Ken,
Do you know what kind of surface treatment Ron Wood specified, nitriding (diffusion of nitrogen into the surface) or tuftriding (salt bath nitrocarburise)? Tuftriding provides less finishing work. Each treatment has their pros and cons.
Nitriding
Advantages & Properties
* The process is conducted in gas (490 - 560 °C) or plasma (400 - 590 °C) for treatment times ranging up to 90 hours.
* Nitriding imparts a high surface hardness which promotes high resistance to wear, scuffing, galling and seizure.
* Fatigue strength is increased mainly by the development of surface compressive stresses.
* Hot hardness and resistance to tempering are improved and corrosion resistance is moderately enhanced.
* The low processing temperature and subsequent slow cooling help minimise distortion.
* Nitriding is most effective when applied to the range of steels containing nitride-forming elements such as chromium, molybdenum, vanadium and aluminum.
Disadvantages and limitations
- The process can only be applied effectively to a limited range of appropriate alloy-containing materials.
- Depending upon process parameters and material, nitrided case depths can range from as little as 0.05mm up to 0.50mm.
- Surface hardness depends upon process parameters, the material and its original condition. For optimum results, a steel for nitriding should be in the hardened and tempered condition and free from decarburisation. (The tempering temperature should be higher than the nitriding temperature).
- A fine-turned or ground surface finish is the most suitable for ensuring a satisfactory nitriding response. Pre-nitriding treatments may be required on certain materials subjected to gas nitriding (e.g. acid pickling/vapour blasting of martensitic stainless steels).
- A stress-relieving treatment ("stabilising") is necessary between the rough and final stages of machining of hardened and tempered high-precision components in order to minimise distortion after nitriding. (The stress-relieving temperature employed should be higher than the nitriding temperature but lower than the tempering temperature).
- On a hardened and tempered/stress-relieved part, nitriding should produce minimal distortion. However, growth can occur, the amount being a function of the treatment parameters and the material. It is fairly predictable and usually less than 0.05mm on a diameter.
Tuftriding
Advantages & Properties
- Gives a very hard (800-1500Hv) wear resistant surface to the metal typically up to a depth of 0.002"(0.05mm) depending upon the material.
- The surface layer is in compression and so not conducive to crack initiation. This increases fatigue resistance
- As the process is carried out at 580 °C no phase changes take place as is the case with higher temperature treatments, and so there is little or no dimensional change in the components.
- When specified with the caustic molten salt quench the surface of the steel becomes very corrosion resistant.
- The hard wear resistant surface also exhibits self-lubricating & anti-seize properties.
Disadvantages and limitations
- The metal must withstand 580 °C. This is particularly important when previously heat treated components that are Tufftrided, as the temperature may lead to softening or distortion.
- As the process is carried out in molten salt, removing this salt from deep small bore holes or hollow components can be difficult.
- Care must be taken with thin section components to ensure that the thin sections do not become Tufftrided all the way through & and hence become brittle.
(Source: Wallwork, UK)
- Knut
