| Heat treatment processes fall into two distinct groups, those which harden and those which soften. They all use time and temperature to alter the microstructure, and hence the mechanical properties of the steel. It is important to recognise that these changes are accompanied by changes in volume and hence part size. With good design, material selection, manufacturing and heat treatment practice it is possible to accommodate and allow for, but never eliminate, these changes. The temperatures are also sufficient to relieve any internal stresses in the component from cold work or prior heat treatment. This too may cause distortion of the part. The Designer's Contribution As far as possible avoid sudden changes of part section. Where this is not possible minimise any stress concentration by the most generous fillet radii possible and the smoothest undercuts. As far as possible avoid mixing thick and thin sections in the same component.
If this is not possible then remove excess metal from the thick section, to equalise the cooling rates in the thin and thicker sections. The layout of any cutouts and holes across the section should be as even as possible, again to equalise cooling rates. Avoid sharp edged slots, stamp marks or rough surface finishes which will act as stress concentrators and crack initiation sites. In collaboration with your in-house metallurgical department, or sub contract heat treater, select a steel with sufficient hardenability to achieve your desired properties in the component section without the need for an over severe quench. Do not economise on inter stage annealing or normalising to relieve machining or cold forming stresses. Pennies saved here may cost pounds in scrap or rectification later. If you are thinking about thermochemical treatments remember that nitriding and nitrocarburising are carried out at lower temperatures than carburising and may cause lower distortion. Again discuss with your in-house or sub contract heat treatment department. The Heat Treater's Contribution Always try to achieve the most uniform temperature, heating and cooling rates across the furnace load. Always try to properly jig and adequately support the part in the furnace to prevent sagging between supports etc., long shafts for example are best heat treated suspended vertically. To ensure uniform heating of the part by allowing sufficient soaking time to minimise warpage. To recommend the steel with the optimum heat treatment characteristics. Where appropriate suggest special processes and steels with lower intrinsic distortion, (such as austempering and martempering). Always use the most economic and appropriate equipment and process for the application. To protect the work from oxidation, decarburisation or other surface degradation as far as possible. |