Common weld defects in thermal power plantsPresentation Transcript
COMMON WELD DEFECTS IN THERMAL POWER PLANTS SHIVAJI CHOUDHURY
The purpose of a weld is to join two metals by fusing them at their interfaces.
A metallurgical bond is formed that provides smooth, uninterrupted microstructural transition across the weldment.
The weldment should be free of significant porosity and nonmetallic inclusions, form smoothly flowing surface contours with the section being joined, and be free of significant residual welding stresses.
ASME design code generally does not describe the strength of weld joints, such as a full penetration butt weld, separate to the strength of base metal in the pressure component.
This implies that the weld joint is expected to have equal or greater strength than the base metal.
A large, utility-size boiler may contain more than 50,000 welds. Each weld is a possible defect site .
COMMON WELDING DEFECTS
INADEQUATE JOINT PENETRATION
Porosity refers to the entrapment of gas bubbles in the weld metal resulting either from decreased solubility of a gas as the molten weld metal cools or from chemical reactions that occur within the weld metal.
The distribution of pores within the weldment can be classified as uniformly scattered porosity, cluster porosity, or linear porosity. Porosity near surfaces seems to have a significant effect on mechanical properties of the weld.
Porosity can be limited by using clean, dry materials and by maintaining proper weld current .
Slag inclusions refer to nonmetallic solids trapped in the weld deposit, or between the weld metal and base metal.
These inclusions may be present as isolated particles or as continuous or interrupted bands.
Slag inclusions are not visible unless they emerge at a surface.
Service failures are generally associated with surface-lying slag inclusions or inclusions that are of such size that they significantly reduce cross-sectional area of the wall.
The number and size of slag inclusions can be minimized by maintaining weld metal at low viscosity, preventing rapid solidification, and maintaining sufficiently high weld-metal temperature
The excess penetration (extrusion) refers to disruption of the weld bead beyond the root of the weld.
This disruption can exist as either excess metal or concavity on the backside of the weld.
These extrusions are not desirable for the following reasons:
The excess material can disrupt coolant flow, possibly causing localized corrosion downstream of the defect (water tube) or localized overheating downstream of the defect (steam tube).
If severe, it can cause root-pass cracking. Such cracks may not be revealed by radiographic examination. Concavity can also substantially reduce fatigue life, and excess concavity has been involved in thermal-fatigue failures.
Excess penetration is frequently caused by improper welding techniques, poor joint preparation, and poor joint alignment. Because disruptions of this type are frequently inaccessible for repair, elimination consists largely in preventing them from occurring.
Incomplete fusion refers to lack of complete melting between adjacent portions of a weld joint. It can occur between individual weld beads, between the base metal and weld metal, or at any point in the welding groove.
This is also related to the inadequate joint penetration. Failures resulting from incomplete fusion of internal surfaces of the weld are infrequent, unless it is severe.
Incomplete fusion at surfaces is more critical and can lead to failure by mechanical fatigue, thermal fatigue, and stress-corrosion cracking.
Incomplete fusion can occur because of failure to fuse the base metal or previously deposited weld metal.
This can be eliminated by increasing weld current or reducing weld speed. Incomplete fusion can also be caused by failure to flux nonmetallic materials adhering to metal surfaces.
This can be eliminated by removing foreign material from surfaces to be welded.
Undercut The undercut refers to the creation of a continuous or intermittent groove melted into the base metal at either the surface (toe of the weld) or the root of the weld.
Depending on depth and sharpness, undercutting may cause failure by either mechanical or thermal fatigue.
Undercutting is generally caused by using excessive welding currents for a particular electrode or maintaining too long an arc.
INADEQUATE JOINT PENETRATION
Inadequate joint penetration involves incomplete penetration of the weld through the thickness of the joint.
It usually applies to the initial weld pass or to passes made from one or both sides of the joint. On double-welded joints, the defect may occur within the wall thickness.
Inadequate joint penetration is one of the most serious welding defects.
Failures by mechanical fatigue, thermal fatigue, stress-corrosion cracking, and simple corrosion have been associated with this defect.
Inadequate joint penetration is generally caused by unsatisfactory groove design, too large an electrode, excessive weld travel rate, or insufficient welding current.
Cracks appear as linear openings at the metal surface. Cracking of weld metal can be critical and has led to frequent failures.
Cracking in weldments can occur in several forms:
Hot cracking in weld deposit: Forms immediately upon solidification of the weld metal.
Cold cracking in weld deposit: Forms after the weld has cooled. Such cracks may form days after the welding procedure
Base metal hot cracking: Forms upon solidification of the weld metal.
Base-metal cold cracking: Forms after the weld metal has cooled.
Cracking normally results from poor welding practice, inadequate joint preparation, improper electrodes, inadequate preheat, and an excessive cooling rate.
If cracking occurs in the weld metal, the following steps may prevent recurrence:
Decrease welding travel speed
Preheat area to be welded, especially on thick sections.
Use low-hydrogen electrodes.
Use dry electrodes.
Sequence weld beads to accommodate shrinkage stresses. especially in thick sections.
Improve heat-input control.
Use correct electrodes.
Use dry electrodes.
Welding debris such as weld spatter, shavings, filings, chips from grinding tube ends, and even welding tools have found their way into tubes as a consequence of tube repair.
If this debris is not removed, it can cause partial blockage of coolant flow and result in overheating failures such as stress rupture.
Such failures can occur months after the completion of the repair.
It is important to establish a “work clean” procedure.
IMPACT OF DEFECTS
The flaws/defects in welding have a major impact on the quality of the boiler pressure components as indicated by past boiler tube failure (BTF) records.
Once the flaws/defects are introduced, the system baseline reliability suffers.
Major economic penalties are incurred to fix these problems. Because problems can be spread everywhere, the detection of these types of flaws after installation is difficult.
BTF REDUCTION PROGRAM FOR OLD BOILERS
100% Radiography of new welding joints to avoid welding defects.
Radiography of old welding joints in phase manner in upcoming AOH (1000 joints per AOH ) to detect erection time old welding defects.
ISO 6520 -1 -2007 :welding and allied processes classification of geometric imperfections in metallic materials –part 1;fusion welding.
ISO 5817 -2007:Welding fusion – welded joints in steel ,nickel ,titanium and alloys (beam welding excluded )Quality level for imperfection.
All welding shall be in accordance with ASME Section I and IX as required.
Melt through, lack of fusion undercut, cracks, linear indications, cold lap, irregular weld beads, slag inclusions, excessive weld reinforcement (over 1/16"), porosity, and blow holes are unacceptable.
Arc strikes and shallow gouges are not acceptable.
Preheat and post-weld heat treatment shall be in accordance with ASME Section I.
All tube welds shall be GTAW welded root pass. Weld rings are not permitted .
All weld repairs shall meet the same quality and criteria as original welds.
Welding of non-pressure parts to pressure parts shall be in accordance with ASME Section I and IX.
It should be realized that welds might not be perfect.
In consideration of this fact, all major welding codes (e g ISO 5817 2007) allow for welding defects, but set limitations on the severity of the defect.
An acceptable weld is not one that is defect-free, but one in which existing defects do not prevent satisfactory service.