Air cooling is a type of quenching method that often goes virtually unnoticed in the heat-treat shop. So, are all air-cooling methods equivalent? The simple answer is no, and it is worth our time to learn why.
We all know that rapid quenching in mediums such as water, oil or polymer generally increases strength and hardness but also produces a significant amount of residual stress and distortion. If the properties (mechanical, metallurgical) of a material (e.g., steel, aluminum) allow air cooling, the result can be a combination of good mechanical properties, proper hardness and a low (residual) stress state.
If done correctly (and this is the real secret to effective air quenching), one can achieve a more or less controllable cooling rate, thus producing a more uniform stress state and less distortion. This is achieved in most cases by proper design of the air-cooling system to produce a uniform temperature distribution (i.e., heat-transfer coefficient) between the part surface and the airflow.
Factors that Impact Air Cooling
There is a relationship (Eq. 1) that describes the convective heat transfer (q') in terms of the heat flux (or heat flow) through the surface of a part, which is proportional to the difference in temperature of the part surface (Ts) and the temperature of the quench gas (Tg) at any given moment in time. This proportionality value, α (“alpha”), is the heat-transfer coefficient.
(1) q' = α (Ts – Tg)
In convective heat transfer, a complex formula (Eq. 2) describes the heat-transfer coefficient (α) as dependent on the gas type (as described by its heat capacity, cp, its heat conductivity, λ, and the dynamic viscosity, η, of the gas) and on a geometric factor (d) that represents the area of free passage of the gas for a specific load arrangement. The physical laws that illustrate how velocity (v) and pressure (ρ) influence the heat-transfer coefficient are given by:
(2) α = C v0.7 ρ0.7 d-0.3 η-0.39 cp0.31 λ0.69
where C is a constant
What this tells us is that there are a number of important factors out on the shop floor to consider when air (or gas) cooling. Some of these include pressure, velocity, temperature, part geometry, part orientation (to the gas stream), material, surface roughness, oxide layer (pre-existing), gas type and design of the cooling equipment.
It has been reported that the heat-transfer coefficient changes significantly with changes in velocity and pressure. Part geometry and orientation also play a significant role, while such factors as humidity appear to have little effect. The type of material, while it influences the resultant microstructure, has a limited effect on the heat-transfer coefficient as does surface roughness or the presence of a pre-existing oxide layer. Meanwhile, gas mixtures (e.g., nitrogen-hydrogen mixture in specific proportions at various pressures) influence the heat transfer.
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