Steam Oxidation of Ferrous PM Materials

Current industry practice for steam oxidation consists of exposing ferrous material to a controlled superheated steam environment at 800-1100 °F (430-590 °C) for 1 to 4 hours. During the exposure the steam reacts with the ferrous material to produce an adherent blue-grey iron oxide (magnetite, Fe3O4):

3Fe + 4H2O (gas) → Fe3O4 + 4H2 (gas)

ASTM B935 is a standard guide for the Steam Treatment of Ferrous Powder Metallurgy (PM) Materials and contains additional information about the process.

Both batch furnaces and continuous belt furnaces are used for the steam treatment of ferrous PM materials. While comparable performance may be obtained from batch and continuous furnaces, the process conditions used may not be exactly the same. Since hydrogen is a by-product of the steam oxidation reaction, suitable safety measures must be taken to eliminate unwanted build-up of hydrogen gas and the possibility of an explosion.

The depth of penetration of the oxide into the part, product mass gain, oxide thickness and hardness of the finished products depends on the steam oxidation process parameters (time and temperature), the density of the material, the type of base powder used to create the part (atomized or sponge) and previous operations that could close-off the surface porosity.

Steam oxidation is applied to ferrous PM materials in order to:

A study was performed to characterize and evaluate the mechanical and physical properties of steam treated ferrous PM materials. Two steam processes were used, identified as Process A and Process B. The Process A cycle was performed for approximately 1 hour in the temperature range of 925-1000 °F (495-540 °C) and the Process B cycle for approximately 2 hours in the temperature range of 1000-1050 °F (540-565 °C). The results of these studies, based on the average of five (5) test samples are summarized in the following tables: