Correlating microstructural features with wear resistance of dual phase steel

In order to explore the tribological potential of the dual phase (DP) steel as a wear resistant material, the wear characteristics of the dual phase (DP) steel have been investigated with varying amounts of martensite from 43 to 81 vol pct, developed by varying holding time at the intercritical annealing temperature of 780 °C. Dry sliding wear tests have been conducted on DP steels using a pin-on-disk machine under different normal loads of 61.3, 68.5, 75.7 and 82.6 N and at a constant sliding speed of 1.20 m/s. At these loads, the mechanism of wear is mainly delamination, which has been confirmed by SEM micrographs of the subsurface and wear debris of the samples. Wear and friction properties have been found to be improved with increasing martensite volume fraction in dual phase steels.     

Annealing behavior of a modified 5083 aluminum alloy

The annealing behavior of a modified 5083 aluminum alloy was studied in the temperature range of 125–375 °C with different holding times. The results shown that the annealing temperature was more sensitive to the mechanical and corrosion resistance properties compared with the annealing holding time. The mechanical and corrosion resistance properties depend on annealing treatment due to different dislocation configuration in the matrix and the second phase interface, annealing temperature and time have been optimized for both of those properties improvement.     

Study of electrical and structural properties of boron doped polysilicon films with a low nitrogen content

In this work we propose, the study of nitrogen doped Silicon films for an application as a poly-Si gate material for metal-oxide-semiconductor devices. Nitrogen doped silicon films have been deposited at amorphous phase by low-pressure chemical vapor deposition (LPCVD) from disilane Si₂H₆ and ammonia NH₃ at low temperature (480^∘C). The films with varied nitrogen contents have been boron implanted, and annealed at several annealing conditions. The influence of the annealing conditions, the nitrogen tenor and the boron dose on the electrical and the structural properties of films are investigated and correlated. Results show that the conductivity is maximal (σ ∼ 10² (Ω ⋅ cm)⁻¹) for higher annealing temperature, a nitrogen content less than 2% and a strong boron dose. These results indicate that under these optimal conditions, although some nitrogen contents is present in the films, these latter have a conducting behavior. The crystallization of films was found to depend principally on the nitrogen tenor. A quasi-totally crystallization was observed for a nitrogen tenor inferior or equal to 2% and for an annealing temperature of 1100^∘C during 120 min. This result is in good agreement with the greatest value of the conductivity obtained under the same conditions.     

Preparation and characterization of yttrium gallium garnet nanoparticles by citrate sol–gel method at low temperature

The Y₃Ga₅O₁₂ (YGG) nanoparticles with pure phase have been successfully prepared with Y(NO₃)₃ and Ga₂O₃ at lower sintering temperature and shorter holding time. The process of citrate sol–gel method was simple and the cost was very low. The thermal behavior of the YGG precursor was characterized by thermogravimetry/differential thermal analysis (TG/DTA). The crystallization temperature was determined by X-ray powder diffraction (XRPD). The morphology of the particles was characterized by transmission electron microscope (TEM). The results illustrate that the crystallization temperature of YGG nanoparticles was about 700 °C and the powders made by this method dispersed uniformly.     

Thermal,magnetic and composition analyses of the reverse transformation of intermetallic sigma phase to ferrite

The reverse transformation of the sigma phase to ferrite in a duplex stainless steel upon heating has been studied by using differential thermal analysis (DTA), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), quantitative metallography and magnetic susceptibility measurement. It has been demonstrated that the reverse transformation of the sigma phase to ferrite is sensitively affected by the morphology of the sigma phase. However, EDS has shown that the composition of the sigma phase does not significantly affect its reverse transformation behaviour. Magnetic susceptibility measurements and TEM have revealed that untransformed, residual ferrite is present even after prolonged annealing between 873 and 1173 K and hence it does play a role in the sigma-to-ferrite reverse transformation. Quantitative metallography was employed to follow the coarsening of the sigma phase upon annealing and DTA was used to estimate its enthalpies of dissolution. As a potential application, a method for temperature monitoring using duplex stainless steels and DTA is suggested.     

Influence of the annealing in nitrogen atmosphere on the XRD, EDX, SEM and electrical properties of chemical bath deposited CdSe thin films

The influence of annealing in nitrogen atmosphere on the structure, optical and electrical properties of cadmium selenide (CdSe) thin films deposited by chemical bath deposition (CBD) onto glass substrates was studied. The samples were annealed in nitrogen atmosphere at various temperatures. A transition from metastable nanocrystalline cubic to stable polycrystalline hexagonal phase has been observed after annealing. The as-deposited CdSe thin films grow in the nanocrystalline cubic phase with optical band gap 1.93 eV. The electrical resistivity of the thin films has been measured in order of 10⁶ Ω cm. The activation energy of the samples has been found to be 0.26–0.19 eV at low temperature region, and 0.36–0.56 eV at high temperature region. It was also found that the activation energy and the resistivity of the films decrease with the increasing annealing temperature.     

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy was studied. The results show that there are obvious yield points on true stress–true strain curves of annealing structures, then a stress drop occurs. The curves show linear work-softening after yielding at annealing temperature of 720–780 °C and linear work-hardening at annealing temperature of 800–840 °C. Elastic energy stored in the α-phase is dramatically released after plastic deformation of the β-phase, which leads to the stress drop.     

Microstructural characterization of precipitates in Cu–10 wt.%Al–0.8 wt.%Be shape-memory alloy

Microstructural characterization of α₁-plate and γ₂ phase precipitated in hypoeutectoid Cu–10 wt.%Al–0.8 wt.%Be shape-memory alloy (SMA) aged at 200 °C for different periods of time (20–160 h) is researched in this study. High-resolution transmission electron microscope (HRTEM) was employed to investigate the α₁-plate with 18R long period stacking order structure (LPSO) in the SMA aged for 20 h. According to the atomic shuffling revealed in HRTEM-micrograph, the atomic model of the 18R LPSO is proposed. The quantitative mapping of electron energy loss spectrometry shows that the α₁-plates in the SMA aged for 160 h contain lower aluminum concentration than the parent phase matrix. The lattice image of the nanometer-sized γ₂ phase precipitated homogeneously in the SMA aged for 160 h is also revealed by using HRTEM. Precipitation of the nanometer-sized γ₂ phase cannot be impeded by means of the addition of beryllium and quenching, and such precipitate does not grow up in the SMA aged for periods of time less than 160 h.     

Study of the decomposition behavior of retained austenite and the partitioning of alloying elements during tempering in CMnSiAl TRIP steels

Tempering approach is designed for better understanding the effects of heat treatment induced by production process when manufacturing on the transformation-induced plasticity steels containing 1.0 wt.% Al. Specific attention is placed on the roles of tempering temperature and the holding time on the decomposition of retained austenite and the redistribution of alloying elements. Using transmission electron microscopy, we found the retained austenite was decomposed into ε-carbide and ferrite in the steels tempered at 300 °C for 9 h. An increase in the temperature of 400 °C and the holding time over 3 h accelerate the nucleation kinetics of cementite formation, leading to the deteriorated thermal stability of austenite. In addition, atom probe tomography studies confirmed the partitioning tendency of alloying elements across the ferrite/cementite interfaces as well as the compositional spikes of Mn at the interfaces during tempering over 400 °C for 9 h.     

Use of a.c. Magnetic Susceptibility for Temperature Measurement and Discrimination Between Spinodal Decomposition and Sigma Phase Formation

Duplex stainless steels are embrittled on exposure to elevated temperatures because of spinodal decomposition (<550°C) and sigma phase formation (between 600°C and 900°C). The sigma phase has been discovered to undergo a paramagnetic-to-ferromagnetic transition at cryogenic temperatures and its Curie temperature has a good dependence on prior annealing temperature. Additionally, it has been found that the room-temperature a.c. magnetic susceptibility also has a good temperature-dependence when spinodal decomposition occurs. It is viable to use room-temperature a.c. magnetic susceptibility and the cryogenic magnetic transition of the sigma phase for 1. temperature measurement and 2. discrimination between spinodal decomposition and sigma phase formation in duplex stainless steels.     

Grain boundary engineering for superplasticity in steels

The microstructure with suitable boundary characters for superplasticity is summarized for the steels which consist of two phases, i.e., ferrite (bcc α) + austenite (fcc γ) or ferrite (α) + cementite (orthorhombic θ -Fe₃C).In (α + γ) duplex alloys, a conventional thermomechanical processing (solution treatment + heavy cold rolling + aging) produces the (α + γ) duplex structure through the competition of recovery/recrystallization of matrix and precipitation. In Fe-Cr-Ni (α + γ) duplex stainless steels with high γ fractions (40–50%), α matrix undergoes recovery to form α subgrain boundaries and γ phase precipitates on α subgrain boundaries with near Kurdjumov-Sachs relationship during aging. By warm deformation, the transition of α boundary structure from low-angle to high-angle type occurs by dynamic continuous recrystallization of α matrix and, simultaneously, coherency across α/γ boundary is lost. Contrarily, α phase first precipitates in deformed γ matrix in Ni-Cr-Fe based alloy during aging. Subsequently discontinuous recrystallization of γ matrix takes place and the (α + γ) microduplex structure with high-angle γ boundaries is formed. The formation of those high-angle boundaries in (α + γ) microduplex structure induces the high strain rate superplasticity.In an ultra-high carbon steel, when pearlite was austenitized in the (γ + θ) region, quenched and tempered at the temperature below A₁, an (α + θ) microduplex structure in which most of α boundaries are of high-angle type is formed through the recovery of the fine (α ′ lath martensite + θ) mixture during tempering. Such (α + θ) microduplex structure with high angle α boundaries exhibits higher superplasticity than that formed by heavy warm rolling or cold rolling and annealing of pearlite which contains higher fraction of low angle boundaries.     

The influence of dynamic strain aging on the low cycle fatigue behavior of near alpha titanium alloy IMI 834

Total strain controlled low cycle fatigue tests on IMI 834 have been conducted in air in the temperature range between 375 and 500 °C at a temperature interval of 25 °C at the nominal strain rate of 6.67 × 10⁻⁴ s⁻¹. The observed maximum peak stress ratio, minimum half-life plastic strain range and lower fatigue life at 425 °C indicates the occurrence of dynamic strain aging (DSA). Pronounced deformation bands, increased dislocation density and non-uniform dispersion of dislocations inside primary α grains observed by the study of transmission electron microscopy supports the occurrence of dynamic strain aging. Initial cyclic softening was attributed to shearing of Ti₃Al precipitates as revealed by TEM evidences.     

Relation between microstructures of martensite and prior austenite in 12 wt% Cr ferritic steel

Tempered martensitic 9–12 wt% Cr ferritic steels are used as heat resistant materials in power plant, where service under conditions of high temperature and pressure for several decades is required, and an adequate resistance to creep is one of the key requirements. In this type of steels, failure has been found to occur preferentially at prior austenite grain boundaries if the prior austenite grains are coarse. It appears that the prior austenite grain boundaries can act as a site of especial weakness in the tempered martensitic microstructure. It would therefore be useful to investigate whether the properties of prior austenite grain boundaries could be modified by some appropriate thermomechanical processing method. One approach to this is to attempt to increase the fraction of annealing twins in the austenite phase and to investigate whether this has an effect on the properties of the martensite after transformation and tempering. In this study, thermomechanical treatments involving hot-rolling have been applied and the fraction of austenite twins produced determined using electron backscatter diffraction analysis. The treatment giving the highest fraction of austenite twins was identified and the effect of the increase in twin fraction on the characteristics of the martensite was investigated. It was found that the fraction of coincidence site lattice boundaries in martensite along prior austenite grain boundaries increased with increasing fraction of prior austenite twin boundaries.     

Effects of heat treatment process and niobium addition on the microstructure and mechanical properties of low carbon steel weld metal

The mechanical properties and microstructure were evaluated and analyzed by optical microscopy (OM) and transmission electron microscopy (TEM) for micro-alloy carbon steel weld metal with and without Nb addition, respectively, under different heat treatment processes including stress relief annealing, normalizing, and no treatment after welding. The strength and elongation of the weld metal without treatment after welding were improved with the addition of Nb element, and the impact toughness was not affected obviously with the Nb addition. After stress relief annealing, the strength decreased for the Nb-free weld metal, while the elongation and impact toughness increased. However, for the Nb-bearing weld metal, stress relief annealing improved the strength of the weld metal significantly, and deteriorated the elongation and impact toughness. In the case of normalizing treatment to the weld metal, it was shown that with the increase of the holding time at the normalizing temperature of 920 °C, for both the weld metals with and without Nb addition, the microstructure of the columnar grain zone (CGZ) was transformed from one of columnar grain into one of equiaxed grain. The grain size of the equiaxed grain zone (EGZ) increased initially, then remained almost unchanged with the prolonging of the holding time. The mechanical properties of the weld metal with and without Nb addition showed no obvious change with the increasing holding time. With the increase of the normalizing temperature, the strength of the Nb-bearing weld metal increased, while the elongation and impact toughness decreased significantly. OM and TEM analysis found that the fine NbC particles were precipitated at the normalizing temperature of 920 °C, which refined the grains of the weld metal and increased the impact toughness. With the increase of the normalizing temperature, the content of widmanstatten ferrite (WF) in the Nb-bearing weld metal increased, whereas the quantity of the NbC particles decreased, which improved the strength and lowered the impact toughness.     

The effect of dynamic strain aging on fatigue properties of dual phase steels with different martensite morphology

Dual phase (DP) steels with network and fibrous martensite were produced by intercritical annealing heat treatment cycles. Some of these steels were deformed at dynamic strain aging temperatures. Room temperature tensile tests of specimens deformed at 300 °C showed that both yield and ultimate tensile strengths for both morphologies increased, while total elongation decreased. Fatigue test results before and after high temperature deformation showed that dynamic strain aging has a stronger effect on fatigue properties of dual phase steels with fibrous martensite. Cracks in DP steels with fibrous martensite propagate in a tortuous path in soft ferrite phase, while they pass of both hard and soft phases in DP steels with network martensite.     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

TEXTURE DEVELOPMENT IN EXTRA LOW CARBON (ELC) AND INTERSTITIAL FREE (IF) STEELS DURING WARM ROLLING

The warm rolling and subsequent annealing textures of an extra low carbon (ELC) and two interstitial free (IF) steels have been investigated using both single pass and multipass rolling in the ferritic temperature range. A total amount of 80% deformation by rolling was given in each case. It was observed that texture development in ELC and IF steels during warm rolling displays striking differences. In general, ELC steels acquire only a weak texture after finish rolling in the upper ferritic range, for both single and multipass rolling. Single pass warm rolling in the lower ferritic range, however, produces very sharp γ fiber in this alloy, as an exception. Recrystallization anneal after warm rolling, in general, has been found to decrease the overall texture. By contrast, much sharper overall texture (and γ fiber) develops in the IF steels during warm rolling, and it generally sharpens after subsequent recrystallization anneal. This is true for both single pass and multipass rolling. The best texture results (highest intensities of γ fibers) are invariably obtained when most of the rolling deformation is given in the ferritic regime. Thus, between the two grades, the IF steels are definitely far better than the ELC steels for the purpose of developing sharp γ fibers by warm rolling process. Out of the two IF steels, the Ti-containing steel appears to develop a perceptibly sharper γ fiber after warm rolling and annealing as compared to the Nb-containing one.     

Effect of heat treatment on characteristics of nanocrystalline ZnO films by electron beam evaporation

Zinc oxide thin films have been grown on glass substrate at room temperature by electron beam evaporation and then were annealed in annealing pressure 600 mbar at different temperatures ranging from 250 to 550 °C for 30 min. Electrical, optical and structural properties of thin films such as electrical resistivity, optical transmittance, band gap and grain size have been obtained as a function of annealing temperature. X-ray diffraction has shown that the maximum intensity peak corresponds to the (002) predominant orientation for ZnO films annealed at various temperatures. The full width at half maximum, decreases after annealing treatment which proves the crystal quality improvement. Scanning electron microscopy images show that the grain size becomes larger by increasing annealing temperature and this result agrees with the X-ray diffraction analysis.     

Effect of annealing temperature on the pitting corrosion resistance of super duplex stainless steel UNS S32750

The pitting corrosion resistance of commercial super duplex stainless steels SAF2507 (UNS S32750) annealed at seven different temperatures ranging from 1030 °C to 1200 °C for 2 h has been investigated by means of potentiostatic critical pitting temperature. The microstructural evolution and pit morphologies of the specimens were studied through optical/scanning electron microscope.Increasing annealing temperature from 1030 °C to 1080 °C elevates the critical pitting temperature, whereas continuing to increase the annealing temperature to 1200 °C decreases the critical pitting temperature. The specimens annealed at 1080 °C for 2 h exhibit the best pitting corrosion resistance with the highest critical pitting temperature. The pit morphologies show that the pit initiation sites transfer from austenite phase to ferrite phase as the annealing temperature increases. The aforementioned results can be explained by the variation of pitting resistance equivalent number of ferrite and austenite phase as the annealing temperature changes.     

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.