Temperature Ductility Research Articles

Hot ductility of an Fe‐10%Ni alloy during penetration of copper

Today's available theories for hot crack formation are based on the fact that hot cracks are formed in the presence of liquid films in the interdendritic areas or at the grain boundaries, which are exposed to tensile stress. Copper is well known to cause hot shortness in steels. In order to study how the liquid embrittles the material, high‐temperature tensile tests were performed at two strain rates during the penetration of liquid copper into Fe‐10%Ni. The penetration distance was measured in samples that were exposed to strain without fracturing. The ductility (area reduction), strain and ultimate tensile stress were determined. Microprobe analysis was performed on the fractured samples. The transition temperature of ductility was found at 1400–1450°C without copper penetration whereas it occurred at 1025–1078°C during the penetration of copper, i.e. copper starts to embrittle at a temperature below its melting point. The microprobe measurements showed that the diffusion rate of copper into Fe‐10%Ni was enhanced when the lattice was strained. The results are discussed in terms of a new theory concerning vacancy formation and condensation as the dominating mechanism for hot crack formation during solidification.

Properties and fracture of structural steels with yield stress of 373–737 MPa in ambient to nil ductility temperature range

By lowering the testing temperature down to below the nil ductility temperature (NDT −20°C) the yield stress, tensile strength and the uniform elongation are increased, while the reduction of area is slightly decreased and the Charpy notch toughness strongly decreased. After strain ageing, the tensile properties are increased, the uniform elongation strongly decreased and the reduction of area slightly decreased. The effect of low temperature on the tensile properties is similar for as-delivered and strain-aged steels. The tensile fracture of as-delivered and strain-aged steels at the NDT −20°C is ductile, while the Charpy-V notch toughness (CVN) fracture is brittle at a significantly higher temperature. The fracture of CVN specimens in the transition temperature range is basically similar to the fracture of notched tensile specimens tested with loading rates in the range 0.1–1500 m/s at the NDT. The mechanism of CVN fracture is discussed.

High Temperature Deformation Behavior of Peritectic Carbon Steel during Solidification.

Phase dependence of tensile strength of Fe-C binary steel and peritectic carbon steel during and after solidification has been studied by a technique for high temperature tensile testing. The experimental technique enabled a sample to melt and solidify without a crucible, and the measurement of a minute load in a solidification temperature range became possible. A numerical model for the analysis of phase transformation during and after solidification was developed with the assumption that local equilibrium holds at liquid/solid interface or δ/γ phase interface.The zero strength temperature was in agreement with zero ductility temperature, and both of these temperatures appeared at the fraction solid of 0.8. Both the tensile strength and elongation of Fe-C binary steel were dependent on the phase state but not on carbon contents. The strain which seems to cause cracks in continuously cast steel slabs for the peritectic carbon steel is predicted to be generated during solidification.

Comparative analysis of pressure vessel integrity for various LOCA conditions

In this study, integrity analysis is performed for a classical four loop PWR pressure vessel fabricated from SA533B type ferritic steel. Pressure vessel behavior is analyzed by deterministic and probabilistic methods under transient conditions, which may cause pressurized thermal shock (PTS). In deterministic analysis, the change of material properties and the mechanical state of the vessel are analyzed against changes in coolant pressure and temperature. Probabilistic analysis is performed to obtain pressure vessel beltline region weld failure probabilities in transient conditions. Overall vessel failure probabilities are evaluated based on the results of deterministic analyses. Computer code VISA-II is utilized for the calculation of vessel failure probabilities. Among three cases considered in this study, a medium break loss of coolant accident induced by a 50 cm 2 break in the hot leg yields the highest vessel rupture probability. The maximum nil ductility temperature in all cases is still below the NRC PTS limit.

A new criterion for internal crack formation in continuously cast steels

To estimate the cracking condition in continuously cast steels, a new model for critical fracture stress given from the measured critical strain has been proposed, which can take into account the brittle temperature range and strain rate. The effects of brittle temperature range and strain rate on critical strain for internal crack formation were analyzed. When the brittle temperature range and strain rate were increased, the possibility of internal crack formation increased due to the decreasing critical strain. To describe the thermomechanical property model of the mushy zone between zero strength temperature (ZST) and zero ductility temperature (ZDT), the yield criterion for porous metals, which can take into account δ/γ transformation, was used. Using the fitting equation for the measured critical strain and the microsegregation analysis, the thermomechanical behavior of the mushy zone could be successfully described by the proposed model, which incorporates the effects of microsegregation of solute elements and δ/γ transformation on hot tear during solidification at the given range of steel compositions and strain rates. A cracking criterion based on the difference of deformation energy in the brittle temperature range is proposed to explain the cracking phenomenon of whole carbon range.

Hot working of commercial Ti–6Al–4V with an equiaxed α–β microstructure: materials modeling considerations

The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s −1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s −1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol −1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol −1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s −1 and these are manifested as adiabatic shear bands in the α–β regime.

Mechanical Behavior of Carbon Steels in the Temperature Range of Mushy Zone.

Tensile strength and ductility of carbon steels have been measured in the temperature range of mushy zone by the in-situ melting tensile test technique with Gleeble system. The specimen was melted and cooled to the test temperature before the tensile deformation in order to get the mechanical properties subject to the continuous casting process. During hot tensile test, a ceramic fiber tube was used to reduce the radial temperature gradient in the heated specimen. Tensile strength of carbon steels in the temperature range of mushy zone increased with decreasing test temperature, and was well described by the modified yield criterion for porous metals. The measured zero strength temperature (ZST) and zero ductility temperature (ZDT) were related to the solid fractions evaluated by the numerical simulation of microsegregation developed by Ueshima et al. The characteristic solid fractions of ZST and ZDT which corresponded to 0.75 and 0.99, respectively, were well described by the prediction equation on ZST and ZDT at given steel compositions and cooling rates.

The effects of ferritic transformation on hot ductility of medium carbon steel

The hot ductilities of Nb-bearing and Nb-free medium carbon steel have been investigated by the hot tensile tests. A high carbon steel (0.4%) was also tested as a reference to determine the relationship between transformation temperature and hot ductility. C and Nb are very effective in delaying the ferrite transformation so that the ductility trough extended to a lower temperature compared to Nb-free medium carbon steel. Decreasing the strain rate promotes the formation of grain boundary ferrite films at higher temperatures and, on the low temperature side, the intergranular failure could still occur even when the amount of ferrite reached 40% or more. The short time temperature oscillation accelerated the precipitation of grain boundary films at higher temperatures, extending the ductility trough to higher temperatures. The samples tested on different machines (direct heating and induction heating) exhibited different hot ductility behavior.

凝固過程および凝固後における高クロム鋼の高温変形挙動

Phase dependence of tensile strength of chromium steel during and after solidification has been studied by a technique for high temperature tensile test. The experimental technique enabled a sample to melt and solidify without crucible, and we can measure a minute load in a solidification temperature range by the experimental technique.A numerical model for the analysis of phase transformation during and after solidification was developed with an assumption of local equilibrium at liquid/solid interface or δ/γ phase interface.The zero strength temperature was in agreement with zero ductility temperature, and both of them appeared at the solid fraction of about 0.8.The tensile strength of chromium steel was dependent on the phase state but not chromium contents. Equations for prediction of tensile strength of δ and γ phases were determined using the experimental results in combination with the previous results for carbon steel and stainless steel.σδ=0.014(Tδ, strat-T)+1.2, MPaσγ=0.067(Tγ, strat-T)+ 6.7, MPaTensile strength of δ phase state is smaller than that of γ phase, and temperature dependence of tensile strength of δ phase is also smaller than that of γ phase.The tensile strength of δ and γ coexisting phase is predicted from the following equation.σ(δ+γ)=σδ·fδ+σγ·fγThese estimated values are in good agreement with experimental results.

Contribution to mechanical metallurgy behaviour of steel during continuous casting

Abstract The paper is devoted to the study of the mechanical metallurgy characteristics of the mushy zone. The basic parameters of the brittle temperature range (TB), the zero strength (ZST), the zero ductility temperature (ZDT) and the liquid impenetrable temperature (LIT) are used for explaining the possibility of cracking. This temperature range was defined in the following form: ZDT

Effects of Mn on fracture behaviour of DO3 Fe3Al-based intermetallic alloy

The fracture behaviour of DO3 Fe3Al-based intermetallic alloys with and without Mn (1.5 at%) were investigated by tensile tests (TT), transmission electron microscope (TEM) and scanning electron microscope (SEM). The results show that the addition of Mn could improve mechanical properties of the alloy, including room temperature ductility and high temperature strength. DO3 Fe–28Al fractured in a transgranular cleavage mode at room temperature and it gradually changed to a ductile style with temperature increasing, whereas the sample with addition of Mn fractured mainly in a mixed intergranular–transgranular cleavage mode. Three major factors are considered to have effect of Mn on fracture behaviour of the alloys: reducing grain size, promoting slip and cross slip and enhancing cleavage strength. © 1998 Chapman & Hall

TiAlの室温延性と高温強度 (1373K)

TiAlの室温延性と高温強度 (1373K)

Effect of Cooling Rate on ZST, LIT and ZDT of Carbon Steels Near Melting Point.

The effect of cooling rate on the characteristic temperatures such as liquidus temperature (TL), zero strength temperature (ZST), liquid impenetrable temperature (LIT) and zero ductility temperature (ZDT) has been investigated by calculating the non-equilibrium pseudo binary Fe–C phase diagram. The effect of cooling rate on TL was not significant. The effect of cooling rate on ZST, LIT and ZDT was significant due to segregation of solute elements at the final stage of solidification. Using the microsegregation analysis proposed by Ueshima, the calculated temperatures at the solid fractions of 0.75 and 0.99 corresponded to the experimentally measured ZST and ZDT, respectively. Prediction equation on ZST, LIT and ZDT, which can take into account cooling rate and steel composition, was proposed. At given steel compositions and cooling rates, the suggested prediction equation on ZST, LIT and ZDT could successfully describe the experimentally measured data and the calculated data from microsegregation analysis.

Microstructural effects on tensile properties of cast TiAl–Fe–V–B alloy

Abstract A gamma titanium aluminide, Ti–46.7Al–1.3Fe–1.1V–0.35B (mol%), has been developed as a casting material for high temperature structural use in aero-engines. This alloy is characterized by the addition of Fe and V which improve castability. The blades for use in a stage 5 low pressure turbine were cast and its engineering applicability was demonstrated by a 500 flight cycles test in a CF6 engine (Nakagawa et al., in: Y.W. Kim, R. Wagner, M. Yamaguchi, Gamma Titanium Aluminides, TMS Annual Meeting, Las Vegas, NV, 1995, pp. 415–424). In the context of wide application, microstructure effects on tensile properties of TiAl–Fe–V–B alloy have been investigated. The optimization of heat treatment conditions and metallurgical features, such as grain structure, grain size as well as aluminum and oxygen content, have been considered to improve the room temperature strength and ductility in this study. As a result, a heat treatment process has been developed to produce TiAl–Fe–V–B alloy, which leads to tensile elongations at room temperature of more than 1.8%.

Hot ductility of an electroslag crucible melted age hardenable Cu-Cr alloy

The electroslag crucible melting process (ESCM), developed on the well established principles of electroslag refining was used to cast age hardening Cu-0.5%Cr alloys using copper scrap. The alloys exhibited superior room temperature tensile strength and ductility compared with conventional wrought material. An important aspect of the ESCM process is that there is considerable chemical refinement of the alloy with respect to both oxygen and sulphur due to reactions inherent in the process. This results in improved hot ductility (to the extent of ∼45%) of the ESCM alloy compared with the material produced by the vacuum melting process.

Thermomechanical processing of mechanically-alloyed Fe-40Al and the influence on mechanical properties

Iron aluminides based on FeAl are being developed as engineering materials for intermediate temperature applications where high-temperature or stainless steels are today used. One of their particular attractions is the excellent resistance to oxidation, particularly in sulphur-containing environments and at elevated temperatures. Such alloys are, unfortunately, not very strong nor very ductile, and effort is required to improve these two properties. An earlier program to develop mechanically alloyed Fe-40%Al (atomic percent) had the objective of improving the room temperature strength and ductility of such an intermetallic by grain refinement through the introduction, during milling, of a dispersion of fine Y{sub 2}O{sub 3} particles. A subsequent study showed that the resulting mechanically-alloyed-and-extruded material was very strong and showed small amounts of ductility when tested at room temperature, but lost its strength rapidly as the test temperature increased. The present study was therefore carried out, examining possibilities to modify the fine microstructure of the extruded material by thermal and thermo-mechanical treatments, much as is done for mechanically-alloyed superalloys, and to examine the influence of such structural modifications on the mechanical behavior.

Modeling the effect of recovery on the room temperature fracture strength of W-0.01% K (non-sag) wire

The room temperature (RT) strength and ductility of potassium doped tungsten wire (i.e. non-sag) varies significantly with annealing conditions. Wire, in an as-drawn condition, exhibits limited ductility ( 1,100 C) of drawn, doped tungsten wire. The result is of interest to those who specify doped tungsten wire for applications where strength retention, after thermal processing (i.e. brazing, annealing, metal-matrix/metal-fiber composites), is important.

Microstructure Control in Iron Aluminides by Phase Decomposition or by Mechanical Alloying for Improved Strength and Ductility

ABSTRACTThe iron aluminides based on Fe3Al or FeAl being developed for intermediate temperature applications suffer from mediocre room temperature strength and ductility and poor high temperature tensile and creep strength. Attempts to overcome these problems have been restricted by the limited possibilities of structure modification by, for example, precipitation of stable strengthening particles. The present study examines two approaches to obtaining two-phase mixtures for improved strength and ductility: by adjusting chemical compositions such that two-phase order-disorder (α-α″) mixtures are obtained, and by mechanical alloying. Two-phase α-α″ mixtures are obtained by heat treatment of Fe-Al alloys with Al content near 20–24% and in ternary Fe-Al-Si alloys with suitably adjusted Al and Si contents. Microstructures of such alloys can be modified during heat treatments by ordering, precipitation or decomposition, and two-phase mixtures similar to those in the γ-γ superalloys obtained. Such two-phase alloys show good high temperature tensile and creep strength with some indication of reasonable ductility and reduced environmental sensitivity. Mechanical alloying can easily produce FeAl alloys of fine grain size reinforced with stable oxide particles. These structures lead to high room temperature strength with some ductility: controlled recrystallization can significantly modify both strength and ductility.

Mechanical Behavior of Mullite Fiber Reinforced Aluminum Alloy Composites

Discontinuously reinforced aluminum alloys are viewed as candidate materials for elevated temperature applications because of their attractive high temperature strength properties and wear resistance. The elevated temperature elastic properties and the failure characteristics in relation to the preform flaws, however, have not received much attention in spite of their potential significance. These issues are studied for an aluminum-silicon alloy reinforced with mullite discontinuous fibers, fabricated using the squeeze infiltration technique. The effect of preform flaws (shot) on room temperature strength and ductility is investigated for composites seeded with different amounts of shot. The Young's modulus of the composite exceeds that of the unreinforced alloy over a wide range of temperatures, and the beneficial influence of the fibers is especially significant at elevated temperatures. The primary contribution to the reduction in the modulus of the composite at higher temperatures is shown to be the degradation in the matrix stiffness. Reinforcing the alloy with mullite fibers results in only a moderate improvement in strength at room temperature but the elongation to failure is reduced considerably. Increasing the amount of shot, although not appreciably degrading strength, further reduces the ductility. Shot is found to play an important role in the damage evolution by fracturing early in the loading process, and thus, the composite integrity when subjected to slow stable crack growth, as in fatigue, for example, could be adversely affected.

連鋳々片の固相線近傍での変形・脆化特性

Tensile strength, ductility and sensitivity of internal crack formation of specimens sampled from continuously cast slabs and experimentaly cast small scale ingots were measured up to their solidification temperature by tensile tests. In the case of experimentaly cast ingots, manganese and sulphur contents were changed widely.The results obtained are summarized as follows.(1) In the brittle region, the tensile strength is below 1kg/mm2 and the apparent elastic modulus is about 100-200kg/ mm2.(2) The brittle-ductile transition mainly depends on the existence of residual (Mn, Fe) S liquid film along the dendritic interface.(3) The critical strain for internal crack formation varies from zero at ZST (Zero Strength Temperature) to about 1% at ZDT (Zero Ductility Temperature) in the brittle region and is found to be independent of strain rate or carbon contents. It was thought that these results come from the lack of molten metal in front of the crack in this tensile tests. On the contrary, it was suggested that the healing behavior of molten metal at the solidification front concerns to the crack formation in real steel strand.