Kenan Genel

Effect of ion nitriding on fatigue behaviour of AISI 4140 steel

Abstract Ion nitriding has become a popular thermo-chemical surface treatment, which is being used to develop fatigue and wear characteristics of steels. Besides the increased strength, the formation of high compressive residual stresses in the case region causes remarkable improvement in fatigue properties of steels. In this study, the effect of case depth on fatigue performance of AISI 4140 low alloy steel has been investigated by conducting a series of rotary bending fatigue tests at the frequency of 95 Hz, on hour glass shaped, 4 mm diameter specimens, which had been ion nitrided at 748 K for 1, 3, 8 and 16 h. Up to 50% improvement in fatigue strength of the steel has been attained by ion nitriding, depending on the case depth formed during the process. The comparison of test results between ion and liquid nitrided specimens having the same case and core properties, has shown that a 12% increase in fatigue strength could be reached by ion nitriding with respect to conventional, salt bath nitriding process. An attempt was made to establish some empirical relationships between the parameters defining relative case depth, which consider case depth and specimen size together, and fatigue strength of ion nitrided steel. It was obtained that the subsurface ‘fish eye’ type crack formation is the dominant fatigue crack initiation mechanism in ion nitrided AISI 4140 steel, and a map has been constructed to illustrate the locations of crack origins depending upon the case depth and the cyclic stress acting on specimens. It was also tried to explain the conditions promoting the ‘fish eye’ crack formation by analysing the combined effects of residual and applied stress patterns as well as inclusion size.

Kinetics of boriding of AISI W1 steel

Abstract A technologically interesting characteristic of boriding is the production of a hard, wear-resistant coating layer on the steel substrate. In this study, case properties of borided AISI W1 steel has been investigated by conducting a series of experiments in Ekabor-I powders at the process temperature of 1123–1323 K at 50 K intervals for periods of 1–8 h. The presence of borides FeB and Fe 2 B formed on the surface of steel substrate was confirmed by optical microscopy and X-ray diffraction. Cross-sectional observation in the optical microscope revealed smooth and compact morphology of the borided layer. The distribution of alloy elements from the surface to the interior was confirmed by energy dispersive X-ray spectroscopy. The hardness of the boride layer formed on the surface of the steel substrate was higher than 1500 HV. It was concluded that the optimum temperature for AISI W1 steel borided in Ekabor-I powders was approximately 1223 K for hardness in 10 μm depth, and the hardness change with boriding temperature was related to the grain size of the treated steel. The kinetics of boriding show a parabolic relationship between layer thickness and process time, and the activation energy for the process is 171.2±16.6 kJ mol −1 . Moreover, an attempt was made to investigate the possibility of predicting the iso-thickness of boride layer variation and to establish an empirical relationship between process parameters of boriding and boride layer.

Boriding response of AISI W1 steel and use of artificial neural network for prediction of borided layer properties

Abstract In the present study, boriding response of AISI W1 steel and prediction of boride layer properties were investigated by using artificial neural network (ANN). Boronizing heat treatment was carried out in a solid medium consisting of Ekabor-I powders at 850–1050 °C at 50 °C intervals for 1–8 h. The substrate used in this study was AISI W1. The presence of borides FeB and Fe 2 B formed on the surface of steel substrate was confirmed by optical microscope and X-ray diffraction analysis. The hardness of the boride layer formed on the surface of the steel substrate was over 1500 VHN. Experimental results indicated that there is a nearly parabolic relationship between boride layer and process time for higher temperatures. Optical microscope cross-sectional observation of the borided layer revealed columnar and compact morphology. Moreover, an attempt was made to investigate possibility of predicting the hardness and depth of boride layer variation and establish some empirical relationship between process parameter of boriding and boride layer, and hardness changes using back-propagation learning algorithm in ANN. Modelling results have shown that hardness and depth of boride layer were predicted with high accuracy by ANN.

Corrosion fatigue behaviour of ion nitrided AISI 4140 steel

Abstract Machine components suffer from corrosion degradation of fatigue characteristics and improvement can be attained by the application of a nitriding treatment, particularly to low alloy steels. In the present study, the effect of ion nitriding on corrosion fatigue performance of AISI 4140 steel has been investigated by conducting a series of rotary bending corrosion fatigue tests at 95 Hz, in 3% NaCl aqueous solution. Hourglass shaped, 4 mm diameter fatigue specimens were ion nitrided at 748 K for 1, 3, 8 and 16 h prior to the tests. It was observed that distinct fatigue limit behaviour of ion nitrided steel in air completely disappeared in corrosive environment besides severe degradation in fatigue characteristics. An improvement reaching to 60% in corrosion fatigue strength can be attained by successive ion nitriding practice based on a fatigue life of 10 7 cycles. An attempt was made to establish an empirical relationship between corrosion fatigue strength and relative case depth, which considers the size of the ion nitrided specimen. It was also determined that a power relationship holds between corrosion fatigue strength and fatigue life of ion nitrided steel. The presence of white layer has resulted in additional improvement in corrosion fatigue resistance, and it was observed that corrosion fatigue cracks were initiated dominantly under the white layer by pit formation mechanism.

Use of artificial neural network for prediction of ion nitrided case depth in Fe–Cr alloys

Abstract In this work, a simple artificial neural network (ANN) model using back-propagation training algorithm for ion nitriding behaviour of Fe–Cr alloys was established. The case depth data were extracted from experimental data and used in the formation of training sets of ANN in order to predict case depth of ion nitrided Fe–Cr alloys, 2.5% Cr intervals for 5–20% Cr. The modelling results confirm the feasibility of this approach and show good agreement with experimental data by Alves et al. (Mater Sci Eng 2002; 279A: 10–15) with high accuracy. A contour diagram as a function of Cr (wt.%) and ion nitriding time for Fe–Cr alloy was constructed for industrial application. It is concluded that a considerable saving in terms of cost and time could be obtained from using the trained ANN model and, it provides more useful data from relatively small experimental databases.

Effect of cathodic polarisation on corrosion fatigue behaviour of ion nitrided AISI 4140 steel

Abstract It is well known that controlled changes in the electrode potential of specimens strongly influence the behaviour of corrosion fatigue for alloys in aqueous media. In other words, the effect of polarisation on the corrosion fatigue performance of steel can be handled depending on the magnitude of applied potential. In the present work, the effect of applied polarisation potential from −750 mV (SCE) to −1500 mV (SCE) on the corrosion fatigue performance of AISI 4140 steel has been investigated by conducting a series of rotary bending, corrosion fatigue tests at 95 Hz, in aerated 3% NaCl aqueous solution. Hourglass shaped, 4 mm diameter fatigue specimens were ion nitrided at 748 K for 1 h prior to tests. It was found that corrosion fatigue life was enhanced by increasing the applied polarisation potential from −750 mV (passive range) to −1080 mV (cathodic range). Extremely rare and shallow pit formation was detected at the surface of specimens under the influence of −1080 mV potential, and the corrosion fatigue strength was increased by 2.6 times the free corrosion fatigue strength based on a fatigue life of 10 6 cycles. An improvement of 93% in fatigue strength could be attained by cathodic protection referring to the data in air for a life of 3×10 6 cycles. An attempt was made to quantify the progress in corrosion fatigue performance by using the parameter of protection ratio, PR. Over-cathodic protection at −1500 mV adversely affected the corrosion fatigue performance of ion nitrided steel due to hydrogen embrittlement, which yields an easy intergranular type of crack initiation and propagation, particularly in the case of the ion nitrided region.

Bending and lateral crushing behavior of a GFRP and PA6 reinforced aluminum square tube

Abstract In this study, the bending behavior of a thin-walled aluminum square tube (AST) were analyzed using the finite element (FE) method, and reinforcing arrangements were decided for the composite beams based on the FE results. Accordingly, bending behavior of thin-walled ASTs with internal cast polyamide (PA6) and external glass fiber reinforcement polymers (GFRP) were investigated via a quasi static three-point bending test experimentally. Moreover, bending performance under impact loading was also investigated experimentally. The results revealed that local buckling has a decisive influence on bending performance of the tube, and the contribution of inner reinforcement is more effective than outer reinforcement. The reinforcements provide 575 % and 312 % increases in bending load and impact energy, respectively. The developed plastic-metal hybrid-composite structure is promising especially for critical supporting parts in vehicles. It is thought that the combination of these materials will offer new focus of attention for designers seeking more appropriate composite beams with high bending load and impact resistance.

FE simulation of plastic collapse and geometrical factors affecting the bending response of a tubular aluminum beam

Abstract In this study, bending response of circular aluminum tube and factors affecting collapse behaviour were investigated by FE simulations. Geometrical parameters such as wall tube thickness, tube diameter, support distances, punch diameter and material properties were considered in analyses systematically. The results of analyses revealed that local buckling plays an important role on collapse load and buckling displacement is directly proportional to tube diameter, regardless of D/t and yield strength of tube material. The ratio of buckling displacement to tube diameter corresponds to 40 % of tube diameter. Moreover, an attempt was made to construct an equation as a function of D, t and yield strength for estimating collapse load. Finally, ANOVA test was used to analyze the influence of some parameters such as diameter of tube, wall thickness and yield strength. It is found from the test that the most significant factor on maximum load is wall thickness (79.6 %).

The effect of foam and PA6 reinforcement on bending performance of 6063-T5 tube

Abstract The influence of cast polyamide (PA6) and polyvinyl chloride (PVC) foam, which have different lengths, on the bending performance of a thin-walled aluminium tube was investigated experimentally. Three-point bending test was performed to determine load carrying and energy absorbing capacity (EAC) of composite tubes. The experimental results indicated that the reinforcement of PVC foam relatively enhanced buckling displacement but its effect on load carrying capacity (LCC) and EAC is negligible. PA6 and PVC foam-reinforced composite beam shows superior bending resistance with higher buckling displacement. The improvement in bending performance of composite specimens arises from the hindering of local buckling as well as increasing bending stiffness. Accordingly, LCC and EAC increased a maximum of 3.6 and 3.9 times, respectively. The developed composite structure is promising for parts serving as support member of vehicles for which light weight is a critical design consideration.