Thursday, March 3, 2016

Welding with Integrity Part Two

by Doug Berger

Two weeks ago, we discussed the importance of weld integrity. This week, we want to spend some time on the weld inspection process. The previous blog can found here.

The goal of having a WPS and trained welders is to achieve a weld which performs to the level the structural design requires.  Non-destructive examination of a weld is a way of determining if a weld will comply with the structural design requirements set out in the WPS without physically breaking the weld to test its strength.  Non-destructive examination may consist simply of a visual inspection of the completed weld, or procedures like radiographic or ultrasonic examination which require specialized equipment.  The focus of this article is on a visual inspection of a weld and some of the key points and inspector will look for.

  • Physical Geometry – size, concavity/convexity, reinforcement.  AWS D1.1 has specific tolerances for these characteristics (the designer of record may specify tolerances for these)
  • Porosity – cavity formed in the weld by gas entrapment during solidification.  Porosity is usually spherical but it may be elongated
  • Inclusions – entrapped foreign solid material such as slag, flux, tungsten or oxide
  • Incomplete Fusion – discontinuity where fusion did not occur between weld metal and the base metals
  • Spatter – droplets of melted metal generated near/at the welding arc
  • Arc Strikes – an arc is outside the weld area and leaves a hard node that could become the start point for a crack of fracture
  • Cracks – a fracture in the weld due to a defect, tensile stress, material with low fracture toughness or temperature changes
  • Incomplete Penetration – when a weld bead that doesn’t start at the root of the groove and then forms channels and crevices in the root of the weld.
Underfill – Condition where the weld face or root surface of a groove weld extends below the adjacent surface of the base metal.

Undercut – Condition where the base metal adjacent to the weld toe or root has been melted and left unfilled by weld metal.

Overlap – Protrusion of unfused weld metal beyond the weld toe or weld root.

Visual inspection is the most cost effective way to verify the weld’s compliance, however, an inspection must be performed at three separate stages: prior to any welding, during welding and after the weld has been completed. The first examination is to check the material specifications and joint fit-up. During the welding process, the inspector checks the equipment settings and welding processes. Finally, after the weld is completed the inspector checks for a variety of surface flaws as specified above. 

A certified inspector has a broad knowledge of many technologies, including welding, destructive and nondestructive testing, metallurgy, and the terminology for each. This in-depth knowledge is why Visual Inspection is so effective in the discovery of a majority of defects allowing them to be repaired before a more expensive incident reveals the problem.

An NTA Certified Weld Inspector can give you the thorough inspection you need to assure yourself of the safety and quality of your weld, and to avoid extra costs down the road. Contact NTA to learn more. 

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.