Common surface hardening methods for gears include flame hardening, induction hardening, contact resistance heating hardening, and advanced laser hardening.
By performing surface heating and quenching, a hardening layer can be created on the surface, maintaining the original microstructure and properties of the gear core. This increases resistance to fatigue and wear while preserving the excellent toughness of the core. Furthermore, this process is energy efficient and reduces distortion during gear hardening.
1. Classification of surface hardening
See Table 1.
Table 1 Surface hardening classification
| Classification | Technology | |
| Approximate separation | Subdivision | |
| Heating method for heating | – | Induction hardening, flame hardening, salt bath heating hardening, electrolyte hardening, contact resistance heating hardening, laser hardening, electron beam hardening, ion beam hardening, current induction hardening high frequency pulse, solar heating quenching, etc. |
| energy density | Heating with lower energy density | Induction hardening, flame hardening, salt bath heating hardening, electrolyte hardening, etc. |
| High energy density heating | Laser quenching, electron beam quenching, ion beam quenching, contact resistance heating quenching, solar heating quenching, etc. | |
| power supply | Internal heat source heating | Induction fire, pulse current induction extinguishing, 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 | Large sprocket | Instruction |
| Hardened layer depth/mm | （0.2-0.4）m① | Effective depth of hardened layer as per GB/T5617-2005 | |
| Tooth surface hardness (HRC) | 50~55 | 45~50 or 300~400HW | If the gear ratio is 1:1, the tooth surface hardness of large and small gears can be the same. |
| Surface organization | Fine needle martensite | Ferrite is not allowed on teeth | |
| Heart hardness (HBW) | Quenching and tempering:Carbon steel 265~280Alloy steel 270~300 | For some gears with low requirements, normalizing can be used as preliminary heat treatment. | |
① m is the gear modulus (mm).
2) Distribution way, strengthening effect and application range of the hardened layer on the gear surface
The. The tooth root is not hardened
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 extinction (including ultra audio frequency) | Medium frequency extinction (2.5KHz, 8kHz) | Flame extinguishing |
| The diameter of the induction processing gear is determined by the power of the equipment;Gear width 10~100mm;m≤5mm。 | The diameter of the processing gear is determined by the power of the equipment, and the width of the gear is 35 ~ 150 mm; Individually up to 400 mm; | The gear diameter can reach 450mm; Special tempering machine; m ≤ 6 mm, m ≤ 12 mm in some cases |
Note: m – gear module (mm)
B. Tooth root hardening
Process method:
Rotary heating quenching method.
Enhancement Effect:
The surface wear resistance of the teeth and the bending fatigue resistance of the tooth roots are improved. The allowable bending stress is 30% to 50% higher compared to a quenched and tempered state. This process can partially replace carbureted gears.
| High frequency extinction (including ultra audio frequency) | Medium frequency extinction (2.5KHz, 8kHz) | Flame extinguishing |
| The diameter of the induction processing gear is determined by the power of the equipment; Gear width 10~100mm; | The diameter of the processing gear is determined by the power of the equipment, and the width of the gear is 35 ~ 150 mm; Individually up to 400 mm; | The gear diameter can reach 450mm; Special tempering machine; m ≤ 6 mm, m ≤ 10 mm in some cases |
w. Tooth root hardening
Process method:
Single tooth continuous heating hardening method.
Enhancement Effect:
The wear resistance of the tooth surface is improved, but the bending fatigue resistance may be affected to a certain extent (generally, the hardening layer ends 2 to 3 mm from the tooth root). The allowable bending stress is lower than that of steel after tempering.
| High frequency extinction (including ultra audio frequency) | Medium frequency extinction (2.5KHz, 8kHz) | Flame extinguishing |
| Gear diameter is not limited, m ≥ 5mm | Gear diameter is not limited, m ≥ 8mm | Gear diameter is not limited, m ≥ 6mm |
d. Tooth root hardening
Process method:
Continuous heating and quenching along the tooth groove.
Enhancement Effect:
The surface wear resistance of the teeth and the bending fatigue resistance of the tooth roots are improved. The allowable bending stress is 30% to 50% higher compared to a quenched and tempered state. This process can partially replace carburized gears.
| High frequency extinction (including ultra audio frequency) | Medium frequency extinction (2.5KHz, 8kHz) | Flame extinguishing |
| Gear diameter is not limited, m ≥ 5mm | Gear diameter is not limited, m ≥ 8mm | Gear diameter is not limited, m ≥ 10mm |
3) Typical machining process of surface hardened gears: blank → normalizing (or annealing) of forging material → mechanical roughing machining → quenching and tempering → mechanical semi-finishing (blank manufacturing) and gear manufacturing → surface quenching → tempering at low temperature → mechanical finishing → finished products.
