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The optimisation of hot isostatic pressing treatments for enhanced mechanical and corrosion performance of stainless steel 316L produced by laser powder bed fusion
Additive Manufacturing, Volume: 58, Start page: 103072
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This work compares the mechanical and corrosion properties of 316L steel manufactured by Laser Powder Bed Fusion (LPBF) and post treated by Hot Isostatic Pressing (HIP) to wrought 316L. HIP is often used by default on LPBF components to reduce porosity and obtain the best mechanical properties, howe...
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This work compares the mechanical and corrosion properties of 316L steel manufactured by Laser Powder Bed Fusion (LPBF) and post treated by Hot Isostatic Pressing (HIP) to wrought 316L. HIP is often used by default on LPBF components to reduce porosity and obtain the best mechanical properties, however, if the HIP temperatures are too high, there is a risk of reducing mechanical strength and corrosion resistance. The purpose of this work was to investigate the HIP parameters and understand the trade-off in properties. By choosing various HIP temperatures (700 °C, 1125 °C, 1200 °C), pressures (100 MPa, 137 MPa and 200 MPa) and hold times, optimal cycles were investigated based on the most favourable mechanical properties (density, hardness, tensile and low-cycle fatigue), and pitting corrosion resistance.Microstructural features associated with LPBF such as melt pools, melt pool boundaries and sub granular cells were observed. These features were found to disappear with longer and higher temperature treatments, accompanied by increased grain sizes. Low and mid temperature point HIP treatments resulted in higher ultimate tensile strength but lower fracture elongation. The decreasing hardness and tensile strength trends were consistent with decreased grain boundary strengthening and decreased dislocation strengthening (with disappearing sub grain boundary and granular cells). Only one HIP condition, consisting of a low temperature and medium pressure, produced samples that achieved runout under low cycle fatigue testing for both the lower and higher stresses. Despite this, most higher temperature HIP cycles reduced the fatigue resistance. This was again attributed to the coarsening of the microstructure at the higher temperature treatments.The spread of the pitting potentials of HIP treated samples was reduced by 52.46 % compared to the as-built material, although none were better overall compared to the wrought material. Of all the properties, porosity appears to play the most influential role on pitting corrosion, and to this extent, despite having a larger variation in results, some of the treated parts demonstrated improved pitting resistance and some demonstrated improved repassivation potentials compared to wrought 316L.
Laser powder bed fusion, Stainless steel 316L, Hot isostatic pressing, Mechanical properties, Corrosion
Faculty of Science and Engineering
This work was funded by the Defence Science and Technology Lab (DSTL) Project code: DSTLX1000128518.