Editing Quenching (section)

==Quench hardening==
Quench hardening is a mechanical process in which steel and cast iron alloys are strengthened and hardened. These metals consist of ferrous metals and alloys. This is done by heating the material to a certain temperature, depending on the material. This produces a harder material by either surface hardening or through-hardening varying on the rate at which the material is cooled. The material is then often [[tempering (metallurgy)|tempered]] to reduce the brittleness that may increase from the quench hardening process. Items that may be quenched include gears, shafts, and wear blocks.

=== Purpose ===
Before hardening, cast steels and iron are of a uniform and lamellar (or layered) [[Pearlite|pearlitic]] grain structure. This is a mixture of [[Allotropes of iron|ferrite]] and [[cementite]] formed when steel or cast iron are manufactured and cooled at a slow rate. Pearlite is not an ideal material for many common applications of steel alloys as it is quite soft. By heating pearlite past its eutectoid transition temperature of 727&nbsp;°C and then rapidly cooling, some of the material's crystal structure can be transformed into a much harder structure known as martensite. Steels with this martensitic structure are often used in applications when the workpiece must be highly resistant to deformation, such as the cutting edge of blades. This is very efficient. {{Why|reason=Why is this more fficient?|date=July 2024}}

===Process===
The process of quenching is a progression, beginning with heating the sample. Most materials are heated to between {{convert|815|and|900|C|F}}, with careful attention paid to keeping temperatures throughout the workpiece uniform. Minimizing uneven heating and overheating is key to imparting desired material properties.

The second step in the quenching process is soaking. Workpieces can be soaked in air (air furnace), a liquid bath, or a vacuum. The recommended time allocation in salt or lead baths is up to 6 minutes. Soaking times can range a little higher within a vacuum. As in the heating step, it is important that the temperature throughout the sample remains as uniform as possible during soaking.

Once the workpiece has finished soaking, it moves on to the cooling step. During this step, the part is submerged into some kind of quenching fluid; different quenching fluids can have a significant effect on the final characteristics of a quenched part. Water is one of the most efficient quenching media where maximum hardness is desired, but there is a small chance that it may cause distortion and tiny cracking. When hardness can be sacrificed, mineral oils are often used. These oil-based fluids often oxidize and form sludge during quenching, which consequently lowers the efficiency of the process. The cooling rate of oil is much less than water. Intermediate rates between water and oil can be obtained with a purpose-formulated quenchant, a substance with an inverse solubility that therefore deposits on the object to slow the rate of cooling.

Quenching can also be accomplished using inert gases, such as nitrogen and noble gases. Nitrogen is commonly used at greater than atmospheric pressure ranging up to 20 bar absolute. Helium is also used because its thermal capacity is greater than nitrogen. Alternatively, argon can be used; however, its density requires significantly more energy to move, and its thermal capacity is less than the alternatives. To minimize distortion in the workpiece, long cylindrical workpieces are quenched vertically; flat workpieces are quenched on the edge; and thick sections should enter the bath first. To prevent steam bubbles the bath is agitated.

Often, after quenching, an iron or steel alloy will be excessively hard and brittle due to an overabundance of martensite. In these cases, another heat treatment technique known as [[Tempering (metallurgy)|tempering]] is performed on the quenched material to increase the [[toughness]] of [[iron]]-based [[alloy]]s. Tempering is usually performed after [[Hardening (metallurgy)|hardening]], to reduce some of the excess [[hardness]], and is done by heating the metal to some temperature below the [[Critical point (thermodynamics)|critical point]] for a certain period of time, then allowing it to cool in still air.

===Mechanism of heat removal during quenching===
Heat is removed in three particular stages:

'''Stage A: Vapor bubbles formed over metal and starts cooling'''

During this stage, due to the [[Leidenfrost effect]], the object is fully surrounded by vapor which insulates it from the rest of the liquid.

'''Stage B: Vapor-transport cooling'''

Once the temperature has dropped enough, the vapor layer will destabilize and the liquid will be able to fully contact the object and heat will be removed much more quickly.

'''Stage C: Liquid cooling'''

This stage occurs when the temperature of the object is below the boiling point of the liquid.