Abstract Maraging steels are a class of ultra high strength steels of special importance due to their extremely high mechanical strength and good toughness. In this work, the effects of the solution annealing temperature on the mechanical properties of the maraging 300 steels were evaluated, in order to maximize the toughness without considerable detriment of the mechanical strength. Five solution annealing temperatures were evaluated. The characterization of the mechanical properties was done by tests of Rockwell C hardness, Charpy-V impact toughness, and tensile and fracture toughness in plane strain. The results obtained show that the fracture toughness increases, and the tensile strength decreases with the temperature of the solution annealing. In this way it was possible to find a heat treatment condition in which it was possible to raise about 20% of the toughness with a loss of only 6% in the tensile strength.

Mӧssbauer spectroscopy, X-ray diffraction and hardness measurements were used to investigate the atomic redistribution and phase transformation of solution-treated and 440°C aged Maraging-300 steel specimens. The ageing temperature that resulted in the best mechanical properties in this steel was 480ºC. At 440ºC, the formation of the precipitates that assure excellent mechanical properties of the maraging steel (Ni3Ti, Fe2Mo) was not expected. The results indicated that Mo and Ti redistribution was severe during the first hour of ageing. The mobility of the Ni atoms probably occurred during all stages of the ageing process, and the Co atoms remained close to Fe atomic sites. An irregular behaviour of the formation of paramagnetic phases containing iron was observed, and some evidence of the formation of these phases without Fe will be discussed. The formation of metastable phases was suggested and the possibility of austenite reversion was not discarded at this ageing temperature.

RESUMO Aços inoxidáveis super ferríticos são utilizados em equipamentos como trocadores de calor e torres de destilação que operam em condições extremas de temperatura, pressão e em ambientes contendo altas concentrações de cloretos, H2S e CO2, encontradas nas novas áreas de exploração de petróleo e gás na costa brasileira. Uma liga super ferrítica com composição baseada em aços comerciais foi desenvolvida para a realização do presente estudo que visa adaptar a composição dos aços super ferríticos, com o objetivo de aprimorá-los. A liga desenvolvida contem 4,4%p de níquel (Ni) para melhorar a sua tenacidade, 25,7%p de cromo (Cr) para que a estabilidade da fase ferrítica fosse mantida devido a adição de Ni, 4,6%p de Mo para aumentar a resistência à corrosão. O diferencial para o estudo desta liga foi a adição de 0,07%p (700 ppm) de boro, visando o aumento da temperabilidade, tenacidade e estabilidade de contornos de grão. Foram realizados cálculos termodinâmicos através do programa computacional Thermo-Calc para a simulação dos diagramas de fase e a fração em peso das fases presentes no material. Os resultados das composições experimentais foram comparados com outros trabalhos já realizados com o aço comercial UNS S44660. Através dos resultados obtidos uma estimativa da temperatura de estabilidade da fase ferrítica foi determinada e as possíveis fases deletérias foram identificadas de acordo com variação da temperatura e da composição química.

ABSTRACT Super ferritic stainless steels are used in equipment such as heat exchangers and distillation towers operating under extreme temperature and pressure conditions and in environments containing high concentrations of chlorides, H2S and CO2 found in the new areas of oil and gas exploration on the Brazilian coast. A super ferritic alloy with composition based on commercial steels was developed for the present study, aiming to adapt the composition of super ferritic steels in order to improve them. The developed alloy contains 4.4 wt.% of nickel (Ni) to improve toughness, 25.7wt.% of chromium (Cr) so that the stability of the ferritic phase is maintained due to the addition of Ni, 4.6wt.% of Mo to increase the corrosion resistance. The differential for the study of this alloy was the addition of 0.07wt.% (700 ppm) of boron, aiming to increase hardenability, toughness and grain boundary stabilization. Thermodynamic calculations were performed using the Thermo-Calc software for the simulation of phase diagrams and the weight fraction of the phases present in the material. The results of the experimental compositions were compared with results from commercial steel UNS S44660. The results obtained an estimate of the stability temperature of the ferritic phase and determined the possible deleterious phases according to temperature variation and chemical composition.

Resumo A soldagem de juntas dissimilares é muito comum em sistemas utilizados na exploração e produção de petróleo em águas profundas. Comumente envolve a soldagem de tubos de aço de baixo carbono com válvulas, bombas ou outros tubos, todos feitos de aços ligados. Vem sendo prática nessas soldagens a utilização de Inconel como material de enchimento; no entanto, esta soldagem é precedida por um processo de amanteigamento no lado de aço baixa liga e um tratamento térmico de alívio de tensões. Uma técnica alternativa apresentada neste trabalho é a utilização de um metal de enchimento de solda que durante o processo de resfriamento sofre uma transformação martensítica em uma temperatura relativamente baixa, de modo que o processamento de deformação resultante compensa a contração que ocorre como resultado do processo de resfriamento. O objetivo deste estudo foi avaliar a aplicação de arame de solda Maraging 250 em uma junta dissimilar típica da indústria do petróleo. Os ensaios foram realizados em juntas de aços ASTM grau A516 70 e AISI 8630M soldadas com Maraging 250. Foram avaliados perfis de tensão residual, dureza, microestrutura, resistência à tração e energia de impacto absorvida. A junta soldada com maraging 250 apresentou altos valores de tensões residuais compressivas. As microestruturas de diferentes regiões de soldagem foram analisadas. O enchimento de solda Maraging 250 mostrou grande potencial de aplicação quando juntas dissimilares dessa natureza estão presentes.

Abstract The welding of dissimilar joints is very common in systems used in oil exploration and production in deep waters. This commonly involves welding low carbon steel pipes with valves, pumps or other pipes, all of them made of alloy steel. Inconel has been used in such welding as filler material; however, this welding is preceded by a process of buttering in the low alloy steel side and a stress relief heat treatment. An alternative technique presented in this paper is the use of a welding filler metal that, during the cooling process, undergoes a martensitic transformation at a relatively low temperature, so that the resulting deformation processing offsets the contraction that occurs as a result of the cooling process. This study evaluates the application of Maraging 250 weld in a dissimilar joint of the oil industry. Tests were performed on joints of ASTM steels A516 grade 70 and AISI 8630M welded with Maraging 250. The residual stress profiles, hardness, microstructure, tensile strength and impact energy absorbed were evaluated. The welded joint with Maraging 250 showed a high intensity of compressive residual stress. The microstructure of different welding regions was analysed and compared.

The supermartensitic stainless steels (SMSS) are a relatively new class of corrosion resistant alloys developed to obtain a better combination of weldability, strength, toughness and corrosion resistance than conventional martensitic stainless steels. The final properties of SMSS are strongly influenced by quenching and tempering heat treatments. In this work, different routes of heat treatments were tested in a Ti-stabilized 12%Cr supermartensitic stainless steel with the objective to improve mechanical properties, specially the low temperature (-46ºC) toughness. Double and triple quenching were tested and compared to single quenching heat treatments. Two tempering temperatures (500ºC and 650ºC) were tested. The results obtained with instrumented Charpy impact tests showed that a triple quenching treatment was able to increase the density of fine TiC particles and improve the mechanical properties of specimens heat treated by quenching and tempering at 650ºC.

In order to improve the mechanical properties of API X70 pipeline steel, a rolling schedule in the recrystallization temperature region associated with a post heat treatment was proposed. Optical and scanning electron microscopy, electron backscattered diffraction, hardness, and tensile tests were carried out to show the effects of microstructure and crystallographic orientation on the final mechanical properties. The normalized hot-rolled sample and the sample subjected to hot-rolling followed by quenching and consequent tempering at 700 °C exhibited an excellent combination of yield strength and elongation. This was attributed to the recrystallized ferrite microstructure and tempered martensite with martensite-austenite constituent dispersion, respectively. In addition, the development of (112)[1̅1̅1] and (111)[101̅] texture components and the high fraction of low angle boundaries have a significant effect on the improvement of the mechanical properties.

Maraging 300 belongs to a family of metallic materials with extremely high mechanical strength and good toughness. Some works have been published about aging temperatures that improve ultimate strength resistance with acceptable toughness levels in this steel family, where the prior austenite grain size obtained by different solution annealing temperature influence in the final mechanical properties. Solution annealing temperatures ranging from 860 °C to 1150 ºC and were kept constant until the aging temperature. These treatments were used in order to investigate their influence on the microstructure and mechanical properties of maraging steel 300, especially with regard to toughness. The characterization of the microstructure was performed by optical microscopy, scanning electron microscope (SEM) and X-ray diffraction (XRD). Mechanical properties were evaluated by Rockwell C hardness and Charpy impact tests. The results showed that there is a temperature range where one can get some improvement in toughness without a large loss of mechanical strength.

A supermartensitic stainless steel with composition 12.2%Cr-5.8%Ni-1.90%Mo-0.028%C (%wt.) was welded by gas tungsten arc welding (GTAW) with superduplex stainless steel filler metal. Post weld heat treatments (PWHT) at 650 ºC for different periods of time were performed in order to decrease the hardness in the heat affected zone (HAZ). This paper deals with the effect of these heat treatments on the microstructure and mechanical properties of the joint. Mechanical strength of the weld joint was slightly inferior to the base metal, but was not affected by heat treatments. Precipitation of intermetallic phases in the weld metal (WM) due to prolonged PWHT was detected by scanning electron microscopy. The impact toughness of the weld metal decreased with the increase of time of heat treatment, due to intermetallic precipitation. However, even in the specimen treated for 1h, the Charpy energy remained above 27J at -46 ºC. It was found that optimum mechanical properties can be obtained with heat treatment for 30 minutes at 650 ºC.

Duplex stainless steels are high strength and corrosion resistant steels extensively used in the petrochemical and chemical industries. The aging at 475 ºC for long periods of time provokes embrittlement and deterioration of corrosion resistance. However, short duration aging at 475 ºC may be used as heat treatment to improve mechanical resistance with small decrease in the other properties. In this work the flow stress curves of lean duplex UNS S32304 and duplex UNS S32205 steels were modeled with Hollomon's equation and work hardening exponents (n) were determined. The analyses were conducted in specimens annealed and heat treated at 475 ºC for short periods of time. The aging at 475 ºC for 4 hours, 8 hours and 12 hours promoted significant hardening with small decrease of ductility. The work hardening exponents of both steels were compared, being higher in the duplex steel than in the lean duplex grade.

One method to face the effects of naphthenic acid corrosion in petroleun refining plants is to use alloys with good resistance to this kind of corrosion. For this purpose, molybdenum additions to chromium containing steels are specially recommended. In this work, experimental Fe-9Cr-xMo (x = 5, 7 and 9 wt. (%)) ingots were cast, forged and hot rolled before being tested in a naphthenic acid environment. Evolution of the mechanical properties with composition is presented. The precipitation and dissolution of phases were investigated with the help of Thermocalc and electron backscattered diffraction (EBSD).

O desenvolvimento de novas ligas de aço requer uma avaliação criteriosa das suas propriedades bem como de seu desempenho frente às condições de operação de acordo com a aplicação a que se destina. Entre estas propriedades pode ser citada a soldabilidade, uma vez que na maioria das vezes estas ligas são empregadas na condição como soldada. O objetivo deste estudo foi analisar o efeito da soldagem MIG em ligas ferríticas contendo 5% de Mo. Corpos de prova da liga solubilizada foram submetidos à soldagem automática pelo processo MIG. Três níveis de energia com diferentes valores de corrente e velocidade de soldagem foram selecionados, resultando em cinco condições de soldagem. A amostra soldada foi submetida à caracterização das regiões termicamente afetadas e do metal de base através de exames metalográficos, por microscopia ótica, MEV e EDX, para verificar o efeito da energia de soldagem na microestrutura das mesmas. Foram realizados ensaios de dureza Vickers visando levantar um perfil de microdureza da liga. Os resultados indicaram a presença de precipitados identificados como sendo martensita. Com exceção aos precipitados observados, nenhum outro defeito, como a presença de trincas, foi observado nos corpos de prova soldado para os diferentes parâmetros empregados.

The development of new steel alloys requires a careful evaluation of their properties as well as its performance against the conditions of operation according to the intended application. Among these properties can be cited the weldability, since in most cases, these alloys are used in the welded condition. The aim of this study was to analyze the effect of GMAW in ferritic alloy containing 5% Mo. Specimens annealed alloy were submitted to automatic GMAW. Three energy levels with different values of current and welding speed were selected, resulting in five welding conditions. The welded samples passed through the characterization of heat-affected regions and the base material through metallographic examination by optical microscopy, SEM and EDX, to determine the effect of welding power in the microstructure. Microhardness tests were performed in order to raise a Vickers hardness profile of the alloy. The results indicated the presence of precipitates identified as martensite. Except for precipitates observed, no other defects, such as the presence of cracks, were observed in specimens welded to the different parameters used.

O objetivo deste trabalho é analisar a influência da energia de soldagem na zona afetada pelo calor (ZAC), de juntas soldadas do aço inoxidável duplex UNS S31803. Foram realizadas soldagens com eletrodo revestido AWS E2209-17 em junta tipo V de Aço Inoxidável Duplex UNS S31803, com dois níveis de energia (15 e 20 kJ/cm). A condição soldada com energia mais elevada apresentou uma ZAC mais extensa e microestrutura mais grosseira nos passes de acabamento. No entanto, nos passes de enchimento e de raiz, as ZAC's destas regiões foram mais refinadas e menos extensa. Em relação à microdureza, a condição soldada com energia de 15 kJ/cm apresentou níveis menores. Em relação à tenacidade, não foi verificada diferença significativa nos resultados.

The aim this work is to evaluate the influence of multipass welding heat input on the microstructure and mechanical properties of the heat affected zone (HAZ) of UNS S31803 duplex stainless steel multipass welded joints. The shielded metal arc welding process using as filler metal the AWS E2209-17 covered electrode were employed had been carried through V joint groove UNS S31803 DSS, so that two levels of energy (15 and 20 kJ/cm) had been used in this experiment. The condition welded with higher energy higher a HAZ extensive and coarser microstructure in the finishing passes. On the other hand, in the wadding passes and root pass, the HAZ this region was more refined and less extensive. In respect of microhardness, the condition welded with energy of 15 kJ/cm got lower levels. In relation to toughness, it was not observed significant differences.

Os aços inoxidáveis duplex (AID) apresentam em sua microestrutura ferrita e austenita, de modo a aliar boas propriedades mecânicas e resistência à corrosão. Estes materiais possuem grande aplicação na indústria petroquímica, sendo o UNS S31803 um dos mais conhecidos. Existem poucos trabalhos referentes ao estudo da influência da restrição na soldagem da junta na formação de Widmanstätten. Logo, este trabalho tem como objetivo analisar a influência da restrição no balanceamento de fases, na morfologia da austenita, na microdureza e na tenacidade da Zona Afetada pelo Calor (ZAC) e da Zona Fundida (ZF). Foram realizadas soldagens em juntas tipo V utilizando eletrodo revestido AWS E2209-17com dois níveis de energia: 15 kJ/cm e 20 kJ/cm, com e sem restrição. De maneira geral, houve maior quantidade de ferrita nos passes de acabamento em relação aos passes de enchimento e de raiz. Foram observadas maiores quantidades de Widmanstätten nas condições soldadas com restrição. A ZAC apresentou uma microestrutura mais grosseira nos passes de acabamento. No entanto, não foram observadas diferenças significativas nas microestruturas quando comparadas as condições com e sem restrição. As condições que apresentaram maior quantidade de Widmanstätten obtiveram menores níveis de microdureza. Não foram observadas diferenças na energia absorvida na ZAC e ZF.

Duplex stainless steels present ferrite and austenite in their microstructure in order to join good mechanical properties and corrosion resistance. These materials are very used in petrochemical industry, being the DSS UNS S31803 one of the most commercially known. There were not much published literature to study the influence of the precipitation of Widmanstätten austenite in joins with some restriction. This work has as objective to analyze the influence of the restriction in the phase balance of austenite and ferrite, in the morphology of the austenite, in the microhardness and the toughness of both HAZ and FZ. Shielded electrode AWS E2209-17 was used to weld V joints in the duplex stainless steel UNS S31803. Welding has been performed in two levels of energy C1 (15 kJ/cm) and C2 (20 kJ/cm), with and without restriction. In a generalized manner, it was observed a greater amount of ferrite in the finishing passes when compared to the first passes. Bigger amounts of Widmanstätten austenite in the conditions welded with restriction had been observed. Generally, the HAZ presented a coarser microstructure in the finishing passes, in way that did not show significant differences when compared with the conditions with and without restriction. Microhardness level was lower when large amount of Widmanstätten has been presented. Differences in energy absorbed in HAZ and FZ has not been observed when comparing conditions with and without restriction. The influence of the use of restraint in welding, especially in balancing phase, microstructure and microhardness has been observed.

Aços Cr-Mo são usados na indústria do petróleo em aplicações com óleos crus ricos em compostos sulfurosos. Aços comerciais como 2.5Cr1Mo ou 9Cr1Mo têm se mostrado ineficientes em consequência de altos índices de corrosão naftênica. Uma estratégia para resolver este problema é o aumento do teor de molibdênio destes aços. Neste trabalho foi estudado o efeito do aumento do teor de molibdênio na microestrutura de ligas Fe-9Cr-xMo, solubilizadas e soldadas. Foram levantados os diagramas de fases com auxílio de um programa comercial para verificar as possíveis fases a serem formadas e identificar os problemas de soldagem. A microestrutura das ligas solubilizadas foi analisada por microscopia óptica e EBSD, além da medição da dureza. Foram realizadas soldagens autógenas para verificar o efeito do aporte térmico na microestrutura e na dureza das ligas. O aumento do teor de molibdênio resultou no aumento da dureza das ligas. A análise microestrutural das ligas soldadas apresentou uma particularidade para a liga com menor teor de molibdênio, a presença de martensita. Já as ligas com maior teor de molibdênio apresentaram uma microestrutura completamente ferrítica. A formação de martensita pode ser um problema na solda da liga com menor teor de molibdênio, uma vez que a mesma pode causar perdas nas propriedades mecânicas comprometendo sua aplicação.

Cr-Mo steels are used in the petroleum industry in applications with crude oils rich in sulfur compounds. 2.5Cr1Mo or 9Cr1Mo do not resist to operating conditions when in contact with crude oils. The increasing of molybdenum content can improve the corrosion resistance of these alloys. This paper studied the effect of increased concentration of molybdenum in the microstructure of Fe-9Cr-xMo alloys, annealed and welded. Phase diagrams were built with the aid of commercial program to check the possible phases to be formed and to identify the problems of welding. Analyses were realized by optical microscopy, EBSD and hardness tests. Autogenous welds were carried out to verify the effect of heat input on microstructure and hardness of the alloys. The results indicated that the increase in molybdenum concentration resulted in increased hardness of the alloys. After welding the alloy with lower molybdenum content presented the formation of martensite. Alloys with molybdenum content above 5% presented a fully ferritic microstructure. The formation of martensite can be a problem in weld alloys with lower content of molybdenum, since it can cause loss in mechanical properties hindering their application.

Deleterious phases precipitation in superduplex stainless steels is the main concern in fabrication by welding and hot forming of this class of material. Sigma, chi and secondary austenite phases are considered deleterious phases because they produce negative effects on corrosion resistance. Besides, sigma and chi phases also promote strong decrease of toughness. In the present work, the precipitations of sigma, chi and secondary austenite under aging in the 800-950 °C interval were studied in two UNS S32750 steels with different grain sizes. The deleterious phases could be quantified by light optical microscopy, with no distinction between them. Scanning electron microscopy was used to distinguish the individual phases in various aging conditions. The results elucidate the influence of the aging temperature and grain size on the kinetics precipitation and morphology of deleterious phases. The kinetics of deleterious phases is higher in the fine grained material in the initial stage of aging, but the maximum amount of deleterious phases is higher in the coarse grained steel.