Effect of Hf additions on phase transformation,microstructural stability,and hardness of nanocrystalline 304L stainless steels synthesized by mechanical alloying

304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 °C. The results showed that face-centered cubic (fcc) phase in 304L transformed to body-centered cubic (bcc) phase during MA. The in-situ studies revealed that bcc-to-fcc phase transformation completed after 105 min annealing at 900 °C for 304L, whereas Hf addition increased the required time and temperature for the complete transformation. The grain size of 304L stainless steel was ~10 nm after MA and remained ~167 and ~293 nm after annealing at 900 and 1100 °C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 °C. The hardness of 304L increased from ~200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 °C with Hf addition as opposed to 195 HV hardness of 304L.     

Photoluminescence characteristics of oxidized hydrogenated nanocrystalline silicon film

We have observed visible photo-luminescence (PL) from an oxidized hydrogenated nanocrystalline silicon (nc-Si:H) film which was prepared in a plasma enhanced chemical vapor deposition (PECVD) system and post-treated by thermal oxidization processes. At low oxidization temperature (T_ox) below 500 °C, silicon oxyhydrides and silicon oxides are formed at the surface of grains; while at high T_ox above 500 °C, the surface of grains is covered by α-SiO₂. PL around 650 nm-750 nm is observed as T_ox ranges from 100 °C to 700 °C during which the grain size (d_c) varies from 2.7 nm to 5.1 nm. At T_ox > 700 °C, the d_c is larger than 5.1 nm and PL peak shifts to 920 nm. The quantum size effect and surface states model was employed to explain our experimental results.     

Acoustic emission monitoring of bainitic and martensitic transformation in medium carbon steel during continuous cooling

Abstract

This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermomechanical simulator. After austenising at a certain temperature, the studied specimen was cooled down and the root mean square (RMS) value of the continuous AE signal was measured. During cooling two distinct peaks in the RMS data were observed at temperatures of 200-300°C and 500-600°C, which have been attributed to martensite and bainite formation respectively. The observed bainite peak strongly indicated that the mechanism of bainite growth is displacive. The AE monitoring of bainite and martensite formation was supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainitic and martensitic transformation was studied by varying the austenising temperature T _a. It was found that upon lowering T _a, i.e. with decreasing austenite grain size, the bainite peak increases while the martensite peak decreases.     

Thermal stability and oxidation behavior of Al-containing nanocrystalline powders produced by cryomilling

Al-containing nanostructured coatings provide excellent protection from high temperature corrosion. Aluminum oxide scales generally provide better oxidation resistance and yield lower oxidation rates than other oxide scale compositions. In this study, nanocrystalline 316L stainless steel containing 6 wt.% Al was synthesized using cryogenic milling (cryomilling). Complete alloying was obtained after 32 h of milling and the average grain size was found to be 7 nm. High temperature thermal stability and oxidation kinetics of the alloyed powders were examined. The powder demonstrated good grain growth stability at 500 °C, at which point, the powders had been heat treated for 120 h and the average grain size was found to be 11.4 nm. The oxidation kinetics of the powder were studied for 48 h at 500, 800, and 1,000 °C, respectively. For comparison, conventional 316LSS powder was also tested. Nanocrystalline 316LSS-6 wt.% Al showed lower weight gain than the conventional 316LSS powders. During the oxidation of nanocrystalline 316LSS-6 wt. % Al at 500 °C, protective aluminum oxide scale formed at the surface. At 800 °C and 1,000 °C, most of the nanocrystalline 316LSS-6 wt.% Al particles showed completed outer aluminum oxide scale. However, at 800 and 1,000 °C, some particles showed growth of chromium oxide scale underneath the aluminum oxide scale. In those samples, Al depletion was also observed due to a non-homogenous distribution of Al during cryomilling. The activation energy of the oxidation reaction was calculated and was found to be affected by the enhancement of the grain boundary diffusion in nanostructured particles.     

Effects of cooling process on microstructure and hardness for 1·0C–1·5Cr bearing steel

Effects of cooling rate (V_cr) and final cooling temperature (T_ft), after hot deformation, on microstructure and hardness for 1·0C–1·5Cr bearing steel were investigated. The results show that if V_cr increases from 2 to 25°C s^?1 and T_ft remains at 650°C, pearlite colony size and grain size both decrease, hardness increases. When V_cr exceeds 8°C s^?1, carbide network can be restrained effectively. TEM micrographs indicate that there exist branches in the local region of lamellar cementite and ferrite, and a ferrite thin film is also found around the proeutectoid carbide. Under the cooling rate of 10°C s^?1, with the increase in T_ft, the microstructure changes from martensite into pearlite, carbide network becomes more serious and hardness decreases.     

The role of grain size in static and cyclic deformation behaviour of a laser reversion annealed metastable austenitic steel

Different grain sizes were created in a metastable 17Cr‐7Mn‐7Ni steel by martensite‐to‐austenite reversion at different temperatures using a laser beam. Two fully reverted material states obtained at 990°C and 780°C exhibited average grain sizes of 7.7 and 2.7 μm, respectively. The third microstructure (610°C) consisted of grains at different stages of recrystallization and deformed austenite. A hot‐pressed, coarse‐grained counterpart was studied for reference. The yield and tensile strengths increased with refined grain size, maintaining reasonable elongation except for the heterogeneous microstructure. Total strain‐controlled fatigue tests revealed increasing initial stress amplitudes but decreasing cyclic hardening and fatigue‐induced α′‐martensite formation with decreasing grain size. Fatigue life was slightly improved for the 2.7‐μm grain size. Contrary, the heterogeneous microstructure yielded an inferior lifetime, especially at high strain amplitudes. Examinations of the cyclically deformed microstructure showed that the characteristic deformation band structure was less pronounced in refined grains.     

Effect of austenitizing temperature and cooling rate on the structure and properties of a ultrahigh strength low alloy steel

A 0.3C-CrMoV(ESR) steel is being developed primarily for making pressure vessels used for aerospace applications. Since it is important to understand the range of microstructures and mechanical properties that will be obtained in the heat affected zone of welds, the steel has been subjected to different austenitizing treatments (temperatures ranging from 925°C to 1250°C) followed by cooling at various rates to room temperature. It has been shown that the austenite grain size increased from about 10 to 250 μm as the austenitizing temperature is increased from 925°C to 1250°C (1 hr) and that the hardness, YS, UTS,% elongation and% reduction in area as well as CVN energy for 450°C tempered condition decrease as the austenitizing temperature is increased for all cooling rates (furnace cooling, air cooling, oil quenching, quenching and tempering at 450°C). This is attributed mainly to the increase in austenitic grain size. The ranges of microstructures that can be obtained in the heat-affected zone are massive ferrite, fine pearlite, upper as well as lower bainite and martensite. The Charpy impact energy for the oil-quenched steel tempered at 200°C, however, did not vary significantly with austenitizing temperature.     

Ball milling of stainless steel scrap chips to produce nanocrystalline powder

Nanocrystalline stainless steel powder was produced by ball milling of austenitic stainless steel scrap chips. The structural and morphological changes of samples during ball milling and after subsequent heat treatment were investigated by X-ray diffractometery, scanning electron microscopy and microhardness measurements. During ball milling the austenite in as-received chips partially transformed to the martensite phase with nanoscale size grains of ∼15 nm. This structure exhibited high microhardness value of about 850 Hv which is much higher than that for original samples. The deformation-induced martensite partially transformed to austenite after annealing at 700 °C for 1 h reducing the hardness of powder particles.     

Effect of tempering temperature on the microstructure and properties of ultrahigh-strength stainless steel

The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M₇C₃ carbide precipitated when the steel was tempered at 400 °C, and M₇C₃ and M₂C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M₂C and M₂₃C₆ carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M₇C₃ carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness.     

Annealing of strain-induced martensite to obtain micro/nanometre grains in austenitic stainless

Micro/nanometre grain sizes appear to improve the biocompatibility of austenitic stainless steel. In order to realise the reverse transformation (from strain-induced martensite) austenite structure control with micro/nanometre size, the influence of annealing parameters on the microstructure evolution and mechanical properties of 316L-Nb austenitic stainless steel were investigated. Furthermore, the role of Nb in the annealing process was also studied. The results showed that the closely 100% reversion transformation austenite structures were obtained in the samples after annealing at 850°C, where the grains with the grain diameter ≤500?nm accounted for 25% and the grains with the grain diameter >0.5?µm accounted for 75%. The micro/nanometre grain steel not only exhibited a high strength level but also exhibited a desirable elongation. Moreover, the Nb demonstrates a remarkable effect on grain-refining and a significant role in improving the stability of the microstructure.     

The residual lifetime of creepdeformed material: Microstructural changes occurring during creep of a 12%CrMoVW steel

The microstructure of specimens of a 12%CrMoVW steel which were creep tested at 500—650°C for 8000—43 000 h has been investigated. The basic microstructure consisted of tempered martensite and about 1% of δ-ferrite. The fine precipitates ( <0.1 J,Lm) found in both phases coarsened only slowly with increasing ageing temperature and time. Coarser carbides (> 0.2 μ,Lm) in the tempered martensite grew up to five times their original size during ageing. The precipitates are probably mainly M₂₃C₆. The size of these carbides, as well as those in the previous austenite grain boundaries, could be related to the Larson—Miller temperature—time parameter corresponding to the ageing.     

Microstructure and property evolutions of a novel Super304H steel during high temperature creeping

High-temperature creep tests of a novel Super304H steel under 650 °C/195 MPa were conducted and the evolutions of microstructure and property with creep time of the material were investigated by using optical microscope, scanning electron microscope, micro Vickers hardness tester and electrochemical workstation. The results show that M₂₃C₆ carbides precipitated along grain boundaries of austenite matrix in a chain distribution and then got coarsened with the increase of creep time. Creep cavities started to form near the surface when the steel was crept for 2500 h. Afterward creep cavities increased, developed, interconnected and finally formed micro cracks along grain boundaries till fracture at the time of 4578 h. The hardness of the steel increased dramatically at the early stage of creeping and reached a high level at 500 h, and then kept a stable state at the succedent stage till fracture. Intergranular corrosion susceptibility of the steel increased first and then declined gradually, indicating the occurrence of sensitization – desensitization process of the steel during creeping.     

Microstructure of as-quenched 3.5NiCrMoV rotor steel. Part I. General structure and retained austenite

The microstructural features have been examined for 3.5NiCrMoV steam turbine rotor steel, in the as-quenched state and tempered at 500 °C. Quenching produces lath martensite, with bands of retained austenite at the lath boundaries and, to a lesser extent, at prior austenite grain-boundaries. Autotempering occurs during the quench, resulting in loss of tetragonality of the martensite and extensive carbide precipitation in the matrix and to a lesser degree at prior austenite grain boundaries, but not at lath boundaries. Tempering at 500 °C leaves the lath structure largely intact, but causes retained austenite to transform to bands of ferrite and cementite. This transformation does not correlate with the reduction in stress corrosion crack velocity which occurs on tempering. The strength of 3.5NiCrMoV steel in the as-quenched and 500 °C tempered conditions is most probably due to the combination of carbide precipitation strengthening and substructure strengthening.     

A study of centrifugal cast high boron high speed steel

In the present study a high‐boron high speed steel (HSS) roll material was designed. Many expensive alloy elements have been substituted by cheap boron alloy, and high‐boron high speed steel roll has been manufactured by centrifugal casting method. The microstructures, mechanical properties and wear resistance of centrifugal casting high‐boron high speed steel roll have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and X‐ray diffraction (XRD) analysis, hardness test, impact test and wear test. The results indicated that the solidification microstructures of high‐boron high speed steel roll consisted of M₂(B,C), (W,Mo)₂(B,C), M₃(B,C), M₂₃(B,C)₆ type borocarbides and martensite, a small amount of retained austenite. Borocarbides were continuously distributed over the grain boundary. After quenching from 1050 °C, local broken network appeared in partial borocarbides, and fine secondary borocarbide precipitated from the matrix. After tempering from 525 °C, the amount of precipitated borocarbide increased significantly. After heat treatment, the hardness of high‐boron high speed steel roll excelled 60 HRC, and its impact toughness excelled 8.0 J/cm². The single groove steel rolling amount of high‐boron high speed steel rolls increases by 500% than that of bainite cast iron roll, when the rolls are used in K₁ mill housing of bar mill.     

Investigation of the formation of Al,Fe, N intermetallic phases during Al pack cementation followed by plasma nitriding on plain carbon steel

Plain carbon steels are not suitable for nitriding as they form an extremely brittle case that spalls off readily, and the hardness increment of the diffusion zone is small. In this research, the effect of plasma nitriding time and temperature variation on the microstructure of the pack cemented aluminized plain carbon steel is investigated. All samples were aluminized at 900 °C for 2 h; the aluminized samples were subsequently plasma nitrided at 500 °C, 550 °C and 600 °C for 2.5, 5, 7.5 and 10 h. The phases formed on the sample surface were detected by X-ray diffraction (XRD). The cross section and samples surface were investigated by optical and scanning electron microscopy (SEM). Microhardness test was conducted to determine hardness change from the surface to the sample core. Results showed that by aluminizing the steel, Fe₃Al phases as well as Fe–Al solid solution were formed on the surface and some aluminum rich precipitates were formed in solid solution grain boundaries. Plasma nitriding of the aluminized layer caused the formation of aluminum and iron nitride (AlN, Fe₄N) on the sample surface. Consequently, surface hardness was improved up to about eight times. By increasing the nitriding temperature and time, aluminum-rich precipitates dissociated. Moreover, due to the diffusion of nitrogen through aluminized region during ion nitriding, iron and aluminum nitrides were formed in aluminized grain boundaries. Increasing nitriding time and temperature lead to the growth of these nitrides in the grain boundaries of the substrate. This phenomenon results in the increment of sample hardness depth. Plasma nitriding of aluminized sample in low pressure chamber with nitrogen and hydrogen gas mixture reduced surface aluminum oxides which were formed in aluminizing stage.     

Thermal stability of ultrafine grained aluminium alloy prepared by large strain extrusion machining

Abstract

In the present study, ultrafine grained (UFG) Al alloy chips with average grain sizes of ～200 nm were successfully prepared by large strain extrusion machining (LSEM) process using a combined cutting tool with rake angle of 10° and chip compression ratio of 1.0. The tests showed that the Vickers hardness of the UFG Al alloy is significantly improved due to grain size reduction. To understand effect of heat on the microstructure and mechanical properties, the UFG chips were subjected to heat treatment at different temperatures and different annealing time durations. When annealed <100°C, most of fine grains within the UFG chips were found to be replaced by elongated grains whose grain sizes increased with a significant increase in the aspect ratio as the annealing time increased. Despite such increase in grain size, the Vickers hardness was not reduced as expected because of the precipitation of secondary phases. When annealed at temperatures up to 200°C, recrystallisation occurred, along with grain growth, but the Vickers hardness did not deteriorate because of precipitation of secondary phases, as before. However, annealing at temperatures of 300°C and above resulted in significant reduction in hardness of the chips due to dominance of grain growth over secondary precipitation. These results indicated that UFG Al alloy chips have a good thermal stability at temperatures <200°C.     

Surface carburised AISI 8620 steel with dual phase core microstructure

Abstract

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.     

Effect of martensite to austenite reversion on the formation of nano/submicron grained AISI 301 stainless steel

The martensite to austenite reversion behavior of 90% cold rolled AISI 301 stainless steel was investigated in order to refine the grain size. Cold rolled specimens were annealed at 600–900 °C, and subsequently characterized by scanning electron microscopy, X-ray diffraction, Feritscope, and hardness measurements. The effects of annealing parameters on the formation of fully-austenitic nano/submicron grained structure and the mechanisms involved were studied. It was found that annealing at 800 °C for 10 s exhibited the smallest average austenite grain size of 240 ± 60 nm with an almost fully-austenitic structure.     

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.     

Preparation and characterisation of nanostructural TiO<Subscript>2</Subscript>–Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> binary oxides with high surface area derived form particulate sol–gel route

Nanostructured and nanoporous TiO₂–Ga₂O₃ films and powders with various Ti:Ga atomic ratios and high specific surface area (SSA) have been prepared by a new straightforward particulate sol–gel route. Titanium isopropoxide and gallium (III) nitrate hydrate were used as precursors and hydroxypropyl cellulose (HPC) was used as a polymeric fugitive agent (PFA) in order to increase the SSA. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) revealed that powders contained both rhombohedral α-Ga₂O₃ and monoclinic β-Ga₂O₃ phases, as well as anatase and rutile. It was observed that the Ga₂O₃ formed from the nitrate precursor retarded anatase-to-rutile transformation. Furthermore, transmission electron microscope (TEM) analysis also showed that Ga₂O₃ hindered the crystallisation and crystal growth of powders. SSA of powders, as measured by Brunauer–Emmett–Teller (BET) analysis, was enhanced by introducing Ga₂O₃. Ti:Ga = 50:50 (at%/at%) binary oxide annealed at 500 °C produced the smallest crystallite size (2 nm), the smallest grain size (18 nm), the highest SSA (327.8 m²/g) and the highest roughness. Ti:Ga = 25:75 (at%/at%) annealed at 800 °C showed the smallest crystallite size (2.4 nm) with 32 nm average grain size and 40.8 m²/g surface area. Ti:Ga = 75:25 (at%/at%) annealed at 800 °C had the highest SSA (57.4 m²/g) with 4.4 nm average crystallite size and 32 nm average grain size. One of the smallest crystallite size and one of the highest SSA reported in the literature is obtained, and they can be used in many applications in areas from optical electronics to gas sensors.