1.25Cr-0.5Mo: (Normalizing + Tempering) and (Quenching + Tempering) 1

[engineermat]

(1) What is the difference of 1.25Cr-0.5Mo subjected to two different heat treatment processing, Normalizing + Tempering, and Quenching + Tempering; such as impact test values?

(2) API 934 requires impact test for Heavy wall 1.25Cr-0.5Mo materials. Why impact test is required for heavy wall (above 50 mm); How about the plate thickness less than 50 mm?

Thanks

 

[metengr]

(1) What is the difference of 1.25Cr-0.5Mo subjected to two different heat treatment processing, Normalizing + Tempering, and Quenching + Tempering; such as impact test values?

The quenching and tempering will improve notch toughness (high impact values at lower test temperature) because quenching will increase the amount of bainite/martensite in these alloys versus ferrite, after austenitizing. The tempered martensite/bainite microstructure has improved notch toughness over ferrite.

2) API 934 requires impact test for Heavy wall 1.25Cr-0.5Mo materials. Why impact test is required for heavy wall (above 50 mm); How about the plate thickness less than 50 mm?

Because heavier wall components have slower cooling rates that results in the formation of more ferrite during cooling from austenitizing. This results in lower notch toughness values, which is why impact testing is required to assure minimum notch toughness in heavy wall components. In some cases liquid quenching followed by tempering may be required to improve notch toughness.

 

[engineermat]

Metengr,

Thanks for your reply. Great answer.

In my mind, the martensite/bainite are brittle phase, which means lower toughness. It looks that I am wrong.

Why the martensite/bainite has improved notch toughness over ferrite?

 

[metengr]

engineermat;
The short answer to this is that the ferrite phase will exhibit cleavage along certain planes of atoms in grains. The martensite and bainite metastable phases do not have a preferred cleavage plane, like ferrite, and as such there is a more torturous path for a cleavage cracks to propagate. This is also why fine grained steels containing ferrite and pearlite exhibit improved notch toughness over coarse grained steels.

 

[EdStainless]

martensite and bainite may be brittle, but they are also strong. When they are finely distributed in a well heat treated structure they improve the properties, even low temp toughness. Residual ferrite adds nothing in the way of mechanical properties.

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