Ana Sofia C. M. D'Oliveira

Microstructural features of consecutive layers of Stellite 6 deposited by laser cladding

Abstract Consecutive layers of a cobalt based alloy, Stellite 6, were deposited onto a 304 stainless steel plate. One, two and five consecutive layers were surface welded with a CO2 CW laser, in order to identify the effect of multilayer laser cladding on microstructure and residual stresses. Depositing two layers upon one another did not affect the microstructure and residual stresses. Further deposition of layers led to changes in hardness profile, microstructure and residual stresses intensity. Results are discussed on the basis of the potential of using laser cladding technology in industrial maintenance, considering that repairing components with worn surfaces often requires the ability to deal with different depth regions.

Phase transformation during cavitation erosion of a Co stainless steel

Abstract Cavitation erosion is a surface degradation mechanism that affects hydraulic turbine blades. Materials have been developed in order to minimise the detrimental effect. This work uses a Co stainless steel specially developed to exhibit improved cavitation resistance. The role of the γ austenite to e-martensite phase transformation during cavitation erosion is assessed and the influence of surface finish on the cavitation erosion behaviour is evaluated as welded surfaces were ground and remelted before ultrasonic cavitation testing. Results reveal that structure refinement due to remelting contributes to a longer incubation period and that phase transformation is responsible for up to 25% of this erosion stage.

Microstructural changes due to laser surface melting of an AISI 304 stainless steel

Several techniques can be used to improve surface properties. These can involve changes on the surface chemical composition (such as alloying and surface welding processes) or on the surface microstructure, such as hardening and melting. In the present work surface melting with a 3kW CO2 cw laser was done to alter surface features of an AISI 304 stainless steel. Microstructure characterisation was done by optical and scanning electron microscopy. Vickers and Knoop microhardness tests evaluated mechanical features after surface melting. Phase transformation during rapid solidification is analysed and discussed.

Effect of temperature and reactivity of molten 55Al-Zn alloy on Co based alloy coatings

Abstract The short service life of bearings in galvanising industry is a result of a complex set of deterioration mechanisms. This work addressed the effect of temperature and molten bath reactivity on the material of the bearings respectively. Three commercial alloys, the Co–Cr–W Stellite 6 and Co–Cr–Mo–Si Tribaloy (T400 and T800) alloy systems, were deposited by plasma transferred arc on AISI 316L plates. Coatings were evaluated for the effect of temperature exposure on hardness, microstructure and sliding abrasive wear. The reactivity with the molten 55Al–Zn alloy was assessed by immersion tests in an industrial bath. Results showed that exposure at 600°C for 168 h resulted in an increase in hardness, microstructure changes and loss of wear resistance for the Stellite 6 coatings. A superior performance to temperature was shown by Tribaloy T800 with a stable abrasive wear resistance. The three alloys exhibited a strong reactivity with the 55Al–Zn molten bath. An intermetallic layer formed on the coatings as the Al from the bath reacted with elements from the Co based alloys. This reactivity consumed the coatings, causing a reduction on thickness particularly on those processed with the T800 alloy.

Hardfacing by Plasma Transferred Arc Process

Em virtude do Processo de Soldagem Plasma com Alimentacao de Po ter similaridades com o Processo de Soldagem Plasma com Alimentacao de Arame, foi realizado um estudo comparativo entre ambos os processos utilizando-se a liga a base de cobalto comercialmente conhecida como Stellite 6, como material de adicao na forma de po e arame. A pesquisa foi realizada com a expectativa de ser aplicada nas operacoes de revestimentos de superficies, em especial em pas de turbinas hidraulicas desgastadas por cavitacao. A selecao do material de adicao a ser empregado depende da natureza do mecanismo de desgaste encontrado. No Labsolda, a liga Stellite 6 vem sendo uma das mais utilizadas, por apresentar uma excelente resistencia ao desgaste erosivo por cavitacao. Foi avaliada a influencia da vazao de gas de plasma a partir dos valores de diluicao, dimensoes do cordao, dureza e microestrutura. O Processo de Soldagem Plasma com Alimentacao de Po foi o que produziu o melhor acabamento superficial, menor diluicao, melhor molhamento e maior largura. Com isto abre-se uma nova perspectiva para revestimentos metalicos e neste contexto se insere a recuperacao por soldagem de partes erodidas de turbinas hidraulicas.

Effect of current and atomized grain size distribution on the solidification of Plasma Transferred Arc coatings

Plasma Transferred Arc (PTA) is the only thermal spray process that results in a metallurgical bond, being frequently described as a hardfacing process. The superior properties of coatings have been related to the fine microstructures obtained, which are finer than those processed under similar heat input with welding techniques using wire feedstock. This observation suggests that the atomized feedstock plays a role on the solidification of coatings. In this study a model for the role of the powders grains in the solidification of PTA coatings is put forward and discussed. An experiment was setup to discuss the model which involved the deposition of an atomized Co-based alloy with different grain size distributions and deposition currents. X ray diffraction showed that there were no phase changes due to the processing parameters. Microstructure analysis by Laser Confocal Microscopy, dilution with the substrate steel and Vickers microhardness were used the characterized coatings and enriched the discussion confirming the role of the powdered feedstock on the solidification of coatings.

Effect of processing on microstructure and properties of CoCrMoSi alloy

CoCrMoSi alloys microstructure is affected by processing and their properties have been associated with the high volume fraction of Laves phase. However, not much has been reported on the effect of structure refinement and morphology on the properties of these alloys. This paper evaluates a CoCrMoSi alloy processed by centrifugal casting and Plasma Transferred Arc (PTA) hardfacing, aiming to understand the correlation between microstructure and properties at room temperature in the as-processed condition and following exposure at 600 °C. Characterization was carried out using scanning electron microscopy, X-ray diffraction, hardness and wear tests. The cooling rates associated with each of the processing techniques used account for the developed microstructures and associated hardness, a 626 HV

Manufatura Aditiva: o papel da soldagem nesta janela de oportunidade

Additive manufacturing (AM) can be considered an evolution from Rapid prototyping as it allows to manufacture a component from a computer file (CAD 3D) though its applications extrapolate the production of prototypes. This technique involves the layered design of a component and subsequent welding deposition of the multilayer structure to produce parts without the need for molds or other tools. Although AM is frequently associated with the use of high density processes the need for higher competitiveness expanded its range of technologies to include arc welding processes. This paper aims to summarize up-to-date information on AM, particularly involving arc welding processes. Emphasis is given on the challenges associated with the building-up of components during multilayered deposition and on post-deposition procedures.

Solidification of PTA aluminide coatings

Abstract Components can be protected with coatings tailored using plasma transferred arc to meet specific demands as mixtures of powders with an almost unlimited chemical composition can be processed. In this study, Ni, Fe and Nb aluminide coatings were processed from mixtures of elemental powders on steel substrates and on a Cu refrigerated mold for comparison purposes. Coatings and mini-billets were characterized by means of microhardness, X-ray diffraction and laser confocal microscopy. Examination of the solidification structures showed that for each alloy system coatings are influenced by the exothermal synthesis of the aluminides and the interaction of the plasma arc with the substrate and with the feedstock. The poor flowability of the Ni–Al powder mixture determined its interaction with the plasma arc and strongly affected the dilution of coatings.

Determination of Residual Stresses in Carburized, Quenched and Tempered AISI 8620 Steel

The residual stresses (RSs), which result from processing, have a great influence on the mechanical behavior of components, particularly when these components are subject to failure mechanisms. The failure mechanisms, in turn, depend on local conditions such as fatigue, brittle fracture or stress-corrosion-cracking. In the case of carburized parts, the influence of RSs becomes particularly critical for thin walls, when overlapping carburized cases may result, having a detrimental influence on the fatigue resistance. This paper presents RSs results, obtained throug x-rays diffraction measured on the surface of carburized sheets of AISI 8620 steel, 0.5mm to 10 mm thick. The results show the influence of core hardness on the RSs on the surface, particularly when the effective case depth is larger than half the thickness. The influence of a stress relieving treatment at 180 C is also reported.