SEDL / STP / STP1046V2-EB / STP49464S

Serrated Flow in Irradiated and Partially Denitrided Mild Steel

Murty, K. Linga
Professor, North Carolina State University, Raleigh, NC

Mahmood, Sheikh T.
Graduate student, North Carolina State University, Raleigh, NC

Pages: 19    Published: Jan 1990

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Abstract

The effects of neutron radiation exposure and nitrogen concentration on dynamic strain-aging characteristics were studied in mild steel. The influence of neutron irradiation was investigated at radiation doses ranging from 4×1016 to 1×1019n/cm2 (>1 MeV). Neutron radiation exposure decreased the concentration of the free, interstitial, impurity elements responsible for locking the dislocations. Neutron irradiation resulted in an increased critical temperature for the onset of serrated flow and a decreased critical temperature for the disappearance of serrations, with narrower temperature regions of unstable plastic deformation. While radiation embrittlement was noted at ambient temperatures, ductility increased (by a maximum of ∼200%) following irradiation (to ∼10¹⁸ n/cm²) in the blue-brittle range for the unirradiated material.

The effect of nitrogen concentration on the dynamic strain-aging characteristics was investigated following dry hydrogen treatment at 948 K with nitrogen concentrations varying from 0.0160 to 0.0014 a/o with three more intermediate values. The effect of decreasing nitrogen atom concentration similar to that of increased radiation exposure—reduced yield drops, an increased critical temperature for the onset of serrations, a decreased critical temperature for the disappearance of serrations, and a decreased range of temperatures of unstable plastic deformation.

The activation energy for the appearance of serrations was identified with that for the diffusion of carbon or nitrogen in steel and was not affected by either neutron irradiation or dry hydrogen treatment. Quantitative correlations of the yield stress, the critical temperature for the onset of serrations, and the density of mobile dislocations with the concentration of nitrogen atoms revealed that Cottrell locking is dominant over Snoek drag for the strain-rates used here.