Exceptional tolerance of austenitic stainless steels to tramp-impurity contamination

The fracture resistance and microstructures of a model material, a highly alloyed austenitic steel 1.4466 (AISI 310MoLN) prepared from re-melted foundry scrap systematically contaminated with varying Sb-concentrations of 250-520 ppm and 0.14 wt % of S were investigated. The deliberately contaminated alloys were also compared with a conventional vacuum-refined austenitic steel. The materials were tested in standard solution annealed condition. The plane strain fracture toughness JIc values of all contaminated steels are surprisingly high, over 390 kJ/m2 (KJIc = 289 MPa m−1/2). Measured provisional fracture toughness JQc, a sample size-dependent analogy to JIc, surpass ∼500 kJ/m2, despite considerable fraction of oxide and sulphide inclusions combined with Sb dissolved in the matrix. Specifically, the JQc for unrefined, S contaminated steel was determined to be 617 ± 54 kJ/m2, whereas the additional contamination by 520 ppm Sb decreased the JQc value only by ∼15 % to 526 ± 43 kJ/m2. The fracture resistance of the vacuum-refined steel is still at least 25–45 % higher compared to unrefined contaminated materials, while the tensile properties remain identical despite considerable contamination. The exceptionally high fracture toughness is enabled by considerable plasticity and strain hardening capacity of the alloy′s matrix that is not altered by the contamination. While the S and O contaminants resulted in the formation of MnS and oxide inclusions, the Sb element was found to be completely dissolved in the austenitic matrix. High fracture resistance and lack of change in tensile properties despite considerable contamination proved high suitability of austenitic steels for recycling-oriented alloy design and utilization in circular manufacturing.