The characteristics of chromized 1020 steel with electrical discharge machining and Ni electroplating pretreatments

A uniform and continuous chromized coating on AISI 1020 steel is produced by low-temperature pack chromization (LTPC) with electrical discharge machining and Ni electroplating pretreatments. The anticorrosive performance of the chromized steels is investigated in a 0.5 M H₂SO₄ solution at room temperature. The testing results indicate that the chromized specimen with electrical discharge machining and Ni electroplating pretreatments exhibits the lowest corrosion current density, 2.16 × 10⁻⁸ A cm⁻², among the tested specimens. The corrosion resistance of all tested specimens are in the order of bare 1020 < 1020-Cr(700-2) < 1020-Ni-Cr(700-2) < 1020-EDM-Ni-Cr(700-2). Moreover, the 1020-Ni-Cr(700-2) specimen have the best conductivity as a result of the less amount of oxides in the superficial coating.     

On the refinement of carbide precipitates by cryotreatment in AISI D2 steel

Refinement of carbide particles by cryotreatment is often proposed as a major factor for the improvement of wear resistance in tool steels. However, this proposition is not substantiated by experimental evidence. This has been examined in this report by (i) detailed micro-structural analyses of the nature, volume fraction, size, population density and distribution of carbide particles, (ii) XRD and EDX micro-analysis on the bulk samples and electrochemically extracted carbides, and (iii) measurement of hardness and wear rate of a series of differently cryotreated AISI D2 steel. The results conclusively establish that (i) cryotreatment, in comparison to conventional treatment, induces precipitation of finer carbides with higher volume fraction and more uniform distribution, and (ii) population density and the size of secondary carbide particles significantly increases with holding time up to a critical duration at 77 K in cryotreatment. The latter observation indicates the pioneering direction towards optimization of cryotreatment design for techno-economic benefit.     

Phase Composition of Oxidised Layers Grown on Steel Exposed to Liquid Lead at 749 K

The corrosion resistance of AISI 316 LN stainless steel was studied in contact with stagnant, oxygen-saturated liquid lead at 749 K for times up to 1200 h. The reaction products were analysed by means of optical and scanning electron microscopy, electron probe microanalysis, X-ray diffraction analysis and Mössbauer spectroscopy. Thin layers of largely variable thickness formed on the surface, mainly consisting of Fe₃O₄, with small amounts of FeO in the inner regions, Fe₂O₃ and an Fe-Pb-O ternary product in the outermost regions. The alloying elements Cr and Ni diffused to a different extent into Fe₃O₄. A mechanism is proposed to explain formation and growth of the reaction products.

     

Improvement of corrosion resistance of nitrided low alloy steel by plasma post-oxidation

Post-oxidizing treatments can be performed to improve the corrosion resistance of nitrided steel samples. In this paper, plasma nitriding treatments were performed at 540 °C for 4 h using ammonia as the working gas, and plasma post-oxidizing treatments were carried out at temperatures ranging from 350 °C to 500 °C for 2 h in oxygen gas. The treated samples were characterized by using optical microscopy, SEM, XRD, and electrochemical polarization. The X-ray analysis revealed the formation of iron-nitride phases of ?-Fe_2-3N and γ′-Fe₄N during plasma nitriding and iron oxide phases of hematite (Fe₂O₃) and magnetite (Fe₃O₄) through the post-oxidizing treatment. In particular, it was found that the very thin magnetite layer 0.8-1.5 μm in thickness on top of the compound layer was obtained by plasma post-oxidized at 400 °C and 450 °C. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment.     

The significance of passivation treatments on AISI 314 foam pieces to be used as substrates for catalytic applications

Several properties of metallic foams such as their low density, high mechanical strength and good coefficients of heat and mass transfer make them attractive for applications in catalysis. Important modifications in the composition and morphology of the metallic foam surfaces can take place when they are submitted to treatments at high temperatures. These surface changes are due to the migration of some elements from the metallic core to the pore surface, thus inducing a passivation via an oxide layer formation. This new layer avoids further metallic segregation and generates a surface roughness, both effects having a significant impact on the catalytic coating quality. This work analyzes the effects of calcination temperature and time on the chemistry and morphology of the metallic surface corresponding to the AISI 314 stainless steel foams of 50 and 60 ppi. The chemical and morphological surface changes were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray (EDX) analysis and Laser Raman Spectroscopy (LRS). The application of high temperature treatments on AISI 314 foams promotes the formation of a surface layer containing chromium oxide and spinel-type compounds of chromium, iron and manganese. The optimum treatment temperature for this type of structures seems to be 900 °C because both the adhesion and thickness of the layer formed are adequate. For the sample with smaller pores (60 ppi) the optimal treatment time is close to 2 h and for that with larger pores (50 ppi) the recommended time is 20 h. Under these conditions, a compromise is found between adhesion, thickness and surface roughness, suitable for the subsequent deposition of catalytic material.     

TC4钛合金及40Cr钢破片中绝热剪切带的TEM分析

 对含有约热剪切带（ASB）的TC4钛合金及40Cr钢破片作了扫描电镜（SEM）和透射电镜（TEM）分析。结果表明，TC4钛合40Cr钢中的ASB是由平均直径不到0.3 μm的微晶组成，微晶列有一定择优取向，电子衍射花样表现为不连续的环状。ASB内位错密度均很低，其邻近基体区含有高密度位错，ASB内没有察到相变发生。     

AISI304钢表面低电压等离子体基离子注入层摩擦磨损性能研究

采用高频低电压等离子体浸没离子注入（ＨＬＰＩＩＩ）技术对ＡＩＳＩ３０４不锈钢表面进行了氮离子注入处理;用球－盘摩擦磨损试验机考察了注入处理后钢表面改性层的摩擦磨损性能;用扫描电子显微镜（ＳＥＭ）、俄歇电子能谱仪（ＡＥＳ）和Ｘ射线衍射（ＸＲＤ）考察了改性层的相组成、Ｎ元素的深度分布及磨损机理。研究结果表明：ＨＬＰＩＩＩ处理能够显著提高样品的摩擦学性能,且其摩擦学性能对温度有较大的依赖性,４００℃下处     

The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 4140 steel by means of surface mechanical attrition treatment (SMAT). Plasma nitriding of SMAT and un-SMAT AISI 4140 steel was carried out by a low-frequency pulse excited plasma unit. A series of nitriding experiments has been conducted at temperatures ranging from 380 to 500 °C for 8 h in an NH₃ gas. The samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that a much thicker compound layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples after nitriding at the low temperature. In particular, plasma nitriding SMAT AISI 4140 steel at 380 °C for 8 h can produced a compound layer of 2.5 μm thickness with very high hardness on the surface, which is similar to un-SMAT samples were plasma nitrided at approximately 430 °C within the same time.     

Characterization of the oxides formed at 1000 °C on the AISI 304 stainless steel by X-ray diffraction and infrared spectroscopy

The aim of this work is to show the contribution of the infrared spectroscopy (FT-IR) to the identification of the oxides formed on the AISI 304 stainless steel during isothermal oxidation at 1000 °C, in air. This work focuses on the differentiation of spinel type AB₂O₄ structures and corundum type M₂O₃ structures. It is shown that after 100 h oxidation, the scale is composed of two subscales. The structural analyses were performed both on the adherent subscale and on the external subscale, which spalled off during cooling to room temperature. In the spalled subscale, the infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) analyses exhibit the presence of two spinel phases: Mn_1.5Cr_1.5O₄ and FeCr₂O₄, as well as hematite Fe₂O₃. The FT-IR and XRD analyses of the adherent subscale enable us to distinguish the spinel Mn_1.5Cr_1.5O₄ and FeCr₂O₄ structures which are difficult to identify by XRD alone. Chromia Cr₂O₃ appears to be slightly present in the adherent subscale. According to our results, the parabolic regime of the kinetic curve corresponds to a scale growth mechanism governed by an inward oxygen diffusion.     

Theoretical and experimental analysis of high power diode laser (HPDL) hardening of AISI 1045 steel

Laser surface hardening makes use of the rapid and cooling cycles produced on metals surfaces exposed to a scanning laser beam without affecting the bulk of the sample. Mechanical and chemical properties of the surface can be enhanced through the metallurgical transformations that take place during the mentioned thermal cycles. Steels and cast irons are the usual materials to be hardened by laser and recently the high power diode lasers (HPDL) became the appropriate tool to carry out this process. In this work, some systematic experiments have been carried out to harden AISI 1045 surface samples by a cw (HPDL) working at different power levels (470, 760 W). The main processing parameters (scanning velocity and density power of the laser beam) were tuned from the prediction realized by the numerical (ANSYS) analysis of the heat conduction involved in the process. Such analysis allowed us to put in evidence the variation of the temperature and the cooling rate of the steel sample surface, affecting the uniformity of the demanding mechanical properties of the surface. In this way, a close-loop temperature control of the surface was justified in order to keep the hardness value within the range required. The formation of martensite phase in the laser treated superficial zone confirmed the hardening of the steel.