Induction Hardening of Large Gears

Induction hardening of large gears can be carried out by one of the following methods: hardening the working surfaces of the teeth (hardening “by the tooth”) or hardening the working surfaces and the hollow of the teeth (hardening “through the hollow”).

We used the second method for hardening the axial gear wheel of the TGMP diesel locomotive (module 10).

After preliminary machining, the gears were improved; then the gears were cut and the teeth were hardened with HDTV heating. Used for heating the engine generator MGZ-250/2500. The operating mode was as follows: power 95-100 kW, generator voltage 450-500 V, generator current 180-220 A, power factor 0.95-0.97. A VTO-500 hardening transformer with 15 turns in the primary winding was used alone in the secondary and upgraded inductor of the construction of the Moscow Automobile Plant (the dimensions of the slats were changed).

The inductor (Img. 1) allows you to simultaneously heat the two surfaces of the teeth and the depression between them: branches 4–4 heat the side surfaces, branch 3 – the depression.

Inductor for hardening gear teeth

Image1. Inductor for hardening gear teeth: 1 and 2 – contact strips; 3 – front tube; 4 – side tubes; 5 – magnetic core; 6 – bar.

Cracks during induction hardening of large gears

We used water for cooling, but after quenching, cracks were observed on the surface of the teeth and on the cavities. When quenched in water heated to 50-60 ° C and in a soap solution, the number of cracks decreased, but the scrap was significant. She did not get rid of cracks and hardening through a tooth proposed by the Leningrad Metal Plant [1]. Cracks were observed in the depressions.

To prevent the occurrence of cracks, we have developed a heat treatment mode for the gear teeth, in which an inductor is introduced into the cavity between the teeth and heats the metal. At the same time, the rear sides of the heated teeth are cooled with water from the sprayer (Img. 2).

Sprayer operation scheme during the period of heating and cooling the teeth

Image2. Sprayer operation scheme during the period of heating and cooling of the teeth: 1 – inductor; 2 – Spraying (hardening) sprayers.

After the cessation of heating (approximately 20 seconds), water is cooled by the backs of the teeth. Then the inductor moves to the next trough. Thus, the coolant is not directed to the parts of the teeth heated by quenching.

To determine the thickness and shape of the hardened layer, the teeth were cut across and along the section (Img. 3).

Hardened layer during hardening of the tooth

Image3. Hardened layer during hardening of the tooth.

The correct method for hardening large gears

As studies have shown, such a boosting of the backs of the teeth contributes to the hardening of their opposite sides to the necessary hardness; The hub mass also contributes to the significant cooling rate of the hollow metal.

After quenching, gears are released for 3 hours at 180-200 ° C. The depth of the hardened layer is 2-6 mm; hardness of the core HB 255-300. Hardness gradually decreases from the surface to the core of the teeth.

Microscopic examination of the hardened area shows that, in the absence of overheating, the structure of the hardened layer is a medium and fine-needle martensite. On the borders of the hardened fate there is troostite.

As a result of this method of hardening gears, the marriage of cracks in the plant was eliminated.

Bibliography:

A. G. SHEKHTMAN
Lyudinovsky Locomotive Plant
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, No. 4. 1963
1. Butkevich, P. I., MITOM, 1961, No. 7.
This article was taken from this resource.