The use of carbon steels is widespread in construction and industry. The so-called technical iron group has many advantages that determine the increased performance of the final products and structures. Along with the optimal characteristics of strength and resistance to stress, such alloys differ in their flexible dynamic properties. In particular, pre-eutectoid steel, which also contains a considerable percentage of carbon mixtures, is appreciated for its high ductility. But this is not all the advantages of this variety of high-strength iron.
Alloy Overview
A distinctive property of steel is the presence in the structure of special alloyed impurities and carbon. Actually, according to the carbon content, a hypoeutectoid alloy is determined. It is important to distinguish between classical eutectoid and ledeburite steel, which have much in common with the described type of technical iron. If we consider the structural class of steel, then the hypereutectoid alloy will belong to eutectoids, but containing alloyed ferrites and perlites. The principal difference from hypereutectoid is the level of carbon, which is below 0.8%. Exceeding this indicator allows us to attribute steel to full-fledged eutectoids. In some ways, the opposite of a hypereutectoid is a hypereutectoid steel, which in addition to perlite also contains secondary carbide impurities. Thus, there are two main factors that make it possible to isolate hypereutectoid alloys from the general group of eutectoids. Firstly, this is a relatively low carbon content, and secondly, it is a special set of impurities based on ferrite.
Manufacturing technology
The general manufacturing process for pre-eutectoid steel is similar to the production of other alloys. That is, approximately the same techniques are used, but in different configurations. Particular attention is required pre-eutectoid steel in terms of obtaining its specific structure. To do this, technology for ensuring the decomposition of austenite against the background of cooling is used. In turn, austenite is a combined mixture, including the same ferrite and perlite. By adjusting the intensity of heating and cooling, technologists can control the dispersion of this additive, which ultimately affects the formation of certain operational qualities of the material.
However, the indicator of carbon provided by perlite remains at the same level. Although subsequent annealing can make adjustments to the formation of the microstructure, the carbon content will be in the range of 0.8%. An obligatory step in the process of formation of the steel structure is normalization. This procedure is required for fractional optimization of grains of the same austenite. In other words, the particles of ferrite and perlite are reduced to optimal sizes, which further improves the technical and physical parameters of steel. This is a complex process in which much depends on the quality of the regulation of heating. If the temperature regime is exceeded, then the opposite effect may well be ensured - an increase in austenite grains.
Steel annealing
The use of several annealing methods is practiced. The techniques of complete and incomplete annealing are fundamentally different. In the first case, austenite is intensively heated to a critical temperature, after which normalization by cooling is carried out. Austenite decays right there. As a rule, a complete annealing of steels is performed in the mode of 700-800 ° . Heat treatment at this level just activates the decay of ferrite elements. The cooling rate can also be adjusted, for example, the furnace maintenance personnel can control the chamber door by closing or opening it. The latest models of isothermal furnaces in automatic mode can carry out delayed cooling in accordance with a given program.
As for incomplete annealing, it is produced by heating with a temperature above 800 ° C. However, there are serious restrictions on the retention time of the critical temperature effect. For this reason, incomplete annealing occurs, as a result of which ferrite does not disappear. Therefore, many deficiencies in the structure of future material are not eliminated. Why is such annealing of steels needed if it does not improve physical qualities? In fact, it is incomplete heat treatment that preserves the soft structure. The final material may not be needed in every application characteristic of carbon steels as such, but it will make it easy to machine. A soft hypereutectoid alloy can be cut without any difficulties and is cheaper in the manufacturing process.
Alloy Normalization
After firing, a series of high heat treatment procedures begins. Allocate normalization and heating operations. In both cases, we are talking about the thermal effect on the workpiece, at which the temperature may exceed 1000 ° C. But normalization of hypereutectoid steels in itself occurs after the completion of heat treatment. At this stage, cooling begins in calm air, at which exposure occurs until the formation of fine-grained austenite. That is, heating is a kind of preparatory operation before bringing the alloy to a normalized state. If we talk about specific structural changes, then most often they are expressed in a decrease in the size of ferrite and perlite, as well as in an increase in their hardness. The strength properties of particles increase in performance compared with similar characteristics achieved by annealing procedures.
After normalization, another long exposure heating procedure may follow. Then the workpiece is cooled, and this step can be performed in various ways. The final pre-eutectoid steel is obtained either in air or in an oven with slow cooling. As practice shows, the highest quality alloy is formed using a full normalization technology.
The effect of temperature on the alloy structure
The temperature intervention in the process of steel structure formation begins from the moment the ferritic-cementite mass is converted into austenite. In other words, perlite goes into a state of functional mixture, which partly becomes the basis for the formation of high-strength steel. In the next step of the heat treatment, hardened steel eliminates excess ferrite. As already noted, it is not always completely disposed of, as in the case of incomplete annealing. But the classic hypereutectoid alloy still involves the elimination of this component of austenite. At the next stage, the existing composition is already being optimized with the expectation of the formation of an optimized structure. That is, there is a decrease in alloy particles with gaining increased strength properties.
Isothermal transformation with a supercooled austenite mixture can be performed in different modes and the temperature level is just one of the parameters that the technologist controls. Peak intervals of thermal exposure, cooling rate, etc. also vary. Depending on the selected normalization mode, hardened steel with certain technical and physical characteristics is obtained. It is at this stage that there is also the opportunity to set special operational properties. A vivid example is an alloy with a soft structure, obtained for the purpose of efficient further processing. But most often, manufacturers nevertheless focus on the needs of the end user and his requirements for the main technical and operational qualities of the metal.
Steel structure
The normalization mode at a temperature of 700 ° C determines the formation of a structure in which the basis will be grains of ferrite and perlite. By the way, hypereutectoid steels instead of ferrite have cementite in the structure. At room temperature, the content of excess ferrite is also observed in the normal state, although this part is minimized with increasing carbon. It is important to emphasize that the structure of steel is slightly dependent on the carbon content. It practically does not affect the behavior of the main components during the same heating process and almost all is concentrated in perlite. Actually, it is possible to determine the level of the carbon mixture by perlite - as a rule, this is an insignificant value.
Another structural nuance is interesting. The fact is that perlite and ferrite particles have the same specific gravity. This means that by the number of one of these components in the total mass, you can find out what is the total area occupied by it. In this way, microsection surfaces are studied. Depending on the mode in which the hypereutectoid steel was heated, the fractional parameters of austenite particles are also formed. But this happens almost in an individual format with the formation of unique values - it’s another matter that the limits for different indicators remain standard.
Properties of pre-eutectoid steel
This metal belongs to low-carbon steels, so you should not expect special performance from it. Suffice it to say that in the strength characteristics this alloy significantly loses to eutectoids. This is due precisely to differences in structure. The fact is that the hypereutectoid class of steel with the content of excess ferrites is inferior in strength to analogues having cementite in the structural set. Partly for this reason, technologists recommend the use of alloys for the construction industry, in the production of which the firing operation with the displacement of ferrites was maximally implemented.
If we talk about the positive exceptional properties of this material, then they consist in plasticity, resistance to natural biological processes of destruction, etc. At the same time, hardening of hypereutectoid steels can add a number of additional qualities to the metal. For example, this can be increased thermal resistance, and a lack of predisposition to corrosion processes, as well as a whole set of protective properties inherent in ordinary low-carbon alloys.
Fields of application
Despite a slight decrease in strength properties due to the metal belonging to the class of ferrite steels, this material is distributed in different areas. For example, in engineering, parts made of hypereutectoid steels are used. Another thing is that high grades of alloys are used, in the manufacture of which advanced firing and normalization technologies were used. Also, the structure of hypereutectoid steel with a low ferrite content allows the use of metal in the manufacture of building structures. Moreover, the affordable cost of some grades of steel of this type can count on significant savings. Sometimes in the manufacture of building materials and steel modules, increased strength is not required at all, but wear resistance and elasticity are required. In such cases, the use of hypereutectoid alloys is justified.
Production
Many enterprises are engaged in the manufacture, preparation and production of hypereutectoid metal in Russia. For example, the Ural Non-Ferrous Metals Plant (UZTSM) produces several grades of steel of this type at once, offering the consumer different sets of technical and physical properties. The Ural Steel Plant produces ferrite steels, which include high-quality alloyed components. In addition, special alloy modifications are available in the assortment, including heat-resistant, high-chromium and stainless metals.
Among the largest manufacturers, the Metalloinvest enterprise can also be distinguished. At the facilities of this company, structural steels with a hypereutectoid structure are produced, designed for use in construction. At the moment, the steel mill of the enterprise is working according to new standards that can improve the weak spot of ferrite alloys - strength indicator. In particular, the company's technologists are working to increase the stress intensity factor, to optimize the toughness and indicators of material fatigue resistance. This allows us to offer alloys of almost universal purpose.
Conclusion
There are several technical and operational properties of industrial and building metals, which are considered basic and are regularly improved. However, as structures and technological processes become more complex, new requirements for the elemental base arise. In this regard, the pre-eutectoid steel, in which various performance qualities are concentrated, clearly manifests itself. The use of this metal is justified not in cases where a part with several ultra-high performance is needed, but in situations where special atypical sets of different properties are required. In this case, the metal shows an example of a combination of flexibility and ductility with optimal impact resistance and basic protective qualities characteristic of most carbon alloys.