Preheating question

[amitloginsites]

Hello all,

I have a tube with certain chemistry which shows CEV 0.45. How do I judge whether this material needs preheating or not? I looked online and it says that the general rule of thumb is that if CEV>0.4 then it needs preheating. If this is true then then how do i calculate how much temperature and how long should i preheat before welding?

 

[stanweld]

Preheating is also based on cooling rate, which is highly dependent on thickness and ambient conditions; it is also based on hydrogen content of the welding filler metals and welding process. Finally, joint restraint will also play a role.

 

[Wrenchbender]

Assume you are arc welding, which means rapid and high localized heating. Purpose of preheating the whole part is to suppress the cooling rate. The hotter the far-field material is, the less likely it will quench the much hotter weld into brittle martensite via conductive transfer. Volumetric strains might then cause cracking. To answer your question about how to calculate, i would:
1. Find a continuous cooling transformation (CCT) diagram for your alloy or get an approximation at

http://calculations.ewi.org/vjp/secure/TTTCCTPlots.asp

2. Find temperature-time when martensite begins, Ms.
3. Manually calc (ugh) or finite element method the temperature-time profile for your part geometry. Vary the starting temp to stay away from Ms. If you're heat treating the part afterward, the formation of martensite doesn't matter as long as the part does not crack before it goes in the furnace. (Beware delayed cracking before it does.)

Alternatively you can use trial-and-error, or just ask the oldest, most experienced guy in the welding shop.

 

[amitloginsites]

Thanks for the responses.

I think i am not able to understand the answers. I have only the chemistry of the material and mechanical properties. The supplier did not provide any information on how the tube was made. I am not a metallurgist but i thoughts there are some recommendations in AWS or some other code to guess whether a material is required preheating or not?

 

[stanweld]

Codes do provide recommended and mandated preheat requirements.

If you were butt welding 4" diameter x .25" wall carbon steel pipe with CE of .45 with a GTAW root pass and E7018 low hydrogen electrodes at temperatures above 32 F, (slow cooling) preheating would not be necessary. If you were to butt weld 36" diameter X 1/2 wall pipe having the same CE with E7010 electrodes with ambient conditions below 40 F,(rapid cooling with high hydrogen weld filler) it would behoove you to preheat. If you were to weld 1" wall pipe having CE of .45, (rapid cooling) with any process, it would behoove you to preheat and most Codes would mandate and define minimum preheat.

In short, it is not only chemistry that dictates the need for preheat.

 

[metengr]

Codes do provide recommended and mandated preheat requirements.

Better look a little closer at AWS D1.1, ASME B31.1 (131.4), Section I (A-100.4), VIII there are recommended preheat requirements.


So, yes indeed recognized Codes and Standards provide recommended preheat requirements based on chemistry and thickness and these should be considered to decide on appropiate temperature.

 

[bhollands]

A carbon equivalent of .45 is not going to require much preheat if any. Assuming that you are using an ER70S6 or a E7018 I would not require a preheat over 100F. You should be able to weld it at room temp (>60F).

Bob

 

[metengr]

I would not require a preheat over 100F. You should be able to weld it at room temp (>60F).

If the material thickness is less than 1" agree with the above. Otherwise, no way would I weld on a 2" thick plate with only a minimum 60 deg F preheat. Plus, most Codes and Standards would require greater than 100 deg F preheat for a thickness over 1".

 

[metengr]

For those that like on-line calculators for estimating preheat and other welding parameters;

http://homepage3.nifty.com/yurioka/exp.html

 

[SJones]

ISO/TR 17844

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04