Aging behaviour in copper bearing high strength low alloy steels

Strengthening due to precipitation of copper and microalloying elements is a phenomenon utilized in the design of some HSLA steels for naval structural applications, In the present work, precipitation of copper and associated property changes during aging of three Cu-containing HSLA steels were investigated. Electrical resistivity change during isothermal aging of one low carbon steel was compared with that for HSLA steels with a view to evaluating the kinetics of copper precipitation in the latter. The results fitted to Johnson-Mehl-Avrami equation indicate that this precipitation occurs in two consecutive stages with activation energy in the range of 114-128 kJ/moI and 64-77 kJ/mol, respectively. These values suggest a dominant role of the high dislocation density of the martensite matrix in the aging process. The peak hardness during isochronal (1 h) aging was observed in the aging temperature range of 500-550degreesC, whereas Charpy impact toughness was lowest for the samples aged at 450degreesC. The fractographs of the Charpy specimens were in good agreement with the impact strength measurements, apparently indicating an adverse effect of coherent copper precipitates on the impact properties.