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A designation is the specific identification of each grade, type, or class of steel by a number, letter, symbol, name, or suitable combination. Unique to a particular steel grade, type and class are terms used to classify steel products. Within the steel industry, they have very specific uses: grade is used to denote chemical composition; type is used to indicate deoxidation practice; and class is used to describe some other attribute, such as strength level or surface smoothness.

In ASTM specifications, however, these terms are used somewhat interchangeably. In ASTM A 533, for example, type denotes chemical composition, while class indicates strength level. In ASTM A 515, grade identifies strength level; the maximum carbon content permitted by this specification depends on both plate thickness and strength level. In ASTM A 302 grade denotes requirements for both chemical composition and mechanical properties. ASTM A 514 and A 5117 are specifications for high-strength quenched and tempered plate for structural and pressure vessel applications, respectively, each contains several compositions that can provide the required mechanical properties. However, A 514 type A has the identical composition limits as A 517 grade.

Chemical composition is by far the most widely used basis for classification and/or designation of steels. The most commonly used system of designation in the United States is that of the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI). The Unified Numbering System (UNS) is also being used with increasing frequency.

SAE-AISI Designations
As stated above, the most widely used system for designating carbon and alloy steels is the SAE-AISI system. As a point of technicality, there are two separate systems, but they are nearly identical and have been carefully coordinated by the two groups. It should be noted, however, that AISI has discontinued the practice of designating steels.

The SAE-AISI system is applied to semi-finished forgings, hot-rolled and cold-finished bars, wire rod and seamless tubular goods, structural shapes, plates, sheet, strip, and welded tubing.

Carbon steels contain less than 1.65% Mn, 0.60% Si, and 0.60% Cu; they comprise the lxxx groups in the SAE-AISI system and are subdivided into four distinct series as a result of the difference in certain fundamental properties among them.

Designations for merchant quality steels include the prefix M. A carbon steel designation with the letter B inserted between the second and third digits indicates the steel contains 0.0005 to 0.003% B. Likewise, the letter L inserted between the second and third digits indicates that the steel contains 0.15 to 0.35% Pb for enhanced machinability. Resulfurized carbon steels of the 11xx group and resulfurized and rephosphorized carbon steels of the 12xx group are produced for applications requiring good machinability. Steels that having nominal manganese contents of between 0.9 and 1.5% but no other alloying additions now have 15xx designations in place of the 10xx designations formerly used.

Alloy steels contain manganese, silicon, or copper in quantities greater than those listed for the carbon steels, or they have specified ranges or minimums for one or more of the other alloying elements. In the AISI-SAE system of designations, the major alloying elements are indicated by the first two digits of the designation. The amount of carbon, in hundredths of a percent, is indicated by the last two (or three) digits.

For alloy steels that have specific hardenability requirements, the suffix H is used to distinguish these steels from corresponding grades that have no hardenability requirement. As with carbon steels, the letter B inserted between the second and third digits indicates that the steel contains boron. The prefix E signifies that the steel was produced by the electric furnace process.

HSLA Steels. Several grades of HSLA steel are described in SAE Recommended Practice J410. These steels have been developed as a compromise between the convenient fabrication characteristics and low cost of plain carbon steels and the high strength of heat-treated alloy steels. These steels have excellent strength and ductility as-rolled.

UNS Designations The Unified Numbering System (UNS) has been developed by ASTM and SAE and several other technical societies, trade associations, and United States government agencies.

A UNS number, which is a designation of chemical composition and not a specification, is assigned to each chemical composition of a metallic alloy. The UNS designation of an alloy consists of a letter and five numerals. The letters indicate the broad class of alloys; the numerals define specific alloys within that class. Existing designation system, such as the AISI-SAE system for steels, have been incorporated into UNS designations. UNS is described in greater detail in SAE J1086 and ASTM E 527.
AMS Designation
Aerospace Materials Specifications (AMS), published by SAE, are complete specifications that are generally adequate for procurement purposes. Most of the AMS designations pertain to materials intended for aerospace applications; the specifications may include mechanical property requirements significantly more severe than those for grades of steel having similar compositions but intended for other applications. Processing requirements, such as for consumable electrode remelting, are common in AMS steels.

ASTM (ASME) Specifications The most widely used standard specifications for steel products in the United States are those published by ASTM. These are complete specifications, generally adequate for procurement purposes. Many ASTM specifications apply to specific products, such as A 574 for alloy steel socket head cap screws. These specifications are generally oriented toward performance of the fabricated end product, with considerable latitude in chemical composition of the steel used to make the end product.

ASTM specifications represent a consensus among producers, specifiers, fabricators, and users of steel mill products. In many cases, the dimensions, tolerances, limits, and restrictions in the ASTM specifications are similar to or the same as the corresponding items of the standard practices in the AISI Steel Products Manuals.

Many of the ASTM specifications have been adopted by the American Society of Mechanical Engineers (ASME) with little or no modification; ASME uses the prefix S and the ASTM designation for these specifications. For example, ASME-SA213 and ASTM A 213 are identical.

Steel products can be identified by the number of the ASTM specification to which they are made. The number consists of the letter A (for ferrous materials) and an arbitrary, serially assigned number. Citing the specification number, however, is not always adequate to completely describe a steel product. For example, A 434 is the specification for heat-treated (hardened and tempered) alloy steel bars. To completely describe steel bars indicated by this specification, the grade (SAE-AISI designation in this case) and class (required strength level) must also be indicated. The ASTM specification A 434 also incorporates, by reference, two standards for test methods (A 370 for mechanical testing and E 112 for grain size determination) and A 29, which specifies the general requirements for bar products.

SAE-AISI designations for the compositions of carbon and alloy steels are sometimes incorporated into the ASTM specifications for bars, wires, and billets for forging. Some ASTM specifications for sheet products include SAE-AISI designations for composition. The ASTM specifications for plates and structural shapes generally specify the limits and ranges of chemical composition directly, without the SAE.AISI designations.

General Specifications. Several ASTM specifications, such as A 20 covering steel plate used for pressure vessels, contain the general requirements common to each member of a broad family of steel products. These general specifications are often supplemented by additional specifications describing a different mill form or intermediate fabricated product.

European and Japanese Designation Systems
Below some basics of European and Japanese designation systems are explained. Please refer to articles about corresponding national and international standards for more details.

DIN standards are developed by Deutsches Institut fur Normung in the Federal Republic of Germany. All West German steel specifications are preceded by the uppercase letters DIN followed an alphanumeric or numeric code. The latter method, known as the Werkstoff number, uses numbers only with a decimal point after the first digit.

JIS standards are developed by the Japanese Industrial Standards Committee, which is part of the Ministry of International Trade and Industry in Tokyo. The JIS steel specifications begin with the uppercase letters JIS and are followed by an uppercase letter (G in the case of carbon and low-alloy steels) designating the division (product form) of the standard. This letter is followed by a series of numbers and letters that indicate the specific steel.

British standards (BS) are developed by the British Standards Institute in London, England. Similar to the JIS standards, each British designation includes a product form and an alloy code.

AFNOR standards are developed by the Association Francaise de Normalisation in Paris, France. The correct format for reporting AFNOR standards is as follows. An uppercase NF is placed to the left of the alphanumeric code. This code consists of an uppercase letter followed by a series of digits, which are subsequently followed by an alphanumeric sequence.

UNI standards are developed by the Ente Nazionale Italiano di Unificazione in Milan, Italy. Italian standards are preceded by the uppercase letter UNI followed by a four-digit product form code subsequently followed by an alphanumeric alloy identification.

Swedish standards (SS) are prepared by the Swedish Standards Institution in Stockholm. Designations begin with the letters SS followed by the number 14 (all Swedish carbon and low-alloy steels are covered by SS14). What subsequently follows is a four digit numerical sequence similar to the German Werkstoff number.

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The study of steels is important because steels represent by far the most widely used metallic materials, primarily due to the fact that they can be manufactured relatively cheaply in large quantities to very precise specifications. They also provide an extensive range of mechanical properties from moderate strength levels (200-300MPa) with excellent ductility and toughness, to very high strengths (2000 MPa) with adequate ductility. It is, therefore, not surprising that irons and steels comprise well over 80% by weight of the alloys in general industrial use.

Steels form perhaps the most complex group of alloys in common use. Therefore, in studying them it is useful to consider the behavior of pure iron first, then iron-carbon alloys, and finally examine the many complexities which arise when further alloying additions are made.

Pure iron is not an easy material to produce. However, it has recently been made with a total impurity content not exceeding 60 ppm (parts per million), of which 10 ppm is accounted for by non-metallic impurities such as carbon, oxygen, sulphur, phosphorus, while 50 ppm represents the metallic impurities. Iron of this purity is extremely weak: the resolved shear stress of a single crystal at room temperature can be as low as 10 MPa, while the yield stress of a polycrystalline sample at the same temperature can be well below 150 MPa.

The phase transformation: α- and γ- iron
Pure iron exists in two crystal forms, one body-centred cubic (bcc) (α-iron, ferrite) which remains stable from low temperatures up to 910°C (the A3 point), when it transforms to a face-centred cubic (fcc) form (γ-iron, austenite). The γ-iron on remains stable until 1390°C, the A4 point, when it reverts to bcc form, (now δ-iron) which remains stable up to the melting point of 1536°C.

The detailed geometry of unit cells of α- and γ-iron crystals is particularly relevant to, for example, the solubility in the two phases of non-metallic elements such as carbon and nitrogen, the diffusivity of alloying elements at elevated temperatures, and the general behavior on plastic deformation.

The bcc structure of α-iron is more loosely packed than that of fcc γ-iron. The largest cavities in the bcc structure are the tetrahedral holes existing between two edge and two central atoms in the structure, which together form a tetrahedron.

It is interesting that the fcc structure, although more closely-packed, has larger holes than the bcc-structure. These holes are at the centers of the cube edges, and are surrounded by six atoms in the form of an octagon, so they are referred to as octahedral holes.

The α↔γ transformation in pure iron occurs very rapidly, so it is impossible to retain the high-temperature fcc form at room temperature. Rapid quenching can substantially alter the morphology of the resulting α-iron, but it still retains its bcc structure.

Carbon and nitrogen in solution in α- and γ- iron
The addition of carbon to iron is sufficient to form a steel. However, steel is a generic term which covers a very large range of complex compositions. The presence of even a small concentration of carbon, e.g. 0.1-0.2 weight per cent (wt%); approximately 0.5-1.0 atomic per cent, has a great strengthening effect on iron, a fact known to smiths over 2500 years ago since iron heated in a charcoal fire can readily absorb carbon by solid state diffusion. However, the detailed processes by which the absorption of carbon into iron converts a relatively soft metal into a very strong and often tough alloy have only recently been fully explored.

The atomic sizes of carbon and nitrogen are sufficiently small relative to that of iron to allow these elements to enter the α- iron and &gamma- iron lattices as interstitial solute atoms. In contrast, the metallic alloying elements such as manganese, nickel and chromium have much larger atoms, i.e. nearer in size to those of iron, and consequently they enter into substitutional solid solution.

However, comparison of the atomic sizes of C and N with the sizes of the available interstices makes it clear that some lattice distortion must take place when these atoms enter the iron lattice. Indeed, it is found that C and N in α-iron occupy not the larger tetrahedral holes, but the octahedral interstices which are more favorably placed for the relief of strain, which occurs by movement of two nearest neighbor iron atoms. In the case of tetrahedral interstices, four iron atoms are of nearest-neighbor status and the displacement of these would require more strain energy. Consequently these interstices are not preferred sites for carbon and nitrogen atoms.

The solubility of both C and N in austenite should be greater than in ferrite, because of the larger interstices available. It is, therefore, reasonable to expect that during simple heat treatments, excess carbon and nitrogen will be precipitated. This could happen in heat treatments involving quenching from the γ state, or even after treatments entirely within the α field, where the solubility of C varies by nearly three orders of magnitude between 720°C and 20°C.

Precipitation of carbon and nitrogen from α-iron. α-iron containing about 0.02 wt % C is substantially supersaturated with carbon if, after being held at 700°C, it is quenched to room temperature. This supersaturated solid solution is not stable, even at room temperature, because of the ease with which carbon can diffuse in α-iron. Consequently, in the range 20-300°C, carbon is precipitated as iron carbide. This process has been followed by measurement of changes in physical properties such as electrical resistivity, internal friction, and by direct observation or the structural changes in the electron microscope.

The process of ageing is a two-stage one. The first stage takes place at temperatures up to 200°C and involves the formation or a transitional iron carbide phase (ε) with a close-packed hexagonal structure which is often difficult to identify, although its morphology and crystallography have been established. It forms as platelets on {100}α planes, apparently homogenously in the α-iron matrix, but at higher ageing temperatures (150-200°C) nucleation occurs preferentially on dislocations. The composition is between Fe2.4C and Fe3C.

Ageing at 200°C and above leads to the second stage of ageing in which orthorhombic cementite Fe3C is formed as platelets on {110}α. Often the platelets grow on several {110} planes from a common centre giving rise to structures which appear dendritic in character. The transition from ε-iron carbide to cementite is difficult to study, but it appears to occur by nucleation of cementite at the ε-carbide/α interlaces, followed by re-solution of the metastable ε-carbide precipitate.

The maximum solubility of nitrogen in ferrite is 0.10 wt %, so a greater volume fraction of nitride precipitate can be obtained. The process is again two-stage with a be tetragonal α” phase, Fe16N2, as the intermediate precipitate, forming as discs on {100}α, matrix planes both homogeneously and on dislocations. Above about 200°C, this transitional nitride is replaced by the ordered fcc γ’, Fe4N.

The ageing of α-iron quenched from a high temperature in the α-range is usually referred to as quench ageing, and there is substantial evidence to show that the process can cause considerable strengthening, even in relatively pure iron. In commercial low carbon steels, nitrogen is usually combined with aluminium, or present in too low concentration to make a substantial contribution to quench ageing, with the result that the major effect is due to carbon. This behavior should be compared with that of strain ageing.

Some practical aspects. The very rapid diffusivity of carbon and nitrogen in iron compared with that of the metallic alloying elements is exploited in the processes of carburizing and nitriding.

Carburizing can be carried out by heating a low carbon steel in contact with carbon to the austenitic range, e.g. 1000°C, where the carbon solubility, c1, is substantial. The result is a carbon gradient in the steel, from c1 at the surface in contact with the carbon, to c at a depth.

The diffusion coefficient D of carbon in iron actually varies with carbon content, so the above relationship is not rigorously obeyed. Carburizing, whether carried out using carbon, or more efficiently using a carburizing gas (gas carburizing), provides a high carbon surface on a steel, which, after appropriate heat treatment, is strong and wear resistant.

Nitriding is normally carried out in an atmosphere of ammonia, but at a lower temperature (500-550°C) than carburizing, consequently the reaction occurs in the ferrite phase, in which nitrogen has a substantially higher solubility than carbon.

Nitriding steels usually contain chromium (≈1%), aluminum (≈1%), vanadium or molybdenum (≈0.2%), which are nitride-forming elements, and which contribute to the very great hardness of the surface layer produced.

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A study of the constitution and structure of all steels and irons must first start with the iron-carbon equilibrium diagram. Many of the basic features of this system (Fig. 1) influence the behavior of even the most complex alloy steels. For example, the phases found in the simple binary Fe-C system persist in complex steels, but it is necessary to examine the effects alloying elements have on the formation and properties of these phases. The iron-carbon diagram provides a valuable foundation on which to build knowledge of both plain carbon and alloy steels in their immense variety.

Fig. 1. The iron-carbon diagram.

It should first be pointed out that the normal equilibrium diagram really represents the metastable equilibrium between iron and iron carbide (cementite). Cementite is metastable, and the true equilibrium should be between iron and graphite. Although graphite occurs extensively in cast irons (2-4 wt % C), it is usually difficult to obtain this equilibrium phase in steels (0.03-1.5 wt %C). Therefore, the metastable equilibrium between iron and iron carbide should be considered, because it is relevant to the behavior of most steels in practice.

The much larger phase field of γ-iron (austenite) compared with that of α-iron (ferrite) reflects the much greater solubility of carbon in γ-iron, with a maximum value of just over 2 wt % at 1147°C (E, Fig.1). This high solubility of carbon in γ-iron is of extreme importance in heat treatment, when solution treatment in the γ-region followed by rapid quenching to room temperature allows a supersaturated solid solution of carbon in iron to be formed.

The α-iron phase field is severely restricted, with a maximum carbon solubility of 0.02 wt% at 723°C (P), so over the carbon range encountered in steels from 0.05 to 1.5 wt%, α-iron is normally associated with iron carbide in one form or another. Similarly, the δ-phase field is very restricted between 1390 and 1534°C and disappears completely when the carbon content reaches 0.5 wt% (B).

There are several temperatures or critical points in the diagram, which are important, both from the basic and from the practical point of view.

* Firstly, there is the A1, temperature at which the eutectoid reaction occurs (P-S-K), which is 723°C in the binary diagram.
* Secondly, there is the A3, temperature when α-iron transforms to γ-iron. For pure iron this occurs at 910°C, but the transformation temperature is progressively lowered along the line GS by the addition of carbon.
* The third point is A4 at which γ-iron transforms to δ-iron, 1390°C in pure iron, hut this is raised as carbon is added. The A2, point is the Curie point when iron changes from the ferro- to the paramagnetic condition. This temperature is 769°C for pure iron, but no change in crystal structure is involved. The A1, A3 and A4 points are easily detected by thermal analysis or dilatometry during cooling or heating cycles, and some hysteresis is observed. Consequently, three values for each point can be obtained. Ac for heating, Ar for cooling and Ae (equilibrium}, but it should be emphasized that the Ac and Ar values will be sensitive to the rates of heating and cooling, as well as to the presence of alloying elements.

The great difference in carbon solubility between γ- and α-iron leads normally to the rejection of carbon as iron carbide at the boundaries of the γ phase field. The transformation of γ to α - iron occurs via a eutectoid reaction, which plays a dominant role in heat treatment.

The eutectoid temperature is 723°C while the eutectoid composition is 0.80% C(s). On cooling alloys containing less than 0,80% C slowly, hypo-eutectoid ferrite is formed from austenite in the range 910-723°C with enrichment of the residual austenite in carbon, until at 723°C the remaining austenite, now containing 0.8% carbon transforms to pearlite, a lamellar mixture of ferrite and iron carbide (cementite). In austenite with 0,80 to 2,06% carbon, on cooling slowly in the temperature interval 1147°C to 723°C, cementite first forms progressively depleting the austenite in carbon, until at 723°C, the austenite contains 0.8% carbon and transforms to pearlite.

Steels with less than about 0.8% carbon are thus hypo-eutectoid alloys with ferrite and pearlite as the prime constituents, the relative volume fractions being determined by the lever rule which states that as the carbon content is increased, the volume percentage of pearlite increases, until it is 100% at the eutectoid composition. Above 0.8% C, cementite becomes the hyper-eutectoid phase, and a similar variation in volume fraction of cementite and pearlite occurs on this side of the eutectoid composition.

The three phases, ferrite, cementite and pearlite are thus the principle constituents of the infrastructure of plain carbon steels, provided they have been subjected to relatively slow cooling rates to avoid the formation of metastable phases.

The austenite- ferrite transformation
Under equilibrium conditions, pro-eutectoid ferrite will form in iron-carbon alloys containing up to 0.8 % carbon. The reaction occurs at 910°C in pure iron, but takes place between 910°C and 723°C in iron-carbon alloys.

However, by quenching from the austenitic state to temperatures below the eutectoid temperature Ae1, ferrite can be formed down to temperatures as low as 600°C. There are pronounced morphological changes as the transformation temperature is lowered, which it should be emphasized apply in general to hypo-and hyper-eutectoid phases, although in each case there will be variations due to the precise crystallography of the phases involved. For example, the same principles apply to the formation of cementite from austenite, but it is not difficult to distinguish ferrite from cementite morphologically.

The austenite-cementite transformation
The Dube classification applies equally well to the various morphologies of cementite formed at progressively lower transformation temperatures. The initial development of grain boundary allotriomorphs is very similar to that of ferrite, and the growth of side plates or Widmanstaten cementite follows the same pattern. The cementite plates are more rigorously crystallographic in form, despite the fact that the orientation relationship with austenite is a more complex one.

As in the case of ferrite, most of the side plates originate from grain boundary allotriomorphs, but in the cementite reaction more side plates nucleate at twin boundaries in austenite.

The austenite-pearlite reaction
Pearlite is probably the most familiar micro structural feature in the whole science of metallography. It was discovered by Sorby over 100 years ago, who correctly assumed it to be a lamellar mixture of iron and iron carbide.

Pearlite is a very common constituent of a wide variety of steels, where it provides a substantial contribution to strength. Lamellar eutectoid structures of this type are widespread in metallurgy, and frequently pearlite is used as a generic term to describe them.

These structures have much in common with the cellular precipitation reactions. Both types of reaction occur by nucleation and growth, and are, therefore, diffusion controlled. Pearlite nuclei occur on austenite grain boundaries, but it is clear that they can also be associated with both pro-eutectoid ferrite and cementite. In commercial steels, pearlite nodules can nucleate on inclusions.

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The simplest version of analyzes the effects of alloying elements on iron-carbon alloys would require analysis of a large number of ternary alloy diagrams over a wide temperature range. However, Wever pointed out that iron binary equilibrium systems fall into four main categories (Fig. 1): open and closed γ-field systems, and expanded and contracted γ-field systems. This approach indicates that alloying elements can influence the equilibrium diagram in two ways:

* by expanding the γ-field, and encouraging the formation of austenite over wider compositional limits. These elements are called γ-stabilizers.
* by contracting the γ-field, and encouraging the formation of ferrite over wider compositional limits. These elements are called α-stabilizers.

The form of the diagram depends to some degree on the electronic structure of the alloying elements which is reflected in their relative positions in the periodic classification.

Figure 1. Classification of iron alloy phase diagrams: a. open γ-field; b. expanded γ-field; c. closed γ-field
(Wever, Archiv, Eisenhüttenwesen, 1928-9, 2, 193)

Class 1: open γ-field. To this group belong the important steel alloying elements nickel and manganese, as well as cobalt and the inert metals ruthenium, rhodium, palladium, osmium, iridium and platinum. Both nickel and manganese, if added in sufficiently high concentration, completely eliminate the bcc α-iron phase and replace it, down to room temperature, with the γ-phase. So nickel and manganese depress the phase transformation from γ to α to lower temperatures (Fig. 1a), i.e. both Ac1 and Ac3 are lowered. It is also easier to obtain metastable austenite by quenching from the γ-region to room temperature, consequently nickel and manganese are useful elements in the formulation of austenitic steels.

Class 2: expanded γ-field. Carbon and nitrogen are the most important elements in this group. The γ-phase field is expanded, but its range of existence is cut short by compound formation (Fig.1b). Copper, zinc and gold have a similar influence. The expansion of the γ-field by carbon, and nitrogen, underlies the whole of the heat treatment of steels, by allowing formation of a homogeneous solid solution (austenite) containing up to 2.0 wt % of carbon or 2.8 wt % of nitrogen.

Class 3: closed γ-field. Many elements restrict the formation of γ-iron, causing the γ-area of the diagram to contract to a small area referred to as the gamma loop (Fig. 1c). This means that the relevant elements are encouraging the formation of bcc iron (ferrite), and one result is that the δ- and γ-phase fields become continuous. Alloys in which this has taken place are, therefore, not amenable to the normal heat treatments involving cooling through the γ/α-phase transformation. Silicon, aluminium, beryllium and phosphorus fall into this category, together with the strong carbide forming elements, titanium, vanadium, molybdenum and chromium.

Class 4: contracted y-field. Boron is the most significant element of this group, together with the carbide forming elements tantalum, niobium and zirconium. The γ-loop is strongly contracted, but is accompanied by compound formation (Fig. 1d).

The distribution of alloying elements in steels. Although only binary systems have been considered so far, when carbon is included to make ternary systems the same general principles usually apply. For a fixed carbon content, as the alloying clement is added the y-field is either expanded or contracted depending on the particular solute.

With an element such as silicon the γ-field is restricted and there is a corresponding enlargement of the α-field. If vanadium is added, the γ-field is contracted and there will be vanadium carbide in equilibrium with ferrite over much of the ferrite field. Nickel does not form a carbide and expands the γ-field. Normally elements with opposing tendencies will cancel each other out at the appropriate combinations, but in some cases anomalies occur. For example, chromium added to nickel in a steel in concentrations around 18% helps to stabilize the γ-phase, as shown by 18Cr8Ni austenitic steels.

One convenient way of illustrating quantitatively the effect of an alloying element on the γ-phase field of the Fe-C system is to project on to the Fe-C plane of the ternary system the γ-phase field boundaries for increasing concentration of a particular alloying element. For more precise and extensive information, it is necessary to consider series of isothermal sections in true ternary systems Fe-C-X, but even in some of the more familiar systems the full information is not available, partly because the acquisition of accurate data can be a difficult and very time-consuming process.

Recently the introduction of computer-based methods has permitted the synthesis of extensive thermochemical and phase equilibria data, and its presentation in the form, for example, of isothermal sections over a wide range of temperatures.

If only steels in which the austenite transforms to ferrite and carbide on slow cooling are considered, the alloying elements can be divided into three categories:

* elements which enter only the ferrite phase
* elements which form stable carbides and also enter the ferrite phase
* elements which enter only the carbide phase.

In the first category there are elements such as nickel, copper, phosphorus and silicon which, in transformable steels, are normally found in solid solution in the ferrite phase, their solubility in cementite or in alloy carbides being quite low.

The majority of alloying elements used in steels fall into the second category, in so far as they are carbide formers and as such, at low concentrations, go into solid solution in cementite, but will also form solid solutions in ferrite. At higher concentrations most will form alloy carbides, which are thermodynamically more stable than cementite.

Typical examples are manganese, chromium, molybdenum, vanadium, titanium, tungsten and niobium. Manganese carbide is not found in steels, but instead manganese enters readily into solid solution in Fe3C. The carbide-forming elements are usually present greatly in excess of the amounts needed in the carbide phase, which are determined primarily by the carbon content of the steel. The remainder enters into solid solution in the ferrite with the non-carbide forming elements nickel and silicon. Some of these elements, notably titanium, tungsten, and molybdenum, produce substantial solid solution hardening of ferrite.

In the third category there are a few elements which enter predominantly the carbide phase. Nitrogen is the most important element and it forms carbo-nitrides with iron and many alloying elements. However, in the presence of certain very strong nitride forming elements, e.g. titanium and aluminum, separate alloy nitride phases can occur.

While ternary phase diagrams, Fe-C-X, can be particularly helpful in understanding the phases which can exist in simple steels, isothermal sections for a number of temperatures are needed before an adequate picture of the equilibrium phases can be built up. For more complex steels the task is formidable and equilibrium diagrams can only give a rough guide to the structures likely to be encountered. It is, however, possible to construct pseudobinary diagrams for groups of steels, which give an overall view of the equilibrium phases likely to be encountered at a particular temperature.

Structural changes resulting from alloying additions. The addition to iron-carbon alloys of elements such as nickel, silicon, manganese, which do not form carbides in competition with cementite, does not basically alter the microstructures formed after transformation. However, in the case of strong carbide-forming elements such as molybdenum, chromium and tungsten, cementite will be replaced by the appropriate alloy carbides, often at relatively low alloying element concentrations. Still stronger carbide forming elements such as niobium, titanium and vanadium are capable of forming alloy carbides, preferentially at alloying concentrations less than 0.1 wt%.

It would, therefore, be expected that the microstructures of steels containing these elements would be radically altered. It has been shown how the difference in solubility of carbon in austenite and ferrite leads to the familiar ferrite/cementite aggregates in plain carbon steels. This means that, because the solubility of cementite in austenite is much greater than in ferrite, it is possible to redistribute the cementite by holding the steel in the austenite region to take it into solution, and then allowing transformation to take place to ferrite and cementite. Examining the possible alloy carbides, and nitrides, in the same way, shows that all the familiar ones are much less soluble in austenite than is cementite.

Chromium and molybdenum carbides are not included, but they are substantially more soluble in austenite than the other carbides. Detailed consideration of such data, together with practical knowledge of alloy steel behavior, indicates that, for niobium and titanium, concentrations of greater than about 0.25 wt % will form excess alloy carbides which cannot be dissolved in austenite at the highest solution temperatures. With vanadium the limit is higher at 1-2%, and with molybdenum up to about 5%. Chromium has a much higher limit before complete solution of chromium carbide in austenite becomes difficult. This argument assumes that sufficient carbon is present in the steel to combine with the alloying element. If not, the excess metallic element will go into solid solution both in the austenite and the ferrite.

In general, the fibrous morphology represents a closer approach to an equilibrium structure so it is more predominant in steels which have transformed slowly. In contrast, the interphase precipitation and dislocation nucleated structures occur more readily in rapidly transforming steels, where there is a high driving force, for example, in microalloyed steels.

The clearest analogy with pearlite is found when the alloy carbide in lath morphology forms nodules in association with ferrite. These pearlitic nodules are often encountered at temperatures just below Ac1, in steels which transform relatively slowly.

For example, these structures are obtained in chromium steels with between 4% and 12% chromium and the crystallography is analogous to that of cementitic pearlite. It is, however, different in detail because of the different crystal structures of the possible carbides. The structures observed are relatively coarse, but finer than pearlite formed under equivalent conditions, because of the need for the partition of the alloying element, e.g. chromium between the carbide and the ferrite. To achieve this, the interlamellar spacing must be substantially finer than in the equivalent iron-carbon case.

Interphase precipitation. Interphase precipitation has been shown to nucleate periodically at the γ/α interface during the transformation. The precipitate particles form in bands which are closely parallel to the interface, and which follow the general direction of the interface even when it changes direction sharply. A further characteristic is the frequent development of only one of the possible Widmanstätten variants, for example VC plates in a particular region are all only of one variant of the habit, i.e. that in which the plates are most nearly parallel to the interface.

The extremely fine scale of this phenomenon in vanadium steels, which also occurs in Ti and Nb steels, is due to the rapid rate at which the γ/α transformation takes place. At the higher transformation temperatures, the slower rate of reaction leads to coarser structures. Similarly, if the reaction is slowed down by addition of further alloying elements, e.g. Ni and Mn, the precipitate dispersion coarsens.

The scale of the dispersion also varies from steel to steel, being coarsest in chromium, tungsten and molybdenum steels where the reaction is relatively slow, and much finer in steels in which vanadium, niobium and titanium are the dominant alloying elements and the transformation is rapid.

Transformation diagrams for alloy steels. The transformation of austenite below the eutectoid temperature can best be presented in an isothermal transformation diagram, in which the beginning and end of transformation is plotted as a function of temperature and time. Such curves are known as time-temperature-transformation, or TTT curves, and form one of the important sources of quantitative information for the heat treatment of steels.

In the simple case of a eutectoid plain carbon steel, the curve is roughly C-shaped with the pearlite reaction occurring down to the nose of the curve and a little beyond. At lower temperatures bainite and martensite are formed. The diagrams become more complex for hypo- and hyper-eutectoid alloys as the ferrite or cementite reactions have also to be represented by additional lines.

http://www.key-to-steel.com/default.aspx?ID=CheckArticle&NM=151

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VOCs or Volatile Organic Compounds are organic chemical compounds that have become vaporised and enter into the atmosphere. Carbon-based molecules like (aldehydes) which are any of a class of highly reactive chemical compounds; used in making resins, dyes, organic acids, and (ketones) which are also any of a class of organic compounds having a carbonyl group linked to a carbon atom in each of two hydrocarbon radicals and other light hydrocarbons are VOCs.

VOCs are sometimes accidentally released into our environment damaging soil and groundwater. Escaping vapours of VOCs into the air contribute to air pollution. VOCs are an important outdoor air pollutant and turns into methane which is an extremely effective greenhouse gas which contributes to enhanced global warming.

There are systems that deal with this type of problem; a VOC Abatement System relies upon the concept of a chemical reaction, and when involving organic hydrocarbons the process is called oxidation. Within this oxidation process, the compounds within the air stream, theses VOCs or Volatile Organic Compounds are broken down from their original composition and reformed into new compounds, and when enough heat and oxygen are added to the hydrocarbons to create the oxidation reaction, this process is called Thermal Oxidation. By breaking the original composition of the VOC Hydrocarbons, carbon and hydrogen, can be regenerate naturally into carbon dioxide and water vapor releasing heat energy. The heat energy is then recouped into the system by use of a heat exchanger, (a device built for efficient heat transfer from one medium to another) while the now clean air stream of carbon dioxide and water is discharged in to the atmosphere.

Needing to find the right systems to answer theses types of environmental issues are not uncommon to the industrial marketplace today. Those within the industry are on the cutting edge of designing systems such as Thermal Oxidizers to help meet this global problem head on. Rest assured that with the new technology in this area of Thermal Oxidizer manufacturing, there are those with an eye finding solutions to this issue.

Derek Lang is with Epcon Industrial, a manufacturer of air pollution control systems, thermal oxidizers, and industrial gas fired ovens. Learn more at http://www.epconlp.com

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Is there really a need for flame resistant work clothes? Or is it all just employer and OSHA hype. Let me help you decide. Flame resistance can mean the difference between survivable burns and major injuries.

Everyday, in a flash or just an electric spark can create a life-threading situation. Are you wearing enough protection? “What is flame resistant material?” you ask.

Protective clothing could be either native flame resistant or treated. Allow me to explain. Native flame resistant work wear can include Nomex III, Nomex IIIA, Kevlar or Firewear.

· Nomex IIIA: An antistatic fiber blend manufactured by Dupont, protects workers from flames and reduces the risk associated with static electricity.

· Kevlar: A process of weaving a liquid into a solid (clear as mud, huh!) It is a process called aramid weave. This weave proposes to be difficult to corrode, resist heat, and has no melting point. However, aramid fiber may be slightly corrosive if exposed to a chemical product known as chlorine.

Flame resistant treated clothing does not lose protection when following manufacturers instructions. Flame Resistant II and Proban are processes used on cotton and cotton blends. Also, treated work clothes is less costly than flame resistant textiles

· Proban & Flamex II: A cotton and cotton-based fabric.

Flame resistance is accomplished with a crossed linked inert (non reacting) polymer in the cotton fabric. Fabrics form an insulating char, when in contact with a fire/flame. They do not smolder, no melting, no afterglow, and flame doesn’t spread outside the charred area.

Yes! You do need flame resistant PPE, personal protective equipment. Untreated clothing when exposed to a combustible flame, electrical arc will continue to burn, even after the source has been extinguished. However, flame resistant work clothes resist the ignition and will self extinguish when the source is removed. Does your skill or craft fall into one of these categories? If so, then you need PPE.

· Work close to an open flame

· Soldering, heating, cutting & burning

· Working with pyrophoric materials

· Oxygen from a compressed gas cylinder

· Working with electricity

If you do not wear PPE, you might as well go through a fire naked.

Take no chances when working around open flames or electrical arc.

Visit http://www.sweetdealpickles.com for a complete line of FR work clothes

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When things go wrong using a Mig Welding machine, its not 100 things going wrong. It’s always 1 or 2 things causing your Mig welding problems… and it’s almost always only one thing at a time.

The absolute #1 mig welding problem is a bad ground. Let me tell you a secret: your Mig welding machine is a cold hearted SOB! It does not care if you have a bad ground. It just keeps pumping out welding wire anyway. If you have ever pulled the trigger on a mig gun and it went Pop! Pop! Pop! and sounded like a drive by shooting, you probably had a bad ground. But take heart! It’s not all your fault and it’s an easy fix.

Mig welding machine manufacturers are in hot competition these days and just seem to cut corners on their ground clamps by not making them out of the best materials. Have you seen them? I mean please! They are so wimpy. But you can easily remedy the problem without even buying a new and better ground clamp.

Just get yourself some old welding lead or ground cable and cut off about two feet of cable. Then Strip off the insulation and tape the ends with electrical tape. Fold it in half and put the ends in a vise. Stick a screwdriver in the loop and twist it pretty tight. (When you are done it will look kind of like a French twist pastry.) Now you have a cool tool that gives you a constant ground. Why?

· Because it’s copper, the best conductor of electricity.

· Its braided wire and has more contact areas than a rigid ground clamp.

When you clamp this thing to what you are welding using your old ground clamp, you have literally hundreds of little contact points that will never lose the ground…not even for a second. This is critical when mig welding because as I already mentioned, your mig machine does not know or care if you lose the ground for a second. It just keeps feeding wire. That is the number one cause for poor starts. Try it! This makes a much better mig welding ground clamp than what comes on most machines.

For more information on this topic and other welding information, visit http://www.weldingtipsandtricks.com/mig-welding-ground.html

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Steel Tip Paint Markers

Steel tip paint markers are available in three different styles: small plastic bottles such as the Ball Point Paint Bottle from Markal Corporation, collapsible tube steel tip markers from Markal and Nissen Company, and the Texpen or Dalo markers available from ITW Dykem. Steel tip markers are ideal for writing on rough metal surfaces such as metal and rusted metal. Paint application is controlled by depressing the steel tip similar to a ball point pen.

Ball Point Paint Bottles

Ball Point Paint Bottles from Markal are work well on vertical applications. The tube must be squeezed and the tip depressed at the same time for paint to flow. The stainless steel tips are durable, the paint is very permanent, and the plastic bottles are very durable and ideal for storing in tool boxes. Paints come in seven colors and one tip size.

Collapsible Tube Markers

Collapsible aluminum tube markers are available from Markal and Nissen companies. Collapsible tubes are available in three different tip diameters and in the case of Makal, low chloride. Tubes must be squeezed and the tips depressed simultaneously to apply paint. The tubes are a little inconvenient to use and you tend to leave a lot of unused paint in the tube.

Texpen and Dalo Style Markers

The Texpen and Dalo style markers employ an aluminum barrel shaped like a marking pen, steel tips in fine, medium, or broad points, and a pump that allows users to pressurize the barrel. Pressurizing the barrel allows user to apply a constant flow of paint in any position-even overhead-without simultaneously squeezing a bottle or tube. The Texpen package solved several problems for users when it was introduced and patented by Marktex Corporation and quickly became the steel tip marker of choice.

Other Considerations

Low chloride or high purity is a popular option for steel tip markers and applications in the welding trade. Low chloride markers come with factory certification by batch number and are used for marking stainless steel, nuclear applications, aircraft parts, and electrical components. High temperature paints is another popular option with paints capable of withstanding ups to 2200 degrees F.

Larry Bly is the founder of http://www.Markingpendepot.com an online distributor of industrial marking products and Competitive Advanage Group, Inc., a Manufacturers Representative agency.

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Here are some tips worth bearing in mind when purchasing used farm equipment.

1. Play by the book

Your best indicators of value on newer equipment are book values and number of hours on the clock, both of which you can get from the dealer manufacturer. Ask for clear evidence of age and usage.

2. An imperfect world

Many dealers admit to the fact that most machinery owners do not take great care of their equipment. It tends to be a case that, if previous owners haven’t needed to repair something on a piece of machinery, then it hasn’t been repaired until absolutely necessary. Therefore, it’s not unreasonable to expect at least some form of minor fault post purchase.

3. Aim low

Sounds obvious but when you’re looking at a piece of equipment that may need repairing do bear in mind that the owner may just want to get shot of it and buy new because they are aware of all the things that need to be fixed before it can be used. Make a low offer and see how you get on. You may pick up a bargain.

Don’t be too suspicious of equipment offered at a low price. Remember that used equipment dealers have lower overheads and expenses than manufacturer dealers, which often accounts for the difference.

4. All that glistens

If a machine has been repainted, don’t automatically assume the seller is trying to cover up a problem. Rather, when the seller applies new paint, it is usually because the old paint has faded from the sun.

On the whole if the paint job is bad, it’s likely to be bad underneath. Check beneath the surface.

5. Brands and quality

There are brand names you’ve heard of and those you haven’t. Buy decent quality brand-name equipment and it’s unlikely you’ll go wrong.

You may have to pay a little over the odds to get hold of it but clean, good conditioned low-hour kit will save you time and money in the long run.

Unless you have plenty of time on your hands and you’re mechanically inclined customized, heavily used or abused kit will end up costing you more than if you had paid more.

6. When you see a chance - take it

The fact is that many people don’t actually know a bargain when they see one. However, if you have done your research and know what equipment you want and its value then you may come across an unexpected bargain.

As a precaution check the item’s paperwork especially if it is being sold by someone not in the used-equipment business for appreciably less than its book value.

7. Check it out

As when buying a car, check the engine for knocks or smoke. Check the bearings, knuckles and joints for damage or indications of wear.

A book price is just that - a price in a book. When considering the value of a specific piece of kit give it a thorough work over.

8. Trust your instincts

Often when you feel something isn’t right, it isn’t and could even be worse than you imagined. Warning bells might ring if the motor sounds odd or doesn’t start properly. Problems tend to only get worse not better when you get your new purchase home. At the same time, just because a piece of kit has only a minor problem doesn’t make it a write off.

9. Stay local

Taking into account hidden costs such as fuel, travel time, hired help, trailer rental and the income they could have earned while staying home you may actually have found a better deal by buying from your local dealer.

10. Look online

There are a number of high profile websites that specialize in farm equipment and machinery - search for phrases such as “used farm machinery” or “second hand farm equipment” and you’ll find a number of reputable dealers with a broad selection of equipment.

Robert Tate - Region Sales Manager of Mascus UK. Mascus is an electronic marketplace for used equipment, farm machinery and trucks. Mascus makes trading in used machines and trucks quicker and more efficient by collecting in one place information about the supply and the demand. All dealers, producers and end users are welcome to offer their used machines and trucks for sale via the marketplace.

For interviews, quotes, images or comments contact:
Robert Tate
Regional Sales Manager - UK
Mob:+ 44 (0) 7970 230055
E mail: robert.tate@mascus.com

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The boom of computers in the several past decades has made a major impact on almost every field of human endeavor. It has introduced new possibilities also for structural engineers, draftsman whose primary task is to design buildings, bridges and other structures. Structural design can be considered to be a combination of art and science.

Structural drawings are part of the language the same structural engineers use to communicate with contractors, fabricators and regulatory bodies. These drawings describe the details of an object’s supporting members, such as beams and columns, which are designed by the structural engineer.

Structural design software is used mostly for developing architectural and complex machine designs or drawings. It has all the primary features available in commonly used engineering software. The most eminent characteristic of this type of software is that it allows users to differentiate components: to mark different components of a design with different color combinations provided with the software. Using additional structural software, structural analysis and structural drawings are now created by teams separated by thousands of miles, faster, more accurately, cost effective and within deadline in India.

Structural drawings are a task to locate, cumbersome to handle and difficult to view. The apprehension on the well being of invaluable Structural drawings is a common phenomenon worldwide. Structural drawings are crucial in the construction of buildings, ships, aircraft, tunnel, bridges, retaining walls, mines, infrastructure projects, automobiles and any other object subject to significant forces. Structural drawings are executed by structural draftsmen. Structural draftsmen typically work under the structural engineer responsible for the structural design. Skilled structural draftsmen convert the engineer’s sketches into CAD drawings, and also check that the designs conform to the relevant statutes and building codes.

Structural engineering services range from designing to building information modeling making us your one stop solution. An amalgamation of an excellent team of structural engineers and state-of-the-art systems making constant endeavors to provide high quality, cost effective and time bound structural drafting design services. Structural engineering services includes civil and structural drafting, architecture structural construction building engineering consultants from structural analysis and 2D and 3D Modeling services with lowest possible rates and within time bound. Structural Vibration analysis & design expertise in structural analysis, structural steel design, Structural design software, wood structural design at affordable cost in India.

At Structural Drafting and Design Services, we utilize our core competencies, combined experience and the latest in technology to provide you high quality civil engineering, 3D modeling services, structural consultants, structural analysis, drafting detailing services and Building Contractors in India, Our Experts team members enable us to deliver value added services across the globe with speed and accuracy and gives you advantages like:

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This article has been offer consideration of Structural Drafting and Design services in http://www.structuraldraftingdesign.com/, in Structural Drafting and Design services more than 7 years.

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