This study investigates the influence of heat treatment on the properties of welded steels 1.4404 and 1.4571 of high thickness concerning the forming properties by rolling and the final properties. The weld seam is a region in the work piece where the material can have a higher resistance to deformation. The strength of the weld seam is often higher than in the parent metal. Therefore a suitable pre-heat treatment (1050°C for 30 min) was applied to the weld seam and heat affected zone within the cold rolling process to achieve a more homogeneous distribution of strength and ductility over the entire work piece. A second heat treatment (1050°C for 60 min) after the rolling was done to obtain the solution annealed structure of the steels. To compare the effect of the heat treatments and of the deformation during rolling (work hardening) tensile tests were performed after each process step. Here both materials behave slightly differently. Metallographic investigations show how the microstructure is influenced and give a clear picture of the fracture occurring. Force – time behaviour during rolling was monitored and provides information about the improvements by the preliminary heat treatment. Cold rolling of welded plates is characterized by a force peak just when the weld seam is within the gap of the rolls. The application of heat treatment has been found to lower that force peak and ensures less distortion in shape during the cold rolling passes.
The influence of post-heat treatment on the mechanical properties of complexly deformed low carbon steel is studied in this work to improve a forming process to ensure and enhance the overall product quality. Since there is no constant deformation degree, there are big differences in the mechanical properties within the part: material seen a high deformation degree shows increased strength but decreased ductility, which carries the risk of formation of cracks. A post heat treatment reduces that risk by reducing inner stresses and work/strain hardening. This study involves two steel component varying in thickness and forming process: stretching plus bending and stretching only. Post heat treatment after cold forming is necessary to provide the safety margin needed in the application. The forming process which consists of stretching only delivers a more uniform deformation over the length of the nozzle. However, adding bending to stretching keeps the deformation degree lower and leads to a more homogenous property distribution after annealing. Material only exposed to stretching shows strong softening by deforming above critical deformation degree with resulting values below initial properties. Even during annealing of around 40 min, recrystallization took part: more globular grains can be seen. Post heat treatment also homogenizes hardness over the cross-section of the work piece. Increased hardness towards the tension and compression surface side as a result of friction between tool and work piece should be avoided to provide the safety margin also on the more local level. Annealing retrieves the yield point phenomenon and strongly increases ductility up to 35 % and reduces the ratio of tensile yield strength to ultimate tensile strength to more moderate levels of around 0.6.
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