Alloy Steels by Definition
A PLAIN carbon steel is iron combined with small amounts of carbon where the carbon content can vary between 0.008% and approximately 2.0% A plain carbon steel can also contain limited amounts of manganese (1.65% max ), silicon (0.60% max ), and copper (0.60% max ) and still be classified as a carbon steel. The more common carbon steels are the standard types such as C1005 with approximately 0.05%C up through the C1095 type with approximately 0.95%C. The C1100 steels are resulfurized and the C1200 steels are resulfurized and rephosphorized.
By simple definition, an alloy steel is a type of steel to which one or more alloying elements have been added to give it special properties that cannot be obtained in carbon steel. The constructional alloy steels are those generally considered to be AISI 1300 through the AISI 9800 series, although modifications of these types are also used in special applications. The chemical limitations of an alloy steel, as defined by the American Iron and Steel Institute (AISI), are as follows:
"Steel is considered to be alloy steel when the maximum of the range given for the content of alloying elements exceeds one or more of the following limits: manganese, 1.65%; silicon, 0.60%; copper, 0.60%; or in which a definite range or a definite minimum quantity of any of the following elements is specified or required within the limits of the recognized field of constructional alloy steels: aluminium ,boron, chromium up to 3.99%, cobalt, columbium' molybdenum, nickel, titanium, tungsten, vanadium, zirconium, or any other alloying element added to obtain a desired alloying effect."
To simplify the designation of alloy steels specified to chemical composition limits, a fournumeral series has been established by the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI). The last two digits of the fournumeral series are intended to indicate the approximate middle of the carbon range. In some cases, five digits are given and the last three represent the carbon content as in the high carbonchromium bearing steels. The first two digits represent the alloy or alloys present. The various types of alloy steel and their numerical designations are shown on the opposite page.
Functions of Alloying Elements in Constructional Alloy Steels
As a general rule, alloy steels are specified when more strength, ductility, and toughness are required than can be obtained in carbon steel in the section or part under consideration. Alloy steels are also used when specific properties such as wear resistance, corrosion resistance, heat resistance, and special low temperature impact properties are desired.
Engineering reasons for the acceptance of any material can usually be justified, but it is often easy to lose sight of the fact that there must also be an economic reason for the widespread use of alloy steels. It is interesting to note that some of the largest users of alloy steels include the most costconscious industries who primarily use alloy steels because of their net advantages over carbon steels. There must, of course, be a mating of the economic and engineering factors in any given application. From a designer's viewpoint, alloy steels give greater latitudes in design, greater safety with larger payloads, and less maintenance.
Large tonnages of the alloying elements nickel, chromium, molybdenum, manganese, vanadium, and othersare now used annually. They are generally furnished as ferroalloys, or in some cases as the pure element, such as nickel, or as oxide compounds of the element, as with nickel and molybdenum. In these forms, they are added to molten steel in specified amounts to produce the modern constructional alloy steels.
Alloying elements are incorporated in steel for one or more of the following reasons, of which the first two are more important:
1. To improve mechanical properties through control of the factors, which influence hardenability, and to permit higher tempering temperature while maintaining high strength and improved ductility
2. To improve mechanical properties at elevated or low temperatures
3. To increase resistance to chemical attack and to elevated temperature oxidation
4. To influence other special properties such as magnetic permeability and neutron absorption.
Microstructure of steel can be considered as being composed of two phases:
1. Ferrite, the magnetic alpha modification of iron possessing high ductility, but poor tensile strength
2. Cementite, the ironcarbide phase, a hard, brittle constituent. The carbide phase in alloy steel is not generally pure iron carbide, hut rather a complex combination of iron and alloy carbides.
Alloying elements have definite effects on the properties of the ferrite and carbide phases present. They improve the mechanical properties in two ways:
1. By changing the state of dispersion of the carbide in the ferrite
2. By changing the properties of the ferrite and carbide phases.
The alloying elements can be classified according to their specific influence on either the ferrite or carbide phase. Some alloying elements, notably manganese, chromium, molybdenum, and vanadium, dissolve in the ferrite phase and also form carbides. Such elements as nickel, copper, and silicon do not form carbides in steel. When present in the amounts usually found in alloy steels, they dissolve in and strengthen the ferrite.
The highest mechanical properties of the constructional alloy steels are developed principally by quenching and tempering. If an alloy steel has been well quenched, maximum surface hardness will be developed. It is important to remember that carbon is responsible for the maximum attainable surface hardness in the section being quenched, but practical considerations necessitate the use of alloys to get maximum surface hardness and depth of hardening as section sizes increase. Alloying elements such as nickel or chromium do not, in the constructional alloy steels, make the steel surface any harder, but rather improve the hardenability and the mechanical properties of the heat treated alloy steel by influencing the rate of austenite transformation so that deephardening characteristics are obtained. In hardening 0.30% carbon steel, a quenching speed of 1800' F per second through the 1300' F temperature level on cooling must be maintained to secure the greatest hardness. Alloying elements reduce the value of this critical quenching speed to 100' F per second or less, thus permitting the steel to be cooled to temperatures of 400 to 500' F before any transformation occurs. In this range, hardening can occur completely through much heavier sections of alloy steel than could possibly be the case with the carbon steels. Depending on alloy content, sections 2, 3, 4 in., and over can be throughhardened by quenching.
The depth of hardness can be determined by direct readings on transverse sections through various diameter bars or can be estimated from the endquench hardenability test.
Even when heat is extracted from alloy steels at slower rates, maximum hardness can be attained because the reaction rates involved in the decomposition of austenite on cooling are slower in the presence of the alloying elements. In practice, this means that a less drastic quench can be employed, and that heavier sections will develop full hardness. The slower quench creates a lower thermal gradient and minimizes quench cracking and warping.
In the practical application of the constructional alloy steels, it has been desirable, in order to obtain the best combination of strength and ductility in the heat treated condition, to have a steel containing both a ferrite strengthener such as nickel and the carbide forming elements chromium and molybdenum, The latter elements allow higher temperatures to be used for tempering while maintaining high tensile strength and a good degree of ductility in addition to their function of promoting full-hardening at slower cooling rates.
Specific Effects of Alloying Elements in the
Constructional Alloy Steels
Manganese
Manganese is normally present in all commercial steels. It is essential to steel production, not only in melting but also in rolling and other processing operations. In hotworking operations, the action of manganese on sulphur improves hot working characteristics and contributes to better surface on the product.
Manganese is soluble in both alpha and gamma iron, and also forms a carbide, Mn3C. Under certain conditions the eutectoid percentage of carbon is lowered, and at 2.0% manganese, becomes 0.67%C. Manganese also lowers the critical points effectively, in a manner similar to that of nickel, and causes a pronounced hysteresis effect between the heating and cooling transformations.
In the constructional alloy steels, manganese very markedly decreases the critical cooling rate and contributes thereby to deep hardening.
With a manganese content of 11.0 to 14.0% and with a carbon content of 1.00 to 1.40%, the alloy is austenitic. This analysis is particularly resistant to wear and abrasion under high impact stresses.
Silicon
Additions of silicon raise the critical temperature on heating by amounts varying with the carbon content. Heating temperatures for quenching treatments are therefore increased. Silicon is not a carbideforming element in steel, but enters into solution in the ferrite. It increases the strength of the ferrite without loss of ductility when added in amounts up to 2.500/0. In these large quantities, however, it introduces processing difficulties, such as poor machinability, and increases susceptibility to decarburization.
One of the most important applications of silicon is its use as a deoxidizer and in molten steel. Silicon is usually present in fully deoxidized constructional alloy steels in amounts up to 0.35%, insuring the production of sound, dense ingots. Silicon increases hardenability and strengthens low alloy steels. In larger amounts it increases resistance to scaling at elevated temperatures and also raises the 500ºF embrittlement range for ultra high strength applications.
Nickel
Nickel as an alloying element in constructional alloy steels is a ferrite strengthener and is soluble in all proportions in both gamma and alpha iron. It depresses the Ac and Ar critical points. It lowers the carbon content of the eutectoid, which, with 3.5% nickel steel, is reduced to 0.70% carbon.
Since nickel does not form any carbide compounds in steel, it remains in solution in the ferrite thus strengthening and toughening the ferrite phase. In either the annealed or untreated condition, nickel will increase the internal strength and elastic limit over corresponding carbon steel. Nickel steels are easily heat treated because nickel very effectively lowers the critical cooling rate necessary to produce hardening on quenching. In heat-treated steel, nickel increases the strength and toughness. Resistance to impact stresses at subzero temperatures is also considerably improved.
In combination with chromium, nickel produces alloy steels with higher elastic ratios, greater hardenability, higher impact, and fatigue resistance than are possible with carbon steels.
Chromium
Chromium as an alloying element in constructional steels is found to form a solid solution with both the alpha and gamma phases of iron. With carbon and iron it forms a complex series of carbide compounds of chromium and iron. It is a strong carbide former, similar in this respect to molybdenum and tungsten. It raises the Ac" critical point, especially when large amounts of chromium are present. The eutectoid carbon content is found to be lowered by chromium additions, by an amount varying with the quantity present. At 2.0%, chromium, the eutectoid forms with 0.62% carbon. With 12.0% chromium, eutectoid carbon drops to under 0.40%.
Complex chromiumiron carbides go into solution in austenite slowly; therefore a sufficient heating time before quenching is necessary when treating the medium carbon grades. If the chromium is sufficiently dissolved in austenite, greater depth of hardening on quenching is obtained because the critical cooling rate required is decreased.
Chromium is essentially a hardening element, and is frequently used with a toughening element such as nickel to produce superior
mechanical properties. At higher temperatures, chromium contributes increased strength, but it is ordinarily used for applications of this nature in conjunction with molybdenum.
Two of the most important properties of steels containing chromium are wear resistance and cutting ability. These effects are primarily due to the high hardness of the chromium carbides.
Molybdenum
Molybdenum as an alloying element in steel can form a solid solution with the ferrite phase and also, depending on the molybdenum and carbon content, can form a complex carbide. Molybdenum slightly increases the electrical resistance of low carbon steels. This indicates it is largely in solid solution in ferrite. In high carbon steels, electrical resistance is decreased, which gives evidence that the molybdenum is largely confined to the carbide phase when the steel is in the annealed condition. Molybdenum raises the Ac3 critical point when added in the usual amounts (0.10 to 0.60%) for constructional alloy steels. When molybdenum is in solid solution in austenite prior to quenching, the reaction rates for the transformation of austenite become considerably slower as compared with a carbon steel, resulting in deeper hardening steel.
Molybdenum steels in the quenched condition require a higher tempering temperature to attain the same degree of softness as comparable carbon or alloy steels. This resistance to tempering conferred by molybdenum contributes to the ability of these steels to retain their strength at elevated temperatures. They show, because of this effect, a considerable resistance to "creep" under sustained loads below their elastic limit at temperatures up to 1100 F.
Alloy steels, which contain 0.15 to 0.30%, molybdenum, show a minimum susceptibility to temper embrittlement. It has been found that some quenched and tempered alloy steels become brittle when slowly cooled from the tempering temperature. This brittleness is made evident by low resistance to impact. The susceptibility of such steels to "temper brittleness" is serious when large parts are being made, for it is difficult to cool large sections rapidly enough to prevent damage. Molybdenum additions in the amounts noted prevent temper brittleness and allow usual cooling rates to be used after tempering.
Vanadium
Vanadium is one of the strong carbideforming elements. It dissolves to some degree in ferrite, imparting strength and toughness. The complex carbides formed by vanadium additions are quite stable.
Vanadium steels show a much finer structure than steels of a similar composition without vanadium. Grain growth tendencies are minimized for temperatures in the heat-treating range, thus allowing higher hardening and normalizing temperatures to be used.
As with other strong carbideforming elements, vanadium raises the critical points and decreases the carbon content of the eutectoid. This latter effect is very pronounced.
Vanadium gives other alloying effects of importance, namely: increased hardenability where it is in solution in the austenite prior to quenching; a secondarv hardening effect upon tempering; and increased hardness at elevated temperatures.
Boron
Boron is usually added to steel to improve hardenability; that is, to increase the depth of hardening during quenching. During World War 11 and the Korean conflict, boron supplanted substantial quantities of nickel, chromium, and molybdenum in a group of alloy steels, primarily to conserve these elements which are ordinarily used to increase hardenability. Boron treated steels will usually have a boron content in the range of 0.0005 to 0.003%. This small] amount of boron appears to be the optimum range for enhancing the hardenability of other alloys and because of this effect boron is designated as an alloy "intensifier." Whenever boron is substituted in part for other alloys, it should be done so only with hardenability in mind because the lowered alloy content may be harmful on some applications. Boron is very effective when used with lowcarbon alloy steels, but its effect is reduced as the carbon increases. When the carbon content is above 0.60%, the use of boron is not suggested. Boron appears to be most detrimental to impact resistance in steels with low transition temperatures; thus, if a nickel steel is to be used for low temperature applications it should not contain boron, but, conversely, boron steels should contain nickel to offset the detrimental effect of boron on low temperature transition.
Boron also has a relatively high nuclear cross section and can be used for neutron absorption.
Aluminium
Aluminium is widely used as a deoxidizer and for control of inherent grain size. When added to steel in controlled amounts, it produces a fine austenitic grain size.
In fact, of all the alloy elements, Aluminium, in prescribed amounts, is the most effective in controlling gram growth. Titanium, zirconium and vanadium are also effective grain growth inhibitors, but have adverse effects on hardenability because their carbide compounds are very stable and difficult to dissolve in austenite prior to quenching.
Aluminium is used as an alloying addition in amounts of 0.95 to 1.30% in the most popular nitriding steel. The aircraft industry uses the nitriding steel, AISI 7140, because of its high surface hardness after nitriding and its distortionfree properties up to approximately the nitriding temperature. The extremely high hardness of the nitrided case is due to the formation of a hard, stable Aluminium nitride compound. The amount of aluminium present in nitriding steels is considerably in excess of the amount necessary to produce a fine austenitic grain size in other steels.
Copper
Copper as a metal has been used since antiquity, but its use as an alloy of steel is not so well known. Copper, like nickel, does not form carbides in steel, but dissolves in and strengthens the ferrite. The use of copper in certain alloys increases the resistance to atmospheric corrosion and increases the yield strength. Low carbon alloy steels alloyed with 1.0 to 1.5% copper show precipitation hardening response during aging at approximately 900' F that markedly increases the yield strength. Copper has been used in some die steel applications and more specifically in the highstrength lowalloy steels where chemical compositions are specially developed to impart higher mechanical property values and greater resistance to atmospheric corrosion than are obtainable from conventional carbon structural steels containing copper.
Columbium
Element 41 of the periodic table has been named "niobium" by several international technical societies; however, prominent metallurgists in this country prefer to use the name columbium.
Columbium influences the properties of steel by (1) imparting a fine grain size and preventing grain coarsening as high as 1875' F, (2) by preventing air hardening, (3) by retarding softening during tempering, (4) by hastening nitriding reactions, and (5) by enabling steel to resist creep and rupture at elevated temperatures. Columbium enters into the ferrite of very lowcarbon steels and strengthens the ferrite by the formation of an intermetallic compound of limited solid solubility. Alloys containing more than 0.5%, columbium can be age hardened when the carbon content is very low. In medium carbon steels, age hardening effects may be obtained if the heat-treating temperature is high enough to effectively dissolve some of the columbium carbides.
One of the advantages of using columbium for grain refinement is that it has a low deoxidizing power and does not introduce undesirable oxide inclusions into the steel. The A3 temperature is raised by columbium and the A4, or upper austenite limit, is lowered, thus restricting the austenite temperature ranges.
Columbium decreases the hardenability of steel by carbon impoverishment as well as by grain refinement and the nucleating effect of the carbides; but columbium can inhibit transformation when in solid solution.
The fine grain size and the decreased hardenability imparted by columbium increase the ductility of steels slightly and the impact strength markedly. These improvements in properties are more noticeable in the lower carbon steels.
Titanium
Titanium in steel is used primarily as a deoxidizer and an effective grain growth inhibitor; but as an alloying element, it has the greatest carbideforming tendency of any of the alloying elements. Its carbideforming tendency is so strong that a 0.50% carbon steel will have practically no tendency to quench harden when 1.5 to 2.0% titanium is added. Titanium's contribution to hardenability is usually negligible because it may only dissolve in minute amounts in the presence of considerable carbon.
Titanium is now being used in some elevated temperature applications where high stress rupture and secondary hardening effects are required. In very low carbon steels, titanium has a pronounced effect on strengthening the ferrite by solid solution effects and ranks next to silicon in this respect. Titanium residuals of 0.02% can cause impairment of impact properties by shifting the transition zone to higher temperatures and may cause increased susceptibility to temper brittleness in steels fully quenched and drawn at 1200' F.
Zirconium
Zirconium is a graingrowth inhibitor and is a more potent deoxidizer than boron, silicon, titanium, vanadium, or manganese. It has been considered as a substitute for manganese in combating hot shortness, but has not been used for economic reasons.
The chief commercial use of zirconium is in lowalloy highstrength steels to improve their hot rolled properties. Improved lowtemperature properties have been reported by some investigators who have shown that zirconium is more effective than vanadium in raising the lowtemperature Izod impact property of certain steels.
Zirconium has a positive effect on hardenability, but usually it is not sufficiently great enough to use the element primarily for this purpose.
The elevated temperature properties are generally improved by the presence of zirconium nitrides which apparently are stable nitrides of low solubility that enable the steel to resist creep.
Cobalt
Cobalt is soluble in all proportions in gamma iron and has nearly 75% solubility in alpha iron. Cobalt hardens or strengthens the ferrite when it is dissolved in ferrite and thus resists softening under elevated temperature conditions. The carbideforming tendency is similar to or slightly higher than iron's carbideforming tendency.
With respect to hardenability, cobalt has a negative effect for 0.40% carbon steels and hastens the transformation to softer products rather than retarding transformation and inducing hardenability, as do most alloying elements. However, in very lowcarbon chromium steels, cobalt has been shown to increase hardenability.
Combination of Alloying Elements
Combinations of the alloys in all possible proportions would, of course, result in a multitude of alloy steels. However, by careful consideration of the characteristics of various alloy combinations, a relatively small list of standard alloy steels meets the requirements of steel producers and consumers. In the standard constructional alloy steels, carbon, manganese, and silicon are elements, which occur normally. A steel containing one additional alloying element, such as nickel, chromium, or molybdenum, is known as single alloy steel. When two such alloying elements are added, it is known as a double (binary) alloy steel; when three are added, it becomes a triple (ternary) alloy steel.
In general, it may be said that the combination of alloys in constructional steels results in no loss of the qualities which are imparted to the steel when alloying agents are added singly. For instance, if nickel strengthens ferrite when used alone, it will act to an equivalent degree when used in combination with other elements. When used in combination, alloys may in fact so complement each other as to give greater overall benefits than when used singly in much larger quantities. Thus steels of the nickel, chromium, or molybdenum single alloy types are used generally in smaller sections where throughhardening and suitable mechanical properties can be attained. When larger sections are to be handled, the double and triple alloy types are preferred because they harden deeper and offer increased resistance to tempering. The triple alloy types afford an interesting example of more uniform properties because the common residual elements are specified as alloys so that unexpected effects due to occasional variations in residual alloys are not encountered. Also, the probability that all specified elements will occur at the low limit of the specified composition is less than that for single or double alloy compositions. Closer control of hardenability and resulting mechanical properties is therefore possible.
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