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Steam Treatment in Powder Metallurgy: Its Process & Purpose

Header_Before and After Steam Treat Image
Powder metal steam treating, also known as steam blackening, is a heat treatment process that enhances the compressive strength, wear, and appearance of iron-based components. With minimal inputs, this secondary operation is an inexpensive, safe, and widely used solution for powder metallurgy (PM) applications in many industries. 

Understanding metal steam treatment basics and benefits will allow you to leverage the advantages of this economical process.

What Is Steam Treatment in Powder Metallurgy?

Steam treating is a secondary operation. In the powder metallurgy process, secondary operations are any additional operations that take place after sintering with the goal of adding functions or features. A steam treatment operation modifies sintered parts by adding a thin, controlled, tightly adherent oxide layer to the surface. 

The outcome of this process depends on:

  • Time
  • Temperature
  • Atmosphere

A component steam-treat operation can be completed as a batch or continuous process. The process used depends on the furnace. While each furnace has its own key points and cycle processes, the end result is still the same:

  1. Parts are placed in a batch or continuous furnace.
  2. The furnace heats the pieces to 600°F and then purges it with steam at a high rate to remove all air. 
  3. The furnace temperature increases to around 1000°F for a predetermined time.
  4. The parts exit the furnace and cool in room-temperature air, creating a layer of black magnetite (Fe3O4m).

Steam Treat_Parts in basket prior to Steam Treat
(Parts getting prepped for a batch steam-treat process.)

Why Choose Steam as Your Metal Heat Treatment Process?

Heat treatment of ferrous metals offers numerous benefits. Based on your part’s requirements, the manufacturer can adjust time, temperature, and atmosphere to achieve a variety of objectives:

  • Increase hardness: This occurs because the pores have been filled and the oxide coating is > HRC45 on the Rockwell hardness scale. 
  • Enhance compressive strength: This property increases as a result of oxide closing off pores 
  • Enhance corrosion resistance: The new oxide layer protects the part from water, chemicals, and oxygen.
  • Improve wear resistance and lubrication retention: This is due to the greater hardness in the oxide. 
  • Fill pores: This occurs at varying levels, with the oxide forming around the particles.
  • Enhance aesthetics: the blue/black hue provides an attractive surface finish. 

Porosity_MAT Lab_Image of microstructure of part steam treated
(A component’s microstructure after a steam-treat. The steam oxide [gray areas] surrounds the surface and fills some of the pores.)

As you can imagine, the benefits of steam-treating metal are applicable to many markets. Automotive, home appliance, and lawn and garden are among the industries taking advantage of this process. 

Considerations for PM Steam-Treat Operation

When considering a steam-treat secondary operation, talk with your powder metallurgy supplier about:

  • Part requirements – These will dictate steam treatment processing time and parameters
  • Corrosion prevention or appearance needs – Oil dips or rust preventative operations can enhance the blue/black color and increase corrosion resistance even more 
  • Surface finish – The magnetite surface layer can be abrasive if the original surface finish has a level of initial roughness 
  • Application or assembly mates – Consider what you’re mating the steam-treated part  with and ensure it won’t impact application performance

Blending Steam Treatment Into Your PM Design Process

Powder metallurgy steam treating is a simple, low-cost process that offers huge benefits. If you’re looking to increase hardness, compressive strength, corrosion and wear resistance, density, or appearance, this secondary operation is worth discussing with your manufacturing supplier. 

To learn more about the opportunities and considerations of the powder metal design process, visit our blog library:

(This blog post was originally published in 2021 and was recently updated with new insights.)