Effect of cold rolling on plastic deformation and microstructure of bearing ring

Bearing ring blanks of the high-speed rail bearings, machine tool spindle bearings and wind power bearings have been manufactured through the ring rolling technology. Yet, the microstructure evolution of bearing ring in cold ring rolling remains unclear. In this work, the plastic strain distribution of the rolled ring is first simulated. Then, the microstructure of bearing ring is studied by the electron backscatter diffraction technique. Furthermore, the microstructure changes in cold ring rolling are investigated through band contrast images, and the texture evolutions after various deformation ratios are analysed by the technique of 3D-Euler space distribution. This work provides valuable guidelines for enhanced understanding the role of the cold ring rolling technology on the microstructure evolution of bearing rings.     

Influence of forging deformation and heat treatment on microstructure of Ti-xNb-3Zr-2Ta alloys

Ti-xNb-3Zr-2Ta alloys (x = 33, 31, 29, 27, 25) (wt.%) were fabricated by vacuum non-consumable arc melting and hot forging. The hot-forging specimens were solid solution treated at 1053 K for 1.8 ks followed by water quenching (WQ) and air cooling (AC) respectively. The microstructure of hot-forging specimens with different deformation rate and solid solution treated at different temperatures was investigated. The result showed that a large amount of α" martensite appeared in the WQ group while only a little amount of α" phase could be found in the AC group. Moreover, for the WQ group, less niobium resulted in more diffraction peaks of α" phase in XRD profiles. This result suggested that the stability of β phase decreased with the decrease of Nb content. The microhardness of Ti-xNb-3Zr-2Ta (wt.%) alloys was improved significantly with the decreasing Nb content in both WQ group and AC group. Increasing deformation ratio could effectively refine β grains for Ti-25N-3Zr-2Ta (wt.%). Both acicular martensite and lath martensite were found in the transmission electron microscope observation of Ti-25Nb-3Zr-2Ta (wt.%) alloy. Martensitic internal twins were identified as well.     

Influence of microstructures on fatigue damage mechanisms in Ti-15-3 alloy

In this study, the fatigue behavior of Ti-15-3 alloy thin plate specimens with two different microstructures was determined. Two kinds of specimens were prepared with different heat treatments: solution treatment (S) and solution treatment followed by aging (S+A). The effects of the microstructures on the fatigue properties and fatigue crack growth behavior were significant in both specimens. The fatigue crack in both specimens propagated in transgranular mode. In the specimen S+A, crack propagation has occurred on non-crystallographic and was closely connected with the configuration of the α-phase platelet, which was caused by the heat treatment. The damage was characterized by dislocation debris clustering ahead of the crack tip.     

Microstructural evolution during friction surfacing of tool steel H13

Coatings of AISI H13 tool steel were made on low carbon steel by friction surfacing. Detailed microstructural studies and microhardness tests were carried out on the coatings. Studies revealed defect-free coatings and sound metallurgical bonding between the coating and the substrate. In addition, mechanical interlocking on a very fine scale was observed to occur between the coating and the substrate. Coatings exhibited martensitic microstructure with fine grain size and with no carbide particles. Coatings in as-deposited condition showed very high hardness (58 HRC) compared to the mechtrode material in annealed condition (20 HRC). Based on these findings, microstructural evolution during friction surfacing of H13 tool steel is discussed. The current work shows that friction surfaced tool steel coatings are suitable for use in as-deposited condition. Further improvements in coating microstructure and properties are possible with appropriate post-surfacing heat treatment.     

Effect of a novel thixoforming process on the microstructure and fracture behavior of A356 aluminum alloy

In the present study, a novel thixoforming process for semi-solid deformation of A356 aluminum alloy is introduced using a continuous hot deformation process to the temperature being lower than the eutectic temperature of the alloy. A new hypothesis was introduced and the deformation mechanism of the alloy was investigated using the presented hypothesis. Microstructure and fracture surfaces of thixoformed samples were investigated using image analyzing technique and scanning electron microscopy. Obtained results indicated that this novel thixoforming process produces fine and compact silicon particles, dispersed uniformly in the microstructure of the alloy, compared to those produced by conventional thixoforming and gravity-cast processes with large and integrated morphology for silicon particles. The production stages of these silicon particles in this process were well documented by mentioned hypothesis. In order to investigate the effect of this novel process on mechanical properties of A356 alloy, tensile tests were conducted on produced samples. It was found that morphological changes of silicon particles as well as increasing the density ratio of samples in this process have a remarkable effect on enhancing the mechanical properties of produced alloy in comparison with other production routes. A new combination parameter, i.e. silicon density ratio (SDR) index was introduced. This parameter correlates the mechanical properties of samples to morphological properties of silicon particles and density ratio of them. Results of the study also indicated that samples with low SDR index have superior mechanical properties and consequently intergranular fracture mode.     

Influence of straining and ageing on the room temperature mechanical properties of dual phase steel

In this work, changes in mechanical properties in dual phase steel containing 20% martensite volume fractions were observed at various ageing temperatures. For example, ΔY (increase in yield strength due to strain ageing), YS and UTS exhibit maximum values at ageing temperature of 100 °C for the pre-strains of 2 and 4%. This is due to the formation of solute atom atmospheres around dislocation. When the ageing temperature increased to 200 °C, yield strength decreased due to overageing resulted from tempering that starts in martensite phase.     

Study of microstructural evolution and mechanical properties exhibited by non alloyed ductile iron during conventional and stepped austempering heat treatment

It was evaluated the microstructural and mechanical response that a non alloyed ductile iron (DI) presented when was subjected to Conventional Austempering (CA) and Stepped Austempering (SA) heat treatments. X-ray Diffraction (XRD) quantification techniques demonstrated to be the more reliable method for monitoring phase transformations taking place during both CA and SA. When CA was applied some intercellular areas remain untransformed even for long time, however when samples were subjected to SA those untransformed areas disappeared and instead finer ausferrite was found. Additionally mechanical properties values obtained from tensile and impact tests confirmed that for all times used, SA was superior to the CA.     

Microstructural evolution in a low carbon steel during cold rolling and subsequent annealing

Cold rolling with subsequent annealing of lath martensite structure could lead to the formation of nanostructures in low carbon steels. In the present work, the microstructural evolution of a 0.13% C steel during this process was studied. The specimens were austenitized at 950 degrees C followed by quenching in ice-brine to get martensitic structure. The quenched samples were aged at 200 degrees C for 30 min. These specimens were cold rolled up to 90% reduction in thickness without any intermediate annealing and then annealed at the temperatures from 400 to 600 degrees C. Scanning and transmission electron microscopy and color metallography was used to investigate the microstructure. Microscopic investigations showed that a multiphase nanostructure composed of equiaxed ferrite grains with the mean grain size of about 188 nm and small blocks of tempered martensite can be achieved under annealing at 400 degrees C for 90 min. Formation of the nanostructure was discussed from the viewpoint of characteristics of the martensite starting structure. Fragmentation of martensite lathes in cold rolling stage play an important role on recrystallization at annealing stage to get the ultrafine structure.     

High-strength and highly-uniform composites produced by compocasting and cold rolling processes

Silicon carbide reinforced aluminum alloy composite materials produced by casting methods are increasingly used in many engineering fields. However, these materials suffer from poor distribution of the reinforcement particles in the matrix and high content of porosity. The effect of subsequent cold rolling process with different reductions on the porosity, microstructure and mechanical properties of cast Al6061/10 vol.% SiC_p composite was investigated in this study. Composites fabricated by compocasting method were rolled at five different reductions of 30, 60, 75, 85 and 95%. The rolled specimens exhibited reduced porosity as well as a more uniform particle distribution when compared with the as-cast samples. Microscopic investigations of the composites after 95% reduction showed an excellent uniform distribution of silicon carbide particles in the matrix. During cold rolling process it was observed that the tensile strength and ductility of the samples increased by increasing the reduction content. After 95% reduction, the tensile strength and elongation values reached 306.7 MPa and 7.9%, which were 4.6 and 3.3 times greater than those of the as-cast composite, respectively.     

Effect of calcium addition on the microstructure and compressive deformation behaviour of 7178 aluminium alloy

Aluminium 7178 alloys containing 1% calcium are used to study the effect of calcium addition on their microstructure and compressive deformation behaviour. The compressive deformation behaviour of aluminium alloy containing 1% calcium is studied at varying strain rates (10⁻²–10/s). The material is prepared using stir casting technique. The yield stress, flow stress and elastic limit are measured from the true stress–strain graph. The strain rate sensitivity and strain-hardening exponent was also determined for each material at different strain rates. Its microstructural characterization reveals that Ca particles act as grain refiners for primary base alloy and helps in improving the strength of the virgin alloy. An empirical relationship has been proposed to predict the flow curve of the alloys as a function of strain and strain rate.     

Effect of brazing temperature on tensile strength and microstructure for a stainless steel plate-fin structure

This paper presented a vacuum brazing technology for 304 stainless steel plate-fin structures with BNi2 filler metal. The effect of brazing temperature on tensile strength and microstructure has been investigated. The tensile strength is increased along with the increasing of brazing temperature. The microstructure is very complex and some Boride compounds are generated in the brazed joint. Full solid solution can be generated in the middle zone of joint when the brazing temperature is increased to 1100 °C. The brittle phases always exist in the fillet no matter how the brazing temperature changes, but the microstructure in fillet becomes more uniform and the tensile strength is increased with the brazing temperature increasing. In total, the brittle Boride compounds are decreased with the brazing temperature increase. Brazing with a filler metal thickness 105 μm and 25 min holding time, 1100 °C is the best suitable brazing temperature and a tensile strength of 82.1 MPa has been achieved for 304 stainless steel plate-fin structure.     

Effects of applied pressure on microstructure and mechanical properties of squeeze cast ductile iron

In this study, the effects of applied pressure during solidification on the microstructure and mechanical properties of cylindrical shaped ductile iron castings were investigated. Magnesium treated cast iron melts were solidified under atmospheric pressure as well as 25, 50 and 75 MPa external pressures. Microstructure features of the castings were characterized using image analysis, optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. Tensile properties, toughness and hardness of the castings were also measured. The results showed that average graphite nodule size, free graphite content and ferrite content of the castings decreased and pearlite and eutectic cementite contents increased as the applied pressure was raised from 0 to 75 Mpa. Graphite nodule count was first increased by raising the applied pressure up to 50 MPa and then decreased. The highest graphite nodule count was obtained at 50 MPa applied pressure. The microstructural changes were associated with the improved cooling rate and the expected changes in the corresponding phase diagram of the alloy under pressure. The ultimate tensile strength (UTS), yield point strength (0.2% offset) and fracture toughness of the castings were improved when the applied pressure was raised from 0 to 50 MPa. Further increase of the applied pressure resulted in slight decrease of these properties due to the formation of more cementite phase in structures as well as reduced graphite nodule count. Hardness of the castings continuously increased with increasing the applied pressure.     

热处理对各向同性热解炭材料微观结构和力学性能的影响

对化学气相沉积法（CVD）制备的各向同性热解炭材料在不同温度下进行热处理，利用金相显微镜、扫描电镜、透射电镜、X射线衍射和显微激光喇曼光谱等表征手段及显微硬度实验、三点弯曲实验，研究了材料的微观结构和力学性能与热处理温度之间的关系。结果表明，随着热处理温度的提高，各向同性热解炭材料的石墨片层间距缩小，石墨化程度增加，晶粒尺寸增大，同时材料中的孔隙结构也发生了较大的变化。材料的显微硬度和弹性模量随热处理温度的升高而降低，抗弯强度在1750℃和2400℃之间没有变化，在2600℃时有显著的增加。     

Influence of the austenitic rolling temperature on the microstructure of a TRIP steel before intercritical annealing

In the present work, the influence of different thermomechanical treatments in the final microstructure of a multiphase-steel (0.2%C–1.5%Mn–1.5%Si) through of morphological characterization of the transformation products was studied. The goal of this work was to investigate the volumetric fraction variation and phase distribution present in the microstructure. The steel was submitted to intercritic annealing treatment (α + γ) with and without previous reheating after the hot-rolling. The microstructure was observed by optical microscopy and mechanical properties were measured by Vickers microhardness. The results show a typical microstructure of multiphase steel (“dual-phase”) with increase of the martensite volumetric fraction and hardness values when the steel was submitted to intercritic annealing without reheating after the hot-rolling.     

Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (T_P1) of 1400 °C, then reheating to different second peak temperatures (T_P2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.     

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb-V microalloyed steel

Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb-V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.     

The microstructure evolution and nucleation mechanisms of dynamic recrystallization in hot-deformed Inconel 625 superalloy

Hot compressions tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator at different strains between 900 °C and 1200 °C with a strain rate of 0.1 s⁻¹. Optical microscope, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the microstructure evolution and nucleation mechanisms of dynamic recrystallization. It was found that both the size and fraction of dynamically recrystallized grains increase with increasing deformation temperature. However, the size of dynamically recrystallized grains almost remains constant with increasing deformation strain. The dominant nucleation mechanism of dynamic recrystallization in Inconel 625 superalloy deformed at 1150 °C is the discontinuous dynamic recrystallization, which is characterized by the bulging of the original grain boundaries accompanied with twining. The continuous dynamic recrystallization characterized by progressive subgrain rotation occurs simultaneously in dynamic recrystallization process, although it can only be considered as an assistant nucleation mechanism at the early stage of hot deformation.