Year by year the user and process site of steelmarket keep growing lay claim to the steelmakers. The researchers, developers and technologists use every effort to try to produce steels with compound properties. From claim of users -especially the Automotive Industry - developed the MP- (Multiphase) and TRIP-steels (Transformation Induced Plasticity). In the Automotive Industry draft mostly the claim of loose of weight and increase the safety reserve. Advanced High Strength Steels (AHSS) such as of the Transformation Induced Plasticity (TRIP) steel type are being used in automotive hybrid designs and present serious problems associated with welding/joining. It has been suspected that interstitial gases such as hydrogen and nitrogen contribute to delayed cracking and embrittlement of welds in sheet steels. The technical literature and experience supports these assumptions in thick plates. Because diffusion distance is short and residual stresses low, sheet metals have not usually exhibited these problems. Preliminary experiments at the LeTourneau and BUTE University revealed very strong nitrogen related problems such as solidification cracking and excessive hardening in the Heat Affected Zone using physical simulation and small scale weldability testing. Nitrogen additions to the welding shielding gas as well as Al and Ti additions to the filler metal are going to control the size and distribution of AlN and TiCN precipitates and reduce the amount of free nitrogen in the weld. Nitrogen is one of the elements suspected of having the strongest ef-fect on variability in base metal properties. One of such AHSS is a multi-phase steel called TRIP, an abbreviation of Transformation Induced Plasticity and is made of a mixture of 70% ferrite + 20% martensite and 10% rest austenite. During forming, the grain boundary retained austenite provides sufficient ductile behavior. Subsequently it is transformed to martensite during plastic deformation due to local strain. In this context nitrogen a strong austenite stabilizer has to be controlled and limited to 200 ppm maximum. This level of nitrogen in TRIP steels is roughly half of that of low alloyed steels and can not be in atomic (free) state, but combined in nitrides. However the exact state of nitrogen in unpredictable and in these heterogenous and non-equilibrium weld fusion and heat affected zones. It is known from the literature that unintentional contamination with nitrogen during welding can exceed the above mentioned levels for homogenous base metals by up to two orders of magnitude. To better understand the potential effect of nitrogen on AHSS a brief overview of gas/metal re-actions in steelmaking is provided below. The main purpose of the study is the understanding fundamental phenomena associated with nitrogen/steel reactions in solidstate and fusion welding of TRIP steels. A combination of physical simulations needed for a comprehensive computer modeling. Ultimately removing barriers in joining and using these high strength steels will improve automobile safety and fuel efficiency by making cars stronger and lighter.


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    Titel :

    Welding investigations of modern high strenght TRIP - steel


    Beteiligte:
    Orosz, C. (Autor:in) / Palotas, B. (Autor:in) / Adonyi, Y. (Autor:in)


    Erscheinungsdatum :

    2006


    Format / Umfang :

    8 Seiten, 6 Bilder, 5 Tabellen, 9 Quellen



    Medientyp :

    Aufsatz (Konferenz)


    Format :

    Print


    Sprache :

    Englisch





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