Mechanical and microstructural analysis of 2205 duplex stainless steel under hot working condition

Hot deformation characteristics of 2205 duplex stainless steel were analyzed by performing hot compression tests at a temperature range of 950–1200 °C and a strain rate of 0.001–1 s⁻¹. Flow stress was modeled by the constitutive equation of hyperbolic sine function. The constants of n, A, α, and the apparent activation energy were determined at different strains. They were then fitted by polynomial equations. Using the hyperbolic sine function and the relations derived between constants and strain flow curves were successfully modeled. Microstructural evolutions were characterized using optical microscopy and electron back scattered diffraction techniques. The results showed that dynamic recovery in ferrite is accelerated at higher temperatures followed by transformation to continuous dynamic recrystallization. Dynamic recrystallization in austenite was postponed by the accommodation of strain in ferrite and very few internal boundaries in austenite. At high strain rates, dynamic recovery in ferrite and dynamic recrystallization in austenite are very slow. Consequently, the total recrystallized fraction decreases. At low temperatures this situation may cause flow instabilities. At low strain rates, softening processes dominate in austenite and ferrite whereas at intermediate strain rates, the formation of substructures is observed in both phases.     

A study on the synthesis of nanostructured WC–10 wt% Co particles from WO<Subscript>3</Subscript>, Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and graphite

The formation of the nanostructured WC–10 wt% Co powder from WO₃, Co₃O₄, and graphite is studied. The effects of the processing parameters of high-energy ball milling, reduction in H₂ atmosphere, and carburization in Ar/CO atmosphere are investigated. The crystallite size of the as-synthesized WC is 30–40 and 40–50 nm for 900 and 1000 °C carburized powders, respectively. The powder is agglomerated with the size of the primary particles ranging from 50 to 700 nm. High-energy ball milling of WO₃–Co₃O₄–C powder mixtures leads to finer particle and crystallite sizes with larger surface area. Such milled powders can be reduced to nanostructured W at 570 °C and carburized to form WC at temperatures as low as 900 °C. Crystal growth has taken place during carburization, particularly at 1000 °C, which results in the formation of truncated triangular prisms and nanoplates of WC at 1000 °C.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.     

Role of strain-induced martensite on microstructural evolution during annealing of metastable austenitic stainless steel

Metastable austenitic stainless steel of type AISI 304L was cold rolled to 90% with and without inter-pass cooling. Inter-pass cooling produced 89% of strain-induced martensite whereas no inter-pass cooling resulted in the formation of 43% of martensite in the austenite matrix. The cold-rolled specimens were annealed at various temperatures in the range of 750–1000 °C. The microstructures of the cold-rolled and annealed specimens were studied by the electron microscope. The grain size and low angle boundaries were determined from the orientation maps recorded by the scanning electron microscope-based electron backscattered diffraction technique. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and volume fraction of martensite. It was noted that large volume fractions of martensite at low annealing temperatures, below 900 °C, were most suitable for the formation of fine grains. On the contrary, reversion of small volume fractions of martensite at critical annealing temperature of 950 °C resulted in grain refinement.     

Spark-plasma-sintering temperature dependence of structural and piezoelectric properties of BNT–BT<Subscript>0.08</Subscript> nanostructured ceramics

(Bi_0.5Na_0.5)TiO₃ doped with 8 mol% BaTiO₃ powder prepared by sol–gel was compacted and sintered by spark-plasma-sintering method. The influence of spark-plasma-sintering temperature on the densification and piezoelectric properties of these ceramics was studied. Starting from BNT–BT_0.08 powder gel with a microstructure consisting of particles with average size of 50 nm, ceramics with grain size of 60–90 nm and density of about 98.9–99.6% of the theoretical density were obtained by spark-plasma-sintering at 800–900 °C. Increasing the sintering temperature by SPS from 800 to 900 °C lead to the increase of d ₃₃, k _p, ε₃₃^T and, decrease of Q _m. Typical d ₃₃ and k _p values of BNT–BT_0.08 ceramics sintered by spark-plasma-sintering at 900 °C were 8 and 0.029, respectively.     

Microstructures and high temperature oxidation resistance of alloys from Nb–Cr–Si system

Oxidation behavior of Nb–30Si–(10,20)Cr alloys have been evaluated in air from 700 to 1400 °C by heating for 24 h and furnace cooling them. The lower weight gain per unit area has been observed for 20Cr alloy at 1200, 1300, and 1400 °C. Pesting has been observed at lower temperatures (700, 800, 900 °C). Analysis indicates that the powder formation at 900, 100, 1100 °C may be associated with β form of Nb₂O₅ (base centered monoclinic form). However the m-monoclinic form of Nb₂O₅ evolves at temperatures above 900 °C while o-orthorhombic Nb₂O₅ forms at below this temperature. The phases in the alloys have been calculated using the Pandat^TM software program at different temperatures using calculated Nb–Cr–Si phase diagrams.     

Numerical analysis for migration of austenite/ferrite interface during carburization of Fe

During isothermal annealing of the ferrite of a binary Fe–C alloy in an appropriate carburization atmosphere at temperatures of 1020–1180 K, the austenite will be formed as a layer on the surface of the ferrite. The migration behavior of the austenite/ferrite interface during the isothermal carburization was numerically analyzed using the diffusion equation describing the flux balance at the interface. However, the diffusion coefficient of C in the austenite monotonically increases with increasing concentration of C. Thus, the composition dependence of the diffusion coefficient was taken into consideration. For the numerical analysis, Crank–Nicolson implicit method was combined with a finite-difference technique. According to the numerical calculation, the parabolic relationship holds between the migration distance of the austenite/ferrite interface and the annealing time. The composition dependence of the diffusion coefficient yields acceleration of the migration. The numerical calculation satisfactorily reproduces the experimental result of the diffusion controlling migration.     

The important role of titanium microalloying in refining austenite grain of heavy-duty H-beam steel during rough rolling produced by a new technology

A new technology of austenite grain refinement, fine austenite enhanced ferrite transformation, is proposed for heavy-duty hot-rolled H-beam steels in this work. Titanium microalloying is very important and necessary for the new technology. The effect of titanium on the prior austenite grain size of steels during simulated rough rolling was investigated. The results show that the prior austenite grain sizes of specimens with titanium and niobium elements are much finer than those of specimens with niobium but without titanium deformed at the same parameters. For the alloying composition of studied steels, titanium nitride particle maybe precipitated in specimens with titanium at above 1,200 °C, however, niobium carbide particles can't form in specimens without titanium at above 1,150 °C. The thermodynamically stable titanium nitride particles can impede the grain growth at high temperature for example furnace heating before rough rolling and bring the epitaxial growth of niobium carbide on pre-existing themselves which induces a large number of titanium nitride-niobium carbide composite precipitates. These fine precipitates can pin austenite grain boundaries effectively and ensure austenite grain refinement.     

Effect of thermomechanical processing on mechanical behavior and microstructure evolution of C–Mn multiphase high strength cold rolled steel

Multiphase (MP) steels have complex microstructures containing polygonal ferrite, martensite, bainite, carbide, and small amounts of retained austenite. This mixture of phases and constituents is responsible for a good combination of strength and ductility in this class of steels. The present work shows how different annealing parameters can be used to create the suitable microstructure to improve mechanical properties of MP steels. Samples were first heated to 740, 760, or 780 °C, held for 300 s, and then quickly cooled to 600 or 500 °C. They were then soaked for another 300 s and finally accelerated cooled in the range of 10–30 °C s⁻¹. The microstructures were examined at the end of each processing route using optical, scanning, and transmission electron microscopy. Hardness values were determined for all conditions. Analysis of the available data allowed to establish the simple and yet useful quantitative relationship between the microstructural parameters, cooling rates, and hardness of the steel.     

Microstructure and crystallography of 20MnCr5 steel tempered at different conditions

The microstructure and crystallography of 20MnCr5 steel tempered at 180 °C for 2 h were examined by electron back scattered diffraction (EBSD) in a field-emission scanning electron microscope. The crystallographic features of martensite in a prior austenite grain area were studied by pole figures (PFs). Compared to the theoretical PFs calculated with Kurdjumow–Sachs (K–S) and Nishiyama–Wassermann (N–W) orientation relationships (ORs), it is shown that both of the ORs exist in the investigated specimens. Misorientations distribution and the result of transmission electron microscope analysis indicating that [011]/54.7° lath boundary existed in the microstructure also prove the coexistence of K–S and N–W ORs. When the steels were further tempered at 350 °C/20 min or 500 °C/20 min, the length of boundaries in equal areas decreased and the crystallographic features still fulfill the coexistence of K–S and N–W ORs.     

Kinetics of austenite formation during continuous heating in a low carbon steel

The kinetics and microstructural evolution of austenite formation in a low carbon steel, with initial microstructure composed of ferrite and pearlite, were studied during continuous heating, by using dilatometric analysis and measurements of microstructural parameters. The formation of austenite was observed to occur in two stages: (a) pearlite dissolution and (b) ferrite to austenite transformation. The critical temperatures of austenite formation in continuous heating increase with increasing heating rate, with greater influence on the finishing temperature of austenite formation. For both the 1 °C/s and 0.1 °C/s heating rates, the formation rate of austenite reaches a maximum at approximately the finishing temperature of pearlite dissolution, and the formation rate of austenite as a function of the temperature is greater at the higher heating rate.     

FCC to HCP transformation kinetics in a Co–27Cr–5Mo–0.23C alloy

In this work the kinetic aspects associated with the FCC → HCP martensitic transformation in a Co–27Cr–5Mo–0.23C alloy processed by powder metallurgy were investigated. In situ X-ray diffraction during isochronous heat treatments in a hot stage indicated that a fully metastable FCC matrix transforms rather fast at temperatures above 725 °C and reaches a maximum transformation into the HCP phase at 940 °C. Alternatively, when the matrix is HCP, some HCP martensite reverts to metastable FCC. Apparently, at low temperatures carbon excess in the HCP martensite promotes the reversal to metastable FCC. In addition, the volume percent of ε-martensite precipitated from stable FCC was determined as a function of time and temperature during isothermal aging between 675 and 900 °C. From these results, TTT diagrams were plotted for a 1% HCP transformed martensite. Maximum transformation rates were found to occur between 825 and 850 °C and activation energies, Q _s of 41–52 kcal/mol were estimated from the experimental outcome. The aged microstructures indicated that below 800 °C, the isothermal transformation was dominated by a lamellar morphology. Nevertheless, aging above 800 °C promoted carbide nucleation and coarsening along the grain boundaries independently of the FCC → HCP martensitic transformation.     

Young’s modulus and damping in dependence on temperature of Ti–6Al–4V components fabricated by shaped metal deposition

Young’s modulus and damping behavior is investigated by the impulse excitation technique in vacuum up to 1100 °C for Ti–6Al–4V components, fabricated by shaped metal deposition (SMD). This is a novel additive manufacturing technique where near net-shape components are built layer by layer by tungsten inert gas welding. The Young’s modulus decreases linearly from 118 GPa at room temperature to 72 GPa at 900 °C, followed by a stronger decrease up to 1000 °C and during the first heating a plateau thereafter. The damping exhibits an exponential increase with temperature superimposed by two peaks around 700 and 900 °C during the first heating. During cooling and follow-up cycles only the damping peak around 700 °C appears. The change in Young’s modulus and the damping behavior is interpreted by different processes like α/β transformation, O alloying and grain boundary sliding. These results indicate that components fabricated by SMD contain a non-equilibrium α phase which transforms to the β phase at higher temperatures than the equilibrium α phase. Furthermore, the vacuum between 2.4 and 5.3 × 10⁻⁴ mbar proved at high temperatures to be not good enough to rule out the contamination by O, which leads to α casing, stiffening, and hardening.     

Investigation on the solution treated behavior of economical 19Cr duplex stainless steels by Mn addition

Effect of Mn on microstructure, mechanical property and pitting corrosion of 19% Cr economical duplex stainless steels with solution temperatures ranging from 1040 to1220 °C has been investigated. The austenite content increases with more Mn addition, but decreases by increasing solution temperature, which can be inferred by trend of partition coefficient K_Mn with solution temperature. Meanwhile, a balanced austenite-ferrite duplex structure of solution-treated specimens was obtained with Mn addition. The impact energy at 20 °C increased with decreasing solution temperature from 1220 °C to 1040 and 1120 °C, and improved by more Mn addition due to more ductile austenite phase formation. These toughness variations were consistent with fracture morphology characteristic changing. The effect of more Mn addition and solution treatment of 1120°Con decreasing of tensile strength and 0.2% offset yield strength were slight. However, the elongation to fracture (%) fell greatly with Mn addition up to 8.1 wt.% for as-rolled and solution treated specimens due to larger deformation strains of austenite than that of ferrite. The decreasing trend of pitting corrosion potential became slower with Mn addition from 3.6 to 8.1 wt.%. The pitting corrosion resistance was lowered by increasing solution temperature due to more weakened repassivation ferrite phase formation.     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Effect of brazing temperature on microstructure and mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/Si<Subscript>3</Subscript>N<Subscript>4</Subscript> joints brazed with Ag–Cu–Ti + Mo composite filler

Mo particles have been introduced into Ag–Cu–Ti brazing alloy for the joining of Si₃N₄ ceramic. Effect of brazing temperature on microstructure and mechanical properties of the joints were investigated. The result shows that a continuous reaction layer which is composed of TiN and Ti₅Si₃ was formed at the Si₃N₄/braze interface. The central part of the joint was composed of Ag-based solid solution, Cu-based solid solution, Mo particles, and Cu–Ti intermetallic compounds. By increasing the brazing temperature, both the thickness of the reaction layer and amount of Cu–Ti intermetallic compounds in the joint increased, being beneficial for the joint strength. Whereas, the reaction between Ti and Si₃N₄ ceramic proceeded excessively and more Cu–Ti intermetallic compounds were precipitated in the joint while elevating the brazing temperature to 950 °C, leading to deterioration of the bending strength. The maximal bending strength reached 429.4 MPa at 900 °C for 5 min when the Si₃N₄ ceramic was brazed with Ag–Cu–Ti + Mo composite filler.     

Microstructures and properties of low-alloy fire resistant steel

Microstructures and properties of weldable quality low-alloy fire resistant structural steels (YS: 287–415 MPa) and TMT rebar (YS: 624 MPa) have been investigated. The study showed that it is possible to obtain two-thirds of room temperature yield stress at 600°C with 0.20–0.25% Mo and 0.30–0.55% Cr in low carbon hot rolled structural steel. Microalloying the Cr-Mo steel by niobium or vanadium singly or in combination resulted in higher guaranteed elevated temperature yield stress (250–280 MPa). The final rolling temperature should be maintained above austenite recrystallization stop temperature (∼ 900°C) to minimize dislocation hardening. In a quenched and self-tempered 600 MPa class TMT reinforcement bar steel (YS: 624 MPa), low chromium (0.55%) addition produced the requisite yield stress at 600°C. The low-alloy fire resistant steel will have superior thermal conductivity up to 600° C (> 30 W/m.k) compared to more concentrated alloys.     

Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding

A Fe-based coating with nano-scale bainitic microstructure was fabricated using laser cladding and subsequent isothermal heat treatment. The microstructure of the coating was observed and analyzed using optical microscope (OM), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results showed that nanostructured bainitic ferrite and carbon-enriched retained austenite distributed uniformly in the coating. Blocky retained austenite was confined to the prior austenite grain boundaries resulting from the elements segregation. The bainitic microstructure obtained at 250 °C had a finer scale compared with that obtained at 300 °C. The volume fraction of austenite increased with increasing transformation temperature for the fully transformed bainitic coating. The bainitic transformation was accelerated as a result of the fine prior austenite generated during the laser cladding. The evolution of the carbon contents in bainitic ferrite and retained austenite revealed the diffusionless mechanism of the bainitic transformation.     

Improvement of the wettability of SiMn IF-HSS by liquid zinc by controlling the dew point of the annealing gas atmosphere

The wettability of low-carbon, 0.3 wt%Si–0.4 wt%Mn interstitial-free steel by liquid zinc at 450 °C was investigated using the dispensed sessile drop method. Before the wetting tests, the steel samples were annealed in a 15%H₂–Ar gas atmosphere at three different dew points, namely −60, −40, and 0 °C. It was found that as the dew point was increased from −60 to −40 °C, the wettability became poorer. However, as the dew point was increased further to 0 °C, the wettability was dramatically improved and was better than that of −60 °C. In order to understand the dramatic change in wettability, the surfaces of the steel samples after annealing were analyzed with SEM and TEM. It was found that the surface oxide changed from randomly distributed hemisphere particles of 20–30-nm high on a very thin oxide film to a film-like layer ~15-nm thick as the dew point was increased from −60 to −40 °C, and at the dew point of 0 °C, internal oxidation was so pronounced that a very thin surface oxide layer 1–2-nm thick was formed. It was believed that the improvement of the wettability at the dew point of 0 °C was caused by the short diffusion distance in the surface oxide layer.     

High temperature mechanical properties of triple phase steels

A 0.15% C–1.2% Si–1.7% Mn steel was intercritically annealed at 780 °C for 5 min and then isothermally held at 400 °C for 4 min followed by oil quenching to room temperature and the annealed microstructure consist of 75% ferrite , 15% bainite and 10% retained austenite was produced. Samples of this steel with triple phase structure were tensile tested at temperature range of 25–450 °C. Stress–strain curves showed serration flow at temperature range of 120–400 °C and smooth flow at the other temperatures. All of the stress–strain curves showed discontinuous yielding at all testing temperatures. Both yield and ultimate tensile strength decreased with increasing temperature, but there exists a temperature region (120–400 °C) where a reduction of strength with increasing temperature is retarded or even slightly increased. The variation in the mechanical properties with temperature was related to the effects of dynamic strain aging, high temperature softening, bainite tempering and austenite to martensite transformation during deformation.