More Hydrogen, Less Oxidization?

One of our atmosphere furnace users recently consulted us regarding the surface brightness of the SS products. The user found the SS product surface had oxidization and then increased H2 flow rate expecting to solve the oxidization, but turned out the oxidization issue was not getting better. It is a little confusing. "More H2, less oxidization", should not it be true? H2 is a common reducing atmosphere. It has been used in applications like annealing, brazing and etc. According to our experience, H2 plays an important role in achieving bright surface finish for metals like SS, silver and etc. In brazing, H2 has the effect to improve the wettability by removing the oxidization surface of the melting filler material.

However, it is not always true that more H2 equals less oxidization. Here's why:

1) H2 source can contain impurities like water, oxygen.

2) H2 can react with oxidizations on the metal belt surface and produce H2O. Then the H2O can react with the SS product in the furnace, causing SS oxidization. In this case, H2 acts like a carrier that transfer the oxygen element from metal belt oxidization layer to the SS product.

Knowing why more H2 may not be beneficial, what can we do to eliminate product oxidization? Here are our suggestions:

1) Choose high purity gas. Dew point of the gas needs to be below -50C for SS bright surface finish. When necessary, use gas purifier before connecting the gas to the furnace

2) Adjust flow rate of N2 to help lower down the O2 level in the furnace chamber. For gas cost and safety concerns, it is recommended to choose N2 as the major gas to control O2 level.

3) Maintain clean operation of the furnace belt. Using in-line ultrasonic belt cleaner to clean the belt is a good option.

After several trials, our furnace user was happy to get the bright surface finish. Have you encountered oxidization issues when using a belt furnace? How do you solve it? We welcome you to share your ideas with us!

Atmosphere Furnace