Sinter-Hardening of Powder-Metal Parts
As a technology for the production of metal parts, one of powder metallurgy’s strongest benefits is the ability to precisely formulate the part’s geometry, material composition and thermal processing in order to produce the most effective solution – in terms of cost and performance – for a particular design.
Capitalizing on the ability to formulate unique metal alloys through the blending of powder metals, the powder metallurgist is able to customize a blend for each application that ideally fits the performance characteristics desired for the end use of the part. Through compaction and tooling design, a very complex, net-shape part can be consistently formed (in large quantities if desired) using the specially designed alloy that may not be produced by any other means. Additionally, a thermal-process cycle can be designed to achieve the desired properties for the part.
The end result is a high-performance, fabricated metal part for a variety of industries that has been custom engineered to satisfy the exact requirements for an application.
The thermal processing required for a conventional powder-metal (PM) part generally involves three distinct phases: de-lubrication, sintering and cooling. Normally produced in a continuous wire mesh-belt furnace, specific processing conditions are generated through the furnace design and the adjustment of controlled conditions.
De-lubrication is necessary to remove the lubricant that is typically added to a PM blend to aid in the compaction and release from the die. In order to thermally remove the lubricant, the part must be held at a certain temperature in an inert atmosphere with a small amount of oxidizer, such as air or water vapor, which combines with the emerging hydrocarbons. The time must be sufficient for the lubricant to vaporize and combine with the available oxygen forming carbon monoxide that exits the furnace.
Sintering is accomplished by moving the part into a higher-temperature zone with a reducing atmosphere that removes the oxides on the individual particles in the compacted part. It is then held at a precise temperature for a defined time period, which is long enough to form the degree of sinter and ultimate physical properties and dimensions required by the part.
Finally, the part is cooled in an inert atmosphere to avoid oxidation and then leaves the furnace once it has cooled sufficiently.
As in any other method of metal part production, powder metallurgy obtains the ultimate physical properties of a part by controlling the chemistry and thermal processing experienced by the part. A major advantage of sinter-hardening is that – unlike conventional heat treating –
it can be performed without reheating a part. This both saves energy and reduces the opportunity for thermal distortion. Another ad-vantage of sinter-hardening is that unlike conventional heat-treat/quench processes, sinter-hardening does not require immersion in a liquid, which can be a problem for PM parts due to their inherent porosity.
Basics of Sinter-Hardening
As shown in this TTT curve for plain-carbon steel (Fig. 1), the cooling rate of the part for hardening must be sufficiently fast to follow Curve 1 in order to form the hard-phase martensitic structure. Note that Curve 3 represents the cooling rate for normalizing, and Curve 2 is an
annealing process. The two primary factors affecting sinter-hardening are the cooling rate and the material composition. Alloying can be used to move the nose of Curve A to the right, allowing a slower cooling rate for the martensite formation.
Alloying elements can be added to the PM blend metal to increase the hardenability of the material so that the required microstructure transformation for hardening can occur at a slower cooling rate. Molybdenum, chromium, nickel and copper can be used to accomplish this. Due to obvious economic impact, the lower the alloying element content, the lower the material cost of the part.
Over the past several years, PM suppliers have developed a variety of alloy blends specifically tailored for use as sinter-hardening materials. Coupled with the correct thermal-processing and cooling system, a large number of high-performance PM parts are being produced with the performance and cost advantages afforded by the use of sinter-hardening.
The other critical factor in the sinter-hardening process design is the cooling rate that can be achieved in a furnace. Typical belt furnaces use water-jacketed coolers that cool the parts through free convection. For generally smaller parts, this cooling rate is sufficient to achieve sinter-hardening.
With a goal of reducing the alloying requirement to achieve the desired hardening, furnace manufacturers have developed a variety of accelerated-cooling units that employ forced convection cooling of the parts. The capital cost is higher, but the material cost is lower, suggesting a tipping point where alloy versus equipment is preferable.
Although varied in their implementation, the primary product offerings from furnace manufacturers for sinter-hardening systems recirculate cooled furnace atmosphere (Fig. 2). Heat is removed from the process atmosphere through some sort of water-cooled heat exchanger, and cooled atmosphere at an elevated velocity is directed on the parts as they exit the heating chamber. This high-er-velocity atmosphere will cool the part at a faster rate than that achieved using free convection due to a higher convective-cooling coefficient.
Variables that contribute to the suitability of sinter-hardening as an acceptable technical solution include the part’s material composition, mass and cross-sectional thickness, as well as the velocity and temperature of the recirculated atmosphere. For example, most of the accelerated-cooling units that are currently available today direct the atmosphere flow from the top of the cooling section using either direct fan(s), vented rotatable tubes or impinging jet arrays. The incident atmosphere flow increases the heat-transfer rate, resulting in a faster cooling rate through forced convection.
Because of the geometry of flowing the cooled atmosphere to the top of the parts resting on the belt, thick parts laying flat on the belt will have a hardness gradient top to bottom since the top cools at a faster rate than the bottom. For this reason, not all parts are suitable for sinter-hardening and could be better suited for conventional heat treating.
Impinging Jet Array Technology
Employing proprietary Impinging Jet Array Technology (Fig. 3), Sinterite’s HyperCooler engineers the accelerated-cooling atmosphere flow in such a way as to exactly balance supply and exhaust flows. This results in all atmosphere flow being vertical to the belt, which induces no lateral atmosphere flow to destabilize the process gases. This results in the horizontal atmosphere flow being decoupled from the blower speed so that changes in cooling rate do not influence the horizontal atmosphere flow.
Additionally, cooling rates are higher than previously possible due to the jet effect. The cooling rate has been demonstrated to be proportional to the blower speed, making it straightforward to return to a previously set-up process, which reduces set-up time. Due to the jet-array design, uniformity of cooling across the belt and in the belt direction is assured. Variability in part parameters is reduced, enabling tighter-tolerance production.
Other technologies exist and have been applied effectively for a variety of sinter-hardening applications. These technologies perform the accerated-cooling function through different designs, which include forced convection fans and rotatable tubes connected to a circulating blower positioned above the parts on the belt (Fig. 4).
Powder metallurgy and sinter-hardening provide an excellent solution to the design and production of metal net-shape parts with controlled hardness, minimal opportunity for heat-treat-induced distortion and an economical high-volume production. Designed and implemented properly, this technology can deliver superior results for certain applications compared to other types of manufacturing technologies. IH
For more information: Contact Jeff Danaher, sales & technical services manager, Sinterite, A Gasbarre Furnace Group Company, 310 State Street, St. Marys, PA 15857; tel: 814-834-2200; fax: 814-834-9335; e-mail: email@example.com; web: www.sinterite.com or www.gasbarrefurnacegroup.com