Static Restoration Research Articles

Annealing of Deformed Materials Developed by Continuous/Discontinuous Dynamic Recrystallization

Annealing behaviour was studied in deformed copper developed by continuous or discontinuous dynamic recrystallization (cDRX or dDRX). Pure copper was deformed to large strains by multi-directional forging at room temperature, resulting in an ultra-fine grained structure due to operation of cDRX. Subsequent annealing of such a fine-grained copper can be controlled mainly by grain growth accompanied with recovery and no texture change, that is continuous static recrystallization (cSRX). On the other hand, 4 kinds of static restoration processes operate during annealing of dDRXed copper, i.e. metadaynamic recovery and recystallization (mDRV and mDRX), and classical static recovery and recrystallization. The stable existence of mDRVed grains containing moderate dislocations leads to incomplete recrystallization even after a long period of annealing time. It is discussed how such various types of annealing processes, occurring in cDRXed or dDRXed matrices, can be connected with the characteristic nature of the deformed microstructures.

Dynamic and Static Restoration Behaviors of Pure Lead and Tin in the Ambient Temperature Range

Dynamic and static restoration behaviors of pure lead and tin were investigated by compression tests, and the deformation temperature and strain rate were varied in the range from 223 to 348 K and from 2×10−3 to 1 s−1, respectively. Lead and tin used had two purity levels of 99.999% (5N) and 99.9% (3N), and 5N and 4N, respectively. The hot working simulator was reformed so as to enable compression tests at low temperatures ranged from 223 to 273 K. S-S curves observed in lead and tin were those in dynamic recrystallization and dynamic recovery types of metals, respectively, of which activation energies were 92 to 119 kJ/mol in lead and 49 to 52 kJ/mol in tin. Steady state flow stress in lead with 5N purity was lower than that of tin with 5N purity. A reduction of purity level from 5N to 3N in lead significantly increased flow stress, but difference in purity level of 5N and 4N in tin exerted tiny influence on flow stress at strain rate below 1×10−1 s−1. Static recrystallization in lead with 5N purity completed in the holding time of less than 600 s even at 273 K, while tin with the same purity showed a slightly retarded recrystallization progress. A reduction of purity level in both metals extended the time period for completion of recrystallization by more than 2 orders.

The static restoration of copper after monotonic or cyclic torsion at 673 K

This paper presents an analysis of the effect of the strain path on the hot flow curves and on the post-deformation static restoration of copper. Hot working was completed either through monotonic (continuous) or cyclic (reversed) torsion with strain amplitude of 0.05, at a temperature of 673 K (400 °C) and a strain rate of 0.1 s −1. The steady state flow stress under cyclic processing was lower than the same stress for monotonic torsion. Softening after straining was analyzed through the double hit technique, where the test is stopped for various periods of time, followed by reloading. Static restoration kinetics after monotonic torsion led to fast and extensive softening, which reached 95% after 100 s. Static softening after cyclic torsion was much slower and more limited than after monotonic torsion and was associated with the occurrence of static recovery as the dominant softening mechanism.

工業用純チタンのα域での動的および静的復旧過程とそれらに及ぼす酸素および鉄の影響

工業用純チタンのα域での動的および静的復旧過程とそれらに及ぼす酸素および鉄の影響

Performance evaluation of optical mesh restoration schemes

This paper investigates the performance of various schemes for restoration of lightpaths in an optical mesh network. We consider schemes in which backup paths for links or paths are pro-computed, but wavelength assignment is either pro-computed (static) or done dynamically upon failure (dynamic). In static restoration we compare the performance of link and path restoration in terms of capacity requirement and restoration time. For the dynamic restoration, we consider several dynamic wavelength reservation schemes such as backward and forward reservation with different portion of wavelength to be reserved in both directions. Their performance is evaluated in terms of restoration speed and restorability. Simulation results indicate that path protection requires less capacity than link protection for 100% restorability, and dynamic restoration with backward wavelength reservation shows better restorability than static restoration schemes, when the extra capacity is relatively small. Forward wavelength reservation shows faster restoration speed with lower restorability than the backward wavelength reservation. In general, the optimal restoration scheme depends on extra capacity available and restoration time limit.

Static restoration processes in warm rolled interstitial free steel

Microstuctural changes after warm rolling of an interstitial free steel have been studied by annealing deformed specimens in a salt bath in the ferrite temperature range. Static recovery and recrystallisation were followed by Knoop hardness measurements, optical microscopy, and TEM. The results show that the kinetics of the static restoration processes are systematically dependent on deformation variables. However, occurrence of some precipitation during static restoration affects the kinetics of the process and the extent to which they proceed.

Continuous recrystallization in austenitic stainless steel after large strain deformation

Static restoration mechanisms operating during annealing were studied in a 304 steel with strain-induced submicron grain structures. The initial microstructure with an average grain size of about 0.3 μm was developed by large strain deformation at 873 K. Early annealing leads to a full relaxation of high internal stresses associated with non-equilibrium strain-induced grain boundaries, while their boundary misorientations and the average grain size barely change. Further annealing results in a transient recrystallization followed by a normal grain growth. The average grain boundary misorientation increases up to around 45° in the former and becomes constant in the latter. This is associated with the change in the grain boundary misorientation distribution from a characteristic strain-induced one to a near random distribution corresponding to a fully recrystallized state. The annealing processes operating in the strain-induced fine grains take place homogeneously in the whole matrix and can be called continuous recrystallization.

Static recrystallization kinetics of 304 stainless steels

Static restoration mechanism during hot interrupted deformation of 304 stainless steel was studied in the temperature range from 900 to 1100°C, various strain rate from 0.05 to 5/sec and pass strain of 0.25–3 times peak strain. It was clarified that the static recrystallization was happened after 3–10 seconds at first deformation. The static restoration was depended on the pass strain, deformation temperature and strain rate and fractional softening (FS) values increased with increasing strain rate, deformation temperature and pass strain. Recystallization kinetics was explained with Avrami equation and Avrami constant was 1.113. This value was independent of deformation variables significantly. The time of 5, 50, 95% recrystallization was evaluated using such equations: t0.05 = 2.9 × 10−12 ∈−1.17\({\dot \varepsilon }\)−0.94D exp(222000 J/mol/RT), t0.5 = 2.0 × 10−10 ∈−1.56\({\dot \varepsilon }\)−0.81D exp(197000 J/mol/RT), t0.95 = 1.9 × 10−8∈−1.63\({\dot \varepsilon }\)−0.76D exp(173000J/mol/RT). The predicted values by use of upper equations had a good agreement with a measurement.

Network restoration using recurrent neural networks

In this article, a method is proposed for network restoration using a centralized, static restoration after failure, where the restoration initiated at the local node or at the source uses a hybrid strategy. © 1998 John Wiley & Sons, Ltd.

Static Restoration of Aluminium During Multi-stage Hot Rolling Simulation

The static softening of aluminium between the deformation steps of multi-stage hot torsion tests has been investigated by analysing the temperature behaviour of the mean flow stress σ and the fractional softening x s throughout the hot rolling simulation. The numerous items of information which the σ - 1/T and x s - T diagrams contain were examined. Four distinct regions of static restoration have been found which display different softening behaviour, depending on the duration of the inter-pass time and the strain per pass. Additionally, the evolution of the microstructure during multistep deformation was examined by SEM. On the basis of the fractional softening, a retained strain e r has been derived which describes the predeformed structure entering a succeeding deformation pass due to incomplete static restoration during the preceding arrest interval.

Effect of Carbon Content on Static Restoration of Hot Worked Plain Carbon Steels.

Effect of Carbon Content on Static Restoration of Hot Worked Plain Carbon Steels.

Bar Mill Torsional Simulation of 304 Stainless Steel

AbstractSimulations of bar mill rolling were performed on worked 304 steel by torsion tests with declining temperature Ti , constant pass strain e i , varying strain rate e i and holding time Ti . In the range 1200–900°C and 2.0–0.1 s−1, the activation energy for the maximum pass flow stresses under these simulated industrial conditions was calculated by sinh-Arrhenius analysis to be 361 kJ/mol which is lower than that determined under continuous isothermal conditions. The fractional softening FS by static restoration declines with decreasing Ti , e i , e i and Ti . In isothermal tests with incomplete recrystallization, FS rises in successive interruptions due to accumulated strain. Under declining T conditions with a pass strain of 0.2, the interpass softening is considerably decreased notably below 1100°C with the result that the pass flow stresses rise regularly as T falls, but faster below 1100°C. The activation energy for static recrystallization was found to be 299 kJ/mol compared to 362 kJ/mol for ...

On the recovery and recrystallization which attend static softening in hot-deformed copper and aluminum

The fundamental nature of the static restoration processes which result in static softening after a hot deformation has been studied in copper and aluminum. The kinetics of static softening were determined using the double-hit technique applied to hot compression while the microstructural changes were characterized by the quantitative metallography of quenched specimens. A static softening parameter based on the area under the compression flow curve was used to describe the static softening kinetics. The static softening curves exhibited a simple sigmoidal shape showing no inflection. The relative softening occurring prior to the initiation of recrystallization was found to be small when compared with that occurring after the onset of recrystallization, and was dependent on deformation temperature, amount of deformation, purity and stacking fault energy. The static softening was related to the fractional recrystallization in a nonlinear manner; the degree of nonlinearity was dependent on the occurrence of recovery and dynamic recrystallization. The recrystallization process in Al was of the classical type with the nucleation stage being either the boundary bulge or subgrain growth mechanism. In Cu twinning appeared to be the major nucleation mechanism for recrystallization. When the applied prestrain was greater than the critical strain for dynamic recrystallization, recrystallization was observed to be completed before the completion of static softening. In this case, the remaining softening occurred by the operation of multiple recrystallization where high-order twins formed in the already twinned regions.

Effect of multipeak dynamic recrystallisation on structure of deformed and annealed copper

Compression tests were carried out on fine grained copper at 870 K and at a constant true strain rate of 1·4×10−3 s−1. Under these conditions, well defined flow stress oscillations followed by steady state flow stress are obtained. Grain size measurements of as deformed material revealed non-monotonic grain coarsening when stress oscillations appear. It was found that grain coarsening is most effective when the flow stress decreases after the first flow stress peak. Periodic flow stress is accompanied by periodic grain coarsening until the latter becomes practically independent of strain when the steady state flow stress region is attained. The structural effects of static processes (recovery and recrystallisation) in dynamically recrystallised material were examined closely. According to the model of periodic dynamic recrystallisation, one would expect periodic changes of the driving force for static restoration processes (mainly metadynamic and/or static recrystallisation). From the present work, conclusions are drawn that are contrary to this concept of structural softening. The critical strain leading to grain coarsening during post-deformation annealing of hot deformed copper was found to be significantly less than the strain corresponding to the first flow stress peak. For higher strains, the grain size of dynamically recrystallised copper was found to be highly stable during annealing for 7 h at 870 K.MST/978

Predictive model for deformation resistance in rolling based on torsion parameters

A mathematical model is proposed to calculate deformation resistance of C–Mn–Nb–V base high strength low alloy (HSLA) steels in finish rolling for temperatures from 700 to 900°C. The new method is based solely on the torsion test results that have been evaluated assuming a non-uniform strain over the sectional area of the specimen and falling strain rate throughout the continuous twisting at a constant rate. However, for commercial applications (involving large cumulative strains) the effective residual strain has been taken into account, as well as an original system of describing stress–strain curves and/or softening kinetics. On the basis of the last mentioned assumptions, the computation program has been written in BASIC language. It is concluded that such a model enables the roll force in plate rolling under conditions of retained static restoration to be predicted precisely.MST/590

Geology and the conservation of antique monuments in Turkey

This article deals with a geological investigation carried out as a fundamental part of the conservation and static restoration of several antique churches excavated in andesitic tuff in the Goreme valley and adjacent areas of Cappadocia in Central Anatolia, Turkey. Two stages have been completed already, in 1982 and 1985, respectively, and the work is still in progress. Seismicity is not regarded as a structural hazard, but the geological history includes a series of volcanic episodes since the Oligocene (about 38 million years ago), which persisted into fairly recent times and laid down a great thickness of tuff rock. Erosion sculptured this tuff preferentially because of irregular harder, basaltic layers, which later capped rock pinnacles, the so-called peribacalars, in which churches were hewn over a thousand years ago. Because the host rock is heavily fissured, precipitation has entered and has damaged many mural paintings. These are also subject to vandalism, and efforts have been made to restore them by the usual conservation intervention. However, grouting is necessary and must take into account the porosity of the tuff, which also permits capillary rise from crypts. Sometimes, during wet episodes, water flushes into these and scours the interior walls as well. The basic problem remains geological, and the contribution of the earth sciences is very important. Thus, an engineering geology study has determined that the tuff rock is suitable both for building retaining walls and acting as a constituent in cement grout. It is expected that the UNESCO/ICCROM program to save the churches will be completed successfully within the next five years or so.

Mathematical model of hot deformation resistance in austenite-ferrite two phase region.

Flow stress of a Si-Mn steel in ferrite plus metastable austenite region and austenite plus ferrite two phase region has been examined by single-stage and multi-stage tension tests. Flow stress of “ferrite” (92% ferrite plus 8 metastable austenite) has been found to be expressed by an equation similar to that applicable to austenite. Strain hardening exponent is smaller in “ferrite” than in austenite region, while strain rate exponent is larger in “ferrite” region. The hot deformation resistance in austenite ferrite two phase region can be approximated by the law of mixture for the hot deformation resistance of austenite and “ferrite”. By adding a term expressing release of strain concurrent with phase transformation to the conventional formula describing static restoration process in austenite, the kinetic of static restoration in austenite-ferrite two phase region can be formulated as a simple mathematical equation. On the basis of experimental results a mathematical model of hot deformation resistance in austenite ferrite two phase region has been developed and successfully applied to roll force prediction in a production plate mill.

The mathematical model of hot deformation resistance with reference to microstructural changes during rolling in plate mill.

Single-stage and multi-stage tension tests of Si-Mn and Nb steels have been conducted in the temperature range of 800 to 1000°C at strain rate of 1 to 10sec-1 in order to evaluate the effect of metallurgical factors on hot deformation resistance. The flow stress for both steels has been found to be expressed by Misaka's type equation. But strain rate exponent, which is constant in Misaka's equation, is temperature dependent. The flow stress increases with decrease in austenite grain size and the addition of Nb. Nb in precipitation is, found to cause the larger increase of the flow stress than Nb in solution. The flow stress in multistage tension test can be expressed by above-mentioned modified Misaka's equation if the strain term is replaced by the term considering strain accumulation. The strain accumulated during rolling can be expressed as a function of the observed deformation resistance and the calculated one for strain free austenite. With this method on-line real time evaluation of static restoration process is practicable. On the basis of the experimental results, a mathematical model for prediction of roll force calculation in plate rolling has been developed. The roll force can be calculated with good accuracy by the model.

2.9%Si 鋼の高温変形後の静的復旧過程

2.9%Si 鋼の高温変形後の静的復旧過程

Recovery and recrystallization of polycrystalline copper after hot working

The restoration processes that take place during the isothermal annealing of hot worked copper, in the temperature range 450–540°C, have been studied in detail. The annealing response was monitored by means of an interrupted compression technique and by quantitative metallography. The study firmly establishes the existence of three distinct mechanisms of static restoration, namely recovery, classical recrystallization and metadynamic recrystallization. The presence of the third process requires the occurrence of dynamic recrystallization during the prior hot working strain. The relative importance of the contribution of each of the three mechanisms to static annealing is shown to be highly sensitive to the degree of prestrain. There exists a critical strain ( ~ 12%) below which no recrystallization occurs during the anneal, and recovery is the only restoration process. At higher strains, both recovery and recrystallization take place, and operate sequentially, because the latter mechanism requires a high degree of substructural rearrangement before it can be initiated. Once the material is worked beyond the critical strain for dynamic recrystallization ( ~ 22%) static restoration proceeds by the operation of metadynamic recrystallization as well as recovery and classical recrystallization. The former process is shown to commence immediately on the cessation of straining, as it requires no induction period for the formation of new grain nuclei. Instead, the nuclei are formed during the prior deformation so that recrystallization proceeds by growth from these centers, until all such sites are exhausted. For all conditions considered in the present work, it was found that the grains produced by both types of static recrystallization were smaller than those produced by dynamic recrystallization. It is argued that potential nuclei that are destroyed by work hardening on continued deformation, become critical when straining ceases, thereby increasing the nucleus density and producing a finer grain size.