After Plasma distribution the work piece surface is uneven

Uniformity of the surface layer of ion nitriding furnace (depth and hardness of the nitriding layer) and color of the work-piece after ion nitriding (luminescent ion nitriding) are common problems. The main reasons can be attributed to the following aspects:

1. Plasma distribution is uneven (electric field, glow distribution):

Core reason: Ionic nitriding relies on the glow discharge between the work-piece (cathode) and the furnace wall (anode) to produce plasma. The density and energy distribution of plasma directly affect the uniformity of heating and the supply of active nitrogen atoms.

Influencing factors:Work-piece shape and layout: Sharp edges, corners, small holes and deep grooves are prone to "edge effect" (luminance concentration), resulting in local overheating and over-seepage. The depths of the grooves and inner holes may have weak or even no glow (hollow cathode effect or shielding), resulting in low temperature, shallow seepage or even no seepage. If the work-piece spacing is too small, it will shield each other's glow. Unreasonable installation method (such as stacking) will also cause shielding. 

2. Differences in heat capacity and heat dissipation of work-pieces: Thin-walled parts heat up faster than thick-walled parts; different parts of the work-piece (such as near the fixture, edge and center) have different heat dissipation conditions. 

3. The atmosphere (gas composition, pressure) is uneven: Cause:Active nitrogen atoms come from the decomposition and ionization of working gases (N2, H2, sometimes a small amount of CH4 or CO2) in the plasma. The flow, mixing and pressure distribution of the atmosphere affect the supply of active particles.

Influencing factors:

1.Air intake mode and location: The position and distribution of the air intake are unreasonable, or the gas mixture is insufficient, resulting in inconsistent gas composition and pressure distribution in different regions of the furnace. 

2.The position of the air outlet: affects the direction of the airflow in the furnace and the formation of the dead zone. 

3.Furnace pressure control: Pressure fluctuations or uneven distribution of pressure in the furnace itself (especially in large furnaces or complex flow fields). Gas purity and proportion: The content of impurity gas (such as O2, H2O) is too high or unstable, or the control of the proportion of process gas is not accurate, affecting the reaction atmosphere.

4. Material factors:

Component analysis: The steel itself has macro or microscopic decomposition of alloy elements (such as Cr, Mo, Al, V, etc.) or impurity elements, resulting in different absorption and diffusion of nitrogen in different regions.

Original tissue state: Tissue after pretreatment (such as conditioning) (uniformity of tempering saustenitic), surface decarburization layer, uncleaned oxide skin, difference in surface roughness, etc., will affect the infiltration and diffusion of nitrogen.

Surface cleanliness: Oil stains, rust, fingerprints and other pollutants form carbonization or oxides under ion bombardment, hindering nitriding or causing abnormal color.

Special instructions for uneven colors:The color after ion nitriding mainly comes from the composition, structure and thickness of the extremely thinmost surface layer of compound layer (white bright layer), as well as the oxide film on its surface. 

The compound layer itself: γ'-Fe4N is brass-colored, and ε-Fe2−3N is silver-gray or slightly blue. Changes in the thickness of the compound layer and the ε phase/γ' phase ratio will lead to color differences. 

Surface oxide film: During the cooling process, the surface of the high-temperature compound layer reacts with the trace amount of oxygen or water vapor left in the furnace to form an extremely thin oxide film (Fe3O4, Fe2O3, etc.). 

The thickness of this oxide film (tens to hundreds of nanometers) is different, and it will present different colors due to the interference effect of light (similar to the color principle of oil film or soap bubbles: blue, yellow, purple, red, etc.). 

The cooling speed, local trace oxygen concentration, and the surface state (smoothness) of the compound layer will significantly affect the thickness and uniformity of the oxide film, resulting in obvious color differences.

How to reduce unevenness:

1. Optimize furnace installation:Reasonably design the tooling fixture to ensure uniform spacing between the yin and yang poles; avoid mutual shielding between workpieces; properly shield sharp corners, small holes, etc.; ensure that thin and thick parts are reasonably matched or treated by furnaces. 

2. Precise temperature control: Increase multi-point temperature measurement (especially key and problem-prone parts) to ensure good thermocouple contact; optimize the temperature control algorithm; consider the actual thermal state of the workpiece. 

3. Improve atmospheric uniformity: Optimize the position of air intake and extraction port; use gas mixer to ensure uniform mixing; stabilize gas flow and pressure; improve gas purity. 

4. Control process parameters:Precisely set and stabilize control voltage, current (or power), temperature, time, gas proportion and flow rate; rational use of pulse technology to improve uniformity. 

5. Ensure the material and surface condition: Use uniformly composed materials; ensure uniform pretreatment structure; thoroughly clean the surface of the work-piece to remove oil stains, rust and decarburization layers.

6. Optimize cooling: Improve cooling uniformity (such as using a fan to stir inert gas); increase the cooling speed as much as possible to reduce oxidation time; ensure that the furnace is well sealed and reduce the residual oxygen content. 

7. Equipment maintenance:Regularly check and maintain the vacuum system (to ensure low leakage rate), cathodic transmission device, anode, power supply, etc. 

In summary, the fundamental reason for the uneven color of the seepage layer after ion nitriding is the inconsistent spatial distribution of physical and chemical conditions (temperature, plasma density, active particle supply, atmosphere composition) on the surface of the work-piece during the process. The color problem is more related to the most surface compound layer and the extremely thin oxidation interference film, while the depth of the seepage layer reflects the depth of nitrogen diffusion. Solving these problems requires comprehensive optimization from multiple aspects such as equipment design, furnace installation method, process control, material preparation and equipment maintenance.

Whether the uneven surface color after ion nitriding affects the use of the work-piece, and whether the light color or insal nitriding will lead to the oxidation of the work-piece, it needs to be discussed according to the situation:

In most cases, simple color unevenness (only oxidation color difference) will not affect the core properties of the work-piece (such as wear resistance and fatigue strength). But you need to judge the cause carefully, because it may be the appearance of a deep problem: 

1. Only the difference in the thickness of the oxide film (no seepage problem):Does not affect the function: The color difference is only caused by the interference of the nano-scale oxide film on the surface (such as uneven cooling), while the depth, hardness and tissue uniformity of the seepage layer meet the standard. At this time, it is a pure appearance problem, which has no effect on the core performance of the work-piece, such as wear resistance and fatigue resistance. Exceptions:Decorative work-pieces:Occasions with strict appearance requirements (such as high-grade knives and decorative parts) may not be acceptable. Precision coating surface: Extremely thick oxide film (although rare) may affect the micron-level coating accuracy. 

2. Uneven color accompanied by seepage/uneven tissue (need to be alert): Seriously affect the function: If the color is uneven due to the insufficient depth of the seepage layer, insufficient hardness, missing or abnormal compound layer caused by uneven temperature, uneven atmosphere, shielding, etc., it will significantly reduce the wear resistance, bite resistance, fatigue strength and other key properties of the work-piece. 

Color is a signal: At this time, the color difference is the external manifestation of the intrinsic qualityproblem, and it must be tested (hardness test, metallography analysis) to confirm whether the seepage layer is uniformly qualified. Conclusion: The "light color" itself seen after nitriding (if the oxide film is thin only due to fast cooling) will not directly cause the work-piece to be more easily oxidized during service. But! If the reason for the "light color" is the nitriding of the area is not in place (especially the compound layer is missing or too thin) , then the area will be very easy to cause serious oxidation (corrosion) failure in the subsequent high temperature or corrosive service environment . This is a serious defect in functionality.