Mixed Microstructure Research Articles

Tailoring the strength and impact toughness of 9Cr1.5Mo1Co steel by optimizing ultra-fine martensite and retained austensite

The urgent demands for more excellent mechanical properties to meet higher steam temperatures for 9Cr steels have been the driving force behind the increasing research efforts on heat treatment. This paper presents a systematic study of the effect of the ultra-fine martensite (ultra-fine M) and retained austensite (RA) obtained by austempering process on mechanical properties are studied by experimental observations and phase-field simulation. The results demonstrate that austempering leads to the split and incompletion of martensitic transition, producing a mixed microstructure consisting of lathy martensite(lathy-M), ultrafine martensite(ultra-M), and retained austensite (RA). Due to the change of local chemical free energy and elastic strain energy caused by the redistribution of C element, two types (isothermal and athermal) of ultra-M form by spontaneous nucleation and interfacial migration during austempering with appropriate temperatures. Further tempering produces dispersed finer M23C6 precipitated at the interfaces of high-dislocated ultra-M than lathy-M. Meanwhile, RA decomposes into fine globular M23C6 and ferrite. Consequently, a larger amount of ultra-M and RA in quenched state results in smaller effective grain sizes and smaller carbides' average size, leading to the enhancement of precipitation strengthening, grain boundary strengthening, and impact toughness. This work establishes the potential for tailoring the strength and impact toughness of 9Cr1.5Mo1Co steel by optimizing the ultra-fine martensite and retained austensite using austempering process.

Dissimilar welding of AISI 201 and 202 low nickel stainless steels by GTA and PA welding processes

This paper presents the results of dissimilar joining techniques for AISI 201 and 202 low nickel austenitic stainless steels. These were welded by Gas Tungsten Arc Welding (GTAW) and Plasma Arc Welding (PAW). The tests were performed to examine the weld bead shapes, microstructure, mechanical properties and corrosive behavior of the weld metals with different welding processes. The weld bead shape by GTAW was larger than that performed by PAW. The microstructure of the weld metal produced by GTAW contained more delta ferrite in the austenite matrix than the weld metal produced by PAW. The volume of hardness in welds produced by PAW higher than the GTAW process same with the ultimate tensile strength and percentage elongation welds produced by PAW higher than the GTAW process. According to the potentiodynamic polarization test results, pitting corrosion resistance of the weld produced by PAW was higher than the weld produced by GTAW. Both weld metals showed the pitted area adjacent to the phase boundary between austenite and delta ferrite in the mixed microstructure. Based on the present work, it was concluded that PAW was the more suitable welding procedure for joining dissimilar 201 and 202 austenitic stainless steels.

Microstructure and mechanical properties of AISI 410S ferritic stainless steel narrow gap weld using flux strip constraining arc welding

The achievement of high comprehensive mechanical properties in welded joints of ferritic stainless steels has attracted considerable attention in the manufacturing industry. Here, a low heat input flux strip constrained arc welding (FSCAW) was performed on a hot-rolled AISI 410S sheet. Defect free joints with a small HAZ width were obtained by one side welding both sides formation. The microstructure of HAZ consists of fine ferrite grains and a small amount of M23C6-decorated martensite due to short high-temperature residence time and rapid cooling rate, resulting in high toughness and hardness. Influenced by the stirring effect of the constraining arc and the promotion of heterogeneous grain nucleation by Ti-rich MX particles originating from the transition of alloying elements in the flux strip, the fusion zone (FZ) exhibits a unique precipitates-strengthened martensite+austensite uniform microstructure with weak martensitic texture. This translates into a simultaneous increase in impact toughness and hardness of FSCAW FZ compared to the MAG joint with a non-uniform mixed microstructure, strong martensitic texture, and large grain size. This work establishes the potential for using FSCAW welding to join ferrite stainless steels.

Utilization of industrial-based PVC waste powder in self-compacting concrete: A sustainable building material

Conventional experimental studies in concrete primarily focus on strength properties and microstructural analysis and ignore the environmental impacts of chosen waste in concrete. The present study addresses this critical research gap and uses industrial-based polyvinyl chloride (PVC) waste to demonstrate and meet the requirements of field practitioners and researchers. In the present study, various proportions of industrial-based PVC waste powder (PWP) (0–30%) by weight of cement were considered to partially replace cement in an M60 grade of self-compacting concrete (SCC). The first part of the study covers the fresh and strength properties of SCC, which was prepared using cement, sand, gravel, water, superplasticizer, SCM-like silica fume, ground granulated blast furnace slag, and PWP. In the second part of the study, the microstructure of various SCC mixes was investigated to understand the effects of PWP on the microstructure of the prepared SCC mixes. The optimum design mix for SCC was chosen based on extensive experimental and microstructural investigations. Further, a life cycle assessment was used to determine the influence of PWP on SCC usage from an environmental impact point of view for the optimized mixes. Considering all criteria, the results show that an SCC containing 5–10% PWP as a replacement for cement can be used. The present study establishes that using PWP in SCC is beneficial for the environment and can help lower the cost of SCC without compromising its strength and durability.

Effects of Finish Rolling Temperature on the Critical Crack Tip Opening Displacement (CTOD) of Typical 500 MPa Grade Weathering Steel

In this study, the effect of finish rolling temperature on the critical crack tip opening displacement (CTOD) of typical 500 MPa grade weathering steel was elucidated. The microstructures were observed via optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and electron back-scattered diffraction (EBSD). The cryogenic fracture toughness and microstructures of steels were analyzed at different finish rolling temperatures (780–840 °C). The results show that a mixed microstructure, i.e., granular bainitic ferrite (GBF), polygonal ferrite (PF), and martensite/austenite (M/A), constituent was formed in each sample. With the decrease of the finish rolling temperature, the GBF content decreased, PF content increased, and the high angle grain boundary (HAGB) number fraction of the matrix increased. Furthermore, the fraction of M/A constituents was increased with reduced average size. The value of CTOD increased significantly from 0.28 to 1.12 mm as the finish rolling temperature decreased from 840 to 780 °C. Both the decrease of M/A constituents and the increase of HAGB increased the cryogenic (−40 °C) fracture toughness of the typical 500 MPa grade weathering steel.

High cycle fatigue properties of high-nitrogen steel hot-rolled with the yield strength level of 1000 MPa

The fatigue properties of an Fe-19Cr-20Mn-0.6N high-nitrogen steel (HNS), which was hot-rolled with the yield strength level of 1000 MPa, were investigated by means of tension–tension fatigue tests. The results showed that the fatigue life increased in a power law manner with the decrease of the maximum stress from 1040 MPa to 880 MPa. The fatigue limit at the 107 cycles was about 840 MPa, with the ratios of the fatigue limit to yield strength and tensile strength being 0.79 and 0.69, respectively. The high fatigue limit could be attributed to its high yield strength level, which was resulted from the mixed microstructure consisting of ultrafine grains and elongated grains with some slip bands.

Corrosion resistance and microstructure analysis of additively manufactured 22% chromium duplex stainless steel by laser metal deposition with wire

Microstructure characteristics and pitting corrosion of a duplex stainless steel (DSS) manufactured by laser metal deposition with wire (LMDw) were studied. The layer-by-layer LMDw process resulted in a mixed microstructure of predominantly ferrite with 2% austenite and chromium-rich nitrides, and reheated regions with ∼33% austenite. The high cooling rate of LMDw restricted the distribution of Cr, Mo, and Ni, in ferrite and austenite, while N diffuses from ferrite to austenite. Subsequent heat treatment at 1100 °C for 1 h resulted in homogenized microstructure, dissolution of nitrides, and balanced ferrite/austenite ratio. It also led to the redistribution of Cr and Mo to ferrite, and Ni and N to austenite. At room temperature, cyclic potentiodynamic polarization measurements in 1.0 M NaCl solution showed no significant differences in corrosion resistance between the as-deposited and heat-treated samples, despite the differences in terms of ferrite to austenite ratio and elemental distribution. Critical pitting temperature (CPT) was the lowest (60 °C) for the predominantly ferritic microstructure with finely dispersed chromium-rich nitrides; while reheated area with ∼33% austenite in as-deposited condition achieved higher critical temperature comparable to what was obtained after heat treatment (73 and 68 °C, respectively). At temperatures above the CPT, selective dissolution of the ferrite after deposition was observed due to depletion of N, while after heat treatment, austenite preferentially dissolved due to Cr and Mo concentrating in ferrite. In summary, results demonstrate how microstructural differences in terms of ferrite-to-austenite ratio, distribution of corrosion-resistant elements, and presence of nitrides affect corrosion resistance of LMDw DSS.

Insight into the Role of Mo Content on the Microstructure and Impact Toughness of X80 Thick-Walled Low-Temperature Pipeline Steel

In this manuscript, the effects of Mo content on the microstructure and impact toughness of X80 thick-walled low-temperature pipeline steel were studied. Two test steels with different Mo content (0.25% and 0.40%) were prepared by the thermo-mechanical control process. The impact properties were measured at −45 °C, and the microstructure evolution was observed via an optical microscope (OM), a scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and a transmission electron microscope (TEM). Each steel showed the formation of a mixed microstructure consisting of polygonal ferrite (PF), granular bainite (GB), and lath bainite (LB). Increasing Mo content resulted in the rise of LB at the expense of PF and GB. At the same time, the morphology of martensite/austenite (M/A) constituents changed from blocky to slender. The dislocation density in the ferrite matrix around the M/A constituents enhanced with an increase in Mo content. This also led to an increase in the microstrains around the M/A constituents. Also, the number fraction of the high angle grain boundary (HAGB) (MTA > 15°) decreased with the addition of more Mo content. Furthermore, with an increase in Mo content from 0.25% to 0.40%, the low-temperature impact toughness decreased from 206 to 57 J. Both an increase in the slender M/A constituents and a decrease in the HAGB number fraction deteriorated the low-temperature impact toughness of the X80 thick-walled low-temperature pipeline steel.

A novel recrystallization annealing method to cooperatively control the grain size and δ phase content for initial aged GH4169 superalloy forging

For initial aged GH4169 superalloy forging, it is important and difficult to find a suitable heat treatment method which can refine the deformed mixed grain and control δ phase content cooperatively. In this study, a novel heat treatment method is proposed, which includes an aging treatment to make many δ phase precipitates come out and a subsequent three-stage recrystallization annealing treatment (TSRT). TSRT consists of a high-temperature (990–1010 °C) annealing, a continuous cooling annealing (CRT) and a low-temperature annealing (930–980 °C). TSRT retains the advantages of CRT which is also proposed by authors' team. Moreover, the highlight of TSRT is that an ideal microstructure can be obtained by adjusting the parameters of low-temperature stage. Compared with CRT, TSRT is easier to control the microstructure. The results show that it is beneficial to obtain an ideal microstructure when the temperature of low-temperature stage is 980 °C and 950 °C. Eventually, two feasible annealing processes are obtained. One is “1000 °C×3 min+1000°C-5 min-980 °C+980 °C×20 min”. Through this annealing process, the deformed mixed grain microstructure can be uniformly refined to an average grain size of 7.02 μm, and the δ phase is distributed in spherical shape with a content of 3.28 vol%. The other is “1000 °C×3 min+1000°C-5 min-980 °C+950 °C×30 min”. Using this process, a grain microstructure with an average size of 4.92 μm can be obtained and the δ phase is distributed in rod-like and spherical shape with a content of 5.69 vol%. Besides, the formation mechanism of recrystallized mixed grains is revealed.

Microstructural characterization and mechanical properties in resistance spot welding of Q&P980 steel involving “effective softening” at the fusion boundary

This study proposes a welding process for Q&P980 steel that involves prolonged post-weld pulses. The process effectively softens the nuggets and results in welded joints with excellent comprehensive mechanical properties. The relationship between microstructural characteristics, element distribution and mechanical properties of weldments was elucidated for the samples subjected to different heat input histories. The microstructure evolution was analyzed using electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), and the mechanical properties of the spot welds were evaluated at room temperature by tensile shear (TS) and cross tension (CT) tests. The findings indicate that the softened area expands from the fusion boundary towards the opposite direction of the nugget as the heat input increases. A mixed microstructure of martensite and retained austenite, characterized by a larger size of prior austenite grains and lower kernel average misorientation, is formed in the “effective softening” region. This region provides serves as the primary location for crack nucleation and governs the propagation path of cracks within the halo ring-like softening zone, which has a significant impact on the fracture mode of the weld. The energy absorption in the TS test was increased by 95.40% compared to the single pulse weld (SPW), while the best combination of strength and energy absorption of the weldment in CT test was 7.2 kN and 64.24 J, respectively.

Complex microstructure induced high thermoelectric performances of p-type Bi–Sb–Te alloys

In this research, we have fabricated the complex microstructure features along with the high density of twin boundaries using the combination of magnetic pulse compaction (MPC) and spark plasma sintering (SPS), which are exclusively improved the thermoelectric transport properties of p-type Bi0.5Sb1.5Te3 alloys via effective scattering of carriers and phonons at the grain boundary interfaces. The texture analysis confirmed the formation of bimodal (mixed grains) microstructural features in MPC + SPS specimens sintered at 350 °C, and 400 °C respectively due to the adhesion of fine grains (grain growth) and initiation of recrystallization at high temperatures. In addition to high density of twin structure, high-angle grain boundaries have also been identified. The power factor of 3.53 mW/m.K2 was obtained for the MPC + SPS at 400 °C due to increased Seebeck coefficient and larger electrical conductivity values. The maximum reduced thermal conductivity of 1.14W/mK was recorded due to a significant reduction in lattice thermal conductivity, which is mainly owing to strong scattering of phonons at the formation of mixed grain microstructures with high-density dislocations. As a result, maximum figure of merit, ZT = 1.17was achieved for MPC + SPS at 400 °C temperature.

Bioinspired fibrous microstructure breaks strength and toughness trade-off in plain carbon steel

Warm rolling at eight temperatures was conducted on two mixed ferrite-pearlite initial microstructures in order to develop strong but tough plain steel. The obtained wood-like fibrous microstructure in AISI 1045 plain steel breaks the strength and toughness trade-off dilemma. After rolling at 500 °C, the tensile strength reaches 1020 MPa, and the impact toughness is as high as 120 J. Intensive α-fiber (<110>//RD) and η-fiber (<100>//RD) textures, as well as necklace-like fine carbide, were developed in the fibrous microstructure. With increasing rolling temperature, the tensile strength and impact toughness decreased due to the reduced texture intensity and the increased carbide size. The high strength of fibrous steels is mainly originated from the dispersion and dislocation strengthening, while the impact toughness is controlled by the carbide size and the occurrence of delamination mechanism.

Study on Preparation of Homogenized Highly Dispersible AlON Powder

The quality of Aluminum oxynitride (AlON) powder is the main factor affecting the properties of ceramics. To accurately control the nitrogen AlON content and improve the dispersion degree of powder, AlON powder was prepared from γ-Al2O3 and activated carbon powder by carbothermic reduction and nitridation. The uniformity of distribution of raw materials and the dimension and microstructure of AlON powder was controlled by changing the time and rotation rate of the ball milling process. The influences of ball milling process parameters on the microstructure and distribution uniformity of Al2O3/C mixed powder, microstructure, and particle size of AlON powder were studied. The results show that powder agglomeration was effectively reduced, and the uniformity of raw material distribution was increased by controlling the time and rotation rate of the ball milling process. The Al2O3/C mixed powder has good particle size and uniform distribution with roller ball milling for 24 h. After holding the powder at 1700 °C for 60 min, the homogenized AlON powder was obtained, and the residual carbon content was about 0.04 wt%. When the time of planetary ball milling at 250 rpm was 16 h, highly dispersible AlON powder was obtained.

Mechanism of mitigating quenching cracking for B-containing 9Cr martensitic heat resistant steel by austempering process

B-containing 9Cr martensitic steels are attractive materials for specific industrial applications due to their excellent high-temperature strength and ductility. However, quenching large forgings with water or oil often results in cracking. The correlation between quenching crack and microstructure was established in this work to evaluate the selection of the quench cooling methods, which make it possible to mitigate the quench cracking by tailoring microstructural evolution. Kinetics dynamic analysis, microstructural characterization, and phase-field simulation were used to clarify the mechanism of mitigating quenching cracking. It is demonstrated that austempering produces a mixed microstructure consisting of lathy martensite, ultra-fine martensite, and a small amount of C-riched retained austensite. This mixed microstructure translates into refining the effective grain size and reducing the internal stress, reducing cracking propagation by reducing phase transformation stress and enhancing the coordinated deformation. Thus, the quenching cracking can be inhibition by tailoring the fraction of ultra-fine martensite and retained austensite in B-containing 9Cr steels.

EFFECT OF ULTRAFAST HEATING ON AISI 304 AUSTENITIC STAINLESS STEEL

This study explores the effects of ultrafast heating on AISI 304 austenitic stainless steel. The research shows that ultrafast heating can lead to fine-grained mixed microstructures in steel, making it a potential alternative for modifying microstructure in stainless steel. The study demonstrates that a minimum temperature of 980 °C is required to achieve a fully recrystallized microstructure. The results also suggest that a lower temperature can result in a finer recrystallized grain size compared to higher temperature results. The study provides valuable insights into the impact of ultrafast heating on the microstructural constituents, recrystallization temperatures, and mechanical properties of investigated steel.

Microstructure and properties of low alloy CrMo steel processed with varied isothermal temperatures

The microstructure and properties of low alloy CrMo steel after isothermal treatment at different temperatures were studied. The experiment process is to rapidly cool the samples that have been held at 940 ℃ for 2 h to a salt bath furnace at different temperatures (300–500 ℃) for isothermal treatment, and then air cooling to ambient temperature. The microstructure after isothermal treatment is granular bainite, which is a mixed microstructure composed of bainitic ferrite (BF) and martensite/austenite (M/A) island. The morphology, size and volume fraction of M/A islands change with the change of isothermal temperature. The mechanical properties are also closely related to isothermal temperature. With the increase of isothermal temperature, the impact toughness and yield strength (YS) decreased. By studying the effects of different strengthening mechanisms and multiphase on YS, the results show that YS mainly depends on BF. According to the SEM and EBSD characterization of the fracture, it is found that the stress in the sample is mainly concentrated near the M/A, which makes the crack easy to initiate on the M/A island or its interface. The brittle fracture tendency of materials is related to M/A island. The larger the size of the M/A island, the more likely brittle fracture will occur and the less ductile it will be.

Effects of wheat gluten addition on dough structure, bread quality and starch digestibility of whole wheat bread

SummaryWhole wheat bread is poor in quality, and it is necessary to use certain methods to improve bread texture and glycaemic index, such as adding exogenous protein. This study evaluated the effects of adding different proportions of gluten (focus on exogenous protein enrichment) on dough structure, bread quality, and starch digestibility, focusing on mixing properties, rheological properties, pasting properties, microstructure, basic quality, and starch digestibility. Dough microstructure results showed that gluten addition helped to form a more complete gluten network. The bread quality results showed that gluten‐enriched bread had larger specific volume, lower baking loss, and better textural quality. Starch digestibility results showed that gluten addition slowed down starch digestion to some extent. The experimental results indicated that 40% added gluten could produce bread samples with optimal quality. The subject systematically studied the effect of gluten enrichment on the quality of whole wheat bread, which is a guide to the application.

The Effect of Stereoregularity and Adding Irganox 1076 on the Physical Aging Behavior of Poly(1-trimetylsilyl-1-propyne).

The effect of the stereoregularity of poly(1-trimethylsilyl-1-propyne) [PTMSP] (cis-content from 50 to 90%) on physical aging was investigated by measurement of the gas permeability. Films from pure PTMSP as well as those with the addition of the antioxidant Irganox 1076 were exposed to the air. The permeability of pure PTMSP films increases with an increase in cis-stereoregularity and correlates with an increase in interchain distances (according to X-ray analysis). For pure PTMSP films, the most significant aging (up to 50% of permeability drop) was observed for polymers with mixed microstructure, and the slowest aging (10-30% of permeability drop) was observed for polymers with cis-regular structure. For PTMSP films with added Irganox 1076, some decrease in permeability with time is also observed. The addition of Irganox 1076 to PTMSP in mixed as well as cis-enriched configurations visibly slows down aging. In the case of cis-regular PTMSP with a slow aging rate, the introduction of an antioxidant does not provide any advantages. The high stability of cis-regular PTMSP demonstrates the possibility of obtaining more stable membrane materials with the highest equilibrium state of the polymer selective layer prepared by casting solution.

Microstructure, hardness and wear performance of quaternary CrNbTiZr refractory medium-entropy alloy coating fabricated on a commercial Zr alloy by pulsed laser cladding

To attain enhanced surface hardness and wear resistance, a quaternary CrNbTiZr refractory medium-entropy alloy (RMEA) coating was fabricated on a commercial Zr alloy (Zr-702) by a pulsed laser cladding technique. Phase constitution, microstructural feature, hardness and wear performance of the RMEA coating were systematically investigated. The specific influence of Zr diluted from the substrate on the coating was explored. The results reveal that the cladding zone is mainly featured by a BCC solid-solution phase and net-like Cr2Zr intermetallics. Such microstructural features agree with those predicted based on atomic radius difference, Allen electronegativity and valence electron concentration. Allotropic solid phase transformation occurs in the heat-affected zone, resulting in a mixed microstructure of blocky grains (α-Zr) and martensitic laths. The hardness of the RMEA coating (637 ± 46 HV) is ∼3.3 times that of the Zr-702 substrate, which results from joint contribution from solid-solution, low-angle grain boundary and second-phase strengthening. The RMEA coating exhibits a wear rate ∼56% lower than the substrate, demonstrating significantly improved wear resistance. Both oxidation and adhesive wear are determined to be dominant wear mechanisms of the RMEA coating, accounting for its enhanced wear performance.

Effect of particle size on the microstructure and consolidation behavior of nickel coating fabricated by laser shockwave sintering

Nickel powders of nominal sizes (i.e., 50 nm, 300 nm, and 2 μm) were pre-coated on copper substrates and then subjected to laser shock processing (LSP) for producing nickel coating. The shockwave consolidation behavior, microstructure, and bonding performance were investigated as a function of particle size. Results indicated that nano-coating had better surface quality, decreased microstructural defects (pores and cracks), and higher bonding strength (approximately twice the hardness value) than its micro-counterpart. Microstructure analysis revealed that nano/submicron particles were consolidated by a mixed solid-liquid sintering behavior, whereas micron particles were consolidated only by solid-state sintering under laser shock. A mixed microstructure of nanocoating was obtained, consisting of solid-state atomic bonds between solid-state sintered particles and amorphous phases between liquid-phase sintered particles. After LSP, the cold shock characteristics of LSP could not only reduce the overall grain growth and oxidation of nano coating, but also induce fine nanocrystalline at the inter-particle interface of the micron coating by dynamic recrystallization.