The composite strength can be theoretically calculated using the Friend and Modified Tsai-Hill formulas. By closer examination, the experimental data agree better with the prediction of the Modified Tsai-Hill 2D (min) or 2D (max) model. Nevertheless, all of the predictions give quite reasonable strength values as well as the trend as a function of temperature and strain rate. Overall, test temperature governs the strengthening efficiency. High temperatures give the best efficiency. Influence from strain rate exists, but is less significant. It is observed that the strengthening effect is more pronounced when the matrix strength is lower, such as at higher temperatures and lower strain rates. Calculations from the critical fiber volume fraction V _crit and load transfer coefficient α both show an increasing trend with increasing temperature and decreasing strain rate, also suggesting that the strengthening effect by adding short fibers into the matrix is more apparent and efficient at high temperatures and low strain rates.     

Tempering kinetics of alloy steels as a function of quench rate and Ms temperature

The effect of quench rate on the as-quenched hardness and carbide or carbon segregate particle size was studied for two alloy steels with M_s temperatures of 310 °C and 300 °C. The quench rate was varied from 681 to 93,000 °C per second. The hardness was studied using the Vickers hardness technique and the particle size was measured using the high-resolution, small-angle X-ray scattering technique. The as-quenched hardness and particle size were both found to decrease as the quench rate increased. Analysis of the data ruled out autotempering of the martensite as a likely basis for the observed quench rate effects. The effects of aging at room temperature (20 °C) on the hardness and particle size for these alloys were also studied. The hardness and particle size were both found to increase as the aging time increased. After 24 hours of aging, the effect of quench rate disappeared in that the hardness and particle size attained were each essentially independent of quench rate. DANIEL G. HENNESSY formerly Research Associate, Department of Materials Engineering, Rensselaer Polytechnic Institute. VIJAY SHARMA formerly Research Associate, Department of Materials Engineering, Rensselaer Polytechnic Institute. This paper is based on a presentation made at the “pcter 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.     

Feeding rate and responses to food deprivation as a function of fasting-induced hypoglycemia.

Examined the relation between hypoglycemia induced by 3 hrs of food removal at various times throughout the day and the amount eaten during the corresponding ad-lib periods in 48 male Wistar rats. Deprivation-induced hypoglycemia differed between the nighttime and the daytime and also between the beginning and the end of the daytime. A highly significant correlation existed between the 3-hr ad-lib intake and this time-dependent fall in blood glucose, which in turn was related to increases in subsequent intakes. It is concluded that rats eat at a rate just required to prevent hypoglycemia under ad-lib conditions and that after blood deprivation they transiently increase this rate to correct the fall in blood glucose and to reestablish the required supply of glucose to tissues. (11 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)