Experimental determination of 3D crack propagation scenario in resistance spot welds of a martensitic stainless steel

International Journal of Fracture - The development of high-performance automotive steels involves increasingly complex chemistries and heat treating sequences, to produce stronger and lighter...     

Austenite Grain Growth in a 2.25Cr-1Mo Vanadium-Free Steel Accounting for Zener Pinning and Solute Drag: Experimental Study and Modeling

Metallurgical and Materials Transactions A - Austenite grain size has been experimentally determined for various austenitization temperatures and times in a 2.25Cr-1Mo vanadium-free steel. Three...     

Damage development in low alloy TRIP-aided steels during air-bending

In order to optimize the metallurgical quality of advanced high strength steels for automotive applications, the present study aims at understanding damage mechanisms involved in air-bending of two low alloy TRansformation Induced Plasticity (TRIP)-aided steels. Air-bending tests were performed together with metallographic investigations of damage development in bent specimens. In order to assess the role of hard bands induced by chemical segregations, air-bending tests on specimens with various locations of the main hard band (with respect to the neutral fibre) were performed. Cracking initiates from the outer surface or from just below, mainly by decohesion between ferrite and secondary (martensite) phases, at hard bands located close to the outer surface. From these examinations, together with a simple kinematics analysis of bending, a quantitative relationship between cracking, local thickness of hard band and local “mesoscopic” strain is proposed.     

Synthesis of few-layered graphene by ion implantation of carbon in nickel thin films

The synthesis of few-layered graphene is performed by ion implantation of carbon species in thin nickel films, followed by high temperature annealing and quenching. Although ion implantation enables a precise control of the carbon content and of the uniformity of the in-plane carbon concentration in the Ni films before annealing, we observe thickness non-uniformities in the synthesized graphene layers after high temperature annealing. These non-uniformities are probably induced by the heterogeneous distribution/topography of the graphene nucleation sites on the Ni surface. Taken altogether, our results indicate that the number of graphene layers on top of Ni films is controlled by the nucleation process on the Ni surface rather than by the carbon content in the Ni film.     

Creep failure model of a tempered martensitic stainless steel integrating multiple deformation and damage mechanisms

A new model considering both deformation and damage evolution under multiple viscoplastic mechanisms is used to represent high temperature creep deformation and damage of a martensitic stainless steel in a wide range of load levels. First, an experimental database is built to characterise both creep flow and damage behaviour using tests on various kinds of specimens. The parameters of the model are fitted to the results and to literature data for long term creep exposure. An attempt is made to use the model to predict creep time to failure up to 10⁵ h.     

Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions

The effect of the tempering heat treatment, including heating prior to the isothermal step, on carbide precipitation has been determined in a 2.25 Cr-1 Mo bainitic steel for thick-walled applications. The carbides were identified using their amount of metallic elements, morphology, nucleation sites, and diffraction patterns. The evolution of carbide phase fraction, morphology, and composition was investigated using transmission electron microscopy, X-ray diffraction, as well as thermodynamic calculations. Upon heating, retained austenite into the as-quenched material decomposes into ferrite and cementite. M₇C₃ carbides then nucleate at the interface between the cementite and the matrix, triggering the dissolution of cementite. M₂C carbides precipitate separately within the bainitic laths during slow heating. M₂₃C₆ carbides precipitate at the interfaces (lath boundaries or prior austenite grain boundaries) and grow by attracting nearby chromium atoms, which results in the dissolution of M₇C₃ and, depending on the temperature, coarsening, or dissolution of M₂C carbides, respectively.     

Analysis of creep lifetime of a ASME Grade 91 welded pipe

A multi-material local approach to creep damage was applied to a ferritic-martensitic ASME Grade 91 steel welded joint at 625 °C. Focus was made on the detailed analysis of the most sensitive area of the weld i.e. the intercritical heat affected zone. Prediction of creep failure of the weld well agrees with experimental results. The model was then applied to the case of a seam-welded pipe exhibiting a roof defect, creep tested at 580 °C, showing consistent results with a more classical engineering assessment.     

Microstructural Characterization of Internal Welding Defects and Their Effect on the Tensile Behavior of FSW Joints of AA2198 Al-Cu-Li Alloy

Internal features and defects such as joint line remnant, kissing bond, and those induced by an initial gap between the two parent sheets were investigated in AA2198-T851 friction stir welded joints. They were compared with the parent material and to defect-free welds obtained using a seamless sheet. The cross-weld tensile strength was reduced by the defects by less than 6 pct. The fracture elongation was not significantly affected in view of experimental scatter. Fracture location, however, changed from the thermomechanically affected zone (retreating side) to the defect in the weld nugget for the welds bearing a kissing bond and for some of the gap welds. The kissing bond was shown by EBSD to be an intergranular feature; it fractured under a normal engineering stress close to 260 MPa during an in situ SEM tensile test. Synchrotron tomography after interrupted tensile testing confirmed opening of the kissing bond. For an initial gap of 23 pct of the sheet thickness, intergranular fracture of copper-enriched or oxide-bearing grain boundaries close to the nugget root was evidenced. The stress and strain state of cross-weld specimens loaded under uniaxial tension was assessed using a 3D finite element, multi-material model, determined on the basis of experimental data obtained on the same specimens using digital image correlation.     

Effect of shear cutting on ductility of a dual phase steel

Both an experimental investigation with an especially designed setup and a mechanical FE analysis of the cutting process showed that for the laboratory dual phase steel investigated, cutting involves positive stress triaxiality and ductile fracture mainly due to void nucleation and coalescence at ferrite-martensite interfaces. Tensile tests on as-cut strip specimens showed a large reduction in ductility due to the presence of damage on the edges of the strips. Tensile tests on strip specimens containing short precracks and mechanical analysis showed that the cutting affected area behaves as a precrack during subsequent mechanical testing.