Determination of mechanical properties is a powerful method for investigating and assessing the performance of service exposed materials. The present paper highlights the trend recently observed in service exposed (38 years) plain carbon steels used as reactor and distillation column materials in a petrochemical industry. Health assessment of both base and welded joints was made using destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. So far as the remaining life at 520 o C is concerned, it is possible to obtain a life of about 1.00 lakh h at the respective hoop stress level of both materials, provided no localized damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 5 years of service exposure at 520 o C.