RU2048547C1 - Low-carbon secondary hardening martensite class steels gear wheels working method - Google Patents

Abstract

FIELD: metallurgy. SUBSTANCE: gear wheels of low-carbon secondary hardening martensite class steels are subjected to 2 or 3-fold cyclical volumetric hardening under temperature of 900 975 C and holding for 1 3 hours with tempering preceding each hardening and exercised under temperature of 600 675 C for 1 5 hours, tempering, working and nitriding for given depth, that is exercised in two steps: first under temperature of 500 540 C for 10 20 hours, second step under temperature of 450 570 C for 20 40 hours. EFFECT: method improves process of low-carbon second hardening martensite class steels gear wheels working. 1 tbl

Description

 The invention relates to metallurgy, in particular to the thermal and chemical-thermal treatment of gears, and is aimed at solving the problem of creating a technology for processing nitrided highly loaded grinding wheels, which ensures their performance comparable with cemented (nitrocarburized) gears.

 Cementation (nitrocarburizing) achieves high values of contact endurance and bending endurance characteristics. However, the use of these types of chemical-thermal treatment followed by heat treatment due to multiple high-temperature heating and cooling causes severe deformation of the gears, reduces accuracy by 2.3 degrees. To eliminate the resulting geometric errors during gear grinding, it is necessary to remove a large allowance (0.15.0.25 mm on each side of the tooth). The need to prevent burns limits the performance of gear grinding, increasing its complexity, which can reach up to 40% of the entire complexity of manufacturing gears.

 Nitriding as a method of surface hardening of gears is widely used in production. However, there is no data on the possibility of using nitriding for highly loaded gear wheels in order to achieve a resistance comparable with that of cemented ones. The levels of permissible stresses recorded in regulatory documents and standards of various countries, including Russia (GOST 21354-87), for nitrided wheels are much lower than for cemented ones.

 The closest technical solution, selected as a prototype, is a method of nitriding parts, which consists in volume hardening, surface hardening by high-frequency currents (HDTV), high tempering, machining and nitriding to a depth of not more than the depth of the surface hardened layer.

 An increase in contact durability when using the aforementioned method is achieved by obtaining fine austenite grain N 11.12 in the surface layer of a part due to the introduction of high-frequency hardening into the heat treatment cycle. Obtaining so many small grains prevents the formation of a coarse nitride network during nitriding and, accordingly, increases the resistance to nucleation and development of fatigue cracks.

However, structural and, accordingly, stress state features arising in the transition zone between the surface hardened layer and the core in a number of alloy steels with yield strength σ _0.2 ≥ 800 MPa cause a decrease in bending fatigue strength. This limits the application of this method.

 The proposed technical treatment of nitrided high-loaded high-precision non-grinded gears, which ensures their performance comparable with cemented gears, is based on the simultaneous fulfillment of a set of requirements: for the choice of steels; heat treatment; on nitriding technology.

The proposed method enables the nitriding process highly loaded precision gears from low-carbon martensitic steels vtorichnotverdeyuschih class and implemented conducting two- or three-fold cyclic volumetric tempering temperature ° ^C at 900,975 exposure time 1.3 hours before each preliminary hardening by tempering at 600,675 ^C for 1.5 hours and nitriding two-stage: a first stage at a temperature of 500,540 ^{° C} for 20.10 hours in the second stage at a temperature of 540. 570 ^{° C} for 40.20 hours.

Nitriding is carried out for gears made of low-carbon secondary hardening steels of the martensitic class:
with a yield strength σ of _0.2 after heat treatment is not lower than 1000 MPa, hardness not lower than 38.5 NRC _e and impact strength KSU not lower than 0.7 MJ / m ² . Higher values of the hardness of the core of nitrided gears improve their performance, but with hardness higher than 42.43 NRC _e , gear milling becomes difficult;
with heat resistance not lower than 560,580 ^of C. With this heat resistance may conduct nitriding process at temperatures providing reinforcement layer is preferably incoherent precipitates, which reduces microdistortions crystal lattice α-phase, and thus increases the contact endurance. Fulfillment of the requirements for heat resistance is also associated with the need to preserve the specified values of yield strength and hardness with a duration of the nitriding process of up to 70.75 hours;
not prone to temper brittleness of the 2nd kind (reversible temper brittleness);
with a low propensity for structural heredity, which makes it possible to obtain austenite grain N 9.10 by volumetric heat treatment;
not prone to the formation in the process of nitriding of a carbide network along the grain boundaries of austenite both in the core and in the nitrided layer. For this, steel should contain at least 2.3% nickel;
with a nitride-forming element content of not higher than 3.4% to obtain a nitrided layer thickness of at least 0.6 mm and a smooth change in hardness from the surface to the core.

Carrying out a two- or three-cycle volumetric heat treatment with the indicated temperature and time parameters preliminary tempering processes and hardening in each cycle (at 600,675 ^C for 1.5 hours by quenching 900,975 + ^C-delayed 1.3 hours) provided a fine austenite grains even N 9.10 in the manufacture of large wheels. In combination with the cyclic processing and final tempering at 560,600 ^C achieved the required level of mechanical properties and heat resistance.

When satisfying the requirements for the composition of steels and the properties of the core after two-stage nitriding, a nitrided layer thickness of at least 0.60 mm is provided;
surface hardness not lower than 800 НV5 when hardening mainly by incoherent precipitates, smooth change in hardness along the depth of the nitrided layer;
effective thickness of the hardened layer is more than 0.30 mm with a hardness of at least 500 HB5; the thickness of the ε-phase layer is not more than 5 μm; the absence of a continuous network of the γ phase and a continuous network of nitrides of alloying elements.

 The proposed method of hardening was tested in laboratory conditions and tested on the gears of high-speed gearboxes made of steel 16Kh2N3MFBAYU-Sh.

 This steel, developed on the basis of the requirements for heat-resistant cemented steels, at the same time satisfies the requirements for nitrided steels for heavily loaded gear wheels and described above.

After quenching from 900 ^{° C} in oil, and tempering at 600 ^{° C} for 2 hours, the steel has the following mechanical properties: σ _B = 1270 MPa, σ _0,2 ≥ = 1100 MPa δ15,6% θ = 63% KSU = 0, 89 MJ / m ² ; HRC _e = 39.

Hardness and toughness are retained unchanged after tempering at 540 ^{° C} for 60 hours. The W-steel 16HN3MFBFYU relates to genetically-grained and at the same time, shows a tendency to structural heredity, so obtaining fine grain austenite N 9.10 requires a special heat treatment.

 The most effective for steels of this type, characterized by high stability of austenite in the pearlite region, is the use of cyclic volume hardening with intermediate tempering.

So, for 16Kh2N3MFBAYU-Sh steel, grain formation N 9.10 at initial N 6. 7 is possible with the following temperature and time parameters:
1st cycle tempering 650 ^о С, 2 h + quenching 975 ^о С, 1 h, oil;
2nd cycle vacation 650 ^о С, 2 h + hardening 900 ^о С, 1 h, oil;
3rd cycle rental 650 ^{° C,} 2 hours + tempering 900 ^{° C,} 1 hour, oil.

 A similar result can be obtained with two-cycle processing, but in this case, the processing parameters must be changed.

 Two- and three-cycle treatments give better stability of properties than single-cycle treatments.

From forged steel blanks 16H2N3MFBAYU-SH, passed three-cycle heat treatment and final rental 600 ^{° C,} 2 hours, were fabricated roller test specimens contact endurance and gears on bending fatigue.

Hardening was carried out on the ion nitriding apparatus according to a two-stage mode: 1st stage, temperature 500 ^о С, time 20 h; 2nd stage temperature 540 ^о С, time 40 h, layer thickness 0.70 mm, surface hardness 824 НВ5, core hardness 39 HRC _e , ε-phase layer thickness varied from 0 to 5 μm.

At the same time, tests were carried out on samples and gears made of 18Kh2N4VA cemented steel with a layer thickness of 1.6 mm, a surface hardness of 824 НV5, and a core hardness of 43 НRC _e .

 The contact endurance limit determined on roller stands according to the rolling scheme with slipping in accordance with GOST 25501-78 for nitrided steel 16X2N3MFBAY-Sh was 2400 MPa, and for cemented steel 18X2N4VA 2200 MPa.

 The flexural endurance of the gears was determined on an electromagnetic pulsator. The cycle of voltage changes was close to zero (the asymmetry coefficient of the cycle is -r = 0.05). The flexural endurance limit of nitrided wheels was obtained equal to 850 MPa and is comparable with the endurance limit of 18Kh2N4VA cemented steel, for which it is 900 MPa.

 An integrated assessment of the health of nitrided gears, replaced instead of cemented and subjected to processing by the proposed method, was obtained during tests on a closed loop installation. The parameters of the tested wheels are shown in the table.

 Reducer gears tested without damage. Thus, while satisfying the entire set of requirements for the composition of steel, its thermal and chemical-thermal treatment, nitrided gears are flexural and contact endurance comparable with cemented ones and do not require gear grinding after nitriding.

Claims (1)

METHOD FOR PROCESSING GEAR WHEELS FROM LOW-CARBON SECOND-CURING MARTENSITY CLASS STEELS, including volume hardening, tempering, machining and nitriding, characterized in that they carry out two or three times cyclic with a continuous temperature of 900 ^° C quenching by tempering at 600 675 ^o C for 1 5 h, and nitriding to a predetermined depth is carried out in two stages: the first at 500 540 ^o C for 10 20 h, and the second at 540 570 ^o C for 20 40 h.

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