stringer or weave for pipe strain 2

[Downhand]

I have an engineer who wants the welder to grind out part of the cap and part of the fill pass (from 3 o'clock to 7 o'clock) and the refill that area in an attempt to cause the pipe to move because it is too far out of alignment with a pump. What would cause more pipe movement; refilling with one weave with a bigger electrode or refilling with stringers with a smaller electrode but more passes.

 

[stanweld]

You want to maximize weld shrinkage stress, so use the larger electrode. Whether you will be able to move the pipe enough to provide acceptable alignment is another issue. In most of these cases, production has had to cut out the weld and refit.

 

[metengr]

What does too far out of alignment mean? Radial, axial or both? Proposing what you want to do will not work, either way. You need to either cut the pipe and realign or use a spool piece or you can try to heat a band and move the pipe (this depends on material and degree of misalignment).

 

[paddingtongreen]

Just heat to a dark cherry, short, narrow, strips across the side that is too long. It will try to expand but will be prevented because it's Fy is reduced with temperature. Because it cannot expand, it will contract and be a little shorter on cooling. keep doing this till it is where you want it.

Better though to cut and use a new spool piece, field fit and welded with the flange bolted to the pump.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[gtaw]

The angular distortion is almost directly related to the number times the carbon steel weld/base metals has been subjected to temperatures above the lower temperature of transformation.

Consider a T-joint welded with a single sided fillet weld. A three-pass fillet weld will exhibit more angular distortion than a single pass fillet weld because the three pass weld has heated and cooled the base metal three times. The large single pass fillet weld only requires the joint to be heated and cooled once.

Another factor is the thermal gradient from one area (the weld and HAZ) and the unaffected base metal. The greater the gradient, the greater the distortion. The stinger beads will result in higher thermal gradients because there is less heat (Q=VxAx60/TS) in comparison to the large single pass fillet. Each pass of the weld deposited with stringers has higher thermal gradients because the heat dissipates into the adjacent base metal quickly. Each pass introduces additional angular distortion. The large single pass fillet weld heats the joint more completely reducing the thermal gradient through the thickness of the base metal and adjacent to the weld. It cools slower, has less thermal gradient, and introduces less distortion.

The temperature gradient required to reach the approximate yield point of the base metal is relatively low. For carbon steel it is on the order of 220F to 230F degrees above the surrounding metal. That temperature gradient is all that is needed to introduce residual stresses on par with the yield strength of the base metal resulting in plastic deformation, i.e., angular distortion.

The trick to flame straightening or cambering is to maximize the delta T from the area heated to the adjacent area. There is no need to heat the base metal above the lower temperature or transformation. A delta T of 230F is sufficient to begin to see some "controlled" distortion. I usually place a barrier between the area being heated and the adjacent area. I pack the area beside the heated area with wet rags to maximize the delta T to maximize the "distortion" without any danger of introducing metallurgical problems because the metal (carbon steel) was heated above the lower temperature of transformation (about 1330F). The idea is the heat one area while not heating the adjacent areas so the unheated areas offer restraint. The heated region expends and deformed by plastic deformation as a result of the heated area developing compressive forces in excess of the yield point of the base metal. Upon cooling the compressive forces become tensile forces if the unheated areas offered sufficient restraint. That is why the entire member is not heated to a uniform temperature. Both delta T and restrain have to work together to cause distortion that can be to your favor as in the case of cambering a beam or against you in the case of a single sided multiple pass fillet weld.


Best regards - Al

 

[GRoberts]

I agree with gtaw, for the same weld volume, more passes (lower heat input per pass, but more heat input overall) will cause more distortion or movement.

 

[Downhand]

Excellent post GTAW. The flange was out about an inch both radially and axially. This is alot considering it needed to be within 12 thousandths of an inch. In any case, the flange was dogged into place and then the flange and a couple of the joints down from the flange were ground down to the first fill pass. Once refilled and the dogs removed the flange alignment was acceptable.

 

[gtaw]

Thanks for the compliments.

Best regards - Al

 

[Tmoose]

thanks GTAW. I had never considered straightening with such low temperature/heat. Might get the chance to try it out this weekend. I will be trying to think deep thoughts about the elongation that I'm inducing locally
Dan T