Bridgeway explain their high quality metal treatments

Bridgeway explains their high quality metal treatments for their engineering products this includes ION-NITRIDING. The basic process of ion nitriding include gas decomposition, sputtering, adsorption, deposition, and diffusion.

Introduction to Plasma Nitriding

Plasma nitriding is an ion-chemical heat treatment process that enhance the surface of metals. Utilizing the phenomenon of glow discharge, nitrogen ions is generated by ionizing nitrogen-containing gases in which then bombards the part’s surface, by heating it and achieving nitriding that results in a surface nitriding layer. Parts are treated with plasma nitriding exhibit significantly increased surface hardness, high wear resistance, fatigue strength, corrosion resistance, and burns resistance. This process is widely applied to cast iron, carbon steel, alloy steel, stainless steel, and titanium alloys.

The basic process of ion nitriding include gas decomposition, sputtering, adsorption, deposition, and diffusion.

During glow discharge, nitrogen ions are accelerated by the electric field towards the surface of the part being treated, creating sputtering. In plasma glow discharge, iron atoms from nitrides with nitrogen in a variety of excitation states. These nitrides are adsorbed on the cathode surface (the surface of the part being treated). Under ion bombardment, these nitrides decompose into nitrogen-containing iron nitrides and nitrogen-containing solid solutions. The surface layer nitrides decompose, allowing nitrogen to diffuse inward, forming an internal nitrided zone, completing the nitriding process.

Advantages of Ion Nitriding

Compared to gas nitriding, ion nitriding offers several advantages:

Faster Nitriding Speed: Ion nitriding offers fast nitriding speeds, which is particularly beneficial for achieving shallow nitrided layers.
Adjustable gas: Ion nitriding enable precise gas adjustment, providing greater control over compound layer properties. The nitrogen and hydrogen can be controlled to obtain less brittle e-phase single-phase layers or tough Y-phase single-phase layers. Ion nitriding can also achieve tougher, compound-free nitriding layers.
Environmental Benefits: Ion nitriding uses nitrogen and hydrogen instead of ammonia. This operates at very low pressures. The absence of ammonia combined with highly-efficient energy and material usage of the process massively reduces it’s overall environmental impact.
Reduced Dimensional Growth: Ion nitriding minimizes dimensional changes and deformation. The bombardment compensates for some dimensional growth, resulting in less deformation.
Low Temperatures: Ion nitriding can be conducted below 450°C, minimizing workpiece distortion.
Versatility: Ion nitriding is suitable for nitriding non-ferrous metals such as stainless steel, titanium alloys, and aluminum alloys. The sputtering and hydrogen ion reduction can remove the passive film on the workpiece surface during ion nitriding.
Better Microstructure Control: Ion nitriding provides better control over microstructure characteristics, particularly in reducing brittleness in the compound layer.

Quality Control of Nitriding

To ensure consistent and high-quality, several critical control points must be addressed throughout the process:

1. Pre-Nitriding Cleaning and Placement

Deburring and Rust Removal: Completely remove excess burrs, iron fillings, and rust before loading the parts into the furnace.
Thorough Cleaning: Ensure the parts are free of oil and other contaminates.
Proper Placement: Maintain adequate spacing between parts. Arrange parts according to the heating characteristics and temperature distribution of ion nitriding to avoid local overhearing. Auxiliary cathodes should be set in areas where temperatures might be lower.
Mechanical Shielding: Areas prone to abnormal discharge, such as narrow gaps, small holes, and non-nitrided areas (e.g., threaded holes), should be masked.

2. Nitriding Process Control

Atmosphere Control: Different nitrogen-to-hydrogen ratios produce different compound compositions. For example, a 75% H₂ and 25% N₂ atmosphere results in a γ’-(Fe₄N) compound layer, while 70% N₂, 27% H₂, and 3% methane produce an ε-(Fe₃N) compound layer.
Pressure Control: The compound layer thickness reaches a maximum at a specific pressure. For instance, 40Cr nitrided at 530°C and 400 Pa pressure has the highest ε-phase content, while at 570°C and 530 Pa, the ε-phase content is maximized. Too low a pressure causes a wide glow discharge layer that can’t follow the surface contours, resulting in low local hardness or no nitriding layer, while too high a pressure increases the risk of arc discharge, potentially scrapping the part.
Temperature Control: Nitriding temperatures affects the phase composition of the nitrided layer. For example, as the nitriding temperature of 40Cr increases from 500°C to 560°C, both γ’ and ε phases increase. Further increase to 580°C-600°C reduces the ε phase and increases the γ’ phase. At 650°C, the compound layer decomposes.
Time Control: Different materials require different nitriding times. For instance, 4340 steel rapidly increases its nitriding layer thickness to about 0.2 mm within 4-6 hours. Extending the time further only slightly increases the thickness, reaching 0.4 mm after 24 hours. Conversely, 38CrMoAl shows a continuous increase in nitriding layer thickness within 24 hours. The nitriding time also affects the phase composition of the nitrided layer. In general alloy steels, with prolonged nitriding time using decomposed ammonia, the ε phase decreases while the γ’ phase increases, eventually forming a nitrided layer composed mainly of the γ’ phase.

3. Post-Nitriding Quality Inspection

Post-nitriding, parts must be inspected. The inspection can be performed on the parts themselves or on accompanying sample blocks (needs to be specified in the report).

A. Surface Hardness Inspection

Surface Hardness: Include hardness testing of individual parts and uniformity testing of the entire batch. Depending on the nitriding layer depth, different Vickers hardness testers with varying test forces should be used. For depths below 0.3mm, a test force not exceeding 49 N is chosen, and for depths above 0.3mm, 49-98 N is selected. If indentations are not allowed on the part surface, accompanying sample blocks can be used and marked in the report. The hardness deviation of individual parts should generally not exceed 45 HV. Uniformity testing of the entire batch requires hardness testing on the part surface, usually using 0.49-1.96 N test force hardness testers, with batch hardness deviations generally not exceeding 70 HV.

B. Nitriding Layer Depth Inspection

Depth Inspection: Use sample blocks with vertical sections of the nitrided layer. A microhardness tester is used to measure hardness at intervals greater than twice the indentation width on the vertical section’s surface. The effective depth of the nitrided layer is determined by the hardness value equal to or greater than the core hardness plus 50 HV.

C. Nitriding Layer Microstructure Inspection

Microstructure Inspection: Using a metallographic microscope at 500x magnification to inspect the nitrided layer. Evaluate the worst-case morphology. Large amounts of vein-like or continuous network structures in the ion nitrided layer are unacceptable.

Normal nitriding layers should appear as shown in accompanying images.(Image 4)