Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《材料研究学报》
Abstract： 18CrNb is a kind of ferritic stainless steel and widely used as parts at the hot end of automobile exhaust system due to its good formability, high temperature strength and oxidation resistance. In this paper, the oxidation of 18CrNb steel in a temperature ranging from 700℃ to 1000℃ was studied through continuous oxidation behavior test. The oxide scale was then characterized by XRD, SEM and EDS, while the oxidation dynamic curve of this steel was plotted based on the weight-gain data after oxidation. The results indicate that 18CrNb steel has excellent oxidation resistance at temperatures lower than 900℃ due to the formation of continuous and dense Cr-rich oxide scale. When the temperature rises up to 950℃, the oxide scale turn to be complex, of which the outer portion composed of Cr-rich Cr-Mn oxide, Mn-rich Mn-Cr oxide, Fe oxide and pure iron oxide; the inner portion composed of Fe-rich Fe/Cr oxide; thereby the scale became loose leading to breakaway oxidation. According to the results of this work, 18CrNb is not suitable for application at temperatures above 950℃.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： High strength micro-alloyed steel has been widely used in automobile and machines because of the remarkable high strength and forming property which are attributed to nano-precipitates and refinement of the organization. Since the nano-precipitates are mostly nucleated between austenite/ferrite transition and ferrite can significantly advance strength, it is important to investigate precipitate behavior in coiling process. Nanoindentation technology provides a chance to study the special influence of nano-precipitates on the micro-properties of ferrite. The effect of cooling rate during continuous cooling process and coiling process on microstructural evolution and micro-hardness of Nb-Ti micro-alloyed steel were studied by using the thermal mechanical simulator, micro-hardness instrument, TEM and nanoindentation instrument. The precipitate behaviors of (Nb,Ti)C in coiling process and its effect on nano-hardnesss of ferrite were discussed. Experiments results indicated that the increase of cooling rate in continuous cooling process and coiling process could promote the microstructure transition from ferrite and pearlite to bainite. The micro-hardness of the tested steel increased with the increase of cooling rate in continuous cooling process, and decreased with the cooling rate in coiling process because of the large number of the dispersive nano-precipitate in ferrite which could improve the strength of matrix. The smaller cooling rate could promote volume fraction of (Nb, Ti)C particles in ferrite because there was enough time for the nucleation and growth of (Nb, Ti)C precipitates. When the cooling rate in coiling process was 0.1 ℃/s, precipitates were dispersive in ferrite matrix with a diameter of less than 10 nm. The nanohardness and Young's modulus of ferrite were 4.13 and 249.3 GPa for Nb-Ti micro-alloyed steel, 2.64 and 237.4 GPa for C-Si-Mn steel. The contribution of nano-precipitates to nanohardness of ferrite reached 1.49 GPa.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： As an important carbide forming element, Nb plays an important role in steel. Precipitated Nb can restrain the austenite grain growth during soaking process and provide precipitation strengthening after g /a phase transformation. Precipitated or dissolved Nb can inhibit recrystallizaton of deformed austenite. Recently, both Nb and Mo are added in steel to enhance the role of Nb. However, these kinds of researches mostly focused on continual cooling process of g /a transformation or isothermal process during tempering, and precipitation behavior of MCtype carbide in steel containing Nb and Mo during reheating process and the effect of Mo on precipitation of NbC in ferrite were rarely reported. Therefore, in this work, precipitation behaviors of MC-type carbide and the synergistic effect of Nb and Mo in steel containing Nb or Nb-Mo during reheating process at the heat rate 20 ℃/min were investigated by means of Vickers hardness test, SEM, HRTEM and DSC. The results show that both Nb and Nb-Mo steels have hardness peaks at 300 and 700 ℃, which are attributed to the precipitation of e-carbide and MCtype carbide, respectively. The MC-type carbide precipitates at about 650 ℃ during reheating process, which is in a good agreement with the nose temperature of MC-type carbide calculated by Avrami equation. (Nb, Mo)C particle forming in Nb-Mo steel during precipitation has a small mismatch with ferrite matrix compared with NbC, leading to the decrease of interfacial energy. Thus, the precipitation kinetic of MC-type carbide in Nb-Mo steel is faster than that in Nb steel, which results in the denser and finer MC-type carbide and higher precipitation strengthening effect.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： TiAl-Nb alloys have been determined as the advanced direction for the development of the high temperature TiAl alloys, so being one of the advanced materials for turbines of aircraft engines and gas-burning power-generation plants. However, highly-Nb addition can lead to the complex solidification behavior of TiAl-Nb alloy and multi- phase microstructure, which is important for the mechanical properties during the alloy design. Bridgman type directional solidification experiments were conducted for Ti-46Al-(8, 9, 10)Nb alloy. The effect of the growth rate and Nb content on the microstructure, phase transition and microsegregation was investigated, and finally the selection diagram of the phase transition and the microstructure of the directional solidified TiAl-Nb alloy were obtained. The results show that the planar-cellular-dendritic evolution of solid-liquid interface can be observed with the increase of the growth rate. Meanwhile the fully b phase solidification changes to the peritectic solidification with the increase of the growth rate, and correspondingly the final microstructure is composed of the a2/g lamellar structure and a multiphase microstructure (B2 phase, a2/g lamellar structure) respectively. The increase of the b-stabilizer Nb content can promote the fully b phase solidification and the formation of the multiphase microstructure (B2 phase, a2/g lamellar structure). The contribution of the growth rate and the Nb content to the phase transition and the microstructure is connected with the solute segregation (S- segregation, b- segregation) closely. The increase of the S-segregation amplitude can easily promote the peritectic reaction, which always leads to the highly solute segregation and the concentrated distribution of plenty of B2 phase in the core of the dendrite. b-segregation is the mainly origin of the B2 phase formation, in which the Nb enrichment in the retained b phase directly determines the morphology and the dimension of the B2 phase. Finally according to the selection diagram of the solidification process and the microstructure of the Ti-46Al alloy with the growth rate and the Nb content, the high Nb content and the low growth rate during fully b solidification should be selected for the prefer microstructure with the homogeneous distribution and the low solute segregation.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： High strength low-alloy (HSLA) steel has been widely used in buildings, bridges, ships and automobiles because of the remarkable high strength and forming property. Conventional HSLA steels are strengthened by a combination of grain refinement, solid-solution strengthening and precipitation hardening, and the contribution of precipitation hardening is considered to be minor, since many of the alloying elements are added to HSLA steels in the past basically for the strengthening of grain refinement. However, in recent research, yield strengths up to 780 MPa have been achieved in Ti and Mo bearing HSLA sheet steels by producing microstructures that consist of a ferritic matrix with nanometer-sized carbides, and the precipitation strengthening has been estimated to be approximately 300 MPa. Nowadays, thermo mechanical controll process (TMCP) is widely used to process HSLA steels, the final temperature of ultra-fast cooling (UFC) plays a decisive role for microstructure evolution and precipitation behavior, and finally determines the mechanical properties of the steels. In this work, the effects of final temperature after UFC on microstructural evolution, precipitation behavior and micro-hardness of Nb-V-Ti bearing low alloy steel were studied by using the thermal mechanical simulator, OM, HRTEM and micro-hardness instrument. The results showed that the microstructure and nucleation sites of micro-alloy carbides changed with final temperature after UFC. The microstructure changed from bainite to pearlite and ferrite and the nucleation sites changed from bainite to ferrite with final cooling temperature increasing. The number density of the precipitates in ferrite matrix was greater than that in bainite. Furthermore, the number density of the nanometer sized carbides got the maximum values at 620 ℃. The aspect ratios of the precipitates were close to 1, which meat that the precipitation morphology close to spherical. The sizes of the carbides were all less than 10 nm and became smaller with the decrease of final cooling temperature. Through the calculation by Orowan mechanism, the contributions of the precipitation strengthening to yield strength could reach 25.6% at the final cooling temperature of 620 ℃.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： High strength low alloy steels utilize chemical composition design of low carbon content and are microalloyed with Nb, V and Ti, or other additions, such as Mo and B, etc. The increase of strength is attributed to grain refinement strengthening, solid-solution strengthening, dislocation strengthening and precipitation hardening. Moreover, the precipitation hardening attracts more and more attentions. However, the detailed results on the sheet spacing, inter-particle spacing, crystallography, composition and the nucleation site of the interphase precipitation carbides in Nb-Ti containing steels have not been reported as yet. In this work, the microstructure, mechanical properties and precipitation behaviors in a low carbon Nb-Ti microalloyed steel were investigated using the dilatometer and TEM. The results show that the interphase precipitation can be observed for different isothermal temperatures and the sheet spacing, inter-particle spacing and size of the interphase precipitation carbides are refined by lowing isothermal temperature. The characteristic sheets of interphase precipitation carbides were identified as planar and curved. The planar sheets of interphase precipitation carbides have been analyzed and found to be parallel with {011},{012},{013}and {035} planes of ferrite. Moreover, the interphase precipitation carbides which have been determined to be (Nb, Ti)C have a NaCl-type crystal structure with a lattice parameter of 0.434 nm and obey the Baker-Nutting (B-N) orientation relationship with respect to ferrite matrix. The contribution of the interphase precipitation hardening to the yield strength of the experiment steel has been estimated above 300 MPa, based on the Orowan mechanism.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Stainless steel is widely used in both industrial production and daily-life due to its anti-corrosion behavior. In view of the shortage in Cr and Ni resources, there has been an increasing interest in developing lowcost stainless steels for several decades. Under the frame of replacing Ni and Cr with Mn and Al, respectively, a recent study indicates that Fe-Mn-Al-C austenitic twinning-induced plasticity (TWIP) steel possesses good comprehensive properties and excellent resistance to oxidation that make it potential in partially replacing conventional austenitic stainless steels. As a viable alternative to low-cost austenitic stainless steel, a new alloy system of highmanganese low-chromium nitrogen-containing TWIP steel was developed in this study. Considering its corrosion resistance, the alloy is not completely free of chromium, yet the Cr content is relatively low. Nitrogen is added, because it is a strong austenite stabilizer that can reduce the tendency to form ferrite and deformation-induced a'- and e-martensites, thereby reducing the amount of nickel required in austenitic stainless steel. Furthermore, nitrogen is beneficial for pitting corrosion resistance. In this study, hot deformation and recrystallization behaviors of this high manganese austenitic TWIP steel were investigated by single-pass compression tests on MMS-300 thermo-mechanical simulator at temperature ranging from 1223 K to 1423 K and strain rate ranging from 0.01 s-1 to 10 s-1. Micro-structure evolution during dynamic recrystallization and the correlation of microstructure change to the stress-strain response were further analyzed by using TEM and SEM equipped with EBSD. The results show that the hot deformation behavior of this steel is more sensitive to deformation rate. Dynamic recrystallization occurs during hot deformation when deformation rate is lower than 0.1 s-1, while dynamic recovery takes place at deformation rate higher than 1 s-1. The hot deformation constitutive equation of the high manganese austenitic TWIP steel was established by regression analysis. There is a close correlation between microstructure evolution and stress-strain curve during dynamic recrystallization.With the increase of strain, the grain boundary migration leads to the nucleation of recrystallization. Sub-grain boundary was also formed with increasing the strain. Dislocations climbing or slipping on the adjacent sub-grain boundary lead to the grain boundary merging, and then, new austenitic grains formed.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： By using high energy ball- milling and vacuum hot-pressed sintering techniques, ultrafine grained carbide cement WC-2.5TiC-10Co were prepared, and the effect of grain growth inhibitors Cr3C2, VC, TaC and NbC on microstructures and mechanical properties were studied by X- ray diffractometer, field emission scanning electron microscope (FESEM) and mechanical performance test. The results indicates that the particle size of the WC powder can be reduced to less than 0.2 mm from 0.6 mm after 30 h of high energy ball-milling at a rotation speed of 350 r/min and ball/powder ratio of 10: 1. After vacuum hot-pressed sintering at 1410℃ for 1h, no new reactive product formed revealed by XRD. When 0.45 % Cr3C2, 0.3 %VC, 0.5 %TaC or NbC were added to the carbide cement, a small quantity of abnormally coarsened WC grains occurred, while the fractured surfaces display loose and smooth. Analyzing the fractured surfaces indicate that cleavage cracking in coarsened WC grains was caused by concentrated stress and become the source of materials fracture. When the contents of inhibitors Cr3C2 and VC were both more increased 0.1%, the grain size of WC can be reduced less than 0.5 mm. In such a case, the fractured surfaces are intimate and step-like, while the bending strength can be increased by 20%. Inhibitors TaC and NbC have not pronounced effect on the preventing the WC grain growth, while the addition of NbC is most effective in improving the relative density of the carbide cement.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：以一种高锰奥氏体孪晶诱发塑性(TWIP)钢为实验材料，采用700~1000 °C保温20 min及800 °C保温10~30 min的退火工艺获得了不同晶粒尺寸分布及晶界特征分布的再结晶组织，结合EBSD技术及动电位极化曲线测试，研究了晶粒度、晶粒均匀性及晶界特征分布对该钢抗腐蚀能力的影响。结果表明，该高锰奥氏体TWIP钢的抗腐蚀能力受组织中的晶粒度及重位点阵(CSL)晶界分布比例的影响，二者的作用在再结晶的组织中因组织的均匀性不同表现出明显差异。当再结晶过程刚刚结束，晶粒组织尚不均匀且未进入晶粒长大阶段时，平均晶粒尺寸对抗腐蚀能力的影响占主导地位。随着平均晶粒尺寸的增大，实验钢的抗腐蚀能力下降。而当再结晶晶粒充分长大，且晶粒尺寸分布均匀，CSL晶界所占的比例对其抗腐蚀能力的影响尤为显著。随着CSL晶界所占晶界比例的提高，实验钢的抗腐蚀能力增加。
Hits
Hits
Downloads
Comment