The composition of the sintering atmosphere is often overlooked as a variable that can optimize the sintered-steel properties of iron-carbon P/M components. This article will examine the effects of CO-containing atmospheres versus non-CO atmospheres in terms of hardness, carbon control and dimensional changes.

Fig. 1. Effect of oxygen pickup during sinter hardening of Cr/Mo-alloyed, structural-steel compacts[1]

Since the late 1970s, the use of nitrogen/hydrogen systems has been the atmosphere of choice for sintering iron and steel powder metallurgy (P/M) parts. Prior to that, a conventional natural-gas-based endothermic atmosphere was common, but the natural gas shortages of the late ‘70s was the catalyst for the switch. The benefits of inherent safety, enhanced properties, better economics and increased reliability led to the wide-scale adoption and popularity of nitrogen-based atmospheres.

With increasing demands on the properties of sintered components used in the automotive industry, however, it was not sufficient only to obtain better properties but imperative to obtain the properties consistently. The final properties of sintered components are determined by a variety of processing variables, including powder size and size distribution, powder composition and purity, and carbon content. Additionally, the type and amount of lubricants as well as compaction densities and furnace parameters (temperature, time at temperature, cooling rates and belt loading) can also influence end results. Often overlooked is the sintering atmosphere as a variable. While most of the other variables are determined during the design stage of the component, the atmosphere can change or be made to change as a function of time during the sintering process, thereby influencing the final properties and their consistency.

The sintering mechanisms and sub-processes taking place inside the furnace with an atmosphere include: reducing specific surface area, increasing number and strength of particle-metallic bonds, decreasing pore volume (increasing density), rounding of pores, alloying and homogenization, and eliminating/reducing lattice defects.

The advent of sinter-hardenable materials presents added challenges to controlling property variations. The surface densification, oxidation and carbon control in the sintering zone can enhance or decrease the cooling rates in the accelerated-cooling zones and, consequently, lead to additional property variations. To increase density, strength, hardenability and service performance, sinter-hardenable powders contain elements such as Cr, Mo, Ni, Mn and/or Cu – many of which can be easily oxidized. In a study about the impact of oxygen on the microstructure and fracture morphology of Fe (Cr, Mo) P/M steels, Sigl and Delarbre concluded[1] that the ductility of chromium P/M steels is seriously deteriorated by high oxygen contents (Fig. 1). In another study, Fillari et al.[2] demonstrated the significant effect cooling rates have on the tensile and yield strength of steels containing 0.69% carbon.

Atmosphere System Fundamentals

In blended nitrogen-based atmospheres, the incoming composition is relatively stable. However, the gas chemistry within the furnace can vary depending on the integrity of the furnace and external factors such as room pressures and exhaust systems. Additionally, gases may be formed within the furnace even if they were not part of the original supply system. It is not uncommon to produce small amounts of ammonia in a nitrogen-plus-hydrogen atmosphere. With flow control panels, it’s possible to precisely monitor and control the mixing of different gases that make up the required atmosphere system. These gas constituents each have their own properties and serve specific roles with regard to the sintering process:
  • N2 – inert, can nitride (SS, Ti, Ta)
  • H2 – reducing, decarburizing, can hydride (Al, Ti)
  • CO – carburizing, reducing
  • CO2 – decarburizing, oxidizing
  • O2/Air – oxidizing
  • H2O – oxidizing, decarburizing
  • Ar – inert
  • CxHy (hydrocarbons) – carburizing, H2 source
  • He – inert
  • NH3 (ammonia) – nitriding
The atmosphere system itself plays an important role in the sintering process. The primary atmosphere functions in a sintering furnace are delubrication; oxide reduction; sintering, bonding and densification; carbon control, including surface and core carbon; microstructure control; oxidation prevention; and heat transfer.

Since atmosphere plays a critical role in the final quality and properties of the sintered part, it is important to be able to directly or indirectly control the variables of an atmosphere system. The primary variables in an atmosphere system are atmosphere composition, including dew point and H2, CO, CO2 and CH4 percentages; atmosphere flow rates and distribution (turbulent vs. laminar flow); atmosphere pressure inside the furnace; atmosphere exit velocity; atmosphere stability in relation to external influences; door openings, such as the part height; and atmosphere curtain design.

Preheat Zone
The key atmosphere reactions take place in the three main zones of the furnace. In the preheat zone, an oxidizing atmosphere is desired to react with the hydrocarbon vapors and prevent sooting or the formation of solid carbon.

Lubricant Removal

Dry atmosphere:
CxHy ® H2 + C (soot)

Humidified atmosphere:
CxHy + H2O ® H2 + CO

High-Heat Zone
In the high-heat zone, the hydrogen-to-moisture ratio determines the oxidation/reduction potential, which determines the rate the surface oxides of the powder particles are reduced, and directly influences the sintering or bonding between the particles.

Fig. 2. Hydrogen/moisture for oxidation/reduction of principal sinter-hardened metals and oxides

Oxide Removal

Fe2O3 + 3H2 ® 2Fe + 3H2O

This ratio needs to be maintained as high as possible at the lowest costs. The equilibrium conditions between metals and their oxides showing the effects of temperature, dew point/H2O content and pH2/pH2O ratio is shown in Figure 2.

Cooling Zone

In the cooling zone, the primary atmosphere function is to prevent oxidation and maximize cooling rates.

Heat Transfer and Oxide Prevention

Any oxygen infiltration into the cooling zone can cause oxidation as per the following equation:
Fe + ½O2 = FeO

Nitrogen by itself generally does an effective job of keeping oxygen out of the furnace. With sinter-hardening considerations, however, cooling rates can be increased significantly by adding hydrogen in this zone. Bowe[3] examined the effect of hydrogen on the heating and cooling rates.

Fig. 3. Effect of %H2 on dimensional change[4]

Effect of Hydrogen and Natural Gas on Dimensional Changes in TRS Bars of Carbon Steel

Garg et al.[4] studied the effect of hydrogen and natural gas concentration on the dimensional changes in transverse rupture (TRS) bars of carbon steels (Fig. 3). Their work showed a linear dimensional growth from 0.24–0.32% as the hydrogen concentration in the N2/H2 atmosphere increased from 1% to 10% followed by a decrease in growth with further increases in hydrogen concentration. Additionally, with natural gas additions of 0.25% and 0.50%, there was no change in growth when hydrogen was increased beyond 10%.

Fig. 4. Combined carbon vs. distance from the surface of steel P/M part in endothermic and H-N atmospheres[5]

Carbon Control

Maintaining the desired carbon level in the P/M part is essential to obtaining consistent part properties. Properties such as hardness, dimensions and yield strength are subject to changes if there is carburization or decarburization occurring either in the surface or in the core of the P/M part. In carbon-neutral atmospheres such as nitrogen-based with H2 or dissociated ammonia, the atmosphere-control parameter is the hydrogen-to-moisture ratio. In such atmospheres, with 5% to 10% hydrogen, maintaining a dew point below -30°F will provide a reducing environment to the iron oxides yet be neutral to carbon. In carbon-rich atmospheres such as endothermic gas, the control parameters are the CO/CO2 ratio and the dew point. In these atmospheres, it is essential to control the carbon potential using natural gas as the controlling gas because of the inherent variations in CO and CO2 resulting from air and natural gas composition variations.

Other causes of atmosphere composition variations include age and condition of the nickel catalyst, temperature control of the generator, instability of the CO in the atmosphere and changes in the cooling-water temperature. Nayar[5] studied the effect of CO-containing and non-CO-containing atmospheres on the variation in carbon content. In the case of atmospheres with low combustibles and low dew point, the changes in carbon level were small and limited to the first 5 mils from the surface of the sintered part. In the case of atmospheres with high combustibles and high dew point, the variations in carbon content more than double. In addition, these variations could be observed as deep as 30 mils into the surface of the P/M part. Figure 4 shows these variations in carbon content.

One explanation for the carbon-content variations is that the P/M parts first undergo decarburization in the hot zone, where the atmosphere is decarburizing in nature. Then the parts undergo a recarburization in the temperature range of 1700–1500°F when using atmospheres with high combustibles and high dew points. In most cases, this decarburization and recarburization is not balanced and leads to variations in the carbon content and, consequently, in the physical properties of the P/M parts.

Other Causes for Property Variations

Atmosphere-related problems such as improper delubrication and oxidation at the front end or exit end of a furnace are also causes for variations in the properties of the final sintered product. Depending on the type and severity of the problem, these variations can be significant. A detailed analysis of atmosphere-related problems and the methodology for identifying and solving these problems can be found in a paper titled “Troubleshooting Guide for Sintering Furnace Atmospheres.”[6] IH

Original source material used with permission from Advances in Powder Metallurgy and Particulate Materials 2006, published by Metal Powder Industries Federation, Princeton, N.J.

For more information: Contact Air Products’ Metals Processing Team, 7201 Hamilton Blvd., Allentown, PA 18195-1501; tel: 800-654-4567 (code 652); e-mail: gigmrktg@airproducts.com.

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