How is heat treatment performed in a nitriding furnace?
To reduce nitriding cycle times ensure that the nitriding process is restricted by the specific grade of alloy structural steel used, two new nitriding techniques—soft nitriding ion nitriding—have been developed over recent decades, building upon the foundation of traditional nitriding processes. These variations in technique are the reason for the differing price points of nitriding furnaces.
Soft nitriding is, in essence, a low-temperature nitrocarburizing process characterized primarily by rapid quenching. During this process, nitrogen atoms permeate the steel simultaneously with a small quantity of carbon atoms. Compared to conventional gas nitriding, the resulting diffusion layer exhibits lower hardness reduced brittleness; hence, the process is termed "soft nitriding."
1. Soft nitriding methods are categorized into three types: gas soft nitriding, liquid soft nitriding, solid soft nitriding. Currently, gas soft nitriding is the most widely utilized method in manufacturing operations within China. Gas soft nitriding involves conducting low-temperature nitrocarburizing within an atmosphere containing active nitrogen carbon atoms. Common media used for this process include urea, formamide, nitrogen dioxide, triethanolamine; these substances undergo thermal decomposition reactions at soft nitriding temperatures to generate the necessary active nitrogen carbon atoms. These active atoms are absorbed by the surface of the workpiece subsequently diffuse into the substrate, thereby forming a nitrocarburized layer in which nitrogen is the predominant element. (Regarding nitriding furnaces continuous soft nitriding production lines): Gas soft nitriding is typically conducted at temperatures ranging 550°C to 570°C, as this temperature range yields the highest hardness values within the nitrided layer. The nitriding duration is typically set at 2 to 3 hours; extending the process beyond this timeframe results in a significantly slower rate of increase in the nitriding layer's depth.
2. Microstructure Properties of the Soft-Nitrided Layer: After undergoing soft nitriding, the steel workpiece acquires a surface "white layer" ranging in thickness a few micrometers to several tens of micrometers. This layer consists of the ε-phase, the γ'-phase, nitrogen-rich cementite [Fe3(C,N)]. Beneath this lies the diffusion layer, which is composed primarily of the γ'-phase the ε-phase.