Inconel 718 (IN 718) is widely used in applications, such as aircraft and power turbine components. Recently, fiber laser welding has become an attractive joining technique in industry for fabrication and repair of service-damaged components. However, a major limitation in the laser welding of IN 718 is that liquation cracking occurs. In the present work, autogenous fiber laser welding of IN 718 was used to study the effects of welding parameters and different pre-weld heat treatments on liquation cracking. Contrary to previous studies, a dual effect of heat input on cracking is observed. A rarely reported effect of heat input is attributed to process instability. Liquation cracking increases with pre-weld heat treatment temperatures that increase grain size and/or, possibly, intregranular boron segregation. The study shows that pre-weld heat treatment at 950oC can be used for repair welding of IN 718 without significant loss in cracking resistance.

Inconel 718 (IN 718) is widely used in applications, such as aircraft and power turbine components. Recently, fiber laser welding has become an attractive joining technique in industry for fabrication and repair of service-damaged components. However, a major limitation in the laser welding of IN 718 is that liquation cracking occurs. In the present work, autogenous fiber laser welding of IN 718 was used to study the effects of welding parameters and different pre-weld heat treatments on liquation cracking. Contrary to previous studies, a dual effect of heat input on cracking is observed. A rarely reported effect of heat input is attributed to process instability. Liquation cracking increases with pre-weld heat treatment temperatures that increase grain size and/or, possibly, intregranular boron segregation. The study shows that pre-weld heat treatment at 950oC can be used for repair welding of IN 718 without significant loss in cracking resistance.

Il est bien connu que la ségrégation intergranulaire du phosphore peut diminuer la cohésion entre les grains, entraînant une fragilisation des aciers. Les aciers bainitiques faiblement alliés utilisés pour la construction des cuves des réacteurs à eau sous pression (REP) contiennent généralement une faible quantité de phosphore (dans la gamme de 100 ppm). L’exposition continue à une irradiation sous un faible flux de neutrons à une température moyenne (environ 300°C) conduit à une fragilisation de l’acier de l'acier de cuve. La ségrégation intergranulaire du phosphore peut potentiellement contribuer à cette fragilisation. Pour assurer la fiabilité des REP en fonctionnement, il est donc important de comprendre les effets des conditions de vieillissement (température, dose d’irradiation etc.), de la composition du matériau et du type de joint de grains (JG) sur l’intensité de la ségrégation intergranulaire du phosphore. La littérature montre que la ségrégation intergranulaire peut fortement varier entre différents joints de grains. Cependant, les études systématiques manquent dans ce champ d’étude.Dans le but d’obtenir une description précise et représentative des joints de grains d’un point de vue structural et chimique, ce travail combine différentes techniques. La principale technique utilisée est la Sonde Atomique Tomographique (SAT). Cette technique permet d’explorer en trois dimensions la distribution atomique d’une grande variété d’éléments dans les joints de grains. La géométrie des joints de grains est déterminée grace à la Diffraction de Kikuchi en Transmission (TKD) associée à une reconstruction de SAT. Pour avoir une meilleure compréhension des mécanismes de ségrégation, des modèles ségrégation d'équilibre et de ségrégation induite par l'irradiation ont été utilisés. / It is well known that the intergranular segregation of phosphorus can diminish the cohesion between grains, resulting in steel embrittlement. Low alloyed bainitic steels used to build nuclear reactor pressure vessel (RPV) generally contain a small amount of phosphorus (in the range of 100 ppm). Continuous exposure to a low neutron dose rate irradiation at intermediate temperature (~300°C) results in radiation embrittlement of RPV steel. Since intergranular segregation of phosphorous can contribute to this embrittlement, for purpose of RPV reliability during operation, it is important to understand the effects of ageing conditions (temperature, irradiation dose etc.), material composition and grain boundary (GB) type on the intensity of phosphorus intergranular segregation. Regarding to literature sources, it was revealed that the intergranular segregation values may strongly vary among different GBs. However, there is a lack of systematic studies in this field.In order to get an accurate and representative description of GB from structural and chemical points of view, different techniques are combined in this work. Atom Probe Tomography technique is utilized as the main tool. This technique is able to explore the 3D atomic distribution of broad variety elements at GB. GB geometry is determined from Transmission Kikuchi Diffraction (TKD) map supplemented by APT reconstruction. To gain a better understanding of segregation mechanisms, models of thermally and radiation-induced segregation were employed.