The aging of the nitrile hearth refers to the phenomenon that the ammonia decomposition rate in the furnace increases significantly after the nitriding tank is used for a long time, resulting in the loss of part of the nitriding ability. Its essence is that the catalytic effect of the ammonia decomposition rate of the tank wall (including the ammonia pipe in the furnace) increases, which increases the consumption of ammonia gas, and may even cause the nitriding to run out of control, and the nitriding product fails to meet the technical requirements. How to deal with the aging of box carburizing furnace?

General household nitriding furnace tanks and ammonia pipes are made of 18-8ti stainless steel. The passive film on the surface of the new nitride tank will prevent nitrogen penetration, but there will still be penetration due to the lower nickel content in 18-8 ti (penetration is generally slow and uneven).

With the increase of nitriding furnace times and nitriding time, the inner wall is gradually nitrided. Under the same process, temperature, time and loading amount, the decomposition rate of ammonia gradually increased. In order to ensure the nitrogen potential NP required by the process, nitrogen denitrification is required.

The basic rule of furnace age is that the flow rate in the furnace increases from slow to fast to the uncontrollable NP stage within a certain period of time. The attached picture is based on the actual production data of a furnace, and the loading capacity of the furnace is quite different from the furnace loading specified by the equipment.

Therefore, the influence of the workpiece on the ammonia decomposition rate is the main influencing factor, and the hearth suction [N] gradually becomes slower, reflecting the actual situation.

It is well known that the lower the ammonia decomposition rate, the higher the NP and the stronger the nitriding ability. But it should be pointed out that the nitrogen atoms produced by the decomposition of ammonia molecules are very active, and if they are not absorbed, they will combine with stable nitrogen molecules at a very fast rate, which cannot be absorbed by the steel surface.

When the decomposition rate of ammonia is very high, that is, the smaller the proportion of ammonia in the atmosphere, that is, the smaller the ammonia concentration, the less the number of ammonia molecules touches the surface area of the steel sheet per unit time, and the number of nitrogen atoms generated by decomposition The less, and the lower the nitriding ability.

It is also believed that the higher the ammonia decomposition rate and the higher the hydrogen concentration in the furnace, the absorption of nitrogen atoms will also be affected. Of course, if the atmosphere is full of ammonia, that is, the decomposition rate is 0, and no nitrogen atoms are produced, then of course there is no nitriding.

Therefore, a reasonable low decomposition rate in production will inevitably lead to a sharp increase in ammonia consumption, and airflow hitting the workpiece will lead to unbalanced, uncontrollable temperatures, resulting in uneven penetration of the layer and deformation of the workpiece, and even some people will cause Nitriding control is difficult.

The decomposition rate and NPs are also controlled by the composite layer. Under reasonable NP conditions, it is beneficial to accelerate the nitriding rate.