The effect of carbides on temper embrittlement of commercial 2.25Cr-1Mo steel

• Main Author: Muhammad, Azali bin
• Published: University of Surrey 1982
• Subjects: 669; Metallurgy & metallography
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.278599

Temper embrittlement has, been studied in samples tempered at different temperatures and for varying times to produce the same hardness (190VPN) and different hardness levels with different types of carbide. The specimens have been embrittled by isothermal ageing at 500°C for times up to 5,000 hrs. The embrittlement has been characterised by Charpy impact testing to determine the ductile-brittle transition temperature together with hardness testing. Types of carbides have been identified from energy dispersive X-ray (EDAX) spectra using carbon extraction replicas and their relative presence determined from transmission electron micrographs. Auger electron spectroscopy (AES) analysis has been used to monitor the change in grain boundary composition after ageing treatment. This study shows that types of carbides and their relative presence in the steel have a marked effect on temper embrittlement. The results indicate that M[2]C and M[23]C[6], which are molybdenum-rich carbides, control the embrittling process. This is in agreement with the AES results that embrittlement is caused by the segregation of phosphorus to grain boundaries which, in turn, are controlled by the concentration of molybdenum and chromium in solid solution and/or in carbides. Chromium was found to be segregated at the grain boundaries and it appears to enhance the segregation of phosphorus; whereas molybdenum was not found to be segregated together with phosphorus to grain boundaries.