ASTM B935-20(2024) is a guide for establishing and maintaining a steam-treatment process for sintered ferrous powder metallurgy materials. The process is also commonly called steam blackening.
Steam treatment deliberately forms an oxide layer on ferrous PM material. ASTM notes that the process can improve resistance to further oxidation and corrosion, provide a decorative blue-gray to blue-black appearance, improve wear behavior and apparent hardness, seal porous parts, and provide a base for additional coatings.
What Is ASTM B935-20(2024)?
The official title is Standard Guide for Steam Treatment of Ferrous Powder Metallurgy (PM) Materials.
ASTM lists B935-20(2024) as the current active edition. The 2020 version was reapproved in 2024 and is identified by ASTM as B0935-20R24. It is published in ASTM Book of Standards Volume 02.05 and developed by Subcommittee B09.05 under Committee B09 on Metal Powders and Metal Powder Products.
ASTM B935 Is a Guide, Not a Test Method
ASTM B935 should not be confused with a test method that produces a single numerical property result. It is a process guide intended to help users establish and maintain an appropriate steam-treatment procedure and evaluate the resulting material.
The guide therefore focuses on process control, material condition and the intended effects of steam treatment rather than defining one universal pass/fail value for every PM component.
Scope of ASTM B935-20(2024)
The guide applies to steam treatment of sintered ferrous powder metallurgy materials. Its stated purpose is to support the establishment and maintenance of a steam-treatment procedure and the appropriate use and evaluation of steam-treated materials.
ASTM also provides additional background information in an appendix concerning the effect of the process on ferrous PM material properties.
What Is Steam Treatment of Powder Metallurgy Parts?
Steam treatment is a controlled oxidation process applied to sintered iron-based PM components. Under suitable processing conditions, the treatment forms magnetite, Fe3O4, on exposed iron surfaces and within accessible surface porosity.
The resulting oxide can create a dark blue-gray to blue-black appearance and modify several surface-related properties of the PM material.
Because sintered PM materials contain interconnected or partially connected pores, steam treatment is particularly relevant to components where oxide formation within near-surface porosity can contribute to sealing and wear behavior.
Why Is Steam Treatment Used?
ASTM identifies several reasons for applying steam treatment to ferrous PM materials.
Decorative Appearance
Steam treatment can produce a blue-gray to blue-black surface. This makes the process useful where a uniform dark appearance is desirable in addition to functional performance.
Improved Resistance to Further Oxidation and Corrosion
ASTM states that steam treatment can reduce the susceptibility of ferrous PM materials to further oxidation and corrosion, improving shelf life.
The degree of protection depends on the material, surface condition, process quality and service environment. Steam treatment should therefore not automatically be treated as equivalent to a heavy-duty corrosion-protection coating for every application.
Increased Apparent Surface Hardness
ASTM notes that the magnetite formed during steam treatment has a hardness typically equivalent to approximately 50 HRC. When this oxide is present in a sintered structure, apparent hardness and wear behavior can improve.
Improved Wear Characteristics
The hard oxide phase formed during treatment can improve wear resistance in suitable PM components. This is one of the more important functional reasons for steam treating parts exposed to rubbing or sliding contact.
Changes in Mechanical Properties
ASTM states that improvements in compressive strength and some other mechanical properties can be observed after steam treatment. The effect is material- and process-dependent and should be evaluated for the specific PM composition and component design.
Sealing of Porous PM Parts
Because powder metallurgy parts contain pores, the oxide formed during steam treatment can help seal surface-connected porosity. This can be useful when leakage resistance or reduced permeability is required.
Base for Additional Coatings
Steam treatment can also be used to provide a surface condition suitable as a base for certain subsequent coatings or finishing operations.
Surface Cleanliness Is Critical
ASTM emphasizes that the performance and quality of steam-treated materials depend strongly on the cleanliness of the surface before treatment and on the adequacy of the processing itself.
This is particularly important for PM parts because residual oil, machining fluid, dirt or other contaminants can interfere with uniform oxide formation.
Effective process control should therefore consider:
- Residual lubricants
- Machining oils
- Handling contamination
- Cleaning chemistry
- Rinsing
- Drying
- Time between cleaning and treatment
An inconsistent cleaning process can produce inconsistent appearance and performance even when the steam-treatment equipment itself is functioning correctly.
General Steam-Treatment Process
The exact operating procedure should be taken from the official ASTM guide and the manufacturer's validated production process. At a general level, steam treatment involves:
- Ensuring that the sintered ferrous PM parts are appropriately cleaned.
- Loading the parts into suitable treatment equipment.
- Establishing controlled thermal and atmospheric conditions.
- Introducing steam under conditions that promote formation of magnetite rather than undesirable oxide products.
- Maintaining the process long enough to achieve the required surface and pore treatment.
- Cooling and handling the parts under controlled conditions.
- Evaluating the treated parts against the applicable product requirements.
Process parameters should be controlled because oxide formation depends on temperature, atmosphere, steam quality, treatment time, part loading and the characteristics of the PM material.
Why Magnetite Matters
The desired oxide associated with steam treatment is magnetite, Fe3O4. It is a hard iron oxide that gives the treated surface its characteristic dark appearance.
The goal is not simply to “rust” the component. Controlled steam treatment is intended to form a useful oxide under controlled processing conditions. Uncontrolled oxidation can create nonuniform or unsuitable corrosion products and should not be confused with the ASTM-guided process.
Why the Process Is Particularly Relevant to PM Materials
Powder metallurgy parts are produced by compacting metal powder and sintering the compact. Depending on density and process route, the resulting material can retain a network of pores.
Those pores influence properties such as:
- Density
- Permeability
- Lubricant retention
- Strength
- Surface behavior
- Corrosion response
Steam treatment can modify the near-surface pore structure through oxide formation, which is why the process has a different functional role in PM components than a simple cosmetic blackening operation on fully dense steel.
Material Properties That May Change
ASTM identifies several material characteristics that can be affected by steam treatment.
| Property | Potential Effect of Steam Treatment |
|---|---|
| Appearance | Blue-gray to blue-black finish |
| Resistance to further oxidation | Can improve |
| Shelf-life corrosion resistance | Can improve |
| Apparent hardness | Can increase |
| Wear characteristics | Can improve |
| Compressive strength | Can improve |
| Porosity sealing | Can improve |
| Suitability as coating base | Can be enhanced for selected subsequent coatings |
These are potential effects rather than universal guarantees. Actual results depend on alloy composition, density, pore structure, prior processing, surface condition and steam-treatment parameters.
Quality Control for Steam-Treated PM Parts
A production steam-treatment process should be controlled using acceptance criteria appropriate to the component and its intended function.
Depending on the product, quality-control checks may include:
- Visual appearance
- Surface condition
- Dimensional verification
- Hardness-related checks
- Leakage or sealing evaluation
- Wear testing
- Corrosion evaluation
- Functional testing
ASTM B935 itself is a guide and does not create one universal acceptance requirement for every steam-treated PM part. The applicable drawing, product specification, customer requirement or internal process specification should define the required acceptance criteria.
Why Process Consistency Matters
Steam treatment is sensitive to both incoming-part condition and processing conditions. Variation can occur if:
- Parts contain different levels of residual contamination
- Loading density changes
- Steam distribution is uneven
- Heating conditions vary
- Treatment time is inconsistent
- Part composition or density changes
- Cooling conditions vary
Manufacturers should therefore treat steam blackening as a controlled manufacturing process rather than only a cosmetic finishing step.
Common Applications
ASTM notes that steam-treated ferrous PM materials are used in several industries, including:
- Automotive
- Marine
- Home appliances
- Lawn and garden equipment
Typical PM components can include gears, structural parts, wear components and other sintered ferrous parts where the surface, wear or sealing benefits of steam treatment are useful.
Steam Treatment vs. Conventional Coating
Steam treatment differs from applying a separate metallic, polymeric or paint coating. The process chemically converts exposed ferrous material into an oxide layer.
This distinction has several practical consequences:
- The treated layer is formed from the component's own iron content.
- The oxide can extend into accessible surface-connected porosity.
- The process can modify wear and apparent hardness rather than only appearance.
- Its corrosion performance should not be assumed to equal that of thicker barrier coatings.
The appropriate surface treatment should be chosen according to the actual service environment and required properties.
Steam Treatment vs. Heat Treatment
Steam treatment uses elevated temperature, but its primary purpose is controlled oxidation rather than conventional bulk heat treatment such as hardening, carburizing or tempering.
The process creates magnetite on and within the near-surface structure. Conventional heat treatments primarily modify the metallic microstructure or composition to achieve mechanical-property changes.
Design and Application Considerations
Before specifying steam treatment, engineers should consider:
- Base PM material
- Part density
- Pore structure
- Dimensional tolerances
- Wear conditions
- Corrosion environment
- Leakage requirements
- Subsequent coating or assembly operations
- Appearance requirements
Steam treatment should be validated for the actual component rather than assumed to produce identical effects across all ferrous PM materials.
Units in ASTM B935-20(2024)
ASTM states that inch-pound units are the standard units in the guide, with one longstanding powder-metallurgy convention: density is expressed in grams per cubic centimetre and the mass used to determine density in grams.
SI values shown in parentheses are mathematical conversions provided for information.
Safety, Health and Environmental Considerations
ASTM B935 does not claim to address every safety concern associated with steam treatment. The user is responsible for establishing appropriate safety, health and environmental practices and determining the applicability of regulatory requirements.
Industrial steam-treatment equipment involves elevated temperatures and steam, so appropriate equipment guarding, pressure control, ventilation, operating procedures and maintenance practices are essential.
ASTM B935-20(2024) Edition Status
ASTM B935-20(2024) is currently listed by ASTM International as the active edition. ASTM shows the document as last updated on October 29, 2024.
The version history listed by ASTM includes B935-20, B935-16, B935-05(2009)e1 and B935-05.
Organizations should verify the revision required by a purchase order, drawing, customer specification or internal quality procedure before processing parts for formal compliance.
Frequently Asked Questions
What is ASTM B935-20(2024)?
It is the active ASTM guide for steam treatment, or steam blackening, of sintered ferrous powder metallurgy materials.
Is ASTM B935 a test method?
No. It is a guide intended to help establish and maintain a steam-treatment procedure and support appropriate use and evaluation of treated PM materials.
What oxide is formed during steam treatment?
The controlled process forms magnetite, Fe3O4, on ferrous PM material.
Does steam treatment increase hardness?
ASTM states that improvements in apparent hardness can be observed. It notes that the magnetite formed during steam treatment typically has a hardness equivalent to about 50 HRC.
Does steam treatment improve corrosion resistance?
ASTM states that it can reduce susceptibility to further oxidation and corrosion and improve shelf life. The actual degree of protection depends on the material, process and environment.
Can steam treatment seal porous PM parts?
Yes. ASTM identifies sealing of PM parts as one use of the process because oxide formation can reduce accessible porosity.
Why is cleaning important before steam treatment?
ASTM emphasizes that performance and quality depend on surface cleanliness before treatment. Oil and other contamination can interfere with consistent oxide formation.
Is ASTM B935-20(2024) the current edition?
Yes. ASTM currently lists B935-20(2024) as the active edition.
Summary
ASTM B935-20(2024) is the active ASTM guide for steam treatment of sintered ferrous powder metallurgy materials. The process, also known as steam blackening, forms magnetite on and within the near-surface structure of the PM component.
Steam treatment can provide a dark decorative appearance, reduce susceptibility to further oxidation, improve apparent hardness and wear characteristics, increase compressive strength in suitable materials, seal porous parts and provide a base for additional coatings.
ASTM emphasizes that the quality of the result depends strongly on surface cleanliness and adequate process control. Because the guide does not establish one universal acceptance value for every component, manufacturers should combine ASTM B935 with the applicable part drawing, customer specification, process qualification and functional requirements.