In this study, in order to accelerate the diffusion of nitrogen into austenitic stainless steel, fine particle peening (FPP) was introduced prior to gas nitriding. The effects of the FPP treatment on gas nitriding behavior, and consequent corrosion and fatigue properties of austenitic stainless steel (SUS316) were evaluated. Gas nitriding was performed at 400°C, 475°C and 550°C. The microstructural characteristics of the treated specimens were examined using a micro-Vickers hardness tester, optical microscope, scanning electron microscope (SEM), Glow discharge optical emission spectrometry (GD-OES), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was revealed that nitrided layer was formed on the FPP-treated surface although passive film existed at the surface. This was because nitrogen diffused into the material through the transferred particles on the surface during the subsequent gas nitriding process. In order to evaluate the corrosion resistance, electrochemical corrosion tests were performed. The specimen nitrided at 400°C showed highest corrosion resistance due to the existence of an S-phase. Fatigue tests were also performed at room temperature using a rotational bending fatigue testing machine. The nitrided specimens pre-treated with FPP showed the higher fatigue strength compared with single surface treated specimens.
Mechanically milled stainless steel powder was applied to hot roll sintering (HRS) or hot isostatic pressing (HIP) processes. The mechanically milled powder has a bimodal structure with a severely deformed surface domain which is named as “Shell”, and an inner domain which is named as “Core”. The shell and core microstructure in the milled powder can be maintained even after sintering. As a result, microstructure of the HRS and HIP materials consists of a shell and core bimodal microstructure. Because severe plastic deformation mainly concentrates to the shell domain, a nano grain structure forms in the shell, while a coarse (meso) grain structure form in the core. Such a nano / meso harmonic structured material demonstrated not only superior strength but also a large elongation. The mechanical properties of the HRS and HIP materials were strongly influenced by the nano/meso harmonic microstructure, such as grain size of the shell/core and shell volume fraction. The shell had role of strengthening and core has role of ductility. Thus, the nano/meso harmonic microstructure has been proved to be very effective to improve mechanical properties of structure materials. In present study, a fracture and deformation mechanism of the harmonic microstructure stainless steel have been investigated through a delicate SEM observation. It was confirmed that fracture was initiated at second phase particles and then propagated along the shell/core interface.
This paper presents the fatigue behavior of a high tensile steel, NHPA-1500. This steel is quenched in press-die and used as pressworked car component without tempering. Axial loading fatigue tests have been performed in laboratory air and the results obtained were discussed through the comparison with those of a high tensile steel, SPC1470. NHPA-1500 after quenching showed very similar static strengths to SPC1470, especially in tensile strength, elongation and Vickers hardness, while fatigue behavior was so different. In NHPA-1500, the S-N curve showed a definite fatigue limit, that is, no fatigue failure occurred until 108 cycles. On the other hand, SPC1470 exhibited a step-wise S-N curve and fatigue failure occurred at stress levels below the conventional fatigue limit of 900MPa. Fracture mechanism operated in low stress region below 900MPa was subsurface crack initiation with a fish-eye. In both steels, the sizes of the maximum nonmetallic inclusions in a standard area were measured using SEM. The results obtained were plotted on an extreme value probability paper, where the sizes of nonmetallic inclusions observed in SPC1470 were statistically larger than those in NHPA-1500. It is considered that the difference in the high cycle fatigue behavior between both steels was due to the size of non-metallic inclusions.
Shelling is the typical rolling contact fatigue (RCF) failure of the railway wheels. In order to evaluate effect of the material strength on the shelling properties of railway wheel steels, RCF tests and FE analyses were conducted. The RCF tests were carried out in the twin disc type testing machine. Wheel specimens made of two kinds of wheel steels with different strength were tested under water lubrications in the RCF tests. The elasto-plastic FE analyses were carried out to calculate the stress intensity factors (SIF) of the surface cracks in the RCF tests. The water penetrations into the surface cracks were taken into consideration in FE analyses. There are two main results in this study. Firstly, the RCF test results showed that the RCF strength of the higher strength wheel steel increased comparing with that of the lower strength wheel steel. Secondly, the SIFs of the branched cracks of the higher strength steel reduced as a result of being the shallower crack depth by decrease of the hydrostatic pressure in the cracks. In addition, these SIFs at the fatigue limits in the RCF tests corresponded to the threshold ranges of the SIF ΔKth of each material. Therefore, these results suggest that the fatigue limits in the RCF tests are influenced by the maximum crack depth.
This paper describes the multiaxial low cycle fatigue life of cruciform specimen of YH61 Ni base single crystal superalloy at high temperature. Biaxial tension-compression low cycle fatigue tests were performed using the cruciform specimen at 1173K under y-directional strain amplitude (εy) constant condition with varying x-directional strain amplitude (εx). Low cycle fatigue lives were influenced by the strain biaxiality and the maximum lives were obtained at εx/εy being equal to 0.5. The maximum principal strain and Mises equivalent strain ranges gave a large scatter in the correlation of the multiaxial low cycle fatigue lives. The principal stress and Mises equivalent stress ranges showed far better correlation than the strain ranges. Cracking mode was also discussed by observing the fracture surface at failure. Cracks propagated in the 45 degree direction to x direction at εx/εy = -1 test while those in the x direction in the other cases. Cracks were initiated at the life ratio less than 0.2 at Ni3(Al+Ta) eutectic alloy part and propagated with cycles.
In order to investigate the effects of humidity change and loading frequency on growth behavior of a fatigue crack of an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out in relative humidity of 25% and 85% at frequencies of 50Hz and 6Hz. At 50Hz, macroscopic appearances of fractures were a shear mode at high stress levels in high humidity, though those were a tensile mode in other conditions of humidity and stress levels. The crack growth rate was accelerated by high humidity. By changing humidity, both of the growth rate and the growth mode of a crack were immediately changed to those corresponding to the changed humidity. That is, the growth behavior and fatigue life under humidity change may be estimated by those in constant humidity. Fatigue life in high humidity at 6Hz was longer than that at 50Hz in spite of accelerations of both of the crack initiation and its growth rate in the early growth process in high humidity at both frequency, though there was no or little influence of frequency on fatigue life in low humidity. Moreover, the crack propagated in the tensile mode even at high stress levels where the shear mode crack propagated at 50Hz. These results were explained by the effect of hydrogen on the crack growth behavior.
This paper presents the Paris law for discontinuous crack growth (DCG) and continuous crack growth (CCG) in polycarbonate under cyclic loading. First, the length and height of the craze zone near the crack tip were expressed on the basis of fracture mechanics. Next, the craze fibril breakdown model for CCG was applied to DCG and correlated with the Paris law. In CCG, the microscopic crack propagation rate dâ/dNequals to macroscopic one da/dN. In contrast, dâ/dN is not equal to da/dN in DCG because the macroscopic crack does not propagate until the craze fibrils break after some number of cycles. Therefore, a failure criterion for unstable crack growth in the craze zone was introduced. As a result, it was concluded that the crack propagation exponent for DCG is the same as that for CCG, while the coefficient in the Paris law for DCG is larger than that for CCG. Finally, the experiment data for DCG and CCG in polycarbonate were fitted to the Paris law to verify the reasonability of the above conclusions. In addition, the number of cycles required for DCG predicted using the model was compared with the experiment result.
Material strengthening can be achieved by means of effectively intercepting the dislocation motion by some obstacles. The representative one is grain boundary (GB) strengthening, called Hall-Petch relationship (H-P effect). Since such an interaction between dislocation and GB always happens at any plastic deformation stage, H-P effect could have plastic strain dependence. Also the dislocation motion is extremely sensitive to temperature, thus temperature dependence should be involved as well. In the present paper, both dependences of H-P effect were investigated using pure aluminum specimens with six different averaged grain sizes at four different temperatures. The former dependence leads to a relationship with work hardening behavior which can be fitted to n-power law, and the latter might successfully induce the activation barrier for the plastic deformation to be evolved across GB. H-P coefficient, which is a coefficient of the term with power -1/2 of averaged grain size on the H-P relationship, was summarized as a function with power m of plastic strain, where m-value is newly defined as plastic strain grain boundary strengthening sensitivity exponent. Considering two simple models for description of fundamental plastic deformation across GB in comparison with experimental results, H-P coefficient is found to be proportional to power n of plastic strain (that is, m-value is equal to n-value of work hardening exponent). This corresponding model suggests that the internal stress due to the piled-up dislocations in front of GB activates the dislocation sources in adjoining grain. However, this relation doesn't hold at elevated temperature. From Arrhenius plots concerning to the relationship between plastic strain and H-P coefficient, activation energy was obtained as 67kJ/mol, which is almost half of that of creep behavior (140kJ/mol).
A hydrophilic polyurethane sand stabilizer (abbreviated as W-OH), was synthesized by our group, which can connect chemical sand fixation with biological sand fixation. This paper is to investigate the durability of W-OH, analyzing mechanism of UV degradation. And several UV stabilizers of hydroxy-benzophenone (W-UA), thioether (W-UB), hindered phenol (W-UC) and hindered amines (W-US) were used to advance W-OH UV degradation resistance. For evaluating on effect of different UV stabilizers on durability of W-OH, firstly variation of weight of W-OH film under UV radiation measured, and then the effect of UV radiation on W-OH sand-fixing layer was studied further. Results demonstrated that W-OH reacted with O2 under UV radiation and transformed to free-radicals of alkyl peroxide or alkoxy or peroxy or hydroxyl and W-OH was gradually degraded. Durability of W-OH can be improved with composing UV stabilizers and the order of effect was W-US > W-UC > W-UB > W-UA. Composition of W-OH and W-US demonstrated excellent UV degradation resistance, without any surface variation for the sand-fixing layer under UV radiation.
This research investigated the corrosion behavior of steel in mortars with 40% blast furnace slag cement replacement and 20% fly ash cement replacement to ordinary Portland cement, considering the concentrations of chloride and other ions present in the mortars saturated pores and around the steel bar. Three methods were used to extract solutions from mortars wherefrom ions were measured. Results showed that reinforcing bar corrosion activity was lower for FA mortars than BFS mortars; and both having better corrosion resistances than mortars with only OPC as binder. Correspondingly, Cl- contents in pore solutions followed the trend OPC > BFS > FA mortars. The Cl-/OH- contents in the solutions obtained from around the steel bar correspond to the mortars corrosion behavior, signifying the influence of Cl- and OH- on corrosion and the greater importance of investigating the ions around the bar than that in the pore solutions of the bulk mortar. Moreover, significant differences in Cl- contents were measured using the 3 methods of solution extraction, which validate that the variations in the method of solution extraction account for the inconclusive Cl- threshold values for reinforcing bar corrosion published in literatures.