This paper presents a specific kind of failure in ethylene pyrolysis furnace tubes. It considers the case in which the tubes made of 35Cr-45Ni-Nb high temperature alloy failed to carburization, causing creep damage. The investigation found that used tubes became difficult to weld repair due to internal carburized layers of the tube. The microstructure and geochemical component of crystallized carbide at grain boundary of tube specimens were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) with back-scattered electrons mode (BSE), and energy dispersive X-ray spectroscopy (EDS). Micro-hardness tests was performed to determine the hardness of the matrix and the compounds of new and used tube material. The testing result indicated that used tubes exhibited a higher hardness and higher degree of carburization compared to those of new tubes. The microstructure of used tubes also revealed coarse chromium carbide precipitation and a continuous carbide lattice at austenite grain boundaries. However, thermal heat treatment applied for developing tube weld repair could result in dissolving or breaking up chromium carbide with a decrease in hardness value. This procedure is recommended to improve the weldability of the 35Cr-45Ni-Nb used tubes alloy.

The effect of welding parameters on GTAW (gas tungsten arc welding) between low carbon steel and 3CR12 ferritic stainless steel with ER308L filler metal was investigated with regard to microstructure and mechanical properties. The welded metal presented good configuration. The microstructure of weldment was found to be of widmanstatten austenite, acicular ferrite and martensite. Martensite and Cr-rich carbide precipitated particles formed under certain conditions in weldment produced results of both better hardness values and tensile strength. The results indicated that influence of these welding parameters have significant effects on microstructure and mechanical properties in welded metal.

This study involves V-groove butt welding of CP Titanium to 304 stainless steel by the gas tungsten arc welding (GTAW) process without and with buttering layer at the 304 stainless steel base metal. ERCuSn-A and ERNiCu-7 were chosen as a filler metals. Investigations including visual testing (VT), microhardness testing and metallurgical analysis were carried out by means of variable welding parameters. The experimental results showed that using the ERCuSn-A filler metal without and with buttering layer, any surface defect was not observed in the dissimilar metals welded specimen but an underbead crack was found at weld metal adjacent to the Ti/weld metal interface. Using the ERNiCu-7 filler metal without buttering layer, linear porosity was observed at weldment. However, using ERNiCu-7 filler metal with buttering layer, defect-free welded specimen could be achieved. The results of EDS analysis indicated that at Ti/weld metal interface, Ti diffused from the Ti base metal to the weld metal. At 304 stainless steel/weld metal interface, Fe, Ni and Cr diffused from the 304 stainless steel base metal to the weld metal.

Abstract. In this paper, size and morphology of the grain refiner and modifier particles in the Al-3%B-3%Sr master alloy production by using different cooling rates were investigated. Two Al-3%B-3%Sr master alloys were produced with 0.2 and 10°C/s, respectively. The grain refinement and eutectic modification efficiency of the Al-3%B-3%Sr master alloy were tested in casting process of A356 alloy by addition of 4wt.% and holding times for 10-120 min. The experimental result showed that microstructure of the M1 alloy (Slow cooling) consisted of larger solidified particles of AlB2, SrB6 and Al4Sr in the matrix of α-Al compared to the M2 alloy (Rapid cooling). The addition of the M1 alloy in cast A356 alloy, it was found that small grain size and fully modify eutectic silicon were obtained from the holding time in a range of 10-60 min. While the addition of M2 alloy, a small grain size was achieved in shorter holding time in a range of 10-30 min but the eutectic silicon was partly modify. From the thermal analyzed result, solidification of un-modified A356 alloy was changed after addition of Al-3%B-3%Sr master alloy. It was clearly observed that both the undercooling of nucleation and eutectic reaction was reduced and the solidification time was shifted to longer.

The aim of this research is to study the effects of welding repair parameters on weld joint of tube 35Cr-45Ni-Nb alloy by using the Gas Tungsten Arc Welding (GTAW) process of two different base metals. One was a new tube material and other was a used tube treated with carburized condition. The 35Cr-45Ni filler metal was sellected for welding repair experiments. The SEM-EDS analysis was used to observe a segregation of atom in the used tube specimen. The non-destructive examination (NDE) tests were done to check the imperfected area of weldment and heat affect zone (HAZ). Data from this research can be used as the guideline suggestion for the welding procedure specificcation (WPS) and the procedure qualification record (PQR) for the welding repair in the peroid of maintenace.