Papers by Keyword: Hot Crack

Abstract: In order to find out the causes of weld cracks in the main steam pipe welded joint of a power plant, a series of relevant tests are specially formulated, which include room temperature tensile and high temperature tensile test, room temperature impact test, the crack tip metallographic observation, crack fracture scanning electron microscope observation, oxide film thickness measurement, and so on. By analyzing the metallographic photographs of the crack, the crack character is preliminarily determined. In addition, the direction of crack extension is determined by the thickness of oxide film in different parts. The crack failure mechanism has been identified. The results show that the welding heat crack occurs in 1# crack initiation position; and the crack initiation in 2# weld crack (primary crack) position is caused by the root defect of welding seam and lead to a high brittleness, which results in the crack failure by crack propagating in operational process gradually.

Abstract: The paper considers the issue of shrinkage stresses of elastic-plastic deformation in the casting ‘Slag bowl’ resulting in a discontinuity and hot crack formation. The stress-strain state of the wall of the as-cast billet during the process of power interaction between the casting and the mould has been analysed according to the existing algorithms. Quantitative indicators of the components of the total stress state in the wall of the casting at the time of hot crack formation were determined. It was found that the bending stress greatly affects the defect, comprising 96 % of the tensile strength of the casting material at the time of destruction. The techniques aimed at reducing hot crack formation in a body of as-cast billet based on the results from the calculations are recommended. In accordance with the proposed recommendations, the changes in the technological parameters in production of a batch of pilot castings showed a decrease in the defect on hot cracks by 73 %.

Abstract: . In this work the physical effects of electromagnetic stirring (EMS) during welding on the microstructure of some commercial 6xxx and 7xxx series of aluminum alloys are investigated. Experiments are carried out with standard GTA equipment in combination with a current carrying coil providing an axial magnetic field. In order to achieve the maximum grain refining effect, a range of optimal stirring parameters is established. It is then demonstrated that the optimal parameters are in the frequency range of 2 to 15 Hz and the corresponding magnetic field intensities of 10 to 25 mT. These parameters are then applied to reduce susceptibility of these alloys to hot cracking. This paper concludes that by application of EMS to GTA welding of aluminum alloys it is possible to refine the grain structure and reduce the hot cracking susceptibility of these alloys, especially those employed in the aerospace and automotive industries.

Abstract: This work aims to develop welding procedure for small diameter longitudinal welded clad pipe made from clad plate. High strength carbon steel base metal bonded with 316L stainless steel clad layer was used in this study. The dissimilar materials at the weld joint and accessibility limitation of small diameter present difficulty in welding process selection to achieve weld soundness. The joint and welding se¬quence are designed to avoid solidification cracking. Nickel base over matching filler is used on the clad side. Typical joint configuration is double V groove weld without clad peel back to minimize the number of passes inside the pipe. Firstly, welding is done on the carbon steel side by using Shielded Metal Arc Welding (SMAW) and Submerged Arc Welding (SAW) with carbon steel electrodes. Then, welding on the clad side is done by using ERNiCrMo-3 filler metal. Two different procedures for the clad side are studied. The first procedure is to weld the clad side by using Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding pulse current (GMAW-pulse) and another procedure is to weld the clad side by using the SAW procedure. Hot cracking was observed in the case of SAW procedure at the clad weld centerline due to high heat input and high level of dilution. Mechanical properties and microstructure are evaluated. Clad weld by use of GTAW and GMAW-pulse could give sound weld metal. The tensile and yield strength of all weld metal were found to be greater than that of base metal and 100% shear failures were observed. Charpy impact energy of weld and HAZ at -10°C was found to be over 100 joules. Hardness of weld and HAZ area are surveyed over the weld cross section to determine local hardening. Additionally intergranular corrosion testing was carried out on the clad weld side and then bend testing was done. No crack was observed. Therefore, GTAW and GMAW-pulse clad weld procedure could give required properties according to clad line pipe standard, reduce cost of production and increase productivity compared to the peel back method.

Abstract: This text does on research on welding behavior of the Ni3Al alloy casting MX246A of based on laser as heat source. First process parameters have effects on crack sensitive in the study. Laser power decided to heat input and the depth of the weld, which has obvious effects on the hot crack. Crack sensitivity of welding at low speed is quite lower than it at high scanning speed; low scanning speed can reduce the sensitivity of weld cracking. Because of the laser absorption and solidification of weld metal for inheritance of the basic material, grain degree of materials makes a significant impact on crack. Crack sensitivity in the fine grain areas is quite lower than it in the coarse grain areas. It is found in the study that the main reason for the weld crack is Mo elements gathering and caused the oxidation of the partial. Welding of MX246A alloy is realized on the basis of the above research, and performance of welding joint is up to more than 75% of base material.

Abstract: In order to investigate the soundness of welds in welding under loads, a series of experiments were carried out under static and cyclic loads. In the case of the experiment under compressive static loads, the defects such as buckling, welding crack, etc. were not observed in all the specimens. On the other hand, in the case of the experiment under compressive cyclic loads, the defects were not observed for the Fillet-type specimen. However, the hot crack was observed for the Butt-type specimen.

Abstract: Laser welding of AISI 304 stainless steel was carried out by pulsed Nd:YAG laser machine under different welding conditions. Geometrical shape of the joints was circular butt. Studies were focused on laser power, welding velocity, beam diameter and pulse duration. Microstructure of welded joints was investigated by optical microscopy (OM) and scanning electron microscopy (SEM). OM and SEM results show no evidence of grain growth in heat affected zone (HAZ). In addition, columnar and equiaxed grains are observed in the weld metal. Various defects such as lack of fusion, underfill, porosity, hot crack and mismatch are observed as a result of improper welding conditions. The best welding conditions to obtain minimum defects are presented.