High Temperature Mechanical Behavior of a 30%Ni-19%Cr Steel

Abstract

The deformation behavior at high temperature of a Type 800 Incoloy alloy reinforced by TiC particles was investigated by tensile and torsion tests at temperatures ranging from 800 to 1 200°C. The as-received material exhibited a microstructure of coarse equiaxic grains with subgrains decorated by small TiC particles. The material showed an activation energy for plastic flow of 400 kJ/mol similar to the activation energy for lattice diffusion in austenitic stainless steels. The stress exponents varied strongly with testing conditions. After testing at temperatures up to 1 000°C, the initial austenitic grains were elongated in the tensile direction, but the subgrain structure did not change with stress. In contrast, a microstructural refinement was observed after tensile testing at 1 100°C, which was associated with a process of dynamic recrystallization that occurs during deformation. At this temperature the deformation behavior of the material can be described by a slip creep mechanism. At the rest of the test temperatures the controlling mechanism is that of constant-structure slip creep.KEY WORDS: iron-base superalloy; microstructure; high temperature mechanical behaviour; constant structure creep.cidate the controlling deformation mechanism, the investigation was completed with a detailed microstructural analysis of the as received and deformed materials. Experimental ProcedureThe as-received material was supplied as a bar of 400 mm, which was prepared by hot rolling a bar of 600 mm in diameter heated to 1 200°C for 45 min and then air cooled. The chemical composition of the alloy used in the present study was determined by Inductively Coupled Plasma, except for nitrogen and carbon that were determined by infrared absorption after combustion in an induction furnace. The composition was the following (wt%): 30Ni-19Cr-0.8Mn-0.5Al-0.5Ti-0.45Si-0.12C-0.01N, balance Fe.The microstructures of the as-received material and after deformation were studied by optical microscopy, and scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) microanalysis. Metallographic preparation included mounting the samples in baquelite and polishing by the conventional method. The microstructure was revealed by etching at room temperature with a solution of 45 mL of HCl, 15 mL of HNO 3 and 20 mL of distilled water.Elevated temperature mechanical properties were characterized by tensile and torsion tests. The tensile tests were performed at strain rates ranging from 3ϫ10 Ϫ6 to 2ϫ 10 Ϫ3 s Ϫ1 in the temperature range 800 to 1 100°C. Tensile samples with a rectangular cross section of 3ϫ5 mm and a gage length of 20 mm were machined in the direction parallel to the bar axis. In order to minimize the effect of oxidation, tensile tests were performed under a protective argon atmosphere. True tensile strains, stresses and strain rates were calculated from the load-deformation curves with the assumption of uniform deformation and conservation of volume.Hot torsion tests were performed at equivalent strain rates ranging f...

Microstructure and stability conditions for hot deformation of a modified iron-based superalloy

“…On the other hand, the value of the activation energy for deformation, 323 kJ mol −1 , is higher than the value of about 280 kJ mol −1 measured for lattice self-diffusion of Fe [30]. However, these high values are similar to those derived in previous studies for the activation energy for plastic deformation in 18-8 stainless steels and high alloy steels [26,28,31,32]. This difference might be attributable to the impurity and minor alloying constituents present, which may affect diffusivity.…”