Lesson 2 HEAT TREATMENT

Heat treatment is a term applied to a variety of procedures for changing the characteristics of metal by heating and cooling. By proper heat treatment, it is possible to obtain certain characteristics in metal such as hardness, tensile strength (ability to resist stretching), and ductility. Heat treatment can be a simple process requiring few tools. In industry, it is a highly scientific and complicated procedure requiring much equipment.

Many of the projects or products made in the machine shop have little or no value until they are heat-treated. This chapter includes only the most elementary information about the heat treatment of steel. Heat treatment can also be done on many of the nonferrous metals such as aluminium, copper and brass. The procedures are different, however, and will not be considered here.

The procedures of heat treatment of steel include hardening, tempering, annealing, and case hardening.

Hardening is a process of heating and cooling steel to increase its hardness and tensile strength, to reduce its ductility, and to obtain a fine grain structure. The procedure includes heating the metal above its critical point or temperature, followed by rapid cooling. As steel is heated, a physical and chemical change takes place between the iron and carbon. The critical point, or critical temperature, is the point at which the steel has the most desirable characteristics. When steel reaches this temperature—somewhere between 1400°F and 1600°F—the change is ideal to make for a hard, strong material if it is cooled quickly. If the metal cools slowly, it changes back to its original state. By plunging the hot metal into water, oil, or brine (quenching), the desirable characteristics are retained. The metal is very hard and strong and less ductile than before.

Heating is done in a furnace fired by gas, oil, or electricity. A device called a pyrometer is attached to the furnace. This accurately registers the exact temperature in the furnace (Fig.1). The temperature of the metal can also be determined by observing its color. You can make use of the colors when heat-treating simple metal parts and tools. Colors are not very accurate, however. Even the expert heat-treater will be off as much as 20°F from the true temperature.

The hardening procedure is:

1. Light the furnace, and allow it to come to the right temperature.

2. Place the metal in the furnace, and heat it to the critical temperature. For carbon tool steel, allow about 20 to 30 minutes per inch of thickness for coming up to heat. Allow about 10 to 15 minutes per inch of thickness for soaking at hardening temperature.

3. Select the correct cooling solution. Some steels can be cooled in water, and others must be cooled in oil or brine. Water is the most widely used material for quenching carbon steels because it is inexpensive and effective. Brine is usually made by adding about 9 percent of common salt to the water. Brine helps to produce a more uniform hardness. The brine cools the parts all over more quickly. Oil is used for a somewhat slower speed of quenching. Most oils used for quenching are mineral oils.

4. Remove the hot metal with tongs, and plunge it into cooling solution. Agitate so that the metal cools quickly and evenly. If it is a thin piece (1ike a knife or blade), cut the cooling solution with the object so it won't warp. If one side cools faster than the other, there will be some warping.

5. A properly hardened piece of steel will be hard and brittle and have high tensile strength. It will also have internal strain. If left in this state, these internal strains could cause the metal to crack.

Tempering is a process of reducing the degree of hardness and strength and increasing the toughness. It removes the brittleness from a hardened piece. It is a process that follows the hardening procedure and makes the metal as hard and tough as possible. Tempering is done by reheating the metal to low or moderate temperature, followed by quenching or by cooling in air. As the metal is heated for tempering, it changes in colour. These colours are called temper colours. You can watch these colours to decide when the correct heat is reached. A more accurate method, of course, is to watch the pyrometer. Many parts and projects are completely tempered. Others are tempered in one section, and the rest remains in the hardened state.

The tempering procedure is:

1. To temper the entire piece, place it in the furnace. Reheat to the correct temperature to produce the hardness and toughness you want. Remove the metal and cool it quickly.

2. To temper small cutting tools:

a. Harden the entire tool. Clean off the scale with abrasive cloth.

b. Heat a scrap piece of metal till red hot.

c. Place the tool on the metal with the point extending beyond the hot piece of metal.

d. Watch the temper colours. When the correct colour reaches the point of the tool, quench it.

Annealing is the process of softening steel to relieve internal strain. This makes the steel easier to machine. The most common method is to place the steel in the furnace and heat it thoroughly. Then turn off the furnace, allowing the metal to cool slowly. Another method is to pack the metal in clay, heat it to the critical temperature, remove it from the furnace, and allow it to cool slowly.

Case hardening is a process of hardening the outer surface or case of ferrous metal. By adding a small amount of carbon to the case of the low-carbon steel, it can be heat-treated to make the case hard, At the same time the centre, or core, remains soft and ductile.

There are many methods of case hardening. In industry, molten cyanide is used (this is called cyaniding). Another industrial method is carburizing. This is a case-hardening procedure in which carbon is added to steel from the surface inward by one of the following methods: (1) pack method, (2) gas method, or (3) liquid-salt method.

This process can be done on such items as hammer heads, piston pins, and other items that must stand a good deal of shock and wear. It can never be used on anything that must be sharpened by grinding.

°F (Fahrenheit) ['færənhait] n. 华氏温度

abrasive [ə'breisiv] a. 研磨的

accuracy ['ækjurəsi] n. 精度, 准确性

agitate ['ædʒiteit] v. 摇动

aluminiun [ˌælju'minjəm] n. 铝

annealing [ə'ni:liŋ] n. 退火

brass [bræs] n. 黄铜

brine [brain] n. 盐水

carburizing ['ka:bjuraiziŋ] n. 渗碳

case [keis] n. 壳, 套

clay [klei] n. 黏土

complicated ['kɔmpləkeitid] a. 复杂的

core [kɔ:] n. 核心

critical ['kritikl] a. 临界的

cyanide ['saiənaid] n. 氰化物

desirable [di'zairəbl] a. 合适的

elementary [elə'mentəri] a. 基本的

extend [eks'tend] v. 伸展

hardness ['ha:dnis] n. 硬度

inexpensive [inik'spensiv] a. 便宜的

mineral ['minərəl] n. 矿物, 矿产

moderate ['mɔdərit] a. 适度的

nonferrous ['nɔn'ferəs] a. 不含铁的, 非铁的

observe [əb'zə:v] vt. 观察

pack [pæk] v. 包装

New Words

plunge [plʌndʒ] v. 投入

point [pɔint] n. 刃口

project [prə'dʒekt] n. 项目, 产品

pyrometer [pai'rɔmitə] n. 高温计

quench [kwentʃ] vt. 淬火

rapid ['ræpid] a. 迅速的

register ['redʒstə] v. 记录, 显示, 记数

relieve [ri'li:v] vt. 减轻, 解除

retain [ri'tein] vt. 保持, 保留

scale [skeil] n. 硬壳

scrap [skræp] n. 小片, 废料

soak [səuk] v. 浸, 泡, 均热

solution [sə'lu:ʃən] n. 溶液

tempering ['tempəriŋ] n. 回火

tensile ['tensail] a. 拉力的, 可拉伸的

thoroughly ['θʌrəuli] adv. 十分地, 彻底地

tong [tɔŋ] n. 火钳

toughness ['tɔfnis] n. 韧性

treatment ['tri:tmənt] n. 处理

warp [wɔ:p] v. 翘曲

Phrases and Expressions

machine shop车间

make use of利用

abrasive cloth砂带，（金刚）砂布

case hardening表面硬化

Notes

1. Heat treatment is a term applied to a variety of procedures for changing the characteristics of metal by heating and cooling.

热处理这一术语指的是各种各样通过加热和冷却以改变金属特性的方法。

2. Colors are not very accurate, however. Even the expert heat-treater will be off as much as 20°F from the true temperature.

然而利用颜色判断不是十分精确。甚至熟练的热处理工人籍此所观察出的温度同真正的温度之间的误差也会达20°F之多。

3. A properly hardened piece of steel will be hard and brittle and has high tensile strength.

正常淬火的钢制工件会变硬变脆并具有很高的抗拉强度。

4. It removes the brittleness from a hardened piece. It is a process that follows the hardening procedure and makes the metal as hard and tough as possible.

它能消除淬火工件的脆性。这是在淬火之后采用的使金属尽可能变硬变韧的方法。

5. Place the tool on the metal with the point extending beyond the hot piece of metal.

把切削工具放在金属上，刃部伸到这块赤热的金属之外。

Reading Material

HEAT TREATMENT OF STEELS

Heat treating refers to the heating and cooling operations performed on a metal for the purpose of altering (改变) such characteristics as hardness, strength, or ductility. A tool steel intended to be machined into a punch (冲头) may first be softened so that it can be machined. After being machined to shape, it must be hardened so that it can sustain (承受) the punishment that punches receive. Most heating operations for hardening leave a scale (锈皮) on the surface, or contribute (造成) other surface defects (缺陷). The final operation must, therefore be grinding (磨) to remove surface defects and provide a suitable surface finish (表面光洁度).

When a steel part is to be either hardened or softened, its temperature must be taken above the critical temperature line; that is, the steel must be austenitized (奥氏体化). Usually a temperature of 50 to 109℃ above the critical temperature is selected, to ensure that the steel part reaches a high enough temperature to be completely austenitized, and also because furnace temperature control is always a little uncertain.

The steel must be held at furnace temperature for sufficient time to dissolve the carbides (碳化物) in the austenite, after which the steel can be cooled. How much residence (停留) time in the furnace is required is to some degree a matter of experience with any particular steel. Usually, for a 3/4 in. bar, 20 minutes or slightly more will do. Double the time for twice the diameter. Alloy steels may require a longer furnace time; many of these steels are best preheated in a lower-temperature furnace before being charged into the hardening furnace.

When the heating time is completed, the steel must be cooled down to room temperature. The cooling method determines whether the steel will be hardened or softened. If the steel is quickly removed from the furnace and quenched into cold water, it will be hardened. If it is left in the furnace to cool slowly with the heat turned off, or cooled in air(small pieces of plain carbon steel cannot be air-softened, however), it will be softened. High-alloy steels may be hardened by air-cooling, but plain carbon steels must have a more severe quench, almost always water.

There are several softening methods for steels, and the word softening therefore does not indicate what softening process or purpose was used. The method of softening by slow cooling from austenite is called annealing, not softening. Annealing leaves the steel in the softest possible condition (dead soft).

To conclude, the difference between hardening and annealing is not in the heating process, but in the cooling process.