Effects of Temperature on Microstructure and Wear of Salt Bath Nitrided 17-4PH Stainless Steel

Salt bath nitriding of 17-4 PH martensitic precipitation hardening stainless steels was conducted at 610, 630, and 650?°C for 2?h using a complex salt bath heat-treatment, and the properties of the nitrided surface were systematically evaluated. Experimental results revealed that the microstructure and phase constituents of the nitrided surface alloy are highly process condition dependent. When 17-4PH stainless steel was subjected to complex salt bathing nitriding, the main phase of the nitrided layer was expanded martensite (????), expanded austenite (??_N), CrN, Fe₄N, and (Fe,Cr) _x O _y . In the sample nitrided above 610?°C, the expanded martensite transformed into expanded austenite. But in the sample nitrided at 650?°C, the expanded austenite decomposed into ??_N and CrN. The decomposed ??_N then disassembled into CrN and alpha again. The nitrided layer depth thickened intensively with the increasing nitriding temperature. The activation energy of nitriding in this salt bath was 125?±?5?kJ/mol.     

Influence of Low-Temperature Pre-oxidation by Ozone on Corrosion Resistance of T91 Steel in Steam at 600 °C

The influence of the low-temperature pre-oxidation by ozone on the corrosion resistance of T91 steel in a steam environment at 600 °C was investigated. XRD, SEM, EDS and XPS were employed to characterize the corrosion products. Results show that the corrosion kinetics of the studied materials obeyed the parabolic oxidation law. Thin (Fe, Cr)₂O₃ oxides were developed on the surface of T91 after pre-oxidation by the ozone at 150 °C. The low-temperature ozone pre-oxidation improved the oxidation resistance of T91 steel in steam, which can possibly be applied to industrial application.     

Materials for ultrasupercritical coal power plants—Boiler materials: Part 1

The efficiency of conventional boiler/steam turbine fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO₂ emission has recently provided an additional incentive to increase efficiency. Thus, steam temperatures of the most efficient fossil power plants are now in the 600 °C (1112 °F) range, which represents an increase of about 60 °C (108 °F) in 30 years. It is expected that steam temperatures will rise another 50 to 100 °C (90 to 180 °F) in the next 30 years. The main enabling technology is the development of stronger high-temperature materials, capable of operating under high stresses at ever-increasing temperatures. Recently, the EPRI performed a state-of-the-art review of materials technology for advanced boiler/steam turbine power plants (ultrasupercritical power plants). The results of the review show that with respect to boilers, high-strength ferritic 9–12Cr steels for use in thick section components are now commercially available for temperatures up to 620 °C (1150 °F). Initial data on two experimental 12Cr ferritic steels indicate that they may be capable of long-term service up to 650 °C (1112 °F), but more data are required to confirm this. For higher temperatures, austenitic steels and Ni-based alloys are needed. Advanced austenitic stainless steels for use as super and reheater tubing are available for service temperatures up to 650 °C (1112 °F) and possibly 700 °C (1292 °F). Ni-based superalloys would be needed for higher temperatures. None of these steels have been approved by the ASME Boiler Code Group so far. Higher-strength materials are needed for upper water walls of boilers with steam pressure above 24 MPa (3400 psi). A high-strength 2-1/2%Cr steel recently ASME code approved as T-23 is the preferred candidate material for this application. Field trials are in progress. This paper will present the results of the EPRI review in detail, relating to boiler material. Results relating to turbine materials are presented in a companion paper as Part 2.     

Oxidation Behavior of Cr2N,CrNbN, and CrTaN Phase Mixtures Formed on Nitrided Cr and Laves-Reinforced Cr Alloys

A series of single-phase Cr(X), single phase Cr ₂X Laves phase, and two-phase Cr(X) + Cr₂ X alloys (X = Nb or Ta) were thermally nitrided for 24 hr at 1100°C in N₂--4H₂ and then oxidized for 2 hr at 1100°C in air. The Cr(X) phase nitrided to form Cr₂N while the Cr₂X phases nitrided to form a complex local mixture of Cr₂N/Cr and CrNbN/CrTaN, Cr₃Nb₃N/Cr₃Ta₃N phases depending on the depth in the nitrided zone. The Ta only slightly increased the isothermal oxidation rate of nitrided Cr(Ta) and Cr₂Ta-reinforced Cr alloys, compared with nitrided, unalloyed Cr. Further, the nitrided two-phase alloys Cr--9.5Ta and Cr--20Ta exhibited improved Cr₂O₃ scale adherence relative to nitrided unalloyed Cr and Cr--1Ta. In contrast, Nb was detrimental to the oxidation resistance of the nitrided Cr(Nb) and Cr₂Nb-reinforced Cr alloys, resulting in the formation of nonprotective Cr--Nb oxides rather than continuous Cr₂O₃. A phenomenological explanation for these effects based on phase chemistry and microstructural distribution is presented. Implications of these results for understanding the oxidation behavior of developmental high-temperature, Laves-strengthened Cr alloys, as well as possible applications as oxidation and wear-resistant coatings are discussed.     

Study of plasma nitriding and nitrocarburising of AISI 430F stainless steel for high hardness and corrosion resistance

ABSTRACT

In order to improve both the hardness and corrosion resistance properties of AISI 430F stainless steel, plasma nitriding (PN) and nitrocarburising processes were carried out at different temperatures ranging from 350 to 500°C for 4?h. After PN, the nitrided layer was found to be thicker compared to that obtained by plasma nitrocarburising process. There was an increase in microhardness values by a factor of six to seven compared to the plasma nitrided and nitrocarburised specimens respectively, treated at 500°C. The electrochemical corrosion behaviour of the plasma nitrided and nitrocarburised AISI 430F specimens show that the plasma nitrided and nitrocarburised specimens treated at 400°C for 4?h showed better corrosion resistance and higher surface hardness than the untreated AISI 430F stainless steel specimens. This is mainly attributed to the presence of nitrogen in the modified layer existing as a solid solution in the ferrite phase.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Effect of Silicon on the Steam Oxidation Resistance of a 9%Cr Heat Resistant Steel

The steam oxidation of 9Cr–0.5Mo–1.8W steels containing 0.06 to 0.49%Si was investigated at 500°, 550°, 600°, 650° and 700°C. The steam oxidation rate of the steel decreased with increasing silicon content. The effect of silicon was most remarkable at 700°C. At 500°, 550° and 600°C, the effect was almost the same, and was smaller than that at 700°C. At 700°C, the formation of a protective amorphous-SiO₂ film reduced the oxidation rate considerably. On the other hand, at 600°C or less, silicon dissolved in the Fe–Cr spinel lattice with no evidence of SiO₂. At 650°C, although amorphous SiO₂ was observed, as at 700°C, at the scale–metal interface, the effect of silicon was the least within the test-temperature range. Thus, 650°C was a peculiar temperature for the effect of silicon on the steam oxidation of 9%Cr steels. The relatively small effect of silicon at 650°C is attributed to the formation of metastable FeO.     

Oxidation Behavior of High-Speed Steel Used for Hot Rolls

Non-isothermal oxidation kinetics of the high-speed steel (HSS) were studied by thermal gravimetric analysis. The surface and cross-sectional morphology of the HSS oxide film formed at different temperatures and durations were observed by scanning electron microscopy, and the corresponding chemical composition was analyzed by using energy dispersive spectrometer. The composition and structure of the oxide film were also investigated by X-ray diffraction. The results showed that the oxide scale of the HSS is mainly composed of Fe₂O₃, Fe₃O₄ and FeCr₂O₄. Temperature is the main factor on the quality of the oxide film. Below 600 °C, the oxidation rate of the steel is slow and the thickness of the oxide film is below 5 µm. However, the oxidation rate sharply increased as the temperature reaches 600 °C. Cr, Mo and V tend to concentrate at the scale/steel interface and form FeCr₂O₄ and other oxides.     

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.     

Effect of Pretreatment of Steel 10 in a Saturated Aqueous NH4F on Its High-Temperature Oxidation in Air

Pretreatment of steel 10 in a saturated aqueous solution of NH₄F substantially affects its high-temperature (250–850°C) oxidation in air. During the oxidation, the pretreated steel becomes covered with a heat-resistant film mainly consisting of Fe₂O₃ or Fe₂O₃ + Fe₃O₄; however, the film can either promote or impede steel oxidation depending on the isothermal exposure time.     

Microstructure and Volatile Species Determination of Ferritic/Martensitic FB2 Steel in Contact With Ar + 40 %H2O at High Temperatures

Volatile species were identified by means of mass spectrometry during the initial stages of oxidation of ferritic/martensitic steel FB2 (Fe–9Cr–1.5Mo–1.1Mn–0.05Si–0.1C–0.1Ni–1Co–0.2V–0.05Nb–0.008B) at 650 °C and 800 °C in a steam atmosphere of Ar + 40 %H₂O for 180 and 150 h, respectively. The greater amount of Cr-containing volatile species detected, (i.e.) CrO₃(g), CrO₂(OH)(g) and CrO₂(OH)₂(g), from the oxidized alloy at 800 °C compared to 650 °C showed that loss and breakdown of chromia (Cr₂O₃) are accelerated by increasing the temperature. For instance, the CrO₂(OH)₂(g), which is responsible for the breakdown of chromia scales, was observed at 650 °C for 150 h of oxidation, while the same species was detected at 800 °C for 75 h. It was found that the hematite (Fe₂O₃) that formed in the initial stage of oxidation was constantly evaporating during the test. Therefore, the sample gradually lost protection against oxidation, with chromia evaporating followed by volatilization of products that formed, such as the magnetite (Fe₃O₄) and wüstite (Fe_1?x O). Eventually, oxidation breakaway of the sample was reached. Simultaneously, the oxidation kinetics of the samples was determined. The morphology/composition and structure of oxidized samples were also studied using the scanning electron microscopy–electron back-scatter diffraction/energy dispersive X-ray spectroscopy and X-ray diffraction techniques.     

On the Erosion–Corrosion Behavior of Active Screen Plasma Nitrided St52 Steel

In this work, samples of St52 steel were plasma nitrided using an iron screen, in an N₂: H₂ gas mixture ratio of 4 : 1, at 500 and 550°C for 5, 10 and 15 h. The X-ray diffraction and optical microscopy methods were used for structural characterization of the coatings. Results indicated that the coatings were composed of Fe_2–3N and Fe₄N phases growing at longer deposition times. Moreover, the Fe_2–3N phase was decomposed to Fe₄N after 10 h of plasma nitriding. The erosion–corrosion behavior of nitrided coatings and a bare substrate were studied in various impact angles: 30, 45, 60 and 90 degrees. Polarization curves of the coated and uncoated samples were recorded between–900 to 600 mV, as a function of the slurry impact angle. Results showed that an active screen plasma nitriding method significantly enhanced the erosion–corrosion resistance of the St52 steel. Moreover, an impact angle of 30° on the sample surface yielded a lower weightloss whereas increasing the impact angle up to 90° caused more weight-loss due to the brittle characteristic of iron nitride coatings. According to SEM micrographs, by increasing the impact angle up to 90°, the depth of the removed mass increased substantially.     

Effect of Chromium Content on the Oxidation Behaviour of Ferritic Steels for Applications in Steam Atmospheres at High Temperatures

Environments containing water vapour are common in many industrial processes, such as power generation systems. Hence, long-term oxidation (1000 h) of P-91 and AISI 430 was studied at 650 and 800 °C, in 100% H₂O atmosphere. The oxidation resistance of the AISI 430 is better than that of the P-91, due to the formation of protective phases on the surface. At 650 °C, a scale composed of Fe₃O₄, Fe₂O₃ and (Fe,Cr)₃O₄ is formed on P-91, although at 800 °C the scale is mainly composed of Fe₃O₄ and (Fe,Cr)₃O₄. On the other hand, on AISI 430 the scale is composed mainly of (Fe,Cr)₂O₃ at 650 °C, and at 800 °C a layer of Cr₂O₃ is formed and remains owing to the higher diffusion rate of Cr at this temperature than at 650 °C, the latter of which compensates the Cr depletion by the degradation of the chromia scale.     

1000 – 1100 °C下多弧离子镀Cr涂层Zr-4合金的高温蒸气氧化行为

Cr涂层锆合金包壳是提高轻水反应堆燃料组件抗高温氧化性能的重要方法。本文旨在研究Cr涂层Zr-4合金在水蒸气环境中的高温氧化行为,采用多弧离子镀工艺制备Cr涂层Zr-4合金样品,在1000 °C和1100 °C水蒸气环境中开展高温氧化实验,采用高精度天平获得样品氧化增重,采用扫描电子显微镜(SEM)、能量色散谱仪(EDS)和X射线衍射仪(XRD)测试分析样品的表面与截面微观形貌、元素分布、物相及Cr-Zr扩散层厚度。结果显示,多弧离子镀制备的Cr涂层Zr-4合金样品表面存在大量尺寸不等的大颗粒,Cr涂层沿着(1 1 0)晶面择优生长。高温蒸气单位面积氧化增重与时间近似遵循抛物线规律,1100 °C的蒸气氧化速率明显高于1000 °C,相同温度下Cr涂层Zr-4合金的氧化速率低于无涂层Zr-4合金。高温蒸气氧化后,样品表面生成针须状氧化物,且氧化物间存在大量微孔。在1100 °C高温蒸气氧化3h和4h后,样品表面被氧化生成蠕虫状的团状物。样品截面呈层状结构,由外向内分别为Cr₂O₃层,Cr涂层,Cr-Zr扩散层,Zr-4合金层。但表面Cr₂O₃层的厚度并不是随着氧化时间的增加而同步增加的,而Cr-Zr扩散层的厚度随氧化温度和氧化时间的增加而增大,且与氧化时间基本呈线性关系。因此,采用多弧离子镀制备的Cr涂层Zr-4合金在1000 °C和1100 °C下表现出良好的抗高温蒸气氧化性能。     

Reaction between a rare earth element and 9Cr-2W steel

We analyzed the reaction between a rare earth element and ferritic-martensitic stainless steel in order to evaluate the role of the rare earth element in the fuel cladding chemical interaction (FCCI). A diffusion couple test between Misch metal (70Ce-30La) and 9Cr-2W (Gr.92) steel at both 660 °C and 800 °C was conducted, and a microstructural analysis was carried out. The result showed that Fe in the Gr.92 material and Ce in the Misch metal mainly diffused and reacted to form intermetallic compounds, namely Fe₂Ce and Fe₁₇Ce phases, above 660 °C, whereas La hardly diffused or reacted in the Gr.92 material. In the specimen tested at 660 °C, the Fe diffused outside of the clad interface so that Cr-rich precipitated beneath the clad interface. In the specimen tested at 800 °C, a local reaction caused by the eutectic transformation of the (Fe,Cr)₁₇Ce phase resulted in a local penetration across the clad.     

Influence of dissolved oxygen concentration and exposure temperature on the steam oxidation behaviour of HR3C

ABSTRACT

Oxidation behaviour of an austenitic steel HR3C at 620～650°C for 1000?h in steam with two different levels of dissolved oxygen (DO), 20?ppb and 5?ppm, was investigated by gravimetry, X-ray diffraction and scanning electron microscopy. Results showed that mainly a single-layered (Cr, Mn)₂O₃ scale was developed. The oxide growth followed near-power kinetics and was accelerated with DO concentration and exposure temperature. At the higher DO concentration, the formation of (Cr, Mn)₂O₃ with a much smaller size was favoured. Besides, the oxide scale became undulated and interfacial voids formed. Higher exposure temperature resulted in the growth of the (Cr, Mn)₂O₃ crystals and the change of their morphology from acicular to granular. Fe₃O₄ nodules and Ni-Cr spinel occurred at 650°C/5?ppm DO. Related oxidation mechanisms were discussed.     

Low-Temperature Nitriding of Nanocrystalline Stainless Steel and Its Effect on Improving Wear and Corrosion Resistance

In this study, an ultrafine-grained surface layer with the average grain size of about 10 nm was fabricated on a stainless steel plate by surface mechanical attrition treatment (SMAT). Plasma nitriding of the samples was carried out by a low-frequency pulse-excited plasma unit. Optical microscopy, x-ray diffraction, scanning electron microscopy, transmission electron microscopy, micro-indentation, and pin-on-disk wear and corrosion experiments were performed for characterization before and after plasma nitriding. It is found that the pre-SMATed sample developed a nitrided layer twice as thick as that on the as-received sample under the same nitriding conditions (300 °C for 4 h), which can be mainly attributed to the fast diffusion of nitrogen along grain boundaries in the nanostructured layer induced by means of SMAT. Results showed that nitriding layers of the as-received and pre-SMATed samples up to 300 °C are dominated by S-phase (γ_N), but its peak intensity for the pre-SMATed sample is sharper than that of the as-received one. During 500 °C nitriding treatment, the nitrogen would react with Cr in the steel to form CrN precipitates, which would lead to the depletion of chromium in the solid solution phase of the nitrided layer. Furthermore, the nitrided layer of the pre-SMATed sample exhibited a high hardness, and an excellent wear and corrosion resistance.     

Effect of Plasma Nitriding and Nitrocarburizing on HVOF-Sprayed Stainless Steel Coatings

In this work, the effects of plasma nitriding (PN) and nitrocarburizing on HVOF-sprayed stainless steel nitride layers were investigated. 316 (austenitic), 17-4PH (precipitation hardening), and 410 (martensitic) stainless steels were plasma-nitrided and nitrocarburized using a N₂ + H₂ gas mixture and the gas mixture containing C₂H₂, respectively, at 550 °C. The results showed that the PN and nitrocarburizing produced a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer depending on the crystal structures of the HVOF-sprayed stainless steel coatings. Also, the diffusion depth of nitrogen increased when a small amount of C₂H₂ (plasma nitrocarburizing process) was added. The PN and nitrocarburizing resulted in not only an increase of the surface hardness, but also improvement of the load bearing capacity of the HVOF-sprayed stainless steel coatings because of the formation of CrN, Fe₃N, and Fe₄N phases. Also, the plasma-nitrocarburized HVOF-sprayed 410 stainless steel had a superior surface microhardness and load bearing capacity due to the formation of Cr₂₃C₆ on the surface.     

The oxidation resistance of Nb-20Mo-15Si-5B-20Cr up to 1,300°C

Nb-20Mo-15Si-5B-20Cr alloy has been oxidized in air in a range of temperatures from 700 to 1,300°C. A 24 hour exposure to air shows a minimum in weight gain per unit area at 900 and 1,000°C. Below and above this temperature the alloy has an oxide scale composed of oxide-metal interface which has been characterized by x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and x-ray mapping. Formation of CrNbO₄ appears to be beneficial for the oxidation resistance while another Cr oxide, Cr₂O₃, may form mainly at temperatures below 1,000°C. SiO₂ formation has been restricted to temperatures above 1,000°C. Long term oxidation consisting of seven successive 24 hour cycles of heating shows better oxidation resistance at 1,300°C for times longer than 96 hours but below that usual lower weight gain per unit area at lower temperature has been observed.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

The effect of dispersed nitrides on the oxidation of ferritic alloys

Using gas-metal equilibration, a nitrogen martensite case can be produced on mild steel which tempers rapidly at 500°C forming incoherent Fe₄N precipitates dispersed in a ferritic matrix. Under oxidizing conditions the finely distributed nitrides provide sites for oxide nucleation resulting in a fine-grained oxide layer. The dispersed nitrides also act as vacancy sinks for inwardly diffusing cation vacancies during the subsequent growth of the oxide scale, thus giving improved metal-oxide adherence. An oxide forming reaction occurs at the metal/oxide interface as well as the oxide/gas interface resulting in a duplex oxide layer and further enhancing metal-oxide adherence. Similar results are found with nitrided FeX alloys (where X is a substitutional alloying element—Cr, Ti or Mo). Nitriding of these alloys produces a dispersion of nitrides in a ferritic matrix, which subsequently influences the oxidation mechanisms. An analogy can be drawn with the influence of subsurface dispersions of incoherent stable oxide particles on the oxidation of superalloys.