....
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.
Solution heat-treatment temperatures for magnesium alloy castings range from 730° F. to 780° F., the exact range depending upon the type of alloy. The temperature range for each type of alloy is hsted in Specification MIL-H-0857. The upper limit of each range listed in the specification is the maximum temperature to which the alloy may be heated without danger of melting the The soaking time ranges from 10 to 18 hours, the exact time depending upon the type of alloy as well as the thickness of the part. Soaking periods longer than 18 hours may be necessary for castings over 2 inches in thickness. Magnesium alloys must never be heated in a salt bath as this may result in an explosion.
A serious potential fire hazard exists in the heat treatment of magnesium alloys. If through oversight or malfunctioning of equipment, the maximum temperatures are exceeded, the casting may ignite and burn freely. For this reason, the furnace used should be equipped with a safety cutoff that will turn off the power to the heating elements and blowers if the regular control equipment malfunc- Some magnesium alloys require a protective atmosphere of sulfur dioxide gas during solution heat treatment. This aids in preventing the start of a fire even if the temperature limits are slightly Air-quenching is used after solution heat treatment of magnesium alloys since there appears to be no advantage in liquid cooling.
Precipitation Heat Treatment After solution treatment, magnesium alloys may be given an aging treatment to increase hardness and yield strength. Generally, the aging treatments are used merely to relieve stress and stabilize the alloys in order to prevent dimensional changes later, especially during or after machining. Both yield strength and hardness are improved somewhat by this treatment at the expense of a slight amount of ductility. The corrosion resistance is also improved, making it closer to the "as cast" Precipitation heat-treatment temperatures are considerably lower than solution heat-treatment temperatures and range from 325° F. to 500° F.
Soaking time ranges from 4 to 18 hours.
HEAT TREATMENT OF TITANIUM Titanium is heat treated for the following purposes:
1. Relief of stresses set up during cold forming or machining.
2. Annealing after hot working or cold working or to provide maximum ductility for subsequent cold working.
3. Thermal hardening to improve strength.
Stress Relieving Stress relieving is generally used to remove stress concentrations resulting from forming of titanium sheet. It is performéd at temperatures ranging from 650° F. to 1,000° F. The time at temperature varies from a few minutes for a very thin sheet to an hour or more for heavier sections. A typical stress-relieving treatment is 900° F. for 30 minutes, followed by an air cool.
The discoloration or scale which forms on the surface of the metal during stress relieving is easily removed by pickling in acid solutions. The recommended solution contains 10 to 20 percent nitric acid and 1 to 3 percent hydrofluoric acid.
The solution should be at room temperature or slightly above.
Full Annealing The annealing of titanium and titanium alloys provides toughness, ductility at room temperature, dimensional and structural stability at elevated temperatures, and improved machinability.
The full anneal is usually called for as preparation for further working. It is performed at 1,200° F. to 1,650° F. The time at temperature varies from 16 minutes to several hours, depending on the thickness of the material and the amount of cold work to be performed. The usual treatment for the commonly used alloys is 1,300° F. for 1 hour, followed by an air cool. A full anneal generally results in sufficient scale formation to require the use of caustic descaling, such as sodium hydride salt bath.
Thermal Hardening Unalloyed titanium cannot be heat treated, but the alloys commonly used in aircraft construction can be strengthened by thermal treatment, usually at some sacrifice in ductility. For best results, a water quench from 1,450° F., followed by re-heating to 900° F. for 8 hours is recommended 217
Casehardening The chemical activity of titanium and its rapid Pressure gage absorption of oxygen, nitrogen, and carbon at relatively low temperatures make casehardening Hydraulic advantageous for special applications. Nitriding, actuating carburizing, or carbonitriding can be used to unit produce a wear-resistant case of 0.0001 to 0.0002 inch in depth.
HARDNESS TESTING Hardness testing is a method of determining the results of heat treatment as well as the state of a Hand metal prior to heat treatment. Since hardness pump values can be tied in with tensile strength values and, in part, with wear resistance, hardness tests are a valuable check of heat-treat control and of material properties.
Practically all hardness-testing equipment now -Penetrator uses the resistance to penetration as a measure of hardness. Included among the better known hardness testers are the Brinell and Rockwell, both - Elevating screw of which are described and illustrated in this section. Also included is a popular portable-type hardness tester currently being used. Microscope Brinell Tester The Brinell tester (figure 6-68) uses a hardened spherical ball, which is forced into the surface of the metal. This ball is 10 millimeters (0.3937 inch) in diameter. A pressure of 3,000 kilograms is used for ferrous metals and 500 kilograms for nonferrous metals. The pressure must be maintained at least 10 seconds for ferrous metals and at least 30 seconds for nonferrous metals. The load is applied by hydraulic pressure. The hydraulic pressure is built up by a hand pump or an electric motor, depending on the model of tester. A pressure gage indicates the amount of pressure. There is a release mechanism for relieving the pressure after the test has been made, and a calibrated microscope is provided for measuring the diameter of the impression in millimeters. The machine has various shaped anvils for supporting the specimen and an elevating screw for bringing the specimen in contact with the ball penetrator. These are attachments for special tests.
In order to determine the Brinell hardness number for a metal, the diameter of the impression is first measured, using the calibrated microscope furnished with the tester. After measuring the diameter of the impression, the measurement is FIGURE 6-68. Brinell hardness tester.
converted into the Brinell hardness number on the conversion table furnished with the tester.
Rockwell Tester The Rockwell hardness tester (figure 6-69) measures the resistance to penetration, as does the Brinell tester. Instead of measuring the diameter of the impression, the Rockwell tester measures the depth, and the hardness is indicated directly on a dial attached to the machine. The dial numbers in the outer circle are black, and the inner numbers are red. Rockwell hardness numbers are based on the difference between the depth of penetration at major and minor loads. The greater 218
Weight pan Scale Penetrator Major load Dial symbol (kg.) number Weights Diamond 60 Black B 16-inch ball 100 Red Diamond 150 Black Penetrator D Diamond 100 Black Specimen E ⅛-inch ball 100 Red Anvil F 16-inch ball 60 Red G 6-inch ball 150 Red Elevating screw- H ⅛-inch ball 60 Red K g-inch ball 150 Red Hand wheel FIGURE 6-70. Standard Rockwell hardness scales.
Zero adjuster Trip lever FICURE 6-69. Rockwell hardness tester.
this difference, the less the hardness number and the softer the material.
Two types of penetrators are used with the Rockwell tester, a diamond cone and a hardened steel ball. The load which forces the penetrator into the metal is called the major load and is measured in kilograms. The results of each penetrator and load combination are reported on separate scales, designated by letters. The penetrator, the major load, and the scale vary with the kind of metal being tested For hardened steels, the diamond penetrator is used; the major load is 150 kilograms; and the hardness is read on the "C" scale. When this reading is recorded, the letter "C" must precede the number indicated by the pointer. The C-scale setup is used for testing metals ranging in hardness from C-20 to the hardest steel (usually about C-70). If the metal is softer than C-20, the B-scale setup is used. With this setup, the 6-inch ball is used as a penetrator; the major load is 100 kilograms; and the hardness is read on the B-scale.
In addition to the "C" and "B" scales, there are other setups for special testing. The scales, penetrators, major loads, and dial numbers to be read are listed in figure 6-70.
The Rockwell tester is equipped with a weight pan, and two weights are supplied with the machine. One weight is marked in red. The other weight is marked in black. With no weight in the weight pan, the machine applies a major load of 60 kilograms. If the scale setup calls for a 100- kilogram load, the red weight is placed in the pan.
For a 150-kilogram load, the black weight is added to the red weight. The black weight is always used with the red weight; it is never used alone.
Practically all testing is done with either the B-scale setup or the C-scale setup. For these scales, the colors may be used as a guide in selecting the weight (or weights) and in reading the dial. For the B-scale test, use the red weight and read the red numbers. For a C-scale test, add the black weight to the red weight and read the black numbers.
In setting up the Rockwell machine, use the diamond penetrator for testing materials known to be hard. If the hardness is unknown, try the diamond, since the steel ball may be deformed if used for testing hard materials. If the metal tests below C-22, then change to the steel ball.
Use the steel ball for all soft materials, those testing less than B-100. Should an overlap occur at the top of the B-scale and the bottom of the C-scale, use the G-scale setup.
Before the major load is applied, the test specimen must be securely locked in place to prevent slipping and to seat the anvil and penetrator properly. To do this, a load of 10 kilograms is applied before the lever is tripped. This preliminary load is called the minor load. The minor load is 10 kilograms regardless of the scale setup.
The metal to be tested in the Rockwell tester must be ground smooth on two opposite sides and be free of scratches and foreign matter. The surface should be perpendicular to the axis of penetration, and the two opposite ground surfaces should be parallel. If the specimen is tapered, the amount of error will depend on the taper. A curved surface will also cause a slight error in the hardness test.
The amount of error depends on the curvature;
i.e., the smaller the radius of curvature, the greater the error. To eliminate such error, a small flat should be ground on the curved surface if possible.
Clad aluminum-alloy sheets cannot be tested directly with any accuracy with a Rockwell hardness tester. If the hardness value of the base metal is desired, the pure aluminum coating must be removed from the area to be checked prior to testing.
Barcol Tester The Barcol tester (figure 6-71) is a portable unit designed for testing aluminum alloys, copper, brass, or other relatively soft materials. It should not be used on aircraft steels. Approximate range of the tester is 25 to 100 Brinell. The unit can be FIGURE 6-71. Barcol portable hardness tester.
Alloy Barcol and temper number 1100-0 35 3003-0 42 3003-H14 56 2024-0 60 5052-0 62 5052-H34 75 6061-T 78 2024-T 85 FIGURE 6-72. Typical Barcol readings for aluminum alloy.
used in any position and in any space that will allow for the operator's hand. It is of great value in the hardness testing of assembled or installed parts, especially to check for proper heat treatment. The hardness is indicated on a dial conveniently divided into 100 graduations.
The design of the Barcol tester is such that operating experience is not necessary. It is only necessary to exert a light pressuse against the instrument to drive the spring-loaded indenter into the material to be tested. The hardness reading is instantly indicated on the dial.
Several typical readings for aluminum alloys are listed in figure 6-72. Note that the harder the material is, the higher the Barcol number will be.
To prevent damage to the point, avoid sliding or scraping when it is in contact with the material being tested. If the point should become damaged, it must be replaced with a new one. No attempt should be made to grind the point.
Each tester is supplied with a test disk for checking the condition of the point. To check the point, press the instrument down on the test disk.
When the downward pressure brings the end of the lower plunger guide against the surface of the disk, the indicator reading should be within the range shown on the test disk.
CHAPTER 7
PHYSICS
GENERAL Physics is the term applied to that area of knowledge regarding the basic and fundamental nature of matter and energy. It does not attempt to determine why matter and energy behave as they do in their relation to physical phenomena, but rather how they behave.
The persons who maintain and repair aircraft should have a knowledge of basic physics, which is sometimes called the science of matter and MATTER Although matter is the most basic of all things related to the field of physics and the material world, it is the hardest to define, Since it cannot be rigidly defined, this chapter will point out those characteristics which are easily recognizable.
Matter itself cannot be destroyed, but it can be changed from one state into another state by chemical or physical means. It is usually considered in terms of the energy it contains, absorbs, or gives off. Under certain controlled conditions, it can be made to aid man in his everyday life.
Matter is any substance that occupies space and has weight. There are three states of matter:
(1) Solids, (2) liquids, and (3) gases. Solids have a definite volume and a definite shape; liquids have a definite volume, but they take the shape of the containing vessel; gases have neither a definite volume nor a definite shape. Gases not only take the shape of the containing vessel, but they expand and fill the vessel, no matter what its volume.
Water is a good example of matter changing from one state to another. At high temperature it is in the gaseous state known as steam. At moderate temperatures it is a liquid, aud at low temperatures it becomes ice, a solid state. In this example, the temperature is the dominant factor in determining the state that the substance assumes. Pressure is another important factor that will effect changes in the state of matter. At pressures lower than atmospheric, water will boil and thus change into steam at temperatures lower than 212° F. Pressure is a critical factor in changing some gases to liquids or solids. Normally, when pressure and chilling are both applied to a gas, it assumes a liquid state. Liquid air, which is a mixture of oxygen and nitrogen, is produced in this manner.
Characteristics of Matter All matter has certain characteristics or general properties. These properties are defined elementally and broadly at this point, and more specifically in applications throughout the text.
Among these properties and relationships are:
a. Volume-meaning to occupy space; having some measurements such as length, width, and height. It may be measured in cubic inches, cubic centimeters, or the like.
b. Mass-the measurement of quantity or the measure of the quantity of matter in a body.
Mass does not vary even though the state changes.
c. Attraction- a force acting mutually between particles of matter, tending to draw them together. Sir Issac Newton called this the "Law of Universal Gravitation." He showed how each particle of matter attracts every other particle, how people are bound to the earth, and how the planets are attracted in the solar system.
d. Weight-the measure of universal gravitation. The pull of gravity on a body is called the weight of the body and indicates how heavy the
e. Density-the mass (weight) of a substance per unit volume. Density can be used to distinguish various types of matter. If a substance is very dense, a large quantity of this matter would occupy a small volume.
f. Inertia-the opposition which a body offers to any change of motion. The property of inertia is common to all matter. It is best expressed in Newton's first law: "A body at rest remains at 221