What is the surface hardening of steel? What is the use of surface hardening?

The art of heat treatment of metals is known to mankind for a long time. They are masters who were engaged in the manufacture of tools, and in particular weapons, mastered independently, or studied for many years with other specialists, more experienced. Secrets were kept secret, which, of course, reduced the speed of the spread of technology, but increased the competitiveness of a specific manufacturer of specific products. One of the techniques of medieval gunsmiths was surface hardening, giving the cutting edges and points of swords and sabers a special hardness combined with the flexibility of the blade. Today, such properties do not surprise anyone anymore, technologies have become massive and have become widespread.

Why does everyone know this to an ordinary person?

This article to the specialists in thermal metal processing seems, most likely, a set of banalities and all known facts. In addition, they may find some inaccuracies in terminology. The information presented is not intended for them; it is addressed to people who are far from metallurgy, amateurs who are interested in the difference in the strength of a good blade from a regular table or folding knife, surface hardening from bulk and similar questions. Buying this or that object in the household, the consumer faces a significant price difference. The seller can not always be qualified and understandable why one tool (a wrench, for example) is much more expensive than another with a general external similarity. He, most likely, will try to "make the brains" understandable concepts and terms for ordinary people. Translated into a common language, these explanations mean that the adjustable wrench will not break or last longer, and sharpening will be required much less often (if the customer wishes to purchase a knife). "Surface hardening!" - mysteriously indicate the cause of the seller, rolling his eyes from an imaginary ecstasy. What is it?

Opposite properties in one product

As is clear from the phrase, in this case only the outer thin layer of the article is subjected to thermal treatment. The fact that steel requires hardening, vaguely guess everything, even those who do not know what it is. It is this that differs from the usual "piece of iron," soft and brittle. But why is it superficial to use such honor? Hardening is used to change the properties of the metal, and not for the sake of some improvement, as it is declared very often. Quality, useful in some cases, becomes harmful in others. The file is hard, because it is easy for them to process iron, aluminum or bronze, but if you try to bend it or hit it with a hammer, it will crack. The same applies to the hacksaw blade, which often breaks at the wrong cutting angle. In order to give firmness in combination with flexibility or plasticity, surface hardening is used. After it, the properties of the product can combine qualities, sometimes opposite, characteristic of different crystal structures. Now we need to go into some material science details.

The simplest ideas about the polymorphism of metals

The same metal can, depending on the shape of the crystal lattice, have different physical properties (hardness, viscosity, ductility, flexibility, elasticity, etc.). This ability to change mechanical parameters is called polymorphism. Very long ago, when manufacturing primitive weapons, people noticed that this or that sword or sword turned out better, it lasts longer and does not break. Of course, our ancestors were not familiar with the molecular structures of the metal, to the point where they reached intuitively and empirically. So, by experience, they discovered that if the tip is heated, then its temperature depends on the shades of luminescence. With rapid cooling in the metal, something changes, it becomes either firmer or acquires flexibility. If it is reheated, it again becomes as it was, and sometimes worse. By that time, there were quite specific ideas about how, for example, an ideal hunting knife should be . Surface hardening, too, was used even then, but more often the so-called local, that is, one with a hard point, flexible - the middle of the blade, and part of the blade adjacent to the handle - plastic (let it bend a little, but not break).

What's going on inside

Without going into special details, it should be noted that the structure of hardened steel is of three main types: martensitic, troostite and sorbitol. The ratio of these crystalline formations determines the mechanical characteristics. In this case it does not matter which of them and how it affects the hardness. The result depends on how much metal is heated and how quickly it is cooled. Thus, surface hardening can occur with an increase in the temperature of the upper layer and subsequent cooling, or as a result of the release of heat to the external environment (liquid, most often oil, water and brine, air or other agents), or by partial removal of it inside the product. In this case, polymorphic transformations occur layer by layer, depending on the degree of achievement of the critical temperature, which influences the formation of a new crystalline structure.

As a result, there is a change in the following zones:

- Upper, hardened.

- Intermediate, hardened partially. It is also called the zone of thermal influence.

- Area of reduced hardness.

- The inside, not subjected to changes.

Methods of surface hardening

You can create a top layer with increased hardness in several ways. Railway wagon spring springs are simply fired with small metal balls (shot), creating a seal of the surface, while the internal volume of the metal remains sufficiently plastic to withstand long mechanical loads. The most ancient is considered a method of rapid heating of an object on an open fire, accompanied by spraying or jet flow. It is for this technology that the traditional eastern curved knife (kerambit) is made. Surface hardening can also be carried out by intensive cooling. Gas-plasma, induction, laser and other methods are also known. On some of them it is worth dwelling in more detail.

HDTV

In the mid-1930s, the Soviet scientist VP Vologdin invented a method of imparting a non-uniform molecular structure to large parts using high-frequency currents. Mechanical engineering developed rapidly, the industry needed technology that provided mass production without compromising quality. Surface hardening of HDTV is based on the phenomenon of induction. The peculiarity of the method lies in the dependence of the thickness of the heated layer on the frequency and the magnitude of the current in the emitting frame. In this case, the result is predicted with a high degree of probability, therefore, quality control is greatly simplified. In addition, the method is applicable for the processing of dimensional products and assemblies, like crankshafts and other large objects that can be moved along the inductor, consistently exposing the entire length. With this technology, it is difficult to select parameters to process small and flat objects, such as a knife. Surface hardening by high-frequency currents is applicable to products relatively voluminous, the strength and durability of which depends on the mechanical properties of the upper layer.

Features of HDTV application

The method was developed in the context of the rapid development of the machine-building industry, the main one for the defense potential of the USSR, which was manifested in the specifics of its application. The most important details of tractors, tanks, cars or airplanes have dimensions that do not allow them to be placed in the frame of a compact inductor, manufacturing equipment for each of them was too expensive, and if done in terms of the largest dimensions, the energy costs became enormous. However, induction surface hardening is used for any products, from relatively small to huge. For example, the gears are subjected to the HDTV in series, turning "tooth after tooth." Continuously progressive elements of the crankshaft and cardan shafts are heated, moving inside the fixed frame of the inductor, while the cooler (sprayer) is included in the process immediately after it. In the final unit of the installation, the sprayed water is immediately irrigated with a spray of water (hence the name that is consonant with the "spray").

Well, products with a small hardening surface are placed in the inductor entirely and are cooled in the same way.

Laser

This device, used in our time quite widely in various fields of human activity, found application in metalworking. The method does not require subsequent cooling, since the beam exposure is short-term and affects the uppermost layer of the metal, causing the desired changes in the crystal structure. "Laser sharpening" really provides for a long time the lack of the need to sharpen the cutting tool (mainly for them and is used), if this method is actually used in its manufacture. However, it should be borne in mind that in our counterfeit age the inscription on the product does not always correspond to the truth. Sometimes such a brand is decorated and sold on the street stall some cheap knife "butterfly". Surface hardening with a laser beam is not cheap, it is only available to leading tool manufacturers.

Cold

The physical basis of the method was the discovery of the phenomenon of increasing the hardness of steel as a result of the transition of the austenite structure to the martensitic structure during deep freezing. This surface hardening is carried out according to the method developed by AP Gulyaev, NA Minkevich, and SS Stenberg in the USSR. It is applicable for carbonaceous (containing more than 0.5 percent C) and alloy steel for special purposes, for example, made for the manufacture of high-speed cutters and other special tools.

Electroheating

In general, it is built on the same principle as induction hardening, with the only difference being that heating is produced resistively, due to the passing current of large values and the resistance of the part. The frequency of the applied voltage in exactly the same way affects the depth of the heated layer, and the higher it is, the thinner it is. The surface of increased hardness can range from fractions of a millimeter to several of its units. This depends on the requirements for the product and its dimensions. Compared with HD in the electroresistive method, a wider range of currents, temperatures and depths of layers. With the help of it can be made, for example, so massive and requiring a special quality object, like a soldier bayonet knife. Surface hardening by electric heating requires a technologically verified cooling regime in oil, water or other heat-receiving agents.

conclusions

So, the main task of surface hardening is the distribution of the crystal structure inside the product, in which there are varieties of sorbitol or troostite inside it, and a layer of martensite is formed on the outside. This can be achieved by several methods, from the simplest and oldest to the most technological and modern. In any case, high-quality steel hardening requires high qualification and accuracy of compliance with production regulations. The product manufactured according to all the rules can not be cheap. For this reason, it is expensive and good kitchen knife, and kerambit. Surface hardening by a laser beam is most common just for cutting tools.