What is the white layer on steel that forms during nitriding?

The white layer consists in a mixture of iron nitrides: Fe4N – γ´ and/or Fe2,3N – ε. The diffusion layer is located bellow the white layer, and it is formed by the diffusion of nitrogen into the metal.

What is Nitride hardening?

Nitriding is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating as defined by the Iron-Carbon Phase Diagram.

Why is nitriding used?

Nitriding (N) and nitrocarburizing (NC) are thermochemical treatments that diffuse nitrogen and carbon into the surface of metals. They improve surface properties of metal components and tools such as scuff and corrosion resistance, and increase the fatigue strength.

What is the purpose of Nitriding?

The main objectives of nitriding are to increase the surface hardness of the material, as well as its wear resistance, fatigue life, and corrosion resistance [30], which are achieved by the presence of the nitrided layer.

What is white layer?

A white layer is a thin phase transformed layer which can appear at the surface of a machined workpiece. This literature study investigates how white layer is created and and in how far it can influence the life of a specific workpiece.

Are there any issues with the white layer of nitride?

The white layer created issues in mechanical usage, given its brittleness, leading to failure and even instances in which the nitride layer fully peeled from the surface.

How are Mn nitrides used to increase core hardness?

In order to limit any potential weakness of the nitride layers, the grain boundary cementite should not form a continuous network, which involves optimization of the processing parameters. The MN nitrides formed are smaller than the initial carbides, which increases the hardness of the nitrided layers.

How are surface hardness and depth of nitride zone measured?

Surface hardness and depth of nitride zone were then recorded from microhardness measurements and metallography. All this data was compiled to establish Nitriding procedures that provide the final desired structure in the minimum cycle time.

What are the advantages and disadvantages of nitriding?

Given the relative simplicity of the process, nitriding (in particular, gas nitriding) remains standard in many industries. The disadvantages of the process, the production of a compound white layer as well as a slightly uncontrolled case depth, are outweighed by the improved mechanical qualities delivered by the case-hardened nitride layer.