Four requirements must be kept in mind when selecting substitute metals. The first and most important of these is maintaining the original strength of the structure. The other three are:
(1) Maintaining contour or aerodynamic smoothness, (2) maintaining original weight, if possible, or keeping added weight to a minimum, and (3) maintaining the original corrosion-resistant properties of the metal.
PRINCIPLES OF HEAT TREATMENT Heat treatment is a series of operations involving the heating and cooling of metals in the solid state. Its purpose is to change a mechanical property or combination of mechanical properties so that the metal will be more useful, serviceable, and safe for a definite purpose. By heat treating, a metal can be made harder, stronger, and more resistant to impact, Heat treating can also make a metal softer and more ductile. No one heat-treating operation can produce all of these characteristics.
In fact, some properties are often improved at the expense of others. In being hardened, for example.
a metal may become brittle.
The various heat-treating processes are similar in that they all involve the heating and cooling of metals. They differ, however, in the temperatures to which the metal is heated, the rate at which it is cooled, and, of course, in the final result.
The most common forms of heat treatment for ferrous metals are hardening, tempering, normalizing, annealing, and casehardening. Most nonferrous metals can be annealed and many of them can be hardened by heat treatment. However, there is only one nonferrous metal, titanium, that can be casehardened, and none can be tempered or normalized.
Internal Structure of Metals The results obtained by heat treatment depend to a great extent on the structure of the metal and on the manner in which the structure changes when the metal is heated and cooled. A pure metal cannot be hardened by heat treatment because there is little change in its structure when heated.
On the other hand, most alloys respond to heat treatment since their structures change with heating and cooling.
An alloy may be in the form of a solid solution, a mechanical mixture, or a combination of a solid solution and a mechanical mixture. When an alloy is in the form of a solid solution, the elements and compounds which form the alloy are absorbed, one into the other, in much the same way that salt is dissolved in a glass of water, and the constituents cannot be identified even under a microscope.
When two or more elements or compounds are mixed but can be identified by microscopic examination, a mechanical mixture is formed.
A mechanical mixture can be compared to the mixture of sand and gravel in concrete. The sand and gravel are both visible. Just as the sand and gravel are held together and kept in place by the matrix of cement, the other constituents of an alloy are embedded in the matrix formed by the An alloy in the form of a mechanical mixture at ordinary temperatures may change to a solid solution when heated. When cooled back to normal temperature, the alloy may return to its original structure. On the other hand, it may remain a solid solution or form a combination of a solid solution and mechanical mixture. An alloy which consists of a combination of solid solution and mechanical mixture at normal temperatures may change to a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original structure, or form a complex solution.
HEAT-TREATING EQUIPMENT Successful heat treating requires close control over all factors affecting the heating and cooling of metals. Such control is possible only when the proper equipment is available and the equipment is selected to fit the particular job. Thus, the furnace must be of the proper size and type and must be so controlled that temperatures are kept within the limits prescribed for each operation.
Even the atmosphere within the furnace affects the condition of the part being heat treated.
Further, the quenching equipment and the quenching medium must be selected to fit the metal and the heat-treating operation. Finally, there must be equipment for handling parts and materials, for cleaning metals, and for straightening parts.
Furnaces and Salt Baths There are many different types and sizes of furnaces used in heat treatment. As a general rule, furnaces are designed to operate in certain specific temperature ranges and attempted use in other ranges frequently results in work of inferior quality. In addition, using a furnace beyond its rated 205
maximum temperature shortens its life and may necessitate costly and time-consuming repairs.
Fuel-fired furnaces (gas or oil) require air for proper combustion and an air compressor or blower is therefore necessary. These furnaces are usually of the muffler type: that is, the combustion of the fuel takes place outside of and around the chamber in which the work is placed. If an open mufler is used, the furnace should be designed so as to prevent the direct impingement of flame on the work.
In furnaces heated by electricity the heating ele- for continuous operation.
ments are generally in the form of wire or ribbon.
Good design requires incorporation of additional heating elements at locations where maximum heat loss may be expected. Such furnaces commonly mosphere.
operate up to a maximum temperature of about 2,000° F. Furnaces operating at temperatures up to about 2,500° F. usually employ resistor bars of sintered carbides.
a pyrometer.
work as possible.
at the desired value.
to high as 2,450° F. used in th The in furnaces. nuch faste They trol. shape, They ca anism such as a relay.
ay rest OI Heating are als surface bath ntain poi furnace ing must be used.
must regardor the temperature.
should as long will 206
accurate method involves the use of commercial Fahrenheit Centigrade crayons, pellets, or paints that melt at various 1500 temperatures within the range of 125° F. to 1,600° 2700 F. The least accurate method of temperature esti- 2600 mation is by observation of the color of the hot • 1400 colors observed are affected by many factors, such hearth of the furnace or of the work. The heat 2500 as the conditions of artificial or natural light, the 2400 character of the scale on the work, etc. • 1300 Steel begins to appear dull red at about 1,000° F., 2300 Color of and as the temperature increases the color changes Hot Body gradually through various shades of red to orange, 220 0 • 1200 to yellow, and finally to white. A rough approxi- White mation of the correspondence between color and 2100 temperature is indicated in figure 6-64.
It is also possible to secure some idea of the 2000 • 1100 temperature of a piece of carbon or low-alloy steel, in the low temperature range used for temper- 190 0 Yellow ing, from the color of the thin oxide film that forms 1800 • 1000 on the cleaned surface of the steel when heated in this range. The approximate temperature-color 1700 Orange relationship for a time at temperature of about one- 900 half is indicated on the lower portion of the scale 1600 in figure 6-64. Light Cherry 1500 Protective Atmospheres 800 It is often necessary or desirable to protect steel 1400 Full Cherry or cast iron from surface oxidation (scaling) and loss of carbon from the surface layers (decarburi- 1300 700 zation). Commercial furnaces, therefore, are generally equipped with some means of atmosphere 1200 Dark Cherry control. This usually is in the form of a burner for burning controlled amounts of gas and air and 110 0 600 directing the products of combustion into the furnace muffle. Water vapor, a product of this com- 1000 Dark Red bustion, is detrimental and many furnaces are 500 equipped with a means for eliminating it. For furnaces not equipped with atmosphere control, a variety of external atmosphere generators are avail- 200 able. The gas so generated is piped into the furnace and one generator may supply several Temper furnaces. If no method of atmosphere control is Colors available, some degree of protection may be secured 300 Blue chips- by covering the work with cast iron borings or Purple Brown Since the work in salt or lead baths is surrounded 400 200 Straw by the liquid heating medium, the problem of preventing scaling or decarburization is simplified. 300 Vacuum furnaces also are used for annealing steels, especially when a bright non-oxidized surtace is a prime consideration.
Soaking The temperature of the furnace must be held constant during the soaking period, since it is during this period that rearrangement of the internal structure of the steel takes place. Soaking 200 100 100
- 0 FIGURE 6-64. Temperature chart indicating conversion of Centigrade to Fahrenheit or visa versa, color temperatures scale for hardening-temperature range, and tempering-temperature range.
temperatures for various types of steel are specified in ranges varying as much as 100° F. (See figure
6-65. Small parts are soaked in the lower part of the specified range and heavy parts in the upper part of the specified range. The length of the soaking period depends upon the type of steel and the size of the part. Naturally, heavier parts require longer soaking to ensure equal heating throughout. As a general rule, a soaking period of 30 minutes to 1 hour is sufficient for the average heat-treating operation.
Cooling The rate of cooling through the critical range determines the form that the steel will retain.
Various rates of cooling are used to produce the desired results. Still air is a slow cooling medium, but is much faster than furnace cooling. Liquids are the fastest cooling media and are therefore There are three commonly used quenching liquids brine, water, and oil. Brine is the most severe medium, water is next, and oil is the least severe. Generally an oil quench is used for alloy steels, and brine or water for carbon steels.
Quenching Media Quenching solutions act only through their ability to cool the steel. They have no beneficial chemical action on the quenched steel and in themselves impart no unusual properties. Most requirements for quenching media are met satisfactorily by water or aqueous solutions of inorganic salts such as table salt or caustic soda, or by some type of oil. The rate of cooling is relatively rapid during quenching in brine, somewhat less rapid in water, and slow in oil.
Brine usually is made of a 5- to 10-percent solution of salt (sodium chloride) in water. In addition to its greater cooling speed, brine has the ability to "throw" the scale from steel during quenching. The cooling ability of both water and brine, particularly water, is considerably affected by their temperature. Both should be kept coldwell below 60° F. If the volume of steel being quenched tends to raise the temperature of the bath appreciably, the quenching bath should be cooled by adding ice or by some means of refrigeration.
There are many specially prepared quenching oils on the market; their cooling rates do not vary widely. A straight mineral oil with a Saybolt 100 k. have the viscosity of about 100 at 100° F. is generally used.
Unlike brine and water, the oils have the greatest cooling velocity at a slightly elevated temperatureabout 100 to 140° F.-because of their decreased viscosity at these temperatures.
When steel is quenched, the liquid in immediate contact with the hot surface vaporizes; this vapor reduces the rate of heat abstraction markedly.
Vigorous agitation of the steel or the use of a pressure spray quench is necessary to dislodge these vapor films and thus permit the desired rate of cooling.
The tendency of steel to warp and crack during the quenching process is difficult to overcome because certain parts of the article cool more rapidly than others. The following recommendations will greatly reduce the warping tendency.
I. A part should never be thrown into the quenching bath. By permitting it to lie on the bottom of the bath, it is apt to cool faster on the top side than on the bottom side, thus causing it to warp or crack.
2. The part should be agitated slightly to destroy the coating of vapor which might prevent it from cooling rapidly. This al lows the bath to convey its heat to the atmosphere.
3. Irregular-shaped parts should be immersed in such a way that the heavy end enters the bath first.
Quenching Equipment The quenching tank should be of the proper size to handle the material being quenched. Circulating pumps and coolers may be used to maintain approximately constant temperatures when a large amount of quenching is to be done. To avoid building up a high concentration of salt in the quenching tank, provision must be made to add fresh water to the quench tank used for molten Tank location in reference to the heat-treating furnace is very important. The tank should be situated to permit rapid transfer of the part from the furance to the quenching medium. A delay of more than a few seconds will, in many instances, prove detrimental to the effectiveness of the heat treatment. When material of thin section is being heat treated, guard sheets should be emploped to retard the loss of heat during transfer to the quench tank. A rinse tank must be provided to remove all salt from the material after quenching if the salt has not been adequately removed in the quenching
HEAT TREATMENT OF FERROUS METALS The first important consideration in the heat treatment of a steel part is to know its chemical 208
Temperatures Tempering (drawing) temperature for tensile strength (p.s.i.)
Quenching Steel No. Normalizing Annealing Hardening medium 100,000 125,000 150,000 180,000 200,000 air cool °F °F °F °F °F 1020 1,650-1,750 1,600-1,700 1,575-1,675 Water 1,650-1,750 1,600-1,700 1,575-1,675 1025 1,600-1,700 1,575-1,650 1,575-1,675 Water 1035 1,575-1,650 1,575-1,625 1,525-1,600 Water 1045 1,550-1,600 1,550-1,600 1,475-1,550 L,475-1,550 1,450-1,500 1,425-1,500 Oil 750 L.475-1,52: 1,425-1,475 1.450-1,500 Oil or 1,600-1,650 1,475-1,525 650 1,600-1,650 1,500-1,550 1,475-1,525 700 1,525-1,575 1,525-1,575 4130(x4130) 1.600-1.700 1,525-1,575 1,575-1,625 4140 .,600-1,650 1,525-1,575 1,525-1,575 4150 1, 550-1,600 4340(x4340) 1,550-1,625 1,475-1,550 4640 1,675-1,700 1,525-1,575 6135 1,600-1,700 1,575-1,625 6150 1, 600-1, 650 800 6195 1,600-1,650 1,525-1,575 1,500-1,550 NE8620 1,525-1,575 NE8630 1,525-1,575 NE8735 1,525-1,575 ,525-1,575 NE8740 1,500-1,550 1,500-1,550 850 30905 51210 1,525-1,575 1,525-1,575 1,775-1,825 51335 1,525-1,575 1,775-1,850 Oil 52100 1,625-1,700 1,525-1,550 Oil Corrosion resisting (16-2) (1) (m) Silicon chro- 1,700-1,725 Oil mium (for springs) NOTES
(a) Draw at 1150° F. for tensile strength of 70,000 p.s.i.
(b) Hardpring emper draw at 800° to 900° F. Rockwell
(c) Bars or torgings may be quenched in water from 1,500°-1,600
(d) Air-cooling from the normalizing temperature will produce a tensile strength of approximately 90,000 p-s.i.
(e) For spring temper draw at 850° to 950° F. Rockwell Hardnesa C-40-45.
(f) Draw at 350° to 450° F. to remove quenching strains.
Rockwell Hardness C-60-65.
(g) Anneal at 1,600° to 1,700° F. to remove residual stresses due to welding or cold work. May be applied only to steel containing titanium or columbium.
(h) Anneal at 1,900° to 2,100° F. to produce maximum softness and corrosion resistance. Cool in air or quench in water.
(i) Harden by cold work only.
Lower side of range for sheet 0.06 inch and under.
Middle of range for sheet and wire 0.125 inch. Upper side of range for forgings
(k) Not recommended for intermediate tensile strengths because of low impact.
(1) AN-QQ-S-770.-It is recommended that, prior to tempering, corrosion-resisting (16 Cr-2 Ni) steel be 1,900 F., after a soaking period of 1/2 hour at thie in oil from a temperature of 1,875° to temperature. To obtain a tensile strength at 115,000
p.s.i., the tempering temperature should be approxi mately 525° F. A holding time at these temperatures of about 2 hours is recommended Tempering tem peratures between 700° and 1,100° F. will not be ap.
(m)
(n) Water used for quenching shall not exceed 65° F Dil used for quenching shall be within the temperatur range of 80°-150° F.
FIGURE 6-65. Heat treatment procedures for steels.
....
composition. This, in turn, determines its upper critical point. When the upper critical point is known, the next consideration is the rate of heating and cooling to be used. Carrying out these operations involves the use of uniform heating furnaces, proper temperature controls, and suitable quenching mediums.
Behavior of Steel During Heating and Cooling Changing the internal structure of a ferrous metal is accomplished by heating to a temperature above its upper critical point, holding it at that temperature for a time sufficient to permit certain internal changes to occur, and then cooling to atmospheric temperature under predetermined, At ordinary temperatures, the carbon in steel exists in the form of particles of iron carbide scattered throughout an iron matrix known as "ferrite." The number, size, and distribution of these particles determine the hardness of the steel.
At elevated temperatures, the carbon is dissolved in the iron matrix in the form of a solid solution called "austenite," and the carbide particles appear only after the steel has been cooled. If the cooling is slow, the carbide particles are relatively coarse and few. In this condition, the steel is soft.
If the cooling is rapid, as by quenching in oil or water, the carbon precipitates as a cloud of very fine carbide particles, and the steel is hard. The fact that the carbide particles can be dissolved in austenite is the basis of the heat treatment of steel.
The temperatures at which this transformation takes place are called the critical points and vary with the composition of the steel. The element normally having the greatest influence is carbon.
certain point. In practice, that point is 0.85 percent carbon content. When the carbon content is increased beyond 0.85 percent, there is no increase in wear resistance.
For most steels, the hardening treatment consists of heating the steel to a temperature just above the upper critical point, soaking or holding for the required length of time, and then cooling it rapidly by plunging the hot steel into oil, water, or brine. Although most steels must be cooled rapidly for hardening, a few may be cooled in still air.
Hardening increases the hardness and strength of the steel but makes it less ductile.
When hardening carbon steel, it must be cooled to below 1,000° F. in less than 1 second. Should the time required for the temperature to drop to 1,000° F. exceed 1 second, the austenite begins to transform into fine pearlite. This pearlite varies in hardness, but is much harder than the pearlite formed by annealing and much softer than the martensite desired. After the 1,000° F. temperature is reached, the rapid cooling must continue if the final structure is to be all martensite.
When alloys are added to steel, the time limit for the temperature drop to 1,000° F. increases above the 1-second limit for carbon steels.
Therefore, a slower quenching medium will produce hardness in alloy steels.
Because of the high internal stresses in the "as quenched" condition, steel must be tempered just before it becomes cold. The part should be removed from the quenching bath at a temperature of approximately 200° F., since the temperature range from 200° F. down to room temperature is the cracking range.
Hardening temperatures and quenching mediums for the various types of steel are listed in figure 6-65.
Pure iron, wrought iron, and extremely lowcarbon steels cannot be appreciably hardened by heat treatment, since they contain no hardening element. Cast iron can be hardened, but its heat treatment is limited. When cast iron is cooled rapidly, it forms white iron, which is hard and brittle. When cooled slowly, it forms gray iron, which is soft but brittle under impact.
In plain carbon steel, the maximum hardness depends almost entirely on the carbon content of the steel. As the carbon content increases, the ability of the steel to be hardened increases.
However, this increase in hardenability with an increase in carbon content continues only to a Hardening Precautions A variety of different shapes and sizes of tongs for handling hot steels is necessary. It should be remembered that cooling of the area contacted by the tongs is retarded and that such areas may not harden, particularly if the steel being treated is very shallow hardening. Small parts may be wired together or quenched in baskets made of wire mesh.
Special quenching jigs and fixtures are frequently used to hold steels during quenching in a manner to restrain distortion.
When selective hardening is desired, portions of the steel may be protected by covering with alundum cement or some other insulating material.
Selective hardening may be accomplished also by 210
the use of water or oil jets designed to direct quenching medium on the areas to be hardened.
This also is accomplished by the induction and flame-hardening procedures previously described, particularly on large production jobs.
Shallow hardening steels, such as plain carbon and certain varieties ot alloy steels, have such a high critical cooling rate that they must be quenched in brine or water to effect hardening.
In general, intricately shaped sections should not be made of shallow hardening steels because of the tendency of these steels to warp and crack during hardening. Such items should be made of deeper hardening steels capable of being hardened by quenching in oil or air.
Tempering Tempering reduces the brittleness imparted by hardening and produces definite physical properties within the steel. Tempering always follows, never precedes, the hardening operation. In addition to reducing brittleness, tempering softens Tempering is always conducted at temperatures below the low critical point of the steel. In this respect, tempering differs from annealing, normalizing, or hardening, all of which require temperatures above the upper critical point. When hardened steel is re-heated, tempering begins at 212° F. and continues as the temperature increases toward the low critical point. By selecting a definite tempering temperature, the resulting hardness and strength can be predetermined.
Approximate temperatures for various tensile strengths are listed in figure 6-65. The minimum time at the tempering temperature should be 1 hour. If the part is over 1 inch in thickness, the time should be increased by 1 hour for each additional inch of thickness. Tempered steels used in aircraft work have from 125,000 to 200,000 p.s.i.
ultimate tensile strength.
Generally, the rate of cooling from the tempering temperature has no effect on the resulting structure; therefore, the steel is usually cooled in still air after being removed from the furnace.
Annealing of steel produces a fine-grained, soft, ductile metal without internal stresses or strains.
In the annealed state, steel has its lowest strength.
In general, annealing is the opposite of hardening.
Annealing of steel is accomplished by heating the metal to just above the upper critical point, soaking at that temperature, and cooling very slowly in the furnace. (See figure 6-65 for recommended temperatures.) Soaking time is approximately 1 hour per inch of thickness of the material. To produce maximum softness in steel, the metal must be cooled very slowly. Slow cooling is obtained by shutting off the heat and allowing the furnace and metal to cool together to 900° F. or lower, then removing the metal from the furnace and cooling in still air. Another method is to bury the heated steel in ashes, sand, or other substance that does not conduct heat readily.
Normalizing air cooling, as compared to furnace cooling, results in a harder and stronger material than that obtained by annealing. Recommended normalizing temperatures for the various types of aircraft steels are listed in figure 6-65.
CASEHARDENING Casehardening produces a hard wear-resistant surface or case over a strong, tough core. Casehardening is ideal for parts which require a wear-resistant surface and, at the same time, must be tough enough internally to withstand the applied loads. The steels best suited to casehardening are the low-carbon and low-alloy steels.
If high-carbon steel is casehardened, the hardness penetrates the core and causes brittleness.
In casehardening, the surface of the metal is changed chemically by introducing a high carbide or nitride content. The core is unaffected chemi- 211
cally. When heat treated, the surface responds to hardening while the core toughens. The common forms of casehardening are carburizing, cyaniding, and nitriding. Since cyaniding is not used in aircraft work, only carburizing and nitriding are discussed in this section.
Carburizing Carburizing is a casehardening process in which carbon is added to the surface of low-carbon steel.
Thus, a carburized steel has a high-carbon surface and a low-carbon interior. When the carburized steel is heat treated, the case is hardened while the core remains soft and tough.
A common method of carburizing is called "pack carburizing." When carburizing is to be done by this method, the steel parts are packed in a container with charcoal or some other material rich in carbon. The container is then sealed with fire clay, placed in a furnace, heated to approximately 1,700° F., and soaked at that temperature for several hours. As the temperature increases, carbon monoxide gas forms inside the container and, being unable to escape, combines with the gamma iron in the surface of the steel. The depth to which the carbon penetrates depends on the length of the soaking period. For example, when carbon steel is soaked for 8 hours, the carbon penetrates to a depth of about 0.062 inch.
In another method of carburizing, called "gas carburizing," a material rich in carbon is introduced into the furnace atmosphere. The carburizing atmosphere is produced by the use of various gases or by the burning of oil, wood, or other materials. When the steel parts are heated in this atmosphere, carbon monoxide combines with the gamma iron to produce practically the same results as those described under the packcarburizing process.
A third method of carburizing is that of "liquid carburizing." In this method the steel is placed in a molten salt bath that contains the chemicals required to produce a case comparable with one resulting from pack or gas carburizing.
Alloy steels with low-carbon content as well as low-carbon steels may be carburized by either of the three processes. However, some alloys, such as nickel, tend to retard the absorption of carbon. As a result, the time required to produce a given thickness of case varies with the composition of Nitriding Nitriding is unlike other casehardening processes in that, before nitriding, the part is heat treated to produce definite physical properties.
Thus, parts are hardened and tempered before being nitrided. Most steels can be nitrided, but special alloys are required for best results. These special alloys contain aluminum as one of the alloying elements and are calied 'nitralloys."
In nitriding, the part is placed in a special nitriding furnace and heated to a temperature of approximately 1,000° F. With the part at this temperature, ammonia gas is circulated within the specially constructed furnace chamber. The high temperature cracks the ammonia gas into nitrogen and hydrogen. The ammonia which does not break down is caught in a water trap below the regions of the other two gases. The nitrogen reacts with the iron to form nitride. The iron nitride is dispersed in minute particles at the surface and works inward. The depth of penetration depends on the length of the treament. In nitriding, soaking periods as long as 72 hours are frequently required to produce the desired thickness of case.
Nitriding can be accomplished with a minimum of distortion, because of the low temperature at which parts are casehardened and because no quenching is required after exposure to the ammonia gas.
HEAT TREATMENT OF NONFERROUS METALS Aluminum Alloys There are two types of heat treatments applicable to aluminum alloys. One is called solution heat treatment, and the other is known as precipitation heat treatment. Some alloys, such as 2017 and 2024, develop their full properties as a result of solution heat treatment followed by about 4 days of aging at room temperature. Other alloys, such as 2014 and 7075, require both heat treatments.
The alloys that require precipitation heat treatment (artificial aging) to develop their full strength also age to a limited extent at room temperature; the rate and amount of strengthening depends upon the alloy. Some reach their maximum natural or room-temperature aging strength in a few days, and are designated as - T4 or - T3 temper. Others continue to age appreciably over a long period of time. Because of this natural aging, 212
the - W designation is specified only when the period of aging is indicated, for example, 7075-W (½ hour). Thus, there is considerable difference in the mechanical and physical properties of freshly quenched (- W) material and material that is in the -T3 or -14 temper.
The hardening of an aluminum alloy by heat treatment consists of four distinct steps:
1. Heating to a predetermined temperature.
2. Soaking at temperature for a specified length of time.
3. Rapidly quenching to a relatively low temperature.
4. Aging or precipitation hardening either spontaneously at room temperature, or as a result of a low-temperature thermal treatment.
The first three steps above are known as solution heat treatment, although it has become common practice to use the shorter term, "heat treatment".
Room-temperature hardening is known as natural aging, while hardening done at moderate temperatures is called artificial aging, or precipitation heat treatment.
SOLUTION HEAT TREATMENT Temperature The temperatures used for solution heat treating vary with different alloys and range from 825° F.
to 980° F. As a rule, they must be controlled within a very narrow range (plus or minus 10°) to obtain specified properties.
If the temperature is too low, maximum strength will not be obtained. When excessive temperatures are used, there is danger of melting the low-melting constituents of some alloys with consequent lowering of the physical properties of the alloy. Even if melting does not occur, the use of higher-than-recommended temperatures promotes discoloration and increases quenching strains.
Time at Temperature The time at temperature, referred to as soaking time, is measured from the time the coldest metal reaches the minimum limit of the desired temperature range. The soaking time varies, depending upon the alloy and thickness, from 10 minutes for thin sheets to approximately 12 hours for heavy forgings. For the heavy sections, the nominal soaking time is approximately 1 hour for each inch of cross-sectional thickness (see figure 6-66).
The soaking time is chosen so that it will be the minimum necessary to develop the required physical properties. The effect of an abbreviated soaking time is obvious. An excessive soaking period aggravates high-temperature oxidation.
With clad material, prolonged heating results in excessive diffusion of copper and other soluble constituents into the protective cladding and may defeat the purpose of cladding.
Quenching After the soluble constituents are in solid solution, the material is quenched to prevent or retard immediate re-precipitation. Three distinct quenching methods are employed. The one to be used in any particular instance depends upon the part, the alloy, and the properties desired.
Cold Water Quenching Parts produced from sheet, extrusions, tubing, small forgings, and similar type material are generally quenched in a cold water bath. The temperature of the water before quenching should not exceed 85° F. A sufficient quantity of water Thickness, in. Time, Minutes Up to .032... 30 .032 to 1/8 30 1/8 to 1/4 40 Over 1/4 60 NOTES: Soaking time starts when the metal (or the molten bath) reaches a temperature within the range specified above.
FIGURE 6-66. Typical soaking times for heat treatment.
should be used to keep the temperature rise under 20° F. Such a drastic quench ensures maximum resistance to corrosion. This is particularly important when working with such alloys as 2017, 2024, and 7075. This is the reason a drastic quench is preferred, even though a slower quench may produce the required mechanical properties.
Hot Water Quenching Large forgings and heavy sections can be quenched in hot or boiling water. This type &f quench minimizes distortion and alleviates cracking which may be produced by the unequal temperatures obtained during the quench. The use of a hot water quench is permitted with these parts because the temperature of the quench water does not critically affect the resistance to corrosion of the forging alloys. In addition, the resistance to corrosion of heavy sections is not as critical a factor as for thin sections.
Spray Quenching High-velocity water sprays are useful for parts formed from clad sheet and for large sections of almost all alloys. This type of quench also minimizes distortion and alleviates quench cracking.
However, many specifications forbid the use of spray quenching for bare 2017 and 2024 sheet materials because of the effect on their resistance to Lag Between Soaking and Quenching The time interval between the removal of the material from the furnace and quenching is critical for some alloys and should be held to a minimum.
When solution heat treating 2017 or 2024 sheet material, the elapsed time must not exceed 10 seconds. The allowable time for heavy sections may be slightly greater.
Allowing the metal to cool slightly before quenching promotes re-precipitation from the solid solution. The precipitation occurs along grain boundaries and in certain slip planes causing poorer formability. In the case of 2017, 2024, and 7075 alloys, their resistance to intergranular corrosion is adversely affected.
Re-heat Treatment The treatment of material which has been previously heat treated is considered a re-heat treatment. The unclad heat-treatable alloys can be solution heat treated repeatedly without harmful The number of solution heat treatments allowed for clad sheet is limited due to increased diffusion of core and cladding with each re-heating. Existing specifications allow one to three re-heat treatments of clad sheet depending upon cladding thickness.
Straightening After Solution Heat Treatment Some warping occurs during solution heat treatment, producing kinks, buckles, waves, and twists. These imperfections are generally removed by straightening and flattening operations.
Where the straightening operations produce an appreciable increase in the tensile and yield strengths and a slight decrease in the percent of elongation, the material is designated -13 temper. When the above values are not materially affected, the material is designated - T4 temper.
PRECIPITATION HEAT TREATING As previously stated, the aluminum alloys are in a comparatively soft state immediately after quenching from a solution heat-treating temperature. To obtain their maximum strengths, they must be either naturally aged or precipitation During this hardening and strengthening operation, precipitation of the soluble constituents from the supersaturated solid solution takes place. As precipitation progresses, the strength of the material increases, often by a series of peaks, until a maximum is reached. Further aging (overaging) causes the strength to steadily decline until a somewhat stable condition is obtained. The submicroscopic particles that are precipitated provide the keys or locks within the grain structure and between the grains to resist internal slippage and distortion when a load of any type is applied.
In this manner, the strength and hardness of the alloy are increased.
Precipitation hardening produces a great increase in the strength and hardness of the material with corresponding decreases in the ductile properties. The process used to obtain the desired increase in strength is therefore known as aging, or precipitation hardening.
The strengthening of the heat-treatable alloys by aging is not due merely to the presence of a precipitate. The strength is due to both the uniform distribution of a finely dispersed submicro- 214
Solution heat-treatment Precipitation heat-treatment Alloy Temp., Quench Temper Temp., Time of Temper °F desig. aging desig.
2017 930-950 Cold water T4 2117 930-950 Cold water T 2024 910-930 Cold water T4 T 6053 960-980 Water T4 445-455 1-2 hr T5 or 345-355 8 hr T6 6061 960-980 Water T4 315-325 18 hr T6 or 345-355 8 hr T6 7075 870 Water 250 24 hr T6 FIGURE 6-67. Conditions for heat treatment of aluminum alloys.
scopic precipitate and its effects upon the crystal structure of the alloy.
The aging practices used depend upon many properties other than strength. As a rule, the artificially aged alloys are slightly overaged to increase their resistance to corrosion. This is especially true with the artificially aged highcopper-content alloys that are susceptible to intergranular corrosion when inadequately aged.
The heat-treatable aluminum alloys are subdivided into two classes, those that obtain their full strength at room temperature and those that require artificial aging.
The alloys that obtain their full strength after 4 or 5 days at room temperature are known as natural aging alloys. Precipitation from the supersaturated solid solution starts soon after quenching, with 90 percent of the maximum strength generally being obtained in 24 hours.
Alloys 2017 and 2024 are natural aging alloys.
The alloys that require precipitation thermal treatment to develop their full strength are artificially aged alloys. However, these alloys also age a limited amount at room temperature, the rate and extent of the strengthening depending upon the alloys.
Many of the artificially aged alloys reach their maximum natural or room temperature aging strengths after a few days. These can be stocked for fabrication in the -T4 or -T3 temper.
High-zinc-content alloys such as 7075 continue to age appreciably over a long period of time, their mechanical property changes being sufficient to reduce their formability.
The advantage of - W temper formability can be utilized, however, in the same manner as with natural aging alloys; that is, by fabricating shortly after solution heat treatment, or retaining formability by the use of refrigeration.
Refrigeration retards the rate of natural aging.
At 32° F., the beginning of the aging process is delayed for several hours, while dry ice (-50° F.
to - 100° F.) retards aging for an extended period of time.
Precipitation Practices The temperatures used for precipitation hardening depend upon the alloy and the properties desired, ranging from 250° F. to 375° F. They should be controlled within a very narrow range (plus or minus 5°) to obtain best results. (See figure 6-67.)
The time at temperature is dependent upon the temperature used, the properties desired, and the alloy. It ranges from 8 to 96 hours. Increasing the aging temperature decreases the soaking period necessary for proper aging. However, a closer control of both time and temperature is necessary when using the higher temperatures.
After receiving the thermal precipitation treatment, the material should be air cooled to room temperature. Water quenching, while not necessary, produces no ill effects. Furnace cooling has a tendency to produce overaging.
ANNEALING OF ALUMINUM ALLOYS The annealing procedure for aluminum alloys consists of heating the alloys to an elevated temperature, holding or soaking them at this temperature for a length of time depending upon the mass of the metal, and then cooling in still air.
Annealing leaves the metal in the best condition for cold-working. However, when prolonged forming operations are involved, the metal will take on a condition known as "mechanical hardness" and will resist further working. It may be necessary to anneal a part several times during the forming process to avoid cracking. Aluminum alloys should not be used in the annealed state for parts or Clad parts should be heated as quickly and carefully as possible, since long exposure to heat tends to cause some of the constituents of the core to diffuse into the cladding. This reduces the corrosion resistance of the cladding.
HEAT TREATMENT OF ALUMINUM ALLOY RIVETS Aluminum alloy rivets are furnished in the following compositions: Alloys 1100, 5056, 2117, 2017, and 2024.
Alloy 1100 rivets are used in the "as fabricated" condition for riveting aluminum alloy sheets where a low-strength rivet is suitable. Alloy 5056 rivets are used in the "as fabricated" condition for riveting magnesium alloy sheets.
Alloy 2117 rivets have moderately high strength and are suitable for riveting aluminum alloy sheets. These rivets receive only one heat treatment, which is performed by the manufacturer, and are anodized after being heat treated. They require no further heat treatment before they are used. Alloy 2117 rivets retain their characteristics indefinitely after heat treatment and can be driven anytime. Rivets made of this alloy are the most widely used in aircraft construction.
Alloy 2017 and 2024 rivets are high-strength rivets suitable for use with aluminum alloy structures. They are purchased from the manufacturer in the heat-treated condition. Since the aging characteristics of these alloys at room temperatures are such that the rivets are unfit for driving, they must be re-heat treated just before they are to be used. Alloy 2017 rivets become too hard for driving in approximately 1 hour after quenching. Alloy 2024 rivets become hardened in 10 minutes after quenching. Both of these alloys may be re-heat treated as often as required;
however, they must be anodized before the first re-heat treatment to prevent intergranular oxidation of the material. If these rivets are stored in a refrigerator at a temperature lower than 32° F.
immediately after quenching, they will remain soft enough to be usable for several days.
Rivets requiring heat treatment are heated either in tubular containers in a salt bath, or in small screen-wire baskets in an air furnace. The heat treatment of alloy 2017 rivets consists of subjecting the rivets to a temperature between 930° F. to 950° F. for approximately 30 minutes, and immediately quenching in cold water. These rivets reach maximum strength in about 9 days after being driven. Alloy 2024 rivets should be heated to a temperature of 910° F. to 930° F. and immediately quenched in cold water. These rivets develop a greater shear strength than 2017 rivets and are used in locations where extra strength is required. Alloy 2024 rivets develop their maximum shear strength in 1 day after being driven.
The 2017 rivet should be driven within approximately 1 hour and the 2024 rivet within 10 to 20 minutes after heat treating or removal from refrigeration. If not used within these times, the rivets should be re-heat treated before being refrigerated.
HEAT TREATMENT OF MAGNESIUM ALLOYS Magnesium alloy castings respond readily to heat treatment, and about 95 percent of the magnesium used in aircraft construction is in the cast form.
The heat treatment of magnesium alloy castings is similar to the heat treatment of aluminum alloys in that there are two types of heat treatment:
(1) Solution heat treatment and (2) precipitation (aging) heat treatment. Magnesium, however, develops a negligible change in its properties when allowed to age naturally at room temperatures.
Solution Heat Treatment Magnesium alloy castings are solution heat treated to improve tensile strength, ductility, and shock resistance. This heat-treatment condition is indicated by using the symbol -T4 following the alloy designation. Solution heat treatment plus artificial aging is designated -T6. Artificial aging is necessary to develop the full properties of the metal.