Sinter hardening of PM components

With many of the normal PM steps eliminated, it is a viable process for OEM operations to eliminate the heat treating and cleaning operation after sintering.

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In this column, I will discuss sinter hardening and compare that process to the more traditional heat-treating quench and temper operation.

Introduction

The use of powder metallurgy (PM) for producing near-net shaped parts has increased in recent years due to performance gains, as well as the advantage of producing a near net shape. PM is not one single process, but a description of processes that start with metal powder as a feed stock. This can include traditional press and sinter, metal injection molding (MIM), powder forging, and hot isostatic pressing (HIP). Additive manufacturing can also be considered a powder metallurgy process, as the process starts with a metal powder as a feedstock.

There are significant differences between heat treating a conventional steel part and a PM part. Porosity, composition, and homogeneity are the primary sources of differences between the heat treatment of a wrought steel part and its PM counterpart. Understanding the differences allows the heat treater to compensate to achieve a part of consistent properties.

In traditional PM processing, the metallic powders of the desired alloy are blended. Additives such as lubricants and binders are added to the mix to increase green strength and reduce die friction. Once the powders are properly blended, the parts are then compacted to form the green part.

Once the part has been compacted, and ejected, the part is then sintered. Sintering involves placing the parts in a controlled atmosphere and heated. Sintering is often accomplished using a mesh belt furnace or pusher furnace. If higher temperatures are needed, then vacuum furnaces are used. A purge is performed to reduce the amount of combustible atmosphere present and remove any binders present by volatilization. The temperature is increased to approximately 75 percent of the melting temperature, to allow solid state diffusion to occur. Adequate time at temperature is necessary to allow solid state diffusion to form adequate strength bridging bonds across the particles, and to form proper metallurgical bonding of the particles. Additional time during sintering improves part density by reducing the number and size of pores. The part is then slowly cooled under the protective atmosphere. The protective atmosphere can be a vacuum, or argon and 10 percent hydrogen. Other atmospheres are also used to reduce the formation of metal oxides at the particle interfaces. A schematic of the PM process is shown in Figure 1.

Figure 1: Traditional PM process.

Parts that have had secondary operations prior to heat treatment may have residual fluids present that are being held in the pores of the PM part. These residual fluids can affect part surface quality, and potentially reduce part strength during heat treatment. Washing and rinsing, with the proper cleaner, is necessary.

The hardenability of PM parts is lower than similar chemistry wrought parts. This is due to the reduced thermal diffusivity of the PM part. The reduced thermal diffusivity is the result of the reduced density of the part. This reduced thermal density requires a faster oil quenchant than would normally be required for a similar chemistry wrought product.

With the porosity present in PM parts, the quenchant can penetrate these pores. A proper cleaning step, usually containing multiple cleaning operations, is often necessary. Cleaning usually includes an initial alkaline clean, followed by a rinsing operation containing a strong corrosion inhibitor. In addition, a pickling operation to remove carbon smut and additional rinse may be required. This also reduces drag-out of the quenchant to the tempering operation, with resultant fumes.

The quenchant can penetrate the surface porosity present in PM parts. It is typical for parts to drag-out 2-3 percent of the part weight during quenching. For instance, a 100-gram part could drag out as much as 3 grams of oil. When this drag-out is multiplied by the large numbers of parts processed per hour, the amount of oil necessary for make-up becomes significant. A proper cleaning step, usually containing multiple cleaning operations, is often necessary. This also reduces drag-out of the quenchant to the tempering operation, with resultant fumes. Further, the cleaner should have adequate splitting capacity, and the oil skimmers present on the washers need to have adequate capacity to keep up with the amount of oil present.

Sinter Hardening

In sinter hardening, many of the normal PM steps are eliminated. After blending and compaction, the green compact is sintered. Instead of slow cooling as in the traditional process, the part is immediately quenched from the sintering temperature using a quenching station, where parts are quenched in high velocity nitrogen, argon, or other gases. This process is shown in Figure 2.

Figure 2: Schematic process of sinter hardening.

This process eliminates many intermediate steps, such as cleaning prior to heat treatment, the heat-treating step, and the cleaning step after quenching, to reduce the amount of oil being dragged out by the parts. It also reduces fumes from the tempering operation.

The high-velocity atmosphere quench, like that used in pressure quenching vacuum heat-treated parts, is less severe than an oil quench, so better dimensional control is achieved. The reheating of the part is eliminated, so energy consumption is further reduced. A schematic of a continuous sinter hardening furnace is shown
in Figure 3.

Figure 3: Continuous mesh belt furnace used for sinter hardening. (Courtesy Cremer Thermoprozessanlagen GmbH)

To achieve the high hardness required, a high carbon content (0.45%C or greater) is necessary. Additional elements such as molybdenum or chromium are added to increase hardenability. However, the high hardness upon exit of the sinter hardening process makes resizing or other secondary operation difficult. Tempering is required to remove residual stresses and to produce tempered martensite.

This process eliminates the heat treating, quenching, and cleaning steps. Sinter hardening is usually limited to large OEM-type operations, where compaction is performed. This eliminates the need for commercial heat-treating operations and keeps operations in-house. This further eliminates supply chain problems, and reduces schedule delays.

Conclusions

Sinter hardening is a viable process for OEM operations to eliminate the heat treating and cleaning operation after sintering. No residual quench oils are present in the hardened part, eliminating often extensive cleaning operations. A reduction in energy consumption, material consumption (oil quenchant and cleaners), and process cycle can be accomplished.

Disadvantages of the process include higher hardenability alloys are required, and secondary sizing operations are difficult due to the high hardness of the parts exiting the sinter hardening operations.

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