Papers by Keyword: Weld

Abstract: Implementation of high strength steels in welded structural designs in the automotive, defence and construction industries is constantly increasing. Prolonged usage of such structures requires deep understanding of welded joint fatigue as well as a reliable and feasible life estimation methods development. Conventional fatigue testing methods often require costly, expensive in maintenance, high loading capacity equipment. They are also commonly restricted to specific specimen geometry and are time consuming due to the limit of a single specimen per test setup. This work presents high cycle fatigue (HCF) testing of high strength MARS 600 welded steel using a quick, simple and efficient resonance fatigue testing (RFT) method. The specimen is a simple cantilever fillet welded to a base plate using austenitic stainless steel 307L consumable electrode. Electrodynamic shaker is used for harmonic base excitation at a constant operating frequency. Several specimens welded to the common base are tested simultaneously, allowing completion of a high number of cycles and statistics in a relatively short time period. A hybrid, practical research approach combining experimental, finite element analysis (FEA), numerical and analytical calculations is presented. Fracture mechanics approach for fatigue life assessment is implemented. Crack growth calculation is based on the Paris - Erdogan law. Reduction in structural integrity due to crack propagation causes a reduction in natural frequency and transmissibility. The change in gain is evaluated via the open crack FEA model and integrated into the crack propagation algorithm. Resonance search, track and dwell module (RSTD) for maintaining constant gain throughout the test is not required. Fatigue life Wohler (SN) curve is constructed. Standard weld fatigue data is often provided for direct loading (tensile stress) and for different stress ratio (R) values. Corrections for mean stress and loading application are required. Current fully reversed (R = -1), indirect loading (bending stress) test results may be readily applied for random vibration fatigue analyses post processing. As expected, actual fatigue life results are higher compared to standard design curves, implying correctness of the manufacturing welding process of examined specimens. The presented procedure is of interest for research as well as for industrial welding processes testing, optimization and qualification.

Abstract: The analysis of factors affecting the state of the metal of pipelines of heating networks on the territory of hazardous production facilities is carried out, of which the effect of mechanical loads should be especially distinguished. The necessity of carrying out additional types of control along with visual and measuring control during routine inspections and after repairs of pipelines of heating networks has been substantiated. The method for measuring the coercive force and its measurement methods are considered, and a decision is made to develop a technique for studying pipelines of a heating network using this method. Samples for laboratory research have been prepared, namely, a fragment of a heating network pipeline, which has worked for over 10 years, and a fragment of a new pipeline with a similar steel grade. A plan for conducting experimental studies of the prepared samples has been prepared, including the developed methods of experimental studies of samples in accordance with the regulatory documentation, including: a method for studying samples of pipelines of heating networks by measuring the coercive force; method for determining the chemical composition of metals and alloys; methodology for carrying out metallographic studies. Measurements of the coercive force were carried out during the loading of the samples. A change in the coercive force with loading was recorded, mainly towards large values. In this case, the values of the coercive force on the sample, which worked in the territory of hazardous production facilities, are higher than those of the sample of the new pipeline. The complex of the studies carried out indicates that it is advisable at enterprises to measure the coercive force of the metal of pipelines of heating networks on the territory of a hazardous production facility during a routine inspection along with visual and measuring, as well as after repair and restoration work.

Abstract: A weld cracking of the outlet pipe occurred during the operation of a low-pressure steam super-heater in the purification device. It was found that many cracks were perpendicular to the weld, which affected the equipment safe operation. In order to investigate the failure cause, the samples containing weld and cracks in the outlet pipe were cut. The experiments, including chemical composition analysis, mechanical properties testing, scanning electron microscope observation, energy spectrum analysis and metallographic analysis, were performed. The results showed that the corrosion products had loose structure, mainly consisting of iron, sodium and phosphorus oxides. The metallographic results of the fracture section indicated that there existed a mixed cracks of trans-granular and inter-granular mode, along with stepped crack characteristics. Therefore, it is believed that the weld transverse cracking in the outlet pipe may be resulted by both of alkali stress corrosion cracking and the hydrogen induced cracking. According to the failure cause analysis, the corresponding suggestions are given, such as welding, operation process and inspection.

Abstract: Designed experimental setup, and a prototype of the original devices for athermic relaxation of thermal residual stresses in openwork welded constructions of large dimensions for the rational use of energy high-frequency acoustic (ultrasonic) fields, conducted laboratory studies and pilot tests in real conditions of production. Ultrasonic relaxation when welding thin-walled constructions standard openwork rolled steel profiles 10 large dimensions (up to 3 x 10 m). It has been established that with a decrease in the time of ultrasonic relaxation, the efficiency of its use decreases somewhat. Therefore, in an industrial environment, it is advisable to use the duration of ultrasonic treatment in the range of 0.02-0.04 min per mm of weld length.

Abstract: Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M₂₃C₆ affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M₂₃C₆. Moreover, the calculations reveals that boron decreases the coherency between matrix and M₂₃C₆, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

Abstract: The heterogeneity of manual arc welded samples of low-carbon steel St3sp meant for bridge construction to be used in Cameroon has been investigated. The chemical analysis of the material was carried out and the arc welding processes used, then the probes were subject to mechanical testing such as tensile test, low-cycle fatigue test, and micro hardness study. A total inclusion of weld defects in the specific welded sample and especially in the cyclic loading specimes has been discovered. Premature fracture was revealed below the plastic limit in some probes during the tension and low-cycle test. A micro structural examination and micro hardness measurements were done in order to determine what exactly could be the reason of this catastrophic failure. The structural analysis revealed the differences in mechanical properties conditioned by phase distribution and carbon content in weld, heat affected zone and base metal. To determine the heterogeneity in mechanical properties the elastic modulus distribution has been calculated due to relationship between the micro hardness and materials strength.

Abstract: Fatigue strength characteristic of laser welded joint between austenitic stainless steel and ferritic stainless steel was studied at room temperature and 300 °C.The results showed that fatigue strength at 300°C was higher than that at room temperature in lower applied stress amplitude region. Fatigue crack mainly initiated at stress concentration part at room temperature. However, fatigue crack origin changed toward to the base material and the heat affected zone with increasing testing temperature. Increase in ductility of the base material and the weld metal induced by increasing testing temperature reduces stress concentration effect and resulted in change in fatigue mechanism. Appropriate weld position in a component with temperature gradient can be considered by minimizing the effect of stress concentration and taking into account degradation of strength due to increase in temperature.

Abstract: In this study, the weldability of thick dissimilar steels to a thin-wall pipe using the electron beam welding has been investigated. In addition, weld quality evaluation in as-welded condition has been conducted. For this purpose, micro and macro examination of the welds has been carried out.

Abstract: Short rows and multiple layers fixture welding is characterized by a non-stationary thermal field, compared to the long welds, characterized by quasi-stationary thermal field, excluding the end zones characterized by a non-stationary thermal field.Up to the present, welded tests are made for homologating welding processes or for welders authorizations or validity prolongations of welders authorizations or certifications, these being made on 500 mm long welded samples, where the 20 mm parts at the beginning and at the end of the sample are removed and no test specimens for mechanical test are made of them. The present article studies the values of HV 10 Vickers hardness in these beginning and end zones of the weld (crater) when the welding is made without putting on the technological plates at the beginning and at the end, compared to the values of HV 10 Vickers hardness measured in the middle part of the weld..

Abstract: The article contains analysis results of destruction of chemical generator corps representing thin-walled pressure vessels working under cyclic loading and in corrosive environments. It is shown that the corps made of steel 321 was destroyed mainly along the weld and heat affected zone. To improve the durability of corps we used materials with shape memory effect (SME) based on NiTi as alloying compound of welds. This allowed us to combine the functional properties of alloys with shape memory effect with increased mechanical, technological and operational properties. Accomplished electrochemical studies of corrosion behavior of steel 321 samples alloyed with NiTi in 3.5% NaCl solution, showed that the corrosion rate of welded joint and the base material are comparable. Fatigue tests in the environment KOH + H2 showed a significant reduction of ductility, which indicates the presence of hydrogen embrittlement. With increasing degree of hydrogenation the fatigue life of alloyed compounds drastically reduces.