Case hardening / carburizing

Durofer-process | One-/two-salt systems | Solid carburizers | Carbonitriding

 

Carburizing and case hardening are thermochemical heat treatment processes by which, through the diffusion of carbon into the surface zone, certain properties such as great hardness and high wear resistance are obtained in the surface zone of case hardening steel.

Variations of the carburizing process

Hardening after carburizing can be carried out by various methods as shown in the diagram, depending on the requirements the structures of the case and the core have to fulfil. The case depth or hardness penetration depth are determined by the treating temperature and duration. The surface carbon content during carburizing and case hardening in salt baths is determined by the carburizing salt or carburizing granulate used and its degree of activation.

In direct hardening A, the parts are either quenched directly from the carburizing temperature into a martemper bath or are first cooled from the carburizing temperature to the surface hardening temperature, before entering the martemper bath.

In single hardening B, the parts are cooled slowly from carburizing temperature to room temperature, for example in air. After that, the temperature is raised to surface or core hardening temperature, held there and the parts quenched.

Position C in the diagram shows a particular kind of single hardening, that is hardening after isothermal transformation in the pearlite range. This method involves carburizing in a salt bath, followed directly by intercritical annealing at ~ 580 °C - 680°C during which the excess dissolved carbon in the austenite is precipitated into cementite. In this way, there is virtually no risk of residual austenite forming during the following surface hardening. Furthermore, there is less distortion during the hardening which follows the intercritical annealing.

Double hardening D consists of hardening the carburized work pieces twice. The first hardening is done from the hardening temperature of the core material, the second from the hardening temperature of the carburized case. After case hardening the components should always be tempered at 140 - 220 °C or stress relief annealing.

 
© Durferrit GmbH, Industriestrasse 3, 68169 Mannheim
Tel.: +49 (0) 621 32224-0, Fax: +49 (0) 621 32224-809
E-Mail: info@hef-durferrit.com