Welding Inspection and Quality Control: Ensuring Excellence in Mechanical Contracting Welds
Whether you are working on HVAC, distribution piping, pressure vessels, chillers, cooling towers, or any other mechanical contracting job, welding inspection and quality control are vital processes for safety and end-product quality.
Why Welding Inspection And Quality Control Is Crucial For Mechanical Contracting
Mechanical contractors are tasked with assembling and welding equipment in various industries that can cause safety hazards and capital loss if the welds were to fail. This is especially the case with structural components, pressure vessels, power boilers, and piping systems.
Welders often have to be certified and follow the ASME (American Society of Mechanical Engineers), AWS (American Welding Society), and API (American Petroleum Institute) codes and standards to ensure weld quality in mechanical contracting. These codes provide guidelines for best practices, travel speed, pre-heat requirements, filler metals, and other vital information.
Welding inspection ensures quality control every step of the way, from pre-welding preparations to post-welding quality control. The primary goals of weld inspection and quality control are preventing welding defects by taking corrective actions early on in the production stage, and ensuring the quality of the end product. For example, a timely inspection could prevent a whole batch of bad welds resulting from poor joint cleanliness or fit-up. Likewise, post-welding X-ray testing could save lives and costly equipment if the weld testing on pressure vessels shows weld defects.
Weld Discontinuities and Defects
A weld discontinuity is not necessarily a defect, while a weld defect is a discontinuity whose nature renders the weldment unable to meet acceptance criteria and must be rejected.
Weld discontinuities are an interruption in the weld structure that range from slag and tungsten inclusions to porosity, cracks, undercuts, and incomplete fusion. Depending on the particular mechanical contracting job and the severity of the discontinuity, some weld discontinuities are acceptable, while others are considered defects.
Pre-Welding Quality Control
Ensuring proper pre-welding quality control can save a significant amount of time and resources by taking corrective action before the weld defects occur. Many mechanical contracting jobs have to follow specific pre-welding inspections. Below are some common pre-welding quality controls that are necessary for most welding jobs:
- Verifying welder qualifications for welding specific weld types, materials, and positions.
- Reviewing welding procedure specification (WPS) to check if data is consistent and error-free. Sometimes, it may be necessary to test the WPS qualification by conducting tests on weld coupons (test pieces).
- Verifying the base metal and welding consumables compliance with the applicable specifications.
- Inspecting for joint cleanliness and proper fit-up.
- Ensuring that the joint design and weld preparation is compliant with the WPS and applicable welding standards (ASME, AWS, API).
- Checking the clearance dimensions of rings, consumable inserts, or backing strips.
Sometimes, pre-welding inspection may find irregularities that are within code limitations but could pose an issue at a later stage in production. That’s why it’s vital for the welding inspector to be familiar with the entire production process and to be experienced with the welded equipment.
Quality Control During Welding
Inspection during the welding process is essential for spotting the weld defects and preventing an incorrect welding approach from causing massive weld rejections across the entire project. For example, not noticing that WPS-defined interpass temperatures were breached on all process piping welds could lead to rejection and rework across the entire pipeline. Another example would be not inspecting for excessive distortion, which would result in weld rejection and costly rework if done on expensive equipment like heat exchangers, distillation columns, and boilers.
Some of the standard quality control checks during welding are:
- Verification of pre-heat and interpass temperatures and ensuring that they are within limits specified in the WPS.
- Confirm that the filler material is as specified in the WPS.
- Confirm the shielding gas and flux are as instructed in the WPS.
- Reviewing sequencing procedures for distortion control.
- Tack weld quality inspection.
- Verifying a proper fit-up before welding.
- Inspecting the root pass and successive weld passes.
- Checking the root condition before welding the other side.
- Ensuring that slag, silica, and other contaminants are removed between passes.
- Visual examination of the finished weld.
- Each weld pass should be visually scrutinized for cleanliness, bead profile, inclusions, undercut, porosity, poor fusion, uneven penetration, and other defects before the next pass is welded.
Quality Control After Welding
A post-welding inspection determines if the weld is rejected or accepted. This is the most important quality control step, which should give positive results if the first two steps are adequately performed. However, pre and during-welding checks mostly rely on visual inspection. So, sometimes, even if everything is done correctly, weld defects that are challenging or impossible to spot without specialized equipment can occur.
However, the post-welding QC depends on the compliance standards, the agreement with the customer, and the class of weld. So, less critical welds can pass this QC step with visual inspection alone.
Non-Destructive Testing (NDT)
The most widely applied form of NDT is visual inspection. While visual inspection can’t detect discontinuities and defects beneath the weld surface, an experienced weld inspector can tell much about the weld quality just from the visual inspection.
However, critical welds must pass NDT inspection that provides the subsurface quality inspection using the testing methods specified by the applicable welding codes and standards.
Some of the most often applied NDT methods are:
- Radiographic examination – While radiation is a safety hazard, x-ray imaging can detect most weld discontinuities and is one of the most widely adopted weld testing methods.
- Magnetic particle testing – Only applicable to ferromagnetic materials and mostly used for crack detection. It’s easy to learn and quick and inexpensive to apply.
- Liquid penetrants – Detects discontinuities open to the surface, like porosity and cracks. Results are quick and easy to read, but weld metal must be clean from surface contaminants before testing.
- Ultrasonic examination – Detects most weld discontinuities, including the planar type discontinuities. Equipment can be battery-powered, and results are immediately obtained. However, the NDT method requires a highly skilled operator and is challenging to perform on thin materials and small welds.
- Electromagnetic testing – Only works with electrically conductive metals. It’s a fast and cost-efficient method for surface discontinuities and some below-surface inclusions.
- Eddy current examination – A special type of electromagnetic testing often used for NDT inspection of piping and tubing systems.
- Proof testing – NDT method where the welded system, equipment, or a single part is tested with one or more tests exceeding actual operating conditions.
- Hydrostatic testing – A form of proof testing for closed containers, like pressure vessels. The container is usually filled with water and pressurized according to the ASME or other applicable codes for hydrostatic testing.
Mechanical contracting jobs sometimes require welding and NDT in confined spaces, like inside boilers, pressure vessels, tanks, service tunnels, and pump rooms. Accessing these spaces often requires certification. Unless certified, inspectors and other personnel should never enter confined spaces, regardless of their work experience. Working in confined spaces must be done according to the regulations provided by the Occupational Safety and Health Administration (OSHA).
Destructive Testing (DT)
While DT is often used for WPS and welder qualification purposes, it’s also sometimes applied to a randomized selection of parts welded for the job.
The most commonly used NDT methods are:
- Guided bend test – The sample weld is bent to a specified radius using a specialized jig with a hydraulic plunger. This test determines the weld metal quality at the root and face of the joint and evaluates the weld ductility and soundness.
- Fillet weld break test – The fillet weld is made on one side of the joint only, while the load is applied to the unwelded side using a press. Once the weld fails, the sample is inspected for weld discontinuities.
- Tension test – The test specimen is pulled apart using a hydraulic machine to determine the maximum tensile load of the weld.
- Macro testing (etching) – Sample welds are removed from the welded joint. Next, they are polished at cross-section and etched with an acid mixture. The acid etch allows a clear visual appearance of the weld’s entire structure, exposing all discontinuities like inadequate penetration, lack of fusion, porosity, and cracking.
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