Nabarro Herring Creep Creep Resistant Materials Materials in which - PowerPoint PPT Presentation

Nabarro – Herring Creep

Creep Resistant Materials Materials in which dislocation movement is controlled Presence of interstitials elements such as C Presence of substitution elements such as Mo, Ti, V, W, Nb and Ta Strong Precipitates Grain Boundary strengtheners B, Zr

Creep Strength as a function of temperature for various steels Temperature o C

Creep Strength of some Low alloy steels vs 304 SS

1000h Creep rupture strength of various superalloys as a function of Temperature Temperature o C .

Positive Effect of Creep Cutting down dislocation multiplication – especially when an oxide is formed on surface which fixes one edge- resulting in dislocation multiplication. If a very stable oxide is formed it does not allow faster oxide growth and hence weight loss and thus result in containment of member diameter. Formation of Internal oxides – if formed in grains results in higher strength due to ppt. hardening and better creep resistance. Certain specific products such as borides, zirconides if formed at grain boundaries can restrict grain boundary sliding

Negative effects of Creep Unprotected oxides formed due to oxidation at high temperatures may spall, leading to metal loss and thus increase in creep rate. Kirkendall voids formed can lead to faster achievement of Tertiary creep thus reducing rupture life. Unwanted corrosion products at grain boundaries such as sulphides, carbides can result in grain boundary failure leading to early failure. Dissolution of gases can lead to formation of ordered ppts. Leading to localized brittleness and failure

Steady State Creep

Creep and Corrosion

Modified Creep Equation