Effects of strain rate and temperature on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel

Abstract

Tension tests have been conducted to evaluate the influence of strain rate (6.3 × 10^-3 to 3.2 × 10^-5 s^-1) and temperature (300-873 K) on tensile deformation and fracture behaviour of 1000 mm diameter and 300 mm thick 9Cr-1Mo tube plate forging in simulated post weld heat treatment condition. Yield strength and ultimate tensile strength decreased gradually up to around 723 K. Beyond 723 K a rapid fall in both the strength values was observed. In the intermediate temperature range (523-673 K), the alloy exhibited higher yield and tensile strength values with decreasing strain rate, indicating negative strain rate sensitivity. In contrast, at temperatures of 723 K and above, the strength values decreased with decrease in strain rate. Serrated flow, a characteristic of dynamic strain ageing, was observed in the temperature range 523-673 K. The upper end temperature of serrated yielding decreased with decrease in strain rate. A detailed analysis of the dependence of critical strain for the onset of serrations on temperature and strain rate yielded an apparent activation energy of 83 kJ mol^-1 for serrated flow at temperatures between 523 and 623 K. Ductility measured in terms of percentage elongation and reduction in area after fracture showed a gradual decrease up to about 673 K and a general increase at high temperatures at all the strain rates, exhibiting a ductility minima in the intermediate temperature range. The alloy has undergone predominantly transgranular ductile fracture at all strain rates and temperatures investigated. Yield and tensile strength of the forged tube plate material is consistently lower than the thin section bar material data. However, the strength values were still higher than the minimum values proposed in the ISO specification for the thin section material. Lower strength values of the forged tube plate material have been attributed to its coarse grain size compared to that of thin section bar material.