EXPERIMENTAL STUDY OF NITRIDED GEARS AT HIGH SPEEDS

A. E. BELYAEV

(Presented by the scientific seminar of the Department of Applied Mechanics)

Reducing the dimensions and weight of high-speed reduction gears while simultaneously striving to increase their load-carrying capacity is unthinkable without introducing strengthening of the working surfaces of the teeth. At present, the two most widespread methods of surface strengthening are nitriding and carburizing.

The use of surface strengthening has made it possible to sharply increase the load-carrying capacity of high-speed gears. Thus, the reducers of floating vessels of modern construction have a coefficient value \(K_{\mathrm{H}} = 25—30\ \mathrm{kg}/\mathrm{cm}^2\) \([6, 9, 11, 13]^*\).

A significant increase in the load-carrying capacity of gear transmissions has been obtained experimentally by a number of researchers. This is well illustrated by the diagram in Fig. 1 \([8]\), where the value of the coefficient \(K_{\mathrm{H}}\) for improved gears made of various grades of steel lies in the interval \(8—18\ \mathrm{kg}/\mathrm{cm}^2\), while the load-carrying capacity of gear wheels subjected to surface strengthening reaches \(85 \div 95\ \mathrm{kg}/\mathrm{cm}^2\).

It was precisely this circumstance that enabled Lloyd’s Register to substantially increase the permissible value of the coefficient \(K_{\mathrm{H}}\) for gears subjected to surface strengthening and manufactured by various firms (Maag, Allen, Brown-Boveri, etc.).

The most economical type of thermochemical strengthening is nitriding, since after it gear transmissions do not require special finishing operations (for example, grinding, used after carburizing) in order to obtain the required degree of accuracy. This is due to the fact that the nitriding process itself requires a comparatively low temperature \((T = 500—540^\circ\mathrm{C})\), which practically does not cause changes in the dimensions of the tooth.

* The coefficient \(K_{\mathrm{H}}\) characterizes the load-carrying capacity of gear transmissions:

\[ K_{\mathrm{H}}=\frac{2M_1}{B\cdot d_{\mathrm{d}1}}\cdot\frac{i\pm1}{i}\ \mathrm{kgf}/\mathrm{cm}^2, \]

where \(M_1\) is the torque on the pinion, \(\mathrm{kg\,cm}\);
\(d_{\mathrm{d}1}\) is the pitch-circle diameter of the pinion, \(\mathrm{cm}\);
\(B\) is the working width of the gear tooth, \(\mathrm{cm}\);
\(i\) is the gear ratio (the minus sign is for internal gearing).
At present, the coefficient \(K_{\mathrm{H}}\) is most often used to compare the load-carrying capacities of high-speed gears.

The extensive use of nitriding to obtain a hard surface in turbine-type gear transmissions is indicated by a number of works [12, 13]. In the latter work, during tests of a full-scale ship installation, the load-carrying capacity of a nitrided pair proved to be equal to 95 kg/cm². At the same time, the working surface of the tooth had no damage whatsoever. Thus, the increasingly established tendency to manufacture gear wheels with increased hardness of the working surface of the teeth predetermines the advisability of testing high-speed nitrided transmissions.

Legend:

• gear wheels made of ordinary improved steels;
• gear wheels made of alloyed improved steels;
• gear wheels subjected to carburizing followed by quenching;
• nitrided gear wheels.

Fig. 1. Coefficient \(K_H\) for gear wheels with different heat and thermochemical treatment (according to data from work [8]).

Until recently, in domestic and foreign machine building only steels of the 38KhMYuA type (GOST 3533—58) with a high aluminum content were used. This was due to the desire to achieve maximum surface hardness (up to 1000–1100 HV). However, the use of aluminum steels also has a number of substantial drawbacks. On the surface a layer of very hard brittle nitrides is formed, 20–30 μm thick, the so-called ε-phase. Under the action of load this layer cracks and crumbles out, distorting the involute profile of the teeth.

I. M. Kilimov [5], who carried out a large number of experiments with nitrided gear wheels, observed cracking and complete spalling of the ε-phase from the tooth surface. In his works R. R. Galper [2] found, on the surface of transmission teeth even at relatively small loads, fatigue cracks in steels 38KhMYuA and 45Kh2N2MFYuA, oriented along a strongly developed network of the ε-phase.

The presence of the ε-phase also adversely affects the fatigue limit in bending. E. A. Kantorovich [4] points out that the lower the probability of formation of the ε-phase, the higher the fatigue limit. In addition, according to the data of work [7], with an increase in the nitrogen content in the layer and an increase in hardness, the static strength of the nitrided layer decreases and destruc-

...fractures are brittle in character. At the same time, the sensitivity of the steel to tensile stresses increases sharply. S. F. Balashov [1] comes to the same conclusions; he recommends that, for heavily loaded parts, the surface hardness should be no more than 650–700 \(HV\) (550–600 HB)*.

There is also the opinion that increasing the hardness of the surface layer does not entail an increase in the bending fatigue limit \(\sigma_{-1b}\). Thus, in [3], an increase in hardness from 41 \(HRC\) to 62 \(HRC\) did not affect \(\sigma_{-1b}\).

What has been said above permits the conclusion that, for turbine-type gear transmissions, it is advisable to choose an aluminum-free grade of steel and a nitriding regime in which the surface hardness would not exceed \(750 \div 800\ HV\).

Experience in the operation of high-speed transmissions, as well as experiments carried out at LKZ, make it possible to adopt for experimental pairs the new steel 30Kh2N2VFMA, whose mechanical properties are given in Table 1.

Table 1

Mechanical characteristics of steel 30Kh2N2VFMA

The nitriding regime for gear pairs made of this steel provides for two periods. In the first, the gear pairs are held in the furnace at a temperature \(T = 500 \pm 5^\circ\) for 40 hours with ammonia dissociation from 16 to 25%; in the second, they are held for 60 hours at a temperature \(T = 540 \pm 5^\circ\) (degree of dissociation \(40 \div 55\%\)).

Such a regime makes it possible to obtain a surface hardness \(HV = 650\text{–}700\) and a layer depth of \(0.3 \div 0.5\) mm. The hardness of the core is \(HB = 240 \div 270\).

Before the experiment, all gear pairs were run in with an abrasive paste consisting of a mixture of 1/4 boron carbide (GOST 3647–59) and 3/4 fatty solidol. Running-in was carried out at a speed of 1.5 m/sec under insignificant loads. Usually, after running-in, a slight metallic gloss was observed on the teeth over the entire width of the gear rim.

During the experiment**, continuous inspection of the tooth surfaces was carried out in order to detect possible spalling of flakes of the hard layer or the occurrence and development of pitting.

It was noted that after \(N_c = 10^8\text{–}2 \cdot 10^8\) on the nitrided surfaces of the wheels, scoring marks appear and the surfaces “brighten.” With further operation, the entire surface of the pinions and wheels becomes shiny...

* Recently, abroad [10], so-called “soft nitriding” has been used. In this process the surface hardness lies in the range \(HV = 500\text{–}650\).

** Comparative experiments were carried out with uncorrected gear wheels whose geometrical parameters were identical: \(m = 3\) mm, \(\alpha = 20^\circ\), \(z_1 = 28\), \(\beta = 19^\circ 11'17''\), \(i = 2\), \(B = 30\) mm, accuracy grade 5, gear milling with roughness \(\nabla 7\). The materials of the improved pinions and wheels were, respectively, 34KhN3M and 34KhN1M, hardness \(HB_{\text{ш}} = 262\) and \(HB_{\text{к}} = 250\).

spalling. On the gears of both transmissions, during the tests, bands oriented in the directions of the sliding velocity were observed.

The tests were carried out with stepwise loading, beginning with a load corresponding to \(K_{\mathrm{H}}=50\ \mathrm{kg/cm^2}\). Operation at the subsequent load stages, at the basic numbers of cycles, introduced no change into the condition of the working surfaces of the teeth.

The load \(K_{\mathrm{H}}=74.5\ \mathrm{kg/cm^2}\) proved, in our experiments, to be the maximum load at which the test rig operated normally. A further increase in load was found to be inexpedient because of design conditions.

Fig. 2. Condition of the working surfaces of the gear teeth of the nitrided wheel after running for three basic numbers of cycles
\((K_{\mathrm{H}}=74.5\ \mathrm{kg/cm^2})\)

After inspection of the teeth at the indicated load with the naked eye, no spalling pits were found; upon inspection, only a matte deposit of particles from the tooth surface was observed. When viewed through a magnifying glass at \(4\times\) magnification, the matte surface of the gear teeth was found to be covered in the near-pitch zone with the finest spalling pits. The surface of the wheel teeth was also affected by a small number of spalling pits (Fig. 2).

Thus, the load evidently did not reach the limiting value. At the same time, the load-carrying capacity of the nitrided transmissions tested at speeds of \(40\text{--}60\ \mathrm{m/sec}\) proved to be \(2\text{--}2.5\) times greater than in transmissions subjected to improvement.

Conclusions

1. The use of surface hardening is one way to sharply increase the load-carrying capacity of high-speed gear transmissions.

2. The load-carrying capacity of nitrided gear transmissions made of steel 30Kh2N2VFMA \((HV=700\text{--}750)\) proved to be \(2\text{--}2.5\) times higher than the load-carrying ...

capacity of equal-strength hardened gears (HB—250–260) at speeds of 40–60 m/sec.

REFERENCES

1. B. F. Balashov. Nitriding as a method for increasing the strength of machine parts. In the collection Increasing the Strength of Machine Parts by Surface Treatment. Mashgiz, 1952.

2. R. R. Gal’per. Contact strength of high-speed gears with surface hardening. LD NTP, L., 1964.

3. I. E. Kantorovich. Increasing the strength of machine parts by thermochemical treatment. In the collection Increasing the Strength of Machine Parts, 1949.

4. I. E. Kantorovich. Modern methods of thermochemical treatment and their influence on fatigue strength. In the collection Increasing the Service Life of Machines, 1960.

5. I. M. Kilimov. Increasing the load-carrying capacity of gears through the use of the hardness-gradient effect. Vestnik mashinostroeniya, No. 3, 1959.

6. O. A. Pyzh. Production of turbine gears and prospects for increasing their accuracy and load-carrying capacity. In the collection Modern Methods for Assessing Quality and Ways of Increasing the Manufacturing Accuracy of Gears. Mashgiz, 1962.

7. S. V. Serensen. Fatigue resistance in connection with hardening and structural forms. In the collection Increasing the Fatigue Strength of Machine Parts, Moscow, 1952.

8. A. Chamberlain. Proceedings of the International Conference of Gearing, London, 1958.

9. W. H. Darlington. Transaction Institute of Marine Engineers, vol. XVIII, 9, 1956.

10. G. Niemann. Softnitrierten Zahnräder, VDI—Z, No. 6, 1960.

11. H. H. Page. Proceedings of the International Conference on Gearing, London, 1958.

12. C. G. Wahl. The Marine Engineering and Naval Architect, May, vol. 83, No. 1007, 1960.

13. P. D. Weavings, W. H. Sampson. Transaction Institute of Marine Engineers, March, 1963.