Welding, cutting, or even grinding on metal produces heat, which it turn has an effect on the structure of the metal. As a steelworker, you need to understand the effect that heat treatment has on metals so you can attain the desired properties for a particular metal. You also need to know what methods can be used to restore a metal to its original condition.
Heat treatment is the process of heating (but never allowing the metal to reach the molten state) and cooling a metal in a series of specific operations which changes or restores its mechanical properties.
Heat treatment makes a metal more useful by making it stronger and more resistant to impact, or alternatively, making it more malleable and ductile.
However, no heat-treating procedure can produce all of these characteristics in one operation; some properties are improved at the expense of others. For example, hardening a metal may make it brittle, or annealing it may make it too soft.
When you have completed this course, you will be able to:
All heat-treating processes are similar because they all involve the heating and cooling of metals. However, there are differences in the methods used, such as the heating temperatures, cooling rates, and quenching media necessary to achieve the desired properties.
The heat treatment of ferrous metals (metals with iron) usually consists of annealing, normalizing, hardening, and/or tempering.
Most nonferrous metals can be annealed, but never tempered, normalized, or case hardened.
To successfully heat treat a metal, you need to have the proper equipment with close control over all factors relevant to the heating and cooling. For example, the furnace must be the proper size and type with the temperatures controlled and kept within the prescribed limits for each operation, and you must have the appropriate quenching media to cool the metal at the correct rate.
The furnace atmosphere itself affects the condition of the metal being heat treated. This atmosphere consists of the gases in the furnace’s heating chamber that circulate and surround the metal being heated.
In an electric furnace, the atmosphere is either air or a controlled mixture of gases.
In a fuel-fired furnace, the atmosphere is a mixture of gases and air. Air combines with gases released by the fuel’s combustion resulting in various proportions of carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H), nitrogen (N), oxygen (O), water vapor (H2O), and other various hydrocarbons (CnH2n). When you vary the proportions of air and fuel in a fuel-fired furnace, you can provide three distinct atmospheres: oxidizing, reducing, and neutral.
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You accomplish heat treatment in three major stages:
In the heating stage, the primary objective is to heat uniformly, and you attain and maintain uniform temperatures by slow heating. If you heat unevenly, one section can expand faster than another, resulting in a distorted or cracked part.
The appropriate heating rate will depend on several factors.
In the soaking stage, the objective is to hold the metal to the proper temperature until the desired internal structural changes take place. “Soaking period” is the term you use for the time the metal is held at the proper temperature. The chemical analysis of the metal and the mass of the part will determine the appropriate soaking period.
For steel parts with uneven cross sections, the largest section determines the soaking period.
Except for the rare variance, you should not bring the temperature of a metal directly from room temperature to soaking temperature in one operation. Instead, heat the metal slowly to a temperature just below the point at which the internal change occurs and hold it at that temperature until you have equalized the heat throughout. Following this process (called “preheating”), quickly heat the metal to its final required temperature.
When a part has an intricate design, you may have to preheat it to more than one temperature stage to prevent cracking and excessive warping. For example, assume an intricate part needs to be heated to 1500°F for hardening.
You may need to heat this part slowly to a 600°F stage and soak it at this temperature for a defined period, then heat it slowly and soak it at a 1200°F stage, and then heat it quickly to the hardening temperature of 1500°F.
Nonferrous metals seldom require preheating; in fact, preheating can cause an increase in their grain size.
In the cooling stage, the objective is self-explanatory, but there are different processes to return a metal to room temperature, depending on the type of metal.
To cool the metal and attain the desired properties, you may need to place it in direct contact with a cooling medium (a gas, liquid, solid, or a combination), and any cooling rate will depend on the metal itself and the chosen medium. Therefore, the choice of a cooling medium has an important influence on the properties desired.
Cooling metal rapidly in air, oil, water, brine, or some other medium is called quenching.
Quenching is usually associated with hardening since most metals that are hardened are cooled rapidly during the process. However, neither quenching nor rapid cooling always results in increased hardness. For example, a water quench is usually used to anneal copper, and some other metals are cooled at a relatively slow rate for hardening, such as air-hardened steels.
Some metals crack or warp during quenching, while others suffer no ill effects; so the quenching medium must fit the metal. Use brine or water for metals that require a rapid cooling rate; use oil mixtures for metals that need a slower cooling rate. Generally, you should water-harden carbon steels, oil-harden alloy steels, and quench nonferrous metals in water.
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“Red-hot” is a term you are probably familiar with as it applies to steel, but steel actually takes on several colors and shades from the time it turns a dull red until it reaches a white heat. Figure 1 shows these approximate colors and their corresponding temperatures.
Figure 1 — Example of approximate heat colors for steel.
Steel is heated through various temperatures during hardening, normalizing, or annealing, and each temperature produces a color change; so by observing these changes, you should be able to approximate the temperature of the steel.
As an example, assume you must quench-harden a steel part at 1500°F. You should heat the part slowly and evenly while watching it closely for any change in color. Once the steel begins to redden, carefully note each change in shades of red as you continue to apply heat. When the steel is bright red, or approximately 1500°F, quench the part.
Your judgment, ability to distinguish shades of red, and the accuracy with which you identify each color with its corresponding temperature can determine the success of your heat-treating operation. If you are one of the 5–8 percent of males, or less than 1 percent of females who are color blind in some way or another, your inability to distinguish colors will be problematic.
Refer again to Figure 1 and you will see that you need to observe the colors closely. To accurately judge a steel’s temperature, you must be able to tell the difference between faint red and blood red, dark cherry and medium cherry, or cherry red and bright red.
Sometimes in various lightings and work environments, this is a difficult task, and to add to the difficulty, your conception of medium cherry may differ from another’s conception. For an actual heat-treating operation, use a printed chart showing the authentic colors of steel at various temperatures. Do not rely on the colors displayed on the monitor you are currently viewing, or on printed versions of this course; there are too many variables in monitors and printers to be able to rely on them for accuracy.
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There are four basic types of heat treatment in use today: annealing, normalizing, hardening, and tempering.
The following sections describe the techniques used in each process and show how they relate to welders and other steelworkers.
The objective of annealing is the opposite of hardening. You anneal metals to relieve internal stresses, soften them, make them more ductile, and refine their grain structures. The process includes all three stages of heat treatment already covered (heat the metal to a specific temperature, hold it at a temperature for a set length of time, cool it to room temperature), but the cooling method will depend on the metal and the properties desired.
You may need to furnace-cool some metals or bury others in ashes, lime, or other insulating materials to achieve the appropriate characteristics.
Under certain job conditions, or without proper preheating, welding can produce areas of molten metal adjacent to other areas at room temperature. Given specific conditions, welding can actually weaken a metal, for as a weld cools, internal stresses occur along with hard spots and brittleness.
Annealing is just one method for correcting these problems and relieving the stresses.
To anneal ferrous metals and produce the maximum softness (ductility) in steel, you slowly heat the metal to its proper temperature, soak it, and then let it cool very slowly by burying the hot part in an insulating material, or by shutting off the furnace and allowing the furnace and the part to cool slowly together.
Soaking periods depend on both the type and the mass of the metal involved. Table 1 provides approximate soaking periods for annealing steel of various thicknesses.
Extremely low-carbon steels require the highest annealing temperature, but as steel’s carbon content increases, its annealing temperatures decrease.
Table 1 — Approximate Soaking Periods for Hardening, Annealing, and Normalizing Steel
|Thickness of metal in inches||Time of heating to required temperature in hours||Soaking time in hours|
|Up to 1||¾||½|
|1 to 2||1 ¼||½|
|2 to 3||1 ¾||¾|
|3 to 4||2 ¼||1|
|4 to 5||2 ¾||1|
|5 to 8||3 ½||1 ½|
Annealing nonferrous metals may or may not follow the same process as ferrous metals. For example, copper becomes hard and brittle when mechanically worked, but it can be made soft again by annealing at a temperature between 700°F and 900°F. However, copper may be cooled rapidly (normally associated with hardening) or slowly since the cooling rate has no effect on the heat treatment.
One drawback experienced in annealing copper is the phenomenon called “hot shortness.” Copper loses its tensile strength at about 900°F and if not properly supported, it could fracture.
Aluminum also has the characteristic of “hot shortness,” and reacts similarly to copper when heat treating. With the large number of aluminum alloys in use, you must provide special care while heat treating aluminum to produce the best properties for each alloy.
The intent of normalizing is to remove internal stresses that may have been induced by heat treating, welding, casting, forging, forming, or machining. Uncontrolled stress leads to metal failure; therefore, you should normalize steel before hardening it to ensure maximum results.
Normalizing applies to ferrous metals only, and it differs from annealing; the metal is heated to a higher temperature, but then it is removed from the furnace for air cooling.
Low-carbon steels do not usually require normalizing, but if they are normalized, no harmful effects result.
Castings are usually annealed rather than normalized; however, some castings require the normalizing heat treatment.
Refer again to Table 1 and note the approximate soaking periods for normalizing steel, which varies with the thickness.
Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility, much tougher than in any other structural condition. Metal parts that will be subjected to impact and those requiring maximum toughness with resistance to external stress are usually normalized.
In normalizing, since the metal is air cooled, the mass of a metal has a significant influence on the cooling rate and hence on the resulting piece’s hardness. With normalizing, thin pieces cool faster in the air and are harder than thick ones, whereas with annealing and its associated furnace cooling, the hardness of the thin and thick pieces is about the same.
The purpose of hardening is not only to harden steel as the name implies, but also to increase its strength. However, there is a trade off; while a hardening heat treatment does increase the hardness and strength of the steel, it also makes it less ductile, and brittleness increases as hardness increases. To remove some of the brittleness, you should temper the steel after hardening.
Many nonferrous metals can also be hardened and their strength increased by controlled heating and rapid cooling, but for nonferrous metals, the same process is called heat treatment rather than hardening.
For most steels, hardening consists of employing the typical first two stages of heat treatment (slowly heat to temperature and soak to time and temperature), but the third stage is dissimilar. With hardening, you rapidly cool the metal by plunging it into oil, water, or brine. (Note: Most steels require rapid cooling [quenching] for hardening, but a few can be air cooled with the same results.)
Refer again to Table 1, and note that the soaking periods for annealing, normalizing and hardening are all the same. The real difference in each heat treatment process occurs in stage three.
The cooling rate required to produce hardness decreases when alloys are added to steel; this is advantageous since a slower cooling rate also lessens the danger of cracking and warping.
The following provides hardening characteristics for a few irons and low-carbon steel.
Adding an alloy to steel to increase its hardness also increases the carbon’s effectiveness to harden and strengthen. Consequently, the carbon content required to produce maximum hardness is lower in alloyed steels than it is for plain carbon steels with the result that alloy steels are usually superior to carbon steels.
When you harden carbon steel, you must cool the steel to below 1000°F in less than one second. When you add alloys to steel and increase the carbon’s effectiveness, you also increase the time limit (more than one second to drop below 1000°F). Therefore, you can use a slower quenching medium to produce the desired hardness.
You usually quench carbon steels in brine or water, and alloy steels in oil.
Quenching steel produces extremely high internal stresses. To relieve them, you can temper the steel just before it becomes cold by removing the part from the quenching bath at a temperature of about 200°F and allowing it to air cool. The temperature range from 200°F down to room temperature is called the “cracking range,” and you do not want the steel to pass through it in the quenching medium. Further information on tempering follows in another section.
The object of case hardening is to produce a hard, wear-resistant surface (case) over a strong, tough core. In case hardening, the surface of the metal is chemically changed by the introduction of a high carbide or nitride content, but the core remains chemically unaffected. When the metal is heat treated, the high-carbon surface responds to hardening and the core toughens.
Case hardening applies only to ferrous metals. It is ideal for parts that must have a wear-resistant surface yet be internally tough enough to withstand heavy loading. Low-carbon and low-alloy series steels are best suited for case hardening. When high-carbon steels are case hardened, the hardness penetrates beyond the surface resulting in brittleness.
There are three principal processes for case hardening: carburizing, cyaniding, and nitriding.
Carburizing — a case hardening process by which carbon is added to the surface of low-carbon steel.
When the carburized steel is heat treated, the case becomes hardened and the core remains soft and tough--in other words, it has a high-carbon surface and a low-carbon interior.
There are two methods for carburizing steel:
The parts can be left in the container and furnace to cool, or they can be removed and air-cooled. In either case, the parts become annealed during the slow cooling. The depth of the carbon penetration depends on the length of the soaking period during heat treatment. Modern methods dictate that carburizing is almost exclusively done by gas atmospheres.
Cyaniding — a case hardening process by which preheated steel is dipped into a heated cyanide bath and allowed to soak.
The part is then removed, quenched, and rinsed to remove any residual cyanide.
This process is fast and efficient. It produces a thin, hard shell, harder than the shell produced by carburizing, and can be completed in 20 to 30 minutes vice several hours. The major drawback is the use of cyanide; cyanide salts are a deadly poison.
Nitriding — a case hardening process by which individual parts have been heat treated and tempered before being heated in a furnace that has an ammonia gas atmosphere.
This case hardening method produces the hardest surface of any of the hardening processes, and it differs from the other methods in that no quenching is required so there is no worry about warping or other types of distortion.
The nitriding process is used to case harden items such as gears, cylinder sleeves, camshafts, and other engine parts that need to be wear-resistant and operate in high-heat areas.
Flame hardening is another process available for hardening the surface of metal parts. In flame hardening, you use an oxyacetylene flame to heat a thin layer of the surface to its critical temperature and then immediately quench it with a water spray. In this case, the cold base metal assists in the quenching since it is not preheated.
Similar to case hardening, this process produces a thin, hardened surface while the internal parts retain their original properties.
The process can be manual or mechanical, but in either case, you must maintain a close watch since an oxyacetylene flame can heat the metal rapidly and temperatures in this method are usually determined visually.
Flame hardening may also be done with automatic equipment.
Whenever possible, automatic equipment is desirable for more uniform results.
Most automatic machines have variable travel speeds to adapt to parts of various sizes and shapes, and the size and shape of the torch will also depend on the part. (Figure 2)
Figure 2 — Typical flame hardening.
A typical flame hardening torch consists of a mixing head, straight extension tube, 90- degree extension head, adjustable yoke, and a water-cooled tip.
Tips are available for hardening flats, rounds, gears, cams, cylinders, and other regular or irregular shapes. Practically any shape or size flame-hardening tip is offered. (Figure 3)
Figure 3 — Typical progressive hardening torch tip.
For hardening localized areas, you can flame harden with a standard hand-held welding torch and the torch flame adjusted to neutral for normal heating. (Figure 4) In corners and grooves, however, you should use a slightly oxidizing flame to keep the torch from sputtering, and exercise particular care against overheating.
If dark streaks appear on the metal surface, this is a sign you are overheating, and you need to increase the distance between flame and metal.
Figure 4 — Example of carburizing, neutral, and oxidizing flames.
Typically, for the best flame-hardening heating results, you should hold the torch with the tip of the inner cone about an eighth of an inch from the surface and direct the flame at right angles to the metal. Occasionally, you may need to change the angle for better results, but you will rarely use a deviation of more than 30°. The speed of torch travel will depend on the type of metal, the mass, the shape of the part, and the depth of hardness desired.
If you have options in selecting the core material for the part to be flame hardened, select the steel according to the properties desired. When surface hardness is the primary factor, select carbon steel; when the physical properties of the core are also factors, select alloy steel.
For good results in flame hardening, plain carbon steels should contain more than 0.35% carbon, and 0.40% to 0.70% is the effective carbon range for water quenching. A carbon content greater than 0.70% is likely to induce surface cracks unless the heating and quenching rate are carefully controlled.
A section that has a flame-hardened surface is equal to a section that was hardened by furnace heating and quenching for the following reasons:
Thus, properly done, flame hardening can produce a hard case that is highly resistant to wear, and a core that retains its original properties.
There are five general methods for flame hardening: stationary, circular band progressive, straight-line progressive, spiral band progressive, and circular band spinning.
Figure 5 — Example of progressive hardening.
Figure 6 — Example of flame hardening with the
circular band spinning method.
When heating and quenching are performed as separate operations, the heating tips may be water cooled internally, but no water sprays simultaneously onto the surface of the part.
When you are flame hardening, follow the same safety precautions that apply to welding:
After hardening by either case or flame, steel is often harder than needed and too brittle for most practical uses, containing severe internal stresses that were set during the rapid cooling of the process. Following hardening, you need to temper the steel to relieve the internal stresses and reduce brittleness.
Tempering consists of:
If this sounds familiar, you are correct; it is the same three-stage process as in heat treatment. The difference is in the temperatures used for tempering, which will affect the resultant strength, hardness, and ductility.
You temper a steel part to reduce the brittleness caused by hardening, and develop specific physical properties; it always follows, never precedes hardening. Tempering reduces brittleness, but it also softens the steel, which you cannot avoid. However, the amount of hardness lost is controllable and dependent on the temperature you subject the steel to during the tempering process. That is true of all steels except high-speed steel; tempering increases the hardness of high-speed steel.
The annealing, normalizing, and hardening processes all include steps at temperatures above the metal’s upper critical point. Tempering is always conducted at temperatures below the metal’s low-critical point.
When you reheat hardened steel, you begin tempering it at 212°F, and continue as the temperature increases toward the low-critical point. You can predetermine the resulting hardness and strength if you preselect the finite tempering temperature. For planning your tempering time, the minimum should be one hour, or if the part is more than one inch thick, increase the time by one additional hour for each additional inch of thickness.
With most steels, the rate of cooling from the tempering temperature has no effect on the steel. After a steel part is removed from the tempering furnace, it is usually cooled in still air, just like in the normalizing process.
However, there are a few anomalies; a few types of steel must be quenched from the tempering temperature to prevent brittleness. Known as blue brittle steels, they can become brittle if heated in certain temperature ranges and cooled slowly. Some nickel chromium steels are subject to this temper brittleness.
Providing there is any hardness to temper, you can temper steel that has been normalized, but you cannot temper annealed steel. What would be the purpose? If you will remember, the purpose of both normalizing (air cooled), and annealing (controlled cooling environment) was to relieve stress, the same as tempering.
Tempering relieves internal stresses from quenching, reduces hardness and brittleness, and may actually increase the tensile strength of hardened steel as it is tempered up to a temperature of about 450°F; above 450°F, tensile strength starts to decrease.
Typically, tempering increases softness, ductility, malleability, and impact resistance, but again, high-speed steel is an exception to the rule. High-speed steel increases in hardness on tempering, provided you temper it at a high temperature (about 1150°F). Remember, to temper a part properly, you need to remove it from the quenching bath before it is completely cold and proceed with the tempering process. Failure to temper correctly can result in a quick failure of the hardened part.
Permanent steel magnets are made of hardened and tempered special alloys whose most important properties are stability and hardness. They are tempered at the minimum tempering temperature (212°F) by placing them in boiling water for 2 to 4 hours, and because of this low-tempering temperature, are very hard.
Do not temper case-hardened parts at too high a temperature or they will lose some of their hardness. A temperature range of 212°F — 400°F is high enough to relieve quenching stresses for case-hardened parts. The design of the part can help determine the appropriate tempering temperature, and some metals do not require tempering at all.
Tempering by color range is similar in concept to heat treating by color range, but one of the first things you will notice is the extreme differences in temperature gradations. Instead of the large 750°— 2350° range with color changes in 100° — 150° (+ –) segments for heat treating, the entire range for tempering by color is only about 170° with color changes in 10° — 20° (+ –) segments. (Figure 7)
In addition, instead of being based on the fundamental metal itself and its alloys as in heat treating, tempering by color is based on surface oxides that change colors as you heat the steel. As you slowly heat a piece of polished hardened steel, you will see the surface turn various colors as the surface temperature changes; this indicates you are making structural changes within the metal.
Once the preplanned color appears, rapidly quench the part to prevent further structural change. The part may be heated by torch, furnace, hot plate, or radiation, but in all circumstances, it must be smooth and free of oil for true indication of color.
Figure 7 — Example of oxide colors for tempering steel.
Cold chisels and similar tools must have hard cutting edges with softer bodies and heads. The heads must be tough but not brittle to prevent shattering when struck, the cutting edge must be twice as hard (or more) as the head, and the zone separating the two must blend the two extremes without a line of demarcation that would encourage breakage.
One method frequently used for tempering chisels and similar tools is one in which the cutting end is heated and tempered by the residual heat of the opposite end of the same tool. To simultaneously harden and temper a cold chisel by this method:
The result is a tough head, a fully hardened cutting edge, and a properly blended structure.
Figure 8 — Typical cold chisel tempering areas.
When the cutting end has cooled:
As soon as the correct shade of blue appears:
By completing this described process, you will have hardened and tempered the chisel, and it only needs grinding.
The oxide color at which you quench the steel during tempering will vary with the properties you want to attain in the part. Refer again to Figure 7. To see the colors clearly, turn the part from side to side under good lighting conditions. While hand tempering can produce the same result as furnace tempering, there is a greater possibility for error, so the slower you perform the operations the more accurate your results will be.
|Test Your Knowledge
1. To make a metal more useful, heat treating can make it stronger, more resistant to impact, malleable, and ductile with one process.
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The rate at which you can cool an object will depend on several factors:
Each is a factor in deciding which quenching medium you should use.
The value of any quenching medium’s cooling rate upon a quenched part will vary with the medium’s temperature; therefore, to get uniform results, you must keep the medium’s temperature within prescribed limits.
The quenching medium’s absorption of heat will also depend on the circulation of the medium or the movement of the part; agitating the liquid or the part breaks up gas that forms an insulating blanket between the part and the liquid, hence increasing the time element to cool to a given temperature.
Normally, when you quench a metal, hardening occurs and the metal’s composition will determine the type of quench to use to achieve the desired hardness.
For example, shallow-hardened, low-alloy, and carbon steels require more severe quenching than deep-hardened alloy steels with large quantities of nickel, manganese, or other elements. Therefore, the shallow-hardening steels are usually quenched in water or brine while the deep-hardening steels are usually quenched in oil.
Sometimes it is necessary to use a combination quench (starting with brine or water and finishing with oil), for in addition to producing the desired hardness, the quench must minimize cracking, warping, and soft spots.
The quenching liquid’s volume needs to be large enough to absorb all the heat during a normal quenching operation.
As you quench more metals, the medium’s temperature will rise as the liquid absorbs the heat.
This temperature rise will cause a decrease in the cooling rate, which in turn will negatively affect your efforts to harden the metal.
Some tanks use mechanical means to keep temperatures at prescribed levels during continuous operations, such as the heat exchanger shown in Figure 9.
Figure 9 — Example of a controlled temperature quench tank.
There are two methods for liquid quenching:
As already mentioned, for satisfactory heat treating results, quenching liquids must be maintained at uniform temperatures; this is particularly true for oil. Many commercial operations that use oil-quenching tanks maintain the oil bath at their proper temperature by circulating the oil medium through coils that themselves are water cooled. Self-contained coolers are an integral part of large quench tanks.
Quenching tanks have a wide range of capabilities, from the large commercial polymer-quenching tank shown in Figure 10 View A, to the small portable water- and oil-quenching tank Figure 10 View B.
Figure 10 — Examples of the wide range of quenching tanks.
The typical portable quenching tank shown in Figure 10 View B can be moved to various parts of the shop for heat treating as needed. These portable tanks may have just one compartment, but typically, they have compartments for containing water in one segment and oil in another, with a liquid-tight partition to prevent mixing.
Each compartment has a drain plug, a screen in the bottom to catch scale and other foreign matter, and a mesh basket to hold the parts. In addition, shops can attach a portable electric pump or a mechanical agitation mixer to the rim of the tank for liquid circulation to aid in uniform cooling.
You can use water to quench some forms of steel, but water is not recommended for tool steel or other alloy steels. Water absorbs large quantities of atmospheric gases, which have a tendency to form bubbles on the metal’s surface when you quench a hot piece. The bubbles tend to collect in holes or recesses causing soft spots that can lead to cracking or warping.
For any given part to be treated, the quench tank must meet the following criteria:
When you quench aluminum alloys and other nonferrous metals, always quench them in clean water.
You prepare brine by dissolving common rock salt in water. The brine solution should contain from 7% to 10% salt by weight or three-fourths pound of salt for each gallon of water. Brine reduces the water’s absorption of atmospheric gases, thus reducing the amount of bubbles and allowing greater surface contact to cool the part more rapidly than water. The correct temperature range for a brine solution is 65°F to 100°F.
You can quench low-alloy and carbon steels in brine solutions, but brine is not recommended for high-carbon or low-alloy steels with uneven cross sections; the rapid cooling rate of brine can cause cracking or stress in the latter.
In addition to rapid and uniform cooling, a brine medium removes a large percentage of any scale that may be present, but do not quench nonferrous metals in brine due to the corrosive action brine has on these metals.
Use oil to quench high-speed and oil-hardened steels and preferably all other steels if you can obtain the required hardness. Practically any type of obtainable oil is acceptable as quenching oil, including the various animal oils, fish oils, vegetable oils, and mineral oils.
Oil is classed as an intermediate quench; its cooling rate is slower than brine or water but faster than air. Keep the quenching oil’s temperature within a range of 80°F to 150°F.
In small amounts, the water that usually collects in the bottom of a quenching oil tank is not harmful, but in large quantity it can interfere with the quenching operations. For example, if the end of a long piece extends through the oil into the water at the bottom of the tank, the more rapid cooling action of the water can cause the piece to crack.
Nonferrous metals are not routinely oil quenched unless called for in the specifications.
Table 2 provides the properties and average cooling rates of various quenching oils relative to water.
Table 2 — Properties and Average Cooling Abilities of Quenching Media
|Quenching Media||Cooling Rate Compared to Water||Flash Point
|Caustic Soda (Sodium Hydroxide) (10%)||2.06||-||-|
|Brine (10%) at 65°||1.96||-||-|
|Water at 65°||1.00||-||-|
Only use caustic soda for specific types of steel that require extremely rapid cooling. Refer to Table 2. Like brine, a solution of water and caustic soda (10% caustic soda by weight) has a higher cooling rate than water. However, caustic soda (note the name “caustic”) requires special attention.
Never quench nonferrous metals in caustic soda Caustic Soda requires special handling because of its harmful effects on skin and clothing.
As the term implies, when you dry quench, you are using materials other than liquids and in most cases only to slow the cooling rate to prevent warping or cracking.
You use air quenching for cooling some highly alloyed steels. If you use still air, place each tool or part on a suitable rack so air can reach all sections of the piece.
If you use circulated air, place them in the same manner in a suitable rack, but ensure that the circulated air from the source reaches the parts equally for uniform cooling.
You can use compressed air to concentrate cooling on specific areas of a part, but to prevent cracking the metal you must first ensure that the air lines are dry and free of the moisture that typically builds in compression tanks and lines.
To quench nonferrous metals, you should use water, but when necessary, you can use forced-air drafts to cool pieces too large to fit into the quench tank. However, you should only use an air quench for nonferrous metal when the part will not be subjected to severe corrosion conditions, and the required strength and other physical properties can be developed by a mild air quench.
The solids you can use for cooling steel parts include cast iron chips, lime, sand, and ashes. Generally, you would use them to slow the rate of cooling; for example, you might place a cast iron part in a lime box after welding to prevent cracking and warping. Regardless of which solid you select, it must be free of moisture to prevent uneven cooling.
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This course has covered just a few elements of the heat treating theory and explained how you can change the properties of a metal. The heat treatment you apply as a steelworker can, if done properly, extend the service life of appropriate TOA parts and equipment. Conversely, if done improperly, you could shorten the service life. To recognize the appropriate treatment for achieving the desired properties for a selected metal is your challenge. However, you should now be able to recognize a reference chart for color temperature, and be able to select a suitable general method of heat treatment with the correct quenching medium to achieve the targeted properties. You may not achieve the ultimate properties on the first try, but repeated practice and experimentation will improve your ability in this set of skills.
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1. What process consists of tempering, normalizing, hardening, and annealing?
2. Most nonferrous metals can be normalized and case hardened but not annealed.
3. Which of the following conditions is required for the successful heat treatment of metals?
4. What type of furnace produces an atmosphere consisting of a gas/air combustion product?
5. Which of these gas mixtures are constituents of a fuel-fired furnace atmosphere?
6. What allows you to provide an oxidizing, reducing, or neutral atmosphere in a fuel fired furnace?
7. What type of furnace(s) allows the atmosphere to consist of air only?
8. What is the primary cause of distortion and cracking of the heat-treated part?
9. How do you determine the soaking period when parts are uneven in cross section?
10. What type of medium is normally used to quench nonferrous metals?
11. What effect is produced when steel is cooled very slowly in a medium that does NOT conduct heat easily?
12. Copper becomes hard and brittle when mechanically worked, but it can be made soft again by annealing. Within what temperature range must you heat it to anneal it?
13. Normalizing is a form of heat treatment applicable to nonferrous metals only.
14. Which of these metals are difficult to harden by heat treatment?
15. What factor almost completely determines the maximum obtainable hardness in plain carbon steel?
16. What case-hardening method produces the hardest surface of any of the hardening processes?
17. If the steel parts are placed in a container packed with charcoal and heated in a furnace, what case-hardening process is being used?
18. On what areas of a part being flame hardened should a slightly oxidizing flame be used?
19. Which of these factors determines the rate at which you move the welding torch when flame hardening a steel part?
20. Flame hardening can produce a hard case that resists wear while the core retains the metal’s original properties.
21. What term is used to describe the process of heating steel to a specific temperature (below its hardening temperature), holding this temperature for a certain length of time, and then cooling the steel in still air to room temperature?
22. Steel can be tempered provided some hardness remains after it has been normalized.
23. In which of the following metals are the softness, ductility, and resistance to impact NOT increased?
24. What are the most important properties to be obtained in tempering permanent steel magnets?
25. Why should you agitate the part or the quenching medium when cooling a part?
26. For which of the following reasons is the flush method of quenching better than other quenching methods for parts having cavities or recesses?
27. What temperature should water not exceed when used as a quenching medium?
28. Which of these quenching media has the highest cooling rate compared to water?
29. What is the correct solution for a brine quench medium?
30. Caustic soda requires special handling because of its harmful effects on skin and clothing.
31. Air quenching should only be used for nonferrous metals
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