Iron ore constitutes about 5% of the earth's crust and is easy to convert to a useful form. Iron is obtained by fusing the ore to drive off oxygen, sulfur, and other impurities. The ore is melted in a furnace in direct contact with the fuel using limestone as a flux. The limestone combines with impurities and forms a slag, which is easily removed.
Adding carbon in small amounts reduces the melting point (2,777°F) of iron. All commercial forms of iron and steel contain carbon, which is an integral part of the metallurgy of iron and steel. Manipulation of atom-to-atom relationships between iron, carbon, and various alloying elements establishes the specific properties of ferrous metals. As atoms transform from one specific arrangement, or crystal lattice, to another, strength, toughness, impact resistance, hardness, ductility, and other properties are altered. The metallurgy of iron and steel is a study of how these atomic rearrangements take place, how they can be controlled, and which properties are affected.
In the iron-carbon alloy system, an important phase transformation takes place between about 1,300 and 1,600°F. The exact temperature is determined by the amount of carbon and other alloying elements in the metal. Iron transforms from a face-centered cubic (FCC) structure -- called the gamma phase, or austenite -- at high temperature to a body-centered cubic (BCC) structure -- alpha phase, or ferrite -- at a lower temperature. In pure iron, this transformation (the A3 transformation) is marked by a distinct increase in length as the metal cools below the critical temperature because the body-centered lattice is less compact than the face-centered lattice.
High-temperature austenite, an FCC structure, allows enough space for carbon to squeeze in between the iron atoms. Iron atoms maintain their place on the lattice and carbon atoms become "interstitials." In the low-temperature ferrite, or BCC structure, however, there is no room for carbon atoms. What happens to these carbon atoms determines many of the properties of iron and steel.
For example, during the slow cooling of a low-carbon steel such as AISI 1020 (0.20% carbon), transformation begins as the metal reaches 1,555°F. The first metal to reach this temperature transforms to ferrite, the BCC structure, and expels the interstitial carbon into the remaining regions of austenite. As the metal cools further, more iron transforms into ferrite, leaving less austenite and more regions rich in expelled interstitial carbon.
Finally, at about 1,350°F, the lower end of the transformation temperature range for 1020 steel, the last remaining austenite tries to transform -- in spite of the rich carbon concentrations. At this point, two things occur: The carbon bonds with available iron atoms to form Fe3C, an intermetallic compound called cementite, or iron carbide, and it precipitates out as a discrete structure; the remaining austenite then transforms to ferrite.
The structure that results from this final transformation is a lamination consisting of alternating layers of ferrite and iron carbide. Of course, the portions of metal that transformed previously remain as large islands of pure ferrite. The laminated structure formed at the last moment is called pearlite. The combined structure of ferrite and pearlite is soft and ductile -- steel in its lowest-strength condition.
In contrast, when ferrous alloys are cooled rapidly, or quenched, expelled carbon atoms do not have time to move away from the iron as it transforms to ferrite. The steel becomes so rigid that, before the carbon atoms can move, they become trapped in the lattice as the iron atoms try to transform to the body-centered cubic structure. The result is a body-centered tetragonal structure in which the carbon atom is an interstitial member. Steel that has undergone this type of transformation is martensitic. Naturally, martensite is in a state of unequilibrium, but it owes much of its high strength and hardness (and lower ductility) to its distorted, stressed lattice structure.
A number of heat-treatment cycles have been developed to alter the structure of steel. For example, when martensite is tempered (heated below A3 temperature) some internal stresses are relieved, and the resulting structure has more ductility than as-quenched martensite.
Other heat treatments change the proportions of pearlite and martensite; some even entrap austenite at room temperature. Others alter or reduce the size of the grains or pattern of these structures, providing improved strength or toughness. And when other alloying elements -- including boron, nickel, chromium, manganese, silicon, and vanadium -- are added to the metal, the behavior of ferrous alloys, as they transform from one structure to another, is further complicated. But because the structure of steel -- and thus, the mechanical properties of steel -- can be altered in so many ways, ferrous alloys can be developed to suite an extremely wide variety of design needs.
Steels and cast irons are both primarily iron with carbon as the main alloying element. Steels contain less than 2 and usually less than 1% carbon; all cast irons contain more than 2% carbon. Two percent is about the maximum carbon content at which iron can solidify as a single-phase alloy with all the carbon solution in austenite. Thus, the cast irons, by definition, solidify as heterogeneous alloys and always have more than one constituent in their microstructure. In addition to carbon, cast irons must also contain silicon, usually from 1 to 3%; thus, they are actually iron-carbon-silicon alloys.
High-carbon content and silicon in cast irons give them excellent castability. Their melting temperatures are appreciably lower than those of steel. Molten iron is more fluid than molten steel and less reactive with molding materials. Formation of lower density graphite during solidification makes production of complex shapes possible. Cast irons, however, do not have sufficient ductility to be rolled or forged.
Iron's carbon content is the key to its distinctive properties. The precipitation of carbon (as graphite) during solidification counteracts the normal shrinkage of the solidifying metal, producing sound sections. Graphite also provides excellent machinability (even at wear-resisting hardness levels), damps vibration, and aids lubrication on wearing surfaces (even under borderline lubrication conditions). When most of the carbon remains combined with the iron (as in white iron), the presence of hard iron carbides provides good abrasion resistance.
In some cases, iron microstructure may be all ferrite -- the same constituent that makes low-carbon steels soft and easily machined. But the ferrite of iron is different because it contains sufficient dissolved silicon to eliminate the characteristic gummy nature of low-carbon steel. Thus, cast irons containing ferrite do not require sulfur or lead additions in order to be free machining.
Because a casting's size and shape control its solidification rate and strength, design of the casting and the casting process involved must be considered in selecting the type of iron to be specified. While most other metals are specified by a standard chemical analysis, a single analysis of cast iron can produce several entirely different types of iron, depending upon foundry practice and shape and size of the casting, all of which influence cooling rate. Thus, iron is usually specified by mechanical properties. For applications involving high temperatures or requiring specific corrosion resistance, however, some analysis requirements may also be specified.
Patternmaking is no longer a necessary step in manufacturing cast-iron parts. Many gray, ductile, and alloy-iron components can be machined directly from bar that is continuously cast to near-net shape. Not only does this "parts-without-patterns" method save the time and expense of patternmaking, continuous-cast iron also provides a uniformly dense, fine-grained structure, essentially free from porosity, sand, or other inclusions. Keys to the uniform microstructure of the metal are the ferrostatic pressure and the temperature-controlled solidification that are unique to the process.
For each basic type of cast iron, there are a number of grades with widely differing mechanical properties. These variations are caused by differences in the microstructure of the metal that surrounds the graphite (or iron carbides). Two different structures can exist in the same casting. The microstructure of cast iron can be controlled by heat treatment, but once graphite is formed, it remains.
Pearlitic cast-iron grades consist of alternating layers of soft ferrite and hard iron carbide. This laminated structure -- called pearlite -- is strong and wear resistant, but still quite machinable. As laminations become finer, hardness and strength of the iron increase. Lamination size can be controlled by heat treatment or cooling rate.
Cast irons that are flame hardened, induction hardened, or furnace heated and subsequently oil quenched contain a martensite structure. When tempered, this structure provides machinability with maximum strength and good wear resistance.
Specification methods: ASTM specifications for iron castings are based on a different method than that of the SAE. ASTM specifications designate the properties of the metal to be obtained in an appropriately sized but separately cast test bar, which is poured under the same conditions as are the castings. SAE specifications, on the other hand, require that the microstructure of the casting be appropriate for the specified grade of metal and that the hardness of each casting at a designated location be within the specified range.
Commercially, the ASTM specification is more commonly used for general engineering applications where the strength of the iron necessary in the part has been established. SAE specifications are usually used for large quantities of smaller cast components such as those used in automobiles, and in agricultural and refrigeration equipment. In these cases, the suitability of a particular grade of iron is established, not only on design considerations, but also on actual proof in operation; the purpose of the specification is to ensure a consistent product comparable to those found, by experience, to be satisfactory.
Gray iron: This is a supersaturated solution of carbon in an iron matrix. The excess carbon precipitates out in the form of graphite flakes. Gray iron is specified by a two-digit designation; Class 20, for example, specifies a minimum tensile strength of 20,000 psi. In addition, gray iron is specified by the cross section and minimum strength of a special test bar. Usually, the test-bar cross section matches or is related to a particularly critical section of the casting. This second specification is necessary because the strength of gray iron is highly sensitive to cross section (the smaller the cross section, the faster the cooling rate and the higher the strength).
Impact strength of gray iron is lower than that of most other cast ferrous metals. In addition, gray iron does not have a distinct yield point (as defined by classical formulas) and should not be used when permanent, plastic deformation is preferred to fracture. Another important characteristic of gray iron -- particularly for precision machinery -- is its ability to damp vibration. Damping capacity is determined principally by the amount and type of graphite flakes. As graphite decreases, damping capacity also decreases.
Gray iron's high compressive strength -- three to five times tensile strength -- can be used to advantage in certain situations. For example, placing ribs on the compression side of a plate instead of the tension side produces a stronger, lighter component.
Gray irons have excellent wear resistance. Even the softer grades perform well under certain borderline lubrication conditions (as in the upper cylinder walls of internal-combustion engines, for example).
To increase the hardness of gray iron for abrasive-wear applications, alloying elements can be added, special foundry techniques can be used, or the iron can be heat treated. Gray iron can be hardened by flame or induction methods, or the foundry can use a chill in the mold to produce hardened, "white-iron" surfaces.
Typical applications of gray iron include automotive engine blocks, gears, flywheels, brake discs and drums, and machine bases. Gray iron serves well in machinery applications because of its good fatigue resistance.
Ductile iron: Ductile, or nodular, iron contains trace amounts of magnesium which, by reacting with the sulfur and oxygen in the molten iron, precipitates out carbon in the form of small spheres. These spheres improve the stiffness, strength, and shock resistance of ductile iron over gray iron. Different grades are produced by controlling the matrix structure around the graphite, either as-cast or by subsequent heat treatment.
A three-part designation system is used to specify ductile iron. The designation of a typical alloy, 60-40-18, for example, specifies a minimum tensile strength of 60,000 psi, a minimum yield strength of 40,000 psi, and 18% elongation in 2 in.
Ductile iron is used in applications such as crankshafts because of its good machinability, fatigue strength, and high modulus of elasticity; in heavy-duty gears because of its high yield strength and wear resistance; and in automobile door hinges because of its ductility. Because it contains magnesium as an additional alloying element, ductile iron is stronger and more shock resistant than gray iron. But although ductile iron also has a higher modulus of elasticity, its damping capacity and thermal conductivity are lower than those of gray iron.
By weight, ductile iron castings are more expensive than gray iron. Because they offer higher strength and provide better impact resistance, however, overall part costs may be about the same.
Although it is not a new treatment for ductile iron, austempering has become increasingly known to the engineering community in the past five to 10 years. Austempering does not produce the same type of structure as it does in steel because of the high carbon and silicon content of iron. The matrix structure of austempered ductile iron (ADI) sets it apart from other cast irons, making it truly a separate class of engineering materials.
In terms of properties, the ADI matrix almost doubles the strength of conventional ductile iron while retaining its excellent toughness. Like ductile iron, ADI is not a single material; rather, it is a family of materials having various combinations of strength, toughness, and wear resistance. Unfortunately, the absence of a standard specification for the materials has restricted its widespread acceptance and use. To help eliminate this problem, the Ductile Iron Society has proposed property specifications for four grades of austempered ductile iron.
Most current applications for ADI are in transportation equipment -- automobiles, trucks, and railroad and military vehicles. The same improved performance and cost savings are expected to make these materials attractive in equipment for other industries such as mining, earthmoving, agriculture, construction, and machine tools.
White iron: White iron is produced by "chilling" selected areas of a casting in the mold, which prevents graphitic carbon from precipitating out. Both gray and ductile iron can be chilled to produce a surface of white iron, consisting of iron carbide, or cementite, which is hard and brittle. In castings that are white iron throughout, however, the composition of iron is selected according to part size to ensure that the volume of metal involved can solidify rapidly enough to produce the white-iron structure.
The principal disadvantage of white iron is its brittleness. This can be reduced somewhat by reducing the carbon content or by thoroughly stress relieving the casting to spheroidize the carbides in the matrix. However, these measures increase cost and reduce hardness.
Chills produce castings with white-iron working surfaces and cores that are a tougher and more easily machinable gray or ductile iron. During chilling, that portion of the casting that is to resist wear is cooled by a metal or graphite heat sink (chill) in the mold. When the molten iron contacts the chill, it solidifies so rapidly that the iron and carbon cannot become dissociated.
Chilling should not be confused with heat-treat hardening, which involves an entirely different metallurgical mechanism. White iron, so called because of its very white fracture, can be formed only during solidification. It will not soften except by extended annealing, and it retains its hardness even above 1,000°F.
White irons are used primarily for applications requiring wear and abrasion resistance such as mill liners and shot-blasting nozzles. Other uses include railroad brake shoes, rolling-mill rolls, clay-mixing and brickmaking equipment, and crushers and pulverizers. Generally, plain (unalloyed) white iron costs less than other cast irons.
Compacted graphite iron: Until recently, compacted graphite iron (CGI), also known as vermicular iron, has been primarily a laboratory curiosity. Long known as an intermediate between gray and ductile iron, it possesses many of the favorable properties of each. However, because of process-control difficulties and the necessity of keeping alloy additions within very tight limits, CGI has been extremely difficult to produce successfully on a commercial scale. For example, if the magnesium addition varied by as little as 0.005%, results would be unsatisfactory.
Processing problems have been solved by the joint development efforts of the Foote Mineral Co. and the British Cast Iron Research Association. An alloy-addition package provides the essential alloying ingredients -- magnesium, titanium, and rare earths -- in exactly the right proportions.
Strength of CGI parts approaches that of ductile cast iron. CGI also offers high thermal conductivity, and its damping capacity is almost as good as that of gray iron; fatigue resistance and ductility are similar to those properties in ductile iron. Machinability is superior to that of ductile iron, and casting yields are high because shrinkage and feeding characteristics are more like gray iron.
The combination of high strength and high thermal conductivity suggests the use of CGI in engine blocks, brake drums, and exhaust manifolds of vehicles. CGI gear plates have replaced aluminum in high-pressure gear pumps because of the iron's ability to maintain dimensional stability at pressures above 1,500 psi.
Malleable iron: Malleable iron is white iron that has been converted by a two-stage heat treatment to a condition having most of its carbon content in the form of irregularly shaped nodules of graphite, called temper carbon. Resulting properties are opposite from those of the white iron from which it is derived. Rather than being hard and brittle, it is malleable and easily machined. Malleable-iron castings generally cost slightly less than ductile-iron castings.
The three basic types of malleable iron are ferritic, pearlitic, and martensitic. Ferritic grades are more machinable and ductile, whereas the pearlitic grades are stronger and harder. Generally, the martensitic grades are grouped with the pearlitic materials; they might be thought of as extensions (at the higher strength end of the range) of pearlitic malleable iron.
In sharp contrast to ferritic malleable iron, whose microstructure is free from combined carbon, pearlitic malleable iron contains from 0.3 to 0.9% carbon in the combined form. Since this constituent can be transformed readily into the hardest form of combined carbon by a simple heating and quenching treatment, pearlitic malleable-iron castings can be selectively hardened. Depth of hardening is controlled by the rate of heat input, time at temperature, and quenching rate. Heat treating can produce surface hardness to about Rockwell C 60.
Carbon in malleable irons helps retain and store lubricants. In extreme-wear service, the pearlitic malleable-iron surface wears away in harmless, micron-size particles, which are less damaging than other types of iron particles. The porous malleable-iron surface traps abrasive debris that accumulates between bearing surfaces. Gall streaks can form on malleable iron but galling does not usually progress.
Malleable-iron castings are often used for heavy-duty bearing surfaces in automobiles, trucks, railroad rolling stock, and farm and construction machinery. Pearlitic grades are highly wear resistant, with hardnesses ranging from 152 to over 300 Bhn. Applications are limited, however, to relatively thin-sectioned castings because of the high shrinkage rate and the need for rapid cooling to produce white iron.
High-alloy irons: High-alloy irons are ductile, gray, or white irons that contain 3 to more than 30% alloy content. Properties by specialized foundries, are significantly different from those of unalloyed irons. These irons are usually specified by chemical composition as well as by various mechanical properties.
White high-alloy irons containing nickel and chromium develop a microstructure with a martensite matrix around primary chromium carbides. This structure provides a high hardness with extreme wear and abrasion resistance. High-chromium irons (typically, about 16%), combine wear and oxidation resistance with toughness. Irons containing from 14 to 24% nickel are austenitic; they provide excellent corrosion resistance for nonmagnetic applications. The 35%-nickel irons have an extremely low coefficient of thermal expansion and are also nonmagnetic and corrosion resistant.
Carbon steel, also called plain carbon steel, is a malleable, iron-based metal containing carbon, small amounts of manganese, and other elements that are inherently present. Steels can either be cast to shape or wrought into various mill forms from which finished parts are formed, machined, forged, stamped, or otherwise shaped.
Cast steels are poured to near-final shape in sand molds. The castings are then heat treated to achieve specified properties and machined to required dimensions.
Wrought steel undergoes two operations. First, it is either poured into ingots or strand cast. Then, the metal is reheated and hot rolled into the finished, wrought form. Hot-rolled steel is characterized by a scaled surface and a decarburized skin. Hot-rolled bars may be subsequently finished in a two-part process. First, acid pickling or shot blasting removes scale. Then, cold drawing through a die and restraightening improves surface properties and strength. Hot-rolled steel may also be cold finished by metal-removal processes such as turning or grinding. Wrought steel can be subsequently heat treated to improve machinability or to adjust mechanical properties.
Carbon steels may be specified by chemical composition, mechanical properties, method of deoxidation, or thermal treatment (and the resulting microstructure).
Composition: Wrought steels are most often specified by composition. No single element controls the characteristics of a steel; rather, the combined effects of several elements influence hardness, machinability, corrosion resistance, tensile strength, deoxidation of the solidifying metal, and microstructure of the solidified metal.
Effects of carbon, the principal hardening and strengthening element in steel, include increased hardness and strength and decreased weldability and ductility. For plain carbon steels, about 0.2 to 0.25% C provides the best machinability. Above and below this level, machinability is generally lower for hot-rolled steels.
Standard wrought-steel compositions (for both carbon and alloy steels) are designated by an AISI or SAE four-digit code, the last two digits of which indicate the nominal carbon content. The carbon-steel grades are:
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The letter "L" between the second and third digits indicates a leaded steel; "B" indicates a boron steel.
Cast-carbon steels are usually specified by grade, such as A, B, or C. The A grade (also LCA, WCA, AN, AQ, etc.) contains 0.25% C and 0.70% Mn maximum. B-grade steels contain 0.30% C and 1.00% Mn, and the C-grade steels contain 0.25% C and 1.20% Mn. These carbon and manganese contents are designed to provide good strength, toughness, and weldability. Cast carbon steels are specified to ASTM A27, A216, A352, or A487.
Microalloying technology has created a new category of steels, positioned both in cost and in performance between carbon steels and the alloy grades. These in-between steels consist of conventional carbon steels to which minute quantities of alloying elements -- usually less than 0.5% -- are added in the steelmaking process to improve mechanical properties. Strength and hardness are increased significantly.
Any base-grade steel can be microalloyed, but the technique was first used in sheet steel a number of years ago. More recently, microalloying has been applied to bar products to eliminate the need for heat-treating operations after parts are forged. Automotive and truck applications include connecting rods, blower shafts, stabilizer bars, U-bolts, and universal joints. Other uses are sucker rods for oil wells and anchor bolts for the construction industry.
Mechanical properties: Cast and wrought products are often specified to meet distinct mechanical requirements in structural applications where forming and machining are not extensive. When steels are specified by mechanical properties only, the producer is free to adjust the analysis of the steel (within limits) to obtain the required properties. Properties may vary with cross section and part size.
Mechanical tests are usually specified under one of two conditions: mechanical test requirements and no chemical limits on any element, or mechanical test requirements and chemical limits on one or more elements, provided that such requirements are technologically compatible.
Method of deoxidation: Molten steel contains dissolved oxygen -- an important element in the steelmaking reaction. How this oxygen is removed or allowed to escape as the metal solidifies determines some of the properties of the steel. So in many cases, "method of deoxidation" is specified in addition to AISI and SAE chemical compositions.
For "killed" steels, elements such as aluminum and silicon may be added to combine chemically with the oxygen, removing most of it from the liquid steel. Killed steels are often specified for hot forging, carburizing, and other processes or applications where maximum uniformity is required. In sheet steel, aging is controlled by killing -- usually with aluminum. Steels intended for use in the as-cast condition are always killed. For this reason, steels for casting are always fully deoxidized.
On the other hand, for "rimmed" steels, oxygen (in the form of carbon monoxide) evolves briskly throughout the solidification process. The outer skin of rimmed steels is practically free from carbon and is very ductile. For these reasons, rimmed steels are often specified for cold-forming applications. Rimmed steels are often available in grades with less than 0.25% C and 0.60% Mn.
Segregation -- a nonuniform variation in internal characteristics and composition that results when various alloying elements redistribute themselves during solidification -- may be pronounced in rimmed steels. For this reason, they are usually not specified for hot forging or for applications requiring uniformity.
"Capped" and "semikilled" steels fall between the rimmed and killed steels in behavior, properties, and degree of oxidation and segregation. Capped steels, for example, are suited for certain cold-forming applications because they have a soft, ductile, surface skin, which is thinner than rimmed-steel skin. For other cold-forming applications, such as cold extrusion, killed steels are more suitable.
Microstructure: The microstructure of carbon and alloy steels in the as-rolled or as-cast condition generally consists of ferrite and pearlite. This basic structure can be altered significantly by various heat treatments or by rolling techniques. A spheroidized annealed structure would consist of spheroids of iron and alloy carbides dispersed in a ferrite matrix for low hardness and maximum ductility, as might be required for cold-forming operations. Quenching and tempering provide the optimum combination of mechanical properties and toughness obtainable from steel. Grain size can also be an important aspect of the microstructure. Toughness of fine-grained steels is generally greater than that of coarse-grained steels.
Free-machining steels: Several free-machining carbon steels are available as castings and as hot-rolled or cold-drawn bar stock and plate. Machinability in steels is improved in several ways, including:
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Steels that contain specified amounts of alloying elements -- other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus -- are known as alloy steels. Alloying elements are added to change mechanical or physical properties. A steel is considered to be an alloy when the maximum of the range given for the content of alloying elements exceeds one or more of these limits: 1.65% Mn, 0.60% Si, or 0.60% Cu; or when a definite range or minimum amount of any of the following elements is specified or required within the limits recognized for constructional alloy steels: aluminum, chromium (to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium or other element added to obtain an alloying effect.
Technically, then, tool and stainless steels are alloy steels. In this chapter, however, the term alloy steel is reserved for those steels that contain a modest amount of alloying elements and that usually depend on thermal treatment to develop specific properties. With proper heat treatment, for example, tensile strength of certain alloy steels can be raised from about 55,000 psi to nearly 300,000 psi.
Subdivisions for most steels in this family include "through-hardenable" and "carburizing" grades (plus several specialty grades such as nitriding steels). Through-hardening grades -- which are heat treated by quenching and tempering -- are used when maximum hardness and strength must extend deep within a part. Carburizing grades are used where a tough core and relatively shallow, hard surface are needed. After a surface-hardening treatment such as carburizing (or nitriding for nitriding alloys), these steels are suitable for parts that must withstand wear as well as high stresses. Cast steels are generally through hardened, not surface treated.
Carbon content and alloying elements influence the overall characteristics of both types of alloy steels. Maximum attainable surface hardness depends primarily on carbon content. Maximum hardness and strength in small sections increase as carbon content increases, up to about 0.7%. However, carbon contents greater than 0.3% can increase the possibility of cracking during quenching or welding. Alloying elements primarily influence hardenability. They also influence other mechanical and fabrication properties including toughness and machinability.
Lead additions (0.15 to 0.35%) substantially improve machinability of alloy steels by high-speed tool steels. For machining with carbide tools, calcium-treated steels are reported to double or triple tool life in addition to improving surface finish.
Few exact rules exist for selecting through-hardening or surface-hardening grades of alloy steels. In most cases, critical parts are field tested to evaluate their performance. Parts with large sections -- heavy forgings, for example -- are often made from alloy steels that have been vacuum degassed. While in a molten state, these steels are exposed to a vacuum which removes hydrogen and, to a lesser degree, oxygen and nitrogen.
Alloy steels are often specified when high strength is needed in moderate-to-large sections. Whether tensile or yield strength is the basis of design, thermally treated alloy steels generally offer high strength-to-weight ratios. For applications requiring maximum ductility, alloys with low sulfur levels (<0.01%) can be supplied by producers using ladle-refining techniques.
In general, wear resistance can be improved by increasing the hardness of an alloy, by specifying an alloy with greater carbon content (without increasing hardness), or by both. The surface of a flame-hardened, medium-carbon steel, for example, is likely to have poorer wear resistance than the carbon-rich case of a carburized steel of equal hardness. Exceptions are nitrided parts, which have better wear resistance than would be expected from the carbon content alone.
For any combination of alloy steel and heat treatment, three factors tend to decrease toughness: low service temperature, high loading rates, and stress concentrations or residual stress. The general effects of these three conditions are qualitatively similar, so low-temperature impact tests (to -50°F) are useful for many applications as toughness indicators under various service conditions and temperatures.
Fully hardened-and-tempered, low-carbon (0.10 to 0.30% C) alloy steels have a good combination of strength and toughness, both at room and low temperature. Care must be taken in heat treatment of certain alloy-steel grades, however, because toughness may be decreased substantially by temper brittleness -- a form of embrittlement developed by slow cooling through the range of 900 to 600°F, or by holding or tempering in this range.
When liquid quenching is impractical (because of the danger of cracking or distortion, or because of cost), various low-carbon nickel or nickel-molybdenum steels in the normalized-and-tempered condition can be used for low-temperature service.
Wrought alloy steels (and carbon steels) are classified by a series of AISI and SAE numbers that designate composition and alloy type. Letters, which are used in addition to the four-digit designations, include the suffix "H," used for steel produced to specific hardenability limits (which allows wider composition ranges for certain alloying elements), and the prefix "E," which indicates a steel made by the basic electric-furnace method. Other specifications, such as those issued by ASTM, specify minimum properties for critical structural, pressure-vessel, and nuclear applications.
ASTM specifications classify cast alloy steels by relating the steel to the mechanical properties and intended service condition. Chemical analysis is secondary. There are ASTM specifications for general use such as A27 or A148 when mechanical properties are critical. For low-temperature service, A352 or A757 is recommended when toughness is important. For weldability, A216 is specified when fabrication is critical, and for pressure service, A217 or A389 is recommended when a number of properties are important. Still other ASTM alloy steels are available for special applications. Other specifications such as SAE J435 are used for cast steels in automotive applications. A summary of steel-casting specifications is available from the Steel Founders' Society of America, Des Plaines, Ill.
One of the features that characterize stainless steels is a minimum 10.5% chromium content as the principal alloying element. Four major categories of wrought stainless steel, based on metallurgical structure, are austenitic, ferritic, martensitic, and precipitation hardening. Cast stainless-steel grades are generally designated as either heat resistant or corrosion resistant.
Austenitic wrought stainless steel are classified in three groups:
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Carbon content is usually low (0.15% or less), and the alloys contain a minimum of 16% chromium with sufficient nickel and manganese to provide an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.
Nitrogen-strengthened austenitic stainless steels are alloys of chromium-manganese-nitrogen; some grades also contain nickel. Yield strengths of these alloys (annealed) are typically 50% higher than those of the nonnitrogen-bearing grades. They are nonmagnetic and most remain so, even after severe cold working.
Like carbon, nitrogen increases the strength of a steel. But unlike carbon, nitrogen does not combine significantly with chromium in a stainless steel. This combination, which forms chromium carbide, reduces the strength and corrosion resistance of an alloy.
Until recently, metallurgists had difficulty adding controlled amounts of nitrogen to an alloy. The development of the argon-oxygen decarburization (AOD) method has made possible strength levels formerly unattainable in conventional annealed stainless alloys.
Austenitic stainless steels are generally used where corrosion resistance and toughness are primary requirements. Typical applications include shafts, pumps, fasteners, and piping in seawater and equipment for processing chemicals, food, and dairy products.
Ferritic wrought alloys (the AISI 400 series) contain from 10.5 to 27% chromium. In addition, the use of argon-oxygen decarburization and vacuum-induction melting has produced several new ferritic grades including 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. Low in carbon content, but generally higher in chromium than the martensitic grades, these steels cannot be hardened by heat treating and are only moderately hardened by cold working. Ferritic stainless steels are magnetic and retain their basic microstructure up to the melting point if sufficient Cr and Mo are present. In the annealed condition, strength of these grades is approximately 50% higher than that of carbon steels.
Ferritic stainless steels are typically used where moderate corrosion resistance is required and where toughness is not a major need. They are also used where chloride stress-corrosion cracking may be a problem because they have high resistance to this type of corrosion failure. In heavy sections, achieving sufficient toughness is difficult with the higher-alloyed ferritic grades. Typical applications include automotive trim and exhaust systems and heat-transfer equipment for the chemical and petrochemical industries.
Martensitic steels are also in the AISI 400 series. These wrought, higher-carbon steels contain from 11.5 to 18% chromium and may have small quantities of additional alloying elements. They are magnetic, can be hardened by heat treatment, and have high strength and moderate toughness in the hardened-and-tempered condition. Forming should be done in the annealed condition. Martensitic stainless steels are less resistant to corrosion than the austenitic or ferritic grades. Two types of martensitic steels -- 416 and 420F -- have been developed specifically for good machinability.
Martensitic stainless steels are used where strength and/or hardness are of primary concern and where the environment is relatively mild from a corrosive standpoint. These alloys are typically used for bearings, molds, cutlery, medical instruments, aircraft structural parts, and turbine components. Type 420 is used increasingly for molds for plastics and for industrial components requiring hardness and corrosion resistance.
Precipitation-hardening stainless steels develop very high strength through a low-temperature heat treatment that does not significantly distort precision parts. Compositions of most precipitation-hardening stainless steels are balanced to produce hardening by an aging treatment that precipitates hard, intermetallic compounds and simultaneously tempers the martensite. The beginning microstructure of PH alloys is austenite or martensite. The austenitic alloys must be thermally treated to transform austenite to martensite before precipitation hardening can be accomplished.
These alloys are used where high strength, moderate corrosion resistance, and good fabricability are required. Typical applications include shafting, high-pressure pumps, aircraft components, high-temper springs, and fasteners.
Cast stainless steels usually have corresponding wrought grades that have similar compositions and properties. However, there are small but important differences in composition between cast and wrought grades. Stainless-steel castings should be specified by the designations established by the ACI (Alloy Casting Institute), and not by the designation of similar wrought alloys.
Service temperature provides the basis for a distinction between heat-resistant and corrosion-resistant cast grades. The C series of ACI grades designates the corrosion-resistant steels; the H series designates the heat-resistant steels, which can be used for structural applications at service temperatures between 1,200 and 2,200°F. Carbon and nickel contents of the H-series alloys are considerably higher than those of the C series. H-series steels are not immune to corrosion, but they corrode slowly -- even when exposed to fuel-combustion products or atmospheres prepared for carburizing and nitriding. C-series grades are used in valve, pumps, and fittings. H-series grades are used for furnace parts and turbine components.
Galling and wear are failure modes that require special attention with stainless steels because these materials serve in many harsh environments. They often operate, for example, at high temperatures, in food-contact applications, and where access is limited. Such restrictions prevent the use of lubricants, leading to metal-to-metal contact -- a condition that promotes galling and accelerated wear.
In a sliding-wear situation, a galling failure mode occurs first, followed by dimensional loss due to wear, which is, in turn, usually followed by corrosion. Galling is a severe form of adhesive wear that shows up as torn areas of the metal surface. Galling can be minimized by decreasing contact stresses or by the use of protective surface layers such as lubricants (where acceptable), weld overlays, platings, and nitrided or carburized surface treatments.
Test results from stainless-steel couples (table) indicate the relatively poor galling resistance of austenitic grades and even alloy 17-4 PH, despite its high hardness. Among the standard grades, only AISI 416 and 440C performed well. Good to excellent galling resistance was demonstrated by Armco's Nitronic 32 and 60 alloys (the latter were developed specifically for antigalling service).
Recent research findings prove that adding silicon to a high-manganese, nitrogen-strengthened austenitic stainless alloy produces a wear-resistant stainless steel. Wear and corrosion resistance are still considered unavoidable trade-offs in stainless, but the new formula promises to resist both conditions.
Beating corrosion is the number one reason for choosing stainless. But in cases where parts are difficult to lubricate, most stainless steels cannot resist wear. Under high loads and insufficient lubrication, stainless often sports a type of surface damage known as galling. In critical parts, galling can lead to seizure or freezing, which can shut down machinery.
Designers typically get around galling by using cast alloys or by applying a cobalt facing to stainless parts. Either way, the fixes can be expensive and may pose new problems that accompany the hard-facing process. These include maintaining uniform facing thickness and ensuring proper adhesion between facing and substrate. A new stainless formula aims to sidestep these difficulties by offering an alternative to expensive wear-resistant materials.
In search of a cost-effective alternative, researchers at Carpenter Technology, Reading, Pa., looked at elemental effects of silicon, manganese, and nickel on galling resistance of nitrogen-strengthened, austenitic stainless steels. Results of an initial test program determined that silicon was a catalyst for galling resistance, while nickel and manganese were not.
The silicon levels in a recently developed gall-resistant stainless alloy are between 3 and 4%. Silicon levels must remain lower than 5% to maintain the proper metallurgical structure. In addition, too much silicon decreases nitrogen solubility. To maintain strength, higher amounts of costly nickel would need to be added.
Researchers can now define optimum composition limits for a gall-resistant stainless steel. To prove the new steel's validity, properties such as galling, wear, and corrosion are evaluated and compared with commercially available stainless steels. Four alloys, a gall-resistant austenitic alloy called Gall-Tough, another austenitic alloys with higher nickel and manganese content (16Cr-8Ni-4Si-8Mn), and Types 304 and 430 stainless steels are included in the comparison.
Results show the galling threshold for gall-resistant stainless is over 15 times higher than that of conventional stainless steels. In addition, gall-resistant stainless withstands more than twice the stress without galling compared to the 16Cr-8Ni-4Si-8Mn alloy. Yet, the new formula sacrifices only a slight amount of corrosion resistance.
For strength and hardness, both gall-resistant stainless and the 16Cr-8Ni-4Si-8Mn alloy beat Types 304 and 430 alloys. The new alloy also shows a uniquely high ultimate tensile strength, possibly due to martensite formation during tensile testing. Ductility for all four alloys is excellent. These findings indicate that gall-resistant alloys can economically bridge the gap between corrosion, galling, and metal-to-metal wear resistance.
The same properties that qualify tool steels for tools and dies are also used for other parts that require resistance to wear, stability during heat treatment, strength at high temperatures, or toughness. Tool steels are increasingly being used for mechanical parts to reduce size or weight, or to resist wear or high-temperature shock.
Tool steels are metallurgically "clean," high-alloy steels that are melted in relatively small heats in electric furnaces and produced with careful attention to homogeneity. They can be further refined by argon/oxygen decarburization (AOD), vacuum methods, or electroslag refining (ESR). As a result, tool steels are often specified for critical high-strength or wear-resistant applications. Because of their high alloy content, tool steels must be rolled or forged with care to produce satisfactory bar products.
To develop their best properties, tool steels are always heat treated. Because the parts may distort during heat treatment, precision parts should be semifinished, heat treated, then finished. Severe distortion is most likely to occur during liquid quenching, so an alloy should be selected that provides the needed mechanical properties with the least severe quench.
Tool steels are classified into several broad groups, some of which are further divided into subgroups according to alloy composition, hardenability, or mechanical similarities.
Water-hardening, or carbon, tool steels, designated Type W by AISI, rely solely on carbon content for their useful properties. These steels are available as shallow, medium, or deep hardening, so the specific alloy selected depends on part cross section and required surface and core hardnesses.
Shock-resisting tool steels (Type S) are strong and tough, but they are not as wear resistant as many other tool steels. These steels resist sudden and repeated loadings. Applications include pneumatic tooling parts, chisels, punches, shear blades, bolts, and springs subjected to moderate heat in service.
Cold-work tool steels, which include oil and air-hardening Types O, A, and D, are often more costly but can be quenched less drastically than water-hardening types. Type O steels are oil hardening; Type A and D steels are air hardening (the least severe quench), and are best suited for applications such as machine ways, brick mold liners, and fuel-injector nozzles. The air-hardening types are specified for thin parts or parts with severe changes in cross section -- parts that are prone to crack or distort during hardening. Hardened parts from these steels have a high surface hardness; however, these steels should not be specified for service at elevated temperatures.
Hot-work steels (Type H) serve well at elevated temperatures. The tungsten and molybdenum high-alloy hot-work steels are heat and abrasion resistant even at 600 to 1,000°F. But although these alloys do not soften at these high temperatures, they should be preheated before and cooled slowly after service to avoid cracking. The chromium grades of hot-work steels are less expensive than the tungsten and molybdenum grades. One of the chromium grades H11, is used extensively for aircraft parts such as primary airframe structures, cargo-support lugs, catapult hooks, and elevon hinges. Grade H13, which is similar to H11 is usually more readily available from suppliers.
High-speed tool steels -- Types T (tungsten alloy) and M (molybdenum alloy) -- make good cutting tools because they resist softening and maintain a sharp cutting edge at high service temperatures. This characteristic is sometimes called "red hardness." These deep-hardening alloys are used for steady, high-load conditions rather than shock loads. Typical applications are pump vanes and parts for heavy-duty strapping machinery.
Other grades, called special-purpose tool steels, include low-cost, Type L, low-alloy steels, often specified for machine parts when wear resistance combined with toughness is important. Carbon-tungsten alloys (Type F) are shallow hardening and wear resistant, but are not suited for high temperatures or for shock service.
Type P mold steels are designed specifically for plastic-molding and zinc die-casting dies. These steels are seldom used for nontooling components.
Many steel mills have formulated their own special-purpose tool-steel alloy. Such alloys may not match a specific AISI designation and must be specified by trade name. Special-purpose tool steels may be superior to the standard grades when used as intended, but they should be specified only after careful evaluation of mechanical properties, heat-treat behavior, and availability in comparison with the standard grades.
Those steel alloys known as high-strength low-alloy (HSLA) steels provide increased strength-to-weight ratios over conventional low-carbon steels for only a modest price premium. Because HSLA alloys are stronger, they can be used in thinner sections, making them particularly attractive for transportation-equipment components where weight reduction is important. HSLA steels are available in all standard wrought forms -- sheet, strip, plate, structural shapes, bar-size shapes, and special shapes.
Typically, HSLA steels are low-carbon steels with up to 1.5% manganese, strengthened by small additions of elements, such as columbium, copper, vanadium or titanium and sometimes by special rolling and cooling techniques. Improved-formability HSLA steels contain additions such as zirconium, calcium, or rare-earth elements for sulfide-inclusion shape control.
Since parts made from HSLA steels can have thinner cross sections than equivalent parts made from low-carbon steel, corrosion of an HSLA steel can significantly reduce strength by decreasing the load-bearing cross section. While additions of elements such as copper, silicon, nickel, chromium, and phosphorus can improve atmospheric corrosion resistance of these alloys, they also increase cost. Galvanizing, zinc-rich coatings, and other rust-preventive finishes can help protect HSLA-steel parts from corrosion.
Grades known as "improved-formability" HSLA steels (sheet-steel grades designated ASTM A715, and plates designated ASTM A656) have yield strengths up to 80,000 psi, yet cost only about 24% more than a typical 34,000-psi plain-carbon steel. Because these alloys must compete with other structural metals such as AISI 1010 steel and aluminum, they must be as inexpensive as possible. However, formulating and rolling a steel that meets this cost requirement is not easy, and the finished product presents a number of trade-offs. For example, the increase in strength from 35,000 to 80,000 psi may be accompanied by a 30 to 40% loss in ductility.
Improved-formability HSLA steels were developed primarily for the automotive industry to replace low-carbon steel parts with thinner cross-section parts for reduced weight without sacrificing strength and dent resistance. Typical passenger-car applications include door-intrusion beams, chassis members, reinforcing and mounting brackets, steering and suspension parts, bumpers, and wheels.
Trucks, construction equipment, off-highway vehicles, mining equipment, and other heavy-duty vehicles use HSLA sheets or plates for chassis components, buckets, grader blades, and structural members outside the body. For these applications, sheets or light-gage plates are specified. Structural forms (alloys from the family of 45,000 to 50,000-psi minimum yield strength HSLA steels) are specified in applications such as offshore oil and gas rigs, single-pole power-transmission towers, railroad cars, and ship construction.
In equipment such as power cranes, cement mixers, farm machinery, trucks, trailers, and power-transmission towers, HSLA bar, with minimum yield strengths ranging from 50,000 to 70,000 psi is used. Forming, drilling, sawing, and other machining operations on HSLA steels usually require 25 to 30% more power than do structural carbon steels.
Most HSLA alloys have directionally sensitive properties. For some grades, formability and impact strength vary significantly depending on whether the material is tested longitudinally or transversely to the rolled direction. For example, bends parallel to the longitudinal direction are more apt to cause cracking around the outside, tension-bearing surface of the bend. This effect is more pronounced in thick sheets. This directional characteristic is substantially reduced in HSLA steels that have been treated for sulfide shape control.
Developed primarily for high-strength applications, these steels are usually heat-treated alloys that provide strengths at least equal to those of as-rolled steel. Heat-treated constructional alloy steels and the ultrahigh-strength steels are used in applications where high strength can be converted to a weight-saving advantage over other steels.
High-yield-strength, quenched-and-tempered constructional alloy steels are usually heat treated at the mill to develop their properties so they require no further heat treatment by the fabricator. Although these heat-treated alloy steels are available in all conventional product forms, they are most common in plate products. Some grades are also available as abrasion-resistant (AR) modifications. In these conditions, high hardness is the desired property, with some toughness being sacrificed. Over 20 types of these proprietary high-strength alloy steels are produced. Some have been developed to combine improved welding characteristics along with high strength. Most have good impact properties in addition to high strength. An example of the high-yield-strength grades in this class is HY-80/100, which is used for naval vessels. This material combines high strength and toughness with weldability.
Ultrahigh-strength steels start with grade 4340 and are modifications of this alloy. When these steels are used for aerospace components, they are usually produced by the vacuum-arc-remelt (VAR) process. Steels commonly considered to be in the ultrahigh-strength category have yield strengths greater than 180,000 psi. They are classified into several broad categories based on chemical composition or metallurgical-hardening mechanisms.
Medium-carbon alloy steels are generally modifications of grade 4330 or 4340 (usually with increased molybdenum, silicon, and/or vanadium). These grades provide excellent hardenability in thick sections.
Modified tool steels of the 5% Cr, 1% Mo, 1% V hot-work die-steel variety (H11 modified, H13) provide the next step in increased hardenability and greater strength. Most steels in this group are air hardened in moderate to large sections and, therefore, are not likely to distort or quench crack. Structural uses of these steels are not as widespread as they once were, mainly because of the development of other steels costing about the same but offering greater fracture toughness.
Maraging steels contain 18% nickel, along with appreciable amounts of molybdenum, cobalt, and titanium, and almost no carbon. These alloys can be strengthened significantly by a precipitation reaction at a relatively low temperature. They can be formed and machined in the solution-annealed condition but not without difficulty. Weldability is excellent. They can be heat treated to 250 to 300-ksi yield strength with a simple 900°F aging treatment. Fracture toughness of the maraging steels is considerably higher than that of the conventional high-strength steels.
Maraging steels are used in a variety of high-performance applications, and most extensively in aircraft and tooling components.
The 9% Ni, 4% Co alloys were designed to provide high strength and toughness at room temperature as well as at moderately elevated temperatures -- to about 800°F. Weldability and fracture toughness are good, but the alloys are susceptible to hydrogen embrittlement. These steels are used in airframes, gears, and large aircraft parts.
Iron, nickel, and cobalt-based alloys used primarily for high-temperature applications are known as superalloys. The iron-based grades, which are less expensive than cobalt or nickel-based grades, are of three types: alloys that can be strengthened by a martensitic type of transformation, alloys that are austenitic and are strengthened by a sequence of hot and cold working (usually, forging at 2,000 to 2,100°F followed by finishing at 1,200 to 1,600°F), and austenitic alloys strengthened by precipitation hardening.
Some metallurgists consider the last group only as superalloys, the others being categorized as high-temperature, high-strength alloys. In general, the martensitic types are used at temperatures below 1,000°F; the austenitic types, above 1,000°F.
The AISI 600 series of superalloys consists of six subclasses of iron-based alloys:
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Iron-based superalloys are characterized by high temperature as well as room-temperature strength and resistance to creep, oxidation, corrosion, and wear. Wear resistance increases with carbon content. Maximum wear resistance is obtained in alloys 611, 612, and 613, which are used in high-temperature aircraft bearings and machinery parts subjected to sliding contact. Oxidation resistance increases with chromium content. The martensitic chromium steels, particularly alloy 616, are used for steam-turbine blades.
The superalloys are available in all conventional mill forms -- billet, bar, sheet, and forgings -- and special shapes are available for most alloys. In general, austenitic alloys are more difficult to machine than martensitic types, which machine best in the annealed condition. Austenitic alloys are usually "gummy" in the solution-treated condition and machine best after being partially aged or fully hardened.
The superalloys are available in all conventional mill forms -- billet, bar, sheet, and forgings -- and special shapes are available for most alloys. In general, austenitic alloys are more difficult to machine than martensitic types, which machine best in the annealed condition. Austenitic alloys are usually "gummy" in the solution-treated condition and machine best after being partially aged or fully hardened.
Crack sensitivity makes most of the martensitic steels difficult to weld by conventional methods. These alloys should be annealed or tempered prior to welding; even then, preheating and postheating are recommended. Welding drastically lowers the mechanical properties of alloys that depend on hot/cold work for strength.
All of the martensitic low-alloy steels machine satisfactorily and are readily fabricated by hot working and cold working. The martensitic secondary-hardening and chromium alloys are all hot worked by preheating and hot forging. Austenitic alloys are more difficult to forge than the martensitic grades.
However, the needs of industries have changed significantly. Removing weight from all products has risen to primary importance. Energy, tooling, and materials costs now figure prominently in parts design, and productivity has emerged as the watchword of the eighties.
With these changes have come changes in powder-metallurgy technology. Through the many manufacturing processes, improvements have been made in the powders themselves -- improvements such as lower levels of inclusions and higher compressibility. In addition to conventional iron and steel metals, the list of available powders has been expanded to include new classes of tool steel, as well as materials such as cermets and alloys of titanium, nickel, and aluminum.
Accompanying these developments has been the growth of new consolidation technologies. As a result, design engineers need current information on which P/M technologies are viable, cost effective, and production effective, and which have potentially wide application.
Although powder metallurgy is used to fabricate parts from just about any metal, the most commonly used metals are the iron-based alloys. Low-density iron P/M parts (5.6 to 6.0 gm/cm³), with a typical tensile strength of 16,000 psi, are usually used in bearing applications. Copper is commonly added to improve both strength and bearing properties. Alloy-steel powders are sometimes hot forged to high or nearly theoretical density to form parts with improved mechanical properties which, when heat treated, may have tensile strengths to 170,000 psi. Powder forging (P/F) is now established as a serious contender for parts formerly made as wrought forgings or machined from mill forms. New vendors with sophisticated automated equipment are increasing the overall P/F capacity as well as contributing to better quality of output.
Iron P/M or sintered iron-copper alloy strength can be varied by adjusting density, carbon content (up to 15%), or all three to satisfy specific design requirements. Each variable can be adjusted by the vendor to satisfy tolerances, mechanical properties, and other requirements of the part being made.
Low-density P/M parts are used in bearing applications because they provide porosity for oil storage. Impregnating sintered-metal bearings with oil usually eliminates the need for relubrication.
For higher strength needs, alloyed (frequently prealloyed Ni/Mo/Fe) iron, compacted to a higher density, is used. When carbon or other alloying elements are mixed with the iron powders and densities exceed 6.2 gm/cm³, the parts are considered to be steel rather than iron. As carbon content is increased up to 1%, the strength of steel P/M parts increases, just as the strength of wrought steel increases with higher carbon content.
Additional applications can be accommodated by sealing the pores in iron P/M parts. The sealing materials used are copper, polyesters, and anaerobics; each requires a different processing system to impregnate the parts. Impregnation of sintered P/M parts is done for any of several reasons:
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Although high precision has been achieved in P/M parts for many years, their application was once restricted because of mechanical property limitations. Now, however, mechanical properties can be increased in steel P/M parts by hot forging in closed dies. Properties of P/M parts forged to 100% theoretical density in production conditions are claimed to be equal, and sometimes superior, to those of wrought steels of similar composition.
Relatively complex carbon or low-alloy steel parts required in large quantities are ideal candidates for P/M forgings. Automakers have been among the first to use these full-dense, precision-forged components in transmissions, accessory mechanisms, and engines.
Adding carbon enables steel P/M parts to be heat treated to increase hardness, toughness, wear resistance, and strength. The addition of alloying elements in the iron-powder mix further enhances the properties of heat-treated steel P/M parts.
Ferrous P/M parts containing 0.3% or higher combined carbon can be quench hardened for increased strength and wear resistance. Surface hardness values of 500 to 650 Knoop, which are file hard, can be obtained by quenching.
In addition, ferrous parts can be carburized by standard means other than liquid salts. Low-density parts carburize throughout while high-density parts develop a distinct carburized case. Very-high-density parts respond favorably to fused salt carbonitriding, but density must be high enough to prevent salt absorption into the pore structure. Tempering for stress relief after quenching is also possible, although oil vapors generated by quench oil in the pore structure of P/M parts must be vented or dispersed.
Sources:
Copyright 1995, 1996 Machine Design Magazine
a Penton Publication
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