Forensic Investigation of Premature Weld Cracking in Stainless Steel Exhaust Ductwork

A Forensic Investigation was Conducted to Evaluate the Quality of Construction, Welds, and Design of Ductwork that had Failed Prematurely.

Weld cracking occurred in corner weld joints and stitch welds used to secure the duct to stiffening angles, approximately 3 weeks after the ductwork was placed in service. Finite element analysis (FEA) was performed to investigate.

System Overview

The exhaust ductwork conveys high-temperature exhaust (approximately 1100°F) from large diesel engines through the roof during engine testing and qualification. The exhaust path includes:

• Vertical entry into the ductwork
• 90-degree turn at the lower elbow
• Horizontal travel for approximately 10 feet
• Second (upper) elbow directing exhaust upward
• Two silencers
• Exit to the atmosphere

The region of weld cracking extends from the exhaust entrance through both elbows up to the first expansion joint.

Weld Inspection Program

Visually examined welded joints included:

• Corner 1/8″ fillet welds connecting stainless steel plates forming the ductwork shell
• 1/8″ fillet welds connecting stiffening angles to the ductwork shell
• The inspection determined that welds appeared thoroughly fused to both sides of the joints, with acceptable weld profiles per 09.1 Section 6, Inspection of Arc Welding Work

Finite Element Analysis

To quantify thermal stresses in the exhaust ductwork, a partial finite element model was built representing the section where temperatures were recorded. The ductwork shell and support ribs are made from 316L stainless steel, with mechanical properties taken from the ASME Boiler and Pressure Vessel Code.

Key Findings

1. 1. High thermally induced stresses at stitch weld locations, nearing 100,000 psi
   2. Corner seam weld stresses approaching 100,000 psi, especially near support ribs
   3. Stress values exceeding twice the material’s yield strength, causing plastic strain concentrations and cyclic plastic straining

Cause of Failure

The investigation determined that the primary cause of failure was the change of ductwork material from carbon steel to stainless steel. This change resulted in:

1. 1. 1. Higher thermal stresses: 316L stainless steel has a thermal expansion coefficient approximately 30% higher than carbon steel, leading to 30% higher thermal stresses under the same conditions.
      2. Greater temperature gradients: Carbon steel has a thermal conductivity approximately 2.5 times greater than stainless steel at temperatures up to 600°F, resulting in higher temperature gradients and thermal stresses in the stainless steel material.

These factors contributed to stress states exceeding material yield strength, causing crack initiation and propagation from the inside corner of the fillet welds.

O’Donnell Consulting Engineers Investigates Various Weld Issues – including Premature Cracking, Distortions, Lack of Fusion and Complete Weld Failure.