Ramón Sigifredo Cortés Paredes

Optimization of the Deposition Parameters of HVOF FeMnCrSi+Ni+B Thermally Sprayed Coatings

HVOF thermal spray process produces coatings with low porosity and low oxide content, as well as high substrate adhesion. Small variations on the parameters of the HVOF process can generate coatings with different characteristics and properties, which also is chemical composition depended of the alloy. FeMnCrSi alloy is a cavitation resistant class of material with a great potential for HVOF deposition use. The main goal of this article is to study the influence of some HVOF parameters deposition, as standoff distance, powder feed rate and carrier gas pressure on three different alloys. FeMnCrSi experimental alloys with some variations in nickel and boron content were studied. Taguchi experimental design with L9 orthogonal array was used in this work. Porosity, oxide content, tensile adhesion strength and microhardness of the coatings were evaluated. The results indicated that all factors have significant influence on these properties. Chemical composition of the alloys was the most important factor, followed by the carrier gas pressure, standoff distance and powder feed rate. The addition of Ni, produces coatings with lower levels of oxide content and porosity. An experiment with improved parameters was conducted, and a great improvement on the coating properties was observed.

Influence of Plasma Remelting on the Microstructure and Cavitation Resistance of Arc-Sprayed Fe-Mn-Cr-Si Alloy

Surface remelting is an important technique for modifying the microstructure of thermally sprayed coatings as it reduces the porosity and promotes a metallurgical bond between substrate and coating. Many studies have been carried out in the field of materials selection and surface engineering in an attempt to reduce cavitation damage. In this work, an Fe-Mn-Cr-Si alloy was deposited by arc spraying and then remelted by a plasma-transferred arc process. The base metal was a soft martensitic stainless steel. The influence of remelting current on coating and base metal microstructure and cavitation resistance was studied. The use of a lower mean current and a pulsed arc reduced the thickness of the heat-affected zone. In specimens remelted with constant arc current, dendrites were aligned parallel to the path followed by the plasma torch; while in those remelted with a pulsed plasma arc, the alignment of the microstructure was disrupted. The use of a higher peak current in pulsed-current plasma transferred arc remelting reduced mass loss due to cavitation. Fe-Mn-Cr-Si coatings exhibited cavitation-induced hardening, with martensite formation during cavitation tests. This transformation helps to increase the cavitation resistance of the remelted coating compared with the soft martensitic stainless steel base metal.

Desenvolvimento da ferrita delta na solda e ZTA resultante de soldagem plasma pulsada em um aço inoxidável supermartensítico

Abstract: The objective of this study was to investigate the effect of pulsed plasma welding on the distribution and amount of delta ferrite which is considered an undesirable phase in a supermartensitic stainless steel. For such weld beads have been obtained by melting the surface of a sample of a high-alloy supermartensitic stainless steel using pulsed and conventional current. Microstructural characterization, especially of delta ferrite, was performed by metallographic analysis by means of optical microscopy and scanning electron microscopy. The experiments showed that for the same heat input, pulsed current promoted the formation of delta ferrite in the form of bands along the weld bead and caused a significant reduction of this phase in HAZ as compared to conventional welding. Key-words: Delta ferrite, Supermartensitic, Pulsed welding. 1. Introducao Os acos inoxidaveis martensiticos de baixo carbono, divididos em martensiticos macios e supermartensiticos tem sido utilizados em diferentes areas da engenharia dentre estas como equipamentos de centrais hidreletricas e em tubulacoes empregadas na producao de petroleo. O uso de tais acos se deve a necessidade de aliar resistencia mecânica, resistencia a corrosao e soldabilidade. Apesar da boa soldabilidade destes acos em comparacao aos martensiticos convencionais, tratamentos termicos apos a soldagem sao normalmente requeridos de forma a minimizar os niveis de tensao residual e ainda garantir propriedades como tenacidade e resistencia a corrosao sob-tensao [1,2]. Em funcao de tais tratamentos nem sempre possiveis de serem realizados em campo, procedimentos especiais de soldagem sao necessarios de forma a minimizar os niveis de tensao residual junto as regioes soldadas. Dentre estes, a soldagem com corrente pulsada. Esta tecnica foi introduzida nos anos 60 como variante do processo com corrente constante e segundo a literatura resulta, dentre outras vantagens, em menor nivel de tensao residual e refino da granulacao da solda quando comparada ao processo convencional. Com relacao a microestrutura de tais acos, a mesma e constituida basicamente da fase martensitica e teores residuais de austenita, a qual promove o aumento da tenacidade. Apos a soldagem a austenita retida e eliminada, vindo a ser substituida pela fase deleteria de ferrita delta [3,4]. O efeito da ferrita delta na resistencia ao impacto dos acos inoxidaveis martensiticos e ainda um assunto controverso. No entanto, Carrouge [3] verificou que a presenca da ferrita delta interfere na resistencia ao impacto aumentando a temperatura de transicao ductil-fragil (TTDF) de -98 °C na condicao de martensita (

Avaliação da relação Creq. / Nieq. para o revestimento AF2209 depositado por aspersão térmica com e sem pré-aquecimento

A oxidacao de parte dos elementos quimicos que compoem o aco inoxidavel duplex durante a aplicacao como revestimento pelo processo de aspersao termica chama arame altera a relacao entre os equivalentes de cromo e niquel, de modo que se torna dificultada a previsao da microestrutura final do revestimento utilizando-se de diagramas constitucionais existentes como Schaeffler. O pre-aquecimento do substrato favorece o processo de deposicao da camada para as particulas aspergidas, mas tambem pode contribuir para a alteracao da relacao Creq./Nieq.. Este trabalho objetiva avaliar como a relacao Creq./Nieq. e modificada devido a oxidacao promovida pelo processo de aspersao, avaliando pelo diagrama de Schaeffler para o aco inoxidavel duplex AF2209 sobre substrato com e sem pre-aquecimento e sua influencia na quantidade final das fases ferrita e austenita solidificadas. Analises por metalografia otica, E.D.S. - M.E.V., D.R.X. e M.F.M. foram utilizadas na caracterizacao da composicao quimica e das fases presentes no revestimento. Como resultado observou-se a diferenca de microestrutura e composicao entre as panquecas aspergidas e a quantidade final das fases solidificadas em funcao do pre-aquecimento do substrato.

Estudo comparativo de três ligas austeníticas com cobalto resistentes à cavitação depositadas por plasma pulsado térmico

A necessidade de diminuicao dos tempos de parada e custos de manutencao de turbinas geradoras de energia eletrica tem motivado o desenvolvimento de novos materiais e processos de recuperacao de areas cavitadas. Atualmente diferentes processos automatizados de soldagem vem sendo estudados, principalmente a soldagem por plasma de arco transferido, PTA (plasma transferred arc), em substituicao a soldagem FCAW (flux cored arc welding). O processo PTA apresenta como principais vantagens a baixa diluicao, estreita zona termicamente afetada, excelente estabilidade de arco e baixo indice de respingos, entretanto, poucas ligas resistentes a cavitacao foram desenvolvidas para este processo. Este trabalho tem por objetivo comparar tres ligas resistentes a cavitacao com Cobalto, depositadas por PTA. A primeira desenvolvida para soldagem FCAW, a segunda desenvolvida para soldagem PTA e uma liga nacional em desenvolvimento do tipo inoxidavel austenitica com cobalto. As amostras foram analisadas atraves de microscopia otica, eletronica de varredura, microdureza Vickers, raios-X e ensaio de cavitacao acelerada, ASTM G-32-95. Os resultados mostraram a formacao de microestruturas austeniticas refinadas nas tres amostras. A liga comercial desenvolvida para soldagem PTA apresentou melhor estabilidade do arco, alem de menor quantidade de defeitos. A liga nacional apresentou bom resultado durante a deposicao, enquanto que a liga FCAW apresentou melhor resultado no ensaio de cavitacao.

Influence of Particle Size Distribution on the Morphology and Cavitation Resistance of High-Velocity Oxygen Fuel Coatings

The cavitation wear process is one of the major wear mechanisms in turbines and rotors of hydroelectric power plants in Brazil. An effective way to increase the cavitation resistance is the use of coatings, applied by thermal spraying. The high-velocity oxy-fuel process (HVOF) is one of the most used thermal spraying processes, and it is widely adopted for applying coatings for protection against wear and in maintenance components. A FeCrMnSiB experimental alloy was deposited onto SAE 1020 substrate by HVOF process, in order to evaluate the influence of the powder particle size range on the morphology and cavitation resistance of the coatings. The morphology of the coatings showed an increase in oxide content with powder size reduction. The increase in the powder particle size reduced the wettability of the particles, observed by the increase in the quantity of non-melted particles. Higher particle size distribution led to an increase in erosion rate, due to higher presence of non-melted particles in the coatings and consequently reduction of splats adhesion. The cavitation damage was perceived mainly by the mechanism of lamellae detachment; however, part of the damage was also absorbed by strain hardening due to the γ-ε martensitic transformation.

Microstructure and fatigue strength in a low-carbon martensitic stainless steel remelted by plasma torch

Abstract This study aimed to evaluate the effect of surface remelting by a plasma torch on fatigue resistance of a soft martensitic stainless steel used in the manufacture of hydraulic turbine rotors. The remelting was performed on the surface of this type of steel using direct and pulsed current with the fatigue tests performed by bending at four points at 750, 850 and 1000 MPa stresses. It was found better performance of the remelted condition in relation to cast material without this treatment and no significant differences in fatigue performance between the two remelted processes. The presence of compressive residual stresses resulting from the martensitic transformation partly explains the better performance of the remelted condition. Microscopic analysis further revealed that the lower the fatigue performance of the base material was also associated with the presence of defects (microshrinkage) from the casting process, which promoted the nucleation of fatigue cracks next to them. The presence of delta ferrite together with martensite laths oriented at 45° to the loading direction promoted fatigue crack nucleation in specimens with surface remelted treatment.

Development of delta ferrite on the weld and HAZ produced by pulsed plasma arc welding on a supermartensitic stainless steel

Delta ferrite is considered an undesirable phase in the microstructure of martensitic stainless steels, given its power to influence the performance of fatigue strength, toughness and corrosion under stress. This study aimed to investigate the effect of pulsed plasma welding on the distribution and amount of delta ferrite on a supermartensitic stainless steel (SMSS). Weld beads were obtained by surface melting on a sample of SMSS alloy using conventional and pulsed current. Then, the microstructural characterization was performed, in particular of the delta ferrite via metallographic analyses by optical and scanning electron microscopy. The experiments showed that for the same heat input, the pulse current promoted the formation of delta ferrite in the form of bands along the weld bead and a significant reduction of this phase in the heat-affected zone compared to conventional welding.

Comparative study of three austenitic alloy with cobalt resistant to cavitation deposited by plasma welding

The necessity to reduce time and maintenance costs in electrical energy generation turbines promotes the development of new materials and processes to recover cavitated areas. Nowadays, different automated welding process have been studied, mainly plasma-transferred arc welding (PTA) in substitution of fluxed core arc welding (FCAW). The PTA process presents as its main advantages: low dilution, a narrow heat-affected zone and excellent arc stability; however, few cavitation resistant alloys are developed for this process. This paper aims to compare three cobalt cavitation resistant alloys deposited with the PTA process. The first alloy is a cobalt stainless steel alloy developed for the FCAW process, the second is a cobalt stainless steel alloy developed for the PTA process and the third is a national developed stainless steel alloy with cobalt. The samples were analysed by optical and electronic microscopy, microhardness and accelerated cavitation testing, ASTM G32-95. Results show that a refined austenitic microstructure was observed in all samples. The commercial alloys, developed for PTA welding, presented a better arc stability and lower quantity of defects. The national alloy demonstrated good results during deposition while the FCAW alloy presented better cavitation resistance.

DESENVOLVIMENTO E USO DO COMPÓSITO DE Nb2O5|Cu COMO REVESTIMENTO APLICADO POR ASPERSÃO TÉRMICA SOBRE O AÇO AISI 1020 PARA PROTEÇÃO CONTRA A CORROSÃO PELO SOLO EM ESTRUTURAS ENTERRADAS

An Nb2O|Cu corrosion-resistant coating was developed and applied onto AISI 1020 steel substrate by Powder Flame Spray. A galvanostatic electrochemical technique was employed, with and without ohmic drop, in four different soils (two corrosively aggressive and two less aggressive). Behavior of coatings in different soils was compared using a cathodic hydrogen reduction reaction (equilibrium potential, overvoltage and exchange current density) focusing on the effect of ohmic drop. Results allow recommendation of Nb2O5|Cu composite for use in buried structure protection.