Corrosion and stress corrosion cracking resistances of the 17-4 precipitation hardened martensitic stainless steel additively manufactured using binder jet printing

The corrosion and stress corrosion cracking (SCC) properties of an additively manufactured (AM) 17-4 PH martensitic stainless steel that was produced using the binder jet printing (BJP) technique are compared with those of the conventionally manufactured (CM) steel, to critically examine the influence of porosity and subtle microstructural variations, introduced due to sintering and hot isostatic pressing (HIP), on them. Microstructures of the BJP and HIP specimens contain porosity, delta-ferrite, and MnS inclusions, while the CM ones do not contain either, but have NbC inclusions. All the corrosion properties of the BJP alloy, investigated using the immersion, cyclic potentiodynamic polarization, and galvanostatic (GL) tests, are inferior (compared to those of the CM alloy) due to the porosity in it, while the NbC inclusions present in the CM alloy adversely affected its corrosion characteristics. Surprisingly, the CM 17-4 PH specimens subjected to the SCC tests failed, while those of the BJP specimens did not (despite the relatively large porosity in it). Detailed analyses (including the X-ray computed tomography) show that crack bridging induced by the delta-ferrite phase, which has superior corrosion resistance compared to the martensite, is the reason for the BJP alloy's SCC resistance. HIP, which reduces both the porosity and delta-ferrite content, resulted in enhanced corrosion and SCC resistances that are even superior to those of the CM 17-4 PH. These results demonstrate, emphatically, that the subtle microstructural variations in the AM alloys can have profound effects on their properties, especially those related to structural integrity and reliability.