We propose to introduce a progressive method of surface plasma hardening, which increases the resistance and durability of tools, rolling rolls and machine parts for various purposes.
1. The essence of plasma hardening
Low-temperature plasma (5000...50000 0 K) is a concentrated energy source and is increasingly used for surface hardening of machine parts and tools made of various alloys.
The essence of plasma hardening is local heating of a surface area at rates of 10 3 ...10 4 0 C/s to high temperatures, followed by cooling at a supercritical speed due to heat removal into the internal layers of the product. In this case, a specific finely dispersed structure with high performance characteristics is formed.
2. Advantages of plasma hardening
When hardening with concentrated energy sources, due to the specificity of the treatment (high heating and cooling rates), it is possible to obtain a structure and properties of the surface layer that are unattainable with traditional methods of heat treatment.
The main advantages of plasma hardening:
The heating is localized, only the surface layer is strengthened, and the core remains viscous, which leads to increased resistance to wear and fatigue;
High hardness and wear resistance of the surface;
Absence or minimal deformation of hardened parts, which makes it possible to increase the accuracy of their manufacture, reduce the labor intensity of machining and the cost of manufacturing parts;
High productivity - 2 - 9 m 2 /hour;
When hardening without surface melting, subsequent mechanical processing (grinding) is not required, i.e. plasma hardening can be used as a finishing operation;
The presence of compressive stresses in the surface layer and large quantity retained austenite (steel, cast iron) increases resistance to crack initiation and propagation;
Hardening is carried out in most cases without forced cooling, i.e. no cooling media or accessories are required.
Like other concentrated energy sources (laser, electron beam), plasma has some new capabilities:
Possibility of replacing scarce high-alloy steels with low-alloy steels, strengthened by plasma hardening;
Possibility of replacing wear-resistant steels with low-carbon steels with a deposited working layer, strengthened by plasma hardening;
Possibility of hardening local areas of the surface (edges of circular knives, cutting and bending dies, saw teeth, electric and chainsaw tires, places for cuffs, bearings, fragments of engravings of stamps and calibers of rolling rolls, etc.);
Possibility of process automation and inclusion of hardening units in flexible production systems and automatic lines.
Compared to laser hardening, plasma hardening has the following advantages:
The cost of equipment of the same power is an order of magnitude lower;
Ease of operation of the installation and its maintenance, i.e. no highly qualified service personnel required;
Mobility of the installation, i.e. the ability to move equipment and quickly install it on any machine that provides the required speed of rotation of the part or movement of the plasma torch;
It is not necessary, as with laser hardening, to apply special coatings to the surface to increase the absorption of laser radiation;
High efficiency, reaching 85%;
Possibility of smooth regulation of mode parameters within a wide range during the hardening process, i.e. changes in the depth, width, structure and properties of the hardened zone.
The disadvantages of plasma hardening include:
Partial tempering in places where hardened strips are applied;
The need to clean the surface of hardened products from various contaminants (scale, rust, oil);
The need for forced cooling of products of small diameter and small thickness to obtain high surface hardness.
3. Plasma hardening equipment and technology
The plasma hardening installation consists of:
Plasmatron (or several plasmatrons);
Power supply;
Oscillator for igniting the plasma arc;
Control panel with instrumentation;
A machine, rotator or manipulator that provides the operating speed of movement of the plasma arc relative to the surface of the product being hardened;
Devices for mounting and adjusting movements of the plasma torch;
Water supply systems for cooling plasmatron components;
Gas supply systems for supplying plasma-forming gas or a mixture of gases.
The main executive body is a plasmatron, in which low-temperature plasma is generated.
Hydrogen, nitrogen, carbon dioxide, air, argon, helium or mixtures thereof are used as plasma-forming gas. In this case, the thermophysical characteristics of the plasma change.
As power sources, you can use specialized welding rectifiers with increased open circuit voltage or conventional welding rectifiers such as VD-306, VDU-504, etc.
The choice of the type and design of the plasma torch, plasma-forming gas and power source are interconnected and depend on the specific task at hand. The power of the installation can be different and ranges from 5 to 50 kW. The installation capacity is up to 2.5 m 2 /hour, depending on the required depth and degree of application of the hardened strips.
Before hardening, the surface of the product is cleaned of contaminants. The hardening process after ignition of the arc occurs when the plasma arc (jet) moves relative to the surface of the product being hardened, which can be carried out in various ways: the part is fixed, the plasma torch moves; the part moves (rotates), the plasma torch is fixed; Both the part and the plasmatron move.
For example, hardening of cylindrical parts is carried out, as a rule, along a helical line, which is achieved by simultaneously rotating the part and moving the plasma torch along the rotation axis. When hardening the entire surface of the product, hardened strips are applied overlapping. To obtain a uniform layer depth and hardness distribution over the surface, the degree of overlap (overlap) is chosen within the range of 45...55%.
The main parameters of the plasma hardening mode, which are established on the basis of studies of prototypes or selected during the hardening process, are:
Linear speed of movement (0.5...6 cm/s);
Plasma arc current (50...1,000 A);
Arc voltage (20...200 V);
Distance from the plasma torch nozzle to the surface of the product (2...100 mm);
4. Some characteristics of the strengthened layer
Geometric characteristics include the depth and width of the plasma impact zone (PLZ). They depend on the parameters of the hardening mode, the thermophysical properties of the alloy being hardened and its structural state.
When plasma hardening with a direct and indirect arc without melting the surface, the depth of the zone can be changed within the range of 0.1...1.8 mm and up to 5 mm, respectively. The width of the zone can be adjusted within 1...40 mm. A larger width of the zone can be obtained by scanning the arc or transverse oscillations of the plasma torch. To obtain greater depth, hardening is carried out with melting, but additional mechanical processing is required, which is not always advisable.
It should be noted that even in modes in which there is no visible melting of the surface of the hardened product, a change in the microrelief occurs: the arithmetic mean deviation of the profile R a decreases, the height of microroughnesses R z decreases, the radius of curvature of the vertices r increases, i.e. Micro-melting of the tops of the irregularities occurs. This has a beneficial effect on changes in roughness parameters and not only increases the hardness of the surface, but also increases its load-bearing capacity and improves the performance properties of hardened products.
5. Materials strengthened by plasma hardening
Plasma hardening from the solid state, i.e. without melting, they are mainly applied to steel, cast iron and titanium alloys. When hardening from a liquid state, i.e. with surface melting, some aluminum and copper alloys are added to these materials.
The hardness values obtained during hardening without melting can vary within wide limits and are in HRC units e:
For low-carbon steels - 32...40;
For medium carbon steels - 52...60;
For cast iron - 50...60.
Hardness and degree of hardening depend primarily on the carbon content. Other factors also have an influence: alloying elements (chemical composition, steel class), the number and shape of graphite inclusions in cast iron, cooling conditions (weight of products, degree of overlap of stripes, presence of cooling media, etc.).
When plasma hardening with melting of steels with a carbon content > 0.4% and cast irons, the hardness is higher. However, it should be noted that in this case the plastic properties deteriorate and the tendency to cracking increases.
According to literary sources and the results of research conducted by employees of the Plasma Laboratory of the Nizhny Tagil branch of USTU-UPI (headed by Berdnikov A.A.) and the Problem Laboratory of Metallurgy of Yekaterinburg USTU-UPI (headed by Prof. Filippov M.A.), plasma hardening can be strengthened with high efficiency:
Carbon structural steels (45, Art. 4, etc.);
Structural low-alloy steels (38ХС, 40Х, 30ХГСА, etc.);
Low-carbon steels of varying degrees of alloying after carburization (20, 12ХН3А, 20Х2Н4А, etc.);
Spring steels (50HFA, 65G, etc.);
Die steels (4Kh5FMS, 5KhNM, etc.);
Roller (50, 60ХН, 9Х, 9Х2МФ, 150ХНМ, etc.);
Carbon instrumental (U8, U10, etc.);
Gray cast iron (with flake graphite);
Malleable cast iron (with flake graphite);
High-strength cast irons (with spherical and vermicular graphite);
Etc.
6. Examples of the effective use of plasma hardening
A) NTMK, RBC and TsPSHB; 1985-1988, N. Tagil.
Hardening of roller straightening machine parts: bandages, rollers, faceplates made of steels 40Х, 34ХН1М, 5ХНМ. More than 700 parts were hardened.
Technical effect: increase in hardness from HB 340...420 to HRC 54...60; increase in durability by 2.5-3 times. Hardening of overhead crane ramps made of steel 38ХГН. 16 pieces hardened. Technical effect: increase in hardness from HB 360 to HRC 53...55.
b) VSMPO, 1989, Verkhnyaya Salda.
Hardening of large dies with complex engravings for semi-hot stamping of titanium. Material - die steels 5ХНМ, 5ХНВ after volumetric hardening and tempering. After several regrinds, the hardened working layer is removed and a non-heat-treated core remains.
Technical effect: increase in hardness from HB 280...380 to HRC 60...63, increase in resistance by 25...100%.
B) Vysokogorsk Mechanical Plant, 1988-1992, N. Tagil.
Hardening of teltomat guides Æ 100 mm, length 2600 mm, steel 45.
Technical effect: increased hardness from HB 420 to HRC 52...54, minimal drives (0.16...0.22 mm), improved grindability.
Hardening of shafts, axles, bearing seats, edges of flat guides and other parts (14 items) made of low-alloy structural and spring steels.
Hardening of saw bars for ELPI electric saws. Over 1000 pieces hardened, steel 7ХНМ. Technical effect: increase in hardness from HRC 41...43 to HRC 59...61.
An installation for plasma heating of hollow copper tubes for high-performance winding of inductors of various sizes was developed, manufactured and implemented at VSW.
d) UVZ, 1991, N. Tagil.
A plasma hardening installation for parts made of structural alloy steels (4 items) has been introduced.
Technical effect: increase in hardness from HB 280...380 to HRC 50...58.
e) Rezhevsky Mechanical Plant, 1990-1991, Dir.
A plasma hardening installation for cylindrical parts Æ 60...150 mm made of structural low-alloy steels has been introduced.
Technical effect: increase in hardness from HB 240...280 to HRC 50...54.
f) Lysva Metallurgical Plant, 1990...1992, Lysva.
2 plasma hardening installations have been introduced for hardening various parts (6 items) made of structural carbon and low-alloy steels.
Technical effect: minimal leads, increasing hardness from HB 260...380 to HRC 50...56.
g) Serov Metallurgical Plant, 1989-1992, Serov.
Hardening of hot rolling rolls of crimping, roughing and semi-finishing stands for rolling circles 180...200, rhombic and hexagonal rolled stock. Roll material - steel 70L, 150Х2Г2НМ.
Technical effect: increased hardness up to HRC 52...56, increased durability of rolls by 20...80%, reduced tendency to form a coarse mesh.
Roller hardening cold rolling for the production of hexagons. The roll material became 9HF after volumetric hardening and low tempering.
Technical effect: increase in hardness from HRC 54...58 to HRC 61...63, increase in roller life by 15...20%.
h) Kachkanarsky GOK, 1999-2000, Kachkanar.
A stationary installation has been introduced for plasma hardening of tire flanges of diesel locomotives, electric locomotives and traction units. As of 2003, more than 1000 bandages have been hardened.
Technical effect: increase to HRC 50...54, increase in durability by 25% compared to high-frequency hardening and 2.0...2.5 times compared to bandages in the as-delivered condition.
i) Krasnouralsk copper smelter, 1998...2001, Krasnouralsk.
Hardening of large-module drive helical gears for mills. Gear material - steel 40X and 45.
Technical effect: increase in hardness to HRC 52...56 and durability by 2.2-2.8 times.
j) NTMK, crimping, large-section, rail and beam shops 1995...2009, N. Tagil.
Hardening of steel and cast iron rolls for rolling channels, angles, circles, squares, rails, center beams, circles. More than 8,000 rolls weighing from 7 to 34 tons were hardened.
Technical effect: increasing the durability of rolls up to 80%, reducing specific consumption by 25...45% kg/t depending on the stand and the rolled profile. Actual Savings 3 - 9 rub. for 1 rub. costs.
k) NTMK, 2000-2009.
Hardening of roller straightening machine tires made of steel 45, 45 XNM for straightening long rolled products. More than 650 bandages have been hardened.
Technical effect: increase in hardness to HRC 52...56, increase in resistance by 1.6-3.1 times.
l) JSC "Gornozavodsktransport", Gornozavodsk, 2003.
A mobile (portable) installation for plasma hardening of diesel locomotive tire flanges has been introduced.
Technical effect
n) JSC "Karelian Okatysh", Kostomuksha, 2004; JSC "Mikhailovsky GOK", Zheleznogorsk, 2006; OJSC "Lebedinsky GOK", Gubkin, 2006
A stationary installation for plasma hardening of flanges and tires of diesel locomotives based on the KZh-20 machine was introduced.
Technical effect: increase in surface hardness to HRC 50...54, increase in durability by 2.0-2.5 times compared to bandages as delivered.
n) OJSC "URALASBEST" (Asbest, Sverdlovsk region 2007) A stationary installation for plasma hardening of flanges and tires of diesel locomotives based on the KZh-20 machine was introduced.
Technical effect: increase in surface hardness to HRC 52...58, increase in durability by 1.8-2.5 times compared to bandages as delivered.
p) OJSC "NTMK" (Nizhny Tagil, 1995-2010) Plasma hardening of hot rolling rolls in OTs-1 (until 1999), in KSC and RBC on an ongoing basis under annual contract agreements.
Technical effect: increased durability, operating time and reduced specific consumption of rolls by 1.2-1.6 times.
Thus, plasma hardening has been introduced at many machine-building, metallurgical enterprises, and mining and processing plants in Russia. Stationary mobile installations for plasma hardening of machine parts and tools on screw-cutting lathes, surfacing lathes, rotary lathes, roll lathes and roll grinders were designed, manufactured and installed. The installations are equipped with plasma torches of our own design for arc hardening of direct and indirect action, with and without arc scanning, including manual plasma torches for hardening of local areas of machine parts and tools.
Plasma surface hardening, as one of the methods of hardening with heating sources with a high power density, is currently used in both small-scale and single, and large-scale and mass production. Its essence lies in thermal phase and structural transformations that occur during rapid concentrated heating of the working surface of a part by a plasma jet and subsequent heat removal deep into the part.
For technological purposes, low-temperature plasma is used, which is a partially ionized gas and has a temperature of the order of 10 3 ... 10 s K. The mechanism of plasma formation, the properties and parameters of the plasma jet depend on the type and properties of the plasma-forming medium, which can be single-component or multi-component. Argon, helium, nitrogen and hydrogen are used as a single-component plasma-forming medium. The following mixtures are used as multicomponent mixtures: argon and hydrogen, argon and helium, nitrogen and hydrogen, air, water, ammonia, nitrogen and oxygen.
The plasma-forming gas must have a high specific heat capacity and thermal conductivity. In this regard, argon has worse electrical and thermophysical characteristics compared to other plasma-forming gases; however, it protects the tungsten electrode well, is easily ionized under the influence of an arc discharge, and does not harmful effects on the surface layer of the metal being processed. However, argon and other inert gases are expensive. In addition, they cannot dissociate in the arc discharge column. Active coolants are di- and triatomic gases, so they are used as an additive to argon. Hydrogen has the best thermophysical characteristics. In the mixture its content usually does not exceed 15-20%. A further increase in the hydrogen content in the mixture leads to a sharp increase in arc voltage. .
Plasma processing of materials has a number of advantages that determine its widespread use for the implementation of all known methods of thermal treatment of materials: the ability to achieve high concentrations of thermal energy; suitability for melting or evaporation of almost any materials known in nature; increased stability of the plasma arc compared to the electric arc; high gas velocity in a plasma jet.
Plasma sources provide a power density of 10 4 ~10 5 W/cm 2, i.e. less than an electron and laser beam, but their unit power can reach 160 kW or more, and the effective heating efficiency is 0.85. Plasma equipment is quite comparable in cost and manufacturing complexity to electric arc equipment, and is characterized by its small dimensions and high maneuverability. It is widely used for cutting, surfacing, spraying, welding and more limitedly for hardening.
2. Patterns of formation of the structure of surface layers of steels under high-energy exposure
All methods of surface high-energy hardening of steels are designed to form hardened layers that provide increased level wear resistance of working surfaces of parts under severe external loading conditions. Despite the fundamental differences in the equipment used for surface treatment, the mechanism for forming the hardened layer is generally the same. It consists of quickly heating a local volume of a part to an austenitic state and subsequent heat removal to neighboring volumes that did not have time to heat up during the period when the heating source was turned on. Due to the fact that the mass of the heated layer is significantly less than the mass of the workpiece, the cooling rate of the surface layer is usually higher than the critical one. Consequently, during the cooling stage, austenite undergoes a martensitic transformation.
The complex of mechanical properties of the surface layer, primarily hardness and strength indicators, is ensured by high heating and cooling rates of steel. This circumstance explains the small size of martensite crystals that appear in small austenite grains and the absence of obvious signs of self-tempering of a supersaturated solid solution. When processing a material, physicochemical processes develop in its surface layers, the nature of which is determined chemical composition, temperature, time, rate of heating and subsequent cooling.
The formation of a high-temperature phase as a result of heating with highly concentrated energy flows, in contrast to slow heating, when the pearlite > austenite transformation occurs under close to isothermal conditions, due to the excess of supplied energy, occurs under conditions of a continuously increasing temperature from A c1 to A c1 end. The shift graph of the critical point is shown in Figure 3. It should be noted that austenite obtained by high-speed heating is characterized by an increased number of defects. A large number of defects is due to their inheritance from the b phase, as well as additional education due to the enhanced effect of phase hardening under transformation conditions at high heating rates. The degree of completion of the austenitization process for a specific composition of the iron-carbon alloy is determined by the heating rate and temperature, the time of thermal exposure, or more precisely, the time that a certain volume of heated metal remains in the temperature range of austenite existence.
Figure 3 - Displacement of the critical point Ac1 during rapid heating of steel.
Since, when treated with concentrated energy flows, different layers of the material are heated to different temperatures, the thermally affected zone can be conventionally imagined as consisting of a number of layers that smoothly transition into each other. The HAZ structure diagram is shown in Figure 4
The first layer is the melting zone, which occurs during hardening from the molten state. The melting zone has a columnar structure with crystals elongated in the direction of the heat sink. The main structural constituent for medium carbon steel is martensite. It should be noted that as the product being hardened moves deeper into the surface, the dimensions of martensite crystals change smoothly. This is due to the fact that the temperature of the material in different zones of the rapidly heated layer is significantly different (despite the fact that the structure in these zones before cooling was the same - austenite).
Figure 4 - Diagram of the structure of the HAZ during plasma hardening: 1 - melting zone; 2- hardening zone; 3 - transition zone
However, the martensite of the main layer is characterized by high dispersion of its constituent elements. This is due to the fact that the maximum length of the martensite crystal corresponds to the size of the austenite grain. Due to the short duration of exposure, the austenite grain does not have time to grow and therefore the martensite formed within it is finely dispersed. In addition, when the process of austenite formation shifts to the region of high temperatures, the carbon concentration decreases, the stability of the nucleus decreases, therefore, the nucleation rate increases sharply, which limits grain growth.
The second layer is the hardening zone from the solid phase, formed in the temperature range Tmel › Tzak › TAc1. In depth, the layer is characterized by strong structural heterogeneity, since along with complete hardening, incomplete hardening occurs. At the upper boundary of the layer, closer to the surface, martensite and retained austenite are observed. At the lower boundary of the layer, closer to the original metal, along with martensite, elements of the original structure are observed: ferrite in hypoeutectoid steels and cementite in hypereutectoid steels.
The third layer is the transition zone, in which the metal is heated to temperatures below the Ac1 point, in which the main structures are tempering structures.
Metallographic studies carried out by the authors of the work showed that the microstructure of the transition zone depends on the initial state of the material being strengthened. Depending on the processing modes, steel grade, and its preliminary heat treatment, the transition zone can have different sizes and structure. In hypoeutectoid steels with an initial ferrite-pearlite structure and hypereutectoid steels with a pearlite-cementite structure, areas of excess phases (ferrite and cementite) are observed after surface hardening. The sizes of conglomerates of these phases increase in the direction from the hardened zone to the zone with the original structure.
The layered structure of the strengthened zone is characteristic of all methods of plasma hardening. The geometric parameters of the plasma heating zone are characterized by the width and depth of the hardened surface layer, which for most methods depend on the parameters of the hardening mode (power of the plasma jet (arc), hardening distance, processing speed).
To ensure high level In order to ensure the structural strength of the product being hardened, it is necessary to carefully control the structure of not only the hardened, but also the transition zone. By changing processing modes, it is possible to reliably control the structural parameters of the main and transition zones, while forming a favorable level of mechanical properties of the material.
Technical sciences/ 8. Materials processing in mechanical engineering
Berger E.E., Larushka N.A.
Kherson National Technical University
PLASMA HARDENING OF MACHINE PARTS
The main methods of increasing the hardness and wear resistance of the surface layer of parts are carburization, nitriding and plasma hardening. Plasma hardening is more preferable because requires significantly less time. Its essence lies in thermal phase and structural transformations that occur during rapid concentrated heating of the working surface of a part by a plasma jet and subsequent heat removal deep into the part.
Since, when treated with concentrated energy flows, different layers of the material are heated to different temperatures, the thermally affected zone can be conventionally imagined as consisting of a number of layers that smoothly transition into each other. The HAZ structure diagram is shown in Fig. 1:
Fig.1. 1 – melting zone; 2- hardening zone; 3 – transition zone.
In order to ensure a high level of structural strength of the hardened product, it is necessary to carefully control the structure of not only the hardened, but also the transition zone. By changing processing modes, it is possible to reliably control the structural parameters of the main and transition zones, while forming a favorable level of mechanical properties of the material.
The studies were carried out on plates made from the following structural steels:
Sample No. 1 – structural carbon steel 45 (casting)
Sample No. 2 – structural alloy steel 30ХНМА (casting)
Sample No. 3 – structural alloy steel 40ХН2МА (forging)
Sample No. 4 – structural alloy steel 40Х (casting)
After the samples were prepared, surface hardening was carried out with a plasma arc. A serial welding rectifier VDU-504 was used as a power source for the plasma arc. Argon was used as the plasma-forming gas.
The structure and hardness of the heat-treated surface layer was studied on transverse microsections using a Neophot-2 microscope at magnifications of 50-1000x. Hardness measurements were carried out using a Duramin-2 microhardness tester under a load of 4.9x103 H. Hardness measurements and studying the structure of the main and hardened sections of transverse microsections made from processed samples gave the following results:
Sample No. 1: Steel 45
The sample was quenched without melting the surface.
A study of the microstructure showed that near the surface the strengthened layer consists of sorbitol and perlite grains (up to 0.84 mm). In the transition layer (0.84-1.04 mm), in addition to sorbitol and pearlite, ferrite veins appear. The structure of the base metal is pearlite and ferrite mesh.
Sample No. 2: Steel 30ХНМА
The sample was quenched without surface melting visible to the naked eye.
The microstructure of the upper layer of the heat-strengthened zone is a layer of martensite with a grain score of 6 (section up to 0.2 mm). This is followed by a layer of martensite with a grain score of 5 and 6 with ferrite veins (section 0.2-0.58 mm). The next layer is martensite with a grain score of 3 and 4 with ferrite veins (section 0.58-1.28 mm).
Fig. 2 – Change in hardness along the depth of the strengthened layer of sample No. 1
In the transition zone, grains of martensite and sorbitol are observed (area 1.28-1.51 mm), in the zone of the base metal - grains of perlite and sorbitol.
Sample No. 2:Becameb30ХН2МА
The sample was quenched with a slight uniform melting of the surface. The results of measuring the hardness of sample No. 3 are presented in Table 1.
Table 1
Change in hardness along the depth of the heat-strengthened layer of sample No. 3
|
Depth, L, mm |
0,35 |
0,64 |
0,89 |
1,14 |
1,47 |
1,77 |
2,08 |
2,35 |
2,79 |
||
|
Hardness, HV |
Microstructural analysis showed the presence of a deep strengthened layer (up to 2.4 mm), the structure of which is martensite with different grain scores.
In the upper layer (up to 0.9 mm) it is grade 7, 8 martensite; then a layer of martensite crystals with a grain score of 6 (area 0.9 - 1.5 mm). Next is a layer of martensite with grains of 4 and 5 points, which in the transition zone (up to 2.5 mm) is replaced by perlite and sorbitol.
Fig. 3 – Change in hardness along the depth of the strengthened layer of sample No. 2.
Sample No. 4: Steel 40 X
The sample was quenched without melting the surface. The results of measuring the hardness of sample No. 4 are presented in Table 2.
Table 2
Change in hardness with depth of the heat-strengthened layer of sample No. 4
|
Depth, mm |
0,04 |
0,11 |
0,17 |
0,29 |
0,43 |
0,58 |
|
|
Hardness, HV |
Microstructural analysis showed that the upper layer of the HAZ (up to 0.11 mm) consists of martensite 5b. Then it transforms into a structure consisting of martensite 5b and sorbitol (section 0.11 - 0.17 mm). Then a layer of martensite, sorbite and perlite is observed (at a depth of 0.17-0.29 mm), which transforms into a structure of sorbitol and pearlite grains. In the zone of the base metal, lamellar pearlite and a ferrite mesh are observed.
Conclusions
After surface treatment of the existing samples, the assumption was confirmed that medium-carbon steels lend themselves well to plasma arc hardening. It was approximately determined that the hardness of the surface layer of the samples increases two or more times compared to the initial values.
It was also shown that by changing the processing modes, it is possible to control the structural parameters of the main and transition zones, thus obtaining the required hardness and depth of the strengthened layer.
With an increase in current strength at a constant processing speed on samples made of 30ХНМА steel, an increase in hardness occurred over the entire depth of the hardened layer. Also, good results in terms of hardness were shown by a sample made of 40ХНМА steel, which was processed at increased current values.
Sample No. 6 made of steel 30ХН2МА, which was also processed at increased current values, attracted attention with the highest hardness and depth of the hardened layer among all samples. This can be explained by the fact that this steel contains a high nickel content, which in turn belongs to the group of austenite-forming alloying elements, i.e. expands the range of existence of austenite. Thus, the austenitization process proceeds quite completely even at a depth of about two millimeters from the surface of the sample, which means that the formation of martensite becomes possible there.
Low current values, according to expectations, did not allow us to obtain a significant increase in hardness in the heat-affected zone (samples made of steel 45, 40ХН2МА).
The experiment also showed that with increasing quenching speed (productivity), the maximum depth of the hardened layer decreases. This is due to the fact that the time of heat propagation into the body of the hardened part is reduced, as a result of which the deep layers do not have time to warm up and undergo austenitization, necessary for the subsequent martensitic transformation.
UDC 621. 791
PLASMA HARDENING OF METALLURGICAL EQUIPMENT PARTS
© Korotkov Vladimir Aleksandrovich, Doctor of Engineering. Sciences, e-mail: [email protected]
Nizhny Tagil branch of the Ural federal university. Russia, Nizhny Tagil The article was received on May 11, 2012.
The development of manual surface hardening with a plasma arc made it possible to harden products that had not previously been hardened, and made it possible to solve many problems in increasing the service life of metallurgical equipment. The service life of casting crane gears, crusher cones, crane rails, and dies for various purposes has been increased several times.
Key words: plasma hardening; parts of metallurgical equipment; wear resistance.
the effect of hardening on the durability of equipment parts; hardening methods. Hardening of cast iron and steel approximately doubles the hardness, while wear resistance, depending on operating conditions, can increase tens of times, which explains its use in the manufacture of metallurgical equipment parts classified as quickly wearing out. However, the use of hardening is hampered by the fact that it is not possible for all parts due to the poor hardenability of massive parts, deformation and cracking in them, and high costs, which increase as a result of mandatory tempering. To overcome these shortcomings, surface hardening methods have been developed using highly concentrated heating sources: electric contact and electrolysis, gas flame, currents high frequency(HDTV), laser and electron beams. They contributed to the expansion of the use of hardening, but, having their own shortcomings, did not solve the problem; Currently, metallurgical equipment is in operation with a large number of contact surfaces (matings) of parts that do not have hardening, which for this reason wear out quickly and lead to frequent and expensive repairs.
One of the previously unused surface hardening methods was plasma arc hardening. The first information about it appeared in the 80s of the last century. The presence in industry of plasma devices for cutting, welding, and spraying was one of the reasons for searching for a method of using them for surface hardening. The microplasma welding installation was used for
hardening of parts of mine equipment, and a plasma spraying installation for hardening parts of rolling rolls. The UPS-501 plasma welding apparatus was modernized for hardening the tires of rail straightening machines, and then rolling rolls. The plasma cutting installation UPR-404 is also adapted for hardening.
Plasma hardening of the 1980-1990s had a significant drawback. It was used only in automatic mode, when the settings were easily maintained unchanged, while manual operation of the process was almost impossible. In the modern age of robots and “unmanned” industries, the development of manual technology may seem misguided. However, manual technologies, due to their versatility, demonstrate viability. In the world, the bulk of welding (more than 80%) continues to be performed using electrodes or semi-automatic machines, i.e. manually. By analogy, it was believed (this calculation was justified) that with the development of the manual method of plasma hardening, the volume of its use will increase, and this will happen at the expense of products
Rice. 1. Hardening using the UDGZ-200 installation
for which it was previously impossible to harden for one reason or another.
Development of a manual plasma hardening method and its characteristics. The problem of manual plasma hardening was solved in 2002 at Kompozit LLC, created in 1990 at the Nizhny Tagil branch of UPI (now UrFU). Here they developed a method and installation UDGZ-200 for manual plasma hardening.
The UDGZ-200 installation (Fig. 1) is equipped with a burner, the small dimensions of which make it convenient for manual manipulation and allow processing the most inaccessible parts of parts.
Technical characteristics of the UDGZ installation
200 is given below
Weight (power supply, burner, unit
burner cooling), kg 20 + 0.5 + 20
Mains voltage, V 380
Power, kW 10
Productivity, cm2/min 25-95
Working gas consumption (argon), l/min 15
Hardening depth, mm 0.5-1.5 Hardness after hardening (depending on
steel grade) up to IKS 65
When hardening, the welder moves the arc along the surface at a speed that ensures “sweating” (the state preceding melting) of the surface under the arc. This process is no more difficult to control than melting during welding, but it provides the necessary heat for hardening and prevents rough melting of the surface. The arc leaves hardened strips 8-16 mm wide on the surface, which the welder places with some overlap. Tarnish colors are observed on their surface as a result of the formation of a thin film of oxides, which does not have a significant effect on the roughness in the range Rz = 4-40 (Fig. 2). In addition, plasma hardening does not produce significant deformations, which, in total,
Rice. 2. Plasma arc and strip hardened by it
with the previous one, it makes it possible to eliminate the labor-intensive finishing machining of a hard hardened layer for many parts and, as a result, reduce the labor intensity and cost of production.
Hardening occurs due to the removal of heat into the body of the part without supplying coolant (water) to the place of heating. Therefore, the UDGZ-200 installation is used at repair sites, at the place of machining and operation of parts, and not only in heat treatment shops and specialized areas. Welders of the 2nd and 3rd categories master the work on it. At the same time, the hardening process can be mechanized, automated and robotic, which makes UDGZ-200 suitable for use in modern high-tech industries. The presence of the UDGZ-200 installation to a certain extent compensates for the lack of furnaces for hardening, carburization, and high-frequency installations, making hardening environmentally friendly.
In Fig. Figure 3 shows a typical microsection of a hardened layer approximately 1 mm thick and the hardness distribution in hardened strips made with overlap. It can be seen that as a result of heating by a plasma arc, the microhardness more than doubled: from -NU 250 to NU 700-800. In the place where the strips overlap (double hardening), the microhardness increases to NU 800-900, and in the zone of thermal influence of the second strip on the first it decreases to NU 600-700 due to tempering processes.
Increasing the durability of metallurgical equipment parts as a result of plasma hardening. Hardening (see Fig. 1) by installing UDGZ-200 gear wheels (steel 35GL, z = 90, t = 24) of a steel-pouring crane (at the Nizhny Tagil Metallurgical Plant) loading
Distance from the edge of the second hardened strip, mm
Rice. 3. Cross section of a sample (45 steel) with two
plasma arc hardening strips (bottom); distribution £
from the surface (top) w
with a lifting capacity of 225 tons (carried out since 2004) increased the hardness of the teeth from HB200 to HB500 and service life from 6 months. up to 17 months, i.e. 2.8 times. The same wheels on a crane with a lower lifting capacity (at the Chelyabinsk Metallurgical Plant) (180 tons) last 10-11 months. (before teeth wear is about 11 mm). After plasma hardening, they worked twice as long and had wear of about 1 mm, i.e. by the thickness of the hardened layer. Since the wear of the teeth has not reached the maximum value (11 mm), the gear was re-hardened directly on the tap, without dismantling it. Savings due to a reduction in the purchase of gears and the cost of replacing them in this case amounted to approximately 4.8 million rubles, while the efficiency of investments in plasma hardening was about 28 rubles. savings for every ruble of hardening costs.
In a similar way (preliminary hardening of new gears and repeated hardening without dismantling as the hardened layer wears out), the service life of the ring gear (steel 40GL) of the pelletizer drum and the under-ring gear (steel 34ХН1М) in the sintering plant is increased seven times. The savings amounted to almost 38 million rubles. with an investment efficiency in plasma hardening of about 5 rubles. savings for every ruble of costs.
Gears (t = 10; g = 16) made of structural steel 40X in “improved” condition, in the open gear of a stacker working with a car dumper in the same sinter production, wore out within one week. Plasma hardening increased the operating time - up to four weeks, i.e. four times. In this case, only the hardened layer (~1mm) was worn out, which made it possible to repeat the hardening directly on the paver and increase the service life of the gears up to eight times.
In the 300 rolling mill drive, torque is transmitted to the rolls through splined couplings (45 steel), the service life of which does not exceed three months. Hardening the splines significantly reduced their wear: after running twice, it was less than 10%.
The dimensions of the hardening burner of the UDGZ-200 installation make it possible to harden the teeth of gears with module t > 6. Hardening is carried out along the side surface of the tooth. The cavities between the teeth are not hardened, since the plasma arc does not operate there. This is a drawback of HDTV hardening, causing tooth breakage during operation. However, plasma hardening of only the side surfaces does not lead to breakdowns, since it is carried out sequentially, whereas high-frequency hardening -
Rice. 4. Plasma hardening of the teeth (on the right) eliminated the breakdowns that occurred during hardening with high-frequency heat (on the left)
simultaneously along the entire profile, inducing high residual stresses. In addition, when hardening high-frequency teeth, one more condition must be observed - do not allow the teeth to be hardened to the full thickness, for which steel of reduced hardenability is used. In cases of breakage of the teeth of the drive gears from
of structural steels (Fig. 4) of railway locomotives (at the Kachkanarsky GOK), it was possible to eliminate breakdowns without replacing the steel by using plasma hardening, while the consumption of gears was halved.
GOLENISCHEV A.A., GOLENISCHEV A.V., PANYCHEV A.P., POLUYAKTOVA T.A., SHAVNINA M.V. - 2015
GOLENISCHEV A.A., GOLENISCHEV A.V., IVANOV V.A. - 2015
The objective of plasma hardening technology is to obtain a hardened layer on a part with specified performance characteristics (wear resistance, strength, crack resistance, endurance, etc.).
Technological processes in which the material is exposed to concentrated energy flows in the form of an electron beam, laser, plasma (welding, surfacing, cutting, hardening, spraying) are currently quite common in industry.
The advantages of electron beam processing in vacuum include high values of effective heating efficiency (h» 0.85) with an overall efficiency of technological electron beam installations of 50%, the ability to transmit energy flows with a power of more than 40-100 kW, no oxidation of the heated surface, high performance process, etc. At the same time, large capital costs for the acquisition and installation of equipment, costs associated with its operation and maintenance, limit the use of electron beam processing of parts to large-scale and mass production in mechanical engineering and the tool industry.
Laser processing is developing intensively, but lasers with powers up to 5 kW have become the most widespread. Higher power lasers are expensive equipment, the operation of which is economically feasible when it is loaded at 80-90%.
Laser radiation provides the highest heating concentration (power density) 10 8 -10 9 W/cm 2, but this advantage cannot be realized for all technological processes. Thus, during hardening without melting, there is a critical power density E cr, above which melting of the surface occurs. For various steels, the value of E cr is in the range (2-6)10 4 W/cm 2, i.e., the power density range characteristic of plasma processing is used.
Plasma sources provide a power density of 10 4 -10 5 W/cm 2, i.e. less than an electron and laser beam, but their unit power can reach 160 kW or more, and the effective heating efficiency is 0.72. Plasma equipment is quite comparable in cost and manufacturing complexity to electric arc equipment. It is widely used for cutting, surfacing, spraying, welding and more limitedly for hardening.
Plasma hardening methods
Two areas of using plasma heating should be highlighted. The first is associated with the use of heating carried out by glow discharge plasma in a vacuum chamber at a residual air pressure of 1.33-13.3 Pa. This process has become widespread for chemical-thermal treatment of tools and other small parts. The disadvantages of this method include the presence of a vacuum chamber and the limitation of the processed parts by its dimensions. In addition, the power density transferred to the workpiece is low.
The technology of electrolytic-plasma hardening should also be included in this direction. An electrolytic heater, included in the electrical circuit as an anode, is connected to the product, which is the cathode. Closure electrical circuit between the anode-electric heater and the surface of the product occurs through an electrolyte (aqueous salt solution). Conversion electrical energy into thermal heat occurs mainly in the layer adjacent to the product. As a result of heating, this layer passes into a vapor-gas state, and microarcs are excited in it under the influence of applied voltage. Power density reaches 2.4× 10 3 W/cm 2 . Since an aqueous salt solution is used as an electrolyte, the same electrolyte can be used to cool and harden heated areas of the surface.
The second direction of application of plasma heating is based on the use of a compressed arc of direct or indirect action generated by a special plasma torch. Under the influence of the walls of the nozzle channel and the jet of plasma-forming gas, the arc column is compressed, its cross section decreases, and the temperature in the central part of the arc column rises to 10,000-50,000 K. As a result, the inner layer in contact with the arc column turns into plasma, and the outer The spray that washes the walls of the nozzle channel remains relatively cold, forming electrical and thermal insulation between the plasma flow and the nozzle channel. This cooled layer of gas prevents the arc column from deflecting from a given direction and closing it on the wall of the nozzle channel. The compressed arc voltage is 60-200 V, which is three to ten times more than in a free arc. The current density of a compressed arc reaches 100 A/mm 2, i.e., an order of magnitude higher than that of a free arc, and the specific power reaches 2× 10 6 W/cm 2 .
Hardening of steels with heating by concentrated energy flows (CEF), by analogy with other types of hardening, consists in the formation of an austenitic structure during the heating stage and its subsequent transformation into martensite during the cooling stage. In this case, the supplied thermal energy is greater than the energy required for the restructuring of the crystal lattice, and the restructuring itself occurs at a certain finite speed. Therefore, the transformation occurs in the temperature range from Ac1init to Ac1kon, i.e., the end of the austenitic transformation shifts to the region of high temperatures T (Fig. 1, region 1).
Due to the high heating rate, the diffusion processes of restructuring the lattice of a body-centered cube of excess ferrite into the lattice of a face-centered cube of austenite may not end on the line GS diagrams Fe - Fe 3 C and shift to the region of higher temperatures (region 2). Micromelting of the cementite-austenite interface can also occur (region 3).
Rice. 1. Section of the diagram Fe-Fe3C with features of structural transformations during high-speed heating
When processed with a welding arc, the heating rate reaches 1000-3000 °C/s. At such heating rates, the shift of all stages of austenitization in steels to the region of higher temperatures can reach 100-300 °C. As a result of heating the CPE, a structure is formed, the features of which are determined by the degree of completion of the austenitization process, determined by the heating rate and temperature, exposure time, initial structure, etc. At a sufficiently high heating temperature or with a relatively long exposure time, the formation of homogeneous austenite is possible. A decrease in heating temperature and exposure time as a result of an increase in critical points and a slowdown in the homogenization process leads to a large heterogeneity of austenite in steel, especially in carbon. In addition to austenite, under these conditions at high temperatures, the existence of undissolved carbides is possible.
The degree of heterogeneity of the structure formed as a result of heating the CPE depends on the dispersion of the initial structure. Moreover, the more dispersed the initial structure, the less heterogeneity of austenite.
The process of processing CPE for the purpose of heat strengthening is characterized by high cooling rates, which lead to hardening of surface areas. To obtain martensite in iron-carbon alloys in the temperature range of minimum stability of austenite (400-600 0 C), it is necessary to ensure a cooling rate greater than the critical one, which for most iron alloys is in the range 50-200 ° C/s. Cooling during heating of the CPE is characterized by significantly higher rates. Thus, the cooling rate during plasma hardening varies from 10 4 to 10 6 °C/s. Plasma hardening is carried out without melting and with melting of the surface of the part.
Energy thresholds that determine the hardening modes have been established (Fig. 2). Energy threshold W 1, corresponds to heating the metal to the temperature at which the austenitic transformation begins. A further increase in power density leads to an increase in the hardness of the processed steel, which reaches its greatest value when heated without melting at the second value of the energy threshold W 2. Then an increase in power density leads to a slight increase in hardness, and the third threshold W 3 corresponds to the beginning of surface melting.
Rice. 2. Effect of power density in the heating spot on surface hardness
Plasma hardening without surface melting is the most common, as it allows the hardness, size and operational characteristics of the treated area to be adjusted within a wide range while maintaining high surface quality. Surface fusion hardening is usually used to achieve special performance properties.
During plasma thermal hardening, individual layers of the treated area are heated in depth to different temperatures, as a result of which the heat-affected zone (HAZ) has a layered structure. Depending on the microstructure and microhardness in steels, three layers are distinguished along the depth of the HAZ (Fig. 3).
Rice. 3. Scheme of the structure of the HAZ during plasma hardening
Melting zone 1 (first layer) occurs during quenching with melting. As a rule, the melting zone has a columnar structure with crystals elongated in the direction of the heat sink. The main structural component is martensite; carbides usually dissolve. Under optimal quenching and melting conditions, decarburization does not occur; there are no pores or slag inclusions. When plasma hardening without reflow, the first layer is absent.
The second layer is hardening zone 2 from the solid phase. Its lower limit is determined by the heating temperature to Ac1. In this case, along with complete hardening, incomplete hardening also occurs. In depth, this layer is characterized by structural heterogeneity. Closer to the surface there are martensite and retained austenite, obtained by cooling from homogeneous austenite. Closer to the original metal, along with martensite, there are elements of the original structure: ferrite in hypoeutectoid steel and cementite in hypereutectoid steel.
In transition zone 3 (third layer), the metal is heated below point Ac1. If the steel has its original state after quenching or tempering, then as a result of plasma treatment, tempering structures are formed in this layer - troostite or sorbitol, characterized by reduced hardness.
The thermally affected zone of the plasma jet (arc) has the shape of a segment; its structure is similar to the HAZ of electron and laser beams.
With plasma heating, it is not always possible to avoid the accumulation of heat in the workpiece. In order to eliminate the accumulation of heat in the product, plasma hardening in liquid media is used. The product being processed is immersed in liquid so that there is a liquid layer of a certain thickness above its surface.
Literature:
Lashchenko G.I. Plasma hardening and sputtering. – K.: “Ecotechnologist i i", 2003 – 64 p.