Ammonia Function in the Carburizing Process

stove

Sometimes we purge some ammonia into the chamber while carburizing in order to prevent microstucture of non-martensite in the surface and grain boundry, it is sometimes useful, but sometimes not.
Who is the expert of this procedure, how the ammonia influencing the non-martensite, which part? alloy solution or the hardability of austensite.
who can give me a hand?
Please use English!Thanks!

reference:
http://www.rclbbs.com/viewthread.php?tid=35395

aaron01

As we know, the non-martensite cause by the loss of alloy element on the surface during carburizing process. It cause the low hardenability, so the austenite on the surface could not be hardened to martensite.
Some documents show it will be improved by purging little volume ammonia in during process. I think it will be good to enhance the hardenability. But in practice,sometimes we can not get obvious improvement.
Let's go back to see what happens on the surface. Where did the alloy element go?
Many alloy element such as Chromium and Titanium are more active than iron, although there is only a little oxygen in the atmosphere, they are easy to reaction with oxygen on the surface of the parts and create alloy oxide. The alloy oxide precipitate from austenite, so there will be a poor alloy content area nearby the oxide network on the surface.
To improve the non-martensite,I think :
the most important thing is to control oxygen content in the furnace;
the second factor is the material, if the material has high content of active alloy element, such as 20CrMnTi and so on, after quenching there will be deeper non-martensite layer,so if could change the material to low oxidated tendency alloy steel;
the third methods is to enhance the cooling speed;
Purging ammonia into furnace will improve the non-martensite in some case when you treat some material without high content of active alloy elemnt, such as low carbon steel, 8620, 4620 and so on. But for high oxidated tendency alloy steel, it has limited effect.