Low-Pressure Carburization for High Hardness & Low Distortion

Immense pressure on industrial manufacturers to produce higher quality parts often results in advances in heat treatment techniques and technology.

Low-pressure carburizing, also called vacuum carburizing, has followed that formula. This case hardening process has matured as industry stakeholders and metallurgists together realized its great potential to improve parts.
The automotive industry has led the way, recognizing that low-pressure carburizing provides enhanced control over part chemistry and significantly reduces distortion risk. And in some select cases, it can save money.

How Low-Pressure Carburization Works

Low-pressure carburizing is completed in vacuum furnaces in millibar (mb) range (the normal atmospheric pressure at the Earth’s surface is about 1 bar, or 100,000 Pa). The process is like other case hardening processes in that a hard case forms around a steel part when carbon is diffused into its surface.

A carbon-carrier gas pumped into the furnace supplies the carbon necessary for carburization. It’s similar to popular nitriding treatments in which nitrogen-bearing gases are applied. Carbon-carriers used in low-pressure carburizing include acetylene (C₂H₂), propane (C₃H₈) and methane (CH₄). Of the three, acetylene is the most commonly used because it contains the best ratio of available carbon.

The high heat of low-pressure carburization breaks the chemical bonds that hold carbon-carrier gases together. It’s also sufficient to cause changes in the steel structure that permit the diffusion of carbon atoms. In fact, low-pressure carburizing can be conducted at higher temperatures than other case hardening treatments.
Low-pressure carburizing offers some key technical advantages compared to other case hardening treatments:

• Case formation and desired case depths are more consistent in parts with complex geometries.
• Intergranular oxidation is eliminated because no oxygen-containing gases are present in the furnace chamber.
• Enhanced process control allows metallurgists to achieve more specific carbon gradients throughout the depth of the case.
• Parts emerge from heat treatment cleaner after gas quenching, reducing the need to clean.

Low-pressure carburizing is best suited for highly hardenable steel parts designed with irregular or intricate shapes. The automotive industry has increasingly turned to the process for heat treatment of critical parts like gears, splines, spindles, shafts, axles, nozzles and injectors.

Common steel types include AISI 4118 steels featuring chromium and molybdenum as primary alloying elements; AISI 8620 steels with nickel, chromium and molybdenum as primary alloying elements and boron steels like AISI 10B20 and 15B21.

Key Advantages of Vacuum Carburizing

When appropriately specified, low-pressure carburizing is faster and more efficient than other heat treatments where diffusion of carbon is required.

First, because low-pressure carburizing occurs in vacuum furnaces, higher temperatures can be achieved. At higher temperatures, treatments can be accelerated. For example, for most materials, a gas carburization treatment completed at 1,750 degrees Fahrenheit for 24 hours in a traditional batch furnace could be done in half the time at 1,850 degrees in a vacuum furnace.

Second, vacuum furnaces allow more precise application of the gases necessary to achieve specific part characteristics. Plus, the closed chamber of a vacuum furnace requires only brief, periodic carburizing gas cycles to achieve results that would otherwise require continuous gas application in a batch furnace. This process control widens the range of case characteristics and case depths that can be achieved. That makes a wider variety of parts in more applications good candidates for low-pressure carburizing.

Third, vacuum furnaces allow for more uniform and controlled gas quenching. That’s opposed to oil quenches that feature steeper thermal gradients and are therefore more severe. As quench severity increases, do does the risk that parts will distort.

Finally, vacuum carburizing with a gas quench is a comparatively “green” process that uses no oil and far less gas than conventional batch furnace treatments.

On the Leading Edge of Vacuum Carburizing

As more advantages of vacuum carburization emerge in automotive and other applications, it’s a good bet the process will continue increasing in popularity. As it does, Paulo will be at the forefront.
We’re an IATF 16949-certified heat treater with broad experience serving the automotive industry, both as a heat treatment provider and as a research and development partner. When we combine that expertise with leading quality systems and the best metallurgists in the business, we make good on our goal to be more like a department within our customers’ own facilities.

If you’re considering a low-pressure carburizing project, request a quote and we’ll talk it over together. Also, see a big-picture view of the role heat treatment plays in industrial operations by reading our Heat treating 101 guide.