HIP treatment of LPBF fabricated Haynes 230: Microstructure evolution and mechanical properties

Abstract

To achieve densification and improve the mechanical properties of LPBF-fabricated Haynes 230, hot isostatic pressing (HIP) was performed at 950, 1050, 1150 and 1250 °C under a constant pressure of 150 MPa for 150 min. The effects of HIP temperature on defect healing, grain evolution, carbide precipitation, and mechanical properties were systematically investigated. The results showed that porosity decreased rapidly with increasing HIP temperature. After HIP at 1050 °C and above, the microcracks were completely healed, the porosity decreased to 0.06% or lower, resulting in a nearly fully dense microstructure. Grain evolution exhibited a distinct temperature threshold. At HIP temperatures below 1150 °C, the grain size, morphology, and orientation remained essentially unchanged from the as-printed state. However, at 1250 °C the grains coarsened markedly and became equiaxed. The carbides progressively coarsened with increasing HIP temperature. The intragranular carbide size increased from 0.14 μm at 950 °C to 1.11 μm at 1250 °C, while grain-boundary carbide size increased from 0.56 μm to 2.54 μm. A 1.8 μm-wide intergranular precipitate-free zone appeared at 1150 °C, followed by complete carbide coarsening at 1250 °C. After HIP at 1050 °C, the room-temperature yield strength, tensile strength, and elongation were 666 MPa, 1118 MPa, and 45%, respectively. The high-temperature (at 900 °C) tensile strength reached 247 MPa and elongation attained 44%. The cellular substructures present in the as-built state contributed substantially to its high yield strength. While after HIP, the precipitated carbides provided additional precipitation strengthening.