Tempering Secondary Hardening Tendency
Secondary Hardening: During tempering of high-alloy steels, fine special carbides form, leading to a temporary hardness increase that compensates for the usual hardness drop.
Physical-Chemical Mechanism of Action
During tempering of high-alloy steels (e.g., martensitic steel), the alloys react with water vapor and fine special carbides such as M23C6 or M7C3 form. These carbides gather at grain boundaries and cause a local hardness increase.
Metallurgical Relevance & Risks in Welding
Secondary hardening can lead to increased hardness and lower toughness of the weld. This can cause cracks or fractures in the material, especially under mechanical stress. Additionally, secondary hardening can reduce formability and make machining more difficult.
Workshop Practice & Instructions
To minimize the negative effects of secondary hardening, the following measures should be taken:
• Cooling the Material: Cool the material slowly and controlled after tempering to avoid the formation of special carbides.
• Use of Protective Agents: Use protective agents that suppress the reaction between water vapor and alloy, e.g., carbon gas or silicon dioxide.
• Optimizing the Tempering Technique: Adjust the tempering technique to reduce the formation of special carbides, e.g., by controlled temperature distribution in the tempering bath.