The steam treatment of powder metallurgy parts help to both seal surface porosity and to increase surface hardness, which improves corrosion resistance, pressure tightness, and wear resistance. Unfortunately, it also reduces tensile strength and ductility from 10 to 20%, depending on conditions and the steel.
Steam treatment usually consists of heating PM parts in a steam atmosphere at a temperature between 510 and 570 C (950 and 1060 F) to form a layer of iron oxide (most often magnetite, or Fe3O4). The oxide forms inside the surface porosity as well, pinching off the opening due to the volume expansion associated with the conversion of the iron. In addition to sealing the surface, steam treating increases surface hardness, as magnetite has a hardness of HRC 50.
For more information, click on the link below (subscription required). Then scroll to Figure 25.
Joseph W. Newkirk, Heat Treatment of Powder Metallurgy Steels, Heat Treating of Irons and Steels, Vol 4D, ASM Handbook, Edited By Jon L. Dossett, George E. Totten, ASM International, 2014, p 253-273, https://doi.org/10.31399/asm.hb.v04d.9781627081689
