Since TD is a high temperature, salt bath process, there are certain factors to be considered.
- Distortion or part geometry change – this is due to unequal sections and uneven cooling. The higher the cooling rate required to harden a part, the higher the tendency for distortion. Combat by selecting steels with high hardenability.
- Size changes – this is due to different microstructures before and after coating. The TD thermal cycle re-creates the internal structure and if the tempering temperatures do not match, size changes (expansion or shrinkage) may result. Inform us of the heat treatment cycle should parts be of critical sizes.
- Carbon content – the carbon content in the steel determines the speed of VC layer formation. Choose steels with > 0.2% free carbon content. Free carbon is that remaining after internal carbides in a steel has formed. For example, AISI D2 has a total C content of 1.55 %C however the free carbon content is ± 0.6%.
- Hardening temperature – the higher the TD treatment temperature, the faster the VC layer forms as this is the basis for diffusion. Therefore O1/K460 although, containing high free carbon, coats very slowly due to its low hardening temperature. Choose steels with higher hardening temperatures.
Typical Low Alloy Steels
|En30B, Bohler K600, 1.2767, 9340||high hardenability steel however with medium %C content and low hardening temperature|
|En24, 4340||medium hardenability steel with medium %C content and low hardening temperature|
|Bohler M200, 1.2312, P20||medium hardenability steel with medium %C content and low hardening temperature|
Typical Tool Steels
|Bohler K460, O1, 1.2510||high carbon, however low hardening temp and limited hardenability. Not recommended.|
|Bohler K110, D2, 1.2379, SDK11||high carbon, high hardenability and high treatment temp. Highly suitable.|
|Bohler K340, DC53, SLD8||high carbon, high hardenabillty and high treatment temp. Highly suitable.|
|Bohler K305, A2, 1.2363 SDK12||high carbon, medium hardenability and high treatment temp. Good suitability.|
|Bohler W302, H13, 1.2344, SKD61||low/medium carbon, high hardenability and high treatment temp. Good suitability.|
|Bohler S600, M2. 1.3343, SKH61||medium/high free carbon and high treatment temp, Parts have to be re-hardened after TD to obtain full hardness. Rather use K340 for 60 – 62 HRc applications.|
|ASP 2060, 2030, 2023 etc||medium/high free carbon and high treatment temp. Good suitability.|
|Bohler M300, M310||Martensitic stainless steels with marginal carbon content. Can be given a thin TD layer for sliding wear resistance. Low suitability.|
Typical Cemented Carbides
|Carbides with a cobalt contact > 10%||Carbide with sufficient binder content can be TD coated|
► Please contact us should you wish to discuss coating of a material not listed above.