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Development of a new martensitic low carbon ODS stainless steel
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School of Chemical Engineering |
Master's thesis
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en
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53
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Abstract
The elaboration and characterization of a new ferritic/martensitic oxide dispersion strengthened (ODS) steel, 13Cr4Ni ODS, with very low carbon content and nickel as the main austenite stabilizer, were investigated. The objective of this work was to assess the suitability of this alloy for fuel cladding applications in sodium-cooled fast reactors (SFR). The material was processed by mechanical alloying followed by hot isostatic pressing (HIP). Two grades were prepared for comparison: an unreinforced 13-4 grade (NR) and an oxide-dispersion-strengthened grade (ODS). Chemical analyses (ICP and EDS) confirmed the effectiveness of this processing route in limiting carbon contamination, while scanning electron microscopy revealed a fine and homogeneous microstructure. Thermal analyses (DSC and dilatometry) showed an austenitic phase transformation around 650 °C, within the targeted operating temperature range, representing a major drawback for the intended application. Heat treatments were applied in order to reduce hardness, but their effect remained limited (−28 HV). The high hardness (415 HV) was identified as the main factor limiting the workability of the material. Furthermore, EBSD analysis revealed the formation of residual austenite grains representing about 8% of the microstructure, and EDS measurements showed that these austenitic domains were correlated with nickel-enriched regions. Room-temperature tensile tests showed a high yield strength (1315 MPa) but limited elongation at fracture (8.4%), confirming the poor formability of the alloy. At 600 °C and 800 °C, tensile tests demonstrated combinations of yield strength and elongation suggesting good creep resistance. Overall, the alloy exhibited low viability for the intended application. Thermodynamic simulations nevertheless indicated that reducing the nickel content could represent a promising optimization route by lowering the fraction of residual austenite and improving the suitability of the alloy for cladding applications.
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Gasik, MichaelThesis advisor
Parry, GuillaumeSornin, Denis