Hello everyone,
My project is to investigate the microstructure of stainless steel 17-4 PH fabricated by selective laser melting. SS 17-4 PH is predominantly a martensite matrix with some delta-ferrite stringers. Parts made by selective laser melting experience complex thermal histories and rapid cooling rates which produce inconsistent microstructures. I am expecting to see a mixture of martensite and retained austenite based on other studies. However, some studies even noted a fully delta-ferrite microstructure so that is also a possibility.
I have been given optical images of 17-4 PH made by SLM to analyse, without a scale bar. I have attached a few of the images. I think it is a martensitic matrix (lath-like) and retained austenite (larger segments). I don't seem to see stringers of delta-ferrite. The black circles are pores from gas entrapment, the gas comes from powder gas atomisation process and building environment. Note: I think the shaded region at the top of image two is an error in metallography sample preparation process.
Could you please help me characterise the microstructure?
Can you please send the references to studies which noted fully delta ferrite microstructures?
Grzegorz Cios - Studies which found fully delta ferrite microstructure
- "Nitrogen Effects in Additively Manufactured Martensitic Stainless Steels: Conventional Thermal Processing and Comparison with Wrought".
Article Nitrogen Effects in Additively Manufactured Martensitic Stai...
- when an argon gas atomised powder feedstock was used with a nitrogen building environment. - "Evidence of Austenite By-Passing in a Stainless Steel Obtained from Laser Melting Additive Manufacturing"
Article Evidence of austenite by-passing in a stainless steel obtain...
- argon building environment
From my research, the gas atomisation and building environment affects the resulting microstructure of SS 17-4 PH. Most other studies demonstrate that argon gas atomised powder or argon building environment promotes a martensite microstructure with lesser amounts of retained austenite. Nitrogen is the other option which is a known austenite stabilising element, hence, it promotes a larger volume fraction of retained austenite in the martensitic microstructure.
Thanks.
The microstructure you are describing is fitting the images quite well on the basis of the images provided. These larger segments I would try to confirm using other technique to say that it is austenite for sure. The shaded region you have mentioned is no error. It is just resin in which the sample was mounted prior grinding and polishing.
Grzegorz Cios Thank you for confirming that with me I really appreciate it! Unfortunately, my project only allows me to qualitatively analyse the optical images. Would you be able to advise how I can describe the retained austenite in the samples? I know martensite is either lath or plate like in appearance, however I am unsure the correct terms to describe retained austenite.
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