Welding & Surface Preparation
Don't fire welders for surface prep failures.
Surface Prep Sets Up Welding Success or Failure
Developing good, experienced welders takes time. Finding qualified replacements is difficult. The average welding professionals in the U.S. as 56 years of age, supporting the assertion that almost 40% of the 1,076,498 people in the welding workforce in the five key SOC Codes will need to be replaced in 2019. Each of these replacement welders will likely need additional training and job experience to attain equivalent expertise of the retiring welders.
Project managers and contractors understand good welders are in short supply. Customers relate a common frustration: being forced to fire good welders due to unacceptable failure rates when welding techniques could not overcome surface contamination and the problem was more likely caused by the surface cleaning contractor’s issues attaining an acceptable clean in profiles or pores and tiny voids metal where post-prep metal remains contaminated by or collecting residue, grit, residual coatings, etc.
For owners, the bottom line is that a failed repair still requires someone to return to that weld and repair it at great expense. Setting standards for 1 repair in 20 joints (5%) may seem reasonable on paper, but it is a bit more complicated in real life. One welder might perform 20 x 25 nb welds, out of which one 25 mm long repair is needed. Another welder might perform 20 x 400 nb welds, out of which one 25 mm long repair is needed. On paper, both have a 5% failure rate (but the second welder achieved more success in the real world).
Due to the critical nature of the industry, it’s understandable that a number of very large oil and gas players have a typical expectation of 95% first pass yield for welds subject to volumetric testing (UT or RT), without taking into account the contribution of extremely challenging working conditions or surface cleanliness to the failure rate.
Seems unfair to put the onus for surface hygiene on welders when a) blasting and cleaning inspection are not their job and b) welders have no control over blast cleaning contractors’ work.
Sometimes Technique Can’t Overcome Contamination
Welders “sight clean” materials prior to welding because slag (including dirt, debris and oxides) will cause problems. However, spot cleaning to visual standards won’t clean out pores or caught under jagged profile edges.
It cannot be overstated. Proper surface cleaning and preparation is critical to ensuring the creation of high quality welds on oil and gas pipelines and minimize the risk of failure. Workpiece surfaces at and adjacent to the location where the weld should be as close to immaculate as possible to ensure the optimal chance for weld success. It’s a fact that if parts to be welded are not clean when welding occurs, the weld joint will lack uniformity and porosity will form around the debris, leading to weld failure. Excessive gas formed from reactions with unremoved paint or primer coatings caught in miniscule pits and dirty crevices cause another type of failure known as “wormholes”.
Surface cleanliness is even more important when welding austenitic stainless steel, aluminum or its alloys, as they are extremely sensitive to the pick-up of hydrogen (from grease, moisture or debris) which contributes to failures caused by weld porosity and solidification cracking.
Autogenous processes like electron beam, TIG or plasma welding, need to be completely free from surface coatings debris that will out-gas or contaminate the weld pool. The small or deep and narrow weld pools inherent in autogenous processes freeze rapidly and are even more sensitive to contamination than welds that require filler metals.
Surface Prep Must Be Comprehensive Prior to Welding
Surface profile and visual cleanliness do not necessarily equal a clean surface.
Surface preparation is the widest gap and weakest link in the critical path of asset maintenance projects.
Surface prep is arguably the most critical point in the process of surface protection and asset integrity but is the least managed variable.
Good surface cleaning improves weld success rates and turnaround times.
Good welders get fired for unacceptable failure rates when welding techniques cannot overcome surface contamination.
“Surface preparation can be defined as steps and/or procedures to be followed prior to welding aimed at ensuring (in conjunction with all the elements of a qualified welding procedure) sound welds.”
A properly selected cleaning process can reduce defects that are often sites for weld failure and corrosive attack in aggressive environments. Following any surface cleaning method, surfaces should be rinsed and dried (to remove soluble salts and moisture) to prevent hydrogen pickup, porosity and lack of penetration in shallow fusion between weld and base metals. Impurities in the base metal such as sulfur and phosphorous in steel, can cause porosity, so additional passivation using a surface sulfide remover may be a wise option to include prior to handing a surface over to be welded.
Balancing Surface Cleanliness, Downtime and Costs
In addition to methods, machines matter. The cleaning process itself can be a source of trouble when using high pressure or invasive mechanical cleaning equipment (i.e., UHP wet jets, sand blasters, metal brushes or grinders) which tend to embed grit, leave gritty dust or create jagged edges with residual coatings and contamination.
Wet abrasive blast equipment types vary. That’s important to note, because some wet abrasive blasters (slurry blasters and hybrid wet/dry blasters) have a poor reputation for breakdowns, clogging and high maintenance, due to the physical constraints of injecting water into the high-pressure air-abrasive stream (making difficult to control and mix properly). High-pressure abrasive flow is also hard on nozzles and valves. Metering problems and frequent Thompson valve repair are common complaints.
Venturi blaster equipment combine grit media and water inside a blast pot. A high-pressure airstream forced through the pot to push the water/media mixture to the blast hose. Limited control and metering can be problematic with these units. They are also rather fussy in terms of required media/water balance. Add too much water, the abrasive cleaning becomes ineffective. Add too little, and the system clogs.
Wet abrasive vapor blasters also combine water and grit in a hopper or pot, depending on the type of vapor blaster. The amount of water added is always whatever it takes to fill the pot after grit is added (just enough to fill voids between grit particles). Air pressurized water introduced from a separate water tank forces the moistened grit mix out and into the blast hose where it is then carried out to the nozzle by consistent hydraulic pressure. Employing low pressure and water-cushioned grit reduces friction and wear on nozzles.
Cost-effective CleanerBlast machines are self-contained equipment similar to wet abrasive vapor blasters, but with certain engineering differences. CleanerBlast machines are precision designed to eliminate clogging and increase intervals between hopper refills. Intuitive redesign of vapor blast technologies translates to continuous operability, reduced setup time and extended “time on the blast hose”. Like several recent generation vapor blasters, CleanerBlast machines achieve proper level surface cleanliness, with no embedding of grit or contaminants back into the surface.
Metal can be cleaned and salt/sulfide removers applied simultaneously, after which the same machine can be used for rinsing off residual grit and drying surfaces to prepare them for immediate welding or coating. These machines operate reliable and well in high-humidity conditions, allowing better scheduling, another major factor in streamlining field maintenance operations.
Unlike sandblasting, low-pressure vapor blast operations do not gouge metal, stress delicate HAZ areas, or hamper most adjacent activities by creating harmful dust clouds, extensive vibration or spark hazards, making side-by-side task performance more possible to reduce the length of time between cleaning and welding or coating tasks.
Good welders are a hot commodity these days. Most welders are proud of their skills, because they’ve earned them the hard way. Give experienced welders what they need to do a superior job, and they’ll do it. Give new welders a higher chance for success, and they’ll achieve it. Firing welders because surface preparation was not up to snuff (and therefore caused insurmountable weld issues) makes no sense. Cleaner surfaces are a win/win situation for project managers, contractors and owners. Welding contractors retain skilled workers. Project managers can better schedule maintenance (surface prep, welding and coating) with less downtime, owners can be assured critical welds failure at or below 5% failure rate, and surface preparation contractors won’t have to redo their work to ensure welders have what the clean surfaces they need to create strong joins.