Stainless Steel Sintering: Intro, Uses, & Benefits

Want to pack a 1-2-3 punch with stainless steel powder metal component design?

The process for producing stainless steel powder metal parts involves compacting loose metal powder into a desired part geometry under high pressure, and then heating the part enough to metallurgically bond the individual metal particles together without melting the part. Stainless Steel powder metal parts manufacturing is a highly specialized, but vital technology for many applications due to its greater precision, lower waste, and reduced energy consumption compared to competing manufacturing technologies.

This article will delve deeper into the benefits of stainless steel sintering, as well as material properties and sintering techniques. Also, you’ll learn best practices for achieving optimal strength, density, and accuracy in the process. 

Sintering Stainless Steel: Step-by-Step Process

Sintering is a heating process that fuses particles of stainless steel – or any viable metal – into a solid structure using heat. The sintering furnace heats the material to just below its melting point. 

Sintering involves a series of carefully controlled steps. Each step is a vital part of the metal’s final strength and quality: 

1. Powder preparation: The metal powders are processed, blended, analyzed, and certified for the desired characteristics & properties. Required additives, like compaction lubricant, and optional additives, like machinability enhancers, are blended with the metal powder at this stage.
   
   
2. Compaction: The loose metal powder is fed into a die and compacted under high pressure into the desired shape.  The dry compaction lubricant contained in the blended powder mix reduces the friction between the metal particles and tool members as the powder is consolidated and then ejected from the die.  The resulting part is called a  “green compact.” “Green” refers to the not-yet-metallurgically-bonded state of the metal particles in the compacted part. The green compact has very little strength at this point, so it is critical that any material handling accounts for the delicate nature of the parts coming out of the compaction press.
   
   
3. De-lubing: The green compact is heated to remove compaction lubricant from the pores of the green compact.  
   
   
4. Sintering: The de-lubed green compact is heated in a furnace to a temperature between 2100°F to 2500°F, allowing the metal particles to bond together. The temperature and atmosphere in the furnace are closely monitored and controlled to prevent oxidation and achieve the desired density and microstructure. Properly sintered parts will have the desired material properties, including density, hardness, strength, impact resistance, elongation, and corrosion resistance.
   
   
5. Post-sintering operations: After being cooled, sintered stainless steel parts may go through further processes to improve mechanical strength or improve dimensional capability. Examples include densification, which reduces the porosity and increases the material’s strength & hardness, or machining and grinding.
   
   

Benefits of Sintered Stainless Steel

Sintered stainless steel is superior in mechanical properties, making it a great choice for harsh industrial environments. Key benefits include:

• Strength and durability: Particles of the net-formed part are bonded at the molecular level. 
  
  
• Corrosion and heat resistance: Stainless steel is a cost-effective, corrosion & heat-resistant material due to its chemical composition. 

• Repeatability: Powder metal technology is ideal for high-volume manufacturing, offering consistent results and meeting design specifications.  
  
  
• Sustainability:  The powder metal process reduces material waste, making production more efficient and producing fewer CO2 emissions. 
  
  
• Cost-effectiveness: Powder metal technology offers material efficiency, little waste, and streamlined production, allowing parts to be produced in one efficient process.
   

Sintering Stainless Steel: Common Challenges and Solutions 

While stainless steel sintering does offer many advantages, it also comes with a few challenges. Here are some of the most common challenges found in the sintering process, as well as possible solutions. 

• Chromium challenges
• Engineered shrinkage 
  
  

Challenge 1: Chromium Challenges

Chromium is a key element to stainless steel’s corrosion resistance due to its inherent passive oxide protective layer, but chromium also presents significant challenges for the powder metal manufacturing process. The particles have higher strength and hardness, making the powder more difficult to compress in the compaction process. Additionally, the passive oxide layer inhibits the bonding between particles during sintering.

Solution: Compaction of stainless steel powder metal requires roughly 20% higher pressure than low alloy steels just to achieve moderate green density.  The compaction tools need to be very robust, and the compaction process carefully monitored. The sintering requires a different atmosphere and a higher temperature than low alloy steels to eliminate oxide layers between particles, promote strong particle bonding, and improve the density.

Challenge 2: Engineered Shrinkage 

As mentioned above, stainless steel and powder metal components rely on high-temperature sintering at 2350°F to increase the density by consolidating the volume, or “shrinking” the parts roughly 10% from their green compact size. The material shrinkage is highly beneficial to the material properties, but presents significant dimensional control challenges. 

Solution: The shrinkage needs to be factored into the tool design, and the green density variation within the part needs to be tightly controlled during the compaction process to ensure proportional shrinkage throughout the cross-section of the part. Furnace temperature and loading need to be closely monitored and controlled to ensure proportional shrinkage of each part and minimal part-to-part and lot-to-lot variation. 

Stainless Steel Powder Metal Components: Technical Considerations

Stainless steel powder metal parts are found in the automotive, aerospace, medical, electronics, and consumer goods industries. If you’re considering stainless steel powder metal parts for your products, there are a few key factors to keep in mind:

• Material Properties: There are different grades of 300 series and 400 series stainless steels that offer varying mechanical properties and corrosion resistance.  The requirements of the application should be considered when determining the material grade that best suits your needs and budget. 
  
  
• Process parameters: The unique challenges of stainless steel powder metal require a deep understanding of the compaction and sintering requirements, and a disciplined approach to monitoring and controlling the parameters.
  
  
• Design and application requirements: Early collaboration between design and manufacturing teams helps ensure the part can be produced cost-efficiently and meets performance expectations. 
  
  

Learn More about Sintered Stainless Steel

Stainless steel sintering delivers precise, durable, and cost-effective products while supporting environmental sustainability. Each part of the sintering process plays a vital role in achieving the optimal results. Because material characteristics can shift throughout each step, maintaining a clear understanding of these variables, while keeping the end goal in mind, is important. 

Learn more about the Sintered Metal Division at Nichols Portland Inc. to see how our innovations and commitment to quality have helped shape the metal parts industry.