Warm Rolled Specimens Research Articles

Development of ultra-fine grain microstructures in a metastable-β titanium alloy via phase transformation assisted recrystallisation

Generally, in metallic alloys, attaining ultra-fine sub-micron grain sizes by recrystallisation requires severe plastic deformation (SPD), however SPD techniques are difficult to apply to large quantities of material. In this work, we explore a strategy to reach an ultra-fine grained microstructure in a metastable-β titanium Ti-20Nb-6Zr (at.%) by controlled recrystallisation after conventional rolling and annealing. The thermally stable sub-micronic dual phase microstructures are associated with the formation of strain-induced martensite during rolling and its subsequent reversion during annealing. Some specimens were cold-rolled to change most of the β microstructure to stress-induced martensite (SIM); others were “warm rolled” just above the critical temperature for formation of SIM (453 K). The role of SIM in grain refinement was isolated by comparing the microstructural evolution of the two sets of specimens during annealing treatments. The cold rolled specimens produced an effective ultrafine grain recrystallisation, which was identified to start from 723 K, and a final equiaxed alpha-beta microstructure of ~250 nm. In warm rolled specimens without SIM, no new beta grains were observed, only an array of intragranular acicular alpha precipitates of approximately 50 nm thickness.

Effect of deformation microstructures on hydrogen embrittlement sensitivity and failure mechanism of 304 austenitic stainless steel: the significant role of rolling temperature

Metastable austenitic stainless steels (ASSs) have excellent ductility but low strength, so that their usage as load-bearing components is significantly limited. Rolling is an effective method of increasing strength, whereas the effect of rolling temperature on microstructural evolution, the hydrogen embrittlement (HE) sensitivity and fracture mechanisms is still unclear. In present study, the effect of cold/warm rolling on detailed microstructural characteristics of 304 ASS was quantitatively investigated, and the corresponding HE sensitivity was evaluated via slow strain rate test (SSRT). The results suggest that cold-rolling led to high strength but poor plasticity and deteriorated HE sensitivity, while warm-rolled samples provided combination of high strength and ductility and also superior HE resistance. Compared with 18% α′-martensite in cold -rolled steel, warm-rolled specimens consisted of complete austenite, less twins and lower dislocation density，moreover, the favorable {112} ND and {110} ND textures replaced the harmful {001} ND texture. Based on in-situ EBSD observation during SSRT, the HE sensitivity was governed by the combined effect of pre-deformation microstructures and the dynamic microstructural evolution. Advanced method of time-of-flight secondary ion mass spectrometry was used to observe the distribution of hydrogen, and the hydrogen content of specimens was determined by the gas chromatograph thermal desorption analysis method. An exceedingly small amount of hydrogen entered the warm-rolled samples, while a large amount of hydrogen was trapped at grain boundaries of cold-rolled sample, leading to complete intergranular fracture. Therefore, warm rolling is an effective pathway for obtaining high combination of strength and ductility together with excellent HE sensitivity.

Substructure Formation, Texture, and Mechanical Behavior of Warm Rolled 2205-Type Duplex Stainless Steel

The microstructural evolution and mechanical behavior of 2205 duplex stainless steel were studied after plate warm rolling at 600 °C with 60 and 80% thickness reduction, using different microscopy techniques, X-ray diffraction, and tensile and hardness testing. The microstructures of the warm-rolled specimens consisted of flattened wavy grains. The texture evolution during rolling in the ferrite domain presented α-fiber and rotated cube components, while the austenite showed brass, copper, and cube components to a lesser extent. Intense formation of entanglement and dislocation forests characterized the microstructure. In ferrite, there was an intense formation of dislocation substructures and cell blocks. In austenite, the substructure was characterized by planar gliding and the formation of dislocation entanglements. After warm rolling, the tensile strength reached 1185 and 1328 MPa at 60 and 80% thickness reduction, respectively. Compared with the as-received steel, the warm work raised the mechanical strength level by between 65 and 72%. These results highlight the prospects for innovative routes to the industrial production of this class of duplex stainless steel, such as cold rolling suppression, considering even ductility reduction.

The dependence of fatigue crack growth on hydrogen in warm-rolled 316 austenitic stainless steel

The fatigue crack growth rate of warm-rolled AISI 316 austenitic stainless steel was investigated by controlling rolling strain and temperature in argon and hydrogen gas atmospheres. The fatigue crack growth rates of warm-rolled 316 specimens tested in hydrogen decreased with increasing rolling temperature, especially 400 °C. By controlling the deformation temperature and strain, the influences of microstructure (including dislocation structure, deformation twins and α′ martensite) and its evolution on hydrogen-induced degradation of mechanical properties were separately discussed. Deformation twins deceased and dislocations became more uniform with the increase in rolling temperature, inhibiting the formation of dynamic α′ martensite during the crack propagation. In the cold-rolled 316 specimens, deformation twins accelerated hydrogen-induced crack growth due to the α′ martensitic transformation at the crack tip. In the warm-rolled specimens, the formation of α′ martensite around the crack tip was completely inhibited, which greatly reduced the fatigue crack growth rate in hydrogen atmosphere.

Microstructure, Tribological Properties, and Hardness of Spray-Deposited and Warm-Rolled Al–Pb Alloys in Peripheral Regions

The Al–18% Pb and Al–20% Pb alloys were processed by a rapid solidification, i.e., the spray deposition technique. The deposit in a shape of Gaussian curve, or bell-shaped deposit, was obtained at a pressure of 9 bar, nozzle-to-rotor distance of approximately 423 mm, and rotor inclination angle of 0°. From obtained spray deposits Al–18% Pb and Al–22% Pb, samples were cut down from the peripheral regions to portions 30 mm long and 20 mm thick. The samples were warm rolled at different percentages of thickness reduction, such as 0, 15, 30, 45, and 60%, at a recrystallization temperature of 0.4Tm. The value Tm refers to the melting point of the alloy at warmrolled condition, i.e., 575°C, which is evident from the Al–Pb phase diagram. As visible from the microstructure, Pb is equally distributed in the aluminum matrix in both spray deposit samples with up to 60% thickness reduction. With lower thickness percentage of warm-rolled specimens, the hardness values decrease in Al–18% Pb and Al–22% Pb alloys. Porosity is lower in Al–18% Pb alloy than that in Al–22% Pb alloy due to the deposition and rolling defects. The tribological tests showed that with the increase of Pb content from 18 to 22%, the wear rate increases abnormally, and the values of wear intensity and friction coefficient of the Al–22% Pb alloy are higher compared with those of the Al–18% Pb alloy.

The Influence of Warm Rolling Reduction on Microstructure Evolution, Tensile Deformation Mechanism and Mechanical Properties of an Fe-30Mn-4Si-2Al TRIP/TWIP Steel

The effects of warm rolling reduction ratio ranging from 20% to 55% on microstructure evolution, the tensile deformation mechanism, and the associated mechanical properties of an Fe-30Mn-4Si-2Al TRIP/TWIP steel were studied. The warm rolling process resulted in the formation and proliferation of sub-structure, comprising dislocations, deformation twins as well as shear bands, and the densities of dislocation and twins were raised along with the increase in rolling reduction. The investigated steel, with a fully recrystallized state, exhibited a single ε-TRIP effect during the room temperature tensile deformation, on top of dislocation glide. However, the formation and growth of twin lamellae and ε-martensite were detected simultaneously during tensile deformation of the warm rolled specimen with rolling reduction of 35%, leading to a good balance between high yield strength of 785 MPa, good total ductility of 44%, and high work hardening rate. As the rolling reduction increased to 55%, the specimen revealed a relatively low work hardening rate, due to the high dislocation density, and dislocation glide was the main deformation mechanism. As a result, a tensile deformation mechanism that started from a single ε-martensitic transformation moved to a bi-mode of ε-martensitic transformation accompanied with deformation twinning, and finally to dislocation glide with the increasing warm rolling reduction was proposed.

Effect of rolling temperature on the microstructure, texture, and magnetic properties of strip-cast grain-oriented 3% Si steel

The effect of rolling temperature on the evolution of microstructure, texture, and magnetic properties of ultra-low-carbon grain-oriented silicon steel was studied in strip-casting process. Dynamic strain-aging (DSA) behavior was observed during warm rolling in the temperature range of ~ 200 to 400 °C based on the fact that majority of the inhibitor elements remained in solution during the strip-casting process. Considering the initial coarse grains with strong {100} fiber prior to rolling, the cold-rolled specimen exhibited pronounced α-fiber and weak γ-fiber texture. However, intense shear bands and high stored energy regardless of the orientation were obtained in the warm-rolled specimens at the DSA temperature, accompanied by weak α-fiber and strong γ-fiber texture. While homogenous microstructure with lower stored energy was observed in the case of high temperature, the differences in shear bands and stored energy governed by the rolling temperature were strongly related to the extent of DSA effect, which is attributed to distinct characteristic of rolling and recrystallization texture. After recrystallization annealing, fine-grained homogeneous microstructure with strong Goss and γ-fiber texture was obtained at the DSA temperature, while relatively random texture with much more α-fiber and θ-fiber components was observed in the case of high temperature. The microstructure and texture of primary annealed sheets exhibited sufficient Goss grains and favorable surrounding matrix with pronounced γ-fiber texture, which was responsible for the perfect secondary recrystallization annealing in the warm-rolled specimens at the DSA temperature. The present study suggests that texture optimization of strip-cast grain-oriented silicon steel can be achieved by warm rolling in the appropriate temperature range, with improved magnetic properties.

Effects of α′ martensite and deformation twin on hydrogen-assisted fatigue crack growth in cold/warm-rolled type 304 stainless steel

The effects of α′ martensite and deformation twin on hydrogen-assisted fatigue crack growth (FCG) were investigated in cold/warm-rolled type 304 stainless steel in 5 MPa hydrogen and argon gas atmosphere. The rate of FCG is reduced in argon gas, while greatly enhanced in hydrogen gas after cold-rolling. The FCG rates of warm-rolled specimens, no matter tested in hydrogen gas or argon gas, are reduced comparing with as-received specimens. After cold-rolling, α′ martensite formed around the grain boundary promotes hydrogen-assisted crack initiation and propagation. The deformation twin plays an important role during FCG besides α′ martensite after warm-rolling, and hydrogen-assisted cracking along the twin boundary and slip band can enhance the FCG rate during cycle loading.

Enhanced strength-ductility relationship in a medium Mn high Al-alloyed multicomponent steel through thermomechanical processing

A medium Mn, high Al, multicomponent steel with duplex ferrite-austenite as the starting microstructure has been designed and the influence of thermo-mechanical processing on the microstructure-texture-tensile property relationship has been investigated. The hot forging-annealing followed by hot rolling-annealing treatment showed poor combination of strength and ductility in the studied alloy. However, a subsequent cold rolling-annealing or warm rolling-annealing treatment has resulted in significant improvement in strength-ductility relationship. An excellent combination of strength (1141–1184MPa) and ductility (18.1–22.2% elongation) has been achieved in both the cold rolled-1073K annealed and warm rolled-1073K annealed specimens due to the presence of fine ferrite grains. Higher fraction of fine M5(Al,Si)C precipitates have restricted the grain boundary migration thus leading to smaller ferrite grains in this annealing temperature. However, dissolution of the M5(Al,Si)C precipitates and the simultaneous increase in the grain size of ferrite at higher annealing temperature (i.e. 1173K and 1273K) deteriorates the strength-ductility relationship in both the cold rolled and warm rolled specimens. The cold rolled-annealed specimens have exhibited better ductility as compared to the warm rolled-annealed specimens due to combined effect of smaller grain size as well as larger volume fraction of gamma (γ) fiber of ferrite.

Microstructural evolution in warm-rolled and cold-rolled strip cast 6.5 wt% Si steel thin sheets and its influence on magnetic properties

6.5wt% Si steel thin sheets were usually fabricated by warm rolling. In our previous work, 6.5wt% Si steel thin sheets with good magnetic properties had been successfully fabricated by cold rolling based on strip casting. In the present work, the main purposes were to find out the influences of warm rolling and cold rolling on microstructures and magnetic properties of the thin sheets with the thickness of 0.2mm, and to confirm which rolling method was more suitable for fabricating 6.5wt% Si steel thin sheets. The results showed that the cold rolled sheet could obtain good surface quality and flatness, while the warm rolled sheet could not. The intensity of γ-fiber rolling texture (<111>//ND) of cold rolled specimen was weaker than that of the warm rolled specimen, especially for the {111}<112> component at surface layer and {111}<110> component at center layer. After the same annealing treatment, the cold rolled specimen, which had higher stored energy and weaker intensity of γ-fiber rolling texture, could obtain smaller recrystallization grain size, weaker intensity of γ-fiber recrystallization texture and stronger intensity of λ-fiber recrystallization texture. Therefore, due to the good surface quality, smaller recrystallization grain size and optimum recrystallization texture, the cold rolled specimen possessed improved magnetic properties, and cold rolling should be more suitable for fabricating 6.5wt% Si steel thin sheets.

Influence of Carbon Content and Rolling Temperature on Rolling Texture in 3 Pct Si Steel

Effects of carbon and rolling temperature up to 453 K (180 °C) on rolling texture of 3 pct Si steel at a reduction of 66 pct were investigated using a single crystal with an initial orientation of {110}〈001〉. With residual-level carbon, uniform slip deformation was observed in the specimen cold rolled at room temperature and most of initial orientation {110}〈001〉 rotated to {111}〈112〉 during the rolling. With carbon addition, the formation of the deformation twins and the shear bands were promoted in the specimen cold rolled at room temperature. Regions with {110}〈001〉 were observed inside the shear bands. Warm-rolled specimen with residual-level carbon had microbands containing tiny {110}〈001〉 regions. Warm-rolled specimen with carbon addition had both the shear bands and the microbands but no deformation twin. Additionally, there were unique band structures with rotated crystal orientation around the rolling direction from initial orientation {110}〈001〉. These experimental results suggest that the carbon addition inhibits dislocation migration by the increase of the critical resolved shear stress (CRSS) and that the high deformation temperature activates multiple slip systems by the reduction of CRSS and further that the carbon addition and high deformation temperature superimposed bring about the activation of symmetrical {110} slip systems additionally.

Crack growth behavior of warm-rolled 316L austenitic stainless steel in high-temperature hydrogenated water

To investigate the effects of warm rolling on the crack growth of 316L austenitic stainless steel, the crack growth rate was measured and the oxide structure was characterized in high-temperature hydrogenated water. The warm-rolled specimens showed a higher crack growth rate compared to the as-received specimens because the slip bands and dislocations produced during warm rolling served as paths for corrosion and cracking. The crack growth rate increased with the dissolved hydrogen concentration. This may be attributed to the decrease in performance and stability of the protective oxide layer formed on the surface of stainless steel in high-temperature water.

Warm deformation and annealing behaviour of iron–silicon–(carbon) steel sheets

Single pass warm rolling and compression experiments were carried out from ambient to 800°C for ultra-low carbon (ULC) steel with ∼100ppm carbon and interstitial free (IF) steels, both with two levels of silicon. Subsequently, annealing was done in order to recrystallize the deformed specimens. The main purpose of this study was to understand the effects of rolling temperature and silicon on stress responses and textures. This study comprises two main themes: flow stress and strain rate sensitivity during compression and shear banding and textures in warm rolled specimens. The effects of deformation temperature on in-grain shear bands were different between ULC-Si and IF-Si steels. As in previous work with more conventional steels, in-grain shear bands in the IF grade had low sensitivity to rolling temperature, while those in the ULC grade depended significantly on the deformation temperature. However, the temperature profile of shear banding in the ULC grade was approximately 150°C higher than in previous work. Deformation and recrystallisation textures for both IF and ULC grades depended on their rolling temperatures. The variation of both grain size and texture after annealing can be explained by the rise and fall of in-grain shear banding activity which is related to the strain rate sensitivity.

Microstructure–texture–mechanical properties relationship in multi-pass warm rolled Ti–6Al–4V Alloy

In the present study, high strength bulk ultrafine-grained titanium alloy Ti–6Al–4V bars were successfully processed using multi-pass warm rolling. Ti–6Al–4V bars of 12mm diameter and several metres long were processed by multi-pass warm rolling at 650°C, 700°C and 750°C. The highest achieved mechanical properties for Ti–6Al–4V in as rolled condition were yield strength 1191MPa, ultimate tensile strength of 1299MPa having an elongation of 10% when the rolling temperature was 650°C. The concurrent evolution of microstructure and texture has been studied using optical microscopy, electron back scattered diffraction and x-ray diffraction. The significant improvement in mechanical properties has been attributed to the ultrafine-grained microstructure as well as the morphology of α and β phases in the warm rolled specimens. The warm rolling of Ti–6Al–4V leads to formation of 〈101¯0〉α//RD fibre texture. This study shows that multi-pass warm rolling has potential to eliminate the costly and time consuming heat treatment steps for small diameter bar products, as the solution treated and aged (STA) properties are achievable in the as rolled condition itself.

The effects of rolling temperature and sensitization treatment on the sulfide stress corrosion cracking of 304L stainless steel

The effects of rolling temperature (25 and 150°C) and sensitization treatment (650°C/1h) on the susceptibility to sulfide stress corrosion cracking of 304L specimens were investigated. Regardless of the rolling temperature, the low hydrogen embrittlement (HE) susceptibility was associated with a high α′-martensite content and/or deformability near the notch-fractured location. In contrast, sensitization treatment increased the HE susceptibility of all rolled specimens. Additionally, cracks were observed only on the cold-rolled specimen in the U-bend immersion tests. The superior performance of the warm-rolled specimen in saturated H2S solution was confirmed by both notched tensile and U-bend immersion tests.

Development of ultrafine-grained magnesium alloy AZ31 by multi-pass warm rolling

High-strength bulk ultrafine-grained magnesium alloy AZ31 bars were successfully developed in this study using multi-pass warm rolling. AZ31 bars of 12 mm diameter and several meters length were produced by multi-pass warm rolling of 45 ×45 mm forged blocks at 250°C and 300°C. The yield strength of warm-rolled rods doubled without any apparent loss of elongation. X-ray diffraction analysis revealed that the fractions of the basal planes oriented along and perpendicular to the rolling directions are nearly similar in intensity levels, indicating no strong texture along the rolling direction. The drastic improvement in mechanical properties is attributed to the ultrafine-grained microstructure in the warm-rolled specimens, whereas the absence of the loss of elongation is attributed to the fewer number of twins.

Texture induced in wavy roll-formed Al–0.73%Mg–0.38%Si alloy

Randomization of texture in Al–0.73Mg–0.38%Si alloy has been attempted by applying the wavy roll-forming method. Rolls having a wavy-shaped surface with about 8 mm in wavelength and 3 mm in amplitude were used. Specimens were solution heat treated at 813 K for 10.8 ks. The specimen was heated at 623 K for 180 s, and then immediately wavy roll-formed. The specimen was heated again and wavy roll-formed with rotating at 90° in plane. This process was repeated 8 times, then the specimen was flattened using conventional flat rolls. Wavy roll-formed specimen at room temperature was also prepared. The roll-formed specimens were annealed and then cold-rolled at room temperature and finally annealed at 773 K for 1.8 ks. For comparison, conventional flat rolled specimens were also prepared at 623 K or room temperature. Textures formed in the specimens were analyzed by electron backscattered diffraction measurements. Grain sizes formed in the warm wavy roll-formed specimen was coarser than that in the warm rolled specimen in final annealing after cold rolling. However, grain refining was accompanied with randomization of texture in the specimen wavy roll-formed at room temperature.

Effect of pre-deformation on the tensile properties of a metastable austenitic steel

The effect of pre-deformation on the tensile properties of an ultrafine-grained metastable austenitic steel was investigated. The pre-deformation greatly improves the yield stress and austenite stability. However, the pre-deformation has little effect on the rate of martensitic transformation with the true strain or on the strain hardening rate at large strains. The higher yield stress and unimproved strain hardening rate in the warm-rolled specimen cause premature plastic instability and smaller uniform ductility.

Grain Boundary Character and Superplasticity of Fine-Grained Ultra-High Carbon Steel

The characteristics and superplasticity of the ( α + θ ) microduplex structures formed by various thermomechanical processings were studied in an ultra-high carbon steel (Fe–1.4Cr–1.0C). After heavy warm rolling of pearlite, an ( α + θ ) microduplex structure with equi-axed α grains of 0.4 µm in diameter and spheroidized θ particles of 0.2 µm in diameter is obtained. The α matrix exhibits a recovered structure in which most of α grain boundaries are low-angle boundaries, resulting in rather smaller elongation at 973 K. Heavy cold rolling and annealing of pearlite produces an (α + θ ) microduplex structure which consists of the coarse-grain region (dα ∼ 0.4 µm) with high-angle α boundaries and the fine-grain region ( dα ∼ 0.2 µm) with low-angle α boundaries. Superplasticity in this specimen is slightly better than the warm-rolled specimen. When pearlite was austenitized in the (γ + θ ) region, quenched and tempered at the temperature below A1 ,a n (α + θ ) microduplex structure in which α and θ grain sizes are nearly the same as in the warm-rolled specimen and most of α boundaries are of high-angle one is formed. Such ultra-fine α grains are formed through the recovery of the fine ( α � lath martensite + θ ) mixture during tempering. This microduplex structure exhibits superior superplasticity. Heavy warm rolling prior to the quenching and tempering improves total elongation further because the distribution of prior γ grain size is more uniform. When cold-rolled pearlite was austenitized and air-cooled, an ( α + θ ) microduplex structure with high-angle α boundary is formed. However, since the α grain size was relatively large (ca. 2 µm), its superplastic performance is poor. Finally, more simplification of processing for superplasticity was attempted. Further improvement of superplasticity was achieved by omitting the tempering in the quenching and tempering treatment.

The effect of rolling conditions on the strength and microstructure of dispersion strengthened copper strips

The effect of rolling conditions on the strength and microstructure of alumina dispersed copper strips was investigated. Commercial strips, Glidcop Al-25, were rolled to similar total reductions by one or multiple passes at room temperature or at 540°C. The specimens rolled at room temperature (cold rolled specimens) had higher strength and hardness than those rolled at 540°C (warm rolled specimen), when rolling schedules were the same. The strength and hardness of the cold rolled specimens were little influenced by the number of rolling passes, whereas the multi-pass warm rolled specimen had lower strength and hardness and higher elongation than the single-pass warm rolled specimen. The structure of specimens is characterized by banded substructures and their strength could be expressed as a function of the band size. In warm rolled strips, no dynamic recrystallization occurred, but the band growth was observed.