RU2381295C2 - Steel for machine components, manufacturing method of machine components from this steel and produced machine components - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: invention relates to ferrous metallurgy field, particularly to receiving of steel, provided for manufacturing of machine components, particularly, gears. For manufacturing of gears it is molten steel, consisting in wt %: carbon 0.19-0.25, manganese 1.1-1.5, silicon 0.8-1.2, sulphur 0.01-0.09, phosphorus from marks from 0.025, nickel from marks up to 0.25, chrome 1-1.4, molybdenum 0.10-0.25, copper from marks up to 0.30, aluminium 0.01-0.045, niobium 0.010-0.045, nitrogen 0.0130-0.0300, if necessary bismuth from marks up to 0.10 and/or lead from marks up to 0.12 and/or tellurium from marks up to 0.015 and/or selenium from marks up to 0.030 and/or calcium from marks up to 0.0050, the rest - iron and conditional by melting admixtures. Steel allows mean values J3m, J11m, J15m and J25m tests by Jominy, satisfying expressions: |J11m-J3mù14/22-J25mù8/22|ëñ2.5 HRC and J3m-J15mëñ9HRC Received steel is mechanically treated and subject to cementation or carbonitriding with following tempering. ^ EFFECT: there are provided required mechanical properties of received components and ability of implementation of cementation or carbonitriding at usual temperature. ^ 8 cl, 1 dwg, 4 tbl

Description

The invention relates to the field of ferrous metallurgy and relates, in particular, to steels for machine parts, such as gears.

Steels for gears must have high fatigue resistance upon contact. For the most part, parts made from such steels are cemented or nitrocarburized to provide sufficient hardness of the surface layer and mechanical strength, while high strength is maintained in the middle due to, in particular, the carbon content of only about 0.10 to 0.30% . The carbon content in the cemented layer can reach up to about 1%.

Various documents describe the steel hardened for gears. You can point to US-A-5518685, in which the content of Si and Mn is kept in relatively low limits (0.45-1% and 0.40-0.70%, respectively) to prevent intercrystalline oxidation during cementation. JP-A-4-21757 describes steels for gears that are cemented by plasma or under reduced pressure, then shot blasted and capable of containing more Si and Mn than the above steels. They have great resistance to pressure on the surface of the gear, which increases their durability.

WO-A-03012156 proposes steel for machine parts, such as gears, having the following composition: 0.12% ≤C≤0.30%; 0.8% ≤ Si ≤ 1.5%; 1.0% ≤Mn≤1.6%; 0.4% ≤Cr≤1.6%; Mo≤0.30%; Ni≤0.6%; Al≤0.06%; Cu≤0.30%; S≤0.10%; P≤0.03%; Nb≤0.050%. This steel has the advantage of minimizing the plastic deformation in the part when it is used, achieved, in particular, by the optimal equilibrium content of silicon and manganese. It is preferable to carry out cementation or nitrocarburization under conditions that exclude oxidation, for example, under reduced pressure, so that a relatively high content of silicon and manganese does not lead to intercrystalline oxidation.

Typically, carburizing or nitrocarburizing is carried out at a temperature of the order of 850-930 ° C. However, at present, attempts are being made to carry out this operation at a higher temperature (high-temperature cementation or nitrocarburizing), in particular of the order of 950-1050 ° C. This increase in processing temperature allows you to either reduce the duration of this treatment with the same depth of the cemented layer, or increase the depth of the cemented layer with the same duration of processing. At the choice of the manufacturer, one can either increase the productivity of the equipment or improve the performance of the products obtained.

However, the use of high temperature carburizing or nitrocarburizing for the described known steels creates some problems. Firstly, high temperature can lead to difficult to control grain growth, which is unfavorable for the mechanical properties of the part. Secondly, after cementation or nitrocarburizing, hardening is carried out, during which the part is subject to deformation. The latter can make it necessary to re-treat the part, and in the most adverse cases even lead to marriage. These problems are even more complicated when the part is quenched after cementation or nitrocarburizing carried out at a high rather than normal temperature.

The aim of the invention is to offer metallurgists performing high-temperature cementation or nitrocarburizing of machine parts, in particular gears, steel, which solves the above problems and which maintains the required mechanical properties, and is also suitable for cementation or nitrocarburizing at ordinary temperatures.

Therefore, the object of the invention is steel for machine parts, characterized in that its composition includes, wt.%:

- 0.19≤C≤0.25;

- 1.1≤Mn≤1.5;

- 0.8≤Si≤l, 2;

- 0.01≤S≤0.09;

- traces≤P≤0,025;

- traces≤Ni≤0.25;

- 1≤Cr≤l, 4;

- 0.10≤Mo≤0.25;

- traces≤Cu≤0.30;

- 0.010≤Al≤0.045;

- 0.010≤Nb≤0.045;

- 0.0130≤N≤0.0300;

- optional: traces≤Bi≤0,10 and / or traces≤Pb≤0,12 and / or traces≤Te≤0,015 and / or traces≤Se≤0,030 and / or traces≤Sa≤0,0050,

the rest is iron and impurities caused by the smelting, and the chemical composition is set so that the average values of J _3m , J _11m , J _15m and J _{25m of the} five Jomini tests are:

α = | J _11m -J _3m × 14/22-J _25m × 8/22 | ≤2.5 HRC and

β = J _3m -J _15m ≤9 HRC

Preferably, the composition was set so that:

β = J _3m -J _15m ≤8 HRC.

Preferably, the composition includes, wt.%:

- 0.19 С C 0 0.25;

- 1.2≤Mn≤1.5;

- 0.85≤Si≤1.2;

- 0.01≤S≤0.09;

- traces ≤P≤0,025;

- 0.08≤Ni≤0.25;

- 1.1≤Cr≤1.4;

- 0.10≤Mo≤0.25;

- 0.06≤Cu≤0.30;

- 0.010≤Al≤0.045;

- 0.015≤Nb≤0.045;

- 0.0130≤N≤0.0300;

- optional: traces≤Bi≤0,07 and / or traces≤Pb≤0,12 and / or traces≤Te≤0,010 and / or traces≤Se≤0,020 and / or traces≤Sa≤0,0045,

- the rest is iron and impurities due to smelting.

It is optimal that the composition includes, wt.%:

- 0.20 С C 0 0.25;

- 1.21≤Mn≤1.45;

- 0.85≤Si≤1.10;

- 0.01≤S≤0.08;

- traces≤P≤0,020;

- 0.08≤Ni≤0.20;

-1.10≤Cr≤1.40;

- 0.11≤Mo≤0.25;

- 0.08≤Cu≤0.30;

- 0.010≤Al≤0.035;

- 0,025≤Nb≤0,040;

-0.0130≤N≤0.0220;

- optional: traces≤Bi≤0,07 and / or traces≤Pb≤0,12 and / or traces≤Te≤0,010 and / or traces≤Se≤0,020 and / or traces≤Sa≤0,0045,

- the rest is iron and impurities due to smelting.

The object of the invention is also a method of manufacturing a machine part from steel subjected to carburizing or nitrocarburizing, characterized in that it uses steel of the above type, which is subjected to treatment, carburizing or nitrocarburizing and hardening.

Preferably, said cementation or nitrocarburizing is carried out at a temperature of 950-1050 ° C.

Also an object of the invention is a steel part of a machine, for example a gear part, characterized in that it is obtained as described above.

As will be shown below, the invention is based on the exact coordination of the content limits of the main alloying elements, as well as on the simultaneous use of aluminum, niobium and nitrogen at their precisely specified content.

The goals set have basically a dual purpose.

Firstly, the choice of the content of the main alloying elements should provide a Jomini curve without a significant inflection point. Compliance with this condition allows to obtain minimal deformation during hardening. From this point of view, high-temperature cementation or nitrocarburizing, as already noted, should be carried out very carefully.

It should be recalled that the Jomini steel curve obtained in the normal standard test characterizes the hardenability of steel. It is obtained by measuring the hardness of a cylindrical sample hardened by a water jet at one of its ends along one of its generators. Hardness is measured in several places at different distances x (mm) from the irrigated end, the corresponding value is denoted by J _x . J _xm is the average value obtained in five experiments on measuring hardness at a distance x.

As indicated in document EP-A-0890653, to which the reader can be referred for more detailed information, the applicant showed that the composition of the steel to obtain the Jomini curve without an inflection point is optimal for a sharp reduction in deformation during quenching following cementation or nitrocarburizing. Such a Jomini curve without an inflection point is obtained if the values of J _11m , J _3m , J _25m and J _15m correspond to the following relations:

α = | J _11m -J _3m × 14/22-J _25m × 8/22 | ≤2.5 HRC;

-β = J _3m -J _15m ≤9 HRC, preferably 8 HRC.

The composition of the steel according to the invention is selected so that it achieves this ratio.

The composition is also selected, in particular, by the simultaneous presence of aluminum, niobium and nitrogen in an amount that provides control over the grain size even during high-temperature cementation or nitrocarburizing.

It goes without saying that the composition of the steel should provide the mechanical properties of the part required during its operation. Among the more carefully controlled parameters, one can indicate the depth of the cemented layer (usually determined by the depth at which the measured hardness is 550 HV), the difference between the hardness on the surface and in the core of the cemented part, which should be as small as possible to minimize deformation during hardening, and hardness of the core, which must be high in order for the part to have high resistance to working loads and, therefore, high fatigue strength.

The invention is explained in more detail below with reference to the attached drawing, which shows Jomini curves for four control types of steel and three types of steel according to the invention.

The steel according to the invention is mainly intended for the manufacture of heavily loaded machine parts, such as gear elements, which are subjected to cementation or nitrocarburizing (preferably at low pressure or in an atmosphere of non-oxidizing gas to prevent oxidation of the most oxidized elements), as at normal temperature from about 850 to 930 ° C, and at high temperatures from about 950 to 1050 ° C. These parts must have high fatigue strength, high resistance and only slight deformability during heat treatments, such as quenching after cementation or nitrocarburizing. Steel has the following composition (all percentages are given in wt.%).

The carbon content is 0.19-0.25 wt.%. This content is common for steels designed for gears. However, this range allows you to set the content of other elements to obtain the Jomini curve of the desired shape. The minimum content of 0.19 wt.% Is determined by the hardness of the core after quenching, carried out to obtain it. Above 0.25 wt.%, The hardness may be excessive so that the steel has the required machinability. The preferred range is 0.20-0.25 wt.%.

The manganese content is 1.1-1.5 wt.%. The minimum content is necessary to obtain the required Jomini curve in combination with the content of other elements. If the content exceeds 1.5 wt.%, There is a risk of segregation and streaky structures during annealing. In addition, with such a high content during smelting, increased corrosion of the heat-resistant lining of the casting ladle can occur. In this case, it is undesirable to further narrow down the specified range of contents, since obtaining at the steel mill the exact required grade of steel would be extremely difficult. A preferred range is a range of 1.2-1.5 wt.%, Preferably 1.21-1.45 wt.%.

The silicon content is from 0.8 to 1.2 wt.%. With this range of contents, the desired shape of the Jomini curve can be obtained in combination with the content of other elements. A minimum content of 0.8 wt.% Is required to provide the necessary hardness of the core, as well as to limit the difference between the hardness of the surface layer and the hardness of the core after cementation or nitrocarburizing. If the content is over 1.2 wt.%, There is a risk of the formation of excess segregations, since silicon, which itself is slightly susceptible to segregation, enhances the segregation of other elements. The risk of oxidation during cementation or nitrocarburizing is also increased. A preferred content range is a range of 0.85-1.20 wt.%, Preferably 0.85-1.10 wt.%.

The sulfur content is 0.01-0.09 wt.%. The minimum content is determined by optimum machinability. If the content is over 0.09 wt.%, There is a risk of a significant decrease in the ability to forge in the hot state. A preferred range of content is a range of 0.01-0.08 wt.%.

The phosphorus content is traces of 0.025 wt.%. As a rule, current standards provide for the maximum phosphorus content from the specified range. In addition, if the content is above this value, there is a risk of a synergistic interaction with niobium, leading to embrittlement of the steel when it is processed in the hot state and / or during continuous casting to produce blooms or ingots. Preferably, the phosphorus content is not more than 0.020 wt.%.

Nickel content is traces of 0.25 wt.%. This element, when introduced specifically in larger quantities, causes unnecessary rise in price of metal. In practice, one can limit oneself to the nickel content naturally present in the raw materials used for melting without its special additive. The preferred content range is a range of 0.08-0.20 wt.%.

The chromium content is 1.00 to 1.40 wt.%. In this range, in combination with the content of other elements, it is possible to obtain a Jomini curve of the required shape. In addition, a minimum content of 1.00 wt.% Allows to obtain good core strength. If the content exceeds 1.40 wt.%, Unnecessary smelting costs increase. The preferred range is 1.10 to 1.40 wt.%.

The molybdenum content is 0.10-0.25 wt.%. In this range, in combination with the content of other elements, a Jomini curve of the required shape is obtained, as well as the necessary hardness of the core part. The preferred range is 0.11-0, 25 wt.%.

The copper content is traces of 0.30 wt.%. In this case, as in the case of nickel, as a rule, its content obtained by melting raw materials is simply kept. If the content exceeds 0.30 wt.%, The viscosity and resistance of the core of the part are reduced. The preferred range is a range of 0.06-0.30 wt.%, Mainly 0.08-0.30 wt.%, In which it is possible to optimize the shape of the Jomini curve and hardness after hardening.

The content of aluminum, niobium and nitrogen should be controlled and precisely maintained within them. Indeed, they are elements that, when interacting, provide control over the fine grain of the metal. Fine grain is necessary to achieve good resistance of the surface layer after cementation or nitrocarburizing, good fatigue strength and to reduce the spread of deformation during hardening. In addition, it is of great importance for obtaining the desired shape of the Jomini curve. The control of grain size, in the framework of the present invention, the more important, the more suitable should be steel for high-temperature cementation or nitrocarburizing, not accompanied by excessive grain growth.

Such control over grains is carried out mainly by the deposition of nitrides and carbonitrides of aluminum and / or niobium. To achieve this, therefore, the presence of a significant amount of both of these elements, as well as nitrogen, in an amount significantly exceeding the amount usual for smelting under normal conditions, is necessary.

The aluminum content should be 0.010-0.045 wt.%. In addition to its above-mentioned function for controlling grain size, this element provides deoxidation of steel and its purity from oxide impurities. When the content is below 0.010 wt.%, Its effectiveness in performing these functions is insufficient. Above 0.045 wt.%, The degree of purity from oxide impurities may not be sufficient for preferred applications. A preferred range of content is a range of 0.010-0.035 wt.%.

The niobium content should be 0.010-0.045 wt.%. When its content is below 0.010 wt.%, The function of controlling the grain size becomes insufficient, especially in the case of a minimum aluminum content. If the content exceeds 0.045 wt.%, There is a risk of cracking during the continuous casting of steel, in particular, in the case when there is a synergistic effect with phosphorus, as noted above. A preferred content range is a range of 0.015-0.045% by weight, preferably 0.015-0.040% by weight.

In combination with the aluminum and niobium content given above, the nitrogen content should be 0.0130-0.0300 wt.% (130-300 ppm) in order to obtain the required grain size and shape of the Jomini curve. A preferred range is a range of 0.0130-0.0220 wt.%.

If necessary, one or more of the known elements traditionally used to increase its workability, namely lead, tellurium, selenium, calcium, bismuth, can be added to steel. Their maximum content is: 0.10 wt.%, Mainly 0.07 wt.% Bi; 0.12 Pb; 0.015 wt.%, Mainly 0.010 wt.% Those; 0.030 wt.%, Mainly 0.020 wt.% Se and 0.0050 wt.%, Mainly 0.0045 wt.% Ca.

Other elements of steel are elements that are usually present in it as melting impurities and which are not specifically introduced. It is necessary, in particular, to ensure that the titanium content does not exceed 0.005 wt.%. In fact, since the steel according to the invention contains a large amount of nitrogen, if this content is exceeded, there is a risk of the formation of large titanium nitrides and / or titanium carbonitrides, distinguishable in the micrograph, reducing fatigue strength and deteriorating workability. In addition, titanium captures nitrogen, which can no longer serve as a means of controlling grain size.

Below the invention is illustrated using examples. The attached drawing shows Jomini curves for four types of steel, the chemical composition of which is given in table 1. Steels A, B, C and D are control. Steels E, F and G are steels according to the invention.

Table 1 The chemical composition of the samples (wt.%) Steel FROM Mn Si S P Ni Cr Mo Cu Al Ti Nb N A (counter) 0.236 0.888 0.224 0.015 0.011 0.011 1,194 0.014 0.010 0,021 Traces Traces 0.0124 In (counter.) 0.195 1,188 0,069 0,023 0.012 0.208 1,228 0,096 0.162 0,021 Traces 0,030 0.0179 C (control) 0.192 1,205 0.845 0,029 0.014 0,080 0,995 0,099 0,110 0,025 Traces 0.011 0.0110 D (control) 0.245 1,215 0.840 0,035 0.012 0,085 0.980 0.103 0,098 0,035 Traces 0.012 0.0090 E (according to the image) 0.230 1,287 0.920 0.018 0.017 0.201 1,269 0,200 0.211 0,032 Traces 0,025 0.0174 F (according to the image) 0.201 1,453 1,191 0,041 0.014 0.139 1,381 0.246 0.122 0,031 0.002 0,038 0,0243 G (according to the image) 0.241 1,254 0.852 0.015 0.010 0.189 1,121 0,111 0.109 0.012 Traces 0.016 0.0141

For sample A, the value of α, described above, is 8.7, and the value of β, also described above, is 19.1. Therefore, these values significantly exceed the maximum value required by the invention. Therefore, it can be seen that the Jomini curve has a pronounced inflection point.

For sample B, α is 2.38, β is 11.1. Therefore, the β value does not meet the requirements of the invention and therefore the Jomini curve also contains a noticeable inflection point, although niobium and nitrogen are contained in this steel within specified limits. The main reason for this is the low silicon content.

For sample C, α is 3.38, β is 10.7. Neither α nor β are kept within given limits, and the Jomini curve contains a distinguishable inflection point. The content of chromium and manganese is less than the required minimum values, the nitrogen content is especially insufficient.

For the sample, D α is equal to 2.845, β is equal to 9.5, which does not lie within the given limits. The Jomini curve contains a noticeable inflection point due to insufficient chromium and nitrogen.

But for the steel sample E according to the invention, α is 0.41, β is 2.7. The necessary conditions are fulfilled, and it can be seen that the Jomini curve is almost straight and does not contain an inflection point.

Also for the sample F of the steel according to the invention, α is 0.23, β is 3.7. And here the Jomini curve is almost straight and does not contain an inflection point.

For sample G of the steel according to the invention, α is 0.83, β is 6.6. The Jomini curve is almost straight and does not contain a noticeable inflection point.

The properties of steels A, B and C, shown in Table 1, were also investigated in case of carburizing at ordinary and high temperatures.

Cementation was carried out at ordinary temperature (930 ° C) and low pressure for cylindrical samples to form a cemented layer with a carbon content of 0.75%. After cementation followed by quenching in a gaseous medium (in this case in a nitrogen medium, a mixture of nitrogen with 10% hydrogen could also be used, for example) at two different pressures: 5 bar and 20 bar. At the same time, they tried to obtain a surface layer strength of 700-800 HV with a cementation depth of 0.50 mm (namely, at a depth at which the hardness is 550 HV). The results are shown in table 2 (test at a pressure of 5 bar) and table 3 (test at a pressure of 20 bar).

table 2 Properties after carburizing and quenching in a gas medium at a pressure of 5 bar Steel Hardness of the surface layer, HV Depth of cemented layer, mm The hardness of the core outside the cementation zone, HV A (counter) 760 0.35 263 In (counter.) 760 0.50 408 E (according to the image) 780 0.48 426 Table 3 Properties after carburizing and quenching in a gas medium at a pressure of 20 bar Steel Hardness of the surface layer, HV Depth of cemented layer, mm The hardness of the core outside the cementation zone, HV A (counter) 780 0.45 318 In (counter.) 720 0.55 423 C (control) 738 0.53 408 E (according to the image) 750 0.55 524

The test data show that the control steel A does not allow to achieve the required depth of the cemented layer. This is due to its lack of hardenability.

All three types of steel, namely the control steels B and C, as well as steel E according to the invention, provided the required depth of carburization carried out under normal temperature conditions.

The difference ΔHV between the hardness of the surface layer and the hardness of the core is very comparable when using a quenching medium with a pressure of 5 bar in the case of control steel B and steel E according to the invention (ΔHV = 352 and 354, respectively), and significantly lower than the difference for control steel A (ΔHV = 497). But when using a quenching medium with a pressure of 20 bar, ΔHV is significantly less optimal for control steels B and C than for steel E according to the invention (ΔHV = 297, 330, 226, respectively). It follows that the residual stresses caused by the difference between the hardness indices and causing the deformation of the cemented parts during hardening under harsh conditions can be minimized by the use of steels according to the invention.

Maximum core hardness values were obtained using steel E according to the invention. Therefore, for gear parts that are heavily loaded during operation, for which high mechanical properties are required (in particular, high hardness values under the cemented layer and in the core), exceeding the working loads acting on them and providing high fatigue strength during operation, an alloy according to invention, which after cementation under the specified conditions provides the highest fatigue strength when used.

Cementation experiments were also carried out at high temperature (980 ° C) on the above-described cylindrical specimens from control steels A, D and steel E according to the invention. And in this case, the carbon content in the cemented surface layer was 0.75 wt.%. In both cases, sought to obtain hardness on the surface from 700 to 800 HV and at a depth of 0.50 mm 550 HV. Hardening in a gaseous medium (nitrogen) after cementation was carried out at a pressure of 20 bar for steels A and D and at a pressure of only 1.5 bar for steel E. The results are shown in table 4. Grain sizes are also shown in accordance with ASTM standard.

Table 4 Properties after cementation and quenching in a gaseous medium at a pressure of 20 bar (steel A, C) and 1.5 bar (steel E) Steel Hardness of the surface layer, HV Depth of cemented layer, mm The hardness of the core outside the cementation zone, HV Grain size in cemented layer according to ASTM standard Grain size outside cemented layer according to ASTM standard A (counter) 740 0.50 312 7/9 8/9 D (control) 735 0.59 461 7/8 8/9 E (according to the image) 740 0.70 500 8/9 9/10

As in the case of cementation at a normal temperature of 930 ° C, both types of steel can achieve the required hardness of the surface layer.

The invention provides a significantly greater depth of cemented layer compared to control steel A, although the latter was tempered in a much more stringent mode, designed to increase the depth of cementation, while the remaining conditions remained the same.

The difference between the hardness of the surface layer and the hardness of the core is significantly less in steel according to the invention than in control steels A and D (ΔHV = 240 for steel E, 428 for steel A and 274 for steel D, respectively). The above advantages with respect to deformation during quenching after cementation at ordinary temperature are manifested in this case, but to an even greater extent.

The hardness of the core is higher for steel according to the invention than for control steel, despite the significantly lower pressure of the quenching medium. The effect on the increase in fatigue strength during operation after quenching at ordinary temperature, which was mentioned above, was noted in this case as well.

Finally, both in and outside the cementation zone, the steel according to the invention contains finer grains according to ASTM than control steels A and D. Therefore, the risk of increasing grain size in high temperature cementation is reduced in this steel. This circumstance is a very important advantage, since the growth of grain in parts after cementation has an extremely negative effect on the fatigue strength of the tooth leg and on the durability of cemented parts. Therefore, the steels according to the invention have excellent machining ability in the manufacture of gear parts (or any other parts that require comparable properties) subjected to high-temperature carburizing or nitrocarburizing, provide economic benefits achieved without any deterioration in the performance of these parts.

Other cementing experiments were also carried out on control steel A and steel E according to the invention at low pressure.

It takes 72 minutes to grind steel A at low pressure and a temperature of 930 ° C, followed by quenching in a gaseous medium at a pressure of 20 bar to obtain the required cementation depth of 0.50 mm at a hardness of 550 HV. When carburizing E steel according to the invention at low pressure and a temperature of 930 ° C followed by quenching in gas (in the same gas as when quenching steel A) at a pressure of 1.5 bar, 30 minutes are required to achieve the same cementation depth 0.50 mm at a hardness of 550 HV.

When carburizing at low pressure and a high temperature of 980 ° C for control steel A, 30 minutes were spent and gas quenching at a pressure of 20 bar was required to obtain the necessary cementation depth of 0.50 mm at a hardness of 550 HV. To achieve the same cementation depth of 0.5 mm with a hardness of 550 HV in steel E according to the invention, 20 minutes of cementation at low pressure and a temperature of 980 ° C are required, while quenching is carried out in a gas medium at a pressure of only 1.5 bar. When hardening steels A and E, the same gas was used, of course.

This indicates that steel E according to the invention allows to reduce the cementation time both at its usual temperature (930 ° C) and at high temperature (980 ° C), resulting in a decrease in the cost of cementation (gas consumption for cementation is reduced, its duration, etc.) and increases productivity in the manufacture of cemented parts.

Owing to its hardenability, the alloy according to the invention also makes it possible to reduce the pressure of quenching gases while obtaining the same depth of carburization, which makes it possible to further reduce or eliminate the deformation of cemented parts and to provide advantages and simplification of gas quenching technology of parts in gas quenching furnaces.

Also at low pressure and high temperature (980 ° C) cemented samples were not cemented for impact testing (dimensions: L = 55 mm, cross section: 10 × 10 mm) made of control steel A, before gas quenching at a pressure of 20 bar , as well as steel E according to the invention, before gas quenching at a pressure of only 1.5 bar. The depth of carburization was the same, and the same quenching gas was used. The cemented and thus hardened samples were then subjected to impact fracture at room temperature. At the same time, the following fracture energy indicators were obtained:

- 19 J for control steel A,

- 29 J for steel E according to the invention.

Simultaneously, cemented at low pressure and normal temperature (930 ° C), impact test specimens made of control steel A were obtained to obtain the same cementation depth as above. Then they were quenched using the same gas at a pressure of 20 bar. These samples were subjected to destruction at room temperature, as described above, while the energy of destruction was 17 J, which is significantly less than the energy spent on the destruction of steel E according to the invention, cemented at high temperature.

This indicates that despite the fact that the hardness of the core of the sample from control steel A (312 HV) was less than the same hardness of the sample from steel E according to the invention (500 HV), the resistance of steel E cemented at high temperature exceeded the resistance of the control steel, cemented at high temperature or at ordinary temperature, at the same final depth of carburization. In other words, the use of steel according to the invention for high-temperature cementation in order to obtain its required depth does not adversely affect the resistance of cemented parts made from this steel compared to the use of control steel also cemented at high or normal temperature to obtain the same depth of cemented layer . The difference between the hardness indices of the core parts of these two types of steel does not have a negative effect. This also indicates the special suitability of the steel according to the invention for high-temperature cementation, due to which the cementation time is reduced at the same time, the productivity is increased and the cost of cementation is reduced in comparison with well-known steel grades cemented at ordinary or high temperatures. The resulting performance properties of parts, such as durability, are not inferior to the same properties of control steels.

Under the conditions described above, cementation at low pressure and high temperature (980 ° C) of bending fatigue samples made of steel E according to the invention and equipped in their central part with an expanding U-shaped notch was also carried out. After cementation, gas quenching was carried out at a pressure of 1.5 bar, while the depth indicators of the cemented layer were the same, the same gas was used for quenching as in the experiments on samples for impact strength testing. In the same way, gas cementation was carried out on steel A of the prior art at a normal temperature of 930 ° C to achieve the same cementation depth as described above, using bending fatigue test specimens similar to those of steel E. After cementation, the samples were quenched oil in order to increase the content and hardness during the bending fatigue test. Then, the fatigue strength limits of both batches of cemented samples from steels A and E were compared at a four-point bending fatigue test, with an expanding U-shaped notch of these samples being placed at right angles to the bending fatigue test. Bending fatigue tests were carried out for steels A and E subjected to carburizing and hardening under the above conditions at 10 million cycles.

Under these conditions, the fatigue limit of steel E according to the invention at 10 million cycles was 1405 MPa, the fatigue limit of steel A was only 1165 MPa.

This shows that the use of steel according to the invention for high-temperature cementation in order to obtain the required depth of the cemented layer is not accompanied by a decrease in bending fatigue resistance, but rather it is very effective compared to traditional cementation at ordinary temperature used for steel known from the prior art for cementing to the same depth even with subsequent oil quenching to increase resistance to bending fatigue.

It should also be added that such bending fatigue tests are used to simulate the fatigue resistance of a gear tooth leg, gear wheel, or gear part when used in an automobile gearbox. This additionally shows that the steels according to the invention have a special ability for high-temperature cementation, while at the same time providing a reduction in the duration of cementation, an increase in productivity and a reduction in the cost of cementation compared to known steels that are cemented at ordinary temperature, while the performance of the parts does not deteriorate, such as resistance to fatigue when bending the legs of the gear tooth or gear after cementation.

Claims (8)

1. Steel for machine parts, characterized in that its composition includes, wt.%:
0.19≤С≤0.25
1.1≤Mn≤1.5
0.8≤Si≤1.2
0.01≤S≤0.09
traces≤P≤0.025
traces≤Ni≤0.25
1≤Cr≤1.4
0.10≤Mo≤0.25
traces≤Cu≤0.30
0.010≤Al≤0.045
0.010≤Nb≤0.045
0.0130≤N≤0.0300
Ti≤0.005
optionally: traces ≤Bi≤0.10, and / or traces ≤Pb≤0.12, and / or traces ≤Te≤0.015, and / or traces ≤Se≤0.030, and / or traces ≤Ca≤0.0050
the rest is iron and smelted impurities,
while the chemical composition is set so that the average
J _3m , J _11m , J _15m and J _25m Jomini tests were:
α = | J _11m -J _3m × 14/22-J _25m × 8/22 | ≤2.5 HRC and
β = J _3m -J _15m ≤9 HRC. 2. Steel for machine parts according to claim 1, characterized in that its composition is selected so that
β = J _3m -J _15m ≤8 HRC. 3. Steel for machine parts according to claim 1 or 2, characterized in that its composition includes, wt.%:
0.19≤С≤0.25
1.2≤Mn≤1.5
0.85≤Si≤1.2
0.01≤S≤0.09
traces≤P≤0.025
0.08≤Ni≤0.25
1.1≤Cr≤1.4
0.10≤Mo≤0.25
0.06≤Cu≤0.30
0.010≤Al≤0.045
0.015≤Nb≤0.045
0.0130≤N≤0.0300
Ti≤0.005
optionally: traces of ≤Bi≤0.07, and / or traces of ≤Pb≤0.12, and / or traces of ≤Te≤0.010, and / or traces of ≤Se≤0.020, and / or traces of ≤Ca≤0.0045
the rest is iron and smelted impurities. 4. Steel for machine parts according to claim 3, characterized in that its composition includes, wt.%:
0.20≤С≤0.25
1.21≤Mn≤1.45
0.85≤Si≤1.10
0.01≤S≤0.08
traces ≤P≤0,020
0.08≤Ni≤0.20
1.10≤Cr≤1.40
0.11≤Mo≤0.25
0.08≤Cu≤0.30
0.010≤Al≤0.035
0.025≤Nb≤0.040
0.0130≤N≤0.0220
Ti≤0.005
optionally: traces of ≤Bi≤0.07, and / or traces of ≤Pb≤0.12, and / or traces of ≤Te≤0.010, and / or traces of ≤Se≤0.020, and / or traces of ≤Ca≤0.0045
the rest is iron and smelted impurities. 5. A method of manufacturing a machine part from cemented or nitro-cemented steel, characterized in that it uses steel according to any one of claims 1 to 4, which is machined and subjected to cementation or nitrocarburizing followed by hardening. 6. The method according to claim 5, characterized in that the said cementation or nitrocarburizing is carried out at a temperature of 950-1050 ° C. 7. Steel part of the machine, characterized in that it is made by the method according to claim 5 or 6. 8. A machine part according to claim 7, characterized in that it is a gear part.

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