Showing papers on "Hot working published in 2001"

Axisymmetric compression test and hot working properties of alloys

Abstract: The paper investigates the use of the hot axisymmetric compression test for the determination of the hot working properties of alloys. Qualitative analysis shows that stress determinations from the test are subject to systematic errors. These arise from frictional effects and deformation heating. The errors are affected by the conditions of the test and the effects of these parameters have been investigated using a fully coupled finite element procedure. Specimen geometry, specimen volume, friction, temperature, strain rate, and strain have been investigated. Procedures outlining methods of error calculation for general testing conditions are given. Methods of using the data to correct stress–strain curves, to validate such curves and to optimise testing conditions are discussed.

Mathematical Modeling of the Recrystallization Kineticsof Nb Microalloyed Steels

Abstract: The recrystallization behavior of Nb microalloyed steels was studied using hot torsion testing with the aim of modeling the recrystallization processes taking place during hot rolling. Continuous and interrupted torsion tests were performed in the temperature range 850 to 1050°C at strain rates of 0.05 to 5/s on selected low carbon steels containing Cr, Mo, Nb, Ni and Ti. The kinetics of static and metadynamic recrystallization were characterized and appropriate expressions were formulated for the recrystallization kinetics. These are shown to depend on steel composition and the processing conditions. The rate of metadynamic recrystallization increases with strain rate and temperature and is observed to be independent of strain, in contrast to the observations for static recrystallization. By means of extrapolations to mill strain rates, it is shown that metadynamic recrystallization will always be more rapid than static recrystallization, even at the largest possible accumulated strains. These calculations support the view that the unexpected load drops occasionally observed in industrial mills (particularly in the final few passes) are probably due to strain accumulation leading to the initiation of dynamic recrystallization, followed by metadynamic recrystallization.

Static recrystallization kinetics in warm worked vanadium microalloyed steels

Abstract: The effect of vanadium on static recrystallization kinetics of vanadium microalloyed carbon steels after simulating warm working conditions has been determined using the stress relaxation method in plane strain compression tests. In the warm working regime, undissolved fine V(C,N) precipitates promote a fine austenite grain size during reheating and interact with the recrystallization process after working, leading to longer recrystallization times in comparison with plain C–Mn steels. The interaction between precipitates and recrystallization is different to that observed for hot working conditions, retarding the total recrystallization process and thus resulting in a lower value of the Avrami exponent and a longer t 0.5 time.

Hot rolling simulations of austenitic stainless steel

Abstract: The dynamic, static and metadynamic recrystallization behavior of austenitic stainless steel during hot rolling was analyzed. In this approach, each of those recrystallization behaviors is described by appropriate kinetics equations. The critical strain for dynamic recrystallization was determined so that a distinction could be made between static and metadynamic recrystallization; then the amounts of strain accumulation compared with the critical strain each pass. The effects of grain size on the fraction recrystallized and of the latter on the flow stress were evaluated for each type recrystallization behavior. In this way, the dependence of the mean flow stress (MFS) on temperature could be analyzed in terms of the extent and nature of the prior or concurrent recrystallization mechanisms. Finally, an example is given of an industrial process in which DRX/MDRX can play an important role. More grain refinement can be achieved by increasing the strain rate, decreasing the interruption time and lowering the temperature of deformation.

Plastic flow and microstructure evolution during thermomechanical processing of laser-deposited Ti-6Al-4V preforms

Abstract: Plastic flow behavior and microstructure evolution during hot working and heat treatment of Ti-6Al-4V synthesized via a laser-deposition, Laser Engineered Net Shaping (LENSℳ), process were established. To this end, isothermal, hot compression tests were conducted on samples in either a deposited + stress relieved condition or a deposited + hot isostatically pressed (hipped) condition. The starting microstructures consisted of columnar grains with fine or coarse Widmanstatten (basketweave) alpha platelets. At subtransus temperatures, the flow curves of both microstructural conditions exhibited a peak stress at low strains followed by extensive flow softening; these curves were almost identical to previous measurements on ingot-metallurgy (IM) Ti-6Al-4V with similar transformed microstructures. In addition, the kinetics of globularization of the alpha phase during subtransus deformation or subsequent static heat treatment were found to be the same as for IM Ti-6Al-4V with comparable alpha-platelet thicknesses. During supertransus heat treatment, moderately fine beta-grain microstructures were developed in samples that had been predeformed below the beta transus. Such a heat treatment for samples previously deformed above the transus gave rise to a nonuniform distribution of coarse beta grains, an effect attributed to critical grain growth.

Effects of titanium addition on the microstructure and mechanical behavior of iron aluminide Fe3Al

Abstract: The microstructure, mechanical properties and deformation behavior of four Fe 3 Al-based alloys containing 0, 5, 10 or 15 at%Ti were investigated. With the addition of Ti, the coarse-grained microstructure of Fe 3 Al was refined and the degree of D0 3 ordering was decreased. Poor bending ductility and a change of fracture mode from cleavage to mixture of intergranular and cleavage were observed in Ti-added alloys. It was shown that the Ti addition decreased the room temperature yield strength, but increased the yield strength at elevated temperatures. Anomalous temperature dependence of yield strength was observed in all the alloys, and the peak strength was shifted to higher temperatures with increasing Ti content. The Ti addition was also revealed to decrease the strain rate sensitivity Curd activation energy for high temperature deformation, Finally, hot working conditions for the Ti-added Fe 3 Al alloys were proposed by taking the stress-strain response, strain rate sensitivity and deformation microstructure into account.

Elimination of intergranular corrosion susceptibility of cold-worked and sensitized AlSl 316 SS by laser surface melting

Abstract: Susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) due to sensitization is one of the major problems associated with austenitic stainless steels. Thermal exposures encountered during fabrication (welding, hot working, etc.) and elevated temperature service may lead to sensitization of components of austenitic stainless steels. Laser surface melting (LSM) is an in-situ method to increase the life of a sensitized component by modifying the surface microstructure without affecting the bulk properties. In this paper, the results obtained in the attempt to improve IGC resistance of coldworked and sensitized 316 SS by LSM are presented. Type 316 SS specimens cold worked to various degrees ranging from 5 to 25% reduction in thickness and sensitized to different degrees by exposing at 898 K for different durations were laser surface melted using continuous wave (cw) CO2 laser. ASTM standard A262 practice A, optical metallography, and ASTM standard G108 were used to characterize the specimens before and after LSM. Influence of prior deformation on the desensitization behavior was evaluated for the laser melting conditions adopted during the investigation. Complete dissolution of M23C6 due to laser melting and suppression of re-precipitation due to rapid quenching result in a desensitized homogenous microstructure, which is immune to IGC. Under identical laser melting conditions, the extent of desensitization decreases with an increase in the degree of cold work, and hence, higher power levels and an extended interaction time must be adopted to homogenize the sensitized microstructure with prior cold work.

Effect of Cooling Rate on Microstructural Development in Alloy 718

Abstract: A variety of cast pyrometR 718 alloy microstructures result from processing of different diameters of industrial scale ingots for producing superalloy forgings used in aircraft turbine engine applications. The hot workability of these cast alloys will significantly depend on the cast microstructure. The current study is focussed on understanding the evolution of the cast microstructure during Vacuum Arc Remelting (VAR) of 718 superalloy as it relates to secondary dendrite arm spacing, second phase evolution and distribution during solidification. The variation microstructure is related to different cooling rates encountered in superalloy production. The effect of cast structure on homogenization and hot working is also discussed.

Coating tool for warm and hot working having excellent lubricant adhesion and wear resistance

Abstract: PROBLEM TO BE SOLVED: To provide a coating tool for warm and hot working which has excellent seizure resistance and wear resistance. SOLUTION: In the tool for warm and hot working, hot die steel or high speed steel is used as a base material, and at least the working face is provided with a coating layer. The outermost surface layer of the coating layer consists of an (a) layer having surface roughness Rz of 4 to 15 μm. Also, a (b) layer consisting of one or more kinds of the nitrides, carbides and carbonitrides essentially consisting of one or more kinds of metallic elements selected from Ti, V, Cr, Al and Si lies directly on the base material. Desirably, the (a) layer essentially consists of one or more kinds of metallic elements selected from Ti, V, Cr, Al, Si and Cu, and whose layer thickness is controlled to 2 to 15 μm. Further, the coating layer is desirably coated by a physical vapor deposition method. Also, it is desirable that, in the coated base material, its hardness in a depth of 25 μm from the outermost surface of the base material is higher by ≥0.2 in 200 HV than the hardness in a depth of 500 μm from the outermost surface of the base material. COPYRIGHT: (C)2002,JPO

Hot working behavior of extruded powder products of B2 iron aluminide alloys

Abstract: The characteristics of hot deformation of hot extruded B2 type iron aluminide alloy powders produced by gas-atomization and water-atomization techniques have been studied using compression tests in the temperature range 850–1150°C and true strain rate range 0.001–100$_{s-^1}$. At strain rates higher than about 1 $_{s-^1}$, the materials exhibited flow softening type of stress–strain curves, while at lower strain rates, the flow curves were of steady-state type. The apparent activation energy for hot deformation has been evaluated and is in the range 430–465 kJ $mole^-1$ both in the gas-atomized and water-atomized materials. The processing map for the gas-atomized material revealed that dynamic recrystallization occurs in wide ranges of temperature and strain rate with its optimum at 1075°C and 0.1 $_{s-^1}$. However, in the case of water-atomized material, the features in the processing map have changed with strain. The gas-atomized material is prone to flow instabilities like flow localization at strain rates higher than 10 $_{s-^1}$, while the water-atomized material exhibits stable flow even at high strains and high strain rates.

Prediction of temperature evolution by FEM during multi-pass hot flat rolling of aluminium alloys

Abstract: In rolling, deformation is applied by a series of passes, which are necessarily separated by holding periods between passes. The formation of the temperature profile therefore has a complex history. Accurate information on the time variation of temperature is essential for predicting microstructural development during hot working conditions and for the design of an acceptable rolling pass schedule. In the present paper, the finite-element method is employed to compute temperature changes under both laboratory and industrial breakdown rolling conditions. The calculated results are then compared with temperature records extracted from the literature. The difference between laboratory and industrial rolling is discussed. By using inverse analysis methods, appropriate heat transfer coefficients can be ascertained.

Model for static recrystallisation critical temperature in microalloyed steels

Abstract: By means of hot torsion tests, the static recrystallisation critical temperature (SRCT) has been determined for 18 microalloyed steels classified into two groups. In one group the metallic microalloying element is vanadium, and in the other it is niobium. In both groups the microalloying element, carbon, and nitrogen contents vary from one steel to another. Tests have been carried out at various strains and strain rates, and recrystallisation–precipitation–time–temperature (RPTT) diagrams have been drawn for each steel in each condition. The SRCT is the asymptote of strain induced precipitation start P s and end P f curves, and its determination has permitted the construction of a model that quantifies the effects of all the external variables implicit in hot working such as strain and strain rate, and the internal variables such as austenite grain size and chemical composition of the steel. Hence, the influence of each of these variables has been quantified, and the model's prediction, comparing ...

BETA Ti ALLOY WITH HIGH STRENGTH AND LOW YOUNG'S MODULUS, AND ITS MANUFACTURING METHOD

Abstract: PROBLEM TO BE SOLVED: To produce a beta Ti alloy in which the problem of increase in Young' s modulus attendant on the increase of strength by aging can be solved and the increase of strength can be attained while maintaining Young's modulus, and also to provide a useful method for manufacturing such beta Ti alloy. SOLUTION: In successively applying hot working, solution heat treatment and cold working to manufacture the beta Ti alloy used in an as-cold-worked state, at least solution heat treatment temperature is regulated so that it is between a value below the β-transformation point and 600°C and also cold working is carried out at ≥30% draft. By this method, the beta Ti alloy having ≤105 GPa Young's modulus can be manufactured. COPYRIGHT: (C)2003,JPO

Influence of Deformation Temperature on Microstructure Evolution and Static Recrystallization of Polycrystalline Copper

Abstract: Static recrystallization was studied in warm and hot deformed polycrystalline copper (99.99%). Samples were compressed to a strain of 0.25 at 573 K and 723K at a strain rate of 2 x 10 -1 s -1 , quenched in water, and subsequently isothermally annealed at 673 K. Static recrystallization kinetics were found to be faster in samples hot-deformed at 723 K in spite of the lower level of stored strain energy. In all cases nucleation at the early stages of annealing takes place by bulging of serrated grain boundaries. Deformation at 723 K introduces higher orientation gradients evolved along serrated grain boundaries than those at 573 K. It is shown that static recrystallization is controlled not only by the level of stored strain energy, but also by the specific features of sub-grain structures formed in local regions.

Identification of processing parameters for Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel using processing maps

Abstract: The processing parameters for hot working of Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel (alloy D9) are identified using processing maps developed on the basis of the dynamic materials model and hot compression data in the temperature range 850–1250°C and strain rate range 0.001–100 s-1. The efficiency of power dissipation increased with increase in temperature and decrease in strain rate. Dynamically recrystallised microstructures resulted when the efficiency of power dissipation was in the range 27–37%, i.e. in the temperature range 1050–1250°C and strain rate range 0.001–0.5 s-1. Flow localisation occurred in the regions of instability at temperatures lower than 1000°C and at higher strain rates. The dynamic recrystallisation regime observed in this alloy is compared with other austenitic stainless steels, namely, AISI type 304L and 316L.

Coating tool for warm and hot working having excellent seizure resistance and wear resistance

Abstract: PROBLEM TO BE SOLVED: To provide a coating tool for warm and hot working which has excellent seizure resistance and wear resistance. SOLUTION: In the tool for warm and hot working, hot die steel or high speed steel is used as a base material, and at least the working face is provided with a coating layer. The outermost surface layer of the coating layer consists of an (a) layer as sulfide. Also, a (b) layer consisting of one or more kinds of the nitrides, carbides and carbonitrides essentially consisting of one or more kinds of metallic elements selected from Ti, V, Cr, Al and Si lies directly on the base material. Desirably, the (a) layer as sulfide consists of one or more kinds selected from Ti and Cr of ≤50% in total by atomic % only of the metallic composition, and the balance substantially Mo, and whose layer thickness is controlled to 0.5 to 10 μm. Further, it is desirable that the above coating layer is coated by a physical vapor deposition method, and, in the coated base material, its hardness in a depth of 25 μm from the outermost surface of the base material is higher by ≥0.2 in 200 HV than the hardness in a depth of 500 μm from the outermost surface of the base material. COPYRIGHT: (C)2002,JPO

Working and bonding of copper sputter target

Abstract: PROBLEM TO BE SOLVED: To provide a method for working a copper sputter target with high purity, in order to reduce generation of particles in a sputtering process and to enhance film uniformity on a substrate used in manufacturing a semiconductor device and circuit. SOLUTION: The working method for the copper sputter target includes steps of; heating a copper billet with purity of at least 99.99%, to 500 deg.C or more; hot working the heated copper billet to apply stress of 40% or higher; cold rolling the wrought copper billet by applying stress of 40% or higher to make a copper sheet; annealing the copper sheet at more than 250 deg.C; and forming a target blank having equiaxial particles. The particles comprise having mean particle sizes of less than 40 μm and orientations of (111), (200), and (311), and that the amount of the particles having each orientation is less than 50%.

Powder Metallurgy Wear-Resistant Materials Based on Iron. Part 1. Materials Prepared by Sintering and Infiltration

Abstract: Available information on the composition and methods of production of iron-based powder metallurgy wear-resistant materials is reviewed. It is shown that most developments in this field are based on the principle of obtaining wear-resistant material by the creation of a pseudoalloy with a clearly defined heterogeneous structure, in which the microhardnesses of the base material and a hard phase are substantially different. In addition to traditional methods, including pressing and sintering porous ingots and (in some cases) infiltration with a lower melting alloy, methods based on hot working sintered porous ingots under pressure have recently found wider application.

Effects of Mn dispersoid on hot working of Al–1Mn

Abstract: The alloy Al–1Mn was deformed in torsion over the range 200–500 °C and 0.1–4 s −1 in the industrially homogenized and extruded condition. At 200 and 250 °C, the flow curves strain hardened to e ≈5 or to fracture; at higher temperatures, a steady state was reached with higher ductilities. The temperature and strain rate dependencies of the peak stress were fitted to a sinh–Arrhenius equation where Q =152 kJ mol −1 . The behavior is compared with Al–0.65Fe in which Al 3 Fe was very effective in pinning dislocations in the range 200–300 °C with much reduced effect at 400–600 °C. Prior to deformation, limited groups of specimens were subjected to solution at 600 °C for 4 h followed by air cooling or water quenching (some specimens stretched 5%) and final aging at 350 °C for 4 h. These treatments failed to raise the strength in the range 200–300 °C.