Shot Blasting or Peening
When tiny steel balls are used the process of blasting parts using this medium will be called shot blasting or peening. Jets of shot are aimed at components while they tumble in a rotating chamber. Some differentiate between “blasting” or “peening”, but the process is the same nevertheless. The former expression is used when the objective is to clean the parts or remove sharp edges, while the latter indicates a desire to mechanically compress the surface layer of the material. In this manner compressive stresses are introduced which in most cases will improve the fatigue resistance of the component.
Sand blasting is used primarily to remove rust, paint, scale or significant blemishes on the surface of components. Depending on the need, the technique is used either before heat treatment (preparation) or after (e.g., removal of oxidation).
Certain types of components, particularly longer objects such as shafts, tend to distort in high temperature treatment. In most cases they can be successfully straightened using special hydraulic presses.
Heat treatment is inherently distorting to the parts. To reduce distortion some components must be restrained during tempering by being “clamped”, and process is then called clamp tempering.
Cryogenic Treatment / Freezing
After a successful quench and temper, it is frequently desirable to subject the work piece to a cryogenic treatment, also referred to as freezing (or sub-zero freezing). This process induces carbide particles to precipitate into voids in the iron lattice, thus creating a denser, more stabilized structure that reduces friction, wear and thermal softening.
Summary of Benefits:
- Conversion of retained austenite (soft) into martensite (hard)
- Increased strength, toughness, stability and durability
- Increased density of the steel structure
- Lower coefficient of friction
- Decreased residual stresses and brittleness
- Significantly improved abrasive wear resistance