There’s no end to the number of metals out there, all of which serve different roles in countless applications. Selecting the materials, it’s up to a metallurgically talented sourcing expert to choose the material classifications that satisfy a customer’s demands. Heat treatment technology then further processes the metal so that it has all of the physical and mechanical characteristics it’ll need to serve its application.
A Blindingly Daunting Task
How many properties can a metal sport when it is first sourced? To answer that question, think about the hundreds of ferrous and non-ferrous materials on the market. Some of them are more workable than others. Some alloys accept certain heat treatment processes, but other classifications refuse thermally active treatment, so the workpiece needs case hardening carburization or nitrocarburizing work. Elsewhere, a mild steel doesn’t possess enough carbon, chromium oxidization problems are plaguing the process, or its melting point doesn’t conform to a furnace’s current workpiece baking configuration. And that’s just the ferrous alloys. For non-ferrous metals, the issues multiply.
The Importance of Material Classification Expertise
A capable material selection professional knows every aluminium series and every steel gauge. Metric or imperial, SAE standard or AISI, alloy connoisseurs know the features to highlight and the factors that’ll facilitate an expertly executed heat treatment run. Stainless steel or aluminium, tool steel or carbon strengthened alloy, even the many non-ferrous alloys, every metal is known to the sourcing professional. And not just by name, either. The composition, austenitic or martensitic microstructure, the forged characteristics and datasheet properties, they’re all intimately recognized by sourcing professionals.
Selected and Classified Alloys: It’s Half the Battle
Correct, by buying in an alloy that serves a customer’s application specs, the heat treatment process is already halfway over. What remains is the engagement of a furnace/tempering procedure that will take the metal workpiece the rest of the way, all the way to the point that its operational parameters absolutely assure it’ll function in its eventual application. In service of this duty, coefficients of thermal expansion are assessed, melting points predetermined, and thermal conductivities recorded. At the end of the day, the primary goal here is to make sure the alloy selection and classification phase picks out an alloy that will be strong but not brittle, hard but not rigid, ductile, but not overly elastic, and environmentally capable but in no way chemically unstable.
To satisfy the above processing parameters, engineer expertise operates as a two-way street. On the one hand, the mechanical and physical specs are supplied. Meanwhile, the shortlisted selection candidates are picked out, but they can only be processed after the heat treatment facility delivers the right equipment. Given the go-ahead, the furnaces and tempering gear convert the raw workpieces into their final form.
Sintered components are produced when fine metallic powders are compacted inside special cavities. As high pressures cause densification, heat is added. Therefore, as a process precursor, this operation employs a finite amount of heat treatment toughening right at the end of the product forming procedure. However, if the tightly packed, materially diffused particles still don’t exhibit sufficient tensile strength and pure hardenability, additional heat treatment work may be deemed necessary.
Powder Metallurgy: Post-Processing Heat Treatment
The atomized powder has its binding agents and lubricants. A slightly porous metal construct has taken shape inside the cavity compaction mechanism, and the presence of material diffusing heat has even introduced a measure of heat treated hardness. Still, the project engineer knows where this product batch is heading. He knows the present fatigue resistance and hardness rating ingrained within each part just won’t cut-the-grade. To maximize hardness, to really address this hardenability issue, the components require the services of a heat treatment furnace.
Unlike Other Product Structures
Let’s say this is a batch of bushings or bearings. They’re heading for a shafting mechanism, where they’ll be placed under great stress. Loaded with their self-lubricating abilities and capillary action, the bushings address their duties with unmitigated ease. But the loads are heavy, the dynamic forces extreme, and the porous metal is under pressure. Using in-house precipitation hardening technology, the friction-mitigating components gain cross-sectional strength and uniform mass hardness. Alternatively, there are all the usual surface hardening techniques, which include nitrocarbonizing, carbonizing, and plasma nitriding. Internal stresses are less likely here, probably because the bearings avoid the machine shop. Unless the parts are taken through a post-production shape refinement phase, there’s not likely to be any tool-produced stress in sintered parts, after all. If cold work stress does rear its ugly head, however, the P/M processed parts can be annealed until those stresses are removed.
Two complex issues hamper heat treatment work, as carried out on sintered components. First of all, several soft materials are used in this sector, including bronze. Powder metallurgy work is also used to control structural porosity. The heat treatment process must account for such unique variables. There could also be a lubricating fluid stored inside those metal pores, which is common enough when the components incorporate a self-lubricating property. So, to answer the title’s question, yes, sintered components should be subjected to heat treatment if their structures lack strength and hardness. For those bearings and bushings alone, the service does supplement and optimize the metal, leaving the friction-handling components ready for the most challenging rotational duties.