Besides carburizing, flame hardening is another common technique for developing a harder surface layer on metals. Flame hardened surfaces are harder, more wear resistant and less likely to deform. Just like other hardening techniques, flame hardening is suitable for gear teeth, drive shafts, railroad wheels, nozzles and other high wear surfaces.

What is flame hardening?

Flame hardening is a common technique for hardening due to its simplicity. Compared to carburization and case hardening, flame hardening is much more easily performed, yet less tightly controlled.

Flame hardened sprocket. Source: Zaereth/CC BY-SA 4.0

Metallurgically speaking, flame hardening is the process of heating the metal above its austenitizing temperature. Heating is typically performed with an open flame, hence the name of the process. Once the surface has been heated thoroughly, it is quenched rapidly to form martensite. Quenching can be performed by spraying or dipping the part in water or oil. The key feature is that the cooling rate is fast enough to turn the surface layer into martensite, rather than pearlite or bainite microstructures. From there, the surface is reheated to temper the martensite.

Temper refers to the heating of the alloy in such a way as to relieve stress. The diffusion of carbon from the surface to deeper in the material is the primary method of relieving stress. While the tempered martensite is a little softer than the quenched martensite, it is much stronger and much less brittle.

Flame hardenable materials

Not all materials are flame hardenable. Flame hardening is only performed on hardenable steel that can easily undergo martensitic transformation, such as mid-carbon steels. However, unlike carburization, many cast irons are also suitable for flame hardening. Because the key component is the austenite phase, some stainless steel alloys can also be flame hardened, as they can be austenitized and quenched to form martensite.

As-quenched martensite is brittle and not suitable for any physical application. It is to be avoided at all costs. Ideally, only the surface layer (up to a few millimeters) is austenitized, quenched and tempered. If the austenitization step is too severe, some of the internal toughness can be affected. Instead of getting a hard, wear resistant layer surrounding a tough core, there is a chance that some material will not be adequately quenched in time, leading to a pearlite or bainite microstructure. That in itself is not a problem, though the hard layer may not be as thick.

Even worse problems arise when the austenitization and the quench are performed correctly and the tempering step is not as long or as hot as it should be. In this case, there is a hard outer layer, a hard but brittle middle layer and a soft but tough inner core. In this case, the part is prone to fracture in the brittle layer, meaning missing teeth on gears and sprockets, chips from crankshafts and other similar problems.

Advantages and limitations of flame hardening

Flame hardening can be performed in many small machine shops or mechanic shops. In the simplest case, steel can be flame hardened with a welding or brazing torch by an experienced mechanic. More complicated shapes can be evenly heated by placing them in a lathe and rotating them through the flame. However, open flame hardening should not be performed in a place where flammable vapors may be present, and more than one mechanic has accidentally set fire to the shop attempting flame hardening in the presence of gasoline fumes.

Another advantage is that the heat-treating process can be simplified by the little surface preparation that is required. In general, epoxies, moisture, oils, grease, wax and other contaminants should be removed from the surface to be hardened. A few passes with the torch will burn off or evaporate most of these types of common contaminants.

Besides small shops, flame hardening can be performed on larger parts under the right circumstances. In some cases, flame hardening can be performed on parts that are too large to put in a furnace for case hardening. This must be done under careful consideration for heat transfer, as a large steel component will transfer heat quickly away from the flame, potentially losing tempers and other heat treatments. It can pull heat away from the surface quickly enough to austenitize a much larger region than anticipated.

Also, heat can be pulled away from the desired part and begin to heat treat other parts, if left attached to an existing assembly. This can cause some other piece to lose its temper, become brittle or cause numerous other problems. Some vehicle components are labeled “Heat Treatable Do Not Weld,” and that statement should include “do not flame harden.” When someone “loses their temper” and gets angry, it is a reference to steel losing its temper, meaning it becomes less hard and less wear resistant.

One limitation of flame hardening is that it is often less tightly controlled than other hardening techniques. The garage mechanic flame hardening a part is barely a repeatable process; it takes skill to ensure that the heating occurs evenly and to the proper depth, and that the quench is performed identically each time. Large parts and complex geometries often require multiple torches in the hands of skilled workers, as it is difficult to measure how the metal is heating internally. Without this information, determining the thickness of the flame hardened layer is, at best, an educated guess, without the help of thermal modeling.

Final thoughts

Flame hardening will always be a useful technique for hardening the surface of materials. Unlike other methods, it can be performed relatively easily, with a simple welding or brazing torch, making it a favorite technique for machine shops.

The flame hardening process is often not as tightly controlled, meaning inconsistencies part to part are common. Statistical testing of repeated parts, often destructive testing, should be performed periodically to ensure that hardening is occurring to the proper depth and that there is no residual quenched martensite left after tempering.