Quench

Coke being pushed into a quenching car, Hanna furnaces of the Great Lakes Steel Corporation, Detroit, Michigan, November 1942.

A quench refers to a rapid cooling. In polymer chemistry and materials science, quenching is used to prevent low-temperature processes such as phase transformations from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and kinetically accessible. For instance, it can reduce crystallinity and thereby increase toughness of both alloys and plastics (produced through polymerization).

In metallurgy, it is most commonly used to harden steel by introducing martensite, in which case the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers and give the illusion that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching.

Extremely rapid cooling can prevent the formation of all crystal structure, resulting in amorphous metal or "metallic glass".

Contents

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, differing upon material, and then rapidly cooling 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 tempered to reduce the brittleness that may increase from the quench hardening process. Items that may be quenched include gears, shafts, and wear blocks.

Process

Quenching metals is a progression; first step is soaking the metal. “Soaking” can be done by air (air furnace), or a bath. The soaking time in air furnaces should be 1 to 2 minutes for each millimeter of cross-section. For a bath the time can range a little higher. 0 to 6 minutes is the recommended time allotment in salt or lead baths. Uneven heating or overheating should be avoided at all cost. Most materials are heated from anywhere to 815-900 degrees Celsius (1500-1650 degrees Fahrenheit).

The next item on the progression list is the cooling of the part. Water is one of the most efficient quenching media where maximum hardness is acquired, but there is a small chance that it may cause distortion and tiny cracking. When hardness can be sacrificed, whale, cottonseed and mineral oils are used. These often tend to oxidize and form a sludge, which consequently lowers the efficiency. The quenching velocity (cooling rate) of oil is much less than water. Intermediate rates between water and oil can be obtained with water containing 10-30% Ucon, a substance with an inverse solubility which therefore deposits on the object to slow the rate of cooling.

The way that an object is placed into the containers to soak is also very important and a step that needs to be discussed. To minimize distortion loss, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles the “quenching bath” should be agitated.

Effect of Quench Hardening

Before the material is hardened, the microstructure of the material is a pearlite grain structure that is uniform and laminar. Pearlite is a mixture of ferrite and cementite formed when steel or cast iron are manufactured and cooled at a slow rate. After quench hardening, the microstructure of the material form into martensite as a fine, needle-like grain structure.[1]

Before using this technique it is essential to look up the rate constants for the quenching of the excited states of metal ions.[2]

Equipment

There are three types of furnaces that are commonly used in quench hardening: salt bath furnace [3], continuous furnace [4], and box furnace. Each are used depending on what other processes or types of quench hardening being done on different materials.

Quenching media

When quenching, there are four types of media: air, brine (salt water), oil, and water. These media are used to increase the severity of the quench.[5]

Role of quenching in scrubbing

In pollution scrubbers, sometimes hot exhaust gas is quenched, or cooled by water sprays, before entering the scrubber proper. Hot gases (those above ambient temperature) are often cooled to near the saturation level.

If not cooled, the hot gas stream can evaporate a large portion of the scrubbing liquor, adversely affecting collection efficiency and damaging scrubber internal parts. If the gases entering the scrubber are too hot, some liquid droplets may evaporate before they have a chance to contact pollutants in the exhaust stream, and others may evaporate after contact, causing captured particles to become reentrained. In some cases, quenching can actually save money.

Cooling the gases reduces the temperature and, therefore, the volume of gases,permitting the use of less expensive construction materials and a smaller scrubber vessel and fan.

A quenching system can be as simple as spraying liquid into the duct just preceding the main scrubbing vessel, or it can be a separate chamber (or tower) with its own spray system identical to a spray tower.

Quenchers are designed using the same principles as scrubbers. Increasing the gas-liquid contact in them increases their operational efficiency. Small liquid droplets cool the exhaust stream more quickly than large droplets because they evaporate more easily. Therefore, less liquid is required. However, in most scrubbing systems, approximately one-and-a-half to two and- a-half times the theoretical evaporation demand is required to ensure proper cooling (Industrial Gas Cleaning Institute 1975). Evaporation also depends on time; it does not occur instantaneously.

Therefore, the quencher should be sized to allow for an adequate exhaust stream residence time. Normal residence times range from 0.15 to 0.25 seconds for gases under 540°C (1000°F) to 0.2 to 0.3 seconds for gases hotter than 540°C (Schifftner 1979).

Quenching with recirculated scrubber liquor could potentially reduce overall scrubber performance, since recycled liquid usually contains a high level of suspended and dissolved solids. As the liquid droplets evaporate, these solids could become reentrained in the exhaust gas stream. To help reduce this problem, clean makeup water can be added directly to the quench system rather than adding all makeup water to a common sump.[6]

See also

References

  1. ↑ [1]
  2. ↑ http://www.rcdc.nd.edu/compilations/Quench/intro.htm
  3. ↑ [2]
  4. ↑ [3]
  5. ↑ Todd, Robert H., Dell K. Allen, and Leo Alting. Manufacturing Processes Reference Guide. 1st. Ed. New York: Industrial Press Inc., 1994
  6. ↑ US EPA Air Pollution Training Institute developed in collaboration with North Carolina State University, College of Engineering (NCSU)