Powder metallurgy is a fascinating science in which customizable powder blends are used to develop unique material properties. It is particularly appealing to manufacturing due to its economics. As such, this technology continues to experience strong growth in an ever-expanding family of component parts. Both sintering and the heat treatment of sintered components require careful understanding and control of the furnace atmosphere because its role must change from one point in the process to another. Let’s learn more.

 

Sintering

In simplest terms, sintering (aka solid-state sintering) is the diffusion bonding of adjacent powder particle surfaces. It can be argued that sintering is not a heat-treatment operation per se but a thermal process applied to so-called “green” compacts in order to impart structural integrity and improve mechanical properties, the foremost of which is strength. Sintering causes the following changes to occur within the part:

  • Particle bonding (resulting in a decrease of pore volume and an increase in density)
  • Grain growth and an increase in the number and strength of interparticle bonds
  • Pore morphology (size, shape)
  • Alloying and homogenization
  • Dimensional (reduced surface area)
  • Reduction of lattice defects

The sintering process is governed by a number of material and process variables that produce a change in the part microstructure, thereby influencing its mechanical properties. These include:

  • Temperature and time
  • Powder particle morphology (i.e., size, shape and distribution of powder particles)
  • Composition of the powder
  • Density
  • Sintering parameters (time-temperature-atmosphere)

 

Sintering Atmospheres

The selection of a sintering atmosphere is most often dictated by the choice of material, desired properties, part design (e.g., density, mass, geometry), production demands and product end-use service application. Other factors – such as furnace design, sintering parameters (e.g., time, temperature, lubricant, loading) and part dimensional tolerances – also play an important role. The actual choice of atmosphere (vacuum, pure gases, mixtures, blended gases, generated gases) further depends on ease of control, cost and possibly facilities-related issues. The choice of a sintering atmosphere must always be taken into consideration due to its influence on final properties.

The purpose of a sintering atmosphere is multifaceted – to aid in lubricant removal from the green compact, reduce residual surface oxides (in order to promote bonding between adjacent powder particles) and protect the compacts from oxidation during the sintering process. For iron-based alloys, the furnace atmosphere may also be called upon to prevent decarburization through hydrocarbon-gas enrichment.

The delubrication operation (aka delube, burn-off, debinding) required by most PM parts can be handled either as an independent functionin a separate furnace or as an integralpart of a continuous sintering furnace. There are several factors toconsider with each approach.

Many stainless steel and some metal injection molded (MIM) components (depending on the feedstock) are delubed or debound in astand-alone batch furnace due to both the nature of the lubricants used, the ability to control the time/temperature/atmosphere profile and to avoid furnace contamination (especially in vacuum and pusher furnaces). Cost of operation of an independent furnace and part handling after delubricationbecome important issues in many cases.

In a continuous furnace with an integral preheat/delube section, muffle designs predominate in order to contain the effluent and provide precise time/temperature/atmosphere control. The atmosphere inside the delubrication chamber must be oxidizing, which is achieved by sending a portion of the gas through water to become highly saturated, or by use of air additions. Meanwhile, the atmosphere inthe high-heat section of the furnace must be highly reducing (Fig. 1).

Designing equipment such that gas enters in the proper location and/or is distributed in selected locations within the chamber (via inlet tubes with holesoriented at variable angles opposite the direction of travel) is critical. Exothermic gas and/or nitrogen are typical delube atmospheres. With nitrogen, the use of air additions or the saturation of the atmosphere (by passingit through heated water) aids greatly in lubricant removal.

Atmosphere requirements in a sintering furnace vary considerably depending on the type of furnace (e.g., mesh-belt conveyor, pusher, walking-beam), style of furnace (batch or continuous) and if delubrication (aka delube, dewax) is required to be performed.

The basic atmosphere requirements for a continuous mesh-belt furnace (Fig. 2) are as follows:

  • In the delubrication zone, where temperatures are typically in the range of 250-700°C (480-1300°F), a high dew-point atmosphere in the range +4.5°C to +20°C (+40°F to +70°F) is generated, often by mixing dry and wet gas or by air additions both intended to aid in lubricant removal.
  • In the sintering zone, where temperatures vary by the material being sintered, a low dew-point atmosphere in the range of -29°C to -40°C (-20°F to -40°F) aids in oxide reduction to promote bonding of the powder-metal particles together.
  • In the post-cool (aka carbon restoration) zone where temperatures are often in the 800-900°C (1475-1650°F) range, or in some instances at the end of the sintering zone, (optional) carbon control of certain materials prevents surface decarburization.
  • In the cooling zone, sufficient gas flow is needed to prevent oxidation. O2 levels in commercial practice often run in the 10-50 ppm range (maximum). The goal is to attain the lowest practical level of oxygen.

The most common sintering atmospheres are mixtures of nitrogen/hydrogen or dissociated ammonia diluted by nitrogen additions. Hydrocarbons are used if carbon restoration is required. H2/N2 ratios vary from as low as 5-7% to as high as 20-30% hydrogen. Stainless steels and some tool steels are often processed in 100% hydrogen as are MIM parts. They are commonly run in either pusher furnaces or vacuum furnaces operating with a partial pressure of hydrogen.

It is also important to recognize that changes to the furnace-atmosphere composition occur while reacting to the metal powder (e.g., reduction of oxides can enrich the atmosphere with water vapor); decarburization enriches the atmosphere with carbon monoxide; and certain types of atmospheres (e.g., endothermic gas) can vary from carburizing to decarburizing as a function of temperature and produce unwanted carbon in the form of soot.

 

Summary

The choice of furnace atmosphere depends in large part on themetallurgy of the materials being sintered in combination with cost, productivity and the properties produced.

Part 2 will discuss the role of furnace atmospheres in sinter hardening and post-heat-treatment sintering.
 


References

  1. Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media Group, 2014.
  2. Herring, Daniel H., Atmosphere Heat Treatment, Volume II, BNP Media Group, 2015.
  3. “Sintering,” Chapter 6, Höganäs PM School
  4. German, Randall M., Powder Metallurgy of Irons and Steels, John Wiley & Sons, Inc., 1998.
  5. German, Randall M., Powder Metallurgy and Particulate Materials Processing, Metal Powder Industries Federation, Inc., 2005.
  6. Pease III, Leander F., and William G. West, Fundamentals of Powder Metallurgy, Metal Powder Industries Federation, 2002.
  7. Nayar, H. S., “Productivity and Quality Improvements via Evolution in Equipment Design and Process Changes in Conventional Sintering,” Industrial Heating, 1994.