ABSTRACT
The expansion of the gas pipeline network makes it necessary, on the one hand, to meet the requirements of standards regarding the materials used, but on the other hand, it is necessary to weld them. In the case of natural gas as a fuel, the welding process is widely used, but in the case of replacing natural gas with a mixture of this gas and hydrogen, the requirements regarding the quality of the process must be significantly increased or the process must be completely changed. This article presents the results of testing welded joints for a newly developed welding technology for the transmission of a hydrogen mixture. Material tests were carried out on a butt-circumferential-welded joint made between two spiral pipes with an outer diameter of 711 mm and wall thickness of 11 mm in the X70 grade. The developed welding technology is distinguished by a change in the beveling method of the edges, which allows the heat input to the material to be limited. The technology was developed for use in natural on-shore and off-shore gas pipelines with the addition of hydrogen. As a result, additional requirements in terms of joint plasticity had to be met during welding. The test results obtained indicate that the joints are characterized by high strength (more than 581 MPa), higher than that of the base material (fracture in the base material) and good impact strength at reduced temperature (more than 129 J). In transverse corrosion, a hardness below 250 HV and a favorable structure of ferrite with different morphologies were obtained.
ABSTRACT
This article presents the results of research on the development of the technology of laser beam butt welding of 17-4 PH stainless steel sheets and the technology of post-weld heat treatment (PWHT). The developed technology allows for favorable conditions to be obtained and for the appropriate microstructure and hardness to exist in the weld area. Moreover, it enables the fulfillment of a number of specific requirements beyond the possibilities of manual welding and other methods. The tests performed include the analysis of the microstructure with the use of light microscopy (LM) for the materials after welding and PWHT. The applied PWHT showed changes in the microstructure and mechanical properties. In all weld areas the martensitic microstructure was observed. The homogeneity of the microstructure in the area of the welded joint after PWTH was revealed. In the as-welded condition and after the PWHT with aging at 481 °C, the hardness was 440 HV5, but after aging at 621 °C, it decreased to 330-340 HV5.
ABSTRACT
This article presents a study on the influence of temperature and time of multi-variant heat treatment on the structure and properties of materials produced in direct metal laser sintering (DMLS) and casting technology. The materials were manufactured in the form of cuboidal elements with a cross-section of 1.5 mm × 15 mm and a length of 60 mm. The samples prepared in this way had a similar volume, but due to the production technology the metal crystallization took place at different rates and directions. In the cast, the direction of heat transfer was toward the mold, and the DMLS was directed locally layer by layer. The small thickness of the cast material allowed reaching conditions similar to the DMLS cooling process. Both DMLS and cast samples show similar mechanical properties (hardness) achieved after long ageing time, i.e., 16 h at 170 °C. The maximum hardness was observed for 8 h. In the DMLS samples, in contrast to cast samples, no lamellar precipitates of silicon were observed, which indicates their better resistance to cracking.
ABSTRACT
Recycling of raw materials and is crucial for the production of new products for the global economy. The aim here is, on the one hand, to reduce energy consumption, and, on the other hand, to obtain materials with similar functional properties. The study undertook research on the possibility of processing AlSi11 aluminum chips by compaction and co-extruding to obtain a product in the form of a flat bar with mechanical properties not lower than those of the cast materials. The performed tests and the developed technique allowed to obtain flat bars with more favorable mechanical properties (Yield Strength YS ≥ 155 MPa; Ultimate Tensile Strength UTS ≥ 212 MPa) than the castings (YS ≥ 70 MPa ≥ 150 MPa). The weldability evaluation tests revealed that the material is susceptible to porosity. The presence of pores, which reduces the cross-section (up to 60%), reduces the tensile strength (up to 20 MPa). The typical joint structure and plasticity is obtained, which indicate the possibility of tensile strength improvement.
ABSTRACT
An analysis was conducted on the possibility of making an assessment of the degree of plastic deformation ε in X2CrNi18-9 steel by measuring three electromagnetic diagnostic signals: the Barkhausen noise features, the impedance components in in-series LCR circuits, and the residual magnetic field components. The impact of ε on a series of different extracted features of diagnostic signals was investigated. The occurrence of two regions of sensitivity was found for all the features of the analysed signals. The two regions were separated by the following critical deformation value: ε ~ 10% for the components of the residual magnetic field and ε ~ 15% for the normalised components of impedance. As for the Barkhausen noise signal, the values were as follows: ε ~ 20% for the mean value, ε ~ 20% for the peak value of the signal envelope, and ε ~ 5% for the total number of the signal events. Metallographic tests were performed, which revealed essential changes in the microstructure of the tested material for the established critical values. The martensite transformation occurring during the plastic deformation process of X2CrNi18-9 austenitic steel process generated a magnetic phase. This magnetic phase was strong enough to relate the strain state to the values of diagnostic signals. The changes in the material electromagnetic properties due to martensitic transformation (γ â α') began much earlier than indicated by the metallographic testing results.
ABSTRACT
The paper presents research results of the influence of heat input into high carbon rail steel during cutting processes on microstructure transformation and cracking. The massive block of steel prepared for rail rolling processes was cut and examined by nondestructive magnetic testing and destructive testing by microscopic examination and hardness measurements. The results show unfavorable microstructure changes where pearlite and transformed ledeburite were obtained. The effects of the presence of such microstructures are high hardness near to cutting surfaces (above 800 HV) and microcracks which grow into low hardness block cores during rolling and rail shaping.
