3/31/99 are determined by the alloying elements and cannot be changed after the metal is cast. In the other, the alloying elements make it possible to heat treat the casting to produce the desired physical properties.
The casting alloys are identified by a letter preceding the alloy number. When a letter precedes a number, it indicates a slight variation in the composition of the original alloy. This variation in composition is simply to impart some desirable quality. In casting alloy 214, for example, the addition of zinc to improve its pouring qualities is indicated by the letter A in front of the number, thus creating the designation A214.
When castings have been heat treated, the heat treatment and the composition of the casting is indicated by the letter T, followed by an alloying number. An example of this is the sand casting alloy 355, which has several different compositions and tempers and is designated by 355-T6, 355-T51, or C355-T51.
Aluminum alloy castings are produced by one of three basic methods: (1) Sand mold, (2) permanent mold, or (3) die cast. In casting aluminum, it must be remembered that in most cases different types of alloys must be used for different types of castings. Sand castings and die castings require different types of alloys than those used in permanent molds.
Sand and permanent mold castings are parts produced by pouring molten metal into a previously prepared mold, allowing the metal to solidify or freeze, and then removing the part. If the mold is made of sand, the part is a sand casting; if it is a metallic mold (usually cast iron) the part is a permanent mold casting. Sand and permanent castings are produced by pouring liquid metal into the mold, the metal flowing under the force of gravity alone.
The two principal types of sand casting alloys are 112 and 212. Little difference exists between the two metals from a mechanical properties standpoint, since both are adaptable to a wide range of products.
The permanent mold process is a later development of the sand casting process, the major difference being in the material from which the molds are made. The advantage of this process is that there are fewer openings (called porosity) than in sand castings.
The sand and the binder, which is mixed with the sand to hold it together, give off a certain amount of gas which causes porosity in a sand casting.
Permanent mold castings are used to obtain higher mechanical properties, better surfaces, or more accurate dimensions. There are two specific types of permanent mold castings: (1) The permanent metal mold with metal cores, and (2) the semipermanent types containing sand cores. Because finer grain structure is produced in alloys subjected to the rapid cooling of metal molds, they are far superior to the sand type castings.
Alloys 122, A132, and 142 are commonly used in permanent mold castings, the principal uses of which are in internal combustion engines.
AC 65-9A CHG 1 Die castings used in aircraft are usually aluminum or magnesium alloy. If weight is of primary importance, magnesium alloy is used because it is lighter than aluminum alloy. Aluminum alloy is frequently used since it is stronger than most magnesium alloys.
A die casting is produced by forcing molten metal under pressure into a metallic die and allowing it to solidify; then the die is opened and the part removed.
The basic difference between permanent mold casting and die casting is that in the permanent mold process the metal flows into the die under gravity. In the die casting operation, the metal is forced under great pressure.
Die castings are used where relatively large production of a given part is involved. Remember, any shape which can be forged can be cast.
Wrought aluminum and wrought aluminum alloys are divided into two general classes, nonheat-treatable alloys and heat-treatable alloys.
Nonheat-treatable alloys are those in which the mechanical properties are determined by the amount of cold-work introduced after the final annealing operation.
The mechanical properties obtained by cold working are destroyed by any subsequent heating and cannot be restored except by additional cold working, which is not always possible. The "full hard" temper is produced by the maximum amount of cold-work that is commercially practicable. Metal in the "as fabricated" condition is produced from the ingot without any subsequent controlled amount of cold working or thermal treatment.
There is, consequently, a variable amount of strain hardening, depending upon the thickness of the section.
For heat-treatable aluminum alloys the mechanical properties are obtained by heat treating to a suitable temperature, holding at that temperature long enough to allow the alloying constituent to enter into solid solution, and then quenching to hold the constituent in solution. The metal is left in a supersaturated, unstable state and is then age hardened either by natural aging at room temperature or by artificial aging at some elevated Aluminum Alloy Designations Wrought aluminum and wrought aluminum alloys are designated by a four-digit index system. The system is broken into three distinct groups: 1xxx group, 2xxx through 8xxx group, and 9xxx group (which is at present unused).
The first digit of a designation identifies the alloy type. The second digit indicates specitic alloy modifications. Should the second number be zero, it would indicate no special control over individual impurities. Digits 1 through 9, however, when assigned consecutively as needed for the second number in this group, indicate the number of controls over individual impurities in the metal.
198-1 (and 198-2)
The last two digits of the Ixxx group are used to indicate the hundredths of 1 percent above the original 99 percent designated by the first digit.
Thus, it the last two digits were 30, the alloy would contain 99 percent plus 0.30 percent of pure aluminum, or a total of 99.30 percent pure aluminum.
Examples of alloys in this group are:
1100-99.00 percent pure aluminum with one control over individual impurities.
1130-99.30 percent pure aluminum with one control over individual impurities.
1275-99.75 percent pure aluminum with two controls over individual impurities.
In the 2xxx through 8xxx groups, the first digit indicates the major alloying element used in the formation of the alloy as follows:
2xxx-copper.
3xxx-manganese.
4xxx--silicon.
5xxx-magnesium.
6xxx-magnesium and silicon.
7xxx-zinc.
8xxx-other elements.
In the 2xxx through 8xxx alloy groups, the second digit in the alloy designation indicates alloy modifications. If the second digit is zero, it indicates the original alloy, while digits 1 through 9 indicate alloy modifications.
The last two of the four digits in the designation identify the different alloys in the group (figure Effect of Alloying Element 1000 series. 99% or higher, excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties, excellent workability.
Iron and silicon are major impurities.
2000 series. Copper is the principle alloying element. Solution heat treatment, optimum properties equal to mild steel, poor corrosion resistance It is usually clad with 6000 or high purity alloy. Its best known alloy is 2024.
3000 series. Manganese is the principle alloying element of this group which is generally non-heat- The percentage of manganese which will be alloy effective is 1.5%. The most popular is 3003 which is of moderate strength, and has 4000 series. Silicon is the principle alloying element. This lowers the melting temperature, Its primary use is in welding and brazing.
used in welding heat-treatable alloys, this group will respond to a limited amount of heat treatment.
5000 series. Magnesium is the principle alloying element. It has good welding and corrosion resistant characteristics. High temperatures (over 150° F.) or excessive cold working will increase susceptibility to corrosion.
6000 series. Silicon and magnesium form mag nesium silicide which makes alloys heat-treatable It is of medium strength, good forming and has corrosion resistant characteristics.
Per cent of alloying elements-aluminum and normal impurities Alloy constitute remainder Cop- Sili- Man- Mag- Zine Nickel Chro- Lead Bisper con ganese nesium mium muth 1100 ....
3003 1.2 ... ... ...
• 2011 5.5 0.5 0.5 2014 4.4 0.8 0.8 0,4 ...
2017 4.0 0.5 0.5 .... ...
2117 2.5 0.3 2018 4.0 0.5 2.0 2024 4.5 0.6 1.5 2025 4.5 0.8 0.8 .....
4032 0.9 12.5 1.O 0.9 6151 1.0 0.6 ... 0.25 5052 2.5 0,25 6053 0.7 1.3 ...... 0.25 8061 0.25 0.6 1.0 0.25 ...
7075 1.6 2.5 5.6 0.3 FIGURE 6-63. Nominal composition of wrought aluminum alloys.
7000 series. Zinc is the principle alloying element. The most popular alloy of the series is 6061.
When coupled with magnesium, it results in heattreatable alloys of very high strength. It usually has copper and chromium added. The principle alloy of this is 7075.
Hardness Identification Where used, the temper designation follows the alloy designation and is separated from it by a dash: i.e., 7075-T6, 2024-T4, etc.
designation consists of a letter indicating the basic temper which may be more specifically defined by the addition of one or more digits. These designations are as follows:
-F As fabricated.
-O Annealed, recrystallized (wrought products only).
-H Strain hardened.
-H1 (plus one or more digits) strain hardened only.
-H2 (plus one or more digits) strain hardened and partially annealed.
-H3 (plus one or more digits) strain hardened and stabilized.
The digit following the designations H1, H2, and H3 indicate the degree of strain hardening, number 8 representing the ultimate tensile strength equal to that achieved by a cold reduction of approximately 75% following a full anneal, 0 representing the annealed state.
Heat Treatment Identification In the wrought form, commercially pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength and does not have the properties required for structural aircraft parts. High strengths are generally obtained by the process of alloying. The resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum.
Alloying is not the only method of increasing the strength of aluminum. Like other materials, aluminum becomes stronger and harder as it is rolled, formed, or otherwise cold-worked.
the hardness depends on the amount of cold working done, 1100 and some wrought aluminum alloys are available in several strain-hardened tempers.
The soft or annealed condition is designated O.
If the material is strain hardened, it is said to be in the H condition.
The most widely used alloys in aircraft construction are hardened by heat treatment rather than by cold-work. These alloys are designated by a somewhat different set of symbols: -T4 and W indicate solution heat treated and quenched but not aged, and T6 indicates an alloy in the heat treated hardened condition.
-W Solution heat treated, unstable temper.
-T Treated to produce stable tempers other than -F, -O, or -H.
-T2 Annealed (cast products only).
-T3 Solution heat treated and then cold worked.
-T4 Solution heat treated.
-T5 Artificially aged only.
-T6 Solution heat treated and then artificially aged.
-T7 Solution heat treated and then stabilized.
-T8 Solution heat treated, cold worked, and then artificially aged.
-T9 Solution heat treated, artificially aged, and then cold worked.
-T10 Artificially aged and then cold worked.
Additional digits may be added to TI through T10 to indicate a variation in treatment which significantly alters the characteristics of the prod- In the wrought form, commercially pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength, however, and does not have the strength required for structural aircraft parts. Higher strengths are generally obtained by the process of alloying. The 200
resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum.
Alloying is not the only method of increasing the condition.
the heat became the first commercial method for producing titanium. The United States Bureau of Mines began making titanium sponge in 1946, and 4 years later the melting process began.
The use of titanium is widespread. It is used in many commercial enterprises and is in constant demand for such items as pumps, screens, and other tools and fixtures where corrosion attack is prevalent. In aircraft construction and repair, titanium is used for fuselage skins, engine shrouds, firewalls, longerons, frames, fittings, air ducts, and fasteners, Titanium is used for making compressor disks, spacer rings, compressor blades and vanes, through bolts, turbine housings and liners, and miscellaneous hardward for turbine engines.
Titanium, in appearance, is similar to stainless steel. One quick method used to identify titanium is the spark test. Titanium gives off a brilliant white trace ending in a brilliant white burst. Also, identification can be accomplished by moistening the titanium and using it to draw a line on a piece of glass. This will leave a dark line similar in from the edge.
Alclad Aluminum The terms "Alclad and Pureclad" are used to designate sheets that consist of an aluminum alloy core coated with a layer of pure aluminum to a depth of approximately 5½ percent on each side.
The pure aluminum coating affords a dual protection for the core, preventing contact with any corrosive agents, and protecting the core electrolytically by preventing any attack caused by scratching or from other abrasions.
Titanium and Titanium Alloys Titanium was discovered by an English priest named Gregot. A crude separation of titanium ore was accomplished in 1825. In 1906 a sufficient amount of pure titanium was isolated in metallic form to permit a study. Following this study, in 1932, an extraction process was developed which Titanium falls between aluminum and stainless steel in terms of elasticity, density, and elevated temperature strength. It has a melting point of from 2,730° F. to 3,155° F., low thermal conductivity, and a low coefficient of expansion. It is light, strong, and resistant to stress-corrosion cracking. Titanium is approximately 60 percent heavier than aluminum and about 50 percent lighter than stainless steel.
Because of the high melting point of titanium, high-temperature properties are disappointing.
The ultimate yield strength of titanium drops rapidly above 800° F. The absorption of oxygen and nitrogen from the air at temperatures above 1,000° F. makes the metal so brittle on long exposure that it soon becomes worthless. However, titanium does have some merit for short-time exposure up to 3,000° F. where strength is not important. Aircraft firewalls demand this requirement.
Titanium is nonmagnetic and has an electrical resistance comparable to that of stainless steel.
Some of the base alloys of titanium are quite hard.
Heat treating and alloying do not develop the hardness of titanium to the high levels of some of the heat-treated alloys of steel. It was only recently that a heat-treatable titanium alloy was developed. Prior to the development of this alloy, heating and rolling was the only method of forming 201
that could be accomplished. However, it is possible to form the new alloy in the soft condition and heat treat it for hardness.
Iron, molybdenum, and chromium are used to stabilize titanium and produce alloys that will quench harden and age harden. The addition of these metals also adds ductility. The fatigue resistance of titanium is greater than that of structural members and aircraft skin, has 110,000
p.s.i. minimum yield strength, and contains 8 percent manganese.
Type A-110AT is a titanium alloy which contains 5 percent aluminum and 2.5 percent tin.
It also has a high minimum yield strength at elevated temperatures with the excellent welding characteristics inherent in alpha-type titanium alloys.
or unalloyed titanium by chemical analysis;
therefore, the grades are determined by mechanical properties.
Titanium Designations The A-B-C classification of titanium alloys was established to provide a convenient and simple means of describing all titanium alloys. Titanium and titanium alloys possess three basic types of crystals: A (alpha), B (beta), and C (combined alpha and beta). Their characteristics are:
weldability; tough and strong both cold and hot, and resistant to oxidation.
B (beta)-Bendability; excellent bend ductility; strong both cold and hot, but vulnerable to contamination.
C (combined alpha and beta for compromise performances-Strong when cold and warm, but weak when hot; good bendability; moderate contamination resistance; excellent forgeability.
Titanium is manufactured for commercial use in two basic compositions; commercially pure titanium and alloyed titanium. A-55 is an example of a commercially pure titanium. It has a yield strength of 55,000 to 80,000 p.s.i. and is a generalpurpose grade for moderate to severe forming. It is sometimes used for nonstructural aircraft parts and for all types of corrosion-resistant applications, such as tubing.
Type A-70 titanium is closely related to type A-55 but has a yield strength of 70,000 to 95,000
p.s.i. It is used where higher strength is required, and it is specified for many moderately stressed aircraft parts. For many corrosion applications, it is used interchangeably with type A-55. Both type A-55 and type A-70 are weldable.
One of the widely used titanium-base alloys is designated as C-110M. It is used for primary Corrosion Characteristics The corrosion resistance of titanium deserves special mention. The resistance of the metal to corrosion is caused by the formation of a protective surface film of stable oxide or chemi-absorbed oxygen. Film is often produced by the presence of oxygen and oxidizing agents.
Corrosion of titanium is uniform. There is little evidence of pitting or other serious forms of localized attack. Normally, it is not subject to stress corrosion, corrosion fatigue, intergranular corrosion, or galvanic corrosion. Its corrosion resistance is equal or superior to 18-8 stainless steel.
Laboratory tests with acid and saline solutions show titanium polarizes readily. The net effect, in general, is to decrease current flow in galvanic and corrosion cells. Corrosion currents on the surface of titanium and metallic couples are naturally restricted. This partly accounts for good resistance to many chemicals; also, the material may be used with some dissimilar metals with no harmful galvanic effect on either.
Copper and Copper Alloys Copper is one of the most widely distributed metals. It is the only reddish-colored metal and is second only to silver in electrical conductivity. Its use as a structural material is limited because of its great weight. However, some of its outstanding characteristics, such as its high electrical and heat conductivity, in many cases overbalance the weight factor.
Because it is very malleable and ductile, copper is ideal for making wire. It is corroded by salt water but is not affected by fresh water. The ultimate tensile strength of copper varies greatly.
For cast copper, the tensile strength is about 25,000 p.s.i., and when cold rolled or cold drawn its tensile strength increases to a range of 40,000 to 67,000 p.s.).
In aircraft, copper is used primarily in the 202
and resistance to both shock and fatigue. Because lockwire. of these properties, they are used for diaphragms, gears, and pumps. Aluminum bronzes are available in rods, bars, plates, sheets, strips, and forgings.
Cast aluminum bronzes, using about 89 percent copper, 9 percent aluminum, and 2 percent of other elements, have high strength combined with ductility, and are resistant to corrosion, shock, and fatigue. Because of these properties, cast aluminum bronze is used in bearings and pump parts. These alloys are useful in areas exposed to salt water and corrosive gases.
Manganese bronze is an exceptionally high strength, tough, corrosion-resistant copper zinc alloy containing aluminum, manganese, iron and, occasionally, nickel or tin. This metal can be formed, extruded, drawn, or rolled to any desired shape. In rod form, it is generally used for machined parts, for aircraft landing gears and brackets.
Silicon bronze is a more recent development composed of about 95 percent copper, 3 percent silicon, and 2 percent manganese, zinc, iron, tin, and aluminum. Although not a bronze in the true sense because of its small tin content, silicon bronze has high strength and great corrosion resistance.
Monel Monel, the leading high-nickel alloy, combines the properties of high strength and excellent corrosion resistance. This metal consists of 68 percent nickel, 29 percent copper, 0.2 percent iron, 1 percent manganese, and 1.8 percent of other elements. It cannot be hardened by heat treatment.
Monel, adaptable to casting and hot- or cold-working, can be successfully welded. It has working properties similar to those of steel. When forged and annealed, it has a tensile strength of 80,000 p.s.i. This can be increased by cold-working to 125,000 p.s.i., sufficient for classification among the tough alloys.
Monel has been successfully used for gears and chains to operate retractable landing gears, and for structural parts subject to corrosion. In aircraft, Monel is used for parts demanding both strength and high resistance to corrosion (such as exhaust manifolds and carburetor needle valves and sleeves).
K-Monel K-Monel is a nonferrous alloy containing mainly nickel, copper, and aluminum. It is produced by adding a small amount of aluminum to the Monel formula. It is corrosion resistant and capable of being hardened by heat treatment.
K-Monel has been successfully used for gears, and structural members in aircraft which are subjected to corrosive attacks. This alloy is nonmagnetic at all temperatures. K-Monel sheet has been successfully welded by both oxyacetylene and electrie-are welding.
treatment is used to put as much of the alloying ingredients as possible into solid solution, which results in high tensile strength and maximum ductility. Aging is applied to castings following heat treatment where maximum hardness and yield strength are desired.
Magnesium embodies fire hazards of an unpredictable nature. When in large sections, its high thermal conductivity makes it difficult to ignite and prevents it from burning. It will not burn until the melting point is reached, which is 1,204° F.
However, magnesium dust and fine chips are ignited easily. Precautions must be taken to avoid this if possible. Should a fire occur, it can be extinguished with an extinguishing powder, such as powdered soapstone, or graphite powder. Water or any standard liquid or foam fire extinguishers cause magnesium to burn more rapidly and can cause explosions.
Magnesium alloys produced in the United States consist of magnesium alloyed with varying proportions of aluminum, manganese, and zinc.
These alloys are designated by a letter of the alphabet, with the number 1 indicating high purity and maximum corrosion resistance.
Many of the magnesium alloys manufactured in this country are produced by the Dow Chemical ds Company and have been given the trade name of is Dowmetal alloys. To distinguish between these alloys, each is assigned a letter. Thus, we have of Dowmetal J, Dowmetal M, and so forth.
of Another manufacturer of magnesium alloys is ed the American Magnesium Corporation, a subnt sidiary of the Aluminun Company of America.
ve This company uses an identification system similar he to that used for aluminum alloys, with the 1g- exception that magnesium alloy numbers are re preceded with the letters AM. Thus, AM240C is a in., cast alloy, and AM240C4 is the same alloy in the ne heat-treated state. AM3SO is an annealed wrought alloy, and AM3SRT is the same alloy rolled after nd heat treatment.
seats.
casting charac-
SUBSTITUTION OF AIRCRAFT METALS In selecting substitute metals for the repair and maintenance of aircraft, it is very important to check the appropriate structural repair manual.
Aircraft manufacturers design structural members such treatments to meet a specific load requirement for a particular aircraft. The methods of repairing these members, apparently similar in construction, will thus vary with different aircraft.
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.