Effect of Microstructure on Tensile Behavior and Mechanical Stability of Retained Austenite in a Cold-Rolled Al-Containing TRIP Steel

In the present study, a quenching treatment prior to two-stage heat treatment was conducted on a Fe–0.28 C–1.55 Mn–2.06 Al transformation-induced plasticity steel to tailor the final microstructure. Compared with the microstructure of the ferrite, bainite and blocky retained austenite obtained by conventional two-stage heat treatment, the microstructure subjected to quenching plus two-stage heat treatment was composed of the ferrite, lath bainite and film-like retained austenite. The corresponding tensile behavior and mechanical stability of retained austenite were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the mechanical stability of blocky retained austenite grains is lower and most of them transform to martensite during the tensile deformation, which leads to higher ultimate tensile strength and instantaneous work hardening exponent. Film-like retained austenite has relatively higher stability, which could cause sustained work hardening and high ductility as well as product of strength and elongation.     

Effect of Isothermal Temperature on Microstructure and Mechanical Properties of High Al–Low Si TRIP Steel

In this study, the effect of isothermal temperature on microstructure and mechanical properties of a high Al–low Si TRIP steel was investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron back scattered diffraction, and tensile test. The results show that typical microstructure containing ferrite, bainite, and retained austenite can be obtained when two-stage heat treatment process was utilized. When annealing temperature is 840 °C and austempering temperature is 400 °C, the tensile strength is 542 MPa and the product of strength and elongation is 17,685 MPa%. The morphologies and stability of the retained austenite in low silicon/high aluminum TRIP steel were finally discussed.     

Effect of Isothermal Temperature on Microstructure and Mechanical Properties of High AI-Low Si TRIP Steel

In this study, the effect of isothermal temperature on microstructure and mechanical properties of a high Al-low Si TRIP steel was investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron back scattered diffraction, and tensile test. The results show that typical microstructure containing ferrite, bainite, and retained austenite can be obtained when two-stage heat treatment process was utilized. When annealing temperature is 840 ℃ and austempering temperature is 400 ℃, the tensile strength is 542 MPa and the product of strength and elongation is 17,685 MPa%. The morphologies and stability of the retained austenite in low silicon/high aluminum TRIP steel were finally discussed.     

Study on microstructure and properties of 590 MPa class welding wire deposited metal on hull

By means of metallographic microscope (OM), scanning electron microscope (SEM), back scattering electron diffraction (EBSD) and transmission electron microscope (TEM), the effect of Cu on microstructure transformation and mechanical properties of deposited metal of 590MPa class steel welding wire was studied. The results show that the microstructure of deposited metal is composed of acicular ferrite, lamellar bainite, granular bainite and residual austenite. With the increase of Cu content, the phase transition temperature of the deposited metal decreases, making the phase transition region of ferrite and pearlite shift to the right, expanding the phase transition region of bainite and shrinking the phase transition region of ferrite and pearlite. The microstructure of deposited metal changed, the content of M-A elements increased but the size decreased, and the ferrite-bainite biphasic microstructure was matched. The reduction of M-A component content in strips and blocks and the reduction of effective grain size will reduce the nucleation probability of microcracks, increase crack growth resistance, and improve the impact toughness of the deposited metal.     

Microstructure and fatigue crack growth behaviour of electron beam welding in 30CrMnSiNi2A steel

The effects of two post-weld heat treatment processes on the microstructure and fatigue properties of the electron beam welded joints of 30CrMnSiNi2A steel were studied. Electron beam local post-weld heat treatment ( EBLPWHT) , in a vacuum chamber, immediately after welding and a traditional furnace whole post-weld heat treatment (FWPWHT) were accepted. The experimental results show that, after EBLPWHT, the main microstructure of weld is changed from coarse acicular martensite into lath martensite, and base metal is changed from ferrite and perlite into upper bainite and residual austenite, however the microstructures of different zones of joints in FWPWHT conditions are tempered sorbite. The fatigue crack growth rate da/dN of welds and base metal are not obviously changed among EBLPWHT, FWPWHT test and as-welded (AW) test, as the mechanical properties of materials have a certain but not large effect on the da/dN of welded joints. The resistance to near threshold fatigue crack growth data of welded joints can be largely improved by EBLPWHT and it is related to microstructure and crack closure effect.     

Microstructure and Mechanical Properties of 1,000 MPa Ultra-High Strength Hot-Rolled Plate Steel for Coal Mine Refuge Chamber

The 1,000 MPa ultra-high strength hot-rolled plate steel with low-carbon bainitic microstructure was developed in the laboratory for coal mine refuge chamber. The static recrystallization behavior, microstructure evolution, and mechanical properties of this hot-rolled plate steel were investigated by the hot compression, continuous cooling transformation, and tensile deformation test. The results show that the developed steel has excellent mechanical properties at both room and elevated temperature, and its microstructure mainly consists of lath bainite, granular bainite, and ferrite after thermal–mechanical control process(TMCP). The ultra-high strength plate steel is obtained by the TMCP process in hot rolling, strengthened by bainitic transformation, microstructure refinement, and precipitation of alloying elements such as Nb, Ti, Mo, and Cu. The experimental steel has relatively low welding crack sensitivity index and high atmospheric corrosion resistance index. Therefore, the developed steel has a good balance of strength and ductility both at room and elevated temperature, weldability and corrosion resistance, and it can suffice for the basic demands for materials in the manufacture of coal mine refuge chamber.     

Effects of cooling rate on the fracture properties of TA15 ELI alloy plates

The effects of cooling rate on the mechanical properties and the fatigue crack growth behavior of TA15 ELI alloy plates with different microstructures were investigated at room temperature. The results indicate that the cooling rate （water quench, air cooling, and furnace cooling） has a pronounced influence on the mechanical properties and on the fatigue crack growth, especially for air cooling and furnace cooling. Optical microstructure observation and scanning electron microscopy of tensile fracture surfaces were performed to gain an insight into the mechanism of properties. The dependence of mechanical properties and fatigue crack growth behavior on the cooling rate can be attributed to the oc lamellae width and the colony size, which induce the change in slip length. The microstmcture produced by air cooling shows the best damage tolerance behavior when compared with water quench and furnace cooling.     

Microstructure and Mechanical Ultra-High Strength Hot-Rolled Refuge Chamber Properties of 1,000 MPa Plate Steel for Coal Mine

The 1,000 MPa ultra-high strength hot-rolled plate steel with low-carbon bainitic microstructure was developed in the laboratory for coal mine refuge chamber. The static recrystallization behavior, microstructure evolution, and mechanical properties of this hot-rolled plate steel were investigated by the hot compression, continuous cooling trans- formation, and tensile deformation test. The results show that the developed steel has excellent mechanical properties at both room and elevated temperature, and its microstructure mainly consists of lath bainite, granular bainite, and ferrite after thermal-mechanical control process （TMCP）. The ultra-high strength plate steel is obtained by the TMCP process in hot rolling, strengthened by bainitic transformation, microstructure refinement, and precipitation of alloying elements such as Nb, Ti, Mo, and Cu. The experimental steel has relatively low welding crack sensitivity index and high atmospheric corrosion resistance index. Therefore, the developed steel has a good balance of strength and ductility both at room and elevated temperature, weldability and corrosion resistance, and it can suffice for the basic demands for materials in the manufacture of coal mine refuge chamber.     

MICROSTRUCTURE TRANSFORMATION IN THE WELDING HEAT AFFECTED ZONE OF 800MPa GRADE ULTRA FINE STRUCTURED STEEL

The transformation behavior and microstructure development in the heat affected zone (HAZ) of 800MPa grade ultra fine structured steel was investigated. It was found that the HAZ has intermediate temperature transformation characteristics in a wide range of cooling rates, with the bainite sheaves consisting of bainite ferrite plates without carbide precipitation and retained austenite in the .fast cooling regime. At relatively high cooling rates, which corresponded to low heat inputs, the hardness o.f the simulated HAZ was above that of the base metal. When the cooling rate was below 9℃/s, the welding HAZ would have an obvious softening. The analysis of transformation rates in continuous cooling processes was completed by numerical differential method. The result indicated that the microstructure transformation rate o.f the HAZ in 800MPa grade ultra fine structured steel changed sharply to slow speeds when the cooling time t8/5 is longer than 7s.     

Influence of Deformation Degree and Cooling Rate on Microstructure and Phase Transformation Temperature of B1500HS Steel

To study the effects of the deformation degree and cooling rate on the microstructure and phase transformation temperature for the B1500 HS steel, the samples were heated at 900 °C for 5 min, compressed by 10, 20, 30 and 40% at the strain rate of 0.1 s~(-1), and then cooled down at the rates of 50, 40, 25, 20 and 15 °C/s by the thermo-mechanical simulator,respectively. The start and finish temperatures of the phase transformation were determined by the tangent method, and the volume fraction of the phase transformation was ascertained by the level principle according to the dilatometric curves.The volume fraction of the retained austenite was determined by X-ray diffraction. The results show that the volume fraction of the bainite rises with an increase in the deformation degree as the cooling rate is lower than the critical rate. At the same cooling rate, the phase transformation temperature rises with an increase in the deformation degree, and the sizes of both the martensite and bainite phases reduce due to the austenite grain refinement induced by the deformation. The volume fraction of the retained austenite reduces as the deformation degree increases. The critical cooling rate of the undeformed samples is approximately 25 °C/s and the critical cooling rate rises as the deformation degree increases.     

In situ observation and electron backscattered diffraction analysis of granular bainite in simulated heat-affected zone of high-strength low-alloy steel

In this study, the formation of granular bainite (GB) which may form in the heat-affected zone of double-sided double arc welding joint was in situ observed. The crystallographic characteristics of GB were also compared with those of microstructure mainly comprising lath bainite (LB). The results show that bainite packets exist at the initial stage of GB transformation and can be distinguished by the distribution of retained austenite and martensite–austenite constituents. The bainite blocks of GB are larger than those of LB which makes both the length and fraction of high-angle grain boundaries in GB less than those of LB which partly lead to the brittleness of granular bainite.     

焊接热输入对Q690高强度桥梁钢焊接热影响区粗晶区组织与力学性能的影响

采用Gleeble-3500热模拟试验机研究了16 mm厚Q690高强度桥梁钢不同焊接热输入(E)条件下焊接热影响区粗晶区(CGHAZ)的组织演变规律,研究了焊接热输入、组织和力学性能之间的关系。结果表明:Q690高强度桥梁钢CGHAZ的组织主要为板条马氏体(LM)、板条贝氏体(LB)和粒状贝氏体(GB)。随着焊接热输入的增大,LM含量逐渐减少,LB和GB含量逐渐增多,组织逐渐粗化;CGHAZ的显微硬度和-40℃冲击吸收能量均逐渐减小;当15 kJ/cm≤E≤30 kJ/cm时,CGHAZ组织为细小的LM和LB,大角度晶界(HAGB)含量较高而GB和M-A组元含量较少,显微硬度较高且冲击韧性较好。     

Influence of Vanadium Content on Bainitic Transformation of a Low-Carbon Boron Steel During Continuous Cooling

The phase transformation behaviors during continuous cooling of low-carbon boron steels with different vanadium contents were studied by means of dilatometric measurement and microstructure observation. The bainite transformation behavior is not noticeably altered when the vanadium content is 0.042 and 0.086 wt%, and these steels exhibit full bainitic microstructure even at a cooling rate of 5 °C/s. When vanadium content is increased to 0.18 wt%,ferrite is still present in the microstructure even at a cooling rate of 40 °C/s. Vickers hardness of the steels with 0.042 and0.086 wt% V is remarkably higher than that of the steel with 0.18 wt% V at a cooling rate higher than 10 °C/s, and the difference is increased with the increase in cooling rate. Moreover, the amount of coarse vanadium precipitates formed in austenite is increased with the increase in vanadium content. The optimum content of vanadium to obtain bainitic microstructure is 0.086 wt% in this experimental low-carbon boron steels.     

Effect of Nanoscale Cu-Riched Clusters on Strength and Impact Toughness in a Tempered Cu-Bearing HSLA Steel

The effect of Cu-riched clusters on strength and impact toughness in a tempered Cu-bearing high-strength low-alloy (HSLA) steel is investigated. With increasing the tempering temperature, it is found that the yield strength increases firstly, achieving the maximum value (~ 1053 MPa) at the tempering temperature of 450 °C, and then decreases significantly with the rise of tempering temperature. The tempering temperature-dependent yield strength is closely related to the precipitation of Cu-riched clusters. When tempering at 450 °C, the peak strength will be reached as the nanoscale Cu-riched clusters with small size and high number density will cause a strong precipitation strengthening (~ 492 MPa) due to the dislocation shearing mechanism. However, the Cu-riched clusters will coarsen with further increasing tempering temperature, resulting in obvious decrement of yield strength owing to the dislocation bypassing mechanism. Compared with the yield strength, the variation in impact energy displays an inverse tendency and the impact energy is only 7 J for the sample tempered at 450 °C. The fracture mode can be well explained by the competition between the cleavage fracture strength (σ_F) and “yield strength” (σ_Y). Although transgranular cleavage fracture can be found in samples tempered at 450 and 550 °C, the crack propagation along the lath boundaries is prevented in the sample tempered at 550 °C. The reason is that the number density of Cu-riched clusters at lath boundaries decreases and the segregation of Mo element at the lath boundaries is induced, which will increase the bonding energy.     

Achieving Superior Superplasticity in a Mg-6Al-Zn Plate via Multi-pass Submerged Friction Stir Processing

A large-scale Mg-6Al-Zn plate was prepared by multi-pass submerged friction stir processing (M-SFSP) and its superplastic deformation behavior was investigated. The maximum elongation of 467%, 332% and 421% was attained for the samples oriented in the processing direction (PD), 45° tilted toward the PD, and transverse direction (TD), respectively, when deformed at 623 K and a strain rate of 1 × 10^-3 s^-1. This was attributed to the homogeneous and fine-grained structure with mostly high-angle grain boundaries facilitating the prevalent deformation mechanism of grain boundary sliding, along with the weakened texture and dynamic recrystallization during hot deformation.     

Corporate Effects of Temperature and Strain Range on the Low Cycle Fatigue Life of a Single-Crystal Superalloy DD10

Low cycle fatigue behavior of a nickel-based single-crystal superalloy DD10 was investigated at 760 and980 °C under different strain ranges. Results show that the fatigue life(Nf) of DD10 alloy exhibits different temperature dependence under various strain ranges. Under low strain range, the alloy exhibits a longer Nfat 760 °C than that at980 °C. However, under high strain range, a reverse result is obtained. This difference can be attributed to the change of dominant damage modes under various test conditions, which is manifested in different modes of crack initiation(crack nucleation and its early propagation). At 760 °C, the crack initiates at pores in subsurface due to local stress concentration.This process is mainly controlled by plastic amplitude and plastic property, but not affected by oxygen-induced damage before the crack propagates to the surface. At 980 °C, the crack initiates at surface instead of pores due to the more homogeneous plastic deformation and the disharmony between the external oxidation layer and the bulk material when the strain amplitude is high. At that temperature, the process is mainly controlled by oxidation damage and strain amplitude simultaneously. Therefore, under high strain range, the crack initiation is much easier at 760 °C due to plastic deformation and the poor plasticity, while under low strain range obvious oxidation damage at 980 °C may accelerate the crack initiation.     

Effect of Post-weld Tempering on the Microstructure and Mechanical Properties in the Simulated HAZs of a High-Strength-High-Toughness Combination Marine Engineering Steel

The effects of tempering temperatures on the microstructure and mechanical properties of the simulated coarse-grain heataffected zone(CGHAZ) and inter-critical heat-affected zone(ICHAZ) were investigated for a high-strength-high-toughness combination marine engineering steel.The results demonstrate that the microstructure of the simulated CGHAZ and ICHAZ after tempering is characterized by tempering sorbites and coarse grain in the simulated CGHAZ.As tempering temperature increases,the tensile strength of the simulated CGHAZ and ICHAZ decreases and the Charpy absorbed energy of the simulated ICHAZ at-50℃increases remarkably,but the impact toughness of the simulated CGHAZ is not improved.After tempering at 550℃,the coarse flake carbides,which distribute at the prior austenite grain and martensite lath boundaries,deteriorate the impact toughness of the simulated CGHAZ.With the increase in tempering temperature,the morphology and the size of the carbides gradually change from coarse flake to fine granular,which is beneficial to the improvement of impact toughness.However,the coarse-grain size of the simulated CGHAZ and the M23 C6-type carbide precipitated along the grain boundaries weakens the enhancing effect of carbides on impact toughness.     

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.     

Influence of Selective Laser Melting Process Parameters on Microstructure and Properties of a Typical Ni-Based Superalloy

The influence of selective laser melting(SLM) process parameters on the microstructure and mechanical properties of a typical Ni-based superalloy was researched. The optimum parameters of P = 170 W, V = 0.8 m/s were determined, under which the SLMed samples exhibited both the largest relative density of 99.57% and the best mechanical properties, including the microhardness(329.3 ± 3.8 HV), yield strength(726 ± 8.1 MPa), ultimate tensile strength(900 ± 5.9 MPa) and elongation((31.9 ± 0.24)%). The...     

Atomic-scale APFIM and TEM investigation of grain boundary microchemistry in Astroloy nickel base superalloys

Electron microscopy and atom-probes techniques, including 3D (three-dimensional) atom-probe, were applied to the investigation of grain-boundary (GB) microchemistry and interfacial segregation phenomena in nickel base superalloys, Astroloy. Diffraction patterns and EDX microanalyses exhibit the presence of intergranular M₂₃C₆ chromium enriched carbides as well as intragranular TiC precipitates. Complex phases containing Zr, C and S were also observed. Three-dimensional images as provided by the tomographic atom-probe revealed the presence of a strong segregation of both boron and molybdenum at grain boundaries. Considerable chromium enrichment at γ′-γ′ grain boundaries and slight carbon segregation to GBs, whatever their chemical nature, were also detected. All these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Experiments show that segregation occurs during cooling and more probably between 1200 and 800°C. Boron, chromium and molybdenum GB enrichments are thought to be mainly controlled by an equilibrium segregation process. No segregation of zirconium was detected.