Delta ferrite to the extent of upto 3% in turbine blade steel grades like X222CrMoV121, K9A62C and some other grades.When finish machined turbine blades are checked for surface defects by LPT (liquid penetrent test) , no surface cracks are found but when magnetic particle inspection is done using head shot method some linear indications are observed in some bladeswhich on microstructural examination and phase micro hardness confirm to be delta ferrite .This problem needs some technological insight to solve the problem.
In an engineering component it is always difficult to alter the microstructures, as you would need to heat-treat the component. If there are stresses within the part upon any subsequent heat treatment, these would be relieved, resulting in components distortion.
So you would need to heat treat your component within a mould. Careful heat treatment is easy to be designed and selected so that you avoid delta ferrite formation. For example if you cool it fast enough you should get a fully martensitic microstructure. You could always get back to the supplier and ask him to modify the post heat treatments to avoid delta ferrite formation. Any subsequent heat treatment is highly likely to distort the part.
Thanks a lot dear Panos Efthymiadis for your valuable answer, as a matter of fact I am looking for reducing amount of delta ferrite in a martensitic stainless steel forged/rolled bars which are raw material for turbine blades making by machining.Some of measures to reduce/eliminate delta ferrite in such case known to me are;
1. It is mainly depending on chemical composition, and thus by balancing chemistry using Scheffler /delong diagrams ( lowest possible ferrite stablizers and highest possible sum of austenite stablizers)
2. It precipitates in segregation areas ,so can be reduced by redistributing the segregating solute atoms by high temperature homogenization of conventional/ESR ingots before forging.
3. By multidirectional forging like upsetting ,drawing and cross forging to break the precipitated delta ferrite and distributing it over a large area.
Regards
Shivraj Singh Kasana
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