Papers by Keyword: High Strength Steel

Abstract: This paper presents an experimental investigation with the objective to determine the root causes for the cracking of a large size bar made of a medium carbon low alloy steel after open die forging and heat treatments operations. The cracks were observed below the surface during the machining step. In order to understand the mechanisms of crack initiation and propagation, micro-CT tomography and scanning electron microscope (SEM) were employed. Microstructural damage analysis revealed oxidation of different alloying elements, more specifically manganese, chromium and silicon. The presence of defects in the form of cavities and porosities were also observed at the grain boundaries. Some of the above defects were observed along the crack path, while others were on both sides of the cracks without any connection to them and finally, a third group completely isolated from any crack. The characteristics of the defects were thoroughly analyzed and it was found that the crack initiation could be attributed principally to the porosities/cavities formed during solidification. The analysis also showed that crack propagation occurred during solidification and/or forging and heat treatment steps.

Abstract: Press-hardening is an intensively developing forming technology which is mainly used for the production of car body parts. Because it is a hot forming technology, small forming forces can be utilized and, due to the lower spring-back effect, more accurate products are achieved. In car bodies, materials with high energy absorption and a sufficient hardening coefficient are mainly used in impacted parts. One of these materials is TRIP multiphase steels with different chemical composition. In these steels, it is possible to achieve an ultimate strength up to 1000 MPa with the ductility of 20-30%. In order to achieve the desired properties, it is necessary to select a suitable heat treatment that allows to achieve a multiphase structure. Phase transformations and mechanical properties are influenced by the use of suitable alloying elements. Three low-alloy, multiphase TRIP steels with different chemical compositions with a carbon content of 0.2% were chosen for the experimental program. The first steel was alloyed only with manganese and silicon, in the second niobium was added, and in the third the influence of chromium on increase of hardenability and strength was investigated. Press-hardening was performed in a heated forming tool. To describe the effect of the cooling rate, the forming was carried out in a tool at room temperature and after preheating to 425°C. The influence of holding time in the tool at 425°C to support the formation of bainite and retained austenite stabilization was also investigated. Mixed ferritic-bainitic-martensitic structures with some retained austenite content were obtained.

Abstract: The hydrogen embrittlement of SK85 high-strength steel sheets was evaluated using a three-point bending test. The effect of electroplating the metal with zinc-based coatings on hydrogen embrittlement was examined by baking treatment of differently electroplated steel specimens. After electroplating, all the specimens underwent hydrogen embrittlement, promoted by hydrogen incorporation into the metal frame, owing to the reduction of hydrogen ions during electroplating. The hydrogen embrittlement of both zinc-and zinc-SiO₂-electroplated SK85 steel continued after baking for 24 hours at 473 K, but that of zinc-nickel-and zinc-nickel-SiO₂-electroplated SK85 steel ceased. Furthermore, TDA revealed that the trapped hydrogen could be released from steel at approximately 473 K. However, after baking, hydrogen embrittlement did not completely disappear, and we suggest that the formation of hydrogen vacancy clusters also accounts for this fracture phenomenon. The hydrogen incorporated into steel during electroplating led to the formation of hydrogen vacancy clusters, which allowed the formation of embrittlement. However, zinc and zinc-SiO₂ films were not permeable enough to release these voids; while the peculiar zinc–nickel and zinc-nickel-SiO₂ film structure enabled the hydrogen vacancy clusters to diffuse from the substrate.

Abstract: One of the disadvantages of using steel powders is the difficulty of producing such materials. In this article, steel powders from various manufacturers were discussed. The analysis of the surface, powder particles and the distribution of the fractional composition and the possibility of reuse was carried out. Direct laser deposition modes, using different laser power, were designed. From the obtained modes, tensile and impact bending samples were grown, and a comparative analysis of the obtained results was carried out.

Abstract: Effect of process parameters of microstructure formation and mechanical properties of direct laser deposited parts of cold-resistant steel 09CrNi2MoCu is studied. Due to local properties of buildup depends on thermal cycle during fabrication simulation of temperature field was carried out. The following cases were analysed: deposition of the first layer on massive substrate and deposition of a layer on the buildup far from the substrate. It was established that one time high temperature reheating of deposited layer has no effect on hardness while additional reheating up to lower temperature leads to considerable decrease in hardness by 87-100 HV. Far from substrate hardness and microstructure bands of 0.7-0.8 mm thickness have a hardness variation in the range of 250-300 HV. The area close to the substrate has a microstructure of upper bainite with higher hardness due to higher cooling rates during deposition. In the process of deposited, at a higher power, a quick process of heating and cooling occurs, and vice versa, which forms various products of bainite transformation. From the obtained modes were presented the results of tests for impact strength at low temperatures.

Abstract: Dissimilar joining of high strength tensile steels are joined using laser beam welding. The selection of the welding conditions for joining of dissimilar materials is highly required to satisfy the quality of the joints. In the present investigation, optimization technique were used to determine the optimal welding conditions. Initially welding conditions were optimized for weld geometry and formation of different zones in the weldment. The metallurgical and mechanical properties of the welds are greatly influenced by the geometry of the welds. The surface response methodology design is carried out for the experimental design by the development of regression equations. Analysis of variance (ANOVA) was used to check the validity of the model. The output of the welding conditions were compared with the predicted values to identify the accuracy of the model. The obtained results from response surface methodology were compared with the experimental results and validated.

Abstract: In order to determine the weldability of a new type of low alloy high strength steel, the feasibility of the proposed welding process was examined and the welding procedure qualification of a new type of ultrahigh strength steel WELDOX 1100 was carried out through the weldability test of the welded joint, the small iron research and the highest hardness test. . The test result satisfies the specification requirements and proves that the proposed welding process plan is correct and feasible. It provides a reliable basis for the preparation of welding process documents and ensures the smooth development of the company's new products.

Abstract: The hole expansion process is a forming process that the fracture easily occurred on the hole edge. Therefore, many research papers presented various processes to make a precision pre-hole shearing to a hole expansion process such as wire cut, EDM, and laser cut. The purpose of this work is to increase a hole expansion ratio on the high strength steel sheet. The conventional piercing by piercing punch and piercing with a counter punch was used to make a pre-hole shearing process in this research paper. The high strength steel grade 590 and 980 MPa of 1.2 mm of thickness were used in the experiments. From experimental results, it was found that the pre-hole shearing by piercing with a counter punch can be increased a shear surface on cutting edge of workpiece. Moreover, the used piercing with counter punch can improve the hole expansion ratio more than the conventional piercing process because sheared surface around the hole edge was increased.

Abstract: In this work, Forming Limit Curves (FLCs) of the conventional and pre-stretched High Strength Steel (HSS) sheet grade 440 (SCGA440-45) were investigated. The conventional forming limit curve was experimentally determined by using the Nakajima stretching test. Subsequently, the non-linear strain path FLCs were precisely developed through the Nakajima stretching test after the specimens were pre-stretched in biaxial direction up to several levels on the Marciniak In-plane stretching test. The gained non-linear strain path FLCs were compared with the conventional FLC.Additionally, the experimental Forming Limit Stress Curve (FLSCs) were calculated using the experimental FLC and non-linear strain path FLCs data from investigated steel sheet. The yield criterion Hill’48 was employed in combination with the Swift strain hardening law to describe anisotropic deformation and plastic flow behavior of the HSS sheet, respectively. Hereby, the influence of pre-stretching levels on the experimentally determined the FLCs and FLSCs were examined. The results prove a significant influence of the pre-stretching levels on the both FLCs and FLSCs of the investigated HSS sheet. For a low pre-stretching in biaxial loading the FLCs demonstrated a reduced formability and the FLSCs exhibited the limited stress levels depending on the experimental FLC data.

Abstract: The weldability of high strength steel of grade 10G2FBY with a yield strength of 500 MPa is conducted in this paper according to the criterion of cold cracks formation in the seam root of plates with a thickness of 18 and 36 mm. The study was carried out in accordance with GOST 26388-84 on samples of type IX at ambient temperatures from –20 to +20 °С at heat input energy from 5.8 to 14.3 kJ/cm. After welding, the plates were cooled in the air for 24 hours. The main criterion for assessing weldability is the presence or absence of cold cracks after welding. The welding was made with a soft wire 4Y42 with a yield strength of 420 MPa. Cold cracks in high strength steel 10G2FBU for thicknesses of 18 and 36 mm are not detected in heat input over 10.3 kJ/cm by welding at the temperature of +20 °С, at the same time when welding at an ambient temperature of -20 °С cracks are not detected when running over energy of 14.7 kJ/cm. Critical cooling rate in which no cracks appeared was 55 °С/s at the temperature of welding +20 °С, and it was 44 °С/s at a temperature of welding –20 °С for both thicknesses. The criterion HV_max<300 HV is a necessary but insufficient condition for the prevention of cold cracking in steels of the type 10G2FBU. In the conditions of the experiment, cold cracks in the weld seam are absent at a hardness of no more than 250-260 HV, which is probably due to the presence of diffusive hydrogen, which is not regulated in the regulations for the supply of this steel grade