真空低压渗碳对304与316L奥氏体不锈钢组织和性能的影响

采用真空低压渗碳技术对304和316L奥氏体不锈钢进行表面强化,利用光学显微镜、扫描电镜、Thermo-Calc热力学软件、X射线衍射仪和显微硬度计等对渗碳层显微组织、相组成及硬度分布进行分析表征,计算了奥氏体不锈钢渗碳层中不同衍射峰的偏移量及渗碳前后晶格常数的变化量。结合钼对奥氏体不锈钢渗碳过程的影响,对比研究了304和316L奥氏体不锈钢渗碳后,在渗碳层深度、表面硬度及碳化物的析出规律等方面的差异。结果表明,经750 ℃真空渗碳2.6 h后,304和316L奥氏体不锈钢晶格常数分别增加了1.33%和1.14%,形成了由膨胀奥氏体和Cr₂₃C₆组成的渗碳层,Cr₂₃C₆在渗碳层中主要以条状沿膨胀奥氏体晶界析出,表面硬度较基体硬度均提升了两倍以上。     

A Hybrid Low Temperature Surface Alloying Process for Austenitic Stainless Steels

This paper describes a novel, hybrid process developed to engineer the surfaces of austenitic stainless steels at temperatures below 450℃ for the improvement in wear and corrosion resistance. The process is carried out in the plasma of a glow discharge containing both nitrogen and carbon reactive species, and facilitates the incorporation of both nitrogen and carbon into the austenite surface to form a dual-layer structure comprising a nitrogen-rich layer on top of a carbon-rich layer.Both layers can be precipitation-free at sufficiently low processing temperatures, and contain nitrogen and carbon respectively in supersaturated fcc austenite solid solutions. The resultant hybrid structure offers several advantages over the conventional low temperature nitriding and the newly developed carburizing processes in terms of mechanical and chemical properties, including higher surface hardness, a hardness gradient from the surface towards the layer-core interface, uniform layer thickness, and much enhanced corrosion resistance. This paper discusses the main features of this hybrid process and the various structural and properties characteristics of the resultant engineered surfaces.     

The wear and corrosion properties of stainless steel nitrided by low-pressure plasma-arc source ion nitriding at low temperatures

Low pressure plasma arc discharge-assisted nitriding of AISI 304 austenitic stainless steel is a process that produces surface layers with useful properties such as a high surface hardness of approximately 1500 Hv_0.1 and a high resistance to frictional wear and corrosion. The phase composition, the thickness, the microstructure and the surface topography of the nitrided layer, as well as its properties, depend essentially on the process parameters. Among them, the processing temperature is the most important factor for forming a hard layer with good wear and corrosion resistance. Nitriding austenitic stainless steel at approximately 420°C for 70 min can produce a thin layer of 7–8 μm with very high hardness and good corrosion resistance on the surface. The microstructure was studied by optical microscopy and both glancing angle and conventional Bragg–Brentano (θ–2θ) symmetric geometry X-ray diffraction (XRD). The formation of expanded austenite was observed. Measurements of the wear depths indicated that the wear resistance of austenitic stainless steel can be improved greatly by nitriding at approximately 420°C using low-pressure plasma-arc source ion nitriding.     

Solid Solution Nitriding Technology of 15Cr-7.5Mn-2.6Mo Duplex Stainless Steel

NEW DEVELOPMENT of stainless steel mainlyoccurs in area of nitrogen-containing austenitic steeland austenitic-ferritic duplex steel.It has been proposedthat a nitrogen-alloyed steel can be considered as anhigh nitrogen steel(HNSs),when the nitrogenconcentration is higher than0.4wt%in austenitic steelsand higher than0.08wt%in martensitic steels'M1.Butthis concept has been developed for the newmanufacturing technologies of HNS,such as pressureor powder metallurgy.In addition,the chemical…     

Stainless steel patterning by combination of micro-patterning and driven strain produced by plasma assisted nitriding

Plasma-assisted nitriding treatments on austenitic stainless steel at low temperature produce the so called “expanded austenite”. The expansion of the nitrided layer that occurs from the initial surface of the substrate in a direction perpendicular to the surface is used here as a mean to produce patterned surfaces by selective nitrogen diffusion through masks. Using grids, a network of well defined square dots can be obtained. In this communication, the results of nitriding treatments on austenitic stainless steel substrates previously covered by a patterned silicon oxide layer are presented. The interactions mechanisms at the interface between fixed silicon oxide mask with several different shapes (circular and square dots) and the expanded austenite are also described. The role of nitrogen diffusion, consistent with the experimental conditions and the mask characteristics, is shown to be very important. Depending on the size of the dots, it leads to a simple uniform mask deformation or to a significant mask deformation with strong distortions at the edges. This phenomenon is noted as a toroidal-shell shape distortion. Optical cross-section seems to prove that it is only the result of the vertical force due to the austenite expansion induced by nitrogen diffusion just under the mask edges.     

450℃渗氮AISI 316L不锈钢中膨胀奥氏体的分解

奥氏体不锈钢等离子渗氮时会形成膨胀奥氏体(γN),其强度、韧性和耐蚀性均高于传统的氮化物层。然而,膨胀奥氏体在热力学上是亚稳的,其性能会因其发生分解而降低。本文对AISI 316L不锈钢进行了等离子渗氮,工艺为450℃×5 h,压力500 Pa。对渗氮后试样采用XRD、OM和TEM等进行了显微组织表征。试验结果证明了面心立方膨胀奥氏体的存在,其晶格常数比未经渗氮奥氏体增加多达9.5%。薄片试样的TEM分析表明,N层中有细小的氮化物形成,并且发现一些区域呈奇特的层片状形貌,与碳钢中的珠光体团很相似。选区电子衍射(SAED)分析表明,这些区域由膨胀奥氏体局部分解产生的体心立方铁素体和立方晶系氮化铬组成。在所研究试样中的某些区域,发现有非晶态膨胀奥氏体。N的分解与膨胀奥氏体层局部区域中铁素体稳定元素(Cr,Mo)的显微偏析和奥氏体稳定元素(Ni)的贫化有关。     

Modern Concept of Nitrogen Potential

Considering the problems of gas nitriding and low-temperature thermochemical treatment the authors treat gas carburizing at 900 – 950°C and gas nitriding at 500 – 600°C as similar processes. This approach is considered from the standpoint of chemical, heterophase, and solid-solution diffusion. The use of specially developed equipment allows the authors to suggest new concepts of nitrogen potential and structure of the gas nitriding process.     

Plasma Nitriding of Austenitic Stainless Steel with Severe Surface Deformation Layer

The dc glow discharge plasma nitriding of austenite stainless steel with severe surface deformation layer is used to produce much thicker surface modified layer. This kind of layers has useful properties such as a high surface hardness of about 1500 Hv 0.1 and high resistance to frictional wear. This paper presents the structures and properties of low temperature plasma nitrided austenitic stainless steel with severe surface deformation layer.     

On the formation of expanded austenite during plasma nitriding of an AISI 316L austenitic stainless steel

Plasma nitriding of an AISI 316L austenitic stainless steel at low (400 °C) and high temperatures (550 °C) was performed under different nitriding gas mixtures. Nitrided surfaces were characterized by XRD using the Rietveld method. Expanded austenite “γ_N” with a special triclinic (t) crystalline structure was formed during the low-temperature nitriding treatment. Minor volume fractions of Fe₃N, Fe₄N and Cr₂N nitrides were also found. The expanded austenite phase showed a distortion ε of the lattice angles due to a very high nitrogen content dissolved in austenite, supersaturating the solid solution and leading to a 10% lattice distortion and to high compressive residual stresses at the surface.After nitriding the specimens at 550 °C the case was composed primarily by a high volume fraction of Fe₄N, Cr₂N and CrN nitrides, leading to a low distortion of the parent austenitic phase, maintaining the original cubic lattice.     

Hard coatings on thermochemically pretreated soft steels: application potential for ball valves

The combination of a thermochemical heat treatment (e.g., nitriding or carburizing, with or without plasma enhancement) and a hard coating [e.g., titanium nitride (TiN), chromium nitride (CrN) or amorphous hydrogenated carbon (a-C:H)] is known as “duplex treatment”. It offers the possibility to improve the functional properties of tools and machine parts compared with a single treatment. The combination of plasma nitriding and physical vapour deposited hard coating has been investigated by various groups, mostly for increasing the wear resistance of tools. However, machine parts and precision components are also very promising candidates for duplex treatments, especially when they are made of soft steels such as stainless steels. This paper shows that the functional behaviour of duplex-treated parts is determined both by the application-oriented optimization of the thermochemical heat treatment and by selection of the appropriate hard coating. The hard coatings include TiN, chromium-containing amorphous hydrogenated carbon (a-C:H:Cr) and “pure” a-C:H. For demonstration purposes, this paper focuses on the ferritic stainless valve steel X20Cr13.     

Corrosion resistance properties of glow-discharge nitrided AISI 316L austenitic stainless steel in NaCl solutions

Glow-discharge nitriding treatments can modify the hardness and the corrosion resistance properties of austenitic stainless steels. The modified layer characteristics mainly depend on the treatment temperature. In the present paper the results relative to glow-discharge nitriding treatments carried out on AISI 316L austenitic stainless steel samples at temperatures ranging from 673 to 773 K are reported. Treated and untreated samples were characterized by means of microstructural and morphological analysis, surface microhardness measurements and corrosion tests in NaCl solutions. The electrochemical characterization was carried out by means of linear polarizations, free corrosion potential-time curves and prolonged crevice corrosion tests. Nitriding treatments performed at higher temperatures (>723 K) can largely increase the surface hardness of AISI 316L stainless steel samples, but decrease the corrosion resistance properties due to the CrN precipitation. Nevertheless nitriding treatments performed at lower temperatures (?723 K) avoid a large CrN precipitation and allow to produce modified layers essentially composed by a nitrogen super-saturated austenitic metastable phase (S-phase) that shows high hardness and very high pitting and crevice corrosion resistance; at the same polarization potentials the anodic current density values are reduced up to three orders of magnitude in comparison with untreated samples and no crevice corrosion event can be detected after 60 days of immersion in 10% NaCl solution at 328 K.     

流态床渗氮工艺的研究

本文对３Ｃｒ２Ｗ８Ｖ，Ｈ１３及３８ＣｅＭｏＡｌ钢在刚玉流态床中进行了渗氮工艺的研究。结果表明，氨气在２５％－６５％范围内变化时，对渗氮结果无明显的影响，在流态床中渗氮可减少工件表面化合物层的形成和加快渗氮的速度。采用脉冲渗氮方法可获得与非脉渗氮同样的效果，同时可降低耗气量。     

A case of nitridation,carburization and oxidation on a stainless steel

A case of corrosion was studied on stainless steel tubes, exposed to a nitriding, carburizing and oxidizing environment (mainly NH₃ and CO₂) at 390–450°C. Due to the high nitriding potential prior formation of internally nitrided layers occurs, at higher temperatures (> about 425°C) under precipitation of CrN in the layer and at lower temperatures under formation of the γ_N‐phase, i.e. austenite with high N‐content and expanded lattice. The latter process causes more severe corrosion, due to the high expansion, the stresses in the nitrided layers lead to bursting and repeated spalling of the scales. Carburization and oxidation are less important. The carburization is slower than nitridation, Fe₃C formation is observed and carbon deposition. Also the oxidation by CO₂ is slow and converts the nitrides and carbides formed before, to unprotective oxide flakes.     

稀土化学热处理进展

综述了哈尔滨工业大学稀土化学热处理研究和应用成果。包括稀土渗碳及碳氮共渗、稀土渗氮及氮碳共渗、等离子体稀土渗氮、稀土渗硼及硼铝共渗、稀土多元共渗、稀土渗金属工艺技术，稀土的催化和微合金化机制，渗层组织和性能的改善，稀土化学热处理在生产中的应用及其过程的数学模型与计算机仿真等。     

Updating of equipment for thermochemical treatment of steel parts in a fluidized bed

Results of updating commercial equipment for thermochemical treatment (TCT) of steel parts in a fluidized bed are presented. The updating has led to the creation of an ecologically safe treatment area with mechanized equipment for carburizing and carbonitriding in a fluidized bed with the use of natural gas, ammonia, and compressed air instead of the equipment for pack cyaniding.     

Plasma nitriding of AISI 304 austenitic stainless steel with pre-shot peening

AISI 304 austenitic stainless steel was plasma nitrided at the temperature ranging from 410 to 520 °C with pre-shot peening. The structural phases, micro-hardness and electrochemical behavior of the nitrided layer were investigated by optical microscopy, X-ray diffraction, micro-hardness testing and anodic polarization testing. The effects of shot peening on the nitride formation, nitride layer growth and corrosion properties were discussed. The results showed that shot peening enhanced the nitrogen diffusion rate and led to a twice thicker nitrided layer than the un-shot peening samples under the same plasma nitriding conditions (410 °C, 4 h). The nitrided layer was composed of single nitrogen expanded austenite (S-phase) when nitriding below 480 °C, which had combined improvement in hardness and corrosion resistance.     

Investigations on the Corrosion Behaviour and Structural Characteristics of Low Temperature Nitrided and Carburised Austenitic Stainless Steel

The wear resistance of austenitic stainless steels can be improved by thermo-chemical surface treatment with nitrogen and carbon. However, it is possible that the corrosion resistance will be impaired by the precipitation of chromiumnitrid or -carbide. The present contribution deals with investigations of the corrosion behaviour and structural characteristics of a low temperature nitrided and carburised austenitic stainless steel.The material investigated was AISI 316L (X2CrNiMol7-12-2) austenitic stainless steel. A commercial plasma-nitriding unit (pulsed dc) was used for the nitriding and carburising process. Additional samples were treated by the gasoxinitriding process for a comparison between plasma- and gasoxinitriding. The nitrided and carburised layer of austenitic stainless steel consists of the nitrogen or carbon S-phase (expanded austenite), respectively. X-ray diffraction investigations show the typical shift of the peaks to lower angles, indicating expansion of the fcc lattice. Also the X-ray diffraction technique was employed to study the residual stresses in the nitrogen and carbon S-phase. The corrosion behaviour of surface engineered samples was investigated with electrochemical methods. Anodic potentiodynamic polarisation curves were recorded for testing the resistance against general corrosion (in H2SO4) and pitting corrosion (in NaCl).     

Hemocompatibility evaluation of plasma-nitrided austenitic stainless steels at low temperature

In this study, plasma-nitrided austenitic stainless steel was prepared using a microwave system as a function of the nitriding duration. The nitride layers were characterized via scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy, atomic force microscopy and a Vickers micro-hardness tester. The effects of plasma nitriding on the compatibility of blood with the austenitic stainless steel were also investigated. Based on the XRD and TEM observations, it was inferred that only a single γ_N phase was detected in the whole process of experiments. The γ_N layer, which was formed on austenitic stainless steel, exhibited lower platelet adhesion and activation than the untreated specimens. The clotting time of the PN specimens was prolonged and increased with the treatment time. This work suggested that the blood-compatible manner of plasma nitriding by controlling surface characteristic on austenitic stainless steel improved the anticoagulant properties, and made austenitic stainless steel suitable candidates in the field of surgical and medical instruments.     

304 不锈钢低温离子渗氮及氮碳共渗处理

目的研究304不锈钢离子渗氮层和氮碳共渗层的组织、硬度及耐磨、耐蚀性能,并考察渗层的磨损机理。方法利用离子渗氮及氮碳共渗工艺在304不锈钢表面获得硬化层,利用XRD,OM及共聚焦显微镜、显微硬度仪、电化学测试仪,分析处理前后渗层的组织、相结构及渗层的硬度及耐磨耐蚀性能。结果 304不锈钢氮碳共渗和渗氮层主要为S相层,在相同工艺条件下,氮碳共渗工艺获得的渗层为γN+γC的复合渗层,且厚度大于单一渗氮层。渗氮层和氮碳共渗层硬度约为基体硬度的3.5倍。在干滑动摩擦条件下,氮碳共渗层比渗氮层具有更好的耐磨性能;渗氮层的磨损机理为磨粒磨损的犁沟效应和断裂,氮碳共渗层的磨损机理为磨粒磨损的犁沟和微切削。电化学测试表明,渗氮层和氮碳共渗层的耐蚀性能均优于基体。结论 304不锈钢在420℃进行离子渗氮和氮碳共渗处理后,硬度和耐磨性能可大幅提高,且氮碳共渗处理效果更佳。     

Low temperature nitriding,nitrocarburising and carburising of AISI 316L austenitic stainless steel

Abstract

AISI 316L grade ASTM F138 austenitic stainless steel specimens were low temperature plasma nitrided (LTPN), nitrocarburised (LTPNC) and carburised (LTPC) using different gas mixtures. Different expanded austenite layers formed after each thermochemical treatment. LTPN and LTPCN led to formation of nitrogen supersaturated expanded austenite (γ_N). After LTPN, a second carbon expanded austenite (γ_C) layer was formed beneath the nitrogen expanded austenite layer (γ_N). LTPC led to formation of a carbon supersaturated expanded austenite (γ_C). Scanning electron microscopy, XRD and microhardness were used to characterise the expanded austenite layers formed on the surface of the specimens. Different mechanisms of formation and growth of the layers are pointed out. XRD results show that the lattice parameter of nitrogen expanded austenite (γ_N) is higher than that calculated for carbon expanded austenite γ_C. As a consequence, the lattice expansion Δa/a for the nitrogen rich (γ_N) phase is higher than the one observed for the (γ_C) phase and the nitrogen rich expanded austenite layer displays higher hardness than the carbon rich expanded austenite layer. The LTPNC bilayer displays a less steep hardness gradient, indicating that the carbon rich expanded austenite layer can grant mechanical support to the harder nitrogen rich layer.