AMIE part A material science and engineering study note for summer 2012 'Tempering'

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys.

For metals, tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to a much lower temperature than was used for hardening.

The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product.

For instance, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.

In glass, tempering is performed by heating the glass and then quickly cooling the surface, increasing the toughness.

Tempering is a heat treatment technique applied to ferrous alloys, such as steel or cast iron, to achieve greater toughness by decreasing the hardness of the alloy.

Tempering is usually performed after quenching, which is rapid cooling of the metal to put it in its hardest state.

Tempering is accomplished by controlled heating of the quenched work-piece to a temperature below its "lower critical temperature".

This is also called the lower transformation temperature or lower arrest (A) temperature; the temperature at which the crystalline phases of the alloy, called ferrite and cementite, begin combining to form a single-phase solid solution referred to as austenite.

Precise control of time and temperature during the tempering process is critical to achieve the desired balance of physical properties.

Low tempering temperatures may only relieve some of the internal stresses, decreasing brittleness while maintaining a majority of the hardness.

Higher tempering temperatures tend to produce a greater reduction in the hardness, sacrificing some yield strength and tensile strength for an increase in elasticity and plasticity.

However, in some low alloy steels, containing other elements like chromium and molybdenum, tempering at low temperatures may produce an increase in hardness, while at higher temperatures the hardness will decrease.

Many steels with high concentrations of these alloying elements behave like precipitation hardening alloys, which produce the opposite effects under the conditions found in quenching and tempering, and are referred to a maraging steels.

In carbon steels, tempering alters the size and distribution of carbides in the martensite, forming a microstructure called "tempered martensite".

Tempering is also performed on normalized steels and cast irons, to increase ductility, machinability, and impact strength.

In tempered glass, tempering is accomplished by creating internal stresses in the amorphous structure, to increase both impact resistance and safety in the event of breakage.

Tempering is an ancient heat-treating technique.

Many different methods and cooling baths for quenching have been attempted during ancient times, from quenching in urine, blood, or metals like mercury or lead, but the process of tempering has remained relatively unchanged over the ages.

Tempering was often confused with quenching and, oftentimes, the term was used to describe both techniques.

Tempering is a method used to decrease the hardness, thereby increasing the ductility of the quenched steel, to impart some springiness and malleability to the metal.

Tempering is used to precisely balance the mechanical properties of the metal, such as shear strength, yield strength, hardness, ductility and tensile strength, to achieve any number of a combination of properties, making the steel useful for a wide variety of applications.

Tempering is sometimes used on normalized steels to further soften it, increasing the malleability and machinability for easier metalworking.

Tempering may also be used on welded steel, to relieve some of the stresses and excess hardness created in the heat affected zone around the weld.

Tempering is most often performed on steel that has been heated above its upper critical (A) temperature and then quickly cooled, in a process called quenching, using methods such as immersing the red-hot steel in water, oil, or forced-air.

Likewise, tempering high-carbon steel to a certain temperature will produce steel that is considerably harder than low-carbon steel that is tempered at the same temperature.

Tempering at a slightly elevated temperature for a shorter time may produce the same effect as tempering at a lower temperature for a longer time.

Tempering times vary, depending on the carbon content, size, and desired application of the steel, but typically range from a few minutes to a few hours.

Tempering quenched-steel at very low temperatures, between 66 and 148 °C (151 and 298 °F), will usually not have much effect other than a slight relief of some of the internal stresses.

Tempering at higher temperatures, from 148 to 205 °C (298 to 401 °F), will produce a slight reduction in hardness, but will primarily relieve much of the internal stresses.

Tempering in the range of 260 and 340 °C (500 and 644 °F) causes a decrease in ductility and an increase in brittleness, and is referred to as the "tempered martensite embrittlement" (TME) range.

Steel requiring more strength than toughness, such as tools, are usually not tempered above 205 °C (401 °F).

When increased toughness is desired at the expense of strength, higher tempering temperatures, from 370 to 540 °C (698 to 1,004 °F), are used.

Tempering at even higher temperatures, between 540 and 600 °C (1,004 and 1,112 °F), will produce excellent toughness, but at a serious reduction in the strength and hardness.

At 600 °C (1,112 °F), the steel experiences another stage of embrittlement, called "temper embrittlement" (TE), so heating above this temperature is also avoided.

Tempering provides a way to carefully decrease the hardness of the steel, thereby increasing the toughness to a more desirable point.

Tempering can further decrease the hardness, increasing the ductility to a point more like annealed steel.

Tempering is often used on carbon steels, producing much the same results.

The process, called "normalize and temper", is used frequently on steels such as 1045 carbon steel, or most other steels containing 0.35 to 0.55% carbon.

These steels are usually tempered after normalizing, to increase the toughness and relieve internal stresses.

Tempering is sometimes used in place of stress relieving to both reduce the internal stresses and to decrease the brittleness around the weld.

Tempering temperatures for this purpose are generally around 205 °C (401 °F) and 343 °C (649 °F).

Tempering was originally a process used and developed by blacksmiths (forgers of iron).

Because few methods of precisely measuring temperature existed until modern times, temperature was usually judged by watching the tempering colors of the metal.

Because tempering often consisted of heating above a charcoal or coal forge, or by fire, holding the work at exactly the right temperature for the right amount of time was usually not possible.

Tempering was usually performed by slowly, evenly overheating the metal, as judged by the color, and then immediately cooling in open air.

As the temperature of the steel is increased, the thickness of the iron oxide will also increase.

These colors appear at very precise temperatures, and provide the blacksmith with a very accurate gauge for measuring the temperature.

Steel in a tempering oven, held at 205 °C (401 °F) for a long time, will begin to turn brown, purple or blue, even though the temperature did not exceed that needed to produce a light-straw color.

These methods consist of quenching to a specific temperature that is above the martensite start (M) temperature, and then holding at that temperature for extended amounts of time.

Depending on the temperature and the amount of time, this allows either pure bainite to form, or holds-off forming the martensite until much of the internal stresses relax.

The steel is then held at the bainite-forming temperature, beyond the point where the temperature reaches an equilibrium, until the bainite fully forms.

The steel is quenched to a much lower temperature than is used for austempering; to just above the martensite start temperature.

The metal is then held at this temperature until the temperature of the steel reaches an equilibrium.

Tempering involves a three-step process in which unstable martensite decomposes into ferrite and unstable carbides, and finally into stable cementite.

Depending on the carbon content, it also contains a small amount of "retained austenite", which is unable to transform into martensite even after quenching below the martensite finish (M) temperature.

If tempered at higher temperatures, between 650 °C (1,202 °F) and 700 °C (1,292 °F), or for longer amounts of time, the martensite may become fully ferritic and the cementite may become coarser or spheroidize (become spherical in shape).

Embrittlement occurs during tempering when, through a specific temperature range, the steel experiences an increase in hardness and a reduction in ductility, as opposed to the normal decrease in hardness that occurs to either side of this range.

The first type is called tempered martensite embrittlement (TME) or one-step embrittlement.

One-step embrittlement usually occurs in carbon steel at temperatures between 230 °C (446 °F) and 290 °C (554 °F), and was historically referred to as "500 °F embrittlement."

Two-step embrittlement typically occurs by aging the metal within a critical temperature range, or by slowly cooling it through that range, For carbon steel, this is typically between 370 °C (698 °F) and 560 °C (1,040 °F), although impurities like phosphorus and sulfur increase the effect dramatically.

The embrittlement can often be avoided by quickly cooling the metal after tempering.

Two methods of tempering are used, called "white tempering" and "black tempering."

Unlike white tempering, black tempering is done in an inert gas environment, so that the decomposing carbon does not burn off.

Instead, the decomposing carbon turns into a type of graphite called "temper graphite" or "flaky graphite," increasing the malleability of the metal.

Tempering is usually performed at temperatures as high as 950 °C (1,740 °F) for up to 20 hours.

Precipitation hardening alloys first came into use during the early 1900s.

Most heat-treatable alloys fall into the category of precipitation hardening alloys, including alloys of aluminum, magnesium, titanium and nickel.

Several high-alloy steels are also precipitation hardening alloys.

These alloys become softer than normal when quenched, and then harden over time.

Although most precipitation hardening alloys will harden at room temperature, some will only harden at elevated temperatures and, in others, the process can be sped up by aging at elevated temperatures.

Although the method is similar to tempering, the term "tempering" is usually not used to describe artificial aging, because the physical processes, (i.e.: precipitation of intermetallic phases from a supersaturated alloy) the desired results, (i.e.: strengthening rather than softening), and the amount of time held at a certain temperature are very different from tempering as used in carbon-steel.

Tempering, or toughening, of glass is a process in which glass is first heated above its annealing temperature (about 720°C), and then rapidly cooled by jets of cool air, thus hardening the surface of the glass and leaving the center soft for a period of time.

Another advantage is that tempered glass can be up to four times stronger than regular glass.