Effect of Mo content on the thermal stability of Ti–Mo-bearing ferritic steel

The effects of tempering holding time at 700°C on the morphology, mechanical properties, and behavior of nanoparticles in Ti–Mo ferritic steel with different Mo contents were analyzed using scanning electron microscopy and transmission electron microscopy. The equilibrium solid solution amounts of Mo, Ti, and C in ferritic steel at various temperatures were calculated, and changes in the sizes of nanoparticles over time at different Mo contents were analyzed. The experimental results and theoretical calculations were in good agreement with each other and showed that the size of nanoparticles in middle Mo content nano-ferrite(MNF) steel changed the least during aging. High Mo contents inhibited the maturation and growth of nanoparticles, but no obvious inhibitory effect was observed when the Mo content exceeded 0.37 wt%.The tensile strength and yield strength continuously decreased with the tempering time. Analysis of the strengthening and toughening mechanisms showed that the different mechanical properties among the three different Mo content experiment steels were mainly determined by grain refinement strengthening(the difference range was 30–40 MPa) and precipitation strengthening(the difference range was 78–127 MPa). MNF steel displayed an ideal chemical ratio and the highest thermodynamic stability, whereas low Mo content nano-ferrite(LNF) steel and high Mo content nano-ferrite(HNF) steel displayed relatively similar thermodynamic stabilities.     

Effect of Mo content on the thermal stability of Ti-Mo-bearing ferritic steel

The effects of tempering holding time at 700℃on the morphology,mechanical properties,and behavior of nanoparticles in Ti-Mo ferritic steel with different Mo contents were analyzed using scanning electron microscopy and transmission electron microscopy.The equilibrium solid solution amounts of Mo,Ti,and C in ferritic steel at various temperatures were calculated,and changes in the sizes of nanoparticles over time at different Mo contents were analyzed.The experimental results and theoretical calculations were in good agreement with each other and showed that the size of nanoparticles in middle Mo content nano-ferrite(MNF)steel changed the least during aging.High Mo contents inhibited the maturation and growth of nanoparticles,but no obvious inhibitory effect was observed when the Mo content exceeded 0.37wt%.The tensile strength and yield strength continuously decreased with the tempering time.Analysis of the strengthening and toughening mechanisms showed that the different mechanical properties among the three different Mo content experiment steels were mainly determined by grain refinement strengthening(the difference range was 30-40 MPa)and precipitation strengthening(the difference range was 78-127 MPa).MNF steel displayed an ideal chemical ratio and the highest thermodynamic stability,whereas low Mo content nano-ferrite(LNF)steel and high Mo content nano-ferrite(HNF)steel displayed relatively similar thermodynamic stabilities.     

Microstructure and mechanical properties of Nb–Mo–ZrB_2 composites prepared by hot-pressing sintering

Nb–Mo–ZrB_2 composites(V(Nb)/V(Mo) = 1) with 15 vol% or 30 vol% of ZrB_2 were fabricated by hot-pressing sintering at 2000°C. The phases, microstructure, and mechanical properties were then investigated. The composites contain Nb-Mo solid solution(denoted as(Nb, Mo)ss hereafter), Zr B, Mo B, and Nb B phases. Compressive strength test results suggest that the strength of Nb–Mo–ZrB_2 composites increases with increasing ZrB_2 content; Nb–Mo–30 vol%ZrB_2 had the highest compressive strength(1905.1 MPa). The improvement in the compressive strength of the Nb–Mo–ZrB_2 composites is mainly attributed to the secondary phase strengthening of the stiffer Zr B phase, solid-solution strengthening of the(Nb, Mo)ss matrix as well as fine-grain strengthening. The fracture toughness decreases with increasing ZrB_2 content. Finally, the fracture modes of the Nb–Mo–ZrB_2 composites are also discussed in detail.     

Microstructure and mechanical properties of Nb–Mo–ZrB2 composites prepared by hot-pressing sintering

Nb–Mo–ZrB₂ composites (V(Nb)/V(Mo)=1) with 15vol% or 30vol% of ZrB₂ were fabricated by hot-pressing sintering at 2000℃. The phases, microstructure, and mechanical properties were then investigated. The composites contain Nb-Mo solid solution (denoted as (Nb, Mo)ss hereafter), ZrB, MoB, and NbB phases. Compressive strength test results suggest that the strength of Nb–Mo–ZrB₂ composites increases with increasing ZrB₂ content; Nb–Mo–30vol%ZrB₂ had the highest compressive strength (1905.1 MPa). The improvement in the compressive strength of the Nb–Mo–ZrB₂ composites is mainly attributed to the secondary phase strengthening of the stiffer ZrB phase, solid-solution strengthening of the (Nb, Mo)ss matrix as well as fine-grain strengthening. The fracture toughness decreases with increasing ZrB₂ content. Finally, the fracture modes of the Nb–Mo–ZrB₂ composites are also discussed in detail.     

Structural refinement of 00Cr13Ni5Mo2 supermartensitic stainless steel during single-stage intercritical tempering

The 00Cr13Ni5Mo2 supermartensitic stainless steel was first tempered at 570–730℃ for 2 h to observe the microstructure and hardness changes. The tempering temperature was set to 600, 650, and 700℃, which is below, equal to, and above the austenite transformation start temperature, respectively, for each holding period to investigate the effects of tempering time on the structure and properties of the steel. The microstructure of the specimens was examined by optical microscopy and transmission electronic microscopy, and the phase composition was detected by X-ray diffraction. As expected, lath refinement was observed in the steel tempered at 700℃, and the refinement degree significantly depended on the tempering time. Contrary to normal steel softening by tempering, the hardness performance of the steel was significantly enhanced primarily because of the refinement of martensite laths after single-stage intercritical tempering. It is believed that the reverse transformation of martensite (α′) to austenite (γ) is responsible for the refinement.     

Variation in retained austenite content and mechanical properties of 0.2C–7Mn steel after intercritical annealing

The effects of annealing time and temperature on the retained austenite content and mechanical properties of 0.2C–7Mn steel were studied. The retained austenite content of 0.2C–7Mn steel was compared with that of 0.2C–5Mn steel. It is found that 0.2C–7Mn steel exhibits a similar variation trend of retained austenite content as 0.2C–5Mn steel. However, in detail, these trends are different. 0.2C–7Mn steel contains approximately 7.5vol% retained austenite after austenitization and quenching. The stability of the reversed austenite in 0.2C–7Mn steel is lower than that in 0.2C–5Mn steel; in contrast, the equilibrium reversed austenite fraction of 0.2C–7Mn steel is substantially greater than that of 0.2C–5Mn steel. Therefore, the retained austenite content in 0.2C–7Mn steel reaches 53.1vol%. The tensile results show that long annealing time and high annealing temperature may not favor the enhancement of mechanical properties of 0.2C–7Mn steel. The effect of retained austenite on the tensile strength of the steel depends on the content of retained austenite; in contrast, the 0.2% yield strength linearly decreases with increasing retained austenite content.     

Mechanical properties and characteristics of nanometer-sized precipitates in hot-rolled low-carbon ferritic steel

The microstructures and properties of hot-rolled low-carbon ferritic steel have been investigated by optical microscopy, field-emission scanning electron microscopy, transmission electron microscopy, and tensile tests after isothermal transformation from 600℃ to 700℃ for 60 min. It is found that the strength of the steel decreases with the increment of isothermal temperature, whereas the hole expansion ratio and the fraction of high-angle grain boundaries increase. A large amount of nanometer-sized carbides were homogeneously distributed throughout the material, and fine (Ti, Mo)C precipitates have a significant precipitation strengthening effect on the ferrite phase because of their high density. The nanometer-sized carbides have a lattice parameter of 0.411–0.431 nm. After isothermal transformation at 650℃ for 60 min, the ferrite phase can be strengthened above 300 MPa by precipitation strengthening according to the Ashby-Orowan mechanism.     

Effects of annealing temperature on the microstructure and properties of the 25Mn-3Si-3Al TWIP steel

Microstructures and mechanical properties of the 25Mn twinning induced plasticity (TWIP) steel at different annealing temperatures were investigated. The results indicated that when the annealing temperature was 1000℃, the 25Mn steel showed excellent comprehensive mechanical properties, the tensile strength was about 640 MPa, the yield strength was higher than 255 MPa, and the elongation was above 82%. The microstructure was analyzed by optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Before deformation the microstructure was composed of austenitic matrix and annealing twins at room temperature; at the same time, a significant amount of annealing twins and stacking faults were observed by TEM. Mechanical twins played a dominant role in deformation and as a result the mechanical properties were found to be excellent.     

AISI 321不锈钢钢钢包精炼过程中钙含量对夹杂物的影响

The effect of three heat processes with different calcium contents on the evolution of inclusions during the ladle furnace refining process of AISI 321 stainless steel was investigated. The size, morphology, and composition of the inclusions were analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. After the addition of aluminum and titanium, the primary oxide in the AISI 321 stainless steel was an Al₂O₃–MgO–TiO_x complex oxide, in which the mass ratio of Al₂O₃/MgO was highly consistent with spinel (MgO·Al₂O₃). After calcium treatment, the calcium content in the oxide increased significantly. Thermodynamic calculations show that when the Ti content was 0.2wt%, the Al and Ca contents were less than 0.10wt% and 0.0005wt%, respectively, which was beneficial for the formation of liquid inclusions in molten steel. Moreover, the modification mechanism of calcium on TiN-wrapped oxides in combination with temperature changes was discussed.     

Effect of ECAP temperature on microstructure and mechanical properties of Al–Zn–Mg–Cu alloy

The effect of equal channel angular pressing(ECAP) at different temperatures(room temperature, 120,150 and 180 °C) on microstructure and mechanical properties of Al-7075 solid solution alloy was investigated. Microstructure of the specimens was examined using orientation imaging microscopy,transmission electron microscopy as well as X-ray diffractometer, and mechanical properties were measured by Vickers microhardness and tensile tests. Microstructural investigations showed that after3 or 4 passes of ECAP, fi ne grains with average grain sizes in range of 300–1000 nm could be obtained at different ECAP temperatures. Increasing ECAP temperature from 120 to 180 °C caused a decrease in mechanical properties as a result of increasing grains and precipitates sizes, decreasing fraction of high angle boundaries and also transformation of η′ into η phase, while increasing ECAP temperature from RT to 120 °C leads to an increase in mechanical properties due to the formation of small η′ precipitates. So it can be concluded that ECAP process at 120 °C is the optimum process for attaining maximum mechanical properties. Quantitative estimates of various strengthening mechanisms revealed that the improvement of mechanical properties was mainly attributed to grain re fi nement strengthening, precipitation strengthening and dislocation strengthening.     

Effect of tempering temperature on the mechanical properties and microstructure of an copper-bearing low carbon bainitic steel

The effect of tempering temperature on the microstructure and mechanical properties of ultra-high strength, copperbearing, low-carbon bainitic steel has been investigated in the experiment. The results showed that the microstructure was mainly the laths of bainite in the as-quenched steel. The bainitic laths were restored and combined after the steel tempered at various temperatures. There were rnartensite/austenite (M/A) islands and numerous dislocations within and between the bainitic laths, while very t-me precipitates of ε-Cu were also observed within the laths. With increasing the tempered temperature from 400 to 600℃, the yield strength (YS) increased from 877 to 957 MPa, whereas the ultimate tensile strength (UTS) decreased from 1020 to 985 MPa. The Charpy V-notch (CVN) varied from 68.5 to 42 J, and the value was minimal for the steel tempered at 500℃.     

A Special TMCP Used to Develop a 800MPa Grade HSLA Steel

The effect of relaxation after finished rolling on structures and properties of four microalloyed steel with different content of Nb and Ti was investigated. By alloy designing and control rolling + relaxation-precipitation-control phase trail storm ati on (RPC) process, a new 800 MPa grade HSLA plate steel could be obtained, the microstructure is composite ultra-fine lath bainite/martensite. The tempering process and mechanical properties of this kind of HSLA steel were investigated. The yield strength can achieve 800 MPa, and the ductility and impact toughness is satisfied.     

Effects of heat treatment on the microstructures and mechanical properties of a new type of nitrogen-containing die steel

Nitrogen can increase the strength of steels without weakening the toughness and improve the corrosion resistance at the same time. Compared with conventional nitrogen-free die steels, a new type of nitrogen-containing die steel was developed with many superior properties, such as high strength, high hardness, and good toughness. This paper focused on the effects of heat treatment on the microstructures and mechanical properties of the new type of nitrogen-containing die steel, which were investigated by the optimized deformation process and heat treatment. Isothermal spheroidal annealing and high-temperature quenching as well as high-temperature tempering were applied in the experiment by means of an orthogonal method after the steel was multiply forged. The mechanical properties of nitrogen-containing die steel forgings are better than the standard of NADCA #207-2003.     

Microstructure and mechanical properties of spark plasma sintered Ti-Mo alloys for dental applications

Ti-Mo alloys with various Mo contents from 6wt% to 14wt% were processed by spark plasma sintering based on elemental powders. The influence of sintering temperature and Mo content on the microstructure and mechanical properties of the resulting alloys were investigated. For each Mo concentration, the optimum sintering temperature was determined, resulting in a fully dense and uniform microstructure of the alloy. The optimized sintering temperature gradually increases in the range of 1100–1300℃ with the increase in Mo content. The microstructure of the Ti-(6–12)Mo alloy consists of acicular α phase surrounded by equiaxed grains of β phase, while the Ti-14Mo alloy only contains single β phase. A small amount of fine α lath precipitated from β phase contributes to the improvement in strength and hardness of the alloys. Under the sintering condition at 1250℃, the Ti-12Mo alloy is found to possess superior mechanical properties with the Vickers hardness of Hv 472, the compressive yield strength of 2182 MPa, the compression rate of 32.7%, and the elastic modulus of 72.1 GPa. These results demonstrate that Ti-Mo alloys fabricated via spark plasma sintering are indeed a perspective candidate alloy for dental applications.     

Analysis on composition and inclusions of ballpoint pen tip steel

Ballpoint pen tip steel, a super free-cutting stainless steel, exhibits excellent corrosion resistance and good machining properties. In this study, inductively coupled plasma spectroscopy, metallographic microscopy, and scanning electron microscopy were used to determine the elemental contents in five ballpoint pen tips and their components, morphologies, and inclusion distributions. The results showed that the steels were all S-Pb-Te super free-cutting ferritic stainless steel. The free-cutting phases in the steels were mainly MnS, Pb, and small amounts of PbTe. MnS inclusions were in the form of chain distributions, and the aspect ratio of each size inclusion in the chain was small. The stress concentration effect could substantially reduce the cutting force when the material was machined. Some of the Pb was distributed evenly in the steel matrix as fine particles (1-2 μm), and the rest of the Pb was distributed at the middle or at both ends of the MnS inclusions. The Pb plays a role in lubrication and melting embrittlement, which substantially increases the cutting performance. PbTe was also usually distributed in the middle and at both ends of the MnS inclusions, and Te could convert the sulfides into spindles, thereby improving the cutting performance of the steel.     

Fabrication of solid-phase-sintered SiC-based composites with short carbon fibers

Solid-phase-sintered SiC-based composites with short carbon fibers (Csf/SSiC) in concentrations ranging from 0 to 10wt% were prepared by pressureless sintering at 2100°C. The phase composition, microstructure, density, and flexural strength of the composites with different Csf contents were investigated. SEM micrographs showed that the Csf distributed in the SSiC matrix homogeneously with some gaps at the fiber/matrix interfaces. The densities of the composites decreased with increasing Csf content. However, the bending strength first increased and then decreased with increasing Csf content, reaching a maximum value of 390 MPa at a Csf content of 5wt%, which was 60 MPa higher than that of SSiC because of the pull-out strengthening mechanism. Notably, Csf was graphitized and damaged during the sintering process because of the high temperature and reaction with boron derived from the sintering additive B₄C; this graphitization degraded the fiber strengthening effect.     

Reaction mechanisms between molten CaF_2-based slags and molten 9CrMoCoB steel

Investigating the reaction mechanism between slag and 9CrMoCoB steel is important to develop the proper slag and produce qualified ingots in the electroslag remelting(ESR) process. Equilibrium reaction experiments between molten 9CrMoCoB steel and the slags of 55 wt%CaF_2–20 wt%CaO–3 wt%MgO–22 wt%Al_2O_3–xwt%B_2O_3(x = 0.0, 0.5, 1.0, 1.5, 2.0, 3.0) were conducted. The reaction mechanisms between molten 9 CrMoCoB steel and the slags with different B_2O_3 contents were deduced based on the composition of the steel and slag samples at different reaction times. Results show that B content in the steel can be controlled within the target range when the B_2O_3 content is 0.5 wt% and the FeO content ranges from 0.18 wt% to 0.22 wt% in the slag. When the B_2O_3 content is ≥1 wt%, the reaction between Si and B_2O_3 leads to the increase of the B content of steel. The additions of SiO_2 and B_2O_3 to the slag should accord to the mass ratio of [B]/[Si] in the electrode, and SiO_2 addition inhibits the reaction between Si and Al_2O_3.     

Effect of prolonged isothermal heat treatment on the mechanical behavior of advanced NANOBAIN steel

The microstructural evolution and consequent changes in strength and ductility of advanced NANOBAIN steel during prolonged isothermal heat-treatment stages were investigated. The microstructure and mechanical properties of nanostructured bainite were not expected to be influenced by extending the heat-treatment time beyond the optimum value because of the autotempering phenomenon and high tempering resistance. However, experimental results indicated that the microstructure was thermodynamically unstable and that prolonged austempering resulted in carbon depletion from high-carbon retained austenite and carbide precipitations. Therefore, austenite became thermally less stable and partially transformed into martensite during cooling to room temperature. Prolonged austempering did not lead to the typical tempering sequence of bainite, and the sizes of the microstructural constituents were independent of the extended heat-treatment times. This independence, in turn, resulted in almost constant ultimate tensile strength values. However, microstructural variations enhanced the yield strength and the hardness of the material at extended isothermal heat-treatment stages. Finally, although microstructural changes decreased the total elongation and impact toughness, considerable combinations of mechanical properties could still be achieved.     

Effects of oxygen content on the oxidation process of Si-containing steel during anisothermal heating

The oxidizing behavior of Si-containing steel was investigated in an O₂ and N₂ binary-component gas with oxygen contents ranging between 0.5vol% and 4.0vol% under anisothermal-oxidation conditions. A simultaneous thermal analyzer was employed to simulate the heating process of Si-containing steel in industrial reheating furnaces. The oxidation gas mixtures were introduced from the commencement of heating. The results show that the oxidizing rate remains constant in the isothermal holding process at high temperatures; therefore, the mass change versus time presents a linear law. A linear relation also exists between the oxidizing rate and the oxygen content. Using the linear regression equation, the oxidation rate at different oxygen contents can be predicted. In addition, the relationship between the total mass gain and the oxygen content is linear; thus, the total mass gain at oxygen contents between 0.5vol%-4.0vol% can be determined. These results enrich the theoretical studies of the oxidation process in Si-containing steels.     

Mechanical properties and wear resistance of ultrafine bainitic steel under low austempering temperature

The mechanical properties and wear resistance of the ultrafine bainitic steel austempered at various temperatures were investigated.Scanning electron microscopy(SEM) and X-ray diffraction were used to analyze the microstructure. The worn surfaces were observed via laser scanning confocal microscopy and SEM. Results indicated that, under low austempering temperatures, the mechanical properties differed, and the wear resistance remained basically unchanged. The tensile strength of the samples was above 1800 MPa, but only one sample austempered at 230°C had an elongation of more than 10%. The weight loss of samples was approximately linear with the cycles of wear and nonlinear with the loads. The samples showed little difference in wear resistance at different isothermal temperatures, whereas the thickness of their deformed layers varied greatly. The results are related to the initial hardness of the sample and the stability of the retained austenite. Meanwhile, the experimental results showed that the effect of austempering temperature on the wear resistance of ultrafine bainitic steel can be neglected under low applied loads and low austempering temperature.