Blog on Heat Treatment of Gears

 Heat Treatment of Gears

Introduction

Heat treatment is a critical and complex element in the manufacturing of gears that greatly impacts how each will perform in transmitting power or carrying motion to other components in an assembly. Heat treatments optimize the performance and extend the life of gears in service by altering their chemical, metallurgical, and physical properties. Heat treatments improve physical properties such as surface hardness, which imparts wear resistance to prevent tooth and bearing surfaces from simply wearing out.

Fig 1. Heat Treatment

Why Gears are Heat Treated?

Heat treatments improve physical properties such as surface hardness, which imparts wear resistance to prevent tooth and bearing surfaces from simply wearing out. It also improve a gear’s fatigue life by generating subsurface compressive stresses to prevent pitting and deformation from high contact stresses on gear teeth.

Fig 2. Heat Treatment of Gears

Iron Carbon Diagram

As steel and its alloys are the general material that are used to make gears, Iron carbon diagram plays an important role in selecting the heat treatment process of gears. It provides knowledge of various heat treatment process and provides foundation for understanding different phases of steel.

Fig 3. Iron Carbon Diagram

Gear Manufacturing Process

                                                                             Fig 4. Gear Manufacturing

Pre-Hardening Heat Treatment

 

Several heat treatments can be performed before or during the gear manufacturing process to prepare the part for manufacturing (to increase machinability).

 

In many cases these steps are essential to the manufacture of a quality gear.

1.       Annealing

2.       Normalizing

3.       Stress relief

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Annealing

 

Annealing is primarily intended to soften the part and to improve its machinability.

There are several annealing processes, all of which involve heating to and holding at a suitable temperature followed by cooling a specific rate usually through a critical range of temperatures.

 

1.    Full or supercritical annealing where a gear blank is heated 90-180°C (160-325°F) above the upper critical temperature (Ac3) of the steel and then slow cooled in the furnace to around 315ºC (600ºF).

2.    Subcritical annealing, where gears are heated to 10-38°C (50-100°F) below the lower critical temperature followed by a slow cool in the furnace.

   

    Fig 5. Full annealing and process annealing ranges 

Normalizing

Normalizing plays a significant role in the control of dimensional variation during hardening and case hardening. Normalizing is a process that involves heating the gear above the upper critical temperature and then cooling at a rate equivalent to that of still air to relieve residual stresses in the gear blank and for dimensional stability in subsequent heat treatment processes. In a microstructural sense, normalizing is intended to produce a more homogenous microstructure. A normalized part is very machineable, but harder than an annealed part.

 

Fig 6. Normalizing 

Stress Relief

It Is intended to relieve internal stresses created in the gear as a consequence of its manufacture.  It is recommended for intricate shapes, especially if aggressive machining methods are used or when large amounts of stock are being removed.  Stress relief involves heating to a temperature below the lower critical temperature, holding long enough to fully soak the part then cooling slowly enough, usually in air, to minimize the development of new residual stresses.

 Hardening Process of Gears

A variety of heat treatment process choices exist for hardening a gear, each designed to increase gear hardness. These usually involve heating and rapid cooling and are typically classified as through-

1.       Hardening

2.       case hardening (carburizing, carbonitriding, nitriding, nitrocarburizing)

3.       surface hardening by applied energy (flame, laser, induction).

 

Neutral Hardening

The hardness is achieved by heating the material into the austenitic range, typically 815-900ºC (1500-1650ºF), followed by quenching and tempering.  It is important to note that hardness uniformity should not be assumed throughout the gear tooth. Since the outside of a gear often cools faster than the inside, there will be a hardness gradient developed. The final hardness is dependent on the amount of carbon in the steel. The depth of hardness depends on the hardenability of the steel as well as the quench severity.

Case Hardening

Case hardening is used to produce a hard, wear-resistant case, or surface layer, on top of a ductile, shock-resistant interior, or core. The idea behind case hardening is to keep the core of the gear tooth at a level around 30-40 HRC to avoid tooth breakage while hardening the outer surface to increase pitting resistance. The higher the surface hardness value the greater the pitting resistance. 

Fig 7. Case Hardened Gear

Carburizing

Carburizing is the most common of the case hardening methods. A properly carburized gear will be able to handle between 30-50 percent more load than a through hardened gear.  Carburizing steels are typically alloy steels with approximately 0.10 to 0.20 percent carbon. Carburizing can be performed in the temperature range of 800ºC (1475ºF) to 1090ºC (2000ºF).

                              

                  Fig 8. Carburizing Furnace                                                 Fig 9. Carbon Layer by Carburizing

 

Carbonitriding 

Carbonitriding is a modification of the carburizing process, not a form of nitriding. This modification consists of introducing ammonia into the carburizing atmosphere in order to add nitrogen to the carburized case as it is being produced. Typically, carbonitriding is done at a lower temperature than carburizing, between 700 and 900°C (1300 and 1650°F), and for a shorter time. Examples of gear steels that are commonly carbonitrided include SAE 1018, 1117, and 12L14.

Fig 9. Carbonitriding Furnace 

 Nitriding

Nitriding is another surface treatment process that has as its objective increasing surface hardness. One of the appeals of this process is that rapid quenching is not required; hence dimensional changes are kept to a minimum. Three factors that are extremely critical in producing superior and consistent nitrided cases and predictable dimensional change are steel composition, prior structure, and core hardness. Nitriding is typically done in the 495-565°C (925 to 1050°F) temperature range.  Examples of commonly nitrided gear steels include SAE 4140, 4150, 4340, 7140, 8640, and Nitralloy alloys.

Fig 10. Gear Nitriding  

Applied Energy Hardening

Various methods of hardening by use of applied energy are used in the manufacture of gears; including

1.       flame hardening

2.       laser surface hardening

3.       Induction hardening

 

Flame Hardening

Flame hardening can be used for both small and large gears by either spinning or by a progressive heating technique. In the progressive heating method, the flames gradually heat the gear in front of the flame head, and sometimes this effect must be compensated for by gradually increasing the speed of travel or by precooling.  A wide range of gear sizes and materials can be hardened by this technique, including plain carbon steels, carburizing grades, cast irons, and certain stainless grades.

                       

Fig 10. Progressive Hardening                                                   Fig 11. Spin Hardening

Laser Hardening

Laser surface hardening is used to enhance the mechanical properties and surface hardness of highly stressed machine parts and as such is of interest to gear manufacturing.  The use of lasers for surface treatments is relatively limited due to the high cost of large industrial lasers and the narrow (4-5 mm) band of material that can be hardened without multiple overlapping passes. Gear materials such as SAE 1045, 4340, and cast irons (gray, malleable, ductile) are good candidates for this technology.

Fig 12. Laser Hardening of Gears 

Induction Hardening

Induction hardening is commonly used in the heat treatment of gears. Induction heating is a process which uses alternating current to heat the surface of a gear tooth.  The area is then quenched resulting in an increase in hardness in the heated area. It is typically accomplished in a relatively short period of time.                 

Fig 13. Heating Coil                                                        Fig 14. Quenching Machine

S45C (Carbon Steel for Structural Machine Usage)

S45C is one of the most commonly used steel, containing moderate amounts of carbon (0.45% ). S45C is easily obtainable and is used in the production of spur gears, helical gears, gear racks, bevel gears and worm gears.

Fig 15. S45C Helical Gear 

SCM 440 (Chrome Molybdenum Alloy Steel)

An alloy steel containing moderate amounts of carbon (0.40% ). It also contains chrome / molybdenum. SCM440 has more strength than S45C and is used with thermal-refining or induction-hardening treatment for producing gears.

Fig 16. SCM 440 spur gear 

Conclusion

●     There are many options for heat treatment of quality gears, but the selection of the right combination of heat treatment processes along with control of process and equipment variables—remains essential.

●     Heat treatment significantly increases the performance and life of gear

●     It is an important process in manufacturing of gears, as it increases the surface hardness and wear resistance of the tooth.

References

1. Pfaffmann, G. “How to Optimize Heat Treatment of Gears to Meet Manufacturing and  Performance Objectives,” Advanced Gear Processing & Manufacturing Conference Proceedings, Nashville, TN, May 9–10, 2000.

2. Breen, D. “Fundamentals of Gear Stress/Strength Relationship–Materials,” SAE Paper No. 841083, Gear Research Institute, 1984, pp. 43–55.

3. Otto, F., and D. Herring. “Gear Heat Treatment,” Heat Treating Progress, Part I—Vol. 2, No. 4, June 2002, pp. 55–59, Part II—Vol. 2, No. 5, July/August 2002, pp. 27–31.

4. Parrish, J. “The Straight Pitch About Gear Materials,” Materials Engineering, November  1982, pp. 71–73.

5. Smith, Y., and G. Eldis. “New Developments in Carburizing Steels,” Metals Engineering  Quarterly, May 1976, pp. 13–20.

6. Chatterjee-Fischer, R. “Internal Oxidation During Carburizing and Heat Treating,” Metallurgical Transactions A, Vol. 9A, No. 11, November 1978, pp. 1553–1560.

7. Pacheco, J., and G. Krauss. “Carburized Steel: Microstructure and High Bending Fatigue Strength,” Heat Treating, Vol. 7, February 1990, pp. 82–87.

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