Manufacturing Processes - Surface Treatments
Shot Blasting Cabinet
Physical Surface Treatments
Surface engineering is a manufacturing processes category that includes both physical surface treatment and chemical surface treatment.
Physical surface treatment primarily involves two processes:
- Shot Peening
- Shot Blasting
These are both methods for cold working the surface layers of materials using a controlled blast of shot or abrasive particles. Peening uses shot and blasting uses abrasive particles.
For high volume production the surface treatment process is often automated using either straight line technique, where the material to be treated is fed linearly through the line of fire of shot material, or a rotational technique where the material is rotated underneath the line of fire of the shot material. Using a consistent rate of feed or rotation ensures a uniform surface treatment.
Shot peening or blasting is normally carried out to relieve tensile stresses by plastically deforming the surface material. This can lead to improved life expectancy of a component by increasing its resistance to stress fatigue.
It is also not unusual to use shot peening or blasting to clean up corroded components for re-use or to prepare and enhance components following manufacturing process that introduce life reducing tensile stresses such as grinding or welding, The process can be used for both aesthetic purposes and improved performance as required.
Shot peeing of threads can also aid the prevention of galling on assembly when it is difficult to select dissimilar materials for the same purpose. Galling is a condition where 2 components, usually of a similar or the same material, have a tendency to cold weld to one another when brought into close contact.
Depending on the material to be treated, the shot size, material type and application forces must be carefully controlled to achieve the desired affect.
Shot Peening Information
Shot Peening Uses and Benefits
Chemical Surface Treatments
There are some heat treatment processes that modify the surface composition and properties of steel. The most common are carburizing, nitriding and induction hardening.
Heat introduced via an induction heating process is very localised which means the surface of a component can be heated very quickly without penetrating the inner core of the component. If this heating process is accompanied with a quenching procedure that is applied before the heat conducts away from the surface, the end result is a case hardened component that benefit from the presence of compressive residual stresses in the surface layers.
The main benefit of this process is increased wear resistance through the conversion of the surface material into a martensitic structure. The inner core retains its 'as machined' characteristics which means the component overall retains a resilience that can withstand externally applied stresses more readily.
Carburizing and Nitriding
Carburizing and nitriding seek the same effect as induction hardening but utilising a different technique. The technique used is to raise the temperature of the component within a carbon or nitrogen atmosphere, the carbon or nitrogen reacts with the surface of the component resulting in a ceramic skin. Carburizing used to be referred to as case hardening and may be more readily recognized by some using this description.
Components being carburized can be exposed to carbon in either a gas or liquid form and tend to have long cycle times due to the rate of diffusion. For that reason the process lends itself more to batch processing. This also applies to nitriding albeit the options for nitriding are gas, salts bath or plazma exposure.
All the treatments are only suited to steels that can be heat treated to form martensite and the heat treatment temperatures need to be carefully controlled, particularly with respect to carburizing and nitriding, to ensure the ductility and toughness of the core components are not compromised.
Vacuum Carburizing and Heat Treating
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