Inflience of Production Methods on Creep Deformation of 329LA Lean Duplex Stainless Steel in Continuous Annealing Condition

Abstract

Duplex stainless steels (DSSs) are stainless steels that are designed to have two phases, austenite and ferrite, by balancing the proper composition. Lean DSSs are DSSs which contain less alloying elements, such as Ni and/or Mo, than conventional DSSs in order to reduce the material’s price. Mn and N, instead, are used to balance the phase fraction and increase corrosion resistance. Two processing methods, continuous and strip casting are used to produce slabs from hot metal. During continuous annealing of the strip after cold rolling, creep deformation occurs even though the line tension is very small. It is well known that DSSs will normally undergo superplastic deformation at the annealing range temperature. However, with different processing methods, this 329LA DSS has different creep characteristics, especially creep rate. In order to investigate, this creep tests are conducted to replicate the continuous annealing condition of 329LA DSSs. From the present results, continuous casted specimens show a higher strain rate compare to strip casted specimens when tested with the same thickness and stress conditions at all temperatures. The strain rate sensitivity values (m) are higher than 0.4 suggesting that these materials undergo superplastic deformation at all conditions while the stress exponents (n) are around 2 corresponding to the grain boundary sliding creep mechanism which is normally the deformation mechanism for most superplastic materials. Optical micrographs show a huge difference in morphology between continuous and strip casted specimens. While it is homogeneous in continuous casted specimens, strip casted specimens have very inhomogeneous structures which lead to larger average grain sizes. From EBSD results show that after the test cold rolled textures are destroyed and become random. The role of the recrystallization in austenite and recovery process in ferrite are to set up a condition for phase boundary sliding making the γ/δ high angle phase boundary which facilitates sliding. Thus, the main mechanism is phase boundary sliding not grain boundary itself. Heat treated specimens were tested to indentify the characteristic of large grain areas in strip casted specimens. The result shows that even though the strain rate is much slower, there is no difference in deformation mechanism and superplastic behavior of the specimens. This means the only possible role for large grain areas is making the average grain size larger which results in a decreasing strain rate. Evolution of microstructures shows grain growth during creep test of continuous cast specimens over time while, grain size and morphology of strip cast specimens become more homogeneous. It also shows that the significant different between continuous cast and strip cast specimens during creep deformation is morphology. Larger initial grain sizes in strip cast specimens results in more γ/γ and δ/δ boundaries after heating up the specimens. Lastly, TEM micrographs show the evidence of dislocation movements at phase boundary which confirm the hypothesis of phase boundary sliding. In conclusion, both continuous and strip casted specimens undergo superplastic deformation under continuous annealing condition. The same deformation mechanism, phase boundary sliding, is the responsible mechanism in both and the role of the recrystallization and recovery process is to set up the condition for phase boundary sliding. The only difference is that the strip casting process produces larger grain slab compared to continuous casting which results in a decreasing strain rate.