What are the common defects of nitriding workpieces

What are the common defects of nitriding workpieces

Surface corrosion

Cause: Ammonium chloride (or carbon tetrachloride) is added too much and evaporates too fast

Solution: Except for stainless steel and heat-resistant steel, try not to add chlorinated dumplings for nitriding of other steel types. Adding silkworm dumplings should be mixed with silica sand to reduce volatilization speed

Surface oxidation color

Causes: insufficient ammonia supply during cooling, negative pressure in the pipe, leaking of the nitriding tank, abnormal pressure, too high temperature of the furnace: failure of the desiccant, excessive water content in the ammonia, and water accumulation in the pipe

Solution: appropriately increase the ammonia flow to ensure that the pressure in the tank is normal, and the workpiece is cooled to below 200 ℃ and discharged

Low hardness of nitriding layer

Cause: The temperature is too high; the first stage temperature is too high during the stepwise nitriding; the ammonia decomposition rate is too high or the ammonia supply is interrupted, and the furnace leaks; the nitriding tank has been used for too long; the surface oil of the workpiece is not removed

Solution: Check the instrument and adjust the furnace temperature; reduce the temperature of the first stage; adjust and stabilize the ammonia decomposition rate; ensure the sealing of the nitriding tank; pre-infiltration of the new nitriding tank, and the nitriding tank (including fixtures) that has been used for a long time After denitrification treatment to ensure normal decomposition rate; strict decontamination and degreasing treatment before nitriding

There are bright spots, soft spots, and uneven hardness (or layers) on the surface of the workpiece

Causes: the surface of the workpiece is not clean; the raw material organization is not uniform; the amount of the furnace is too much or the workpiece is improperly hung; the furnace temperature is not uniform; the ion nitriding shield is improper, and there are tin, copper, zinc, water glass, etc. To the workpiece surface

Solution: clean and decontaminate, remove the oxide scale on the surface of the workpiece; install the furnace reasonably; improve the furnace design; reduce the temperature difference in the tank and strengthen the furnace cycle; during ion nitriding, no slits or small holes on the workpiece shielding are allowed

Nitrided parts are brittle

Cause: The surface of the workpiece is decarburized during nitriding, or decarburization occurs before tempering treatment (such as 38CrMoAIA) before nitriding; the nitrogen potential is too high

Solution: Improve the tightness of the nitriding tank, reduce the water content in ammonia, and improve the quality of preliminary heat treatment; increase the ammonia decomposition rate and reduce the oxygen potential. The ion nitriding parts can be cooled by hydrogen bombardment to repel the nitrogen

Nitrided layer network or needle-like, vein-like nitride

Causes: Nitriding temperature is too high, nitrogen potential is too high, ammonia water content is large, raw material structure is coarse; the surface decarburization layer is not removed; the surface of the infiltration part is rough, there are sharp corners, edges, and free ferrite after quenching and tempering >5% (volume fraction)

Solution: control the temperature, dance force and ammonia water content, the workpiece is not allowed to have sharp corners, reduce the non-smooth transition of the workpiece; increase the machining allowance after preheating, and control the quality of quenching and tempering

Nitriding layer is too shallow

Causes: low temperature; insufficient holding time; unstable nitrogen potential (ammonia decomposition rate); the workpiece has not been quenched and tempered; the new nitriding tank or nitriding tank (or fixture) has been used for too long; improper furnace installation, air flow Poor circulation; low vacuum of ion nitriding

Solution: Increase the temperature appropriately. Calibrate the instrument and thermocouple; extend the holding time as appropriate; adjust the nitrogen potential: the workpiece should be quenched and tempered before nitriding; the new nitriding tank should be pre-infiltrated; the long-term nitriding tank or fixture should be denitrified; reasonable installation of the furnace, Adjust the distance between the workpieces; check whether the furnace has leaks, gas sources, etc.