Role of microstructural condition on fatigue damage development of AISI 316L at 20 and 300 °C

At 300 °C, when dynamic strain ageing takes place, the fatigue life of AISI 316L for lower strain amplitudes is lower than under equivalent conditions at 20 °C. Exhaustive examination of the changes in: (1) apparent elastic modulus, (2) microstructural condition, and (3) fractographic features has been performed to reveal the reason for the life reduction. The analysis of apparent elastic modulus variations and the results of fractographic observations show that the propagation rates for fatigue cracks at 20 °C are faster than for 300 °C. Crack initiation however occurs earlier at 300 °C, in particular for lower strain amplitude tests, due to the activity of localised deformation bands as a consequence of cyclic loading. In addition to persistent slip bands, a form of ladder-free deformation bands is also present at 300 °C, in particular at low strain amplitudes. When the fatigue life is rather short, the influence of the ladder-free deformation bands on cyclic endurance is negligible. The ladder-free type of localised bands have a strong influence on crack initiation once the material endurance increases with lowering strain amplitude, leading to the relative life reduction at the elevated temperature. In addition, the incidence of secondary cyclic hardening for lower strain amplitude tests at 300 °C partly contributes to the more evident life reduction. The influence of dislocation walls on the propagation of microstructurally short fatigue cracks is also examined.     

Anomalous creep behaviour of 316 stainless steel at 550°C

The results offifteen constant-load creep tests at 550°C, with nominal stresses in the range 200 to 360 MPa and with test durations ofup to 14 000 h, are presented. The usual primary, secondary and tertiary creep behaviour was exhibited for nominal stresses greater than about 330 MPa. At lower stresses, ‘renewed’ primary and secondary creep regions were observed. The renewed secondary creep strain rates were found to be about an order of magnitude greater than the initial secondary creep strain rates. The results indicate that the occurrence ofthe renewed primary and secondary creep regions is associated with time-dependent exposure to a temperature of 550°C. The presence or magnitude of the prior stress level does not appear to have any significant effect.

The results are relevant to design procedures because extrapolation of short duration or high stress data to long-term design lifetimes is often required. Unless the possibility ofthe occurrence ofrenewed primary and secondary creep is taken into account, gross errors in strain predictions could occur.     

High temperature corrosion studies on friction welded low alloy steel and stainless steel in air and molten salt environment at 650 °C

The investigation on high-temperature corrosion resistance of the weldments is necessary for prolonged service lifetime of the components used in corrosive environments. This paper reports on the performance of friction welded low alloy steel AISI 4140 and stainless steel AISI 304 in air as well as molten salt environment of Na₂SO₄–60%V₂O₅ and NaCl–50%Na₂SO₄ at 650 °C. This paper reports several studies carried out for characterizing the weldments corrosion behavior. Initially thermogravimetric technique was used to establish the kinetics of corrosion. For analyzing the corrosion products, X-ray diffraction, scanning electron microscopy/energy-dispersive analysis and electron probe micro analysis techniques were used. From the results of the experiments, it is observed that the weldments suffered accelerated corrosion in NaCl–Na₂SO₄ environment and showed spalling/sputtering of the oxide scale. Furthermore, corrosion resistance of weld interface was found to be lower than that of parent metals in molten salt environment. Weight gain kinetics in air oxidation studies reveals a steady-state parabolic rate law while the kinetics with salt deposits displays multi-stage growth rates. Moreover NaCl is the main corrosive species in high temperature corrosion, involving mixtures of NaCl and Na₂SO₄ which is responsible for formation of internal attack.     

Effects of aging at 700°C on ductile fracture of AISI 316 stainless steel

Abstract

The micromechanisms of ductile fracture have been studied in a commercial AISI 316 austenitic stainless steel. Tensile, Charpy impact, and ductile fracture toughness testing have been performed on unaged material and samples aged at 700°C for times up to 4380 h. Examination of the specimens after testing has shown that the microstructural changes occurring at grain boundaries are responsible for the observed losses of ductility and crack growth resistance. The relative magnitude of the observed changes in mechanical properties has been explained using a simple model to describe the ductile fracture process.

MST/1001     

Extensometer probe indentation during low-cycle fatigue of plain and circumferentially notched cylindrical bars at 550 °C

Cyclic stress–strain tests were undertaken at 550 °C on (i) plain and (ii) notched specimens of different acuities in several low- and highly-alloyed ferritic steels (1CrMoV, NF616, TB12M and HCM12A). Axial strains were measured between the minimum sections of the (semi-circular) notches using a longitudinal extensometer, while surface hoop strains were measured by means of a diametral extensometer with probes located at the notch root. Over a period of 100 cycles, softening occurred in both specimen types. Cycle-by-cycle embedding was observed for the (diametral) probe tips and was entirely due to testpiece working in the cyclic state, the sprung-load extensometer force being far lower than that encountered in conventional indentation experiments. An interpretation of the effect is offered (a) in terms of the internal stress state produced during cycling and (b) by an energy argument to explain the ‘ranking order’ of resistance to penetration. The effects of cyclically-induced oxidation, possible mechanisms for the embedding effect and likely reduction in endurance are considered.     

Martensitic stainless steel AISI 420—mechanical properties,creep and fracture toughness

In this paper some experimental results and analyses regarding the behavior of AISI 420 martensitic stainless steel under different environmental conditions are presented. That way, mechanical properties like ultimate tensile strength and 0.2 percent offset yield strength at lowered and elevated temperatures as well as short-time creep behavior for selected stress levels at selected elevated temperatures of mentioned material are shown. The temperature effect on mentioned mechanical properties is also presented. Fracture toughness was calculated on the basis of Charpy impact energy. Experimentally obtained results can be of importance for structure designers.     

Solvothermal synthesis of graphene sheets at 300 °C

Graphene nanosheets (GS) had been solvothermally synthesized through reducing hexachloro-1,3-butadiene (C₄Cl₆) by metallic sodium (Na) in polyethylene glycol-600 (PEG-600) at 300 °C. Atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) investigations indicated that 1–3 graphite layers could be observed. The Raman spectrum showed that the peak of 2D band at 2693 cm^? 1 of GS had a smaller wave number and stronger intensity compared to the 2717 cm^? 1 of commercial graphitic flakes. Meanwhile, the I_D/I_G value of GS was 0.40 indicating a lower density of defects of GS. The possible reaction process was that C₄Cl₆ was dechlorinated by Na in the presence of PEG-600 to produce carbon framework, then these newly produced carbon framework would connect to each other to form the hexagonal network of graphene.     

Fatigue crack growth behaviour of A533B steel in pressurized water at 288 and 28 °C

Fatigue crack growth behaviour of A533B steel was investigated in pressurized water at 288 °C using specimens machined from four different orientations. When inclusions were oriented along the direction of crack propagation, fatigue crack growth rate (FCGR) was enhanced compared to when they were perpendicular to the direction of crack propagation. At low ΔK levels FCGR in ambient water was slightly higher than that in 288 °C water. This may be attributed to the occurrence of intergranular cracking in ambient water tested specimen. Though mainly ductile striations were observed on the fracture surfaces, isolated intergranular facets (in a specimen tested in ambient water) and fan shaped features were also present. Hydrogen induced damage was clearly evident in the ambient water tested specimen in the form of isolated intergranular facets.     

Dependence of dynamic strain ageing on strain amplitudes during the low-cycle fatigue of TP347H austenitic stainless steel at 550 °C

Low-cycle fatigue (LCF) tests are carried out on TP347H stainless steel at a strain rate of 8 × 10⁻³ s⁻¹ with total strain amplitudes (Δε_t/2) of ±0.4% and ±1.0%, at room temperature (RT) and 550 °C. It is found that the stress responses and dislocation structures under cyclic loading strongly depend on the value of strain amplitude at 550 °C. Compared with those at the same strain amplitude at RT, the material shows a rapid strain softening, and finally attains a stabilized state at Δε_t/2 = ±0.4% and 550 °C, but the one presents an anomalous behavior, i.e., first a rapid hardening to the maximum stress, followed by a reducing softening at Δε_t/2 = ±1.0% and 550 °C. More cells resulting from dislocation cross-slip and planar structures due to dynamic strain ageing (DSA) restricting cross-slip develop at low strain amplitude of ±0.4% at the first cycle. However, there are more complicated dislocation structures, such as cells, elongated cells, walls/channels and planar structures at Δε_t/2 = ±1.0%. The observations of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) exclude the effects of martensitic transformation, creep, oxidation, and precipitations on these stress responses and microstructure evolutions, which result from DSA appearing at 550 °C.     

The wear and corrosion resistance of shot peened–nitrided 316L austenitic stainless steel

316L austenitic stainless steel was gas nitrided at 570 °C with pre-shot peening. Shot peening and nitriding are surface treatments that enhance the mechanical properties of surface layers by inducing compressive residual stresses and formation of hard phases, respectively. The structural phases, micro-hardness, wear behavior and corrosion resistance of specimens were investigated by X-ray diffraction, Vickers micro-hardness, wear testing, scanning electron microscopy and cyclic polarization tests. The effects of shot peening on the nitride layer formation and corrosion resistance of specimens were studied. The results showed that shot peening enhanced the nitride layer formation. The shot peened–nitrided specimens had higher wear resistance and hardness than other specimens. On the other hand, although nitriding deteriorated the corrosion resistance of the specimens, cyclic polarization tests showed that shot peening before the nitriding treatment could alleviate this adverse effect.     

The effect of σ-phase precipitation at 800°C on the mechanical properties of a high alloyed duplex stainless steel

High alloyed duplex stainless steels, referred to as super duplex, are being used increasingly for applications in corrosive environments. The higher Cr and Mo contents of these materials can lead to susceptibility to σ-phase precipitation, even during short time exposures to high temperatures. The 800°C aging of a high alloyed duplex stainless steel was investigated. Optical microscopy with image analysis, X-ray diffraction, and SEM were used to determine the σ-phase and to estimate the amount. Mechanical properties such as tensile strength, impact strength and hardness were investigated. It was found that the impact strength is most sensitive to aging at 800°C. The σ-phase transformation is discussed and the microstructural aspects are described.     

Thermal expansion of hydroxyapatite between − 100 °C and 50 °C

Hydroxy apatite (HAp) ceramic was synthesized using traditional sintering. Dilatometric and lattice thermal expansion properties of a HAp ceramic were evaluated at temperatures of ? 100–50 °C. In that temperature range, the dilatometric thermal expansion coefficient and the lattice thermal expansion coefficient of the HAp ceramic were, respectively, 10.6 × 10^? 6/°C and 9.9 × 10^? 6/°C. Furthermore, thermal expansion properties of a human tooth were measured. The thermal expansion coefficient of the horizontal direction perpendicular to the growing direction of a tooth was 15.5 × 10^? 6/°C; that of the vertical direction along with the direction of tooth growth was 18.9 × 10^? 6/°C at the temperature range described above.     

Mechanical properties of ultra-lightweight cement composite at low temperatures of 0 to −60 °C

This paper presents an experimental study on mechanical properties of an innovative ultra-lightweight cement composite (ULCC) at low temperatures down to −60 °C in comparison with those at ambient temperature. Those properties include stress-strain curve, ultimate strength, elastic modulus, Poisson ratio, and flexural tensile behavior. Effect of curing condition is also evaluated. In addition, the performance of the ULCC is compared with that of a normal weight concrete (NWC) and a lightweight concrete (LWC) with similar 28-day compressive strength. The cylindrical compressive strength of the NWC and LWC was increased generally with the reduction in temperature. However, the same phenomenon was not observed for the ULCC. The elastic modulus of the ULCC did not change much, whereas the elastic modulus of the NWC increased significantly with the reduction of temperature from 30 °C to −60 °C. Strain of the ULCC at the peak load was generally much higher than that of the NWC and LWC, and was generally not affected by the temperature. The flexural strength of the three concretes was increased with the reduction in temperature. Duration of the moist curing did not affect the performance of the ULCC under compression significantly, but influenced its flexural strength significantly.     

Hot corrosion behavior of porous nickel-based alloys containing molybdenum in the presence of NaCl at 750 °C

The hot corrosion of porous Ni-23Cr-xMo (0%, 4.5%, 9.0%, 13.5%, mass fraction) alloys tested at 750 °C under cyclic procedure was investigated in order to elucidate the effect of Mo addition on hot corrosion in the presence of NaCl. The hot corrosion experiments were performed at 750 °C in air with 4 mg cm^− 2 NaCl deposit. The performance of the alloys was evaluated by the results of weight change kinetics. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize the corrosion products. The results indicate that NaCl accelerated the oxidation of the alloys by chloridized elements Mo and Cr. Among the porous Ni-23Cr-xMo alloys, Ni-23Cr-9Mo alloy exhibited the best hot corrosion resistance due to the formation of NiO-NiCr₂O₄-Cr₂O₃ oxide scales. Furthermore, these oxide scales were confirmed more effective to protect the alloys after adding of Mo.     

Effect of vanadic corrosion on creep-rupture properties of Superni-600 at 650–750°C

The effect of vanadic corrosion on creep-rupture properties of a nickel base superalloy Superni-600, at 650–750°C, has been investigated. Sodium metavanadate and sodium metavanadate plus 15 wt% sodium sulphate were used as the corrodent salts. Weight change studies have also been made to understand the mechanism of corrosion. Both sodium metavanadate and the sodium metavanadate/sodium sulphate mixture are found to be aggressive and to reduce the creep-rupture life. The degradation of creep-rupture properties and possible hot corrosion reactions are discussed. Cracking and fluxing of the protective scales, together with easier crack nucleation and growth at grain boundaries in the presence of a liquid deposit accounts for the enhanced creep rates and reduced rupture lives.     

High temperature oxidation behavior of ferritic steel in supercritical water at 550–700°C

Oxidation tests were conducted on ferritic steel T22 exposed to deaerated supercritical water at 550–700°C and 25 MPa. Oxide films formed on T22 had a double-layered structure with the outer layer consisting of iron oxide and the inner layer consisting of spinel oxide. Pores formed on the surface of samples initially but healed at longer exposure time and higher temperature. Cracks occurred along the grain boundaries in the oxide scale at 600–700°C for 200 and 400 h. The oxidation kinetics obeyed a near-parabolic law in all cases. The data of activation energy of T22 indicated that the likely oxidation rate-controlling step may be the outward diffusion of iron along the magnetite bulk.     

Fatigue behavior of coated and uncoated cast Inconel 713LC at 800 °C

Cylindrical specimens of Inconel 713LC in as-received condition and with surface treatment by Al diffusion coating were cyclically strained under strain control at 800 °C in air. Surface treated layer was characterized and the hardness depth profile was measured. Cyclic stress–strain response and fatigue life of both materials were assessed. The stress response of the coated superalloy specimens is lower in comparison with the untreated specimens. Beneficial effect of surface treatment on the Manson–Coffin curve is documented. Specimen sectioning and fracture surface studies revealed fatigue damage mechanisms both in coated and uncoated specimens. Propagation path of the principal crack is predominantly interdendritic.     

Structural,mechanical, and tribological properties of AISI 304 and AISI 316L steels submitted to nitrogen–carbon glow discharge

Glow discharge (GD) nitrocarburizing, at low-carbon content and different working temperatures, was performed on AISI 316L and AISI 304 stainless steels. Structural compositions were studied by X-ray diffraction. Instrumented indentation and conventional Vickers method allowed hardness profiles to be determined. Tribological behavior was studied by means of reciprocating sliding and nanoscratch tests. After nitrocarburizing, both steels showed similar embedded nitride and carbide formations. The layer formed by GD in nitrogen–carbon atmosphere is thicker than those consisting solely of nitrogen particularly for AISI 316L. At working temperatures higher than 400 °C, roughness increased and wear was limited to asperity compaction. Wear mechanisms were similar in both steels. However, wear was reduced by up to a factor of 5 in treated steels. No difference in elastic surface recovery was observed after nitrocarburizing in either steel.     

High temperature cyclic oxidation and hot corrosion behaviours of superalloys at 900°C

Oxidation and hot corrosion are serious problems in aircraft, marine, industrial, and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the degradation of turbine engines. To obviate these problems, superalloys, viz. Superni 75, Superni 718 and Superfer 800H superalloys (Midhani grade), are the prominent materials for the high temperature applications. It is very essential to investigate the degradation mechanism of superalloys due to oxidation and hot corrosion and substantiate the role of alloying elements for the formation of protective oxide films over the surface of the superalloys. Therefore, the present work investigates the oxidation and hot corrosion behaviour of superalloys exposed to air and molten salt (Na₂SO₄–60% V₂O₅) environment, respectively, at 900°C under cyclic conditions. The weight change measurements made on the superalloys during the experiments are used to determine the kinetics of oxidation and hot corrosion. X-ray diffraction (XRD), X-ray mapping and field emission scanning electron microscope (FESEM, FEI, Quanta 200F company) with EDAX Genesis software attachment, made in Czech Republic are used to characterize the corroded products of the superalloys. It is observed that the formation of scale rich in Cr₂O₃, NiO and spinel NiCr₂O₄ has contributed for the better oxidation and hot corrosion resistance of Superni 75; whereas relatively lesser hot corrosion resistance of Superfer 800H is due to the formation of non-protective oxides of iron and sulphides of iron and nickel. The parabolic rate constants calculated for the superalloys show that the corrosion rate is minimum in air as compared to molten salt environment.     

The isothermal section of the Al-Si-B ternary system at 700 °C and its applications

The isothermal section of the Al−Si−B ternary system at 700 °C has been calculated using FactSage software and experimentally determined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray powder diffraction(XRD). The calculated results show that there are six three-phase regions in the isothermal section: Liquid-Al+AlB₂+(Si), AlB₂+AlB₁₂+(Si), (Si)+AlB₁₂+SiB₃, AlB₁₂+SiB₃+SiB₆, AlB₁₂+SiB₆+SiB₁₄, AlB₁₂+SiB₁₄+(B). Four three-phase regions of Liquid-Al+AlB₂+(Si), AlB₂+AlB₁₂+(Si), (Si)+AlB₁₂+SiB₃ and AlB₁₂+SiB_n+(B) were confirmed experimentally, which are consistent with calculated results. The small AlB₂ particles in the Al-3B alloy have good refining effect on the primary α-Al phase in the Al-10Si alloy, which greatly refines the microstructure and improves the mechanical properties of the Al-10Si alloy.