Hypoeutectoid steel: structure, properties, production and application

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

The use of carbon steels is widespread in construction and industry. The group of so-called technical iron has many advantages that lead to increased performance of final products and structures. Along with the optimum characteristics of strength and resistance to stress, these alloys are also distinguished by flexible dynamic properties. In particular, hypoeutectoid steel, which also contains a considerable percentage of carbon mixtures, is valued for its high ductility. But this is not all the advantages of this variety of high-strength iron.

General information about the alloy

A distinctive feature of steel is the presence of special alloyed impurities and carbon in the structure. Actually, the hypoeutectoid alloy is determined by the carbon content. Here it is important to distinguish between classical eutectoid and ledeburite steels, which have much in common with the described variety of technical iron. If we consider the structural class of steel, then the hypoeutectoid alloy will refer to eutectoids, but containing alloyed ferrites and pearlites. The fundamental difference from hypereutectoids is the level of carbon below 0.8%. Exceeding thisindicator allows us to classify steel as full-fledged eutectoids. In some way, the opposite of the hypoeutectoid is the hypereutectoid steel, which, in addition to pearlite, also contains secondary impurities of carbides. Thus, there are two main factors that make it possible to distinguish hypoeutectoid alloys from the general group of eutectoids. Firstly, this is a relatively small carbon content, and secondly, this is a special set of impurities, the basis of which is ferrite.

Production technology

The general technological process for the manufacture of hypoeutectoid steel is similar to the production of other alloys. That is, approximately the same techniques are used, but in different configurations. Hypoeutectoid steel requires special attention in terms of obtaining its specific structure. For this, a technology is used to ensure the decomposition of austenite against the background of cooling. In turn, austenite is a combined mixture, including the same ferrite and pearlite. By regulating the intensity of heating and cooling, technologists can control the dispersion of this additive, which ultimately affects the formation of certain performance qualities of the material.

However, the carbon provided by perlite remains the same. Although subsequent annealing may correct the formation of the microstructure, the carbon content will be in the range of 0.8%. An obligatory stage in the process of steel structure formation is normalization. This procedure is required for fractional optimization of grains of the sameaustenite. In other words, ferrite and pearlite particles are reduced to optimal sizes, which further improves the technical and physical performance of the steel. This is a complex process in which much depends on the quality of the heating regulation. If the temperature regime is exceeded, then the opposite effect may well be provided - an increase in austenite grains.

Steel annealing

The use of several annealing methods is practiced. There is a fundamental difference between full and partial annealing techniques. In the first case, the austenite is intensively heated to a critical temperature, after which normalization is carried out by means of cooling. This is where the decomposition of austenite occurs. As a rule, full annealing of steels is carried out in the mode of 700-800 °C. Heat treatment at this level just activates the processes of decay of ferrite elements. The cooling rate can also be adjusted, for example, the oven operating personnel can control the chamber door by closing or opening it. The latest models of isothermal ovens in automatic mode can carry out slow 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 limitations on the time of holding the critical temperature effect. For this reason, incomplete annealing occurs, as a result of which the ferrite does not disappear. Consequently, many shortcomings in the structure of the future material are not eliminated. Why is such annealing of steels necessary if it does not improve the physicalquality? In fact, it is the incomplete heat treatment that makes it possible to preserve the soft structure. The end material may not be required in every application specific to carbon steels per se, but will allow easy machining. The soft pro-eutectoid alloy is easy to cut and less expensive to manufacture.

Alloy normalization

After firing comes the turn of procedures of increased heat treatment. There are operations of normalization and heating. In both cases, we are talking about a thermal effect on the workpiece, at which the temperature can exceed 1000 °C. But in itself, the normalization of hypoeutectoid steels occurs after the completion of heat treatment. At this stage, cooling begins under conditions of still air, during which exposure takes place until the complete formation of fine-grained austenite. 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 a decrease in the size of ferrite and pearlite, as well as in an increase in their hardness. The strength qualities of the particles are increased in terms of compared to those achieved by annealing procedures.

After normalization, another long exposure heating procedure may follow. The workpiece is then cooled, and this step can be performed in different ways. The final hypoeutectoid steel is obtained either in air or inslow-cooling ovens. As practice shows, the highest quality alloy is formed using the full technology of normalization.

The effect of temperature on the structure of the alloy

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

Isothermal transformation with a supercooled mixture of austenites can be performed in different modes and the temperature level is just one of the parameters controlled by the technologist. Peak intervals of thermal action, cooling rate, etc. also vary. Depending on the chosen normalization mode, hardened steel is obtained with certain technical and physical characteristics. It is at this stage that it is also possible to set special operational properties. A striking example is an alloy with a soft structure, obtained with the aim of efficient further processing. But most oftenmanufacturers still focus on the needs of the end consumer and his requirements for the main technical and operational qualities of the metal.

Structure of steel

The normalization mode at a temperature of 700 °C causes the formation of a structure in which the grains of ferrites and pearlites will form the basis. By the way, hypereutectoid steels instead of ferrite have cementite in the structure. At room temperature, in the normal state, the content of excess ferrite is also noted, although this part is minimized as carbon increases. It is important to emphasize that the structure of steel depends to a small extent on the carbon content. It practically does not affect the behavior of the main components during the same heating, and almost all of it is concentrated in perlite. Actually, perlite can be used to determine the level of carbon mixture content - as a rule, this is an insignificant value.

Another structural nuance is also interesting. The fact is that pearlite and ferrite particles have the same specific gravity. This means that by the amount of one of these components in the total mass, you can find out what the total area it occupies. Thus, microsection surfaces are studied. Depending on the mode in which the hypoeutectoid 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 - another thing is that the limits for various indicators remain standard.

Properties of hypoeutectoid steel

This metal belongsto low-carbon steels, so you should not expect special performance from it. Suffice it to say that in terms of strength characteristics, this alloy is significantly inferior to eutectoids. This is due to differences in structure. The fact is that the hypoeutectoid class of steel with the content of excess ferrites is inferior in strength to analogues that have cementite in the structural set. Partly for this reason, technologists recommend using alloys for the construction industry, in the production of which the firing operation with the displacement of ferrites was implemented to the maximum.

If we talk about the positive exceptional properties of this material, then they are plasticity, resistance to natural biological processes of destruction, etc. At the same time, hardening of hypoeutectoid steels can add a number of additional qualities to the metal. For example, it can be increased thermal resistance, and the absence of a predisposition to corrosion processes, as well as a whole range of protective properties inherent in conventional low-carbon alloys.

Application areas

Despite a slight decrease in strength properties due to the fact that the metal belongs to the class of ferritic steels, this material is common in different areas. For example, in mechanical engineering, parts made of hypoeutectoid steels are used. Another thing is that high grades of alloys are used, in the manufacture of which advanced technologies of firing and normalization were used. Also, the structure of hypoeutectoid steel with a reduced ferrite content is quiteallows the use of metal in the production of building structures. Moreover, the affordable cost of some steel grades of this type allows you to 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 necessary. In such cases, the use of hypoeutectoid alloys is justified.

Production

Many enterprises are engaged in the manufacture, preparation and production of hypoeutectoid metal in Russia. For example, the Ural Non-Ferrous Metals Plant (UZTSM) produces several steel grades of this type at once, offering the consumer different sets of technical and physical properties. The Ural Steel Plant produces ferritic 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.

Metalloinvest can also be singled out among the largest manufacturers. At the facilities of this company, structural steels with a hypoeutectoid structure are produced, designed for use in construction. At the moment, the steel plant of the enterprise is working according to new standards, which allows improving the weak point of ferrite alloys - the strength indicator. In particular, the company's technologists are working to increase the stress intensity factor, to optimize the impact strength and fatigue resistance of the material. This allows us to offer almost universal alloys.

Conclusion

There are several technical and operational properties of industrial and building metals that are considered basic and are regularly improved. However, as designs and technological processes become more complex, new requirements for the element base also arise. In this regard, hypoeutectoid 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 ultra-high performance is needed, but in situations where special atypical sets of different properties are required. In this case, the metal exemplifies the combination of flexibility and ductility with optimum impact resistance and the basic protective qualities found in most carbon alloys.