Valve Casting RT accepance
In Valve castings as per B 16.34 Coverage all Client Prefers Radography Level II in critical areas.
My question is in critical areas we got Level IV Shrinkage or Inclusions . Is accecatable to reduse the defect level to Level II by welding upgradation and accept the castings.
|P V SASTRY|
R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D, India, Joined Jan 2003, 195
Re: Valve Casting RT accepance In Reply to slimmoon25 at 18:50 Oct-06-2009 (Opening).
In my opinion you cannot do partial repair welding on the defective area and bring it down to acceptable limits in radiography, particularly in critical areas.
I always maintained that repair weld in a casting is a defect by itself. It is done as a necessity and considered to be technically acceptable defect. This is mainly because of the HAZ which is at best stress relieved. The coarse HAZ grain can never be refined and brought to the original cast size.
Then let us consider a defect at a depth of about 30 to 40 mm below the surface. The repair procedure starts with gouging of all the sound metal above the defect and finally removing that 20 or 30 mm of defect. The digging cannot be obviously with vertical walls and a proper angle has to be maintained to facilitate further gouging as well as subsequent welding.
Then imagine the size of depression/cavity that is gouged to remove a 20 or 30 mm defect at a depth of 30 to 40 mm. After due grinding of the profiled walls, the welding process starts with all its human and technological fallibility. Please remember that there are many small physical weld defects like small slag inclusions which may not be detected in the subsequent NDT either due to test sensitivity or become acceptable nothings.
Then there are other metallurgical defects which any way are not detected in the NDT but supposed to be non existing if the established welding procedure is followed. (For example if these valves are being used in high temperature service the first runs are supposed to be made with lower diameter electrode and the welding done at very shallow angles at minimum voltage. All this to reduce the extent of HAZ and prevent reheat cracking during SR or subsequent service).These are not the easiest things that are always followed.
I am not at all implying that there will omissions on your side. I am only referring to some of the inherent aspects that are not in anybodys control or extremely difficult to control.
In the light of this imagine that you are making a repair weld on a left over defect in a critical area. In my opinion it is only compounding the problem.
Also it is difficult to make a sound welding on a defective area like shrinkage. I do not know the type of shrinkage you got. Spongy shrinkage is more difficult because of the gasses it can hide and release them during solidification of the next layer welding.
I said it is difficult to weld on a defective area like shrinkage but it is almost impossible to weld on a cracked area and mask the cracks (I am not referring to very small and shallow cracks but lengths of about 10 to 15mm and above). Any no. of times shop supervisors tried to quickly weld without removing the crack completely to save time and score some incentive points.
Every time they ended up leaving a tip of the ice berg on the surface. I.e. when the weld is being inspected next day by Wet Fluorescent Magnetic particle testing, we used to find just a small innocuous indication of about 2 mm or less on the surface. When we started digging it by progressive grinding it used get gradually enlarged and finally lead up to the original crack from where they started welding. This happened dozens of times and people who gained experience did not repeat but always there were guys who wanted to get first hand experience of getting bitten.
Coming back to the original point where we started I firmly believe that welding itself is a technically acceptable defect. Do not (repair) weld if it can be avoided. Based on the location and other criticalities I was always accepting defects on the border line or even slightly beyond the border line, rather than going for repair welding for all the adverse reasons I mentioned above. But once decided to weld the defect should be completely removed and the welding should be religiously done with motivated and educated people. This is more so in critical areas.
A little more information on the intended service and the material of the valves, the critical areas (particularly if these areas are to be process welded subsequently), the type of shrinkage you got etc would have been more helpful.
All the best in your pursuit.
With best wishes
P V SASTRY
Portugal, Joined Sep 2009, 2
Re: Valve Casting RT accepance In Reply to P V SASTRY at 15:20 Oct-07-2009 .
In reference to P V. Sastry explaination, i would suggest that prior to repair of defective material by welding, a pre-heat and a post heat treatment of the material to be weld repaired should be considered and be done properly. This can be achieve as long as cracks are totally removed and it should be in a certain depth that can be considered repairable. In my experience a defect having a depth of 1/4" can be repaired as long as you have the relevant guiding datas such hardness, welding rods, electric current, build-up materials,etc.
I doubt if this can be done to casting material.
I hope this contribution may find useful too.
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