What is surface hardening of steel? What is surface hardening?

The Art of heat treatment of metals known to mankind for a long time. They are masters in manufacturing of tools, especially weapons, possessed by themselves or were trained for years by other specialists, more experienced. Secrets were kept secret, which of course, reduces the speed of spread of technology, but increased the competitiveness of a specific manufacturer's product-specific destination. One of the methods of medieval armourers were surface hardening, giving the cutting edges and the points of swords and sabers special hardness in combination with flexibility of the blade. Today such properties is no surprise, technologies have become widespread and was widely spread.

What is all this know to the ordinary person?

This article is the specialists in thermal metal processing seem likely to set platitudes and all of the known facts. In addition, they may find some inaccuracies in terminology. The information provided is not for them, it is addressed to people far from the industry, Amateurs who are interested in what is different in strength from a good ordinary table blade or folding knife, surface hardening of the bulk, and similar issues. Buying one or the other need in the household item, the consumer is faced with a significant price difference. The seller may not always competently and clearly explain why one tool (a wrench, for example) is much more than another when the General external similarity. He is likely to try and "bullshitting" incomprehensible to the ordinary layman concepts and terms. Translated into ordinary language, these explanations would mean that the wrench will not break or last longer, and sharpening will be required much less frequently (if a customer wants to purchase the knife). “Surface hardening!” - mysteriously indicate why the seller, rolling his imaginary eyes of delight. What is it?

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Opposite properties in one product

As is clear from the phrase, heat treatment in this case, subject only the outer thin layer of the product. That steel requires hardening, are all dimly aware, even those who do not know what it is. It differs from the usual “hardware”, soft and brittle. But why such a honor is it superficial? Annealing is used to change the properties of the metal, and not for any improvement, as it is declared very often. Quality, useful in some cases and is detrimental in others. File solid, because they are easy to handle iron, aluminum or bronze, but if you try to bend or hit it with a hammer, it will crack. The same applies to the saw blade, often breaking down at the wrong angle cut. In order to impart hardness in combination with flexibility, or plasticity, applied surface hardening. After it properties products can combine the quality, sometimes the opposite characteristic of different crystal structures. Now we need to delve into some material details.

Simple notions of polymorphism of metals

One and the same metal can, depending on the shape of the crystal lattice, possess different physical properties (hardness, viscosity, malleability, flexibility, elasticity, etc.) This ability to change mechanical parameters is called polymorphism. A very long time, making primitive weapons, people noticed that a sword or cleaver turned out better, it retains its sharpness longer and not break. Of course, our ancestors were not familiar with the molecular structures of the metal, until they reached intuitively and empirically. So, empirically, they discovered that if you heat the tip, then from shades of light depends on its temperature. At fast cooling in metal something changes, it becomes either harder or gaining flexibility. If it is re-heated, it is again exactly the same, and sometimes worse. By the time formed very specific ideas about what should be, for example, an ideal hunting knife. Surface hardening is also used even then, but more often used the so-called local, that is, in which solid was the cutting edge, flexible - the middle of the blade and of the blade adjacent to the handle – plastic (better a little bent, but not broken).

What's going on inside

Without going into much detail, it should be noted that the structure of hardened steel there are three main types: martensitic, troostite and sorbitol. The ratio of these crystal structures and mechanical properties depend. In this case, no matter which of them and how it affects the hardness. The result depends on how warmed up the metal and how fast it cooled. Thus, the surface hardening can occur with increasing temperature of the upper layer and subsequent cooling either by heat emission of the external environment (fluid, most often oil, water and brine, air or other agents) or bypartial leaving it inside the unit. While polymorphic transformation occurs in layers, depending on the achievement of critical temperatures influencing the formation of a new crystalline structure.

In the end, there is a change in the following areas:

- Upper, subjected to hardening.

- the intermediate, partially hardened. It is also called the heat affected zone.

the Area of low hardness.

- the Inner part, which was not amended.

Methods of surface hardening

To Create a top layer with increased hardness in several ways. The railway wagon plate springs just fired small metal balls (shot) that creates a seal surface, wherein the internal volume of the metal remains ductile enough to withstand prolonged mechanical stress. The most ancient method is the rapid heating of the subject on an open fire, accompanied by a spray or jet flow. Using this technology made the traditional Eastern curved knife (karambit). Surface hardening may be performed by intensive cooling. Also known gas-plasma, induction, laser and other methods. Some of them is to dwell.

HDTV

In the mid 30-ies of the Soviet scientist V. P. Vologdin invented a method of making specified-the uneven molecular structure of large parts using high frequency currents. Engineering developed rapidly, industry was required technology for mass production without compromising quality. Surface hardening with high frequency current based on the phenomenon of induction. Feature of the method is based on the thickness of the heated layer on the frequency and amount of current in the radiating frame. The result with a high degree of probability of the predicted, therefore, quality control is greatly simplified. Besides, the method is applicable for the processing of dimensional products and components, like crankshafts and other large items that can be moved around the inductor, sequentially exposing the effects of full length. According to this technology, it is difficult to find options to handle small and flat objects such as a knife. Surface hardening by high frequency currents applicable to the products on a relatively large scale, the strength and durability of which depends on the mechanical properties of the upper layer.

Features of application of method HDTV

The Method was developed in the rapid development of the engineering industry, important for the defense potential of the USSR, which found manifestation in the specifics of its application. The most important items of tractors, tanks, cars or planes have dimensions that do not allow to place them in the frame of a compact inductor, manufacturing equipment for each of them was too expensive, and if it is to make, based on the largest dimensions, the energy expenditure was huge. However, induction surface hardening is applied for all products, from relatively small to huge. For example, gears are exposed to high frequency sequentially turning “a tooth for a tooth». Continuously-consistently heated elements crankshafts, cardan shafts moving inside a stationary frame of the inductor, wherein the cooler (sprayer) are included in the process immediately following it. In a leaf node setup is immediate irrigation of the workpiece in the spray (hence the name that sounds similar to the "spray").

And products with short surface hardening are placed in the inductor as a whole and cooled as well.

Laser

This device, nowadays used widely in various fields of human activities, has found application in metal processing. The method does not require subsequent cooling, because the effects of the beam short and it affects on the upper layer of the metal, causing desirable changes in the crystalline structure. “Laser-sharpening” really delivers long-no need to make jokes cutting tool (mostly for them and applied), if the manufacture of it really applied to this method. Note, however, that our counterfeit century inscription on the product is not always true. Sometimes the stigma is decorated and sold in a street stall of some cheap knife-“butterfly”. Surface hardening by laser beam – technology is not cheap, it's available the leading manufacturers of tool.

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 austenitic structure to martensite during deep freezing. Produced such a surface hardening method, developed by A. P. Gulyaev, N. A. Minkevich and S. S. Sternberg in the USSR. It is applicable for carbon (containing over 0.5 percent) and alloy steel for special purposes, for example, produced for the manufacture of high-speed cutting tools and other special tool products.

Electric heating

In General andgenerally it is built on the same principle as induction hardening, with the only difference that is made by resistive heating, by passing a current of large value and the resistance of the part. The frequency of the applied voltage similarly affects the depth of the heated layer, and the higher it is, the thinner. The surface of high hardness can range from fractions of a millimetre to several units. It depends on the requirements of the product and its size. Compared to HDTV from electroresistive method over a wide range of currents, temperatures and depths of the layers. Using it can be manufactured, for example, such a mass and requiring particular qualities of a piece, like a soldier's bayonet. Surface hardening by electric heating requires a technologically superior cooling in oil, water or other teploperenosa agents.

Insights

So, the main objective of surface hardening is that the distribution of the crystalline structure inside the product, where within it are the species of sorbitol or troostite, and outside, a layer of martensite. This can be achieved in several methods, from the simplest and oldest up to the most technologically advanced and modern. In any case, high-quality hardening steel requires high skill and accuracy of compliance with production regulations. The product made according to the rules, can not be cheap. For this reason, expensive and good kitchen knife and karambit. Surface hardening by laser beam is most common for cutting tools.