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Quench

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".

Quench Hardening
Quench hardening is a mechanical process in which steel and cast iron alloys are strengthened and hardened. This 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, Wear Blocks, Etc.

Process of Quench Hardening
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 min for each mm of cross-section. For a bath the time can range a little higher. 0 to 6 min 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 (time to cool) 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 maximize 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, needlelike grain structure. [1]

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

Equipment Used in Quench Hardening
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]

淬火　 

    钢的淬火是将钢加热到临界温度Ac3（亚共析钢）或Ac1（过共析钢）以上某一温度，保温一段时间，使之全部或部分奥氏体[1]化，然后以大于临界冷却速度的冷速快冷到Ms以下（或Ms附近等温）进行马氏体（或贝氏体）转变的热处理工艺。
　　通常也将铝合金、铜合金、钛合金、钢化玻璃等材料的固溶处理或带有快速冷却过程的热处理工艺称为淬火。
　　淬火的目的是使过冷奥氏体进行马氏体或贝氏体转变，得到马氏体或贝氏体组织，然后配合以不同温度的回火，以大幅提高钢的强度、硬度、耐磨性、疲劳强度以及韧性等，从而满足各种机械零件和工具的不同使用要求。也可以通过淬火满足某些特种钢材的的铁磁性、耐蚀性等特殊的物理、化学性能。
　　淬火能使钢强化的根本原因是相变，即奥氏体组织通过相变而成为马氏体组织（或贝氏体组织）。
　　钢淬火工艺最早的应用见于河北易县燕下都遗址出土的战国时代的钢制兵器。
　　淬火工艺最早的史料记载见于《汉书.王褒传》中的“清水焠其峰”。
　　“淬火”在专业文献上，人们写的是“淬火”，而读起来又称“蘸火”。“蘸火”已成为专业口头交流的习用词，但文献中又看不到它的存在。也就是说，淬火是标准词，人们不读它，“蘸火”是常用词，人们却不写它，这是我国文字中不多见的现象。
　　淬火是“蘸火”的正词，淬火的古词为蔯火，本义是灭火，引申义是“将高温的物体急速冷却的工艺”。“蘸火”是冷僻词，属于现代词，是文字改革后出现的产物，“蘸”字本义与淬火无关。“蘸火”本词为“湛火”，“湛”字读音同“蘸”，而其字形又与水、火有关，符合“水与火合为蔯”之意，字义与“淬火”相通。“湛火”为本词，“蘸火”则为假借词。
　　淬火
　　将金属工件加热到某一适当温度并保持一段时间，随即浸入淬冷介质中快速冷却的金属热处理工艺。常用的淬冷介质有盐水、水、矿物油、空气等。淬火可以提高金属工件的硬度及耐磨性，因而广泛用于各种工、模、量具及要求表面耐磨的零件（如齿轮、轧辊、渗碳零件等）。通过淬火与不同温度的回火配合，可以大幅度提高金属的强度、韧性及疲劳强度，并可获得这些性能之间的配合（综合机械性能）以满足不同的使用要求。另外淬火还可使一些特殊性能的钢获得一定的物理化学性能，如淬火使永磁钢增强其铁磁性、不锈钢提高其耐蚀性等。淬火工艺主要用于钢件。常用的钢在加热到临界温度以上时，原有在室温下的组织将全部或大部转变为奥氏体。随后将钢浸入水或油中快速冷却，奥氏体即转变为马氏体。与钢中其他组织相比，马氏体硬度最高。钢淬火的目的就是为了使它的组织全部或大部转变为马氏体，获得高硬度，然后在适当温度下回火，使工件具有预期的性能。淬火时的快速冷却会使工件内部产生内应力，当其大到一定程度时工件便会发生扭曲变形甚至开裂。为此必须选择合适的冷却方法。根据冷却方法，淬火工艺分为单液淬火、双介质淬火、马氏体分级淬火和贝氏体等温淬火4类。
　　淬火效果的重要因素，淬火工件硬度要求和检测方法：
　　淬火工件的硬度影响了淬火的效果。淬火工件一般采用洛氏硬度计，测试HRC硬度。淬火的薄硬钢板和表面淬火工件可测试HRA的硬度。厚度小于0.8mm的淬火钢板、浅层表面淬火工件和直径小于5mm的淬火钢棒，可改用表面洛氏硬度计，测试HRN硬度。
　　在焊接中碳钢和某些合金钢时，热影响区中可能发生淬火现象而变硬，易形成冷裂纹，这是在焊接过程中要设法防止的。
　　由于淬火后金属硬而脆，产生的表面残余应力会造成冷裂纹，回火可作为在不影响硬度的基础上，消除冷裂纹的手段之一。
　　淬火对厚度、直径较小的零件使用比较合适，对于过大的零件，淬火深度不够，渗碳也存在同样问题，此时应考虑在钢材中加入铬等合金来增加强度。
　　淬火是钢铁材料强化的基本手段之一。钢中马氏体是铁基固溶体组织中最硬的相(表1)，故钢件淬火可以获得高硬度、高强度。但是，马氏体的脆性很大，加之淬火后钢件内部有较大的淬火内应力，因而不宜直接应用，必须进行回火。
　  淬火工艺在现代机械制造工业得到广泛的应用。机械中重要零件，尤其在汽车、飞机、火箭中应用的钢件几乎都经过淬火处理。为满足各种零件干差万别的技术要求，发展了各种淬火工艺。如，按接受处理的部位，有整体、局部淬火和表面淬火；按加热时相变是否完全，有完全淬火和不完全淬火(对于亚共析钢，该法又称亚临界淬火)；按冷却时相变的内容，有分级淬火，等温淬火和欠速淬火等。
　　工艺过程 包括加热、保温、冷却3个阶段。下面以钢的淬火为例，介绍上述三个阶段工艺参数选择的原则。
加热温度 以钢的相变临界点为依据，加热时要形成细小、均匀奥氏体晶粒，淬火后获得细小马氏体组织。碳素钢的淬火加热温度范围如图1所示。由本图示出的淬火温度选择原则也适用于大多数合金钢，尤其低合金钢。亚共析钢加热温度为Ac3温度以上30～50℃。从图上看，高温下钢的状态处在单相奥氏体(A)区内，故称为完全淬火。如亚共析钢加热温度高于Ac1、低于Ac3温度，则高温下部分先共析铁素体未完全转变成奥氏体，即为不完全(或亚临界)淬火。过共析钢淬火温度为Ac1温度以上30～50℃，这温度范围处于奥氏体与渗碳体(A+C)双相区。因而过共析钢的正常的淬火仍属不完全淬火，淬火后得到马氏体基体上分布渗碳体的组织。这-组织状态具有高硬度和高耐磨性。对于过共析钢，若加热温度过高，先共析渗碳体溶解过多，甚至完全溶解，则奥氏体晶粒将发生长大，奥氏体碳含量也增加。淬火后，粗大马氏体组织使钢件淬火态微区内应力增加，微裂纹增多，零件的变形和开裂倾向增加；由于奥氏体碳浓度高，马氏体点下降，残留奥氏体量增加，使工件的硬度和耐磨性降低。常用钢种淬火的温度参见表2。
　  实际生产中，加热温度的选择要根据具体情况加以调整。如亚共析钢中碳含量为下限，当装炉量较多，欲增加零件淬硬层深度等时可选用温度上限；若工件形状复杂，变形要求严格等要采用温度下限。
　　保温时间 由设备加热方式、零件尺寸、钢的成分、装炉量和设备功率等多种因素确定。对整体淬火而言，保温的目的是使工件内部温度均匀趋于一致。对各类淬火，其保温时间最终取决于在要求淬火的区域获得良好的淬火加热组织。
　　加热与保温是影响淬火质量的重要环节，奥氏体化获得的组织状态直接影响淬火后的性能。-般钢件奥氏体晶粒控制在5～8级。
　　冷却方法 要使钢中高温相——奥氏体在冷却过程中转变成低温亚稳相——马氏体，冷却速度必须大于钢的临界冷却速度。工件在冷却过程中，表面与心部的冷却速度有-定差异，如果这种差异足够大，则可能造成大于临界冷却速度部分转变成马氏体，而小于临界冷却速度的心部不能转变成马氏体的情况。为保证整个截面上都转变为马氏体需要选用冷却能力足够强的淬火介质，以保证工件心部有足够高的冷却速度。但是冷却速度大，工件内部由于热胀冷缩不均匀造成内应力，可能使工件变形或开裂。因而要考虑上述两种矛盾因素，合理选择淬火介质和冷却方式。
　　冷却阶段不仅零件获得合理的组织，达到所需要的性能，而且要保持零件的尺寸和形状精度，是淬火工艺过程的关键环节。
　　分类 可按冷却方式分为单液淬火、双液淬火、分级淬火和等温淬火等。冷却方式的选择要根据钢种、零件形状和技术要求诸因素。
　　单液淬火 将工件加热后使用单一介质冷却，最常使用的有水和油两种，其变、温曲线如图2中的曲线1。为防止工件过大的变形和开裂，工件不宜在介质中冷至室温，可在200～300℃出水或油，在空气中冷却。单液淬火操作简单易行，广泛用于形状简单的工件。有时将工件加热后，先在空气中停留-段时间，再淬入淬火介质中，以减少淬冷过程中工件内部的温差，降低工件变形与开裂的倾向，称为预冷淬火。
　　双液淬火 工件加热后，先淬入水或其他冷却能力强的介质中冷却至400℃左右，迅速转入油或其他冷却能力较弱的介质中冷却。变温曲线如图2中曲线2。所谓“水淬油冷”法使用得相当普遍。先淬入冷却能力强的介质，工件快速冷却可避免钢中奥氏体分解。低温段转入冷却能力较弱的介质可有效减少工件的内应力，降低工件变形和开裂倾向。本工艺的关键是如何控制在水中停留的时间。根据经验，按工件厚度计算在水中停留的时间，系数为O.2～O.3s/mm，碳素钢取上限，合金钢取下限。这种工艺适用于碳素钢制造的中型零件(直径10～40mm)和低合金钢制造的较大型零件。
　　分级淬火 工件加热后，淬入温度处于马氏体点(ms)附近的介质(可用熔融硝盐、碱或热油)中，停留一段时间，然后取出空冷。变温曲线如图2中曲线3。分级温度应选择在该钢种过冷奥氏体的稳定区域，以保证分级停留过程中不发生相变。对于具有中间稳定区(“两个鼻子”)型TTT曲线的某些高合金钢，分级温度也可选在中温(400～600℃)区。分级的目的是使工件内部温度趋于一致，减少在后续冷却过程中的内应力及变形和开裂倾向。此工艺适用于形状复杂，变形要求严格的合金钢件。高速钢制造的工具淬火多用此工艺。
　　等温淬火 工件加热后，淬入温度处于该钢种下贝氏体(B下)转变范围的介质中，保温使之完成下贝氏体转变，然后取出空冷，变温曲线如图2中的曲线4。等温温度对下贝氏体性能影响较大，温度控制要求严格。常用钢种的等温温度和时间列于表3。等温淬火工艺特别适用于要求变形小、形状复杂，尤其同时还要求较高强韧性的零件。