1- Heat Treating - The Heat Treating Process
|A machinist will often be required to machine steels that have been previously heat-treated so it is important to understand how the heat treatment changed the steel. Armed with this knowledge the machinist can select the best machinery, cutting tools, and cutting conditions.
If a machinist works as a tool and die maker they will be working quite a bit with tool steels. These metals are machined in a relatively soft condition, heat-treated to gain hardness, annealed to relieve internal stresses from the hardening process, and then finish machined with a grinding or lapping process that can handle the hardness.
Other times a machinist may need to work on a weldment that - due to the welding process - has many internal stresses. They must stress relieve the weldment prior to machining to prevent distortion after machining. In this case the heat treating process is not to make the work piece harder but is to make the work piece more machinable and, in truth, makes the metal a bit softer.
Regardless of the reason for the heat treating, the basic process is the same and has three steps.
The stress relieving process is almost never so thurough as to remove all stresses. A weldment like the one above would be very difficult to stress relieve entirely. Therfore if a large surface needs to be machined flat it is best to go slow, take minimal size cuts measure for warp (lack of flatness), and machine a little more. If the stresses seem to never let go then send it out again for more stress relieving.
Alternatively the heat trearting facility can give you a multliple soak or longer soak. Meaning maybe three seperate annealings or one very very long soak which eats into oven time and is very expensive. Look at the video below and try to imagine how expensive it is to keep the oven door open while it is burning up all that natural gas. RS