Effect of gas mixture of plasma post-oxidation on corrosion properties of plasma nitrocarburised AISI 4130 steel

Plasma post-oxidizing is a process to improve the corrosion resistance of nitrocarburised parts. In this study, the effect of gas mixture of post-oxidation process on corrosion resistance of AISI 4130 plasma nitrocarburised steel has been studied. Plasma nitocarburising was carried out at 520 °C for 5 h in an atmosphere containing 49 vol% nitrogen, 49 vol% hydrogen and 2 vol% carbon dioxide. The nitrocarburised samples were post-oxidized at 450 °C for 1 h under different O₂:H₂ ratios (5:1, 1:1, 1:3, 1:5 and 1:7). The treated samples were studied using XRD, SEM, surface roughness measurement, microhardness and potentiodynamic methods. X-ray diffraction patterns revealed that with decreasing the O₂:H₂ ratio from 5:1 to 1:5, the amount of magnetite phase and the corrosion resistance of the samples were increased. It was seen that with increasing the O₂:H₂ ratio, the thickness of oxide layer was increased and the surface roughness was decreased. Furthermore, a model for oxide formation during plasma post-oxidation is proposed.     

Effect of treating atmosphere in plasma post-oxidation of nitrocarburized AISI 5115 steel

In this study quenched and tempered AISI 5115 steel was plasma-nitrided and nitrocarburized at 550 °C for 5 h in atmospheres of 80% N₂ balanced with various amounts of CO₂ and H₂ gases. The amount of CO₂ varied from 0 to 10 vol%. The highest amount of ε phases was formed in the compound layer after treating in atmosphere containing 7 vol% CO₂. Optimized compound layer was post-oxidized for 1 h at 450 °C under O₂/H₂ volume ratios of 1/1 and 3/1 as well as 100% oxygen. The treated samples were characterized using metallographic techniques, XRD, SEM, roughness measurement and potentiodynamic methods. The results showed that the growth rate of the oxide layer increased with increasing O₂ in the oxidizing gas mixture. X-ray diffraction analysis of oxidized layers confirmed the formation of highest amount of magnetite at post-oxidation in an atmosphere with the O₂/H₂ volume ratio of 1/1. Electrochemical polarization tests proved the enhancement of corrosion resistance by plasma post-oxidation and the highest corrosion resistance obtained after oxidizing under an O₂/H₂ volume ratio of 1/1.     

Prediction of dynamic recrystallisation behaviour of AISI type 4140 medium carbon steel

Abstract

The aim of the present work was to establish quantitative relationships between the flow stress and the volume fraction of dynamic recrystallisation (DRX) as a function of processing variables such as strain rate, temperature, and strain for AISI type 4140 medium carbon steel, by means of torsion tests. Torsion tests were carried out in the temperature range 900-1100°C and the strain rate range 5·0 × 10­2 -5·0 × 10⁰ s^­1 to study the high temperature softening behaviour. For the exact prediction of flow stress, the effective stress—effective strain curves were divided into two regions, the work hardening and dynamic recovery region and the DRX region. The flow stress of the DRX region could be expressed in terms of the volume fraction of DRX. It was found that the calculated results were in agreement with the experimental flow stress and microstructure of the steel for any deformation condition.     

Comparative study on corrosion behaviour of Nitinol and stainless steel orthodontic wires in simulated saliva solution in presence of fluoride ions

Localized corrosion and effects of pre-passivation treatment of Nitinol and SS304 orthodontic wires in simulated saliva solution in the presence and absence of fluoride ions were investigated by means of potentiodynamic and potentiostatic polarisations. Results revealed that Nitinol does not show pitting corrosion in saliva solution however, SS304 shows pitting corrosion. Meanwhile fluoride ion has deteriorative effect on pitting corrosion of Nitinol, while its effect on SS304 was marginally constructive. Additionally, the presence of artificial crevice has no effect on corrosion behaviour in the presence of fluoride. Pre-passivation treatment has positive influence on pitting corrosion of both alloys in the presence of F^? ions.     

Oxidation and corrosion behaviour of laser-alloyed mild steel with chromium

The oxidation and corrosion of mild steel coated with chromium powder followed by laser treatment has been investigated. Chromium powder was deposited on mild steel surface using atmospheric plasma spraying. The chromium-coated surface was irradiated separately using a pulsed Nd:YAG laser and a continuous CO₂ laser. The oxidation tests were carried out in air at 700°C and anodic polarization tests in 1N H₂SO₄ solution. There was significant difference in the morphologies of the surface alloys formed using CO₂ laser and Nd:YAG laser. Samples treated with CO₂ laser showed better oxidation resistance whereas the surface alloys formed using Nd:YAG laser showed poor oxidation resistance. Anodic polarization tests carried out on samples laser-treated with CO₂ laser showed improved corrosion resistance.     

Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N₂–15% H₂ and that for plasma nitrocarburizing was 85% N₂–12% H₂–3% CO₂. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 °C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 °C for both treatments, the amount of ε phase decreased and γ′ phase increased. Nitrocarburizing treatment resulted in formation of a more amount of ε phase with respect to nitriding treatment. However, the highest amount of ε phase was observed in the ferritic nitrocarburized sample at 570 °C. The sample nitrided at 620 °C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 °C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 °C, corrosion potential decreased due to the less ε phase development in the compound layer and more porous compound layer formed at 620 °C with respect to the treated samples at 570 °C.     

Fatigue strength improvement of a hard chromium plated AISI 4140 steel using a plasma nitriding pre-treatment

The effect of a plasma nitriding (PN) pre‐treatment on the fatigue performance of hard chromium (HC) plated AISI 4140 steel has been investigated by conducting a series of rotary bending fatigue tests at a frequency of 95 Hz. hourglass shaped test specimens of 4‐mm diameter had been plasma nitrided at 510°C for 4, 8 and 12 h. It was found that HC‐plated specimens with a coating layer of 23 ± 2 μm thickness showed approximately 33% reduction in fatigue strength when compared to quenched and tempered (Q&T) specimens. An application of the PN pre‐treatment before the plating process was effective in improving the fatigue performance of HC‐coated steel. An improvement of 71% in the fatigue strength of pre‐treated specimens was recorded as compared with the specimens, which were HC plated only. The results also indicated that prolonged nitriding time did not cause better improvement in the fatigue performance.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

Influence of fusing temperature on microstructure, wear and corrosion resistance of induction melted bimetal of Co-based alloy and AISI 4140 steel

Two bimetals composed of a Co-based alloy and AISI 4140 steel were fabricated by induction fusing at 1200 °C and 1250 °C, respectively. Their microstructures were examined and their wear and corrosion resistances were investigated using a ball-on-disc system and immersion tests. The results show that the phases of the Co-based alloy of the bimetal, fused at 1200 °C, are inherited from its original powder and consist of a Co-rich phase, a Cr-rich phase and W₃CoB₃; however, the effect of iron dilution causes the formation of σ-CrFe and Co_0.72Fe_0.28. When fusing at 1250 °C, W₃CoB₃ decomposes to form Co₃B and increases the W content in the Co-rich phase thereby reducing the corrosion rate of the Co-rich phase and resulting in improved corrosion resistance of the bimetal. However, a higher fusing temperature causes a significant drop in hardness due to severe alloy dilution by iron diffusing from AISI 4140 steel and results in declined wear resistance. The associated wear behavior also changes from abrasion wear to oxidation wear.     

Effect of post-weld heat treatment and electrolytic plasma processing on tungsten inert gas welded AISI 4140 alloy steel

Post-weld heat treatment (PWHT) is commonly adopted on welded joints and structures to relieve post-weld residual stresses; and restore the mechanical properties and structural integrity. An electrolytic plasma process (EPP) has been developed to improve corrosion behavior and wear resistance of structural materials; and can be employed in other applications and surface modifications aspects. In this study the effects of PWHT and EPP on the residual stresses, micro-hardness, microstructures, and uniaxial tensile properties are explored on tungsten inert gas (TIG) welded AISI-4140 alloys steel with SAE-4130 chromium–molybdenum alloy welding filler rod. For rational comparison all of the welded samples are checked with nondestructive Phased Array Ultrasonic Testing (PAUT) and to ensure defect-free samples before testing. Residual stresses are assessed with ultrasonic testing at different distances from weld center line. PWHT resulted in relief of tensile residual stress due to grain refinement. As a consequence higher ductility but lower strength existed in PWHT samples. In comparison, EPP-treated samples revealed lower residual stresses, but no significant variation on the grain refinement. Consequently, EPP-treated specimens exhibited higher tensile strength but lower ductility and toughness for the martensitic formation due to the rapid heating and quenching effects. EPP was also applied on PWHT samples, but which did not reveal any substantial effect on the tensile properties after PWHT at 650 °C. Finally the microstructures and fracture morphology are analyzed using scanning electron microscopy (SEM) and optical microscope to study the evolution of microstructures.     

Effect of plasma electrolytic oxidation treatment on the corrosion and stress corrosion cracking behaviour of AM50 magnesium alloy

The effect of a silicate-based plasma anodization treatment on the corrosion and stress corrosion cracking behaviour of a cast AM50 magnesium alloy was studied. Electrochemical tests revealed the beneficial effect of the plasma electrolytic oxidation (PEO) in improving the corrosion resistance of the alloy. Although the coating had provided an improved resistance to stress corrosion cracking in this test environment at a nominal strain rate of 10⁻⁶ s⁻¹, it could not completely eliminate the SCC susceptibility of the alloy. Cracking of the coating under conditions of straining was found to be the reason for SCC of PEO-coated alloy.     

Mechanical and corrosion behaviour of powder metallurgy stainless steel based metal matrix composites

Abstract

Stainless steel matrix composites were manufactured using powder metallurgy techniques. Matrixes of AISI 316L stainless steel were reinforced with yttria or alumina particles. Chromium diboride was added in some cases and boron nitride in others to obtain steels with densities close to theoretical, using reactive (liquid phase) sintering techniques. The composites showed very good densification and better hardness than the 316L stainless steel without additions. The 316L steel reinforced with 4 wt-% yttria chromium diboride showed the highest density and strength, with an acceptable corrosion resistance.     

Comparative hot corrosion studies of EB-PVD and plasma sprayed coatings*

The use of directly fired heat engines, i. e. diesel and gas turbine engines, is likely to grow in the future. However, economic requirements will necessitate the development of highly efficient engines that operate at high temperatures and utilize low-grade fuels. An increase of combustion gas temperature and the utilization of low-grade fuels contaminated with, amongst others, sulphur and vanadium, may be accomplished by careful selection of the materials, e.g. by applying corrosion-resistant coatings on heatresistant alloys. The main purpose of the present work has been to test various coatingsubstrate combinations in a highly corrosive environment. The comparison of various coating techniques, e.g. electron beam physical vapour deposition (EB-PVD), air plasma spraying (APS), and low pressure plasma spraying/vacuum plasma spraying (LPPS/VPS), has been emphasized.     

Evaluation of hot corrosion behaviour of HVOF sprayed NiCrAl coating on superalloys at 900 °C

In the present investigation, NiCrAl coating was deposited on Ni- and Fe-based superalloy substrates by using high velocity oxy-fuel (HVOF) process to study the hot corrosion behaviour in molten salt (Na₂SO₄–60% V₂O₅) environment at 900 °C under cyclic conditions. The mass gain measurements were performed after each cycle to establish the kinetics of corrosion using thermogravimetric technique. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopic analysis (SEM/EDS) and X-ray mapping techniques were used to analyse the corrosion products. The bare superalloys experienced higher weight gain. The NiCrAl-coated Superni 750 alloy (SN 750) provided a better protection among the coated superalloys investigated. The formation of oxides and spinels of nickel, chromium and aluminum may be contributing better resistance to hot corrosion.     

Effect of BTA on electrochemical corrosion and stress corrosion cracking behaviour of type 304 stainless steel in 1 M HCl

Effect of benzotriazole (BTA) on polarization and stress corrosion cracking (SCC) behaviour of type 304 stainless steel in 1 M HCl was investigated. The anodic polarization curves showed that with BTA additions the anodic polarization kinetics in the active region was not affected, though a reduction in critical current density, i _crit, and passive current density, i _p, was observed. However, BTA was found to influence significantly the cathodic reaction kinetics. SCC results using smooth tensile test specimens showed an increase in time-to-failure, t _f, with BTA additions. Crack growth rate studies using single-edge notched (SEN) specimens showed an increase in threshold stress intensity for SCC, K _ISCC, and a decrease in crack growth rate, da/dt, with BTA additions. While the adsorption isotherms derived from weight loss data followed a Langmuir adsorption isotherm signifying a monolayer adsorption, the adsorption isotherms derived from SCC test data deviated from this at higher BTA concentrations. The paper discusses the possible reason for this deviation.