Welding & NDT

Expansion is necessary in all cases because the tube hole in the tube sheet is slightly higher than the tube OD. So if we do not expand it will cause crevice and ultimately tends to leakage.

For Heavy expansion, normally grooves shall be provided in the tube sheet as per TEMA RB-7.24. Tubes will be expanded in such a way that it will flow in these grooves. In Heavy expansion normally 5% thinning of tube thickness occurs.

For strength welded tube to tube sheet joints, contact expansion is necessary as explained in (1) above.

1.what are all the Quality checkpoints while welding with SS?

Welding of SS special compared to CS. Do not mix up CS with SS welding. Avoid using the same hand gloves, grinding wheel, tongs or even the same work area used for CS to avoid Carbon migration on SS. Handle SS carefully. Stake it vertically to avoid walking over it which forms serrations. All the above may lead to corrosion. Use special SS grinding wheel to avoid clogging of gr. wheels. Do not use yellow-oil chalks(sulfur-rich)  for writing on SS to avoid corrosion. Check currents as recommended by the manufacturers.  Pickle and passivate  S after welding. There are plenty more.

2.how to check the discoloration while welding?

Use low currents (15 % or less) compared to MS welding. The weld or HAZ should not turn to blue, which means depletion of chromium (reaction with Oxygen to form Cr -Oxides) The weld should be straw / light yellow, white to show nonreaction. Stop the welding if required, to avoid overheating while welding.

3. purge time calculation?

It is difficult as it depends upon the length of the plate, pipe dia, etc.  Purge till you make sure the oxygen is removed fully. Use a burning match stick (easier method) to check the outflow of gas. Any left out oxygen may create oxidation at the root side and is not acceptable..

4. if the thickness of the base material is less than 3mm what are all aspects shall be followed to maintain good welding?

Use Pulsed TIG welding processes as it gives very low heat input compared to normal /continuous weld. Pulsing is good for higher thicknesses also.

5. what is the distance calculation for purging?

Purging could be localized or to the full length of the pipe if required. Purging could be 40 -50 % less to shielding pressure. gas inlet end could be around 50 to 75 mm or so from the weld Cir-seam end if it is a pipe.

6.how to determine the electrical parameter and shielding gas parameters, preheating details for GTAW while preparing proposed WPS for SS and NON-FERROUS MATERIALS?

SS does not require any preheating. The inter pass temp. could be around 75 to 150 deg.C max. The total heat input should be less or nearer to 1.0 Kj /mm. Use stringer runs only. Do not use weave pass to avoid more heat input. The shielding gas pressure could be bet. 8 -10 -12 LPM. For nonferrous metals, heat input could be around 2.0 Kj/mm. Use only DC EN for both SS & MS.

Ultrasonic phased arrays use a multiple element probe whereby the output pulse from each element is time delayed in such a way so as produce constructive interference at a specific angle and a specific depth. These time delays can be incremented over a range of angles to sweep the beam over the desired angular range. For example, 40 to 75 degree beam sweep would be produced by calculating the time delays to produce constructive interference at 40, 41, 42 ...75 degs. This NDT technology is also referred as Swept Beam Ultrasonic testing. The main advantages of phased array in NDE are:

1. Ability to sweep a range of angles

2. Ability to display the image in real time for the swept angles

3. Ability to focus

The above figure displays the concept of phased arrays. Time delays to the eight elements control focusing and beam sweep. Focal spot size (shown by the shaded orange area) is controlled by beam spread.