This study describes design and construction of a novel flexible heat treatment line for processing customer-oriented small batch steels. The induction heater (600 kW) developed is suitable for the sheet thickness in the range 3.2 30 mm and the width of 85 1250 mm. Sheets are fed using an electrical motor (1.5 kW) and a chain drive, the speed being in the range 0.3 7 m/min, depending on the power and the sheet dimensions. At this study, 4.5 (WR-1) and 10 mm (WR-2) thick wear resistant steels were tempered at different peak temperatures to compare the effect of rapid tempering on mechanical properties. Results showed that the heat treatment line is capable of producing tempered steel grades with adequate properties at industrial product rate. For example, 4.5 thick WR-1 tempered at 550 oC provided a yield strength (YS) over 1000 MPa with minimum bending radius of 6 mm (in the delivered condition YS = 1605 MPa and Rmin = 12). Tempering of WR-1 at 700 oC provided YS of 762 MPa and Rmin of 1 mm. Results were similar between two test materials, but the enhancement in bendability was slightly more effective with the thinner sheet.

Sandwich panel structures are increasingly used in applications where the most important demands are the weight saving and long service life. Utilizing sandwich panels, extremely light-weight, stiff and robust structures can be manufactured. In this study, sandwich panels were produced by specially designed cost-effective forming tools. Various kind of test materials were used for corrugated cores and skin plates: conventional low-carbon steel grade EN 10130 and ferritic stainless steel grade 1.4509 with plate thicknesses of 0.6 and 0.75 mm. A common S355 structural steel was used as a reference for bending strength comparison.For measuring the stiffness, MTS tensile and fatigue testing machine was selected to determine the bending resistance of the sandwich panels. The bending force, needed for yielding and fracture, related to the bending length and intensity was compared with the results from bending of the reference plates. Results showed that the bending force of the panels is significantly higher than that of a plate having similar intensity. The best results were obtained with the stainless steel (SS) panel that had 27% higher bending force at the yield point than 5 mm thick S355 plate having 3 times larger intensity. The carbon steel panel was approximately 40% weaker than the SS-panel and both panel types lost strength when loading direction was changed from transverse to 45 degree and further to 90 degree load (longitudinal).

The main aim of the study was to develop forming tools for wide (over 1.2 meter) sandwich panels. Longitudinal laminating technology was selected for tool manufacturing due to its flexibility and cost efficiency. Laminating technology enables easy modification of the tool dimensions afterwards. The function to optimize or vary the dimensions of the tool was set as a secondary objective for the study. Forming tools for sandwich panels are usually complicated structures and joining of the plates can be difficult in some cases. Typically sandwich forming tools are capable to produce only narrow panels (less than 1 meter) and optimization must be done during designing of the tool. In this study, a rapid designing and manufacturing of a flexible sandwich panel forming tool was investigated. Sandwich panels are usually applied in light structures or voice covers due to their very low weight, high stiffness, durability and production cost savings. Designing of the forming tool was made by using a 3D CAD program. Conventional steel plates were used for the forming tool and the assembly was done by fixing the plate parts longitudinally together (laminating). Most important criterion for the forming tool was its capability to produce high quality geometry for the core. Laser welding assembly showed that the quality of the core was good enough for welding the lap joints properly. Both of the objectives were fulfilled: 1) forming tools were suitable for forming of wide cores (1.2 meter) and 2) the structure of the laminated tool enables to change or add new plate parts to change the dimensions of the final product.

The aim of the study was to investigate correlation between bendability and tensile properties of high-strength steels. Strength and elongation in tensile test have traditionally been basic values in evaluating bending properties. Advanced high-strength (AHS) steels often have tendency for strain localization, causing risk of fractures and impairing the shape of the bend. Practice has shown that tensile test results, indicating bendability well, are not sufficiently accurate when using AHS steels. Since tensile test is a fast, simple and cheap testing method, it would be beneficial to rework it to suit better for predicting of bendability. In this study, the usability of tensile test results to predict failures in bending AHS steels has been investigated. The most common failures and failure mechanisms in bending are also presented. Test materials used were 6 mm thick AHS wear-resistant, protection and structural steels with good and poor bending properties. Minimum bending radii were determined and then compared with ten-sile test results to estimate the correlation. Conventional tensile test results, fracture surfaces and necking through width and thickness were analyzed. Correlation coefficient for measured tension properties and minimum bending radius was calculated. Results showed that in tensile test, have the best correlation with minimum bending radius with necking through the thickness and actual strain in necking area.

In this work laser ablation was used for the determination of residual stress of a formed plate. Manufacturing processes, like bending, create residual stresses in the product and those can be very disadvantageous for fatigue durability. Residual stresses that are generated during the manu-facturing of products can cause distortions, dimensional errors or can even break the products. The research material was ultra-high-strength steel (UHSS) with a yield strength of 1100 MPa. Speci-mens with a 90 degree bent angle were made by air bending using a press brake. Air bending causes variable residual stress patterns in the cross section of the material. The residual stresses of the formed area were determined by removing material and measuring strains caused by the release of stresses. A slot with the width of 1 mm has been manufactured parallel to the edge, both on the outer and on the inner side of the bend. True residual stress distribution can be calculated from the measured strains. A pumped 1064 nm Nd:YVO4 ablation laser, whose pulse length is 90 ns, was used for the removal of material. This process creates a negligible heat affected zone (HAZ) and laser ablation doesn’t increase stresses in the specimen. The results were compared with those ob-tained when slots were produced by milling and wire-EDM, as well as with the stress values meas-ured by X-ray diffraction.