Furnace door open 'x' minutes = 'y' scale

[dbooker630]

I'm writing a procedure on how to handle tray wrecks in a pusher carburizing furnace. History has told me that wrecks will never be completely eliminated, but it is how you handle them that minimizes loss of product in the furnace.

The furnace operates at 1700F with endo+NG atmosphere. Like I said above, when a wreck occurs the endo is replaced with nitrogen, preferably after some cooling. At some point a furnace door will have to be opened to assess the wreck and how easily it can be cleared.

If time was not a factor and I was the production manager I would let the furnace cool to below 1000F and then analyze the wreck. However this is the auto business and I'm just the metallurgist, so things don't always work the way I like it. This means the wreck sometimes must be assessed AND cleared while hot, and parts are cherry-red.

The rule that I learned from the old-timers years ago was to work at the wreck no more than five minutes, then close the door to allow the N2 to recover. I'm being asked to publish a maximum time that a door can be opened which I'm reluctant to do on just my opinion. Five minutes always worked and even if some decarb formed it was easy to rework.

You may have figured out by now that the door was opened for far longer than five minutes, creating a lot of rework and loss of product due to excessive scale...What I'm looking for is any published evidence of an empirical relationship between duration of air entry and how much decarb or scale forms in the process. The material is 8822 or 8620 steel. Thanks for your help!

 

[redpicker]

My thinking is that you will find it hard to find any published information on this, emperical or theoritical. There are just too many factors (temperature, door size, part size, wind velocity, etc...) that would make any correlation so dependent on the specific installation that published information would be of little use.

I think, however, you have the best source of information at your disposal; the old-timers. While they may not fully understand what is going on with the steel, they do have a very good idea of what works (or better, what doesn't work). As an old boss of mine once told me, "we've screwed things up just about every way possible", meaning they've done everything the right way, the wrong way, and the "God-knows-how" way and seen the results. Often, this information is much more valuable than all the scientific analysis you can perform.

My suggestion, use the 5 minute limit in your procedure citing the old-timer's experience as your justification. If you later gain emperical evidence (the amount of rework time or scrap, for example) that 5 minute limit is too long (or too short), you can certianly change it. Based on the best information you have, though, it seems to be the best choice.

rp

 

[rmw]

Can you even if only occasionally (maybe even weekends or third shift) run a load of scrap-meaning pieces that are already out of tolerance prior to the furnace process that otherwise would have been scrapped and never would have seen the furnace and see what happens to them with the door opened at various time intervals in excess of 5 minutes?

rmw

 

[tripleZ]

Maybe unrelated, but maybe not. What material are you using for your furnace elements? Our pusher furnaces (for sintering) are running under either straight H2 or a NH3 mixture. Furnace jam-ups shut off the H2 values and throw the furnace into purge mode with N2. Now, we're running about 800F hotter than you are. But from what I've gathered, the moly elements in the furnace at that high of a temp are easily damaged through oxidation. So the time you wait is more to protect the elements than the parts within it (especially with moly prices these days).

But your temps and your atmosphere are completely different from mine, so the issue may not exist.