| Date | January/February 2004 |
| Volume | 10 |
| Issue | 1 |
| Return to Journal Index | |
January/February 2004 Inspectioneering Journal Article Index
-
January/February 2004 Inspectioneering Journal
DMW cracking is another fabrication issue that can and does result in equipment failure. It usually occurs at the weld juncture where carbon steel or low alloy steels are welded to austenitic (300 series) stainless steels in high temperature applications. The large difference in coefficient of expansion of the two steels, sometimes exacerbated by thermal cycling, results in cracking at the toe (HAZ) of the weld joining the two materials. Using an austenitic stainless filler material for the DMW junction also increases the stress intensification on the toe of the weld on the ferritic side of the weldment. This type of cracking is most common when temperatures above 800F (425C) are involved, such as in FCCU reactor/regenerations systems, superheaters, reheaters, fired heaters, and hydroprocess equipment. Use of bolted joints, if possible, or nickel base filler materials helps to avoid the DMW cracking problem.
-
January/February 2004 Inspectioneering Journal
When we specify that some equipment (vessels, flanges, fittings, etc.) be overlaid with a corrosion resistant alloy, we need to pay attention to making sure that the chemistry of the top layer of alloy welding, that will be exposed directly to the process fluid, is sufficient to resist degradation from the process environment. This may sound logical, but I’m aware of several cases where weld overlaid surfaces are ground or machined to meet specification tolerances, and in so doing the most resistant part of the alloy overlay is removed.
-
January/February 2004 Inspectioneering JournalBy John Reynolds at Intertek
Inadequate PWHT is one of our pressure equipment nemeses. We normally specify PWHT for a variety of pressure equipment integrity reasons including when we need to lower residual stresses, increase resistance to cracking or soften weld hardness. All for the purpose of prolonging the service life of our equipment and preventing unexpected failures. But this issue is clearly where those old sayings of "Buyer beware" and "You don't get what you expect, you get what you inspect" apply quite often.
-
January/February 2004 Inspectioneering JournalBy John Reynolds at Intertek
Repair welds can be another undetected and insidious "fabrication defect" that eventually results in equipment failure. Any experienced metallurgist that has completed numerous failure analyses over the years will tell you that periodically they see failures that initiated at the site of a repair weld. Sometimes those repair welds are field repairs, but not infrequently they occurred during original fabrication and were unknown to the purchaser. Typically our standards and specifications do not cover repairs completed by the fabricator, so they believe they are free to do whatever they want to repair a manufacturing or fabrication defect, then grind it flush, finish the fabrication and ship the product.
-
January/February 2004 Inspectioneering JournalBy John Reynolds at Intertek
Speaking of stress raisers, they are another insidious type of flaw that can and do lead to equipment failures. Stress raisers (aka stress intensification sites) can be mechanical or metallurgical notches. Undercutting, physical weld flaws, mismatched thicknesses, and sharp geometric intersections can all become stress raisers. But so too can so-called "metallurgical notches" like one finds at the edge of a weld where the cast structure of the weld pool meets the wrought structure of the heat-affected zone. Equipment in many static services without significant cyclic stresses can tolerate the presence of some stress raisers because of hefty code design margins. But equipment in services with significant fatigue stresses (thermal and mechanical) are especially vulnerable to such notches and deserve special QA/QC attention during design, specification, fabrication, and repairs.
-
January/February 2004 Inspectioneering JournalBy John Reynolds at Intertek
Cracks along the toe of a weld are not uncommon during fabrication, and can occur for a wide variety of reasons involving the metallurgy and process control of the the same issues covered above on repair welds can apply to repair welds on castings; especially if you are unaware that the foundry or fabricator is trying to salvage a defective casting by covering up porosity and shrinkage cracking with a big glob of weld metal.
-
January/February 2004 Inspectioneering JournalBy Dan Quinn at In TANK Services, Inc.
Storage tank owners can reap large financial benefits by shifting from a reactive tank repair strategy to a proactive inspection and maintenance program. In reaction to well publicized tank failures in the 80's and the development of API 653 guidelines in 1991, many tank owners started to proactively inspect their tanks and perform significant tank repairs. However, after a lengthy period of focus on inspections and repairs in the 90's, many tank owners have become complacent with respect to tank maintenance.
