UA54030A - A process for SURFACE hardening of LOW-HARDENING steels - Google Patents

Abstract

A process for surface hardening of low-hardening steels includes preheating the surface layers without fusion by arc with unfusible electrode. The heat penetration to be carried out for many times, and surface layers preheated to austenitic state temperature 1200-1250 °С are cooled at speed of 450-500 °С/s. Quantity of sequential preheatings depends on carbon diffusion path length lкр for previous steel ferrite-pearlite structure, which is determined depending on cube size A, calculated according the formula mm, wherein D is mean diameter of the pearlite spherical grain as conditioned by the grain point, mm; - pearlite volume part in steel which is defined with formula ; С – carbon percentage in steel; is equal to 3.14.

Description

Description of the invention

The invention relates to the field of metallurgy, namely, to heat treatment of steels and is used in 2 local processing (surface hardening) of low-carbon and low-alloy steels with an initial ferrite-pearlitic structure and low hardenability to obtain a hardened layer of high hardness at a depth of up to mm and a hardness of up to 400ОНМ .

It is known about the methods of thermal treatment of steels using local heating sources, which are based on the formation of an austenite structure during heating of the surface and its transformation into martensite during 710 subsequent cooling. A laser and an electron beam, an arc with non-fusible electrodes - tungsten or carbon - can be used as heating sources. Such local heating sources are characterized by a high concentration of energy and in most cases are used for fusion welding and thermal cutting of various metals and alloys. Their use in heat treatment of steels is used for surface strengthening of tools and machine parts that are subject to frictional wear | (Samogutyn S.S. 72 Plasma treatment of instrumental steels // Automatic welding. -- 1996. -- Mo. 8. -- p. 48 - 51.;

Dombrovsky Yu.M. Effects of plasma thermal surface treatment on the strength of steel structures // Welding production. -- 1999. -- Mo. 3, -- p. 14 - 16.; Berdnikov A.A., Filippov M.A., Studenok

E.S. The structure of hardened carbon steels after plasma surface heating // MYTOM. -- 1997. - M96.--p.2-44

It is better to use arc heating with a non-fusible electrode for surface hardening. The arc discharge has a larger section of the heating spot and a lower specific power compared to electron and laser beams, which allows to increase the time the metal stays at high temperatures during its heating-cooling process. At the same time, the cooling rates are still sufficient for the transformation of austenite by the martensitic mechanism. Recently, much attention has been paid to the processes of surface 22 plasma arc hardening of steels. This is due to the fact that of all sources of heating with concentrated energy flows, the use of a direct action plasma arc for heat treatment is better from both a technical and an economic point of view. With this method of hardening without remelting the steel surface, it is difficult to obtain a strengthened layer more than mm thick. Increasing the effective heating power beyond the limit inevitably leads to melting, which is undesirable for machined parts to size. By the method of special 30 o scanning with a plasma arc, it is possible to achieve the thickness of the hardened layer to a depth of Zmm. co

It is known about surface heat treatment methods mainly for medium and high carbon steels, as well as alloy steels with preliminary or subsequent volume heat treatment. The best analogue is the electric arc surface treatment of carbon and alloy steels with a non-fusible electrode. "AND

The closest in terms of technical essence is the method of plasma-arc surface treatment of carbon and 35 alloy steels without surface melting, which consists in surface treatment with an arc without scanning the arc: steel 20, o 45, U8 I - 200A, plasmatron movement speed m - 2cm/s , consumption of plasma-forming gas 7.5 l/min | Influence of plasma-arc processing on structural transformations and surface hardening of carbon and alloy steels. / Stavrev D.S., Kaputkina L.M., Kirov S.K. etc./ Metal science and thermal "metal processing". -- 1996. -- Mo. 9, -- P. 16 - 19). Plasma-arc surface treatment makes it possible to increase - 70 wear resistance due to the formation of a structure characterized by significant grinding of austenite grain, c rails, packets of martensite plates, the presence of small carbides of the RezC, Re(Sg)3C, StozSv type. However, this method is applied to medium, high-carbon and alloyed steels and is not applicable to steels with low hardenability, because it does not ensure the production of surface layers of high hardness. Hardening of steel occurs after its rapid cooling from a temperature that ensures partial or complete dissolution of 45 carbides. A fairly rapid cooling of the heated surface layer is ensured by changing the speed and movement of the heat source within wide limits, since it is determined by the high speed of heat removal into the cold, unheated, metal. Along with this, the dissolution of carbides requires a certain time, which depends on the heating temperature, the type of carbides, and the nature of the structure in the initial state. o The invention is based on the task of developing a method of surface hardening of steels with low

Ge) 20 hardenability, in which, when changing the processing mode, an increase in strength is achieved in the surface layers of low-carbon and low-alloy steels with an initial ferrite-pearlite structure, which will increase the wear resistance of machine parts and structures made of such steels, and also makes it possible in some cases abandon the use of more expensive alloy steels of increased hardenability.

The problem is solved by the fact that in the method of hardening steels with low hardenability, which includes heating the surface layers without arc melting with a non-fusible electrode, according to the invention, heating in are carried out repeatedly, and the surface layers, heated to the temperature of the austenitic state of 1200 - 12507С, are cooled at a rate of 450 - 500"C/s. The number of consecutive heatings depends on the length of the carbon diffusion path Ikr for the original ferrite-pearlite structure of the steel, which is determined depending on the size of the cube A, calculated according to the formula "E: mm, where OO is the average diameter of a spherical pearlite grain, 60 AB, and is determined by the grain score, mm; p - volume fraction of pearlite in steel, determined by the formula p - 129.90. C - 2.99; C - percentage of carbon in steel; x is a constant equal to 3.14.

When hardening low-carbon steels with an initial ferrite-pearlite structure by heating without melting the surface with an arc with a non-fusible electrode, it was established that a significant increase in the hardness of the surface layers occurs with a uniform distribution of carbon in austenite. This can be achieved when the boundaries of the carbon diffusion path from the location of the former pearlite grains to the former ferrite grains overlap. A necessary condition for obtaining a high hardness of the surface layer of low-carbon steels with an initial ferritic-pearlitic structure is to reach a state during heating when the carbon concentrated in the pearlite grains has time to distribute itself throughout the austenite volume. This is achieved due to the high cooling rate, which prevents the pearlitic transformation into steel. The cooling rates during local heating without melting the steel surface by an arc with a non-fusible electrode are quite sufficient to transform such austenite into martensite. Since the residence time of austenite above the temperature of the transformation of ferrite into austenite (temperature Az) with such a single heating is insufficient for such a 7/0 redistribution of carbon in austenite, then for the successful surface hardening of low-carbon steels, either the selection of steel with a very fine grain of ferrite and pearlite is required, or additional preparation of the surface layers of coarse-grained steel is required, which ensures a more uniform distribution of carbon-intensive structures. It is possible to achieve a more even distribution of carbon in the volume of steel by additional heating without melting the surface of low-carbon steel followed by rapid cooling. At the same time, the cooling mode /5 should ensure the beginning of the transformation of austenite, in which the concentration of carbon exceeds its concentration in the former grains of ferrite, into bainite or martensite. The number of such warm-ups for a given steel can be determined theoretically when, during the final hardening to martensite, a hardness value is reached in the surface layer, close to the hardness of martensite at the average value of the carbon content in the steel.

Figures 1 - 4 show the microstructures of the base metal (Figure 1) and surface hardened layers after one-time (Figure 2), two-time (Figure 3) and three-time heating (Figure 4) by an arc with a carbon non-fusible electrode at a driving energy of X00J/ see The microstructure was photographed at a distance from the surface where the hardness was determined. These pictures of the microstructure of the strengthened surface layers of St.3 steel were obtained on a raster electron microscope-microanalyzer RZMMA-102 at a magnification of 1500 times.

The implementation of the method of surface hardening is shown in the following example. Verification of the above provisions was carried out by repeatedly heating the surface of St. Z steel with an arc with a carbon non-fusible electrode on a direct current of direct polarity with one arc in the mode I - 85A, Od - 218, Mu - 2cm/s with a heating efficiency equal to 0.55 at a running energy of ЧО00J/cm. The table below presents data on the hardness of heated surface layers depending on the number of heatings. "c) from 0 bsnovny metal disposable progov| two-time warm-up Three-row rubbing Fo

The depth of the cutting edge mm 000030000000000210000002 ovo o «

One-time heating of the surface followed by air cooling of this steel leads to an increase in | ! ; ! ! And) hardness at a depth of 0.2 mm up to 27 NM (Fig. 2), which is 120 NM higher than the hardness of the base metal. Hardness was measured by Vickers on a TPK-10 hardness tester with a load of 10Okg. Double heating due to longer residence time of austenite in the temperature range, above the critical temperature Az, increases the path of carbon diffusion in the steel and leads, after cooling, to an increase in the hardness of the surface layer at a depth of 0.2 mm already to ZVONM « du (Fig. 3). The use of three-time heating increases the hardness of St.Z steel at a depth of О.Змм to 40ООНМУ, and from at a depth of ОБММ - to ЗбОНМ. This hardness of the surface layer is close to the maximum possible for martensite with a carbon content close to its average composition in the studied steel. From the comparison of the microstructures, it can be seen that as the number of heatings increases, there is an increase in the needle-like component of martensite. After three times of heating in the structure of the surface layer at a depth of 0.3 mm, almost complete hardening of the steel to martensite is achieved (Fig. 4). p. 15 The application of the method of surface hardening of low-carbon steels with an initial ferrite-pearlite structure can be implemented both for flat surfaces and for surfaces of cylindrical parts. t. The developed method of surface hardening of steels by heating them without melting with an arc with a non-fusible electrode expands the range of steels that are hardened by hardening without changing the technology of their heating. The difference is that for medium-carbon steels, the strengthening of the surface layers (95) 50 by quenching is achieved with a single heating, and for low-carbon steels with an initial ferrite-pearlite structure, multiple heating. In the latter case, the multiplicity of heating is determined by the specific carbon content in low-carbon steel and its grain score.

Strengthening surface treatment by heating with an arc with a non-fusible electrode can be applied not only during the manufacture of parts and structures, but also after restoration by surfacing of worn parts. At the same time, surfacing can be carried out with low-carbon and low-carbon low-alloy steels. bonding wires, which ensure high resistance of the deposited metal to the formation of hot and cold cracks. The ferrite-pearlite structure of the deposited metal obtained when using such welding wires can be strengthened in the surface layer with the use of additional mechanical heating of the treated surface of the part with an arc with a non-fusible electrode.

Claims (2)

The formula of the invention 1. The method of surface hardening of steels with low hardenability, which includes heating the surface layers without remelting by an arc with a non-fusible electrode, which is characterized by the fact that the heating is carried out repeatedly, and the surface layers, heated to the temperature of the austenite state of 1200-1250 "С, are cooled at a speed 450-500 7С/. 2. The method according to claim 1, which differs in that the number of successive heatings depends on the length of the carbon diffusion path fr for the previous ferrite-pearlite structure of the steel, which is determined depending on the size of the cube A, calculated according to the formula x mm, where O is the average diameter of spherical pearlite grain, A-ROI-, 6 due to grain score, mm; B - the volume fraction of pearlite in steel, which is determined by the formula зо в-12990:7-9а; C - percentage of carbon in steel; x is equal to 3.14. « «c) so «c) « IS c) - with . and? 1 ch" (c) o 50 (42) 60 b5