Number of welding Pass, Heat Input and Distotion

By Lawrence (*****)

Date 09-02-2009 15:06 Edited 09-02-2009 15:08

The larger electrodes must carry more current, so greater heat input logically follows. (assuming the greater number of passes for the smaller electrodes are applied in such a way that a maximum interpass temp is not exceeded)

The grain refinement is directly linked to cooling rates.. The larger electrodes (in most senarios) will have a slower cooling rate, due to the greater heat input per individual pass, that will encourage a larger <less refined> grain structure.

By 3.2 Inspector (***)

Date 09-02-2009 17:46

Sorry for going off topic here ;(

"The grain refinement is directly linked to cooling rates.."
Are you 100% sure about this Lawrence?

3.2

By Lawrence (*****)

Date 09-02-2009 17:56

Maybe I should have said there is a relationship?

Cooling rates and grain size (refinement) are often related in carbon steels don't you think?

By 3.2 Inspector (***)

Date 09-02-2009 18:00

I would think its more like heat input and grain size are often related.

Since heat input and cooling rates also often is related, I will somehow agree with you.

3.2

By Lawrence (*****)

Date 09-02-2009 18:13

You said it better than I did. :)

By 3.2 Inspector (***)

Date 09-02-2009 18:36

I am just glad that I did not misunderstand the basics all of a sudden :)

3.2

By GRoberts (***)

Date 09-03-2009 00:47

a. is the correct answer. Because the 2nd weld was made with 12 passes instead of 5, there will be more grain refinement due to the heat of subsequent passes, and less unrefined columnar grained weld metal. It doesn't really have to do with the diameter, but in this case, the number of passes. Answer c. is not correct because the weld with more passes will have more distortion. You will get less distortion with fewer high heat input passes as long as the volume of weld metal is the same as the more lower heat input passes.

By hrezvany (*)

Date 09-03-2009 06:28

I have read that in terms of angular distortion less passes is better than more passes. Because more passes produce more angular distorion. with respect to this issue you're rigth.
But regarding longitudinal and transverse shrinkage, more passes produce less longitudinal shrinkage assuming constant deposited weld.

What's your opinion?

By GRoberts (***)

Date 09-04-2009 21:45

I believe that more passes cause more lateral (angular) and longitudinal distortion. Why would the amount of distortion change based on which direction it is measured with regards to the weld travel direction. More passes (i.e. higher heat input) means that more base metal heats up. So when the weld shrinks, the thermal gradients are smaller. It should have some of the same affects as more preheat, which also reduces distortion in both directions.

By hrezvany (*)

Date 09-05-2009 09:36 Edited 09-06-2009 10:02

First of all I want to say that I read it in Cswip 3.1 book. I explain this phenomenon as follow:
longitudinal and transverse shrinkage (not distotion) is in accordance to initial heat input. In the subsequent passes due to increase of inertial momentum of the workpiece section, amount of shrinkage will be decreased.
This is my opinion, please share your ideas to investigate the correctness of mentioned statement in the book.

By GRoberts (***)

Date 09-09-2009 23:54

I believe that I agree with the book, but a without a whole lot of context, it is hard to know for sure. I think they are saying that a large root pass (i.e. high heat input) will reduce overall shrinkage. (Although I don't know how you can get shrinkage without distortion. Perhaps it is just a funny way to differentiate between angular distortion and longitudinal or transverse distortion). The root pass does have the most affect on the total amount of shrinkage/distortion as compared to other passes. However, whether it is the root pass or any other pass, the heat input correlates (i.e. higher heat input passes/larger passes) to the amount of stiffness provided to the joint by the weld metal in order to resist distortion/shrinkage on subsequent passes.