Fitness for Service Analysis for a Cast Iron Dryer Head
A paper manufacturer contracted Metallurgical Consulting to perform a fitness for service analysis on several very old cast iron dryer rolls. These dryer rolls were manufactured for one of their paper machines in 1915. This analysis would be their first modern inspection.
A wet fluorescent magnetic particle inspection (WFMP) showed indications running 360⁰ around two of the bolted heads; which resulted in their removal from service. Others heads exhibited smaller indications and evidence of active crack growth which was discovered with acoustical emission testing (Figure 1). The customer needed to know if the indications would allow the rolls to remain in service.
To provide this information, replicas were made of the indications by polishing and etching the heads in place and using dental replicating material. One of the rejected heads was also made available for destructive inspection. Metallurgical evaluation of the replicas and actual cross sections revealed that the indications were probably entrapped bifilms or oxide/cold shut type imperfections from the original casting process. No metallurgical evidence for active crack growth was observed, and graphite flake separation from the matrix was observed only in some cases, which is evidence for low stresses in general. Stress analysis and fracture mechanics evaluation of the indications also revealed stresses to be low and stress intensities at fatigue crack growth threshold levels. Removing the original old dryer head bolts with active AE signals removed the evidence for active cracking.
Metallurgical evaluation concluded that the indications were casting imperfections and that they had been present for the life of the dryer.
It was recommended that the rolls were fit for continued service with the warning that excessive condensate levels, paper breaks, and sudden temperature changes could cause crack growth and or rupture in these or new cast iron dryer rolls. Periodic inspections were recommended. For more information on this project or the process involved in the testing, contact Metallurgical Consultants directly.
Images From The Metallurgical Services Provided For These Cast Iron Dryer Rolls
(click on thumbnail to enlarge)
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