Failure Analysis of Furnace Tubes

This project highlights two furnaces, nominally identical in design and operation, which were built to heat air for drying wood products. Bark from logs used for the wood product was burned in the furnaces. Both furnaces were fed from a common source of the bark chips, and were designed for 1650⁰F maximum tube wall temperature. One furnace, however, was clearly hotter and had much lower tube life for the first bank of type 310 stainless steel tubes facing the combustion chamber.

Metallurgical Consulting performed an inspection of the furnace while it was down to determine conditions and to select tubes for evaluation. A hot spot was apparent on the front bank of tubes facing the combustion chamber (Fig. 1). A tube above the hot spot and a tube from the most distorted area, which had been in service for 6 months, were removed for analysis, as well as fly ash samples. Metallographic evaluation of the tubes revealed ferrite and sigma phases in the microstructure, which is normal for 1650⁰F design. However, ferrite and sigma were absent at the OD surface in the hottest area. This is evidence for higher temperatures. This is shown in Figures 2 and 3. X-ray analysis revealed high levels of sulfur in the fly ash. Figures 4A, 4B, and 4C show sulfur penetration into the tubes and chromium depletion along grain boundaries. The evidence for 1) sigma phase absence at the OD surface, 2) abnormal grain growth at the surface, and 3) rapid tube wastage provided evidence for outer tube wall temperatures well in excess of 1900⁰F.

Severe oxidation and sulfidation attack at these excessively high temperatures was wasting the tubes. Time spent exploring the bark feed to the two furnaces revealed probable reasons for the different conditions in the two furnaces. A longer distance for moving the bark to the hotter furnace allowed more time to break the bark down to a smaller size and possibly put more dirt (source of sulfur) on the bark and into the furnace. Recommendations were made to correct the feed system, and to remove the source of the sulfur. Modifications to reduce furnace temperatures and fly ash accumulation made by another consultant were supported by these findings.

Images From The Metallurgical Services Provided For This Furnace Tube Failure Analysis

(click on thumbnail to enlarge)

Figure 1: Tubes in the first bank were distorted after 6 months of service.

Figure 2: A cross section of one tube shows heavy attack. The dark phase to the right is ferrite.

Figure 3: Bright grain boundary platelets of sigma phase (white symbol) can be seen on the left side. The phase is largely absent closer to the tube surface. Black arrows point to annealing twins. (Crossed polarizers and sensitive tint)

Figure 4A: An SEM micrograph of the OD wall of a tube with damage.

Figure 4B: An X-ray map of sulfur concentration in figure 4A is shown.

Figure 4C: Chromium X-ray mapping from Figure 4A shows chromium leaving the tube and concentrating in the scale.

Figure 5A: Ferrite and sigma phases as seen by SEM.

Figure 5B: Both phases are rich in chromium as seen by the chrome X-ray map.