Metallurgical evaluation was performed on a fractured column tube of the reformer furnace in an ammonia plant. The tubes were manufactured from centrifugally cast heat resistant steel HK 40. Optical and scanning electron microscope (SEM) were used for microstructural and fracture analysis. For composition determination of the microconstituents energy dispersive X ray spectroscopy (EDS) was used. To evaluate mechanical properties, hardness and microhardness measurements were performed. Investigations based on the microstructural features with the idea to indicate suitability of weld repair of the column were performed in this study. It was observed that the crack initiation, caused by oxidation/corrosion and thermal stresses induced by temperature gradient, appeared in the inner side of the tube wall and propagation occurred along grain boundaries. The results clearly showed the presence of an irregular microstructure which contributed to crack propagation through the tube wall. An occurrence of precipitated needle-shaped carbides/carbonitrides and brittle σ phase was also identified in the microstructure. Results of the microstructural and fracture analysis clearly indicate that reformer furnace columns made of heat resistant steel HK 40 were unsuitable for weld repair. KeywordsMrostructural evaluation; Fracture analysis; Steel HK40; Weld repair. Análisis micro estructural como indicador de la idoneidad de reparación por soldadura de aceros al Cr-Ni resistentes al calor ResumenLa evaluación metalúrgica se realizó en un tubo de columna con fracturas, que es parte del horno reformador en una planta de amoníaco. Estos tubos son fundidos centrífugamente y fabricados en acero resistente al calor, de tipo HK-40. Para el análisis microestructural de la fractura se ha utilizado microscopía óptica y electrónica de barrido (SEM). La composición de los micro-constituyentes se determinó por espectrometría de rayos X de energía dispersiva (EDS). Las propiedades mecánicas se evaluaron mediante mediciones de microdureza Vickers. Las investigaciones en este estudio se han llevado a cabo con el fin de demostrar la idoneidad de reparación por soldadura de columnas en base a sus características micro-estructurales. Se ha observado que el inicio de la rotura, causada por el efecto de la oxidación/corrosión y el choque térmico ocasionado por el gradiente de temperatura, aparece en la pared interna del tubo y se propaga a lo largo de los bordes de grano. Los resultados demuestran la presencia de una microestructura irregular que contribuyó a la propagación de la rotura a lo largo de la pared del tubo. En la microestructura también se ha registrado la formación da la precipitación de fases de carburo/carbón nitrito que aparecen en forma de agujas y fase σ frágil. Con base en los resultados obtenidos, se puede concluir que la microestructura investigada no es idónea para aplicar la reparación por soldadura. Palabras claveEvaluación microestructural; Análisis de fractura; Acero de tipo HK40; Reparación por soldadura.

Prealloyed copper powder containing 2.5wt. %Al was processed in a planetary ball mill to evaluate matrix hardening due to formation of Al2O3 particles in situ by internal oxidation. After milling, the powders were heat treated in hydrogen in order to reduce copper oxide formed on particle surfaces during milling. The compacts were made by hot pressing. The examinations show that the compacts possess a good combination of high strength and high electrical conductivities. After 5h of milling the microhardness of the prealloyed compact was 3 to 4 times higher than that of the as-received electrolytic copper compacted under the same conditions, while the conductivity was 68% IACS. Also, it was found that the prealloyed compacts preserved much of the hardness after exposure to high temperature in inert atmospheres.