Gear steel

Application Area

Designed for highly stressed gears requiring sufficiently high surface hardness on the tooth flanks, high bending strength in the tooth root, and good impact toughness to withstand sudden loads. Gears made from CV58Z exhibit high wear resistance and are resistant to pitting formation. This steel is also suitable for other components, such as cam discs.

Description

Typically, components with these requirements are made from case-hardening steel. However, for economic reasons, flame hardening or inductive hardening of quenched and tempered steels is often used. In these processes, the tooth flanks are heated and quenched in a feed process. However, this hardening method is only effective for spur gears with module > 6. Bevel gears and gear modules below 6 cannot be hardened with satisfactory results using this method. To overcome this limitation, the circumferential hardening process was developed, enabling consistent hardness results even for smaller gears. A crucial requirement was developing a steel with high surface hardness while maintaining sufficient elasticity to absorb shock loads during operation.

Properties

• Oil-hardening steel, specifically designed for circumferential hardening.
• Not suitable for processes requiring water quenching.
• Requires strict metallurgical conditions and suitable testing procedures to ensure consistent steel properties for circumferential hardening.
• Supplied in either annealed or quenched and tempered condition.

Strength Properties in Delivery Condition

• Annealed to spheroidized cementite.
• Quenched and tempered for higher strength (although higher strengths are not recommended due to machining difficulties).
• During circumferential hardening, a tooth root strength of approximately 1470 N/mm² is achieved.
• Quenched and tempered condition is recommended for smaller gears (outer diameter ≤ 200 mm) and when the bore is broached (e.g., keyway profiles) or the teeth are hobbed.

• Lower distortion during hardening.
• Achieves higher hardness of 62 – 64 HRC (approx. 2–3 HRC higher than annealed material).

• Annealed material can be used for larger gears since longer heating times during hardening ensure proper hardness results.

Warmformgebung und Wärmebehandlung

*) Annealing temperature depends on the required microstructure and processing conditions.

To ensure consistent and reproducible results, CV 58 Z is produced using special melting techniques and carefully controlled heat treatment.

Distortion During Hardening

In practical applications, hardening distortion is measured by the change in the pressure angle and the major pitch deviation before and after hardening.

Comparative studies between CV 58 Z and 16 MnCr 5 show that:

• Hardening distortion in CV 58 Z is on average 45% and 30% lower, respectively, than in 16 MnCr 5.
• This results in improved dimensional stability after heat treatment.

Circumferential Hardening

Circumferential hardening, also called rotational hardening, is used to harden the circumference of rotating components.

Process:

• This method is similar to stationary hardening but with the key difference that the workpiece rotates during heating and quenching.
• A machined gear blank, in either an annealed or pre-hardened state, is locally heated and then quenched in oil.

Objectives:

• High surface hardness for wear resistance
• High bending strength
• Good impact toughness

Hardened Zones

In case-hardening steel 16 MnCr 5, the hard case layer and the soft core are clearly visible as two distinct structural zones. In CV 58 Z, the gear tooth exhibits an almost fully hardened microstructure, with a gradual transition into the softer structure of the gear rim. This results in higher load-bearing capacity and better impact resistance compared to conventional case-hardened steels.