Sérgio Luiz Henke

Effect of post-weld heat treatment on fatigue crack propagation in welded joints in CA6NM martensite stainless steel

Welding International Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t778164493 Effect of post-weld heat treatment on fatigue crack propagation in welded joints in CA6NM martensite stainless steel A. G. M. Pukasiewicz a; S. L. Henke b; W. J. P. Casas c a CEFET-PR Unidade de Ponta Grossa/Coordenacao de Mecânica, Ponta Grossa, Parana, Brazil. b LACTEC Instituto de Tecnologia para o Desenvolvimento, Curitiba, Parana, Brazil. c UFRGS Universidade Federal do Rio Grande do Sul/Departamento de Mecânica, Porto Alegre, Rio Grande do Sul, Brazil.

Microstructural control of Co-based PTA coatings

Cobalt-based alloys are widely used as hardfacing materials when wear resistance is required at room temperature or high temperature applications. However, their performance is a consequence of their microstructures that depends on the processing conditions. This work focused on the influence of solidification rate on the structure development by processing the alloys with and without the interference of the substrate. The coatings were characterized by scanning electron microscopy, energy dispersive spectrometer, optical microscopy and instrument indentation tests. Results showed that despite the same phases developed in tested conditions, differences in the solidification microstructure and the influence of Fe diffusing from the substrate accounted for the measured variation in hardness. Higher hardness values were obtained for the samples processed free-standing (mini billets) with respect to the coatings and they were independent of the processing parameters, indicating that the substrate compromise the properties of hardness, as expected.

Efeito da temperatura interpasse na microestrutura, tenacidade ao impacto e propagação de trinca por fadiga de uniões soldadas por GTAW do aço ASTM A743-CA6NM

Martensitic stainless steels have been used for hydraulic turbines manufacturing, because its high toughness. However, these steels have some restrictions regarding regions recovered by welding, mainly by non-tempered martensite formation, promoting toughness reduction. Concerning hydraulic turbine repairs, there is a great interest in welding procedures development that promotes a better toughness, without post welding heat treatment (PWHT). The mainly objective of this paper is analyze the influence of interpass temperature on the microstructure, impact toughness and fatigue crack propagation in CA6NM martensitic stainless steel multipass welded joints, with AWS410NiMo filler metal, with GTAW (gas tungsten arc welding). It was observed the interpass temperature influence on ferrite δ formation, observing intergranular ferrite d formation on the d+g field in 80 oC interpass temperature, while the sample welded at 150 oC the formation of ferrite d occurs mainly in the δ monophase field. Ferrite d formation with the lowest temperature interpass promoted an increase in impact toughness and a decrease in the fatigue crack propagation when compared with 150oC interpass temperature sample. It was observed that GTAW process can be an excellent alternative for CA6NM hydraulic turbine repair, it was also observed a significant interpass temperature influence.

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 (

Fracture toughness of CA6NM alloy, quenched and tempered, and of its welded joint without PWHT

CA6NM quenched and tempered steel is used in hydraulic turbine rotors, pumps and compressors. The objective of this research is to determine the fracture toughness of tempered and quenched CA6NM alloy, and of its welded joints without post-welded heat treatment (PWHT). To this end, compact tension (CT) test pieces are milled from pieces of CA6NM steel for evaluation of the toughness of the alloy used in a hydraulic turbine. Due to the elasto-plastic condition of the material, the test pieces are tested by means of the J integral concept, setting out the resistance curve J–R and the crack initiation J IC. In welded joints produced from ingots, without PWHT, the fragility they show does not allow the J–R curve for the CT test pieces to be drawn up, and the toughness is characterized by means of the K IC concept. The welding procedure looks at the probable conditions for repair of cavitation wear to the turbine, where PWHT cannot be carried out. The results confirmed the higher toughness for the CA6NM steel, with values approximately three times higher than those obtained in the welded joints without PWHT. In terms of the fracture, the CA6NM steel shows ductile behaviour while the welded joint without PWHT shows fragile behaviour.

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.

Effect of Cold Work on Cavitation Resistance of an Austenitic Stainless Steel Coating

Machining procedures of welding deposits are usual and result on cold work hardened surfaces. The cold work effect on cavitation erosion of an austenitic stainless steel surface is assessed. FeCrMnSiB coatings were processed by PTA on AISI 304 plates. Specimens were grouped as the cold work deformed surface (CWHS) and the undeformed polished surface (UPS) specimens. Top surface and transverse section of coatings were analysed for slip lines and hardness changes by light microscopy and Vickers microhardness measurements. Ultrasonic cavitation tests were conducted in accordance to ASTM G32-10. CWHS specimens exhibited slip lines and hardened surfaces while UPS specimens did not show traces of slip lines and had insignificant changes on microhardness. Cold work prior to cavitation indirectly increased the nominal incubation time and reduced the maximum erosion rate. Cold work increases the duration of the acceleration period postponing the onset of the maximum erosion rate and enhancing cavitation resistance.

Effect of interpass temperature on microstructure, impact toughness and fatigue crack propagation in joints welded using the GTAW process on steel ASTM A743-CA6NM

Mainly due to their great toughness, martensitic stainless steels are used for manufacturing hydraulic turbines. However, these steels have some restrictions regarding regions recovered by welding, mainly due to the formation of non-quenched martensite, which causes a reduction in toughness. Considering repair of hydraulic turbines, there is a great interest in developing welding procedures that increase impact toughness and avoid post-welding heat treatment (TTPS). This study aims to analyse the influence of interpass temperature on microstructure, impact toughness and fatigue crack propagation in multipass welded joints on martensitic stainless steel CA6NM, using AWS410NiMo filler metal and the gas tungsten arc welding (GTAW) process. In the sample with interpass temperature of 80°C, influence of the interpass temperature on the formation of ferrite δ, with intragranular formation in the two-phase δ field, was observed, while in the sample welded at 150°C, the formation of ferrite δδ occurred mainly in the single-phase field. The change in the formation of ferrite δ, with the low interpass temperature, promoted an increase in impact toughness and a decrease in the fatigue crack propagation when compared with the sample welded with a higher interpass temperature. The results obtained indicate that the TIG process is an excellent alternative for the repair of CA6NM steel, with a significant influence from the interpass temperature.

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.

First Results of Cavitation Erosion Behavior of Plasma Nitrided Niobium: Surface Modification

This work presents the first results of the plasma nitriding study performed in pure niobium in order to increase its cavitation erosion resistance. Samples were prepared from 98.9% purity and 90% reduction cold-rolled niobium bars. Annealing treatment of the cold-worked niobium samples was carried out in vacuum furnace at 1.33 Pa pressure, in the temperature of 1000 °C, for a time of 60 min. Annealed samples showing hardness of 80 HV were cut to dimensions of 20 × 30 × 4 mm3. Nitriding treatment was conducted at 1080 °C, gas mixture of 90% N2 + 10% H2, flow rate of 5 × 10–6 Nm3s–1, and pressure of 1200 Pa (9 Torr), for a total time of 4 h comprised by two treatment steps of 2 h each. For comparison purpose, results for nitrided and non-nitrided niobium are confronted. Samples were characterized by XRD, nanoindentation, microhardness, SEM, and 2D surface topography and 3D interferometry profile analysis techniques. Cavitation testing was conducted according to ASTM G32-09. Comparatively, promising results based on the formation of niobium nitride phases in treated surfaces are presented and discussed in the present work.