An overview of heat treatment techniques

Editor’s note > This is the first in a five-part series.

I recently had the wondrous experience of being invited to tour a very modern state-of-the-art heat-treatment system fabrication facility. I want to say thanks to my friends at SECO/ Warwick and SECO / Vacuum for the tour, the technical discussions regarding their technology, and finally the opportunity to present in their e-Seminar 4.2.

This article is the first in a five-part series wherein I will cover all the most common, major heat-treating techniques and equipment as well as taking a look ahead to the future of heat-treating metals. This will be an extension of a series of articles and technical summaries of industry today.

There are four main categories of heat-treatment techniques and equipment. The order in which I have presented does not mean to imply a chronology or precedence in terms of the end result.

Carburizing is a case hardening process in which a metal part or component of low carbon content is heated in a carbon-rich gas atmosphere. The process of heating the metal component in a high carbon environment allows for diffusion of carbon atoms directly into the surface of the part that needs to be case hardened. The enhanced performance of case carburized parts, especially gears, is that the process induces tensile stress in the surface of the material.

Carburizing is a thermochemical process in which carbon is diffused into the surface of low carbon steels to increase the carbon content to sufficient levels so that the surface will respond to heat treatment and produce a hard, wear-resistant layer. There are three traditional types of carburizing commonly used:

• Gas carburizing.
• Liquid carburizing (or cyaniding).
• Solid (pack) carburizing.

All three processes rely on the transformation of austenite into martensite during quenching. The increase in carbon content at the surface must be high enough to give a martensitic layer with sufficient hardness, typically 58 to 62 HR_C to provide a wear-resistant surface. The required carbon content at the surface after diffusion is usually 0.6 percent to 1.00 percent carbon. These processes can be carried out on a wide range of plain carbon steels, alloy steels, and cast irons where the bulk carbon content is a maximum of 0.40 percent and usually less than 0.25 percent carbon. Incorrect heat treatment can lead to oxidation or de-carburization. Although a relatively slow process, carburizing can be used as a continuous process and is suitable for high-volume surface hardening.

An overview of the four major techniques:

Gas Carburizing: In gas carburizing, the component is held in a furnace containing an atmosphere of methane or propane with a neutral carrier gas, usually a mixture of N₂, CO, CO₂, H₂ and CH₄. At the carburizing temperature, methane (or propane) decomposes at the component surface to atomic carbon and hydrogen, with the carbon diffusing into the surface. In most cases, the carburizing atmosphere is created from methane or propane and is produced in a special device (the endothermic generator) by gases partially oxidated with air. The reaction for methane is: 2CH₄ + O₂ (+N₂) => 2CO + 4H₂ (+N₂) and similar for propane. The atmosphere consists mainly of CO, H₂, and N₂, while the main C carrier is CO (reaction 2CO => C + CO₂), while CO₂, H₂O, O₂, and CH₄ are residual gases. The carburizing atmosphere is delivered to a furnace from the endothermic generator (not directly), and methane or propane is injected directly into the furnace in small amounts to compensate for the carbon absorbed by steel. Methanol is the only agent which can create the carburizing atmosphere directly in a furnace and decomposes accordingly: CH₃OH => CO + 2H₂. The temperature is typically 925°C and carburizing times range from five to six hours for a 1 (mm) depth case to as many as of around 90 hours for a 4 (mm) case. The quenching medium is usually oil, but can be water, brine, caustic soda, or polymer, and neutral gas under high pressure.

Liquid Carburizing (or Cyaniding): Liquid or cyanide carburizing is carried out by placing the component in a salt bath at a temperature of 845°C to 955°C. The salt is usually a cyanide-chloride-carbonate mixture and is highly toxic. The cyanide salts introduce a small amount of nitrogen into the surface, which further improves its hardness. Although it’s the fastest carburizing process, it is suitable only for small batch sizes.

Solid (Pack) Carburizing: In solid or pack carburizing, the components are surrounded by a carburizing medium and placed in a sealed box. The medium is usually coke or charcoal mixed with barium carbonate. The process is really one of gas carburization since the CO produced dissociates into CO₂ and carbon, which diffuses into the components’ surface. Temperatures are usually 790°C to 845°C for times of two to 36 hours. Pack carburizing is the least sophisticated carburizing process and as a result remains a very limited use method, as such I will not cover this process or method in any detail.

Carbonitriding (Carbo-Nitriding): Carbonitriding is undertaken on a similar range of steels although the bulk carbon content can be as high as 0.4 to 0.5 percent carbon. The process is particularly suited for hardening the surface of components that need a through-hardened core, such as gears and shafts. Carbonitriding is a modification of gas carburization where ammonia is added and is the source of nitrogen. By carbonitriding the metal, technicians increase the strength of the material. It is stronger and more resistant to wear and tear. The carbon and nitrogen diffused on the surface creates increased strength on the surface while maintaining a lesser hardness at the core.

What are carburized gears? Carburizing is a widely used, effective technique to increase surface hardness of steel used in gears and achieve compressive residual stresses. There are several methods of carburizing, which is the addition of carbon to the surface of low carbon steels. In gears, and specifically thinking about high cycle fatigue, the load on the surface of the tooth is applied directly on the face of the tooth. This induces a compressive stress within the contact zone or patch (as it is commonly called). In the unloaded condition, the non-heat-treated face of the gear tooth is at zero compressive stress. When a contact load is applied to one gear tooth, from contact with its gear mate, the center of the contact patch deflects to provide enough area in the contact patch that the surface stress is near the yield stress of the material.

In previous articles I have discussed the additional effect of the induced shear stress due to the sliding component of tooth-to-tooth interaction and will not repeat it here. Suffice it to say that any induced shear stress on the surface of the tooth face will theoretically push the state of stress over the yield material limit. We heat treat gear materials, especially case carburizing, to induce a component of tensile stress as a means to offset or prestress the surface of our gear. However, a highly hardened material becomes functionally brittle. To mitigate this adverse effect, we only case carburize the surface over a ductile core. The ductile core supports the hard (brittle) surface that provides a means to deal with the applied compressive stress.

So, let’s review some of the common and very effective heat-treatment methods and equipment.

LPC: Low Pressure Carburizing

Low Pressure Carburizing (LPC), also known as Vacuum Carburizing, is similar to other case hardening processes and is a carburizing process, a part of case hardening process similar to other carburizing or carbonitriding, followed by quenching. The goal of LPC or vacuum carburizing is to obtain a part with a solid, tough core and a hard, wear-resistant surface. LPC has been established as one of the most popular case-hardening processes. Low-pressure vacuum carburizing is a state-of-the-art thermal process where carburizing is put into the furnace under very low pressures (7-13 mbar). Parts are first heated in vacuum to above the transformation temperature of the alloy and then exposed to carbon-carrying gas, or gas mixtures, under partial pressure. There are three types of carburizing commonly used: gas carburizing, liquid carburizing (or cyaniding), and solid (pack) carburizing. Similar to other case hardening processes, the goal of Vacuum Carburizing is to obtain a part with a solid, tough core and a hard, wear-resistant surface. LPC has been established as one of the most popular case hardening processes. It is applied to increase the fatigue limit of dynamically loaded components. Case hardening essentially consists of three steps: Parts are first austenitized, then carburized, and once the required carbon profile is reached, they are quenched, which is followed by a tempering step.

The LPC process takes place in a temperature range between 870 to 1,050°C with a pressure range between 5 and 15 (mbar). In most cases, the vacuum carburizing temperature is between 920 and 980°C. During the complete process, the treated components are not exposed to any traces of oxygen.

Oxygen-free hydrocarbons such as acetylene C₂H₂ (ethyne) are used as the carbon source. The hydrocarbons are injected into the furnace chamber, creating a pressure of a few millibars. The use of acetylene is recommended since it provides a homogenous carburizing even for complex shapes, such as bores and even blind holes. Once the targeted carbon profile is obtained, the parts are quenched. In most cases, oil quenching is applied, but High Pressure Gas Quenching (HPGQ) becomes more and more popular after LPC with pressures up to 25 bar of nitrogen or helium.

This is a case-hardening process, carried out in a vacuum furnace using hydrocarbon gases at very low pressure and elevated temperatures to obtain a hardened surface layer of tempered martensite and a tough core. The equipment used to do this is sometimes referred to as an LPC Boiler. The treatment is used to increase the wear resistance and fatigue life of components, and we are likely aware of case carburizing steels such as 8620H, which is used often in high-performance gears. LPC produces parts with high hardness below the surface, which compares with more conventionally carburized parts with faster cycle times. Parts can be carburized between 930°C and 1,000 °C / 1,700°F and 1,830°F. Penetration of carbon in deep blind holes results in uniform hardness on the entire profile.

The benefits of carburizing over conventional hardening heat treatment are that carburizing increases resistance to wear by creating a strong shell while maintaining a softer interior. It is often performed after the parts are rough or green machined and before finish machining or grinding. The benefits are the gears remain durable, the process enhances wear resistance and durability, increases corrosion resistance, improves the effect of the ductile core, and enhances the reliability of the surface hardness. There are other benefits as well:

• Excellent carburizing homogeneity even for components with complex shapes.
• Avoiding Inter-Granular Oxidation (IGO) and surface oxidation.
• Clean surfaces of parts after heat treatment; no washing of parts necessary.
• Environmentally friendly process (small consumption of resources; no disposal of oil, salt bath residues, or detergent residues).
• Potential to reduce heat treat distortion (unwanted changes of the part-geometry during heat treatment in form and size).

Common or standardized case hardening steels such as 4120, 4320, 5120, 5130, 52100, 8625, 9310, 18CrNi8, 20MnCr5, 27MnCr5, 18CrNiMo7-6, 8620 (for small parts), 16MnCr5 (for small parts), can be treated with LPC and HPGQ. The core hardness after treatment will, of course, depend on the geometry of the treated components and on the hardenability of the chosen steel grade. If necessary, the components can be partially mechanically shielded from carburizing during the LPC process. As an example, if threads need to be kept soft by masking that area of the part, it will avoid subsequent expensive hard machining operations.

VOQ: Vacuum Oil Quench

In vacuum heat treatment, air (specifically oxygen) is removed from the furnace to create a vacuum. The part may be heated in the vacuum with or without an inert gas (typically nitrogen and argon) to achieve the desired properties while protecting the part’s surface. As in traditional heat treatment, the metal is quickly cooled. Quenching is the cooling part of the heat-treatment process that comes after other heat treatment has been performed on a part. By submerging hot metal into a quench media, it can be rapidly cooled and therefore retain the beneficial properties it gained through the heating process.

Quench oils have two primary functions. First, they harden the component by controlling heat transfer during quenching. Second, they enhance the wetting of the component in order to minimize undesirable conditions that may cause distortions and even cracking. Quench oils can range from a very generic motor oil (which is a common quenching oil used in both blacksmithing and blade-smithing applications) to new and used motor oils, which are both widely available. New motor oil is typically cheaper to use than commercial quenching oils.

Parts made of low-carbon steel and low-hardenability alloys quench better in fast oils. Hot oils are kept at much higher temperatures and are used to ensure that the core temperature and surface temperature of the part do not vary too greatly during a quench. This controls distortion and reduces the risk of cracking.

The process of heating and then quickly cooling parts is a way to achieve added hardness in the part. The heating causes changes in the crystalline structure in the surface layer(s) of the metal. The rapid cooling “freezes” those changes in the crystalline matrix and makes the surface harder. The first stage in a quench is known as the vapor stage. The submerged part is so much hotter than the quenchant, a vapor blanket forms around the part. Cooling of the part occurs during this stage, but it is impeded by the vapor, which acts as an insulator.

The second stage is the boiling stage, which is characterized by the violent boiling of the quenchant. Parts cool fastest in this stage because the temperature of the part has decreased enough during the previous stage for the vapor blanket to dissipate. With the quenchant able to contact the part unimpeded, it can carry away the most heat through boiling.

The third stage is the convective stage, during which convection and conduction further carry heat away from a part. Convection refers to the movement of a liquid due to the tendency of hotter, less dense liquids to rise while cooler, more dense liquids sink. Conduction refers to the tendency of heat to dissipate throughout a substance when there are temperature differences in the liquid. Oils are aggressively agitated, typically by mechanical means (rotating impeller, etc.), during the quench. The intent of this dynamic fluid (oil quenchant) is to force it to flow upward through a workload; natural convection becomes insignificant in the overall thermal reduction of the part.

HPGQ: High Pressure Gas Quench

The most common definition for High Pressure Gas Quenching is accelerating the rate (speed) of quenching by densification and cooling of gas. One of the reasons for the intense interest in this quenching technique is related to improved part distortion with full hardness. Gas quenching is an important step in the treatment of steel parts. The process consists of cooling the parts down from their critical temperature quickly in order to strengthen and harden the metal. This process is best suited to ferrous metals and those with high alloy content.

One benefit of the high-pressure gas quenching technique is it provides a means to reduce part distortion while still achieving the full hardness potential of the material. A critical concern is to avoid sacrifice of metallurgical, mechanical, or physical properties, while retaining the ability to transform a material to a microstructure that is similar, identical, or superior to that of a known quenching medium (e.g. oil or salt). Common quench gases include nitrogen, argon, and helium. Hydrogen, although excellent with respect to heat transfer, is seldom used as a quench gas due to safety considerations and hydrogen embrittlement concerns.

HPGQ offers a number of attractive benefits including unprecedented part cleanliness and less overall dimensional change. Fixed or variable cooling rates are applied as required to control hardness and distortion with the ability to vary quench pressure depending on load size, material type, and part section thickness. Product consistency and repeatability are excellent using high pressure gas quenching.

The goal of the gas quenching process is to improve hardness. Upon completion of austenitization, the parts are subjected to HPGQ in order to change the microstructure from austenite into martensite, thus obtaining the desired increase in hardness. One advantage to using high-pressure gas is that it treats material in a much more uniform way. While all forms of heat treatment will cause some level of distortion, there is typically 50–75 percent less distortion with gas than with oil quenching. There is also a difference in ductility between gas and oil methods. When comparing identical AISI 4140 parts that have been heat treated using the different methods, there is a higher yield strength and greater elongation in the sample treated with high-pressure gas over the oil-quenched part.

4DQ: 4D Quench

The 4D Quench is a modern alternative to a quench press. 4D quenching is a vacuum heat-treatment solution for single piece quenching with distortion control and reduction. The process replaces oil quenching with a clean, and environmentally friendly, cost-effective nitrogen quenching technology. It also provides a distortion-free alternative to oil and quench presses and the problems associated with their operation. The benefits are single piece flow vacuum heating (through hardening only). The process also provides a means to heat treat parts without generating an oxide layer due to the vacuum environment of the process. Since it does not use the press methodology, there are no press maintenance issues, which also means that the process can be a continuous, or through-flow, process. This allows for parts to be conventionally carburized, slow cooled, then processed through the 4D Quench system.

The 4D Quench system is a unique single-piece flow vacuum furnace designed to heat or re-heat products such as gears on a single conveyor deck. Each part is then individually transferred out of the vacuum furnace and into a quench chamber that uses a proprietary arrangement of cooling nozzles to surround the part (3D cooling). The part is then rotated (the 4th dimension) during the quench. This additional step to the typical process provides a uniform flow of cooling gas from all sides; top, bottom, and surround to ensure high-speed cooling. The pattern of cooling nozzles can be adjusted to fit the particular parts size and shape to provide optimum cooling rate and uniformity during the cooling process. The entire system provides uniform quenching which results in very repeatable finished part geometry with significant reduction of distortion.

Looking ahead

So, what’s next in this series? In the next four articles I intend to go into depth on each of the major heat-treating processes I have outlined:

• Gas carburizing.
• Liquid carburizing (or cyaniding).
• Solid (pack) carburizing.
• Carbonitriding (carbo-nitriding).

 For each article I am planning to discuss these aspects for each of the processes:

• The technical details of the process.
• What the process does to the material.
• What is the benefit of this / these processes and the effects they create within the material.
• Why one would choose this process over the others in the list.
• The applicability and common areas of use for the particular process.

What I want to help all of us understand is determining the applicability, effectiveness, and best choice for heat treating, depending on the final properties of the gear we need to produce.