What is surface hardening of steel? What is surface hardening used for?

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

The art of heat treatment of metals has been known to mankind for a long time. The craftsmen involved in the manufacture of tools, and especially weapons, mastered it on their own, or studied for many years with other more experienced specialists. Secrets were kept secret, which, of course, slowed down the spread of technology, but increased the competitiveness of a particular manufacturer of products for a specific purpose. One of the techniques of medieval armorers was surface hardening, which gives the cutting edges and points of swords and sabers a special hardness, combined with the flexibility of the blade. Today, such properties no longer surprise anyone, technologies have become massive and have become widespread.

Why would an ordinary person know all this?

This article to specialists in thermal metalworking will most likely seem like a set of platitudes and well-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,those who are interested in how an ordinary table or folding knife differs in strength from a good blade, surface hardening from volume hardening, and similar questions. When buying one or another item needed in the household, the consumer is faced with a significant price difference. The seller cannot always explain in a qualified and understandable way why one tool (a wrench, for example) is much more expensive than another with a general external similarity. He, most likely, will try to “powder his brains” with concepts and terms that are incomprehensible to the average layman. Translated into ordinary language, these explanations will mean that the adjustable wrench will not break or last longer, and sharpening will be required much less often (if the client wants to purchase a knife). "Surface hardening!" - the seller will mysteriously indicate the reason, rolling his eyes with imaginary delight. What is it?

Opposite properties in one product

As is clear from the phrase, in this case only the outer thin layer of the product is subjected to heat treatment. The fact that steel requires hardening is vaguely guessed by everyone, even those who do not know what it is at all. This is what distinguishes it from the usual "piece of iron", soft and brittle. But why is it the superficial that enjoys such honor? Hardening is used to change the properties of the metal, and not for the sake of some kind of improvement, as is often declared. 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 itor 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 impart hardness combined with flexibility or ductility, surface hardening is applied. After it, the properties of the product can combine qualities, sometimes opposite, characteristic of different crystal structures. Now we need to delve 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. A long time ago, when making primitive weapons, people noticed that one or another sword or cleaver turned out to be more successful, it retains its sharpness longer and does not break. Of course, our ancestors were not familiar with the molecular structures of the metal, they came to everything intuitively and empirically. So, empirically, they discovered that if the tip is heated, then its temperature depends on the shades of the glow. With rapid cooling, something changes in the metal, it becomes either harder or becomes more flexible. If it is reheated, it becomes the same again, and sometimes even worse. By that time, quite specific ideas were formed about what, for example, an ideal hunting knife should be. Surface hardening was also used then, but more oftenthe so-called local was used, that is, one in which the point was solid, the middle of the blade was flexible, and the part of the blade adjacent to the handle was plastic (let it bend a little, but not break).

What's going on inside

Without going into particular details, it should be noted that the structure of hardened steel is of three main types: martensitic, troostite and sorbitic. The mechanical characteristics depend on the ratio of these crystalline formations. In this case, it does not matter which of them and how affects the hardness. The result depends on how hot the metal is and how quickly it cools. Thus, surface hardening can occur with an increase in the temperature of the upper layer and subsequent cooling, either as a result of heat transfer to the external environment (liquids, most often oil, water and brine, air or other agents), or due to its partial escape into the product. In this case, polymorphic transformations occur in layers, depending on the degree of reaching the critical temperature, which affects the formation of a new crystal structure.

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

- Upper hardened.

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

- Area of reduced hardness.

- Unmodified interior.

Surface Hardening Methods

Create a top layer withincreased hardness, in several ways. Railroad car springs are simply shot at with small metal balls (shot) that create a surface seal, while the internal volume of the metal remains plastic enough to withstand long-term mechanical stress. The most ancient method is considered to be the rapid heating of an object on an open fire, accompanied by spraying or jet flow. It is by this technology that a traditional oriental curved knife (karambit) is made. Surface hardening can also be carried out by means of intensive cooling. Gas-plasma, induction, laser and other methods are also known. Some of them are worth dwelling on.

HDTV

In the mid-1930s, the Soviet scientist V. P. Vologdin invented a method for imparting a given non-uniform molecular structure to large parts using high-frequency currents. Mechanical engineering developed rapidly, the industry needed technologies that ensured mass production without sacrificing 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 magnitude of the current in the radiating loop. In this case, the result is predictable with a high degree of probability, therefore, quality control is greatly simplified. In addition, the method is applicable to the processing of overall products and assemblies, such as crankshafts and other large objects that can be moved along the inductor, sequentially.exposing the entire length. With this technology, it is difficult to choose the parameters to process small and flat objects, such as a knife. Surface hardening with high frequency currents is applicable to relatively bulky products, the strength and wear resistance of which depend on the mechanical properties of the top layer.

Features of using the HDTV method

The method was developed in the conditions of 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 parts of tractors, tanks, automobiles or airplanes are not large enough to be placed in the frame of a compact inductor, it was too expensive to manufacture equipment for each of them, and if it was made based on the largest dimensions, then the energy costs became enormous. However, induction case hardening is applied to any products, from relatively small to huge ones. For example, gears are exposed to HDTV sequentially, turning tooth by tooth. The elements of the crankshafts and cardan shafts are continuously and sequentially heated, moving inside the fixed frame of the inductor, while the cooler (sprayer) is included in the technological process immediately after it. At the end of the machine, the workpiece is immediately sprayed with water (hence the name, consonant with “spray”).

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

Laser

This devicein our time, used quite widely in various fields of human activity, has found application in metalworking. The method does not require subsequent cooling, since the impact of the beam is short-term and it affects the uppermost layer of the metal, causing the desired changes in the crystal structure. "Laser sharpening" really ensures that there is no need to sharpen the cutting tool for a long time (it is mainly used for them), if this method is really 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 some cheap “butterfly” knife sold at a street stall is also decorated with such a brand. Surface hardening with a laser beam is an expensive technology, it is available only 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 austenitic structure to martensitic during deep freezing. Such surface hardening is carried out according to the method developed by A. P. Gulyaev, N. A. Minkevich and S. S. Shtenberg in the USSR. It is applicable to carbon (containing more than 0.5 percent C) and alloy steel for special purposes, such as those produced for the manufacture of high-speed cutters and other special tool products.

Electric heating

In general, it is built on the same principle as induction hardening, with the only difference that the heating is resistive, due topassing current of large values and the resistance of the part. The frequency of the input voltage in 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. It depends on the requirements for the product and its dimensions. Compared to HDTV, the electroresistive method has a wider range of currents, temperatures, and layer depths. With the help of it, for example, such a massive and requiring special quality item as a soldier's bayonet-knife can be made. 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 such a distribution of the crystal structure inside the product, in which varieties of sorbite or troostite remain inside it, and a layer of martensite is formed outside. This can be achieved by several methods, from the simplest and most ancient to the most technologically advanced and modern. In any case, high-quality hardening of steel requires high qualification and accuracy in compliance with production regulations. A product made according to all the rules cannot be cheap. For this reason, both a good kitchen knife and karambit are expensive. Surface hardening with a laser beam is most common just for cutting tools.