Cracked ammonia and synthesis gas (N₂ and H₂) from an ammonia cracker outlet flow through an internally insulated conical inlet to waste heat boiler at a pressure of 125 kg/mm² with a temperature of around 803 K in a Heavy Water Plant. At normal working conditions, the outer surface temperature is not expected to go more than 393 K. But high temperature (around 623 K) was noticed on the outer surface of the cone near the inlet side. Since the material has experienced elevated temperature service for a considerable period of time, it was suspected that metallurgical degradation could have taken place. To evaluate the component integrity and to assess its residual life, detailed NDT&E of the component was carried out and remedial measures were taken up for extending its life.

During ultrasonic normal beam inspection of the component, heavy sound scattering and attenuation was observed from the material around 300 mm length from the inlet side (reduced portion of the cone). Angle beam examination with a 70°, 4 MHz probe revealed presence of micro cracks and pores upto a depth of around 4 mm from the inner surface in this region. Hardness measurements were also carried out on the outer surface of the cone at various points. The hardness values observed in the range 115 to 135 VHN were well below the nominal specified range (145-192 VHN as per ASTM standard A182/A-182 M-93b for this material ASTM A182 GR F1). The reduction in the hardness of the material clearly showed the degradation of the material which was subjected to elevated temperature. Around 20% reduction in hardness from the minimum required hardness value was noticed indicating the reduction in the strength of the material.

To evaluate the microstructural condition of the cone, insitu metallography was also carried out. The study revealed that a typical ferrite, pearlite and bainite structure was present at the unaffected region. Whereas, at the heat affected region, considerable changes have taken place. More ferrite content, absence of pearlite and presence of spheroidal carbides were observed at this region indicating the softening of the material. The insitu metallographic results also revealed intergranular micro pores and cracks. Liquid penetrant examination also revealed presence of micro cracks and pore.

Based on the NDT results, the plant authorities had taken a decision to repair the reduced portion of the cone by removing the defective material from the inner side of the cone. Subsequently the material was removed by scooping up to a depth of 4 mm in steps of 0.5 mm. Liquid penetrant examination was carried out at each stage of the material removal till no defect indication was seen on the inner surface. At this stage, the depth of material removed is 4 mm, which is in agreement with the ultrasonically determined depth to which presence of micropores and cracks were predicted. After the material removal ultrasonic flaw detection and insitu metallography were again carried out and it was confirmed that the area in free from micro cracks and pores. Systematic NDT & E methods have been successfully employed for assessing the extent of degradation of the conical inlet, and thus helped in employing a suitable repair procedure.