By Howard Ferguson
Anyone who buys P/M parts in the heat-treated condition or who performs heat treating in-house knows the difficulty of testing consistent properties and size control.
Most commercial heat treaters base their pricing on the weight capacity of their furnace baskets and how many furnace hours it will take to process the load. This is usually where the problem begins. P/M parts, because of their porous nature, have lower thermal conductivity than wrought steel and must be considered in a different light. The most common problem in heat treating P/M parts is overloading. The more parts you put into the basket, the more variability you impart to the hardening response. Another important factor is shadowing of parts. This occurs when parts are touching each other in the load. Consequently, these closed surfaces are not exposed to the agitated quenching medium and therefore do not cool at the same rate as the exposed surfaces. This can cause soft spots and unusual distortion in the P/M parts since they do not fully transform to the hard martensite phase upon quenching.
Hardenability is the main consideration when selecting a steel for any application that requires hardening. In wrought steels, this property is controlled by composition and grain size. In P/M, however, the density of the part also determines hardenability, which increases in direct proportion with density for a given composition.
When maximum strength and hardness are a requirement for your P/M component, it would be wise to make heat treatment one of your prime considerations when selecting the material and finalizing the design. Secondary operations, including heat treatment, should get the same consideration as your primary processes during the product
In this way, you will minimize unwelcome surprises during this very critical process.