Nitrogen uptake of nickel free austenitic stainless steel powder during heat treatment—an XPS study

In austenitic stainless steel nitrogen stabilizes the austenitic phase, improves the mechanical properties and increases the corrosion resistance. Nitrogen alloying enables to produce austenitic steels without the element nickel which is high priced and classified as allergy inducing. A novel production route is nitrogen alloying of CrMn‐prealloyed steel powder via the gas phase. This is beneficial as the nitrogen content can be adjusted above the amount that is reached during conventional casting. A problem which has to be overcome is the oxide layer present on the powder surface which impedes both the sintering process and the uptake of nitrogen. This study focuses on whether heat treatment under pure nitrogen is an appropriate procedure to enable sintering and nitrogen uptake by reduction of surface oxides. X‐ray photoelectron spectroscopy (XPS) in combination with scanning electron microscopy (SEM) and energy dispersive X‐ray spectrometry (EDS) are used to investigate the surface of powdered FeMn19Cr17C0.4N heat treated under nitrogen atmosphere. The analyses showed reduction of iron oxides already at 500 °C leading to oxide‐free metallic surface zones. Mn and Cr oxides are reduced at higher temperatures. Distinct nitrogen uptake was registered, and successful subsequent sintering was reached. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of minor alloying elements on the initial stages of oxidation of austenitic stainless steel materials

Surface oxidation of Fe‐19Cr‐17Ni, Fe‐19Cr‐18Ni‐1Al and TiC‐enriched Fe‐19Cr‐18Ni‐1Al alloys was investigated by photoelectron spectroscopy (PES). The experiments were conducted at 323 K in pure O₂ (2.7 × 10^?6 mbar). Composition and morphology of the nanoscale surface oxides were determined quantitatively by inelastic electron background analysis. Moreover, use of synchrotron radiation facilities were necessary to obtain improved sensitivity for studying minor alloying elements such as Al and Si. The results indicate oxygen‐induced segregation of Al, which significantly hinders the oxidation of the major alloying elements Fe and Cr. Ti remains in its inert carbide form. The relative concentration of Fe within the oxide layer was found to increase with the oxide‐layer thickness, indicating greater mobility of Fe relative to other alloying elements. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of atomization on surface oxide composition in 316L stainless steel powders for additive manufacturing

The initial oxide state of powder is essential to the robust additive manufacturing of metal components using powder bed fusion processes. However, the variation of the powder surface oxide composition as a function of the atomizing medium is not clear. This work summarizes a detailed surface characterization of three 316L powders, produced using water atomization (WA), vacuum melting inert gas atomization (VIGA), and nitrogen atomization (GA). X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy analyses were combined to characterize the surface state of the powders. The results showed that the surface oxides consisted of a thin (~4 nm) iron oxide (Fe₂O₃) layer with particulate oxide phases rich in Cr, Mn, and Si, with a varying composition. XPS analysis combined with depth-profiling showed that the VIGA powder had the lowest surface coverage of particulate compounds, followed by the GA powder, whereas the WA powder had the largest fraction of particulate surface oxides. The composition of the oxides was evaluated based on the XPS analysis of the oxide standards. Effects of Ar sputtering on the peak positions of the oxide standards were evaluated with the aim of providing an accurate analysis of the oxide characteristics at different etch depths.     

Surface chemistry and diffusion of trace and alloying elements during in vacuum thermal deoxidation of stainless steel

Removal of the native surface oxide from steel is an important initial step during vacuum brazing. Trace and alloying elements in steel, such as Mn, Si, and Ni, can diffuse to the surface and influence the deoxidation process. The detailed surface chemical composition and grain morphology of the common stainless-steel grade 316L is imaged and spectroscopically analyzed at several stages of in-vacuum annealing from room temperature up to 850°C. Measurements are performed using synchrotron-based X-ray photoemission and low-energy electron microscopy (XPEEM/LEEM). The initial native Cr surface oxide is amorphous and unaffected by the underlying Fe grain morphology. After annealing to ~700°C, the grain morphology is seen at the surface, persisting also after the complete oxygen removal at 850°C. The surface concentration of first Mn and then Si increases significantly when annealing to 500°C and 700°C, respectively, while Ni and Cr concentrations do not change. Mn and Si are not located only in grain boundaries or clusters but are distributed across over the surface. Both Mn and Si appear as oxides, while Cr oxide becomes metallic Cr. Annealing from 500°C up to 850°C leads to the removal of first the Mn and then Si oxides from the surface, while Cr and Fe are completely reduced to metals. Deoxidation of Cr occurs faster at the grain boundaries, and the final Cr metal surface content varies between the grains. The findings are summarized in a general qualitative model, relevant for austenite steels.     

Evolution of surface chemistry during sintering of water-atomized iron and low-alloyed steel powder

Water-atomized iron and steel powder is commonly used as the base material for powder metallurgy (PM) of ferrous components. The powder surface chemistry is characterized by a thin surface oxide layer and more thermodynamically stable oxide particulates whose extent, distribution, and composition change during the sintering cycle due to a complex set of oxidation–reduction reactions. In this study, the surface chemistry of iron and steel powder was investigated by combined surface and thermal analysis. The progressive reduction of oxides was studied using model sintering cycles in hydrogen atmospheres in a thermogravimetric (TG) setup, with experiments ended at intermediate steps (500–1300°C) of the heating stage. The surface chemistry of the samples was then investigated by means of X-ray photoelectron spectroscopy (XPS) to reveal changes that occurred during heating. The results show that reduction of the surface oxide layer occurs at relatively lower temperature for the steel powder, attributed to an influence of chromium, which is supported by a strong increase in Cr content immediately after oxide layer reduction. The reduction of the stable oxide particulates was shifted to higher temperatures, reflecting their higher thermodynamic stability. A complementary vacuum annealing treatment at 800°C was performed in a furnace directly connected to the XPS instrument allowing for sample transfer in vacuum. The results showed that Fe oxides were completely reduced, with segregation and growth of Cr and Mn oxides on the particle surfaces. This underlines the sequential reduction of oxides during sintering that reflects the thermodynamic stability and availability of oxide-forming elements.     

Influence of soaking duration on the selective oxidation and galvanizability of a high‐strength dual phase steel

High‐strength dual phase steels readily exhibit bad galvanizability and coating defects because of selective oxides formed on steel surface during the annealing process prior to galvanizing. To investigate selective oxidation of alloying elements and their effects on glavanizability, a high‐strength dual phase steel was annealed with soaking duration for 45, 90, and 120 s, respectively, and then galvanized using a hot‐dip simulator. Field‐emission scanning electron microscopy characterization revealed that when dual phase steel was soaked for 45 s, selective oxides mainly precipitated along grain boundaries, while only a few of the oxides formed on grains. With soaking duration increased, oxides were so dense that nearly all steel surface was covered, leaving little bare area of the steel surface. Further XPS analysis showed that selective oxides mainly consisted of MnO and Cr₂O₃. In addition, the chemical nature of oxides did not change at all although soaking duration prolonged. Scanning Auger microprobe depth profiles presented that Mn had a much higher tendency to segregate than Cr and Mo. Oxygen penetration depth to subsurface was promoted as soaking duration increased. The formation of interfacial inhibition layer was founded to be greatly influenced by the density and size of surface oxides. The widely spaced small oxides had virtually no adverse effect on wettability because of aluminothermic reduction of oxides by the bath dissolved Al. As the oxides became dense and considerably big, the grains of the inhibition layer in some certain zones became coarse and the galvanizability tended to deteriorate. Copyright © 2011 John Wiley & Sons, Ltd.     

Mechanochemical synthesis and high-frequency induction heated consolidation of nanostructured Ti5Si3 and its mechanical properties

Pure Ti and Si powders were milled in a horizontal-rotation ball mill and in a high-energy ball mill to synthesize Ti₅Si₃ powders. The high-energy ball milling produced nanosized single-phase Ti₅Si₃ particles. Meanwhile, no reaction occurred during the horizontal milling. The two milled powders were consolidated using the high-frequency induction heated sintering method. A dense nanostructured Ti₅Si₃ compact was consolidated within 2 min using the mechanically synthesized Ti₅Si₃ powder. The retainment of nanoscale structure during sintering is believed to be the reason for the good mechanical properties of the Ti₅Si₃ compact. In comparison, the horizontally milled powder reacted to form Ti₅Si₃ partially on sintering. It is believed that the enhanced toughness of the horizontally milled samples may be due to the crack-deterring effect of softer Si/Ti grains.     

Einsatz der Abriebmethode für die Verwechslungsprüfung hochlegierter Stähle

The method of abrasion of small sample quantities on Al₂O₃-disks is applicable to alloy and high-alloy steels. For example the contents of the elements Si, Cr, Ni, Mn, V, W, Mo, Ti and Nb may be determined quantitatively. The possible sorting of high-alloy steels and scarp is an important thing of economy. The method of abrasion with a X-ray spectrometric detection system is simple and of low costs.     

Einsatz der Abriebmethode für die Verwechslungsprüfung hochlegierter Stähle

The method of abrasion of small sample quantities on Al₂O₃-disks is applicable to alloy and high-alloy steels. For example the contents of the elements Si, Cr, Ni, Mn, V, W, Mo, Ti and Nb may be determined quantitatively. The possible sorting of high-alloy steels and scarp is an important thing of economy. The method of abrasion with a X-ray spectrometric detection system is simple and of low costs.     

Surface Characterisation of Water-Atomised Al–Zn–Mg–Cu Alloy Powders by SIMS and AES

 The present paper focuses on the characterisation of surface composition and alloying element in-depth distribution of water-atomised Al–Zn–Mg–Cu alloy powders by secondary ion mass-spectrometry and Auger electron spectroscopy. A pronounced segregation of Mg and some impurities (Fe, Ca, S) concurrently with some Zn depletion are observed on the powder surface. The oxide film formed on the powder surface mainly consists of Al and Mg oxides. The film is non-uniform in thickness: rather coarse surface oxide islands coexist with surface areas covered by a thin (<1.8 nm) oxide layer. The extent of surface oxidation is strongly affected by solidification conditions: The average thickness of the surface oxides increases with increasing particle size or with decreasing cooling rate. All alloying elements are homogeneously distributed in the bulk of individual particles. No significant differences in chemical composition between different particles of a given powder are observed. Received November 26, 1999. Revision September 25, 2001.     

Determination of Active Metal in Ultradispersed Iron Powders and TG Study of Their Oxidation

Two oxidation stages of electrolytic ultradispersed iron powder at the temperature range of 90–450°C have been stated. The contribution of increasing mass and evolving heat at the first oxidation stage due to changing Fe⁰ into Fe₂O₃ in the total oxidation effect is predominant. The thermal method of active metal determination in electrolytic iron powders has been developed. The coarse-grained reduced iron powder was not oxidized completely just to 900°C because of local sintering of big iron particles as a result of evolving heat at oxidation of high-dispersed iron particles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of atmosphere and heating rate on mechanism of MoSi<Subscript>2</Subscript> formation during self-propagating high-temperature synthesis

To study the effect of atmospheric type and heating rate on formation mechanism of MoSi₂, the Mo + 2Si powder mixture was exposed to simultaneous thermal analysis (STA) in air atmosphere at different heating rates (10, 15, and 20 °C/min). To further study the changes, thermal analyses of molybdenum powders and consumed silicon were also performed separately. An amount of aluminum powder (5 wt.%) was also added to Mo + 2Si powder mixture and exposed to thermal analysis at different heating rates (10, 15, and 20 °C/min) to study the effect of the presence of active elements (like aluminum) on the trend of the performance of changes. To perform phase studies on the products of the thermal analysis at a later stage, each product was separately tested by an X-ray diffraction (XRD) method. Contrary to expectations, the XRD patterns showed that the trends of changes during thermal analysis were not in the direction of MoSi₂, and the DTA–TG peaks obtained from these analyses were in fact related to other changes. Ultimately, the results showed that the peaks on the DTA curves resulted from the oxidation of molybdenum particles; and the (MoO₃) melt of the product, and in continuation of the reduction of a part of this oxide, it resulted during the silicothermic and aluminothermy reactions. The results of this research also showed that with regard to the presence of intensive oxidation tendency of molybdenum particles, there is no chance for the formation of MoSi₂ by heating the powder mixture of Mo + 2Si in air atmosphere and at low heating rates.     

Analysis of the oxidation process of powders and sinters of the austenitic stainless steel

Nowadays, austenitic stainless steels due to its properties are widely used in various applications. In powder metallurgy technology, one of the key factors, affecting the pressing process, the sintering phenomena and the final properties of sintered parts, is the shape and particle size of the powders. The article presents the results focused on analysis of the oxidation process of austenitic stainless steel. Powders of the same chemical composition, however of different shapes, depending on the manufacturing technique, i.e. spherical after inert gas atomization and sponge after water atomization, were investigated. Moreover, the influence of particles’ size from different ranges 40–56 and 80–100 µm on the oxidation behaviour was analysed. Thermal measurements, differential scanning calorimetry and thermogravimetry, were performed by the STA 409 CD (Netzsch) advanced coupling techniques. Moreover, dilatometry technique has been used for the analysis of the influence of the size and shape of particles on the sintering process. Sinters oxidation phenomena have also been determined. Based on the obtained results, it was found that both shape and size of powders have a significant influence on the oxidation processes of powders as well as sinters of the austenitic stainless steel.     

Experimental study and thermodynamic analysis on the selective oxidation of DP1180 dual-phase steel

The selective oxidation process of dual-phase 1180 steel was investigated in this study. Annealing was carried out in Cr/Cr₂O₃ Rhines pack at 820°C for 60, 120, and 240 s, under the oxygen partial pressures of 10⁻²³, 10⁻²⁵, and 10⁻²⁹ atm, respectively. Field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD), and X-ray photoelectron spectroscopy (XPS) were used to analyze the experimental results. The oxides involved SiO₂, MnO, MnSiO₃, Mn₂SiO₄, and MnCr₂O₄. The amount of oxides augmented along with the increasing oxygen partial pressure. The Fe–Mn–Cr–Si–C–O₂ oxidation diagram was calculated by thermodynamic software and was used to explain the selective oxidation process of dual-phase 1180 steel. Experimental results combined with the thermodynamic computations well.     

Direct multielement determination of trace elements in boron carbide powders by direct current arc atomic emission spectrometry using a CCD spectrometer

The use of direct current arc atomic emission spectrometry (DC-arc-AES) with a CCD spectrometer for the direct determination of the trace impurities Al, Ca, Cr, Cu, Fe, Mg, Mn, Na, Ni, Si, Ti, and Zr in three well characterized boron carbide powders is described. The detection limits obtained by the procedure were found to be between 0.2 (Mg) and 25 (Na) ??g?g^?1 for the above elements. Three boron carbide powder samples with trace element concentrations between 0.9 (Cu) and 934 (Si) ??g?g^?1 for Al, Ca, Cr, Cu, Fe, Mg, Mn, Na, Ni, Si, Ti, and Zr ?? including the standard reference material ERM?-ED102 ?? were analyzed by DC-arc-AES. The relative standard deviations for 9 measurements when using 5.0?±?0.3?mg of the respective samples were found to vary from 6.2 to 27% for Al and Cu, respectively. The trace elements Al, Ca, Cr, Cu, Fe, Mn, Ni, Si, Ti and Zr could be determined in the standard reference material and their concentrations determined by DC-arc AES were found to be between 89 and 116% of the accepted values. Fe and Ti were determined by DC-arc AES in the three boron carbide samples as well as in Al₂O₃, BN, SiC, coal fly ash, graphite and obsidian rock. The correlation coefficients of the plots of the net intensities versus the accepted values over the concentration ranges from 18 to 1750 and from 6 to 8000???g?g^?1 are 0.999 and 0.990 for Fe and Ti, respectively.

电感耦合等离子体原子发射光谱法测定不锈钢中8种合金元素

采用硝酸-盐酸-水(1+3+6)混合酸溶液溶解不锈钢样品,用电感耦合等离子体原子发射光谱法同时测定试样溶液中铬、镍、铜、锰、磷、硅、钼和钛等8种合金元素。选择钇元素作为内标元素,选择波长为357.869,231.604,327.396,257.610,178.284,251.611,202.030,337.280 nm8条谱线依次作为铬、镍、铜、锰、磷、硅、钼和钛的分析线。方法用于分析了12种标准物质,测定值同证书值一致,各元素的相对标准偏差(n=7)均小于5.5%。     

High-temperature oxidation behaviour of low-entropy alloy to medium- and high-entropy alloys

The high-temperature oxidation behaviour of CoCrNi, CoCrNiMn, and CoCrNiMnFe equimolar alloys was investigated. All three alloys have a single-phase face-centred cubic structure. Thermogravimetric analyses (TGA) were conducted at temperatures ranging from 800 to 1000 °C for 24 h in dry air. The kinetic curves of the oxidation were measured by TGA, and the microstructure and chemical element distribution in different regions of the specimens were analysed. The oxidation kinetics of the three alloys followed the two-stage parabolic rate law, with rate constants generally increasing with increasing temperature. CoCrNi displayed the highest resistance to oxidation, followed by CoCrNiMnFe and CoCrNiMn exhibiting the least resistance to oxidation. The addition of Mn to CoCrNi increased the oxidation rate. The oxidation resistance of CoCrNiMn was enhanced by the addition of Fe. Less Mn Content and the formation of more Cr₂O₃ were responsible for the reduction in the oxidation rates of CoCrNiMnFe. The calculated activation energies of CoCrNiMn and CoCrNiMnFe at 800, 850 and 900 °C were 108 and 137 kJ mol^?1, respectively, and are comparable to that of Mn diffusion in Mn oxides. The diffusion of Mn through the oxides at 800–900 °C is considered to be the rate-limiting process. The intense diffusion of Cr at 1000 °C contributed to the formation of CrMn_1.5O₄ spinel with Mn in the outer layer of CoCrNiMn and Cr₂O₃ in the outer layer of CoCrNiMn.     

Production of doped ZnO powders for varistor applications using sol-gel techniques

The electrical behaviour of ZnO varistors is controlled by the characteristics of the ceramic microstructure which depends strongly on the properties of the initial powder. This paper describes a study of the effect of different chemical methods to synthesise doped ZnO powders. The ceramic powders were synthesised by the following routes: a) classical mixing of oxides (for comparison purposes), b) aqueous solution of inorganic salts and c) hydrolysis and polycondensation of metalorganic compounds in an organic solvent. The physicochemical characteristics of these powders were evaluated using a scanning electron microscope and thermoanalytical instrumentation. Standard spray drying technology was used to pelletise the powders to obtain an agglomerated powder suitable for uniaxial pressing. The discs were sintered in an electric furnace under air atmosphere using several temperature programmes. The ceramic microstructure was characterised using SEM and X-ray diffraction. The effects of powder processing route on sintering and microstructural development are discussed. Powders prepared by the metalorganic route exhibited somewhat lower sintering temperatures than conventional powders. However, the rate of sintering was slower for metalorganic and aqueous solution powders. These observations were related to powder morphology. The ZnO–Bi₂O₃–CoO system exhibited the best varistor characteristics as it was expected, whereas the binary systems supported much lower voltages at low currents than ternary systems.     

Structural and thermal investigation of Ta–25 mass% Cu alloy prepared by mechanosynthesis route

A mixture of Ta and 25 mass% Cu elemental powders was subjected to mechanical alloying in a high-energy ball mill up to 60 h. The results are composite particles formed by nanocrystalline Cu and amorphous Ta phases. Thermal stability of amorphous was investigated by DSC. The XRD, FTIR and EDX analyses of Ta–25 mass% Cu powder milled for 60 h performed after DSC at 800 and 900 °C have revealed large amounts of Ta nitride and Ta oxides even though the milling process was done in Ar atmosphere. This is due to high reactivity of Ta fine particles with oxygen and nitrogen from air. During manipulations of the powder (taking samples from vials and its investigation), the adsorption phenomena on its surface occur, and both surface-adsorbed N₂ and O₂ are processed with powder and embedded in it. While heating of Ta–25% Cu milled powder in DSC, nitrogen and oxygen diffusion into tantalum is activated, and Ta₂N and TaO₂/Ta₂O₅ compound forms.