Irregular Grain Growth Research Articles

Influence of hot-working conditions on grain growth of superalloy 718

The grain growth behavior of superalloy 718 was investigated using high-temperature compression tests and subsequent heat treatment. Abnormal grain growth (AGG) induced by low plastic strains was observed during thermal processing, which included solution heat treatment, and annealing, causing the matrix grains to coarsen from ∼12 μm to 80 μm in diameter. Both the plastic strain and displacement rate during deformation had large influences on the occurrence of AGG, which was mainly observed in regions of low plastic strain; the plastic strain range where AGG occurred increased with decreasing displacement rate. In particular, deformation at low displacement rates (within the same range as dynamic recrystallization) easily produced grain growth during solution heat treatment. Such grain growth was referred to as irregular grain growth (IGG) to distinguish it from AGG. Both the AGG and IGG processes were initiated when the intragranular misorientation exceeded a critical level, causing an increase in the twin boundary ratio and a decrease in intragranular misorientation. The nuclei of AGG grains were the nucleation of newly recrystallized grains without dislocation.

INFLUENCE OF DOUBLE SOLUTION TREATMENT ON HARDNESS IN 17-4 PH STEEL

The investigated material is a corrosion-resistant, Cu precipitation hardened steel 17-4PH,which undergoes a macroscopic contraction, as a result of applying the following heat treatment:double solution treatment at 1028°C for 1 h (condition A), ageing at 540°C for 4 h (conditionH1025). The second solution treatment at 1028°C was found to eliminate the retained austenite,being the evidence of a completely finished martensitic transformation.Indeed, the only phaseidentified in all samples was fcc lath martensite exhibiting a parallel striped structure.Unfortunately, this additional heat-treatment operation leads likewise to significant and irregulargrain growth, which consequently causes a drop in material hardness. Moreover, the secondsolution annealing, caused a shift in the XRD peaks to higher 2θ angles, resulting from a latticeparameter decrease by0,25%. The two subsequent heat-treatment procedures bring the latticeparameter back to its initial value. This seemingly reversible process of decrease and increase of thelattice parameter was observed for samples subjected to all the heat treatment operations, stronglysuggesting the existence of a relation between the microstructural changes and the macroscopiccontraction of the steel material. In addition to the martensitic phase, in the unaged samples, a δ-ferrite phase could be identified by TEM and electron diffraction, which is favorable for ductilityand toughness of the material. In all samples, non-coherent fcc-NbC precipitates identified byelectron diffraction and EDX mapping having sizes up to 70 nm were found.