Bainite top and bottom are established terms used to describe the microstructures that can be easily distinguished by using microscopy routine, and whose formation mechanisms are well understood. There are, however, a number of other microstructures description of the steel that include the word 'bainite. These descriptions can be useful in the Commission's form of the microstructure. But this must be done with care, avoiding the natural tendency to imagine a mechanism of transformation simply because someone has chosen piece of terminology.
Granules Bainite
Of all the descriptions of unusual bainitique microstructures, granular bainite is probably the most useful and widely used nomenclature. In early 1950, shortly cooled continuously carbon steels were found to reveal microstructures which consisted of thick plates and those with a granular almost entirely "with pockets of martensite and kept ustenite, (Habraken, 1956 , 1957, 1965; Ridal and McCann, 1965; Habraken and Economopolus, 1967). Habraken and co-workers called this granular bainite and terminology has become popular because many industrial-heat treatments involve continuous cooling rather than isothermal processing. energy generation in particular, the industry uses huge amounts of microstructures bainitique generated by large steel components to cool naturally. Bainite granular is supposed to happen in steels were cooled to permanence, it can be produced by processing insulated.
The thick plates ferrite mentioned earlier, do not really exist. They are, in fact, beams ferrite bainitique regions with very thin austenite between subunits because of the low concentration of carbon steels involved (Leont'yev and Kovalevskaya, 1974; Josefsson and Andren, 1989). Thus, on a scale perspective, they give the appearance of thick plates. Many of the original conclusions were reached from the observation of the microstructures that are not of sufficient resolution to determine the fine structure in the jets of bainite. Indeed, the evidence of this interpretation of the so-called thick plates appeared in the literature from 1967, when thin sheet TEM observations were made by Habraken and Economopolus, revealing the thin ferrite bainitique platelets in the pulleys.
A feature (but not unique) granular bainite characteristic is the lack of carbides in the microstructure. The carbon that is partitioned from the ferrite bainitique stabilizes the residual austeite, so that the final miscostructure contains both retained austenite and some high carbon martensite. Consistent with the observation on conventional bainite, there is no redistribution of fluid replacement during the formation of granular bainite (Azevedo and Tenuta-Galvao-da-Silva, 1978).
The magnitude of the transformation of granular bainite is found to depend on the undercooling below the temperature bainite-start (Habraken and Economopolus, 1967). This is a reflection of the fact that the microstructure, and classic bainite, reflects a phenomenon reaction incomplete.
The evidence indicates that granular bainite is no different from ordinary bainite in the mechanism of transformation. The peculiar morphology is the result of two factors: continuous cooling transformation gradual bainite during cooling to room temperature. The low concentration of carbon ensures that all the movies or parts of austenite carbide that may exist between subunits of bainite are minimal, which makes the identification of platelets in the sheaves rather difficult using optical microscopy.
Finally, it is interesting to note that in an attempt to infer a mechanism for the formation of granular bainite, Habraken (1965) proposed that the austenite prior to the transformation is divided into regions which are rich in carbon, and those which are relatively exhausted. These regions are exhausted then supposed to transform itself into granular bainite. The idea is the same as Klier and Lyman (1944) and showed beeb be thermodynamically impossible in steels (Aaronson et al. 1966a)
Inverse Bainite
Ferrite is the major phase in conventional bainite; carbide precipitation when there is a secondary event. In the so-called 'inverse bainite' which is in the steels hypereutectoid is cementite which is the first phase of the form (Hillert, 1957). A plaque-like spine of cementite grows directly from austenite (Henemann, 1970) and becomes surrounded by a layer of ferrite. The term "reverse" reflects the fact that, unlike conventional bainite, cementite is the first phase of austenite rush.
The mechanism of the transformation is virtually unknown, there is no evidence that the growth of ferrite is a coordinated by the movement of atoms, and no crystallographic or chemical composition data. In judging from the form alone, the ferrite probably through the development of a mechanism for reconstructive transformation. It is premature to classify as bainite transformation.
Columnar Bainite
"Columns bainite 'is a description of a non-lamellar total of cementite and ferrite, the general shape of which is an irregular and slightly elongated colony (Fig. 11.3). The distribution of particles cementite within the colony is rather strange, the majority of the needle-shaped particles are aligned with the longest dimension of the colony. The latter region is surrounded by a layer of a different microstructure, in which coarse particles of cementite respond to austenite / ferrite edge on the interface (Nilan, 1967). The structure is normally observed in steels hypereutectoid (Greninger and Troiano, 1940; Vilella, 1940; Jellinghaus, 1957: Speich and Cohen, 1960), but it was found lower carbon steels with high-pressure processing (Nilan, 1967). It may be pertinent to stress that the eutectoid composition is shifted to lower concentrations of carbon per hydrostatic pressure.
The microstructure can be obtained at processing temperatures comfortable than those associated with conventional bainite, but there is no invariant surface strain relief that accompanies the growth of "columns bainie. It is probable that the columns is similar bainite more than that bainite perlite, but further investigations are required to make reasonable decisions about the mechanism of growth.
Perlite Bainite
In steel carbide containing elements forming strong, it is possible to obtain perlite, in which the carbide phase is an alloy carbide (M7C3) instead of cementite. The perlite alloy can be formed at a temperature above Bs, or slightly below that temperature, but only after holding the transformation temperature for very long periods of time (usually several days). Throughout optical microscopy, perlite engravings dark as nodules, but the settlements tend to have faceted crystal rather than the normal colonies nicely rounded perlite. This is probably a reflection of the dependence on the orientation of the interfacial energy of the alloy carbide.
Because of this aspect, transmission electron microscopy observations can be misleading. The crystallographically facets nodules perlite high resolution gives the appearance of parallel plates with ferrite intervene carbides, a microstructure of this magnitude similar to the upper bainite. The terminology 'perlite bainite' given to the transformation of this product is misleading. It is crude partitioning substitutional solutes during processing, there is no surface relief Indeed, the carbide phases of growth and ferrite in collaboration, and there is no reason to associate this with bainite microstructute .
Grain Boundary Bainite lower
Bainite nucleation occurs in most steel mixed with austenite grain boundaries. The lowest rate of nucleation bainite can be great at temperatures close to Mrs; the austenite grains become surfaces covered by the decline bainite subunits (Fig.11.5). The rate at which carbon partitions ferrite supersaturated is slow when such transformations is at low temperatures. Therefore, the sub-units are able to form in the tables without any intervention austenite (Bhadeshia and Edmonds, 1979a). These layers of sub-units are in the shape of allotriomorphs, but there is no doubt that they form individually.
The microstructure has caused some concern in the context of the 300M, which is an ultra-high-strength steel used in quenched and tempered condition (Padmanabhan and Wood, 1984). The alloy is very high hardenability sections of 10 cm in diameter can be carried out by air cooling martensitic austenitisation temperature. However, optical microscopy revealed the presence of allotriomorphs surprising on the detailed examination which proved to be the lower limit grain bainite described above.
Summary
Bainite granular bainite regular is essentially generated by continuous cooling transformation steels with low carbon. The mechanism of bainite reverse is not clear, but it is the formation of cementite as the main phase. It is not clear whether the ferrite, when he finished forms and ruin the cementite, for the development of a mechanism displacive or reconstructive.
If there is any doubt on the mechanism of bainite Conversely, the terms of columns and perlite bainite are misnomers and are undoubtedly best avoided. Columnar bainite is simply an aggregate of cementite and ferrite, which pushes a mechanism for reconstructive transformation. Perlite bainite is simply an alloy crystallographically facets perlite.
A high supersaturations, the tables below bainite subunit can quickly decorate the austenite grain surfaces, which gives an impression of allotriomorphs. This "grain bainite lower limit is much harder than allotriomorphic ferrite, and therefore is easy to distinguish.
Showing posts with label alloy. Show all posts
Showing posts with label alloy. Show all posts
Thursday, March 13, 2008
Influence of Alloying Elements on Steel Microstructure
Abstract
It is a long tradition to discuss various alloying elements in terms of giving them the properties of the steel. For example, the rule is that chromium (Cr) makes it difficult steel while nickel (Ni) and manganese (Mn), it is rude. In saying that, there were certain types of steel and transferred to mind the mechanical properties of steel particularly at the alloying element, which was thought to have the most influence on the steel under consideration . This mode of reasoning can give false impressions and following examples illustrate this point.
It is a long tradition to discuss various alloying elements in terms of giving them the properties of the steel. For example, the rule is that chromium (Cr) makes it difficult steel while nickel (Ni) and manganese (Mn), it is rude. In saying that, there were certain types of steel and transferred to mind the mechanical properties of steel particularly at the alloying element, which was thought to have the most influence on the steel under consideration . This mode of reasoning can give false impressions and following examples illustrate this point. When we say that chromium makes steel hard and wear-resistant probably associate ourselves with 2% C, 12% Cr tool steel grade, which makes it tougher become very tough and resistant. But if, instead, we choose a steel containing 0.10% C and 12% Cr, hardness obtained upon curing is very modest. It is true that increasing Mn steel tenacity if we have in mind the 13% manganese steel, called Hadfield steel. In between concentration% and 5%, however, Mn variable can produce an effect on the properties of steel, it is used with. The toughness can either increase or decrease. A property of great importance is the ability of alloying elements to promote the formation of a certain phase or stabilize it. These elements are grouped as austenite formation, the formation of ferrite, carbide formation and training nitride elements.
Austenite-forming elements
The elements C, Ni and Mn are the most important ones in this group. Sufficiently large amounts of Ni or Mn render a steel austenitic even at room temperature. An example of this is the so-called Hadfield steel which contains 13% Mn, 1,2% Cr and l% C. In this steel both the Mn and C take part in stabilizing the austenite. Another example is austenitic stainless steel containing 18% Cr and 8% Ni.
The equilibrium diagram for iron-nickel, An alloy containing 10% Ni becomes wholly austenitic if heated to 700°C. On cooling, transformation from g to a takes place in the temperature range 700-300°C.
Ferrite-forming elements
The most important elements in this group are Cr, Si, Mo, W and Al. The range of stability of ferrite in iron-chromium alloys. Fe-Cr alloys in the solid state containing more than 13% Cr are ferritic at all temperatures up to incipient melting. Another instance of ferritic steel is one that is used as transformer sheet material. This is a low-carbon steel containing about 3% Si.
Multi-alloyed steels
The great majority of steels contain at least three components. The constitution of such steels can be deduced from ternary phase diagrams (3 components). The interpretation of these diagrams is relatively difficult and they are of limited value to people dealing with practical heat treatment since they represent equilibrium conditions only. Furthermore, since most alloys contain more than three components it is necessary to look for other ways of assessing the effect produced by the alloying elements on the structural transformations occurring during heat treatment. One approach that is quite good is the use of Schaeffler diagrams. Here the austenite formers are set out along the ordinate and the ferrite formers along the abscissa. The original diagram contained only Ni and Cr but the modified diagram includes other elements and gives them coefficients that reduce them to the equivalents of Ni or Cr respectively. The diagram holds good for the rates of cooling which result from welding.
A 12% Cr steel containing 0,3% C is martensitic, the 0,3% C gives the steel a nickel equivalent of 9. An 18/8 steel (18% Cr, 8% Ni) is austenitic if it contains 0-0,5% C and 2% Mn. The Ni content of such steels is usually kept between 9% and 10%. Hadfield steel with 13% Mn (mentioned above) is austenitic due to its high carbon content. Should this be reduced to about 0,20% the steel becomes martensitic.
Carbide-forming elements
Several ferrite formers also function as carbide formers. The majority of carbide formers are also ferrite formers with respect to Fe. The affinity of the elements in the line below for carbon increases from left to right. Cr, W, Mo, V, Ti, Nb, Ta, Zr. Some carbides may be referred to as special carbides, i.e. non-iron-containing carbides, such as Cr7C3 W2C, VC, Mo2C. Double or complex carbides contain both Fe and a carbide-forming element, for example Fe4W2C. High-speed and hot-work tool steels normally contain three types of carbides, which are usually designated M6C, M23C6 and MC. The letter M represents collectively all the metal atoms. Thus M6C represents Fe4W2C or Fe4Mo2C; M23C6 represents Cr23C6 and MC represents VC or V4C3.
Carbide stabilizers
The stability of the carbides is dependent on the presence of other elements in the steel. How stable the carbides are depends on how the element is partitioned between the cementite and the matrix. The ratio of the percentage, by weight, of the element contained in each of the two phases is called the partition coefficient K. The following values are given for K:
Al Cu P Si Co Ni W Mo Mn Cr Ti Nb Ta
0 0 0 0 0,2 0,3 2 8 11,4 28 Increasing
Note that Mn, which by itself is a very weak carbide former, is a relatively potent carbide stabilizer. In practice, Cr is the alloying element most commonly used as a carbide stabilizer. Malleable cast iron (i.e. white cast iron that is rendered soft by a graphitizing heat treatment called malleablizing) must not contain any Cr. Steel containing only Si or Ni is susceptible to graphitization, but this is most simply prevented by alloying with Cr.
Nitride-forming elements
All carbide formers are also nitride formers. Nitrogen may be introduced into the surface of the steel by nitriding. By measuring the hardness of various nitrided alloy steels it is possible to investigate the tendency of the different alloying elements to form hard nitrides or to increase the hardness of the steel by a mechanism known as precipitation hardening. The results obtained by such investigations are shown in Figure 4, from which it can be seen that very high hardnesses result from alloying a steel with Al or Ti in amounts of about 1,5%.
On nitriding the base material, hardness of about 400 HV is obtained and according to the diagram the hardness is unchanged if the steel is alloyed with Ni since this element is not a nitride former and hence does not contribute to any hardness increase.
It is a long tradition to discuss various alloying elements in terms of giving them the properties of the steel. For example, the rule is that chromium (Cr) makes it difficult steel while nickel (Ni) and manganese (Mn), it is rude. In saying that, there were certain types of steel and transferred to mind the mechanical properties of steel particularly at the alloying element, which was thought to have the most influence on the steel under consideration . This mode of reasoning can give false impressions and following examples illustrate this point.
It is a long tradition to discuss various alloying elements in terms of giving them the properties of the steel. For example, the rule is that chromium (Cr) makes it difficult steel while nickel (Ni) and manganese (Mn), it is rude. In saying that, there were certain types of steel and transferred to mind the mechanical properties of steel particularly at the alloying element, which was thought to have the most influence on the steel under consideration . This mode of reasoning can give false impressions and following examples illustrate this point. When we say that chromium makes steel hard and wear-resistant probably associate ourselves with 2% C, 12% Cr tool steel grade, which makes it tougher become very tough and resistant. But if, instead, we choose a steel containing 0.10% C and 12% Cr, hardness obtained upon curing is very modest. It is true that increasing Mn steel tenacity if we have in mind the 13% manganese steel, called Hadfield steel. In between concentration% and 5%, however, Mn variable can produce an effect on the properties of steel, it is used with. The toughness can either increase or decrease. A property of great importance is the ability of alloying elements to promote the formation of a certain phase or stabilize it. These elements are grouped as austenite formation, the formation of ferrite, carbide formation and training nitride elements.
Austenite-forming elements
The elements C, Ni and Mn are the most important ones in this group. Sufficiently large amounts of Ni or Mn render a steel austenitic even at room temperature. An example of this is the so-called Hadfield steel which contains 13% Mn, 1,2% Cr and l% C. In this steel both the Mn and C take part in stabilizing the austenite. Another example is austenitic stainless steel containing 18% Cr and 8% Ni.
The equilibrium diagram for iron-nickel, An alloy containing 10% Ni becomes wholly austenitic if heated to 700°C. On cooling, transformation from g to a takes place in the temperature range 700-300°C.
Ferrite-forming elements
The most important elements in this group are Cr, Si, Mo, W and Al. The range of stability of ferrite in iron-chromium alloys. Fe-Cr alloys in the solid state containing more than 13% Cr are ferritic at all temperatures up to incipient melting. Another instance of ferritic steel is one that is used as transformer sheet material. This is a low-carbon steel containing about 3% Si.
Multi-alloyed steels
The great majority of steels contain at least three components. The constitution of such steels can be deduced from ternary phase diagrams (3 components). The interpretation of these diagrams is relatively difficult and they are of limited value to people dealing with practical heat treatment since they represent equilibrium conditions only. Furthermore, since most alloys contain more than three components it is necessary to look for other ways of assessing the effect produced by the alloying elements on the structural transformations occurring during heat treatment. One approach that is quite good is the use of Schaeffler diagrams. Here the austenite formers are set out along the ordinate and the ferrite formers along the abscissa. The original diagram contained only Ni and Cr but the modified diagram includes other elements and gives them coefficients that reduce them to the equivalents of Ni or Cr respectively. The diagram holds good for the rates of cooling which result from welding.
A 12% Cr steel containing 0,3% C is martensitic, the 0,3% C gives the steel a nickel equivalent of 9. An 18/8 steel (18% Cr, 8% Ni) is austenitic if it contains 0-0,5% C and 2% Mn. The Ni content of such steels is usually kept between 9% and 10%. Hadfield steel with 13% Mn (mentioned above) is austenitic due to its high carbon content. Should this be reduced to about 0,20% the steel becomes martensitic.
Carbide-forming elements
Several ferrite formers also function as carbide formers. The majority of carbide formers are also ferrite formers with respect to Fe. The affinity of the elements in the line below for carbon increases from left to right. Cr, W, Mo, V, Ti, Nb, Ta, Zr. Some carbides may be referred to as special carbides, i.e. non-iron-containing carbides, such as Cr7C3 W2C, VC, Mo2C. Double or complex carbides contain both Fe and a carbide-forming element, for example Fe4W2C. High-speed and hot-work tool steels normally contain three types of carbides, which are usually designated M6C, M23C6 and MC. The letter M represents collectively all the metal atoms. Thus M6C represents Fe4W2C or Fe4Mo2C; M23C6 represents Cr23C6 and MC represents VC or V4C3.
Carbide stabilizers
The stability of the carbides is dependent on the presence of other elements in the steel. How stable the carbides are depends on how the element is partitioned between the cementite and the matrix. The ratio of the percentage, by weight, of the element contained in each of the two phases is called the partition coefficient K. The following values are given for K:
Al Cu P Si Co Ni W Mo Mn Cr Ti Nb Ta
0 0 0 0 0,2 0,3 2 8 11,4 28 Increasing
Note that Mn, which by itself is a very weak carbide former, is a relatively potent carbide stabilizer. In practice, Cr is the alloying element most commonly used as a carbide stabilizer. Malleable cast iron (i.e. white cast iron that is rendered soft by a graphitizing heat treatment called malleablizing) must not contain any Cr. Steel containing only Si or Ni is susceptible to graphitization, but this is most simply prevented by alloying with Cr.
Nitride-forming elements
All carbide formers are also nitride formers. Nitrogen may be introduced into the surface of the steel by nitriding. By measuring the hardness of various nitrided alloy steels it is possible to investigate the tendency of the different alloying elements to form hard nitrides or to increase the hardness of the steel by a mechanism known as precipitation hardening. The results obtained by such investigations are shown in Figure 4, from which it can be seen that very high hardnesses result from alloying a steel with Al or Ti in amounts of about 1,5%.
On nitriding the base material, hardness of about 400 HV is obtained and according to the diagram the hardness is unchanged if the steel is alloyed with Ni since this element is not a nitride former and hence does not contribute to any hardness increase.
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