Microstructures in the sub-zones of butt joint consisting of dual phase DP980 steel and complex phase CP780 steel with a thickness of 1.5 mm welded using fibre laser process were evaluated. The microstructure in the fusion zone was built of an approximately equal portion of martensite and lower bainite. In both coarse-grained heat affected zones near DP980 and CP780 steel dominant presence of lath martensite and a minor portion of lower bainite were observed. Similar microstructures were created in both fine-grained heat affected zone near DP980 and near CP780 steel but with the presence of ferrite and tempered martensite in region heated to lower peak temperatures. Mainly M3C carbides precipitated in lower bainite, and mostly MX carbonitrides precipitated in martensite. Created microstructures corresponded with microhardness profile across the joint. The highest microhardness of 476 HV0.1 was measured in the coarse-grained region of the heat affected zone near DP980 steel with a high portion of martensite.
The paper is dedicated to forming and properties of passenger car´s B-pillar reinforcement drawn from simple blank and alternatively tailor-welded blank (TWB). Drawn part is characterised by a place with a large strain, while forming process simulation did not confirm the creation of crack using the TWB consisting of dual phase HCT980X instead of previous HCT600 steel. It is because HCT980X steel has higher strength and lower ductile properties. The analysis of properties of drawn parts is focused on the simulated crash test in Dynaform software. Obtained sizes of drawn forces in simulated frame of the drawn parts and their comparison proved the possibility of the replacement the 1.2 mm thick simple blank from HCT600 steel with the 1 mm thick TWB consisting of HCT600 and HCT980X steel. The changed thickness of the simple blank caused 20% weight saving while containing the same properties of the drawn part.
The microstructure of fibre laser welded joint of TRIP 690T and HSLA LAD340 steel sheets with different thicknesses was evaluated. The microstructure in the fusion zone was built of martensite, lower bainite and a small portion of upper bainite. Martensite and lower bainite created in both the coarse-grained and fine-grained heat affected zones (HAZs) near 690T steel, but some portion of ferrite was also identified in the fine-grained HAZ near 690T steel. Acicular ferrite, upper bainite, and equiaxed ferrite were observed in both the coarsegrained and fine-grained HAZs near LAD340 steel, but lower bainite was occasionally found in the coarse-grained HAZ near LAD340 steel. Creation of hard microconstituents caused an increase in microhardness values in the fusion zone and both HAZs compared to the base metals. The highest microhardness of 457 HV0.1 was measured in the coarse-grained HAZ near 690T steel with martensitic and lower bainitic microstructure.
This article is focused on an analysis of factors negatively affecting the tube production process of tubes made from austenitic stainless steel with a very small diameter of ϕ 0.34 mm. The analysis was concentrated on factors that affect the drawing process stability of the seam tubes where the desired final dimensions—a diameter of ϕ 0.34 mm and a wall thickness of 0.057 mm—are limiting factors. Seam tubes made from steel 1.4306 and 1.4301, from producers KT and EW with a longitudinal weld line made by tungsten inert gas (TIG) welding, were used as blanks for constituent drawing operations. It is desirable to provide sufficient inert gas flow and cooling during the formation of a weld joint in a protective atmosphere chamber. A significant temperature gradient prevents the formation of undesirable Cr23C6 carbides in the heat-affected zone (HAZ) which negatively affects the plasticity and formability of the steel and is the cause of technological fractures.
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