Surface Hardening of Gears: How Many Do You Know?

The surface quenching methods commonly used for gears include flame quenching, induction quenching, contact resistance heating quenching, etc. in addition, there are advanced laser quenching.

After the surface hardening layer is obtained by surface heating and quenching, the original microstructure and properties of the gear core can still be maintained, so as to improve the fatigue strength and wear resistance and maintain the excellent comprehensive properties of the core toughness.

And it can save energy and reduce gear quenching distortion.

Surface Hardening of Gears: How Many Do You Know? 1

1. Classification of surface hardening

See Table 1.

Table 1 Classification of surface hardening

Classification

Technology

Rough separation

 

 

Subdivision

Heating method for heating

Induction quenching, flame quenching, salt bath heating quenching, electrolyte quenching, contact resistance heating quenching, laser quenching, electron beam quenching, ion beam quenching, high-frequency pulse current induction quenching, solar heating quenching, etc.

energy density

Lower energy density heating

Induction quenching, flame quenching, salt bath heating quenching, electrolyte quenching, etc.

High energy density heating

Laser quenching, electron beam quenching, ion beam quenching, contact resistance heating quenching, solar heating quenching, etc.

energy source

Internal heat source heating

Induction fire, pulse current induction quenching, etc.

 

External heat source heating

Flame quenching, salt bath heating quenching, electrolyte quenching, contact resistance heating quenching, laser quenching, electron beam quenching, ion beam quenching, solar heating quenching, etc.

2. General technical requirements for surface hardened gears

1) See Table 2 for the technical requirements of surface hardened gears.

Table 2 technical requirements for surface hardened gears

Project

Pinion

Big gear wheel

 

 

Instruction

Depth of hardened layer / mm

(0.2-0.4)m①

Effective hardened layer depth, as per GB / T5617-2005

Tooth surface hardness (HRC)

50~55

45~50 or 300~400H

W

If the transmission ratio is 1:1, the tooth surface hardness of large and small gears can be equal.

Surface organization

Fine needle martensite

Ferrite is not allowed on the teeth

Heart hardness (HBW)

Quenching and tempering:

Carbon steel 265 ~ 280

Alloy steel 270 ~ 300

For some gears with low requirements, normalizing can be used as preliminary heat treatment.

① m is the gear module (mm).

2) Distribution form, strengthening effect and application range of hardened layer on gear surface

a. The tooth root is not hardened

Surface Hardening of Gears: How Many Do You Know? 2

Process method:

Rotary heating quenching method.

Enhancement effect:

The wear resistance of the tooth surface is improved;

The bending fatigue strength has little influence, and the allowable bending stress is lower than the level of the steel after tempering.

High frequency (including ultra audio frequency) quenchingMedium frequency (2.5KHz, 8kHz) quenching Flame quenching 
The diameter of induction processing gear is determined by the equipment power;Gear width 10 ~ 100mm;m≤5mm。The diameter of the processing gear is determined by the equipment power, and the gear width is 35 ~ 150mm;Individually up to 400mm;m≤10mThe gear diameter can reach 450mm;Special quenching machine;m ≤ 6mm, m ≤ 12mm in some cases

Note: m – gear module (mm)

b. Tooth root hardening

Surface Hardening of Gears: How Many Do You Know? 3

Process method:

Rotary heating quenching method.

Enhancement effect:

The wear resistance of tooth surface and the bending fatigue strength of tooth root are improved;

The allowable bending stress is 30% ~ 50% higher than that in the quenched and tempered state;

It can partly replace the carburized gear.

High frequency (including ultra audio frequency) quenchingMedium frequency (2.5KHz, 8kHz) quenching Flame quenching 
The diameter of induction processing gear is determined by the equipment power;Gear width 10 ~ 100mm;m≤5mm.The diameter of the processing gear is determined by the equipment power, and the gear width is 35 ~ 150mm;Individually up to 400mm;m≤10mThe gear diameter can reach 450mm;Special quenching machine;m ≤ 6mm, m ≤ 10mm in some cases

c. Tooth root hardening

Surface Hardening of Gears: How Many Do You Know? 4

Process method:

Single tooth continuous heating quenching method.

Enhancement effect:

The wear resistance of the tooth surface is improved;

The bending fatigue strength is affected to a certain extent (generally, the hardened layer ends at 2 ~ 3mm from the tooth root);

The allowable bending stress is lower than the level of the steel after tempering.

High frequency (including ultra audio frequency) quenchingMedium frequency (2.5KHz, 8kHz) quenching Flame quenching 
Gear diameter is not limited, m ≥ 5mmGear diameter is not limited, m ≥ 8mmGear diameter is not limited, m ≥ 6mm

d. Tooth root hardening

Surface Hardening of Gears: How Many Do You Know? 5

Process method:

Continuous heating and quenching along the tooth groove.

Enhancement effect:

The wear resistance of tooth surface and the bending fatigue strength of tooth root are improved;

The allowable bending stress is 30% ~ 50% higher than that in the quenched and tempered state;

It can partly replace the carburized gear.

High frequency (including ultra audio frequency) quenchingMedium frequency (2.5KHz, 8kHz) quenching Flame quenching 
Gear diameter is not limited, m ≥ 5mmGear diameter is not limited, m ≥ 8mmGear diameter is not limited, m ≥ 10mm

3) Typical machining process of surface hardened gears: blank → normalizing (or annealing) of forging stock → mechanical rough machining → quenching and tempering → mechanical semi finishing (blank making) and gear making → surface quenching → low temperature tempering → mechanical finishing → finished products.

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