Induction Heat-Treating Metal: Common Side Effects

Induction heat treating is a fast, effective means of improving mechanical properties of carbon steels and carbon alloy steels. It improves strength and hardness. The depth of the heated and hardened surface will depend upon several factors, including the carbon content and alloy contents of the steel, process used and more. Most companies familiar with the process are aware of the side effects of induction heat treatment, but for those new to the process, a short primer from Zion Industries, Inc., a premier induction heat treating company, can let you know what to expect.

1. The parts will change color. The first and most obvious result of heat treatment is that the portion of the material that has been heat treated will be darker than the rest of the part. This is a natural result of heat treating. Metal starts to change colors at around 300 degrees Fahrenheit. The progression of color as the temperature rises is first golden, then a bronze color, progressing to blue and eventually, black. As heat treated material is heated to 1500 degrees F and above, the heat treated portion will be quite dark, the surrounding areas will also change color in the tempering operation.
 
After induction another necessary step is the tempering process. Depending on the heat levels during tempering, even areas of the material that were not induction treated will take on a golden color. For example, if the teeth on a gear need heat induction treatment the teeth will be dark, the rest of part will be lighter will change color corresponding to the temperatures used for tempering.

2. This color can be remedied. Although induction heating will change the color of the metal portions being treated, if a part is highly visible and needs to retain its original tone, additional stock can be left on the part for cosmetic purposes to return the part to a shiny surface color. A grinding operation removes the discolored layer at the surface.
 
This step may not be necessary if the part being heat treated is on the interior of an assembly and the cosmetic requirements do not apply.

3. Some flaking may occur. When the metal surface interacts with oxygen and induction heating temperatures or tempering temperatures rise above 1,000 degrees, some flaking of small pieces can develop. A descaling operation can be performed to remove flakes.
 
Some materials may scale more than others. This scaling can be detrimental to the overall system performance in closed system such as an engine or transmission which is why descaling is a necessary, precautionary step.

4. Size changes and distortion may occur. The fast, localized heating and rapid quenching involved in the induction heat treating process changes the structure of carbon steel. All heat-treating processes change the structure of the material. The material’s chemical structure locks into a new, stronger formation called martensite. The heating and cooling process must be controlled and repeatable to ensure consistent transformation to martensite. Martensite can leave the steel brittle and unstable, tempering will stabilize the structure and achieve a hardness in the required range. Zion Industries works with our customers to ensure that the heat treating, and tempering process specified are best suited for the application.
 
This change into martensite produces size differentials and distortion in the metal as the martensitic structure is larger. Steel shafts and bars as well as flat geometrical planes in gears or sprockets can be affected. A straightening operation or flattening operation will help return shafts and plates to their specified size and with some parts, a grinding operation may be required to solve the issue. The best time to address potential distortion is during the design phase. This is why…

5. Samples are critical. Induction heat treating a sample part can help determine how much distortion will occur. In order to have a finished part that is the right size and fits within specifications, the initial design must factor in the distortion that occurs during heat treating and tempering.
 
There are different terms for the distortion that occurs in certain geometric shapes once induction heat treated. A flat plate design, such as a gear or sprocket that requires heat induction for the outside teeth can suffer distortion in the interior section of the gear.  Stress from the process creates a “dish” in the center or “potato chipping” due to the final shape after induction. A shaft may “bow” or “twist” after the induction process and the part may “grow” or “shrink” in size.
 
A design element often used to counteract distortion involves putting holes or slots in the center of the gear or sprocket so when the metal experiences the stress of heat induction, it has a place to move. Distortion can and should be factored into your part’s design phase.

Factors that can impact distortion during heat induction can include:

  • Geometries that include thin cross section or slots
  • Sulfur inclusions in re-sulfurized material can open into seams during the heat-treating process.
  • Material specification—Certain materials can be more prone to cracking during the heat-treating process and the resulting size and hardness changes in the material.
  • Design phase—Secondary operations such as grinding or other machining operations should be factored into the design to ensure the finished part meets specifications after treatment

Zion Industries has multiple locations to serve you, with operations in Ohio, Michigan and North Carolina. Call us today at 330-483-4650, request a quote or contact us to collaborate on your next heat induction project.
 

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