The shaft furnace pre-reduction of chromite pellets may offer a cost-effective alternative to the capital-intensive grate-kiln system for ferrochrome production. This study investigated the temperature-dependent reduction and consolidation behaviour under simplified shaft-furnace-oriented conditions in a high-temperature tube furnace. Using a coke reductant (C/O molar ratio = 0.8) under an argon atmosphere, we evaluated the effects of temperature (1000-1400 degrees C) on reduction degree and crushing strength of pellets. At 1300 degrees C, the pellets simultaneously achieved a high reduction degree of 66.23% and a cold crushing strength of 864.6 N, thereby meeting the stringent feed requirements for submerged arc furnaces (SAFs). Microstructural characterisation revealed that the strength enhancement was linked to the formation of a continuous metallic iron network and subsequently to an FeCr alloy. The reduction mechanism proceeded through three distinct stages: initial pore formation via O2- removal, growth of a metallic iron layer, and final chromium reduction. The last stage was governed by solid-state diffusion of Fe2+, Cr3+, and O2- ions after the dense product layer formed. These results clarify the micro-mechanisms controlling pellet quality and provide a basis for further assessment of shaft-furnace-oriented chromite pellet pre-reduction. La pr & eacute;-r & eacute;duction des boulettes de chromite dans un four vertical pourrait offrir une solution rentable par rapport au syst & egrave;me de four & agrave; grille, & agrave; forte intensit & eacute; de capital, pour la production de ferrochrome. Cette & eacute;tude a examin & eacute; le comportement de r & eacute;duction et de consolidation en fonction de la temp & eacute;rature dans des conditions simplifi & eacute;es, orient & eacute;es vers un four vertical dans un four & agrave; tube & agrave; haute temp & eacute;rature. En utilisant du coke comme r & eacute;ducteur (rapport molaire C/O = 0.8) sous atmosph & egrave;re d'argon, nous avons & eacute;valu & eacute; les effets de la temp & eacute;rature (1000 & agrave; 1400 degrees C) sur le degr & eacute; de r & eacute;duction et la r & eacute;sistance & agrave; la compression des boulettes. & Agrave; 1300 degrees C, les boulettes ont atteint en m & ecirc;me temps un degr & eacute; de r & eacute;duction & eacute;lev & eacute; de 66.23% et une r & eacute;sistance & agrave; la compression & agrave; froid de 864.6 N, r & eacute;pondant ainsi aux exigences strictes d'alimentation des fours & agrave; arc submerg & eacute; (SAF). La caract & eacute;risation microstructurale a r & eacute;v & eacute;l & eacute; que l'am & eacute;lioration de la r & eacute;sistance & eacute;tait li & eacute;e & agrave; la formation d'un r & eacute;seau continu de fer m & eacute;tallique et subs & eacute;quemment, & agrave; un alliage FeCr. Le m & eacute;canisme de r & eacute;duction s'est d & eacute;roul & eacute; en trois & eacute;tapes distinctes: formation initiale de pores par & eacute;limination d'O2-, croissance de la couche de fer m & eacute;tallique et r & eacute;duction finale du chrome. La derni & egrave;re & eacute;tape & eacute;tait contr & ocirc;l & eacute;e par la diffusion & agrave; l'& eacute;tat solide des ions Fe2+, Cr3+ et O2- apr & egrave;s la formation de la couche de produit dense. Ces r & eacute;sultats clarifient les micro-m & eacute;canismes contr & ocirc;lant la qualit & eacute; des boulettes et pr & eacute;sentent une base pour une & eacute;valuation future de la pr & eacute;-r & eacute;duction des boulettes de chromite destin & eacute;es au four vertical.
Pipeline welding plays an important role in oil and gas transportation. The use of new materials in pipeline applications reflects the importance of improved understanding of welding requirements to reduce the occurrence of future failures. This is driving the development and formulation of appropriate welding fluxes for making effective welding joints of these materials. The present study investigates and compares the microstructural evolution and mechanical properties of the weldments of API X70 steel fabricated by submerged arc welding with a commercial flux and laboratory-prepared agglomerated fluxes. The laboratory basic fluxes were formulated to determine their effects on the weld-metal chemistry and performance as compared to a standard commercial type of flux. Microstructural investigation showed that the acicular-ferrite and the martensite-austenite (MA) constituents strongly depend on the flux composition. Maximum microhardness observed in the fusion zone for laboratory-prepared and commercial flux was 233 HV & 224 HV, while in the heat-affected zone (HAZ) it was 257 HV & 251 HV. Laboratory-prepared fluxes exhibited moderately low-temperature impact toughness (-55C) for both fusion and heat-affected zone but comparable with the commercial flux. Mechanical properties of laboratory-prepared fluxes were comparable to those of the commercial flux. Le soudage des pipelines joue un r & ocirc;le important dans le transport du p & eacute;trole et du gaz. L'utilisation de nouveaux mat & eacute;riaux dans les applications de pipelines refl & egrave;tent l'importance d'une meilleure compr & eacute;hension des exigences de soudage afin de r & eacute;duire la fr & eacute;quence des d & eacute;faillances futures. Ceci stimule le d & eacute;veloppement et la formulation de flux de soudage appropri & eacute;s pour r & eacute;aliser des assemblages soud & eacute;s efficaces de ces mat & eacute;riaux. L'& eacute;tude courante examine et compare l'& eacute;volution microstructurale et les propri & eacute;t & eacute;s m & eacute;caniques d'assemblages soud & eacute;s d'acier API X70 fabriqu & eacute;s par soudage & agrave; l'arc submerg & eacute; avec un flux commercial et des flux agglom & eacute;r & eacute;s pr & eacute;par & eacute;s en laboratoire. On a formul & eacute; les flux de base du laboratoire afin de d & eacute;terminer leurs effets sur la chimie et les performances du m & eacute;tal soud & eacute; par rapport & agrave; un flux commercial standard. L'examen microstructural a montr & eacute; que la ferrite aciculaire et les constituants martensitique-aust & eacute;nitique (MA) d & eacute;pendent fortement de la composition du flux. La microduret & eacute; maximale observ & eacute;e dans la zone de fusion pour le flux pr & eacute;par & eacute; en laboratoire et le flux commercial & eacute;tait de 233 HV et 224 HV, alors que dans la zone thermiquement affect & eacute;e (HAZ), elle & eacute;tait de 257 HV et 251 HV. Les flux pr & eacute;par & eacute;s en laboratoire pr & eacute;sentaient une r & eacute;sistance au choc mod & eacute;r & eacute;e & agrave; basse temp & eacute;rature (-55 degrees C) pour la zone de fusion et la zone thermiquement affect & eacute;e, comparable & agrave; celle du flux commercial. Les propri & eacute;t & eacute;s m & eacute;caniques des flux pr & eacute;par & eacute;s en laboratoire & eacute;taient comparables & agrave; celles du flux commercial.
In this study, Ni-Al2O3-TiO2-WC composite coatings were fabricated using both direct current (DC) and multi-step pulse current (PC) electroplating methods. The metallurgical features, as well as the wear and corrosion behaviours of the deposited layers were evaluated. The results showed that the coatings generated by the pulsed current (PC) method exhibit superior wear and corrosion resistance. Specifically, wear weight loss improved from 12.4 mg (best DC condition) to 8.8 mg in the PC method, while corrosion current density decreased from 7.02E-7 A.cm-2 to 5.95E-7 A.cm-2. The nickel matrix deposits produced via the pulsed method, which included multi-step positive and negative currents with hardness values of 379 and 320 microVickers, were recognised as the most wear-resistant coatings, with a recorded wear weight loss of 7.5 and 6.7 mg, respectively. The incorporation of more than 30 vol.-% of reinforcing particles, together with a compact morphology, significantly enhanced the wear performance. Conversely, the coating produced with a plating current density of 8 A/dm2, with positive, zero, and negative plating current cycles exhibited the lowest corrosion current density (J corr = 4.74E-8) despite only containing 10 vol.-% reinforcement. This improvement is attributed to the reduced of micron-cracks and pores and the high compactness of morphology. Dans cette & eacute;tude, on a fabriqu & eacute; des rev & ecirc;tements composites Ni-Al2O3-TiO2-WC en utilisant des m & eacute;thodes d'& eacute;lectrolyse en courant continu (CC) et en courant puls & eacute; (CP) & agrave; plusieurs & eacute;tapes. On a & eacute;valu & eacute; les caract & eacute;ristiques m & eacute;tallurgiques ainsi que les comportements & agrave; l'usure et & agrave; la corrosion des couches d & eacute;pos & eacute;es. Les r & eacute;sultats ont montr & eacute; que les rev & ecirc;tements obtenus par la m & eacute;thode du courant puls & eacute; (CP) pr & eacute;sentent une r & eacute;sistance sup & eacute;rieure & agrave; l'usure et & agrave; la corrosion. Sp & eacute;cifiquement, la perte de masse par usure est pass & eacute;e de 12.4 mg (meilleure condition CC) & agrave; 8.8 mg avec la m & eacute;thode de CP, tandis que la densit & eacute; de courant de corrosion a diminu & eacute; de 7.02E-7 A & centerdot;cm-2 & agrave; 5.95E-7 A & centerdot;cm-2. On a observ & eacute; que les d & eacute;p & ocirc;ts de matrice de nickel produits par la m & eacute;thode de CP, qui comprenait des courants positifs et n & eacute;gatifs & agrave; plusieurs & eacute;tapes avec des duret & eacute;s de 379 et 320 microVickers, constituaient les rev & ecirc;tements les plus r & eacute;sistants & agrave; l'usure, avec une perte de masse par usure enregistr & eacute;e de 7.5 et 6.7 mg, respectivement. L'incorporation de plus de 30% en volume de particules de renfort, associ & eacute;e & agrave; une morphologie compacte, a am & eacute;lior & eacute; significativement la r & eacute;sistance & agrave; l'usure. Inversement, le rev & ecirc;tement obtenu avec une densit & eacute; de courant de placage de 8 A/dm2, avec des cycles de courant de placage positifs, nuls et n & eacute;gatifs, pr & eacute;sentait la plus faible densit & eacute; de courant de corrosion (Jcorr=4.74E-8), en d & eacute;pit de sa teneur en renfort de seulement 10% en volume. Cette am & eacute;lioration est attribu & eacute;e & agrave; la r & eacute;duction des microfissures et des pores, ainsi qu'& agrave; la grande compacit & eacute; de la morphologie.
This study presents a systematic multi-objective optimisation approach for friction stir-welded TiO2-reinforced AA2024-T3 joints. The Box-Behnken design (BBD) is incorporated within the response surface methodology (RSM) framework along with a desirability function technique. The aim is to improve ultimate tensile strength (UTS) and percentage elongation (%E) at the same time. This helps in balancing the trade-off between strength and ductility. A quadratic regression model is developed at a 95% confidence level to explain the tensile and elongation behaviour of the welded joints. Analysis of variance (ANOVA) is used to check model accuracy and identify the important process parameters and their interactions. The optimisation results show that tool rotational speed (TRS) and welding speed (WS) strongly affect joint performance. The tool tilt angle (TTA) also plays an important role in producing defect-free joints with better mechanical properties. Fractographic analysis of the joint without nanoparticles (WN) revealed a relatively smooth fracture surface with fewer shallow dimples and occasional flat facets, indicating limited plastic deformation. At the optimum conditions of TRS 1366 rpm, WS 40 mm/min and TTA 2 degrees, the joint achieved 366.67 MPa UTS and 8.34% elongation. Nanoparticle addition increased UTS and % elongation by 15.30% and 27.90%, respectively.
Crevice corrosion resistance of stainless steels (SSs) in chloride environments is governed by the coupled effects of alloy chemistry, passive-film stability, and localised corrosion kinetics. This study systematically correlates alloy composition, electrochemical behaviour, passive-film chemistry, and crevice corrosion resistance of AISI 304, AISI 317, and UNS S2205 SSs using potentiodynamic polarisation, ASTM G48 immersion testing, FE-SEM/EDS, optical profilometry, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Localised corrosion resistance increased progressively from AISI 304 to AISI 317 and UNS S2205, with pitting potentials of 0.081, 0.853, and 0.978 VSCE, respectively. AISI 304 exhibited the highest corrosion rate (4.079 +/- 0.16 mm/y), and crevice penetration depth (similar to 2.01 mm), whereas UNS S2205 showed the lowest values (2.798 +/- 0.35 mm/y, and similar to 0.77 mm). AISI 317 displayed intermediate behaviour owing to Mo-assisted passive-film stabilisation and repassivation. Raman spectroscopy identified Fe oxides and oxyhydroxides as the principal corrosion products, while XPS revealed a compact Cr- and Mo-enriched passive film on UNS S2205, in contrast to the Fe-rich passive films formed on the austenitic grades. The superior corrosion resistance of UNS S2205 is attributed to the synergistic effects of higher Cr, Mo, and N contents which collectively enhance passive-film stability and suppress chloride-induced localised corrosion. La r & eacute;sistance & agrave; la corrosion par crevasse des aciers inoxydables (SS) en milieux chlor & eacute;s est contr & ocirc;l & eacute;e par les effets combin & eacute;s de la chimie de l'alliage, de la stabilit & eacute; du film passif et de la cin & eacute;tique de la corrosion localis & eacute;e. Cette & eacute;tude & eacute;tablit une corr & eacute;lation syst & eacute;matique entre la composition de l'alliage, le comportement & eacute;lectrochimique, la chimie du film passif et la r & eacute;sistance & agrave; la corrosion par crevasse des SS AISI 304, AISI 317 et UNS S2205 en utilisant la polarisation potentiodynamique, les essais d'immersion ASTM G48, la FE-MEB/EDS, la profilom & eacute;trie optique, la spectroscopie Raman et la spectroscopie de photo & eacute;lectrons X (XPS). La r & eacute;sistance & agrave; la corrosion localis & eacute;e a augment & eacute; progressivement de l'AISI 304 & agrave; l'AISI 317 et & agrave; l'UNS S2205, avec des potentiels de piq & ucirc;ration de 0.081, 0.853 et 0.978 VSCE, respectivement. L'AISI 304 pr & eacute;sentait le taux de corrosion le plus & eacute;lev & eacute; (4.079 +/- 0.16 mm/an) et la plus grande profondeur de p & eacute;n & eacute;tration des crevasses (similar to 2.01 mm), alors que l'UNS S2205 a montr & eacute; les valeurs les plus faibles (2.798 +/- 0.35 mm/an, et similar to 0.77 mm). L'AISI 317 pr & eacute;sentait un comportement interm & eacute;diaire gr & acirc;ce & agrave; la stabilisation et & agrave; la repassivation du film passif assist & eacute;es par le Mo. La spectroscopie Raman a identifi & eacute; des oxydes et des oxyhydroxydes de Fe comme principaux produits de corrosion, alors que l'XPS a r & eacute;v & eacute;l & eacute; un film passif compact enrichi en Cr et Mo sur l'UNS S2205, contrairement aux films passifs riches en Fe form & eacute;s sur les aciers aust & eacute;nitiques. La r & eacute;sistance sup & eacute;rieure & agrave; la corrosion de l'UNS S2205 est attribu & eacute;e aux effets synergiques des teneurs plus & eacute;lev & eacute;es en Cr, Mo et N qui am & eacute;liorent collectivement la stabilit & eacute; du film passif et suppriment la corrosion localis & eacute;e induite par les chlorures.
