Do Sintered Bearings Need Heat Treatment?

August 17, 2018

Powder metallurgy represents something of an ideal production model. There are no costly castings to produce, and the bearings possess a highly desirable self-lubricating feature. It’s also possible to make sintered bearings without incorporating a post-production stage, although this part of the operation is often recommended. With that said, apart from the end-phase jobs that machine and polish the pressed-metal rings, is heat treatment work an obligatory step?

Heat Treatment Pitfalls 

The sintering process creates an elaborate system of pores within the bearings. Through those pores, a lubricant is discharged until a fine film encircles the bearing ring. This is capillary action, a principle that occurs when bearings spin. Now, this intricate cluster of pores, penetrating the entire bearing, function very well indeed. Unfortunately, a poorly configured heat treatment cycle can impede capillary action. Furthermore, certain metals and chemical additives are known to oxidize when they’re exposed to great quantities of heat.

Reviewing the Benefits 

As a desired post-production phase, heat treatment does deliver certain essential benefits. The sintered metal, loaded with pores, could be fracture-prone. A case hardening or induction hardening furnace would clearly remove that underlying flaw. Further heat work and an accompanying tempering stage would take this operation further, perhaps by making the sintered bearings entirely fatigue-resistant. In conclusion, this is a powerful tool, one that’s designed to improve the mechanical characteristics of sintered bearings. However, the process can also impede product capillary action or even oxidize the alloying metals within the pressed metal because of some unpredictable catalyzing event.

Heat Treatment Alternatives 

Again, heat treatment is an essential tool to have on hand, but it’s not always the best way forward. For example, standard self-lubricating bearings are capable of handling light to moderate loads. For heavier loads and transient radial forces, employ a more durable alloy or a lower porosity ratio. As long as the shaft rotates at a high enough velocity, hydrodynamic action will spread the lubricating agent. Lastly, again as an alternative to heat treatment, research steam treatment options. Designed to intelligently add an oxidized layer of magnetite, this process does not mechanically alter the makeup of the bearing alloy.

Heat treatment processing is conducted at the end of a sintered bearings production journey, but it’s still considered an elective phase. Granted, harder bearings do overcome load-limiting porosity problems, but the heat can also create as many problems as it solves. For light to moderate applications, purely sintered bearings are often more than good enough. Then, for heavier applications, consider stronger alloys or non-transformative steam treatment instead of a full-out material conversion operation.

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