Constitutive analysis of hot metal flow behavior of virgin and rejuvenated heat treatment creep exhausted power plant X20 steel

Abstract

This paper presents constitutive equations that describe the hot flow behaviour of Virgin (VG) X20 and rejuvenated heat-treated creep exhaust (CE) X20 steels. The study provides a foundation for determining the effect of rejuvenation heat treatment on CE steels by making comparisons to the VG steel. Hot compression tests were conducted in the temperature range of 900 °C to 1050 °C, at strain rates of 0.1–10 s⁻¹ to a total strain of 0.6, and stress–strain curves were obtained. The flow stress curves of both steels exhibited dynamic recovery (DRV) characteristics as the main softening mechanism. Constitutive constants of steady-state stresses were determined. The stress exponents, n, were 6.62 (VG) and 5.58 (CE), and the apparent activation energy values were 380.36 kJmol⁻¹(VG) and 435.70 kJmol⁻¹ (CE). Analysis of the activation energies showed that VG steel had better workability properties than CE steel and was easier to deform at high temperatures. Constitutive equations for predicting the flow stress in the two steels were established. This were verified by statistical tools: Pearson’s correlation coefficient (R) and Absolute Average Relative Error (AARE). The results showed R-values were, 0.98 (VG) and 0.99 (CE), and the AARE values for VG were 4.17% and 9.01% for CE. The statistical parameters indicated a good correlation between the experimental and predicted values. The constitutive equations therefore adequately described the flow stress behaviour of both steels and can therefore efficiently analyse industrial metal forming schedules.