weld rod oven temps

Date 06-09-2009 18:20

Hello Donnie, About 7 or 8 years ago my facility was slated for re-certification for WABO testing. At that time they came through and expected me to have "calibrated" thermometers in the ovens. After much bantering about various requirements for "certified and calibrated" thermometers, gauges, flowmeters, etc., etc., etc., they finally backed off on many of those requirements. Yes, it could certainly be a requirement under certain circumstances by various code bodies. I do not believe however that AWS has that requirement in the code. If I'm wrong, I'm sure that there are plenty of folks out there who will correct me. Best regards, Allan

By G.S.Crisi (****)

Date 06-09-2009 19:05

The "calibration of weld rod oven temperature" is achieved by means of a simple and cheap instrument called "temperature switch" or "thermostat".
I don't know in the States, but here in Brazil by paying a little more you can buy a temperature switch certified by Inmetro (our local equivalent to the American Bureau of Standards). Of course, the certification doesn't last for ever and you should renew it when its validity voids. In practice, it's easier and cheaper to buy and install a new switch.
Giovanni S. Crisi
Sao Paulo - Brazil

By 803056

Date 06-09-2009 21:21

You should be able to purchase a thermometer or simple check the oven with a thermocouple that has been calibrated.

Best regards - Al

By MBSims

Date 06-09-2009 22:35

We just use a tempstick on a sample rod end. The tempstick has a CofC for the temperature it is marked for. This avoids "calibrated" thermometers.

By mdagg

Date 06-10-2009 11:46

Use a calibrated infared thermometer to verify the temperature at a certain interval. Make sure you incorporate this in your "inspection" procedure and you will be fine.

By js55

Date 06-10-2009 12:42

Marty is dead on here. Unless forced to by a customer or particular governing body it is silly (my words not Marty's of course) to get too carried away with monitoring your rod oven. Temp stick the thing.
I mean really, does it matter that much if the rod is 250 or 175? Verify it at the min in as simple a process as you can and let it go at that.

By G.S.Crisi (****)

Date 06-10-2009 15:37

Yes, it matters, because water boils at 212 ºF. So, at 250 you're sure that there's no moisture into the electrode coating, it's all evaporated. At 175 there's still some moisture into the coating, which you don't want.
Giovanni S. Crisi

By ravi theCobra (**)

Date 06-10-2009 16:24

This all works well unless you are doing Electric Boat work and you have to send electrode ( 11018 ) coatings
to the lab and they must be < .1 % by weight water -

Even with 225 to 250 rod heating there is some moisture in t he coating -

By js55

Date 06-10-2009 16:59

Giovanni,
I think your response is correct, but perhaps in need of clarification from my perspective. 175, it seems to me, would prevent the ingress of moisture if it wasn't already in the rod. The other thing is, I do not believe the moisture needs to boil to be removed. Wouldn't the removal of moisture be evaporative as well? Just taking longer?

By DaveBoyer (*****)

Date 06-11-2009 06:07

Moisture chemicaly combined with the coating will need much higher temperatures to drive it out, that is why re-conditioning temperatures are so high. Even at 175f water is still water, it needs to be above 212 at sea level to be steam, and a hydroscopic material will even absorb it at that temperature.

By G.S.Crisi (****)

Date 06-17-2009 21:06

js55,
excuse me for the delayed answer; I've been out of town for a few days.
Dave is right at saying that when moisture combines with the coating material it'll need a higher temperature to get rid of. This is the case of basic (or low hydrogen) electrodes whose coating is made predominantly of calcium oxyde (chemical formula CaO), usually called "lime". When it absorbs moisture, i.e., water vapor, the calcium oxyde combines with water to give calcium hydroxide, usually called " hydrated lime", which is a new chemical substance. The chemical reaction is CaO + H2O ----> Ca (OH)2. In order to split it again into lime and water vapor a higher temperature is required, as Dave correctly says.

The case is different when the coating material doesn't react chemically with water, as is the case of cellulosic and rutilic coatings. They'll absorb moisture but won't react with it. In this case, the absorption or release of moisture will depend, as physical chemistry says, on the vapor pressure of the moisture within the coating and the partial vapor pressure of moisture in the atmosphere. Both of them depend on temperature and the latter depends also, as Dave correctly points out, on the altitude above sea level. If the vapor pressure of moisture within the coating is higher than the partial pressure of moisture in the atmosphere, then the moisture within the coating will be released. If the partial vapor pressure of moisture in the atmosphere is higher than the vapor pressure of moisture within the coating, then the coating will absorb moisture.

In order to make those calculations you must know the altitude of your oven above sea level, the atmospheric pressure at that altitude, the moisture vapor pressure and the relative humidity (% humidity) in the environment in that particular day. For that, you should have at hand the psychrometric chart, steam/water tables and a table showing the normal atmospheric pressure at different altitudes, which is commonly found on compressed air manuals. Of course, you should have some basic knowledge of physical chemistry.

Knowing that water boils at 212 F at sea level, (i.e., at 212 F all water has evaporated), instead of making calculations it's easier to state a temperature higher than that (250 F in this case) to bake the electrodes and make sure that there will be no moisture into the coating (or there will be very little).
Giovanni S. Crisi

By G.S.Crisi (****)

Date 06-17-2009 21:08

js55, take a look at my answer above.
Giovanni S. Crisi

By SWP

Date 06-10-2009 13:17 Edited 06-10-2009 13:22

I like the tempstick idea with the certificate of compliance (CofC), it seems accurate and painless. The IR thermometer method is not accurate unless you go through a lot of pain to prove you have calibrated it and are using it consistent with the calibration. What I mean is, the IR thermometer has many variables that have to be known and controlled. There is emissivity affected by surface finish, focal distance, and measurement spot size. Essentially you need a calibrated thermometer to verify that the IR sensor is reading correctly at the chosen measurement location in the oven or on the rod, so in this case, why not just use the calibrated thermometer?

I believe it does matter what the oven temperature is. There is a reason that codes specify temperature and rod handling requirements, because hydrogen embrittlement is well documented.