Pre-eutectoid steel: structure, properties, production and application

The use of carbon steels is widespread in construction and industry. A group of so-called technical iron has many advantages, which determine the increased performance of finished products and structures. Along with the optimum characteristics of strength and resistance to loads, such alloys also have flexible dynamic properties. In particular, the pre-eutectoid steel, which also has a considerable percentage of carbonaceous mixtures, is valued for its high ductility. But this is not all the advantages of this type of high-strength iron.

General information about the alloy

A distinctive property of steel is the presence in the structure of special alloyed impurities and carbon. Actually, the content of carbon is determined by the pre-eutectoid alloy. Here it is important to distinguish between the classical eutectoid as well as the lebedebury steel, which have much in common with the described variety of technical iron. If we consider the structural class of steel, the pre-eutectoid alloy will refer to eutectoids, but containing alloyed ferrites and perlites. The principal difference from hypereutectoid is the carbon level, which is below 0.8%. Exceeding this indicator allows us to refer steel to full-value eutectoids. In some way, the opposite of the pre-eutectoid is the hypereutectoid steel, which also contains secondary impurities of carbides in addition to perlite. Thus, there are two main factors that make it possible to isolate the pre-eutectoidal alloys from the general group of eutectoids. First, this is a relatively small content of carbon, and secondly, it is a special set of impurities, the basis of which is ferrite.

Manufacturing technology

The general technological process of manufacturing the pre-eutectoid steel is similar to that of other alloys. That is, the same techniques are used, but in other configurations. Special attention is paid to the pre-eutectoid steel in terms of obtaining its specific structure. To do this, the technology of ensuring the decomposition of austenite against the background of cooling is involved. In turn, austenite is a combined mixture that includes 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 properties of the material.

However, the carbon index provided by perlite remains at the same level. Although subsequent annealing can correct the formation of the microstructure, the carbon content will be within 0.8%. The obligatory stage 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 optimum sizes, which further improves the technical and physical properties of steel. This is a complex process, in which much depends on the quality of heating regulation. If the temperature regime is exceeded, a reverse effect may be provided-an increase in the austenite grains.

Annealing of steel

Practice is the use of several annealing methods. The techniques of complete and incomplete annealing are fundamentally different. In the first case, an intense heating of the austenite to a critical temperature occurs , after which the cooling is normalized. Here, austenite decomposes. As a rule, complete annealing of steels is performed in the 700-800 ° C mode. Thermal treatment at this level just activates the processes of decay of ferrite elements. The cooling rate is also adjustable, for example, the maintenance staff can control the door of the camera by closing or opening it. The newest models of isothermal furnaces in automatic mode can carry out slow cooling in accordance with the specified program.

As for incomplete annealing, it is produced when heated with a temperature above 800 ° C. However, there are serious limitations on the retention time of the critical temperature effect. For this reason, incomplete annealing occurs, as a result of which ferrite does not disappear. Consequently, many shortcomings in the structure of the future material are not eliminated. Why do we need such annealing of steels, if it does not improve physical qualities? In fact, it is the incomplete heat treatment that allows to keep the soft structure. The final material may not be required in every sphere of application typical for carbon steels as such, but it will allow to easily machinish. The soft pre-eutectoid alloy can be cut without any difficulties and is cheaper in the manufacturing process.

Normalization of the alloy

After firing, a series of procedures for increased heat treatment occurs. Isolate the operations of normalization and heating. In both cases, it is a thermal action on the workpiece, at which the temperature can exceed 1000 ° C. But in itself, the normalization of the pre-eutectoid steels occurs after the completion of the heat treatment. At this stage, cooling begins in a calm air, at which the aging takes place until the fine austenite is completely formed. That is, heating is a kind of preparatory operation before bringing the alloy into a normalized state. If we talk about specific structural changes, then most often they are expressed in decreasing the dimensions of ferrite and perlite, as well as in increasing their hardness. The strength properties of particles increase in terms compared to the similar characteristics achieved by annealing procedures.

After normalization, another heating procedure with a long exposure may follow. The blank is then cooled, and this step can be performed in various ways. The final pre-eutectoid steel is obtained either in air or in a slow-cooling furnace. As practice shows, the best quality alloy is formed by using the full technology of normalization.

The influence of temperature on the structure of the alloy

The temperature intervention in the process of formation of the structure of steel begins with the moment of transformation of the ferritic-cementite mass into austenite. In other words, perlite transforms into a state of a functional mixture, which partly becomes the basis for the formation of high-strength steel. At the next stage of thermal exposure, hardened steel gets rid of excess ferrite. As already noted, not always get rid of it completely, as in the case of incomplete annealing. But the classical pre-eutectoid alloy still suggests the elimination of this austenite component. At the next stage, there is already optimization of the existing composition with the expectation of forming an optimized structure. That is, there is a decrease in the alloy particles with the acquisition of increased strength properties.

Isothermal transformation with a supercooled austenite mixture can be performed in different modes and the temperature level is only one of the parameters that is controlled by the technologist. Also the peak intervals of thermal influence, the cooling rate, etc. vary. Depending on the selected normalization mode, hardened steel with some technical and physical characteristics is obtained. It is at this stage that it is also possible to set specific operational properties. A vivid example is an alloy with a soft structure, obtained for the purpose of efficient further processing. But most often manufacturers are still oriented towards the needs of the end user and his requirements for the basic technical and operational qualities of the metal.

Structure of steel

Normalization at a temperature of 700 ° C causes the formation of a structure in which the basis will be the grains of ferrites and perlites. By the way, hypereutectoid steels instead of ferrite have cementite in the structure. At room temperature, the content of excess ferrite is also noted in the usual state, although this part is minimized with increasing carbon. It is important to emphasize that the structure of steel to a small extent depends on the carbon content. It practically does not affect the behavior of the main components in the same heating process and almost all is concentrated in the perlite. Actually, perlite, and you can determine the level of the content of the carbon mixture - as a rule, this is an insignificant amount.

Another structural nuance is also interesting. The fact is that the particles of perlite and ferrite 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 the total area occupied by it is. Thus, the surfaces of microsections are studied. Depending on the mode in which the pre-eutectoid steel was heated, fractional parameters of the austenite particles are formed. But this happens almost in an individual format with the formation of unique values - it's another matter that the limits on different indicators remain standard.

Properties of the pre-eutectoid steel

This metal belongs to low-carbon steels, so it is not worth waiting for special performance qualities. Suffice it to say that in the strength characteristics this alloy significantly loses to eutectoids. This is due to differences in the structure. The fact is that the pre-eutectoid class of steel with the content of excess ferrites is inferior in strength to the analogs 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 displacement of ferrites was maximally realized.

If we talk about the positive exceptional properties of this material, they consist in plasticity, resistance to natural biological processes of destruction, etc. Along with this, quenching of the pre-eutectoid steels can add a number of additional qualities to the metal. For example, it can be increased thermal stability, and a lack of predisposition to corrosion processes, as well as a variety of protective properties inherent in conventional low-carbon alloys.

Applications

Despite some decrease in strength properties due to the metal's membership in the class of ferrite steels, this material is common in various fields. For example, in the engineering industry, parts made of pre-eutectoid steels are used. Another thing is that high grades of alloys are used, in the manufacture of which advanced technologies of roasting and normalization were used. Also, the structure of the pre-eutectoid steel with a reduced content of ferrite makes it possible to use metal in the production of building structures. Moreover, the affordable value of some brands of steel of this type allows you to expect significant savings. Sometimes in the manufacture of building materials and steel modules does not require increased strength, but it requires durability and resilience. In such cases, justified the use of pre-eutectoid alloys.

Production

Many enterprises are engaged in the manufacture, preparation and production of the pre-eutectoid metal in Russia. For example, the Ural Non-ferrous Metals Plant (UZTSM) produces several types of steel of this type, offering different sets of technical and physical properties to the consumer. The Ural Steel Plant produces ferrite steels, which include high-quality alloyed components. In addition, special modifications of alloys are available in the range, including high-temperature, high-chromium and stainless metals.

Among the largest producers can be identified and the enterprise "Metalloinvest". The capacities of this company produce structural steels with a pre-eutectoid structure, designed for use in construction. At the moment, the steel plant of the company operates according to new standards, allowing to improve the weak point of ferrite alloys - strength index. In particular, the company's technologists are working on increasing the voltage intensity factor, on optimizing the toughness and the fatigue resistance of the material. This makes it possible to offer alloys of almost universal purpose.

Conclusion

There are several technical and operational properties of industrial and construction metals, which are considered basic and regularly improved. However, as structures and technological processes become more complex, new requirements arise for the element base. In this respect, pre-eutectoid steel clearly manifests itself, in which different performance qualities are concentrated. The use of this metal is justified not in cases where a part with several ultrahigh indices 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 plasticity with optimal impact resistance and the basic protective properties characteristic of most carbon alloys.