Preheat to remove moisture before welding. 2

[winai]

Hi,

I have some WPS mention a bout minimum preheat temperature: Preheat to remove moisture
I,m finding in AWS D1.1 CODE. But cant find it. What is meaning of this moisture need to remove?any idea pls help me to clarify about this” Moisture.

Thanks
winai

 

[phighfill]

Steel retains moisture during climate changes. It goes back to dew point and preheat will remove this moisture. If you ever experience, or see, when the preheat is performed, you will notice the line of moisture that is on the outide of the flame.
You will also notice in D1.1 that there are requirements for preheat depending upon the type of material and the thickness. Some materials, i.e. A36, that do not require some type of prehat. I work to D1.1 (2000 editiion) and the table to look at is Table 3.2. On the right column of the table there will be a "Minimum Preheat and Interpass Temperature) that is required.

 

[winai]

More information FCAW process plate grade AH 36 T-JOINT web pate is 1/2”(12.7 mm) thick and face plate 5/8”(16 mm) thick. Ambient temperature 25-30 degree C. according to D1.1 code minimum preheat is 0 degree C.So preheat is not required by the code.BUT WPS have mention about minimum preheat temperature;( preheat to remove moisture ). how to identify moisture in the joint surface that need or no need preheat to remove. i see joint surface is clean and dry.not see any evidence moisture like vapor or water there.in my opinion the term (preheat to remove moisture) is wrong concept to write on WPS.

Thanks
winai

 

[phighfill]

Most of the WPS's that are written do not state "preheat to remove moisture", they will just state preheat. If the WPS states to preheat, whether to remove moisture or not, it is to achieve the correct temperature for welding. You can achieve the moisture removal without getting to the proper preheat temperature for welding. Preheat to remove moisture is just a surface condition where as preheat for welding is an internal condition.

 

[gtaw]

Any one that passed chemistry 101 should recollect that one byproduct of combustion is H2O. The moisture observed on a cold plate or member is simply condensatation of the water that is a byproduct of the chemical reaction between the fuel (typically a hydrocarbon) and the oxygen. All the "smoke" observed during a rocket launch, where the propellants are liquid oxygen and liquid hydrogen, is water in the gaseous state (steam).

Preheat is used as a means of reducing the cooling rate to mitigate undesirable microstructures that can result if the cooling rate is too high. The preheat temperature required is dependent on the chemistry, i.e., carbon equivalency, the base metal thickness, and the degree of restraint. AWS D1.1 is relatively conservative compared to other welding standards such as some NAVSEA fabrication standards or ASME B&PV code.

Moisture will stop collecting on the surface of the base metal as soon as the temperature of the steel is above the dew point. Think of the moisture that collects on the surface of a glass containing cold ice tea. Once the ice melts and the temperature of the tea is the same as the surrounding air, moisture no longer collects on the outside of the glass. The moisture isn’t collecting on the surface of the glass because it is leaking or because there are pores in the glass, it is simply the moisture in the air condensing on the colder surface of the glass container. I would have used a pitcher of cold beer as the analogy, but rarely does the beer last long enough for the temperature to come into equilibrium with the surrounding air.

We did an experiment in Thermogoddamics when I was a kid. We had to determine the BTU of natural gas. We passed water through a boiler and monitored the temperature of the water going in and the temperature of the water going out. We also monitored the volume of water passing through the boiler. As we passed the water through the boiler, we heated it with a Bunsen burner. The volume of the gas burned was measured. Several of my classmates asked for new boilers as the experiment got underway because their boilers had obvious leaks. The professor turned to me as asked, “Al, my little welder friend, tell all these genius’ where the water is coming from.”

Considering all the students were in their fourth year and majoring in Physics I concluded there is no hope for society.

Back to the question; water can be trapped between lapped members, it can be a byproduct of combustion, it can even be surface moisture because it rained the evening before, but it ain’t coming out of the base metal! If it did, we wouldn’t be able to buy beer in cans, submarines would only submerged, and ships would always sink.


Best regards - Al

 

[winai]

Hi thanks for all of u in put.
Because some of CWI tell me that steel is absorbed moisture in a night time. So In the morning before start welding we need to apply preheat to remove moisture even code not required. wherever in work shop or out side . I think I need to tell the CWI go back to study and passed chemistry 101 like Gtaw said,

Thanks again

winai

 

[gtaw]

Like all professionals, some are better than others. As long as they pass the exams, they are called CWIs. Then again, not everyone claiming to be a CWI is.

Best regards - Al

 

[winai]

Hi All,

Again confessing about preheat with heating torch.

In morning time when apply heat by heating torch. We will see water( byproduct of combustion is H2O). But start from 10 o' cock or even in the afternoon. Why we didn’t see any water when during preheating?
Any one who know or have experience. Pls help to explain.

Thanks
winai

 

[gtaw]

The temperature of the base metal is above the dew point, so the water which is in the gaseous state does not condense on the surface of the "warm" base metal.

Think of a cold glass of beer or iced tea. Moisture collects on the outside of the container because it is cold, i.e., its temperature is below the dew point of the surrounding air. Any moisture in the air condenses on the on the cold surface of the container. Once the beer or the ice melts in the iced tea, the liquid and container is no longer cold. Once the liquid and the container is at the same ambient temperature as the surrounding air the moisture no long condenses and collects on the outside surface of the container.

In the case of preheat; the source of moisture is the byproduct of combustion. Once the temperature of the base metal is warmed, the moisture no longer condenses and wets the surface.

Base metal that is heated by a resistance blanket or by friction as is the case when the base metal is prepared by grinding does not collect moisture on the surface and it doesn’t' get wet as is the case when the material is heated with a torch.

Back to the moisture that collects on the outside surface of a cold glass of beer or iced tea; the moisture that collects on the surface is not the result of leakage through the glass. If it were the result of leakage, it would continue to leak until the glass is empty. I believe we can agree that the liquid that does leak out is via the top of the glass and it ends up in our belly and not the table top. The glass container is not porous nor are the metal vessels we fabricate to contain liquids like water, oil, petrochemicals, or gases such as oxygen, helium, and hydrogen to name just a few of the fluids we store in metal vessels.

As long as the base metal being preheated by a method that involves the combustion process, moisture will condense on the cold surface. I have had cases in the cold of winter where the moisture continued to form on the cold steel some distance from the flickering flame of the torch until I completed the weld. Let me explain. Due to the very cold temperature, I had to turn the propane/air preheating torch down to a flickering flame to maintain the required preheat temperature. The preheating torch was leaning up against the inside of the column between the flanges. About 12 inches (900 mm) from the area being welded the moisture produced by the flame continued to condense and it froze to form a rather large icicle hanging between the column flanges. The foreman, who knew little of science, accused me of melting the snow from the upper stories of the building we were erecting. Silly man, he probably still believed the earth was flat and the sailors fell off into space when they sailed to close to the edge.



Best regards - Al