RU2553764C1 - Nitrated steel for gear wheels - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel has the following component ratio, wt %: carbon of more than 0.40 to 0.43, silicon 0.17-0.37, manganese 0.50-0.65, chrome of 1.10 to less than 1.20, molybdenum 0.20-0.30, vanadium 0.05-0.08, nickel ≤0.30, copper ≤0.25, sulphur ≤0.020, phosphorus ≤0.020, and iron is the rest.

EFFECT: specified stable characteristics both of a nitrated layer: its thickness, hardness, structural viscosity and obtaining strength and viscosity of the core at satisfactory processibility and accuracy of large-sized gear wheels and increasing resistance of bending and contact fatigue of a nitrated layer and the whole part.

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Description

The invention relates to metallurgy and mechanical engineering and can be used for the manufacture of highly loaded large-sized gears, mainly with internal gearing, in particular for the manufacture of large-sized nitrided crown gears of planetary gearboxes used in marine vessels, gas and oil industries, and designs of particularly heavy-duty dump trucks ( over 100 tons) and in other machines and mechanisms generating and transmitting large capacities.

For the manufacture of large nitrided wheels with internal gear teeth for mining dump trucks, steel grade 38X2MUA GOST 4543-71 [1] is used, containing, wt.%:

Carbon 0.35-0.42 Silicon 0.20-0.45 Manganese 0.30-0.60 Chromium 1.35-1.65 Molybdenum 0.15-0.25 Aluminum 0.7-1.1 Nickel ≤0.30 Copper ≤0.30 Sulfur ≤0.025 Phosphorus ≤0.025 Iron rest

At a nitriding temperature of 500-520 ° C, this steel provides a maximum hardness of 1100-1200 HV, which allows it to be used for the manufacture of cylinder liners of engines, spindles, and often gears. Preliminary heat treatment is carried out in the form of quenching from 930-950 ° C and tempering 600-650 ° C.

The disadvantages of steel are that due to the high aluminum content it is low-tech. It is characterized by defects of metallurgical origin: the presence of non-metallic inclusions, the formation of small cracks, hairlines and others [3].

The nitrided layer of large wheels made of this steel, although it has high values of hardness and wear resistance, is characterized by a small thickness (relative to the tooth module) and increased brittleness, which occurs during operation of large gear wheels and unavoidable dynamic loads (especially when the hardness of the tooth core is lower than 240 HB ) leads to cracking of the layer and its peeling - “peeling”.

In the manufacture of large gears with internal gearing, the problem of ensuring accuracy in the formation of teeth is associated with a large amount of metal being removed and the inability to replace a tool that has become dull during the process of cutting teeth.

The machining of the aluminum-containing steel under consideration, especially during gear grinding, is difficult, which leads to a loss of accuracy and, consequently, a decrease in the structural strength of the material.

Therefore, for large gears, machinability of steel is of particular importance.

Closest to the proposed steel and selected as a prototype steel 38XMA [2] of the following composition, wt.%:

Carbon 0.35-0.42 Silicon 0.17-0.37 Manganese 0.35-0.65 Chromium 0.90-1.3 Molybdenum 0.20-0.30 Nickel ≤0.30 Copper ≤0.30 Sulfur ≤0.025 Phosphorus ≤0.025 Iron rest

The steel composition is well processed.

The disadvantage of steel is that a wide range of chemical elements and their composition do not provide the necessary stable strength and operational qualities of nitrided oversized gears with internal gearing.

The nitrided layer of large wheels made of this steel, although viscous, is not sufficiently wear-resistant.

The composition of the steel does not allow for the hardenability of the material of the part at cross sections characteristic of large gears.

The objective of the invention is to develop the composition of steel, which allows to increase the technological and operational properties of large gears.

The problem is solved due to the fact that the nitrided steel for gears contains carbon, silicon, manganese, chromium, molybdenum, vanadium, nickel, copper, sulfur, phosphorus, iron in the following ratio of components, masses. %:

Carbon from more than 0.40 to 0.43 Silicon 0.17-0.37 Manganese 0.50-0.65 Chromium 1.10 to less than 1.20 Molybdenum 0.20-0.30 Vanadium 0.05-0.08 Nickel ≤0.30 Copper ≤0.25 Sulfur ≤0.020 Phosphorus ≤0.020 Iron rest

The presence of the specified narrowed limit on the carbon content in this steel in combination with alloying elements is sufficient to obtain through hardenability of large gears and ensure the stability of their properties. A content of less than 0.40% reduces hardenability, more than 0.43% degrades workability.

Silicon deoxidizes steel, but at a concentration higher than that indicated, during nitriding it promotes the development of a brittle γ′-phase in the layer, thereby increasing its brittleness.

A silicon content of less than 0.17% reduces hardenability, increases the ductility of steel, thereby degrading machinability by cutting.

Manganese deoxidizes and hardens steel, binds sulfur, however, with a manganese content of more than 0.65%, it increases the tendency to grain growth.

The manganese content of 0.5-0.65% ensures complete desulfurization of steel, thereby eliminating the unwanted release of sulfur in the form of a eutectic.

Chrome increases the strength of steel. A chromium content of more than 1.2% impairs machinability; less than 1.10 does not provide the necessary hardenability and strength.

Chrome, molybdenum and vanadium are the main nidride-forming elements in the formation of the diffusion layer during nitriding of products from this steel. The introduction of alloying elements of chromium, molybdenum, vanadium increases the speed of the nitriding process, the solubility of nitrogen in iron, and contributes to the release of finely dispersed nitrides.

Molybdenum and vanadium are required in nitrided steel to provide resistance against tempering and temper brittleness.

In nitrided steel without aluminum, the hardness of the diffusion nitrided layer depends on the initial hardness of the core: the higher the hardness of the core (lower tempering temperature), the greater the hardness of the nitrided layer throughout its thickness.

The content of molybdenum 0.20% and vanadium 0.05% provides sufficient resistance against tempering and temper brittleness: at a lower content of these elements such properties of steel are not provided to the necessary extent.

The upper limits of the content of molybdenum and vanadium in steel are taken in connection with the need to prevent the appearance of intermetallic phases of these elements that impair the machining, especially cutting teeth of large gears with internal gearing.

The presence of molybdenum and vanadium in the steel within the specified limits increases the resistance to bending and contact fatigue of the nitrided layer and the entire part by braking under stress the process of localization of plastic deformation that accelerates fracture.

Nickel and copper - impurities entering the charge with scrap metal, and their content, above mentioned, reduce nitrogen diffusion during nitriding.

Sulfur and phosphorus are inevitable harmful impurities and a decrease in their content of less than 0.02% makes the steel cleaner and better.

A number of melts of the inventive steel compositions 1-6 and prototype, the chemical composition of which is presented in table 1.

Steel prototype and the inventive composition was obtained as follows.

Steel was smelted in an AC steelmaking furnace with a capacity of 6 tons with a main lining. The composition of the charge is alloyed scrap and waste gr. B11, steel scrap and category 3A waste. Deoxidants - secondary aluminum grade AB91, ferromanganese 78A, ferrosilicon FS-45.

The melt temperature is up to 1750 ° C, the smelting of the intermediate product is 60 min, the secondary furnace treatment (including evacuation is 60 min), the temperature of the steel is 1600-1680 ° C. Casting temperature - 1555 ° C. Bucket type - with the ability to purge metal in the bucket with argon, equipped with a slide gate.

The diameter of the ingots is 277 ± ⁵ , 340 ± ⁵ , 411 ± ⁵ .

The ingots are either pre-forged, or immediately cut in length and stitched. From stitched blanks by the method of hot rolling on ring rolling mills rings are made - blanks of gear wheels.

From the rings of the claimed steel and the prototype of the above compositions, pilot industrial batches of ring gears (internal gearing gears) of planetary gear motors of mining dump wheels with a carrying capacity of 55, 130, 240 and 320 tons are made.

The machinability of the gears of the prototype and the inventive steel compositions 1, 2, 3, 4 is satisfactory.

Mechanical tests were carried out on samples cut from the core of the gears, after the final operation, nitriding.

Heat treatment: annealing, quenching with cooling in oil, high tempering with cooling in the chill chamber, and nitriding were carried out according to known methods [3, 4].

Strength, plastic properties and impact strength at both plus and minus temperatures of gears made of the inventive steel, are stable and higher values in comparison with the prototype.

Steel is well nitrided. Hardness of nitrided surface 700-770 HV ₅ .

Layer thickness: up to a core structure of 0.4-0.5 mm; to a core hardness of 0.65-0.75 mm. The layer is strong, viscous and wear-resistant.

The accuracy of the gears with module 7, made of the inventive steel, high, table 3.

Toothed wheels with internal gearing (m = 7 mm and m = 10 mm) provided the given reliability and operability of planetary gearboxes for mining dump trucks with a lifting capacity of 55, 130 and 240 tons.

Information sources

1. GOST 4543 “Rolled products from alloy structural steel”, p. 44.

2. Marochnik steels and alloys. Edited by V.G. Sorokina, 1989. M.: "Engineering", p. 221-222.

3. Yu.M. Lakhtin, Y.D. Kogan, G.I. Shpis, Z. Bemer. "Theory and technology of nitriding." 1991. M.: Metallurgy. S. 237-238.

4. I.V. Firger. "Heat treatment of alloys." Leningrad: Mechanical Engineering. 1982.- S. 27-35.

Claims (1)

Nitrided steel for gears containing carbon, silicon, manganese, chromium, molybdenum, vanadium, nickel, copper, sulfur, phosphorus and iron, characterized in that it contains components in the following ratio, wt.%:
carbon from more than 0.40 to 0.43 silicon 0.17-0.37 manganese 0.50-0.65 chromium 1.10 to less than 1.20 molybdenum 0.20-0.30 vanadium 0.05-0.08 nickel ≤0.30 copper ≤0.25 sulfur ≤0.020 phosphorus ≤0.020 iron rest