The effect of grain size and martensitic transformation on the wear behavior of AISI 304L stainless steel

In the present study, a combination of cold rolling and subsequent annealing was used to produce an AISI 304L stainless steel with different grain sizes (650 nm, 3 μm and 12 μm). Wear behavior of the steel was subsequently examined using dry sliding wear test under different loads of 10 N, 20 N and 30 N. Different microstructural characterizations were conducted on the samples. The results demonstrated that the ultra-fine grained steel (650 nm grain size) had better wear resistance under normal loads of 10 N and 20 N, whereas under the normal load of 30 N, it showed weak wear resistance as compared to the steel with larger grain size (3 μm and 12 μm). This behavior can be attributed to the amount of induced martensitic transformation formed during the wear test. This transformation was evaluated using XRD analysis and quantified by Ferritescope measurements. Wear mechanism was recognized as delamination in the early stages of the wear test and the mixture of delamination and abrasion for higher distances.     

Gigacycle fatigue behavior by ultrasonic nanocrystalline surface modification

Nanocrystalline surface layer up to 84 microm in thick is produced on a specimen made of Al6061-T6 alloy by means of surface treatment called ultrasonic nanocrystalline surface modification (UNSM) technique. The refined grain size is produced in the top-layer and it is increased with increasing depth from the top surface. Vickers microhardness measurement for each nanocrystalline surface layer is performed and measurement results showed that the microhardness is increased from 116 HV up to 150 HV, respectively. In this study, fatigue behavior of Al6061-T6 alloy was studied up to 10(7)-10(9) cycles by using a newly developed ultrasonic fatigue testing (UFT) rig. The fatigue results of the UNSM-treated Al6061-T6 alloy specimens were compared with those of the untreated specimens. The microstructure of the untreated and UNSM-treated specimens was characterized by means of scanning electron microscopey (SEM) and transmission electron microscopey (TEM).     

Fretting wear of nitrogen-implanted AISI 304 stainless steel

Fretting wear resistance of nitrogen-implanted AISI 304 stainless steel was measured and compared to that of unimplanted steel. After 5 × 10³ cycles, unimplanted steel revealed severe damage on non-slip area and adhesive-type wear on microslip region whereas nitrogen-implanted stainless steel was still undamaged. The improved fretting wear resistance is explained to be due to the increased load carrying capacity and decreased adhesion of nitrogen-implanted steel.     

Influence of ultrasonic cavitation on surface residual stresses in AISI 304 stainless steel

The effect of ultrasonic cavitation in water on residual stress changes in AlSl 304 stainless steel has been investigated. Studies indicate that high-intensity ultrasonic cavitation introduces a very high compressive residual stress at the surface (due to work-hardening) even for short durations of exposure at ambient temperatures. With increased exposure, the stresses become more compressive; however, they tend to reach a saturation value. Different combinations of temperature, time and cavitation intensity were tried out and the best effects were noticed for a treatment temperature of 5 °C. 304 stainless steel was chosen for the present study on account of its amenability to strong work-hardening. The test specimen was attached to the tip of an ultrasonic vibrator and immersed in the cavitating liquid, i.e. water. However, even in situations where the specimen was kept in a stand-off position close to the vibrator tip (with water in between) similar effects were noticed. The maximum depth of hardening was found to be about 70 m. During this process, there was also a mild roughening of the surface. An incidental observation pertains to the formation of both and martensites at the surface detectable by X-ray diffractometer recordings for specific conditions of cavitation treatment. The required high intensities of vibration in this study were obtained through an in-house built highpower ultrasonic generator working at a frequency around 20 kHz.     

Wear behavior of Cu-Zn alloy by ultrasonic nanocrystalline surface modification

The ultrasonic nanocrystalline surface modification (UNSM) was applied to disk specimens made of Cu-Zn alloy in order to investigate the UNSM effects under five various conditions on wear of deformation twinning. In this paper, ball-on-disk test was conducted, and the results of UNSM-treated specimens showed that surface layer dislocation density and multi-directional twins were abruptly increased, and the grain size was altered into nano scale. UNSM delivers force onto the workpiece surface 20,000 times per second with 1,000 to 4,000 contact counts per square millimeter. The UNSM technology creates nanocrystalline and deformation twinning on the workpiece surface. One of the main concepts of this study is that defined phenomena of the UNSM technology, and the results revealed that nanocrystalline and deformation twinning depth might be controlled by means of impact energy of UNSM technology. EBSD and TEM analyses showed that deformation layer was increased up to 268 microm, and initial twin density was 0.001 x 10(6) cm(-2) and increased up to 0.343 x 10(6) cm(-2). Wear volume loss was also decreased from 703 x 10(3) mm3 to 387 x 10(3) mm3. Wear behavior according to deformation depth was observed under three different combinations. This is related to deformation depth which was created by UNSM technology.     

Influence of pad span on fretting fatigue behaviour of AISI 304 stainless steel

The present work deals with the influence of pad span on fretting fatigue behaviour of AISI 304 stainless steel. Relative slip is one of the three primary variables influencing fretting fatigue behaviour. The relative slip can be modified by changing the pad span and/or cyclic stress. In the present study, the effect of relative slip was studied at different cyclic stress levels and by using fretting pads with three different pad span values (15, 20 and 30 mm). The relative slip increased with an increase in pad span and cyclic stress. Samples tested with fretting pads having longer pad span (30 mm) exhibited longer lives. Though the specimens tested with pads having longer pad span experienced higher frictional stress and tangential force coefficient compared with those tested with pads having smaller pad span (15 or 20 mm), the relative slip values were larger in the former. Due to larger relative slip values it was assumed that small cracks initiated by fretting fatigue would have been worn away due to wear damage. Due to this the specimens tested with pads having longer pad span exhibited enhanced fretting fatigue lives. More deformation-induced martensite formed in the samples tested with pads having longer pad span owing to longer lives.     

Using Mather-type plasma focus device for surface modification of AISI304 Steel

A 2.8 kJ plasma focus device with a nitrogen gas filling and a copper anode capsulated by aluminum was used to modify the surface of AISI304 steel substrate, in order to improve its properties. The treatment was carried out using a various number of nitrogen plasma focus shots at a pressure of 0.5 mbar and at two steel sample distances (20 and 40 mm) from the anode. The plasma diagnostics was made using the voltage and current curves recorded by a voltage divider, Rogowskii coil, accompanied with calculations using a five phase radiative Lee model (RADPF5.15a) to determine the temperature and plasma density.The surface hardness of AISI304 steel was increased by ∼175% after plasma treatment and the thickness of the treated layers was about 1-2 μm. Results show that the surface hardness is increased with increasing shot number and decreased with increasing distance from the anode. Changes in surface morphology and the elemental composition were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX).     

Corrosion resistance of the layers formed on the surface of plasma-nitrided AISI 304L steel

Austenitic stainless steels have good corrosion resistance, but their low hardness and low wear resistance limit their use whenever surface hardness is required. Nitriding treatments have been successfully applied to stainless steels to improve their mechanical and tribological properties; however, at temperatures above 723 K, gas or salt bath nitriding processes decrease the corrosion resistance due to the formation of CrN and other phases within the modified layer. Chromium compounds draw chromium and nitrogen from the adjacent regions, degrading the corrosion resistance. The plasma nitriding technique permits the use of treatment temperatures as low as 623 K without promoting degradation in the corrosion resistance of stainless steel. In this work, the pulsed glow discharge (PGD) technique was used for nitriding steel (AISI304L) in order to investigate the effect of the temperature of this treatment in the morphology and, as a consequence, in the anodic behavior of the formed layers, in solution with and without chloride ions. Four different temperatures were employed (623, 673, 723, and 773 K). The samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), microhardness measurements, and electrochemical tests with potentiodynamic anodic polarization curves. The nitriding temperature alters the anodic behavior due to a displacement of the polarization curve towards higher currents, in a solution free of chloride ions. In a chloride solution, the nitriding temperature increases the pitting potential up to the oxygen evolution region.     

Effect of ultrasonic nanocrystal surface modification on the characteristics of AISI 310 stainless steel up to very high cycle fatigue

Effects of ultrasonic nanocrystal surface modification (UNSM) on the very high cycle fatigue response of AISI 310 stainless steel have been investigated. The higher impact force used in UNSM treatment showed a higher fatigue life improvement. The fatigue life improvement was higher in crack initiation from the surface of specimens. The subsurface crack initiation depth in the alloy increased with increase in the fatigue failure cycles. It was concluded that UNSM treatment can increase the life of the alloy significantly up to very high cycle fatigue.     

The edge-cracking of AISI 304 stainless steel during hot-rolling

The hot-rolled plates of AISI 304 stainless steel, containing edge cracks of different intensities, were examined. The austenitic matrix of the steel contained small amounts of ferrite inhomogeneously distributed across the width and the thickness of the plate. A correlation was found between ferrite content and edge cracking: the higher the ferrite content the longer the edge cracks. Among the chemical elements present in the steel, the most critical effect on ferrite content was exerted by carbon and nitrogen. The longest edge cracks were observed for plates with the lowest content of carbon and nitrogen. A possible contribution of steel chemistry and heating temperature to changes in the steel phase composition and the probability of edge cracking is discussed.     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

The thermal conductivity of AISI 304L stainless steel

A compilation and critical analysis of the thermal conductivity () of AISI 304 stainless steel (SS) between 100 and 1707 K has been given in the literature. The author represented his recommended values of by an inflection in the A versus temperature relationship between 300 and 500 K. Because a physical mechanism had not been identified that would produce such a temperature dependence in of 304 SS, interest was generated in the possible existence of an as yet undiscovered phenomenon that might cause such an inflection. Consequently, experimental verification of the inflection was sought. The present paper presents recent measurements of , the electrical resistivity, and the absolute Seebeck coefficient of 304L SS from 300 to 1000 K and of the thermal diffusivity () from 297 to 423 K. The values computed from the a measurements were within ± 1.6% of the directly measured An inflection was not observed in the temperature dependence of between 300 and 500 K. After careful evaluation and because a physical mechanism still has not been identified which would produce such an inflection, the authors conclude that the inflection in the vs T relationship reported in the literature was caused by the data analysis technique.     

Modelling the effect of strain rate on the thermomechanical behaviour of AISI 304 stainless steel

The present work deals with the interdependences between the strain rate and the strain hardening on stainless austenitic steels. Uniaxial tension tests were conducted on a 304 stainless steel at different strain rates in order to analyse the influence of this parameter on the strain hardening and on the material formability. For the strain rates levels analysed (10^‐4 to 10^‐1 s^‐1) it was also observed that increasing the strain rate from 10^‐4 up to 10^‐1 s^‐1 leads to a 25 % difference in uniform tension elongation revealing the curve‐crossing phenomenon. Namely, strain rates equal or higher than 10^‐2 lead to a stagnation of strain hardening after a tensile strain of about 0.2. In order to investigate the results obtained, microstructural and thermal analyses were conducted and numerical simulations were performed. It was observed that the decreasing of formability of the material is essentially due to thermal aspects. In the discussion, the experimental and numerical results are analysed in terms of thermal softening, phase transformation and strain rate sensibility.     

Effect of strain rate on nanocrystalline surface formation in controlled ball impact process in AISI 304 SS

Strain rate during severe plastic forming processes affects the grain size formation. Nanocrystalline grain size formation at and near the surface in stainless steel, AISI304, during the controlled ball impact process carried out at a high strain rate are reported. High strain rate exerted by using a small diameter ball resulted in the formation of grains of size ~ 80 nm on the surface. The peening coverage and surface roughness depend on the sample traveling velocity for a given ball diameter and impact velocity.     

Determination of the best behavior among AISI D3 steel, 304 stainless steel and CrN/AlN coatings under erosive-corrosive effect

The erosive-corrosive effect of aqueous NaCl slurries on metals and metals coated with a multilayer system was analyzed. The erosion-corrosion experiments were performed in a test machine in which the impingement velocity, impact angle, concentration of solids and pH of the solution were controlled. Polarization curves were simultaneously obtained to correlate the electrochemical effects to the erosive wear mechanisms. The slurry used consists of silica particles suspended in a mixture of acid solution and 3.5% NaCl, with a pH value of 5.6. Electrochemical results showed the best corrosion resistance for steel coated with CrN/AlN system deposited with 50 bilayer. Additionally, the surface analysis by SEM micrograph revealed formation of cracks in CrN/AlN multilayers coating and plastic deformation in both steel substrates (AISI D3 steel and 304 stainless steel), especially when the mean impact angle is a critical value of 90°. Measurements of critical and passive current densities showed that the behavior of coated materials differed depending on the substrate that is used. Nonetheless, in a general way, by increasing the impact angle and by changing its incidence from normal to grazing, it led to an improved resistance to erosion-corrosion processes.     

Fatigue behaviour of AISI 304 steel to AISI 4340 steel welded by friction welding

In the presented study, The weldability of AISI 304 austenitic stainless steel to AISI 4340 steel joined by friction welding in different rotational speeds and fatigue behaviour of friction-welded samples were investigated. Tension tests were applied to welded parts to obtain the strength of the joints. The welding zones were examined by scanning electron microscopy (SEM) and analyzed by energy dispersive spectroscopy (EDS). The Vıckers microhardness distributions in welding zone were determined. Fatigue tests were performed using a rotational bending fatigue test machine and the fatigue strength has been analysed drawing S-N curves and critically observing fatigue fracture surfaces of the tested samples. The experimental results indicate that mechanical properties and microstructural features are affected significantly by rotation speed and the fatigue strength of friction-welded samples decrease due to chromium carbide precipitation in welding zone with increasing rotation speed in choosen conditions.     

Bias effects on the tribological behavior of cathodic arc evaporated CrTiAlN coatings on AISI 304 stainless steel

In this study, CrTiAlN coatings were deposited on AISI 304 stainless steel by cathodic arc evaporation under a systematic variation of the substrate bias voltage. The coating morphology and properties including surface roughness, adhesion, hardness/elastic modulus (H/E) ratio, and friction behavior were analyzed to evaluate the impact of the substrate bias voltage on the coating microstructure and properties. The results suggest that for an optimized value of the substrate bias voltage, i.e. − 150 V, the CrTiAlN coatings showed increased Cr content and improved properties, such as higher adhesion strength, hardness, and elastic modulus in comparison to the coatings deposited by other substrate bias voltage. Moreover, the optimum coatings achieved a remarkable reduction in the steel friction coefficient from 0.65 to 0.45.     

Strain-rate effect on high temperature low-cycle fatigue deformation of AISI 304L stainless steel

The effect of strain rate on the behaviour of high temperature low-cycle fatigue is investigated for AISI 304L stainless steel. Regardless of the test temperature of 873 or 973 K, the fatigue life is saturated in the strain-rate range of slower than 4 × 10^–3 sec^–1. Also it is interesting to note that serrated flow, which is evidence of the occurrence of dynamic strain ageing, is clearly observed in the load-elongation hysteresis loops for strain rates that are slower (at 873 K) and faster (at 973 K) than 4 × 10^–3 sec^–1. Since the combination of temperature and strain rate is concerned with the phenomenon of dynamic strain ageing, it is considered that the above-mentioned saturated fatigue life at 873 K is caused by dynamic strain ageing and that the hardening effect due to dynamic strain ageing abnormally increases the fatigue life. However, even though the behaviour of fatigue life under strain rates slower than 4 × 10^–3 sec^–1 at 973 K has nothing to do with the dynamic strain ageing, it has been found that the failure life is also saturated in this slower strain-rate range. This behaviour is considered to be caused by the effect of creep, because the deformation under the low strain-rate activates the recovery process and as a result it causes saturation of the inelastic strain range.     

The effect of post-heat treatment of laser surface melted AISI 1018 steel

The effect of laser surface melting (LSM) and following heat treatment on the microstructure of AISI 1018 steel was investigated. The microstructures of the samples (as-received, laser treated, and heat treated) were characterized using optical microscopy, scanning electron microscopy with energy-dispersive spectrometry, and hardness testing techniques. The post-heat treatment of laser-treated samples was carried out at 900°C for 10 min followed by quenching universe water and oil. As the quenching rate decreased, the hardness of the melted layers decreased significantly, although the grain size of the melted region was still very small compared to the unaffected material. This was due to the very small austenite grain size which offered more grain-boundary area where decomposition can nucleate. Heavy Mn-Si precipitates were observed in the laser-treated layers, and post-heat treatment resulted in increased diameters of these particles owing to the lengthy time for diffusion.