Preheating with Acetylene

Date 03-04-2016 16:28

Interesting thoughts about the fuel...A properly adjusted torch/flame should be fine. A properly sized rose bud tip, if kept moving, should preheat fairly evenly. You can keep a watch on the heat with successive tempil sticks to make sure the critical temperature hasn't been over shot. (ie. structural steel carbon steel is 1200°F, 1100°F quenched or tempered AWS D1.1 Clause 5.26.2)

By welderbrent (*****)

Date 03-05-2016 00:58

A comment to supplement John's answer, for 'PREHEAT' you won't want or need to be anywhere close to 1200°. For heavier members 225° is the top except for special situations which can require temps on up to the 4-600° range.

Now, for heat cambering, straightening, and other operation you may well need more heat and even close to 1000-1200°. But, not for preheat.

Acet works just fine. Sounds like some old wives tale to me.

He Is In Control, Have a Great Day, Brent

By Milton Gravitt (***)

Date 03-05-2016 07:50

You have to use both acetylene and oxygen to do the preheating if you use only acetylene it will are could cause carburization.
M.G.

By TimGary (****)

Date 03-07-2016 14:52

Agreed, for the most part.

A trick I learned an eon or two ago was that when welding on threaded pipe, close to the threads, the sooty residue left behind from a pure acetylene flame makes a good anti-spatter.
I'm not sure what this residue actually is. Carbon seems like a good descriptive name as it is deep black and powdery, but I don't know if there is actually any carbon in it.

Does anyone know what the residue actually is?

Tim

By Superflux (****)

Date 03-07-2016 18:31

TimGary,
DON'T do it!!! You'll inject carbon into the puddle and "crystalize" the weld!!!!!!!!!!
;)
lol
and yes... it is carbon

By TimGary (****)

Date 03-07-2016 19:47

Yup, I should have clarified that the anti spatter / soot was only to protect exposed threads, and was to to be cleaned from, or not applied to weld joints.

Tim

By Lawrence (*****)

Date 03-07-2016 19:54

I use that acetylene "smut" to cover sheet aluminum when annealing. When using the planishing hammer or even a shot bag and mallot, that aluminum gets stiff and cracks quick... Just cover with smut and then use an indirect neutral flame until the smut disappears.... Not very scientific, but the black stuff disappears right before the aluminum goes liquid.

This is for artwork not codework :)

By TimGary (****)

Date 03-07-2016 20:13

Ok, so my question of "What is the soot left from an acetylene flame, actually" has been bugging me, so I did a little research...

Acetylene gas is a byproduct of a chemical reaction that occurs when calcium carbide is mixed with water.
This can be expressed as:

CaC2 + 2 H2O → C2H2 + Ca(OH)2

So Acetylene is made up of Carbon, Calcium, Oxygen and Hydrogen.

Calcium, Oxygen and Hydrogen can break down, but carbon doesn't.
(I'm told that every atom of carbon that has been on, or in, or added to our planet since the beginning is still here, being used over and over in various molecules until they break down)

So, it makes sense now that yes, the soot left behind from an acetylene flame must be almost pure carbon.

Tim

By Superflux (****)

Date 03-07-2016 23:57

Just funnin' with ya there TimGary.
We have done post weld (SMAW E7018) mechanical tests with bevels that were air arced (No grinding just chipped of any dross! Then welded over the "Frosty" surface) and a control set with ground bevels and there was no discernable difference in tensile or charpy results.

By TimGary (****)

Date 03-08-2016 14:12

Wow, that sounds like the basis for a fully repeatable procedure that will never backfire and cause porosity issues.
Just think of the money that can be saved by eliminating grinding!

By Superflux (****)

Date 03-08-2016 18:32

This was a classroom project to prove that a skilled and competently performed air arc or O/A torch cut is not "That" dirty.

By jwright650 (*****)

Date 03-07-2016 14:22

"....for 'PREHEAT' you won't want or need to be anywhere close to 1200°. For heavier members 225° is the top except for special situations which can require temps on up to the 4-600° range.

Now, for heat cambering, straightening, and other operation you may well need more heat and even close to 1000-1200°. But, not for preheat." -Brent

Agreed, preheat need not to exceed 225°F for low hydrogen electrodes/processes, and 300°F for others(AWS Table 3.2-- speaking of preheating structural steel, mild steel). Also agree with the special situations requiring a bit more heat.

I answered pretty quickly on the way out the door to another job.....I should have explained things a bit better. Thanks for the help Brent.

By Superflux (****)

Date 03-05-2016 20:10

It all depends on size, material and geometry.
When on large heavy parts such as adding a 1" liner plate to a D-10 bulldozer a couple or 4 "Weed Burners" are required. You would never get all of the components (efficiently) up to temp with the concentrated heat of O/A rosebuds.
On the other hand small intricate assemblies weighing only a few pounds might require small O/A blowpipes to get the heat where needed and not melt areas with thin cross sections.
Carburization is only the result of improper cleaning of parts and as mentioned, a carburizing flame.

By Tyrone

Date 03-11-2016 11:56

Thanks for all the input.
I will not ban the use of acetylene.
Just make it idiot resistant. :wink:

Tyrone