Metal parts manufacturers today utilize a variety of heat treatments to change some of the physical or chemical properties of metals. One category of heat treatment relates to hardening only a designated portion of the surface (as opposed to the entire piece). Flame hardening represents just one of these specialized forms of selectively applied heat treatments.
In the ancient world, metal workers and blacksmiths likely discovered the benefits of flame hardening and utilized this process widely. As a heat treatment, it does not require extensive technology to implement, simply a flame source and a quenching capability. Individual blacksmiths may have developed skill in implementing this heat treatment to harden portions of the surface of items such as blades, shields, and metal armor.
Following the invention of welding equipment, manufacturers developed far greater control over the flame hardening process because they could regulate the temperatures involved more accurately. This heat treatment assisted in the generation of mass-produced goods. Today, some companies design and sell flame hardening machinery to industrial manufacturing facilities.
During flame hardening, a manufacturer either applies a heat source to a metal workpiece or passes the part through a stationary heat source. Quenching follows this step. It helps halt surface transformations abruptly.
The high temperatures generated by the heat source cause the exposed surface to begin melting very briefly, producing visible changes at a microscopic level within the superficial grain structure of this impacted material (typically limited in depth). Quenching prevents the metal from cooling slowly and resuming its previous surface grain structure, helping ensure the impact of the intense brief heat remains permanent. The flame hardening process contributes to selectively more uniform, harder (and more brittle) metal and metal alloy surfaces.
Since flame hardening does not involve protracted exposure to high temperatures, the interior of the workpiece remains unchanged. However, this heat treatment will produce a thin, hard crust over the portion of the surface directly exposed to the flame. The process helps produce a selectively harder and more abrasion and scratch-resistant exterior.
Manufacturers have devised several types of flame hardening technologies. Some sources distinguish these processes using two broad categories:
During this heat treatment, a manufacturer both heats and quenches cast metal materials simultaneously, with selected portions of the workpiece receiving heat immediately in advance of a cooling spray jet directed at the heated metal zone. This process works well in automated environments. Popular types of line hardening include:
During shell hardening, a manufacturer first heats a designated portion of a workpiece (usually a casting) and then subsequently quenches the entire flame hardened area. Many “low tech” production environments utilize this simple form of flame hardening. Some widely used types of shell hardening include:
The materials used during flame hardening and the applications for this technology elicit interest today on the part of manufacturers. While not designed to harden parts subjected to extreme environmental stresses or heavy wear, this heat treatment has become a highly cost-effective way to create more durable consumer goods.
A variety of metals and metal alloys tolerate flame hardening. Many metals utilized by blacksmiths and jewelers develop selectively harder surfaces through the use of this heat treatment, for example. In a manufacturing environment, cast iron, and low and medium carbon or alloy steels all sometimes undergo flame hardening. It potentially serves to toughen metal surfaces selectively without regard to the shape or dimensions of workpieces.
Today, materials used in flame hardening often include oxygen and acetylene gas welding torches and a potentially wide selection of quenching mediums (e.g. water, oil, and proprietary products). Additionally, designers have developed a variety of machines to perform flame hardening in production settings. Some of this equipment automates quenching via directed water jets or sprays.
While companies typically use other types of heat treatments to harden parts generated during high volume production runs, they may still employ flame hardening to treat metal parts generated in lower volumes and to harden some irregularly shaped surfaces. The limited depth of hardening constrains the use of this heat treatment for some purposes.
Popular applications for flame hardening occur in the manufacture of consumer housewares. Some manufacturers also employ flame hardening on specific industrial parts, such as rollers pulleys and cable sheaves.
This heat treatment offers numerous benefits. First, it creates selectively harder, more wear resistant surfaces in components of many sizes and shapes. Second, it helps minimize dimensional distortion. Third, this process offers a highly cost-effective way to harden parts generated in low volume production runs. Fourth, today manufacturers exert greater control over the parameters required to produce uniform selective surface hardening within low tolerance ranges using this heat treatment.
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