Jussi A. Karjalainen

Bendability of Ultra-High-Strength Steel

Utilisation of ultra-high-strength (UHS) steels is rapidly spreading from the automotive industry into many other application areas. It is necessary to know how these materials behave in common production processes such as air bending. The bendability of UHS steels is much lower compared to normal and high-strength construction steels. In this work, experimental tests were carried out using complex phase (CP) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250) and S650MC HS steel in order to inspect material bendability and possible problems in the bending process. Mechanical and geometrical damages were registered and classified. The bending method used was air bending and press brake bending with an elastic lower die. The FE analysis was used to understand the stress state at different points in the material and build-up of failure. As UHS steels cannot stand large local strains, a large radius must be used in air bending. The results show that even when a large radius is used in air bending, the strain is not evenly distributed; there is a clear high strain area in the middle of the bend. It was also possible to simulate the other phenomena occurring in experimental tests, such as losing contact with the punch and ‘nut-like’ geometry, using FE analysis. Experimental test results also show that by using an elastic lower die, it is possible to avoid unwanted phenomena and obtain an almost 50% smaller punch radius, but the required force is 50% bigger than that required in air bending.

Laser-Assisted Bending

When sheets of high-strength (HS) and ultra-high-strength (UHS) steels are bent by a press brake the process suffers from large bending forces, considerable springback, and eventual cracks. Additionally, some unpredictable effects, such as lost contact to the punch, caused by strain hardening may occur producing a bend with erroneous radii. The strain hardening of the bending line may make further processes, such as forming or welding, more complex. One solution to these problems is to anneal the bending line with a laser in advance. Of course, it is also possible to utilise other types of heat sources, but the laser can offer the most precisely controlled heat treatment. The proper process parameters depend on the material, and it has been noticed that inadequate process parameters may harden the material instead of annealing. In this work some experiments on bending sheet metal samples of HS or UHS steel with previously laser-annealed bending lines have been carried out and the outcome analysed. The results show that the annealing produces better bending results compared to the conventional procedure. This includes lower springback, less hardening in the bending line and more precise geometry of the bend. It can be even suggested that proper annealing with strain hardening in bending will produce the original material structure. Obviously, more theoretical and experimental work is required to optimise the process parameters including the laser power and speed for each pair of material strength and thickness.

Cutting Edge and its Influence on the Fatigue Life of High Strength CrMn-Austenitic Stainless Steel

In the field of product design the fatigue behavior is one of the most challenging issues, especially in the case of high strength steels. It is well known that surface quality has significant influence on a steel component fatigue properties. Different cutting methods have dissimilar effects on cutting edge’s surface properties and quality. Thus different cutting methods have distinct influence on fatigue life of steels. For example, while thermal cutting is often fast and effective, it induces heat to work piece and thereby influences on microstructure. Unlike thermal cutting methods, waterjet cutting does not create any heat in the material, but it impairs the surface quality. In this work, the influence of laser-, high-definition plasmaand waterjet cutting on fatigue life has been studied. The results obtained by these cutting methods were compared to ones reached with machined specimens. The test material was 2.27 mm thick sheet of a temper rolled CrMnaustenitic stainless steel grade AISI 201 LN TR with the yield strength of 666 MPa and tensile strength of 814 MPa. Load-controlled low-cycle fatigue tests were performed using the R ratio (R=σmin/σmax) of -1 to flat type specimens. Specimens were stressed on transverse orientation at the stress amplitude of 70%, 80% and 90% of the yield strength. The qualities of cutting edge surfaces were evaluated by measuring surface roughness (Ra-value and Rz-value). Length of heat affected zone was measured from thermally cut edges and also hardness of cut edges was measured.

Mechanical Properties of Laser Heat Treated 6 mm Thick UHSS-Steel

In this work abrasion resistant (AR) steel with a sheet thickness of 6 mm was heat treated by a 4 kW Nd:YAG and a 4 kW Yb:Yag–laser, followed by self‐quenching. In the delivered condition, test material blank (B27S) is water quenched from 920° C. In this condition, fully martensitic microstructure provides excellent hardness of over 500 HB. The test material is referred to AR500 from now onwards. Laser heat treatment was carried out only on top surface of the AR500 sheet: the achieved maximum temperature in the cross‐section varies as a function of the depth. Consequently, the microstructure and mechanical properties differ between the surfaces and the centre of the cross‐section (layered microstructure). For better understanding, all layers were tested in tensile tests. For a wide heat treatment track, the laser beam was moved by scanning. Temperatures were measured using thermographic camera and thermocouples. Laser heat treated AR500 samples were tested in hardness tests and by air bending using a pres...

FEM - Modeling of Bendability of Ultra-High Strength Steel

Ultra-high strength steels have been widely used in different industrial applications. It is necessary to understand the behavior of these materials in common forming processes such as air bending. It is known that the bendability of ultra-high strength steels is lower than other high-strength steels but what are yet to be discovered are the parameters that define the limits of bendability of these steels. The aim of this study was to investigate the factors affecting the bendability of ultra-high strength steel using optical strain measurements and FEM-modeling of the bending process. By using the true stress-strain relation measured by optical strain measuring system the bendability of ultra-high-strength steel was modeled fairly accurately. As a result, it was noted that the strain distribution at the bend of a steel possessing better uniform strain was more widely distributed and there were no highly localized strains. On the other hand as the failure occurred the strains were considerably smaller than the true failure strain of the material in uniaxial tension. As a conclusion it was stated that the ability to withstand the localization of deformation might describe the bendability of ultra-high-strength steel better than the values of the uniform or true failure strain in uniaxial tension test.

Effect of Convex Sheared Punch Geometry on Cutting Force of Ultra-High-Strength Steel

Extremely high strength of the ultra-high-strength steels leads to increased load factors on the tooling machines and punching tools. This experimental study examines how much convex punch geometry affects cutting forces when punching ultra-high-strength steels. Tools used in punching tests were four different convex sheared rooftop punches and one conventional flat end punch, to which rooftop punches were compared to. The material in punching tests was ultra-high-strength steel Ruukki Optim 960 QC, with a thickness of 4 mm. The test material in punching tests was sheared with rooftop punches and a flat end punch and occurred cutting forces were measured. The qualities of punched holes were evaluated visually and the roundness measurements were also performed. The results show that the cutting forces of Optim 960 QC can be reduced radically with optimal convex punch geometry. With using 14-degree shear angle of the punch end, the cutting forces reduced up to 57 % compared to forces of the conventional flat end tool. However, largest tested shear angles caused several negative effects on the cutting quality of the holes and therefore they are not suitable in all applications. Punching tests proved that the cutting clearance had no appreciable effect on cutting forces when punching ultra-high-strength steel. Instead there was a noticeable effect on the quality of the punched hole, especially when large shear angles were used.

Improving accuracy of aging CNC machines without physical changes

This study focused on the systematic error compensation of CNC machines via software way. The method is based on machining test pieces, measuring same for finding the systematic errors of a machine, forming an error map and using the map for correcting the tool paths of the NC-program. Technically, it was shown that by measuring the systematic errors of machines and numerically calculating new, compensated parameters, the precision of aging machines could be substantially improved. Resultingly, the method extends the lifetime of an older machine, postponing the need for new investment for several years.

UHS Steel Formability in Flexible Roll Forming

Ultra-high-strength (UHS) steels are very interesting materials for many applications where high strength can be utilized to create lighter and more effective constructions. The poor formability of UHS steels, however, may limit the usefulness of these materials in many applications. In this work, some experiments using a flexible roll forming machine developed at the Chair of Manufacturing Technology (LFT), University of Erlangen-Nuremberg were carried out using 4mm-thick bainitic martensitic UHS steels (YS/TS 960/1000 and 1100/1250) and the outcomes have been analysed. Results of these experimental tests show that using roll forming it is possible to bend test materials to an angle of 90º without damages with an evidently smaller radius than in air bending. The radius obtained using roll forming can be as small as 40% of the value used in air bending. Tests also show that with the method used in these tests it is possible to make roll forming for the whole length of the plate. The tests proved that the NC-controlled single-step roll forming method has potential for manufacturing small batches of bend profiles; however, more development has to be carried out if the process is to be made suitable for industry.

Punching Force Reduction with Wave-Formed Tools

Flat-end tools are the most general types used in sheet metal punching and nibbling. They are geometrically simple and easy to sharpen but, on other hand, their cutting forces are relatively large, and hence the cutting process is frequently noisy. In order to reduce both cutting force and noise tools with one-way or two-way shearing have been utilised. The major drawbacks of these tools are the asymmetry of cutting easily causing non-circular holes with round tools, lateral forces with one-way shearing, excessive forming during cutting and more complex tool geometry to maintain. Here a new geometry for a punch is employed. The shearing edge is a sinus curve with several peaks making the cutting edge circularly symmetric and the phenomenon totally balanced. This means smaller forming forces, particularly in cases when also the radial form is concave. The geometry is without doubt more complex compared to the flat-end tool but rather easy to produce by multi-axis milling and electro-discharge machining. In the current work a set of experimental punches has been designed, manufactured and tested. A simple analytical theory for cutting force has also been derived and compared with the test results. The results show that the new geometry produces very precise hole geometry with a lower cutting force compared to conventional flat-end tools. Of course, more theoretical and experimental work is required to optimise the tool geometry including the tool clearance for each pair of material strength and thickness.

Influence of Predetermined Surface Defect to the Bendability of Ultra-High-Strength Steel

The use of ultra-high-strength steels (UHS) has become more and more popular within last decade. Higher strength levels provide lighter and more robust steel structures, but UHS-steels are also more sensitive to surface defects (e.g. scratches). Practically this means that the critical crack size decreases when the strength increases. The aim of the study was to study if the formula of critical crack size is valid on forming processes of UHS-steels. Surface cracks with different depths were created by scratching the surface of the sheet by machining center. Effect of the scratch depth was determined by bending the specimens to 90 degrees. Bents were then visually compared and classified by the minimum achieved bending radius. Test materials used were direct quenched (DQ) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250). Results from the bending tests were compared to the calculated values given by the formula of critical crack size.