There is all kinds of "heat treating". If you heated them to cherry red then quenched them that is flame surface hardening. Done with an electromagnet it is induction hardening.
While there are many methods and processes to heat treat metal, most ferrous metal requires a second step to increase ductility and fracture resistance.
Although the source is questionable, this info from Wikipedia speaks in laymans terms:
Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. It is also a technique used to increase the toughness of glass. 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.
Very few metals react to heat treatment in the same manner, or to the same extent, that steel does. Steel can be softened to a very malleable state through annealing, or it can be hardened to a state nearly as rigid and brittle as glass by quenching. However, in its hardened state, steel is usually far too brittle, lacking the structural integrity to be useful for most applications. 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. This allows the metal to bend before breaking. Depending on how much temper is imparted to the steel, it may bend elastically (the steel returns to its original shape once the load is removed), or it may bend plastically (the steel does not return to its original shape, resulting in permanent deformation), before fracturing. 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. Tools such as hammers and wrenches require good resistance to abrasion, impact resistance, and resistance to deformation. Springs do not require as much rigidity, but must deform elastically before breaking. Automotive parts tend to be a little less rigid, but need to deform plastically before breaking.
Except in rare cases where maximum rigidity and hardness are needed, such as the untempered steel used for files, quenched steel is almost always tempered to some degree. However, steel is sometimes annealed through a process called normalizing, leaving the steel only partially softened. 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.[3]
Quenched-steel
Tempering is most often performed on steel that has been heated above its upper critical (A3) 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. The quenched-steel, being placed in, or very near, its hardest possible state, is then tempered to incrementally decrease the hardness to a point more suitable for the desired application. The hardness of the quenched-steel depends on both cooling speed and on the composition of the alloy. Steel with a high carbon-content will reach a much harder state than steel with a low carbon-content. 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. The amount of time held at the tempering temperature also has an effect. 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. This range is usually avoided. 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.[3]
Normalized steel
Steel that has been heated above its upper critical temperature and then cooled in standing air is called normalized steel. This produces steel that is much stronger than full-annealed steel, and much tougher than tempered quenched steel. However, added toughness is sometimes needed at a reduction in strength. Tempering provides a way to carefully decrease the hardness of the steel, thereby increasing the toughness to a more desirable point. Cast-steel is often normalized rather than annealed, to decrease the amount of distortion that can occur. Tempering can further decrease the hardness, increasing the ductility to a point more like annealed steel.[8] 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. This can make the metal more suitable for its intended use and easier to machine.[9]
