Structure and properties of nitrocarburized diffusion layers generated on high-speed steels

This work analyzes the structure and properties of nitrocarburized diffusion cases generated on M2 type high-speed and 321 stainless steels in a thermochemical. Application of variable process temperatures in the range of 450–600 °C and a variable process duration (2–6 h) enabled observation of growth kinetics of the layers on tested steel grades. Evaluation of properties of the cases obtained comprised hardness measurements and wear tests, carried out by the 3 cylinder-cone method. The evaluation showed that the nitrocarburizing process developed for high-speed and stainless steels yields hard surface layers with beneficial functional properties.     

The effect of erosion on the electrochemical properties of AISI 1020 steel

The mechanisms and characteristics of the mechano-electrochemical effect were investigated for eroded AISI 1020 steel specimens. The effect of erosion on open-circuit potential, corrosion potential, linear polarization resistance, and corrosion rate was determined. The corrosion rate was exponentially related to the decrease in corrosion potential and increase in the strain energy. The corrosion rate was quantitatively calculated using a mechano-electrochemical dynamic equation, which is based on the decrease in corrosion potential and increase in strain energy. The calculated rate agrees well with the experimentally measured corrosion rate.     

Plasma post oxidation of nitrocarburized AISI 4140 steel

Plasma nitrocarburizing and plasma oxidizing treatments were performed to improve the wear and corrosion resistance of AISI 4140 steel.Plasma nitrocarburizing was conducted for 3 h at 570 ℃ in the nitrogen, hydrogen and methane atmosphere to produce the ε-Fe2-3(N,C) phase.It was found that the compound layer produced by plasma nitrocarburising was predominantly composed of ε-phase, with a small proportion of γ'-Fe4(N,C) phase.The thickness of the compound layer was about 10 μm and the diffusion layer was about 300 μm in thickness, respectively.Plasma post oxidation was performed on the nitrocarburized samples with various oxygen/hydrogen ratio at a constant temperature of 500 ℃ for 1 h.The very thin magnetite (Fe3O4) layer 1-2 μm in thickness on top of the compound layer was obtained by plasma post oxidation.It was confirmed that the corrosion characteristics of the nitrocarburized compound layer can be further improved by the application of the superficial magnetite layer.     

Effects of boronizing process on the surface roughness and dimensions of AISI 1020, AISI 1040 and AISI 2714

In this study, the effects of boronizing treatment on material's dimensional changes and surface roughness were investigated. The parameters chosen were substrate material composition, surface roughness before boronizing treatment, and boronizing time. The AISI 1020, AISI 1040 and AISI 2714 were chosen as substrate materials whereas Ekabor I was selected as boronizing powder. Materials were boronized at 900 °C by using a solid boronizing method for 2 or 4 h. The gradual growth of boride layer on the surface, dimensional changes and their effects on surface roughness were investigated. Variations in topographical surface roughness were determined by SEM. With the boronizing treatment, dimensional increases of the material's were observed. The dimensional increase was one fifth of boride layer thickness for AISI 1020 or AISI 1040, whereas it was one third of boride layer thickness for AISI 2714. Boronizing treatment had also a significant effect on surface roughness of materials. A “threshold roughness” term was defined in our study. This term is a surface roughness value for smooth surfaces, which received boronizing treatment. For the same material and with the same boronizing conditions, the threshold roughness value was achieved after boronizing, when surface roughness of material was below the threshold roughness value, before boronizing was applied. However, when surface roughness of a material was above that threshold roughness value, surface roughness value decreased with boronizing treatment. The thereshold roughness value depended on substrate material composition and boronizing parameters.     

Microbiologically influenced corrosion of AISI type 304 stainless steels under fresh water biofilms

The influence of biofilms formed by microorganisms on the corrosion behaviour of stainless steel (SS) in various fresh water environments was studied using electrochemical techniques. Biofilm characterization studies showed an algal‐dominated biofilm in the normal dark and light exposures, whereas, in the continuous dark exposures, bacterial‐dominated biofilm was observed. The open circuit potential (OCP) and the breakdown potential (E_b) of the specimens with biofilms showed a positive shift compared to those of the freshly polished specimens. Though ennoblement of breakdown potential showed protective nature of biofilm, ageing of the algal‐dominated biofilm was found to initiate localised corrosion of the substratum due to the heterogeneities in the biofilm, creating anodes and cathodes. In the case of specimens containing 123‐day old algal‐dominated biofilm, steady increase in current density was observed during polarization starting from OCP which is indicative of initiation and steady propagation of crevice corrosion.     

Atomic Force Microscopy, Scanning Kelvin Probe Force Microscopy and magnetic measurements on thermally oxidized AISI 304 and AISI 316 stainless steels

Thermally oxidized AISI304 and AISI316 stainless steels are studied by Atomic Force Microscopy, Scanning Kelvin Probe Force Microscopy (SKPFM) and Magneto-Optical Kerr effect as a function of their growth temperature. The surface roughness is a competition between the roughness of the bare substrate and the roughness resulting from the oxide layer growth. Cr oxide is present at some places on the surface at low growth temperature as shown by SKPFM. The observed decrease of surface potential with the oxide layer thickness indicates an effective protection against corrosion. Magnetic measurements demonstrate that the outer layer contains a magnetite phase (in-plane magnetization).     

Microstructure and wear properties of AISI 1020 steel surface modified by HARDOX 450 and FeB powder mixture

The effects of gas tungsten arc-processing (GTA) was used for developing wear resistance of AISI 1020 steel substrate. Appropriate quantities of FeB powder and Hardox 450 were combined to create conditions that synthesize particles into reinforced Fe-based composite surface coating. The phase transformations on new created coated surfaces were comprehensively examined by using a combination of scanning electron microscopy (SEM), microanalysis by energy dispersive spectrometry (EDS), X-Ray diffraction (XRD), microhardness and abrasive wear tests. The microstructure studies of the superficial layers of the coating revealed presence of a mixture of the dendritic phase structure of ferrit (α) and FeB-Fe₂B-Fe₃B borides. The results show that; the size of dendrites formed in the coated surface, the change of hardness of the coated surfaces, the borides volume rate and thickness of the coating changed by the variation of the processing parameters. The studies concluded that Hardox 450 + 40 wt % FeB composite coating was the most appropriate combination in terms of hardness and wear performance.     

Influence of processing parameters on the characteristics of surface layers of low temperature plasma nitrocarburized AISI 630 martensitic stainless steel

Plasma nitrocarburizing was performed on solution-treated AISI 630 martensitic precipitation hardening stainless steel samples with a gas mixture of H₂, N₂, and CH₄ with changing temperature, discharge voltage and amount of CH₄. When nitrocarburized with increasing temperature from 380 °C to 430 °C at fixed 25% N₂ and 6% CH₄, the thickness of expanded martensite (α'_N) layer and surface hardness increased up to 10 μm and 1323 HV_0.05, respectively but the corrosion resistance decreased. Though the increase of discharge voltage from 400 V to 600 V increased α'_N layer thickness and surface hardness (up to 13 μm and 1491 HV_0.05, respectively), the treated samples still showed very poor corrosion behavior. Thus, to further improve the corrosion resistance, the influence of variation of the amount of CH₄ in the nitrocarburizing process was investigated. Increasing the CH₄ percentage aided higher corrosion resistance, although it decreased the α'_N layer thickness. The most appropriate conditions for moderate α'_N layer thickness, high surface hardness and better corrosion resistance than the solution-treated bare sample were established, which is plasma nitrocarburizing at 400 °C with 400 V discharge voltage and containing 25% N₂ and 4% CH₄.     

Influence of peening on the corrosion properties of AISI 304 stainless steel

The effects of peening treatment on the microstructure and corrosion behavior of AISI 304SS were investigated. Shot and ultrasonic peening were performed on the austenitic stainless steel, and peened specimens were compared in terms of microstructure, surface roughness and corrosion resistance. Nano-sized grains, multi-directional mechanical twins and strain-induced martensite were formed on the surfaces, and the volume fraction of strain-induced martensite in the ultrasonically peened specimen was higher than that of the shot-peened specimen. The ultrasonically peened specimen which had smoother surface and contained more strain-induced martensite showed superior general and localized corrosion resistance to the as-received and shot-peened specimens.     

离子渗氮AISI 420马氏体不锈钢耐蚀行为研究

采用不同温度对AISI 420马氏体不锈钢进行离子渗氮处理.借助光学显微镜和X射线衍射(XRD)技术分析了渗氮层的微观组织结构,利用显微硬度计测试了渗氮层的硬度分布,通过电化学极化曲线测试和盐雾腐蚀试验研究了离子渗氮AISI 420不锈钢在模拟工业环境中的腐蚀行为.结果表明:AISI 420不锈钢350℃低温离子渗氮层由ε-Fe3N和N过饱和固溶体αN相组成,其化学稳定性高,加之固溶Cr元素的联合作用,明显提高了AISI 420不锈钢基材的腐蚀抗力.AISI 420钢经450℃和550℃渗氮处理,渗氮层中的αN分解成了α相和CrN,造成基体贫Cr,降低了基材的耐蚀性能.马氏体不锈钢低温离子渗氮处理不仅可以提高表面硬度,而且可以获得良好的耐蚀性能.     

Effect of processing temperatures on characteristics of surface layers of low temperature plasma nitrocarburized AISI 204Cu austenitic stainless steel

The influence of processing temperatures on the surface characteristics of AISI 204Cu austenitic stainless steel was investigated during a low temperature plasma nitrocarburizing.The resultant layer was a dual-layer structure,which comprises a N-enriched layer on the top of C-enriched layer.The surface hardness and the layer thickness increase up to about HV 0.05 1000 and 20μm with increasing temperature.The specimen treated at 400°C shows a much enhanced corrosion resistance compared to the untreated steel.A loss in corrosion resistance was observed for specimens treated at temperatures above 430°C due to the formation of Cr2N.     

Microstructure and corrosion behaviour of plasma‐nitrocarburized sintered steel

Powder characteristics and manufacturing processes determine the microstructure, and therefore, the physical, chemical, and mechanical properties of sintered steels. In particular, porosity and corrosion resistance are intimately related, since the contact area between substrate and electrolyte significantly affects the corrosion resistance of sintered steels. This study addresses the effect of powder characteristics and pressing parameters on the microstructure and corrosion resistance of low‐carbon sintered and sintered/plasma‐nitrocarburized steel. The results indicated that the corrosion resistance increased with increasing density and decreasing specific surface area. Additionally, plasma‐nitrocarburizing was highly effective in coating open pores of the material.     

17-4PH钢等离子体稀土氮碳共渗层的组织结构和性能

利用OM、XRD、扫描电镜(SEM)、能谱仪(EDS)、显微硬度计和摩擦磨损试验机研究了17-4PH马氏体沉淀硬化不锈钢在500 ℃脉冲等离子体稀土氮碳共渗层的组织结构和性能.结果表明,稀土添加使共渗层致密;共渗层主要由α'、ε-Fe3N、γ'-Fe4N及CrN组成;添加稀土或减小氮氢比,渗层中ε-Fe3N相的比例降低.表面形貌SEM观察和EDS成分分析结果表明,共渗表面主要由弥散分布的(Fe,Cr)xNy化合物颗粒、颗粒团和颗粒团簇,以及少量"柱状"生长的γ'-Fe4N相组成;当氮氢比为1:6时,稀土氮碳共渗层表面硬度可达1580 HV0.1.随磨损时间延长,稀土添加较无稀土添加的共渗层摩擦系数变化较小.     

稀土对42CrMo钢等离子体氮碳共渗组织及性能的影响

在渗剂中添加稀土,在42CrMo钢表面进行了等离子体氮碳共渗试验,并对渗层的组织形貌、显微硬度及接触疲劳强度进行了测试和分析,研究了稀土对等离子体氮碳共渗层的影响.结果表明,稀土在短时间内的催渗效果优于长时间,本次试验以低于8 h的效果最为明显;稀土有细化晶粒,使渗层组织结合得更加致密的效果,还可提高次表层的显微硬度和接触疲劳强度.     

The semiconducting properties of passive films formed on AISI 316 L and AISI 321 stainless steels: A test of the point defect model (PDM)

The semiconductor properties of passive films formed on AISI 316L in 1 M H₂SO₄ in three temperatures and AISI 321 in 0.5 M H₂SO₄ were studied by employing Mott–Schottky analysis in conjunction with the point defect model (PDM). Based on the Mott–Schottky analysis in conjunction with PDM, it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. Also, the results indicated that donor densities increased with temperature. By assuming that the donors are oxygen ion vacancies and/or cation interstitials, the diffusion coefficients of the donors for two stainless steels are calculated.     

A study of martensitic stainless steel AISI 420 modified using plasma nitriding

We studied martensitic stainless steel AISI 420, modified using glow discharge plasma nitriding. Microhardness measurements, X-ray diffraction (XRD), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to investigate the surface microhardness, crystal structure, microstructure and chemical bonding in the modified surfaces. High surface microhardness (1300 HV) over a case depth of about 60 microns is obtained. Glancing incidence X-ray diffraction (GIXRD) indicates the presence of a predominantly Fe₃N phase with dispersed CrN within 2–5 microns on the surface. In addition, using Bragg–Brentano geometry, we measured the presence of a minor phase of Fe₄N in the case depth. SEM confirms that the microstructure within 2–5 microns of the surface is different from that of the bulk. XPS shows nitride phase formation on the surface. AES measured over the cross-section of the case depth shows a direct relation of the increased surface microhardness to the high nitrogen content.     

Influence of Ti,C and N concentration on the intergranular corrosion behaviour of AISI 316Ti and 321 stainless steels

Intergranular corrosion behaviour of 316Ti and 321 austenitic stainless steels has been evaluated in relation to the influence exerted by modification of Ti, C and N concentrations. For this evaluation, electrochemical measurements – double loop electrochemical potentiokinetic reactivation (DL-EPR) – were performed to produce time–temperature–sensitization (TTS) diagrams for tested materials. Transmission (TEM) and scanning electron microscopy (SEM) were used to determine the composition and nature of precipitates. The addition of Ti promotes better intergranular corrosion resistance in stainless steels. The precipitation of titanium carbides reduces the formation of chromium-rich carbides, which occurs at lower concentrations. Also, the reduction of carbon content to below 0.03 wt.% improves sensitization resistance more than does Ti content. The presence of Mo in AISI 316Ti stainless steel reduces chromium-rich carbide precipitation; the reason is that Mo increases the stability of titanium carbides and tends to replace chromium in the formation of carbides and intermetallic compounds, thus reducing the risks of chromium-depletion.     

Microstructural features and corrosion properties in laser welded duplex stainless steels

Synopsis

The most common techniques of underwater welding and cutting used in practice are described. In spite of the adverse effect of pressure, present‐day technology enables good quality joints to be made. The local dry chamber, and wet‐welding with a shielded electrode methods, characterised by their low cost, are used at shallow depths down to 60 m. The hyperbaric, dry method of mechanised TIG and MIG welding is used at depths down to 200 m. Results of underwater automatic cutting by an arc‐oxygen method at depths down to 600 m, and the equipment used are discussed.     

A corrosion prediction method for weathering steels

Japanese Specification for Highway Bridges [Japan Road Association: Specification for Highway Bridges—I & II, 2002.3] has been revised in 2002, which clearly states that degradation of bridge members, including those of weathering steels, to occur in prolonged period of time must be taken into account at the design stage to realize long term structural durability. To cope with the revised paradigm, SABI chemistry committee in Japan Society of Corrosion Engineers has proposed durable state concept [SABI Chemistry Committee of Japan Society of Corrosion Engineers, in: Proc. 132nd Symposium for Corrosion and Protection, 2001.6.25, p. 3], which is defined as a condition in which corrosion rate of steel is slow enough not to form thick rust [H. Kihira, K. Shiotani, H. Miyuki, T. Nakayama, M. Takemura, Y. Watanabe, Doboku Gakkai Ronbun-shu (J. Japan Soc. Civil Engineers) No. 745/I-65, 2003.10, pp.77-87]. Since corrosiveness of atmosphere differs by location, a corrosion prediction method for both conventional and advanced weathering steels [H. Kihira, A. Usami, K. Tanabe, M. Ito, G. Shigesato, Y. Tomita, T. Kusunoki, T. Tsuzuki, S. Ito, T. Murata, in: Electrochemical Society Proceedings, vol. 99-26, 2000, pp. 127-136] is needed to ensure structural durability for future. Thus, versatile computational scheme for it has been derived through mathematical modeling based upon the durable state concept.     

Study of corrosion protection of different stainless steels by nanocrystalline plasma electrolysis

The effect of nitrogen and carbon bombardment on the corrosion behavior of three different stainless steels (AISI 304, AISI 316L and AISI 430) is discussed in this work. Nitrogen and Carbon was entered into the lattice in order to generate a rich region near the surface by a relatively new method called plasma electrolysis. Different ion bombardment doses under different applied voltages and thus different electrical fields (230, 245, 260 volts) at 330 mS/Cm of electrical conductivity of specific electrolyte have been tested to optimize the ion bombardment doses for each steel. The corrosion measurements were carried out in sodium chloride solution by using potentiodynamic scanning (PDS) and electrochemical impedance spectroscopy (EIS). The experimental results showed that the effect of N and C ion bombardment mainly depends on the microstructure and/or composition of the stainless steels. Less compact structures and/or less amount of alloying elements (as occurs with the body centered cubic, ferritic AISI 430) achieve bigger changes with this modification, whereas on stainless steels with a larger amount of alloying elements and / or more compact structures (like the face centered cubic, austenitic AISI 316L) ion bombardment slightly modifies the corrosion behavior. Original Russian Text ? M.Kh. Aliev, A. Sabour, T. Shahrabi, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 4, pp. 432–437. The text was submitted by the authors in English