Marina Tikhonova

Σ3 CSL boundary distributions in an austenitic stainless steel subjected to multidirectional forging followed by annealing

The effect of processing and annealing temperatures on the grain boundary characters in the ultrafine-grained structure of a 304-type austenitic stainless steel was studied. An S304H steel was subjected to multidirectional forging (MDF) at 500–800°C to total strains of ~4, followed by annealing at 800–1,000°C for 30 min. The MDF resulted in the formation of ultrafine-grained microstructures with mean grain sizes of 0.28–0.85 μm depending on the processing temperature. The annealing behaviour of the ultrafine-grained steel was characterized by the development of continuous post-dynamic recrystallization including a rapid recovery followed by a gradual grain growth. The post-dynamically recrystallized grain size depended on both the deformation temperature and the annealing temperature. The recrystallization kinetics was reduced with an increase in the temperature of the preceding deformation. The grain growth during post-dynamic recrystallization was accompanied by an increase in the fraction of Σ3n CSL boundaries, which was defined by a relative change in the grain size, i.e. a ratio of the annealed grain size to that evolved by preceding warm working (D/D0). The fraction of Σ3n CSL boundaries sharply rose to approximately 0.5 in the range of D/D0 from 1 to 5, which can be considered as early stage of continuous post-dynamic recrystallization. Then, the rate of increase in the fraction of Σ3n CSL boundaries slowed down significantly in the range of D/D0 > 5. A fivefold increase in the grain size by annealing is a necessary condition to obtain approximately 50% Σ3n CSL boundaries in the recrystallized microstructure.

Development of Σ3 n CSL boundaries in austenitic stainless steels subjected to large strain deformation and annealing

The development of annealing twins was studied in chromium–nickel austenitic stainless steels subjected to cold or warm working. The annealing behavior can be characterized by an austenite reversal, recrystallization, and grain growth, depending on the deformation microstructures. The grain coarsening during recrystallization followed by a grain growth was accompanied by the development of twin-related Σ3n CSL boundaries. The fraction of Σ3n CSL boundaries and their density are defined by a unique parameter that is a relative change in the grain size, i.e., a ratio of the annealed grain size to that one evolved by preceding plastic working (D/D0). The fraction of Σ3n CSL boundaries rapidly increased at early stage of recrystallization and grain growth while the ratio of D/D0 attained 5. Then, the rate of increase in the fraction of Σ3n CSL boundaries slowed down significantly during further grain coarsening. On the other hand, the density of Σ3n CSL boundaries increased to its maximum at a ratio of D/D0 about 2.5 followed by a gradual decrease during subsequent grain growth.

Regularities of Grain Refinement in an Austenitic Stainless Steel during Multiple Warm Working

The structural changes that are related to the new fine grain development in a chromium-nickel austenitic stainless steel subjected to warm working by means of multiple forging and multiple rolling were studied. The multiple warm working to a total strain of 2 at temperatures of 500-900C resulted in the development of submicrocrystalline structures with mean grain sizes of 300-850 nm, depending on processing conditions. The new fine grains resulted mainly from a kind of continuous reactions, which can be referred to as continuous dynamic recrystallization. Namely, the new grains resulted from a progressive evolution of strain-induced grain boundaries, the number and misorientation of which gradually increased during deformation. In contrast to hot working accompanied by discontinuous dynamic recrystallization, when the dynamic grain size can be expressed by a power law function of temperature compensated strain rate as D ~ Z-0.4, much weaker temperature/strain rate dependence of D ~ Z-0.1 was obtained for the warm working.

On Regularities of Grain Refinement through Large Strain Deformation

The recent studies on grain refinement in austenitic stainless steels during large strain deformations are critically reviewed. The paper is focused on the mechanism of structural changes that is responsible for the development of submicrocrystalline structures that can be interpreted as continuous dynamic recrystallization developing under conditions of warm working. The final grain size that is attainable by large strain warm working can be expressed by a power law function of temperature compensated strain rate with an exponent of about -0.15. The development of submicrocrystalline structures is assisted by the deformation microbanding and dynamic recovery, which are characterized by opposite temperature dependencies. The grain refinement kinetics, therefore, are characterized by a weak temperature dependence for a wide range of warm working conditions.

The Formation of Fine-Grained Structure in S304H-Type Austenitic Stainless Steel during Hot-To-Warm Working

The dynamic process of grain evolution in a Super304H austenitic stainless steel was studied in compression tests. The tests were carried out to a strain of 0.7 at temperatures ranging from 700 to 1000°C and strain rate of 10-3s-1. In addition to single pass compression the multiple compressions with changing the loading direction in 90o and decreasing the temperature with step of 100°C from 1000 to 700°C in each pass were utilized to achieve large cumulative strains. Under multiple compression the values of flow stresses were lower than those at single-pass compressions under the same temperatures. The fraction of dynamically recrystallized grains decreased from 1.0 to almost zero with decreasing temperature in single-pass compressions. On the other hand, almost fully recrystallized structure developed under conditions of multiple compressions. The size of dynamically recrystallized grains decreased with decreasing the deformation temperature, approaching a submicrometer scale level at 700°C. The relationship between the deformation conditions and operating mechanisms of dynamic recrystallization is discussed in some details.

Submicrocrystalline Austenitic Stainless Steel Processed by Cold or Warm High Pressure Torsion

The formation of submicrocrystalline structure during severe plastic deformation and its effect on mechanical properties of an S304H austenitic stainless steel with chemical composition of Fe – 0.1C – 0.12N – 0.1Si – 0.95Mn – 18.4Cr – 7.85Ni – 3.2Cu – 0.5Nb – 0.01P – 0.006S (all in mass%) were studied. The severe plastic deformation was carried out by high pressure torsion (HPT) at two different temperatures, i.e., room temperature or 400°C. HPT at room temperature or 400°C led to the formation of a fully austenitic submicrocrystalline structure. The grain size and strength of the steels with ultrafine-grained structures produced by cold or warm HPT were almost the same. The ultimate tensile strengths were 1950 MPa and 1828 MPa after HPT at room temperature and 400°C, respectively.

Effect of SPD Processing Technique on Grain Refinement and Properties of an Austenitic Stainless Steel

The formation of nanocrystalline structures and mechanical properties were studied in a nitrogen-bearing 304-type stainless steel subjected to severe plastic deformation (SPD). The steel samples were processed at ambient temperature using three different methods, i.e., caliber rolling, multidirectional forging and high pressure torsion. All these techniques resulted in pronounced grain refinement. The microstructures consisting of austenite/ferrite crystallites with transverse dimensions of 50 and 30 nm evolved in the rolled and forged samples, respectively. The austenite fractions comprised approximately 0.4. In contrast, the microstructure consisted mainly of austenite with an average grain size of about 25 nm evolved after high pressure torsion. All samples of the stainless steel subjected to severe plastic deformation demonstrated significant strengthening. The ultimate tensile strengths of 2065 MPa and 1950 MPa, were obtained after rolling and high pressure torsion, respectively. The ultimate tensile strength of samples subjected to multidirectional forging was 1540 MPa.

Dynamic Recrystallization Mechanisms Operating under Different Processing Conditions

Dynamic recrystallization (DRX) is one of the most important mechanisms for microstructure evolution during deformation of various metals and alloys. So-called discontinuous DRX usually develops in structural materials with low to medium stacking fault energy during hot working. The local migration, i.e. bulging, of grain boundaries leads to the formation of recrystallization nuclei, which then grow out consuming work-hardened surroundings. The cyclic character of nucleation and growth of new grains during deformation results in a dynamically constant average grain size. The dynamic grain size is sensitively dependent on temperature and strain rate and can be expressed by a power law function of flow stress with a grain size exponent of about-0.7 under conditions of hot working. Recent studies on DRX phenomenon suggest that a decrease in deformation temperature changes the structural mechanism for new grain formation. As a result, the grain size exponent in the relationship between the dynamic grain size and flow stress approaches about-0.25 under warm working conditions.

GRAIN BOUNDARY PLANE DISTRIBUTIONS IN 304 STEEL ANNEALED AT HIGH TEMPERATURE AFTER A PARALLEL PROCESSING OF MULTIPLE FORGING AND DIRECT ROLLING

It is well-recognized that low ∑-CSL boundaries are highly populated in the grain boundary character distribution(GBCD) for austenitic stainless steel(SS) processed by low strain and subsequent annealing.However,large-strain plus annealing typically tends to introducing numerous random high angle grain boundaries(RHABs) instead of producing high fraction of∑3,∑9 and∑27 boundaries.In this case,the distribution of grain boundary planes of RHABs must be very relevant to the properties of material.The current study is to explore the evolution of GBCD and grain boundary plane distribution(GBPD) in 304 austenitic SS after large strain and subsequent annealing using electron backscatter diffraction(EBSD) and five-parameter analysis(FPA).After solid solution treatment,304 steel samples were separately processed by multiple forging(MF) and direct rolling (DR) with true straine=2 followed by same annealing at 900℃for 2—120 min.Then the GBCDs and GBPDs of the two groups of samples were examined.The results show that the total∑3~n(n=1, 2,3) special boundaries in any sample as processed take a length fraction of lower than 45%out of the entire boundaries,and with annealing proceeding the incoherent∑3 boundaries tend to be tuned into coherent ones and consequently the summation fractions of∑9 and∑27 boundaries decrease accordingly.In the two samples which were separately processed by MF and DR but followed by the same annealing at 900℃for 120 min,their random boundaries or general high angle boundaries(∑3~n special boundaries filtered) mostly appeare to be the 111 twist and 110 tilt boundaries,indicating there exist grain boundary textures(GBT) in both samples.However,in the condition of some misorientations, the GBPDs of random boundaries are quite different in the two samples.For grain boundaries of 111/30—40°misorientation,more grain boundaries of twist type nearly on the exact {111} plane are found in the specimen processed by DR and annealing for 120 min(DR120) compared to that processed by MF and annealing for 120 min(MF120).For the grain boundaries of 110/50°misorientation,it was found that most of such boundaries in MF120 are tilt type and positioned on {112},{113} and {115} planes,whereas those in DR120 are tilt or mixed type positioned on {001}, {111} and {012}.It was suggested that there are distinct effects of pre-processing on the GBPDs of annealed 304 steel.

Kinetics of Grain Refinement by Warm Deformation of 304-Type Stainless Steel

The dynamic process of grain evolution in an S304H-type austenitic stainless steel was studied in multiple forging tests at temperatures of 500°C, 600°C and 700°C. The deformation microstructure with a grain size of about 100 to 400 nm resulted from continuous dynamic recrystallization. The size of new grains and the recrystallization kinetics decreased with decreasing the deformation temperature. The dynamically equilibrium grain size evolved at large strains followed a power law function of the flow stress with a grain size exponent of about-0.2. The formation of new fine grains was assisted by dynamic recovery, which leads to an apparent steady state flow at large total strains.