Heat Treating
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.
- Heat the metal to a specific temperature
- Hold the metal at that temperature for a specific amount of time
- Cool the metal in a specific manner
The stress relieving process is almost never so thorough as to remove all stresses. A weldment like the one above would be very difficult to stress relieve entirely. Therefore, 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 treating facility can give you a multiple soak or longer soak. Meaning maybe three separate 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. R.S. (The Virtual Machine Shop. 2011)
Video
Watch this 2:12 video on Heating Treating from Tooling U-SME (2014).
Watch this 8:59 old video about Grain Structure of Steel by Total Training Support, December 20, 2016.
Watch this 18:16 video Heat Treatment – Types (Including Annealing), Process and Structures (Principles of Metallurgy) by Matallurgy Data, May 26, 2020
Watch this 7:45 video What is the Different Types of Heat Treatment in Metallurgy? by James Sword Engineering, July 16, 2022
Watch this 16:50 video Heat Treating For Beginners by Blondihacks, December 16, 2023.
The following chapter relies on the basic understanding of the chemical structure and how heat affects that structure. That structural change is the reason why physical and mechanical properties change. Heat treating is the area where all metals tradespeople need to understand in order to manufacture, fabricate, and produce quality metal parts.
Derived from The Virtual Machine Shop via the WayBack Machine Heat Treating – The Heat Treating Process(2011) accessed and available 17 January 2024.
