Reading Material 9
Heat Treatment of Steel
Types of Heat Treating Operations Five operations are detailed in this lesson as the basis of heat treatment. Explanations of these operations follow.
Full annealing Full annealing is the process of softening steel by a heating and cooling cycle* so that it may be bent or cut easily. In annealing* steel is heated above a transformation temperature and cooled very slowly after it has reached a suitable
temperature. The distinguishing characteristics of full annealing are: (a) temperature above the critical temperature and (b) very slow cooling, usually in the furnace.
Normalizing Normalizing is identical with annealing, except that the steel is air cooled; this is much faster than cooling in a furnace. Steel is normalized to refine grain size, make its structure more uniform, or to improve machinability.
Hardening Hardening is carried out by quenching a steel, that is, cooling it rapidly from a temperature above the transformation temperature. Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels. After steel is quenched, it is usually very hard and brittle; it may even crack if dropped. To make the steel more ductile, it must be tempered.
Tempering Tempering consists of reheating a quenched steel to a suitable temperature below the transformation temperature for an appropriate time and cooling back to room temperature. How this process makes steel tough will be discussed later.
Stress relieving Stress relieving is the heating of steel to a temperature below the transformation temperature, as in tempering, but is done primarily to relieve internal stress and thus prevent distortion or cracking during machining. This is sometimes called process annealing.
Reasons for Heat Treating Heat treatment of steel is usually intended to accomplish any one of the following objectives?
? Remove stresses induced by cold working or to remove stresses set up by nonuniform cooling of hot metal objects; ? ? ?
Refine the grain structure of hot worked steels which may have developed coarse grain size; Secure the proper grain structure;
Decrease the hardness and increase the ductility;
? Increase the hardness so as to increase resistance to wear or to enable the steel to withstand more service conditions;
? Increase the toughness; that is, to produce a steel having both a high tensile strength and good ductility, enabling it to withstand high impact; ? ? ?
Improve the machinability;
Improve the electrical properties;
Change or modify the magnetic properties of steel.
Hea# TruUawnt The hardest condition for any given steel is obtained by quenching to a fully martensitic structure. Since hardness is directly related to strength, a steel composed of 100% martensite is at its strongest possible condition. However, strength is not the only property that must be considered in the application of steel parts. Ductility may be equally important. Tempering Ductility is the ability of a metal to change shape before it breaks. Fleshly quenched martensite is hard but not ductile; in fact, it is very brittle. Tempering is needed to impart ductility to the martensite, usually at a small sacrifice in strength. In addition, tempering greatly
increases the resistance of martensite to shock loading.
The effect of tempering may be illustrated as follows. If the head of a hammer were quenched to a fully martensitic structure, it probably would crack after the first few blows. Tempering during manufacture of the hammer imparts shock resistance with only a slight decrease in hardness. Tempering is accomplished by heating a quenched part to some point below the transformation temperature, and holding it at this temperature for an hour or more, depending on its size. Most steels are tempered between 205 and 595°C. As higher temperatures are employed, toughness or shock resistance of the steel is increased, but the hardness and strength decrease.
Annealing The two-stage heat treating process of quenching and tempering is designed to produce high strength steel capable of resisting shock and deformation without breaking. On the other hand, the annealing process is intended to make steel easier to deform or machine. In manufacturing steel products, machining and severe bending operations are often employed. Even tempered steel may not cut or bend very easily and annealing is often necessary.
Process annealing Process annealing consists of heating steel to a temperature just below the lowest transformation temperature for a short time. This makes the steel easier to form. This heat treatment is commonly applied in the sheet and wire industries, and the temperatures generally used are from 550 to 650°C.
Full annealing Full annealing, where steel is heated 50 to 100°C above the third transformation temperature for hypoeutectoid steels, and above the lowest transformation temperature for hypereutectoid steels, and slow cooled, makes the steel much easier to cut, as well as bend. In full annealing, cooling must take place very slowly so that a coarse pearlite is formed. Slow cooling is not essential for process annealing, since any cooling rate from temperatures below the lowest transformation temperature will result in the same microstructure and hardness.
During cold deformation, steel has a tendency to harden in deformed areas, making it more difficult to bend and liable to breakage. Alternate deforming and annealing operations are performed on most manufactured steel products.
Normalizing The process of normalizing consists of heating to a temperature above the third transformation temperature and allowing the part to cool in still air. The actual temperature required for this depends on the composition of the steel, but is usually around 870\the term normalize does not describe the purpose. The process might be more accurately described as a homogenizing or grain-refining treatment. Within any piece of steel, the composition is usually not uniform throughout. That is, one area may have more carbon than the area adjacent to it. These compositional differences affect the way in which the steel will respond to heat treatment. If it is heated to a high temperature, the carbon can readily diffuse throughout, and the result is a reasonably uniform composition from one area to the next. The steel is then more homogeneous and will respond to the heat treatment in a more uniform way.
Because of characteristics inherent in cast steel? the normalizing treatment is more frequently applied to ingots prior to working, and to steel castings and forgings prior to hardening.
Stress Relieving When a metal is heated, expansion occurs which is more or less proportional to the temperature rise. Upon cooling a metal, the reverse reaction takes place. That is, a contraction is observed. When a steel bar or plate is heated at one point more than at another, as in welding or during forging, internal stresses are set up. During heating, expansion of the heated area cannot take place unhindered, and it tends to deform. On cooling, contraction is prevented from taking place by the unyielding cold metal surrounding the heated area. The forces
attempting to contract the metal are not relieved, and when the metal is cold again, the forces remain as internal stresses. Stresses also result from volume changes, which accompany metal transformations and precipitation. Internal or residual stresses are bad because they may cause warping of steel parts when they are machined. To relieve these stresses, steel is heated to around 595°C ? assuming that the entire part is heated uniformly, then cooled slowly back to room temperature. This procedure is called stress relief annealing, or merely stress relieving. (Selected from: Heat Treater's Guide- Standard Practices and Procedures for Steel, 1982. )
阅读材料9
钢的热处理
各种热处理操作五种操作是说明以这种经验作为热处理的基础。这些操作解释如下。 全退火全退火是通过循环加热和冷却的程序来软化钢，以便达到容易地弯曲或切割，在退火中，钢受热达到一个适温之后，上面的温度将慢慢转变和冷却。全退火明显的特征是：（1）温度达到临界点以上和（2）在熔炉里非常缓慢的冷却。
规格化规格化等同于退火，除了那些空冷的钢，这是比在熔炉里更加快的。钢是规格化到加工精度我使它的建筑物更统一或是去改善机械加工性。
变硬变硬是通过对钢淬火，也就是，快速地从一个温度冷却到转变温度。为了最快的冷却钢可以用水或是盐水来淬，一些高合金钢可以用油和空气应用于较高强度的高合金钢。钢淬火后，通常是很坚硬和非常脆的；如果跌落甚至可以产生裂缝，为了制造更多地可塑性钢，它必须得回火。
回火回火包括二次加热淬火钢到一个低于转变温度的适温是为了提供适当的的时间和冷却回到室温。怎么样使炼钢坚韧这个工艺将会不断讨论。
应力消除应力消除就是把钢加热到低于转变温度得一个温度，,在回火中，在加工中这样做可以减轻内应力和防止变形或产生裂缝.有时这也叫退火。热处理的理由，钢的热处理通常是为了完成任意的下列目标
切削应力诱发是由对非均匀冷却的熔融金属冷加工或切削加压引起的； 细化的晶粒可通过钢的热加工来发展发展成为粗粒； 获得自身特有颗粒结构：
降低其硬度和增强其延展性：
增加硬度来提升它的耐磨性或使这种钢经得起更多使用条件；
增强韧性,也就是，生产具备高拉伸强度和良好的延展性的钢、使它能经得起高冲击； 改善电的特性。
替换或变质处理钢的磁性
热处理最难的是通过淬火使任何给定的钢完全变为马氏体组织。因为硬度跟力量是直接联系的,钢强度是由马氏体组织决定的。然而、力量不是在钢局部应用的仅有性质。延展性也同样重要。
回火延展性就是一种材料在它破裂前所能够改变形状的能力。肉体的淬火马氏体是硬的
而不是坚韧的；事实上、那马氏体是很脆弱的.回火就是提高马氏体的延展性，通常在强力前会出现小的亏损。此外。回火大大增强了马氏体受冲击的抵抗力。
回火的影响以下举例说明。如果一个锤子地头被淬火成完全马氏体的结构，它可能在打击几次以后破裂。回火在锤子的加工过程中提高了抗冲击的性能，并且硬度稍微降低。回火是通过加热淬火后的钢到转变温度以下来完成的，并且保持这个温度一个小时或者更久，这由钢的大小来决定的。大多数钢的回火温度在205℃~595℃。随着温度的提高，钢的韧性和抗冲击强度也增强，但硬度和强度下降。
退火在回火和淬火两个阶段的热处理中，其目的是制造高强度的钢板，使它能够抵抗冲击和变形但不破裂。令一方面，退火工艺的目的是使钢容易变形和机械制造。在钢制品的机械加工中，机械制造和剧烈弯曲是正常的。回火钢可能不易切削和弯曲，所以回火往往是必要的。
退火工艺退火工艺包括加热钢到低于最低转变温度上的一段时间。这使钢更容易成型。这种热处理一般适用于钢铁和电线产业，这个温度一般在550~650℃。
完全退火完全退火，钢加热到第三转变温度以上50~100℃的成为亚共析钢，加热到最低转变温度以上为过共析钢，和缓慢冷却后，使钢容易切削和弯曲。在完全退火中，冷却过程必须很缓慢，从而形成粗糙的珠光体。缓慢冷却并不是退火过程中比不可少的过程，因为加热到最低的转变温度以下的任何冷却速度都会导致同样的结构和硬度。
在冷变形中，钢有一中在变形中变硬的趋势，使之难于弯曲和易破坏。大多数的机械加工钢产品都需要交替变形和退火。
正火正火过程包括加热到第三温度以上，并且让其在空气中冷却。正火实际需要的温度取决于钢的组成成分，但通常在870℃左右。这个过程可能会更准确的描述为均匀或晶粒细化处理。在任何一块钢的组成通常是不统一的。也就是说，一个地方可能有更多的碳在它周围。这些成分的差异影响着钢的热处理。如果加热到一个高的温度，碳可能扩散到各处，其结果是得到一个均匀的组成。这时的钢的组织均匀并且对热处理有一个好的反应方式。
由于铸铁的固有性质，正火处理对工作前的铸铁块，硬化前的钢的铸件和锻件使用更频繁。
去应力当金属被加热时，就会发生膨胀，膨胀的多少于温度成比例。这样金属会发生相反的反应。也就是说，可以观察到钢的缩小。当一个钢的一个点加热的温度高于其它地方时，就会在那点产生内应力，就像焊接或锻造一样。加热过程中，加热膨胀的部位因无法自由膨胀，而往往会发生变形。冷却时，缩小被阻止发生。使金属变形的力并没有减弱，当金属在次冷却时，这个力与内应力一样不变。体积变化也可以产生应力，这种变化伴随着金属的转变和沉淀。内应力和残余应力是不好的，因为在加工时，它们可能会导致钢铁部件的变形。为消除这些应力，钢铁要加热到595℃左右，让整个部分被加热均匀，然后慢慢冷却到室温。这种方法被称为应力退火，或者为去应力。
（选自：热处理指南；钢的标准操作规程与程序，1982）
Reading Material 10
Corrosion Control
Corrosion problems can be solved in the following ways:
(1) Select a material that is resistant to the corrosion environment.
(2) Give metal a protective coating. (3) (4) (5) (6)
Change the service conditions, such as temperature, pressure, or velocity.
Change the environment chemistry, such as pH, concentration, aeration, or impurities. Add a corrosion inhibitor.
Shift the electric potential of the metal by cathodic or anodic protection.
(7) Modify the design of the equipment or system.
(8) Let it corrode and replace it (often a viable alternative!).
The methods listed above are the accepted ways of dealing with a corrosion problem, but not all of them apply in a given situation. In particular, the corrosion engineer often cannot change the service conditions or environment chemistry. These may be as unalterable as the ocean, or nearly as unalterable: an industrial process that is running fairly smoothly where any change will be fanatically opposed by the production people.
Most corrosion problems originate with either improper design or improper material selection. However, a good choice of material can overcome severe environmental conditions and even some deficiencies in design.
Once the engineer has determined that there is no danger of a catastrophe, deciding which way to combat corrosion usually comes down to the economics of the situation. Material Selection
Stainless steel are usually the first choice for a \properties, because these alloys are resistant to a wide range of oxidizers, but they cannot withstand strong reducing solutions, such as hydrochloric acid. Stainless steels can be corroded, despite their name. The stainless steels are classified into five general groups (martensitic, ferritic, austenitic, duplex and precipitation-hardenable strainless steels) according to their metallurgical structures, with the choice of which one to use depending not only on corrosion resistance but also on required strength and cost.
Commercially pure nickel has high corrosion resistance, especially to alkalies, combined with mechanical properties similar to mild steel, and good weldability. Nickel and nickel alloys are widely used in the food industry and are frequently selected for service in chlorine, hydrogen chloride, and chlorinated hydrocarbons. They are very resistant to high-temperature air and to stress-corrosion cracking.
Aluminum is a very reactive metal in the standard electromotive force series; it immediately reacts with air to form a passive film consisting of two layers; an inner, compact, amorphous oxide and an outer, thicker, more permeable hydrated oxide. Aluminum is naturally compatible with the atmosphere and withstands many solutions well if the pH lies between about 4 and 9. Strong acids and moderately strong bases destroy aluminum's passive film. Chloride ions are particularly damaging because they attack the film only at weak spots and pit the aluminum. Many chlorinated organic solvents and alcohols can attack aluminum alloys disastrously, sometimes explosively. Protective Coatings
The major purpose of coating a metal is to protect it from a corrosive environment when the metal is otherwise suitable for the service conditions in terms of mechanical and physical properties. Coating metal with good mechanical properties (usually steel) is often more practical in terms of cost and required life than selecting a more corrosion resistant but expensive material.