preheat/postweld heating

Date 02-26-2010 16:49

I am assuming you are talking preheat

Electric Resistance
Pros: Greater control over temperature
   Easier to facilitate chart recording
   Electric outlets often more convenient

Cons: Pain in the azz to hook up
   Pain in the azz on roll outs
   Leads everywhere creating tripping hazards
   More expensive generally
   Loss of controlling thermocouples

Torch
Pros: Roll outs easy
Multiple set ups cheaper
Multiple set ups more portable

Cons: Direct flame impingment
   Gas safety issues
   Gas bottle exhanges
   Gas headers may be required

I suppose there's much I've left out.

By RonG

Date 03-01-2010 15:05 Edited 03-01-2010 15:08

Its real simple, If you need a controlled ramp (rise & fall rate); soak time and chart for records; your best bet is is to use CHP's (ceramic heating pads). If you do not need any of for mentioned, a torch is the way to go.

For gradient: You make the set up to control gradient as prescribed say 100'F per foot in all directions is what we normally shoot for.

Truth be known, if you do not have a record, you did not do a Preheat or PWHT.

Heat is heat (BTU's) if it comes from a CHP or a Rose bud and "structure change" will be in how you apply it.

Time &Temperature are horse's of a different color though and must be controllled and recorded.

Just for posterity you said "TORCH" not an oven with a flame.

By zambrota (**)

Date 03-01-2010 21:59

Ron,

Don't you think that CHP's can cause hot spots, i.e. non-uniform heat distribution? Is there any recommendation how many you need to install (per sqm)?

Thanks

By RonG

Date 03-02-2010 13:16

Is this just a point of curiosity or do you really have a problem with this process?

You place the TC (Thermal Couple) in the center of the pad. In case you are not aware it is common practice to tack weld the TC to the work by useing a tack welder designed for that purpose. This allow’s you to control & monitor the work temp not the CHP.

The controlling TC is placed in the center of the pad with a monitor close to 1 edge. The heat is uniform if done correctly, more so than a torch (assuming the work is larger than the flame).

There are different sizes; shapes and voltage of heaters thus requiring different arrangements. We use 40 volt heaters that are 144 sq/in no matter the shape. That gives you uniform heat and wattage, which can be converted to BTU’s.

That is where experience comes in handy. Call a contractor; I am sure they would be glad to explain the process. All my welding personnel are good at it and we do all our PWHT in house, save when the work load won’t permit it.

By zambrota (**)

Date 03-02-2010 22:41

Thanks Ron,

FYI

I have a problem with a heavy Cr-Mo spindle. In client's specification the maximum preheat temperature difference in preheat zone is 30°C. I used CHP's but I could not achieve it even with heaters installed next to each other. I don't know what's wrong: material characteristics, CHP's or somethig else? How to get uniform heat all around the shaft and temperature recording chart in order to prove ±30°C tolerance?

By RonG

Date 03-02-2010 22:56

Maybe I can help some more if you give me a little more info. We stress relieve large Turbine rotors (#100,000 is the largest) regularly and we turn them up on end to do it. Getting enough heat where you want it can be a challenge.

We work to +/-25'F.

If you want you can e-mail me with info if it will be any easier for you, but I need to know the mass you’re dealing with and the temp you want to reach.

By zambrota (**)

Date 03-03-2010 04:02

Why do you turn turbine rotors up?

My shaft is approx 50 tonnes. Middle section is 2m dia. by 1m length (I need to preheat that section for repair) and two end sections are 1m dia. and 2m long. Preheat temperature shoud be 300°C with ±30°C allowed tolerance during measurement. Customer insisted on the preheat temperature record evidence. I don't know how to control uniform temperature distribution. Is it try & error method or there are some rules that I'm not familiar with?

Thanks

By RonG

Date 03-03-2010 13:59

To keep them straight with in .0005".

I guess what you described is the main area of your concern, roughly 79”OD x 40” long trying to get to 600’F, sounds more like a disk. Is there more to it?

I only get about 30 US tons from what you describe but no matter it’s a large mass and will require a bunch of KW’s.

Can you tell me a little about your equipment? Power source: Available KW; secondary out put; and number of available CHP’s.

By zambrota (**)

Date 03-03-2010 22:45

That's correct. But it is only the weight of middle section. If you include 2 shaft journals, that adds up to 50 tonnes. Some more information: Maximum equipment power is 45KW (6 units,i.e. 6x45=270KW); Primary input is 400V,63A/500V,52A. I have 8 CHP-s (power 1.35/2.7KW, voltage 30/60, Amps 45A)

By RonG

Date 03-04-2010 18:48

You need some darn good insulation. Your CHP's @ 50 Amps can put out roughly 3.7KW x 8 = 30KW.
To raise 1 pound of steel 1 degree F you need .145 BTU's. The conversion from BTU's to Electrical power in KWhrs is 3412 BTU/KWhr multiplied by a factor or 4 to allow for heat loss.

In short----youre a little short on power, but if you have some really good insulation and a lot of time it can be done but not likely. You will need more CHP's and splitters (3 way cables) to allow you use 3 CHP's per station (zone) of which you have 6 makeing a total of 18 CHP's giving you a better chance.

I suggest you call a contractor like Cooper heat. What part of the world are you in? There companys that rent equipment.

By zambrota (**)

Date 03-04-2010 21:39

Thanks Ron,

You help is enormous.

Regards