Effect of service simulated ageing on microstructure and creep rupture properties of I CrMoV (Casing Casting) steel vis-a-vis remnant life assessment in view of latent creep strength

Abstract

In many cases, it was observed that service exposed aged material delivers creep life comparable to the original material. In order to understand this unusual behavior, 1 Cr1Mo 0.25V casing casting steel has been given service simulated ageing treatment covering up to 150,000 hours. Room temperature tensile strength, hardness and FATI'are evaluated for each time-temperature pairs. Stress-rupture tests have been carried out following iso-stress methodology. Microstructural changes subsequent to ageing and after stress rupture tests are evaluated by transmission electron microscopes (TEM). The results have shown that though there is a drop in mechanical properties of aged material, but stress-rupture property behaved derently.Life steadily declines at intermediate simulation ageing conditions up to 100,000 hours, where as, improves onfurther ageing (__ 150, 000hrs). Thus, extrapolated rupture life at 530°C (service temperature) produced a parabolic 'C' shape in ageing time vs. rupture life plot. Metallographic observation revealed that at intermediate ageing condition, metastability and transitional behaviour of alloy carbides are responsible for the drop in rupture life. On the contrary, improvement in rupture strength at extremely aged condition (.150,000 hours) is attributed to the evenly dispersion of stable carbides, viz. MC, M23C6, in-situ formation of (V, Mo) C ('H' type carbide) and secondary precipitation of MC and M2C carbide in a ferrite matrix possessing low dislocation density. The phenomenon may be attributed to the latent creep strength of the material.using scanning electron (SEM) andtransmission electron microscopes (TEM). The results have shown that though there is a drop in mechanical properties of aged material, but stress-rupture property behaved derently.Life steadily declines at intermediate simulation ageing conditions up to 100,000 hours, where as, improves onfurther ageing (__ 150, 000hrs). Thus, extrapolated rupture life at 530°C (service temperature) produced a parabolic 'C' shape in ageing time vs. rupture life plot. Metallographic observation revealed that at intermediate ageing condition, metastability and transitional behaviour of alloy carbides are responsible for the drop in rupture life. On the contrary, improvement in rupture strength at extremely aged condition (.150,000 hours) is attributed to the evenly dispersion of stable carbides, viz. MC,M23C6, in-situ formation of (V, Mo) C ('H' type carbide) and secondary precipitation of MC and M2C carbide in a ferrite matrix possessing low dislocation density. The phenomenon may be attributed to the latent creep strength of the material.