Composite tube cracking in kraft recovery boilers: A state-of-the-art review

Beginning in the mid-1960s, increasing energy costs in Finland and Sweden made energy recovery more critical to the cost-effective operation of a kraft pulp mill. Boiler designers responded to this need by raising the steam operating pressure, but almost immediately the wall tubes in these new boilers began to corrode rapidly. Test panels installed in the walls of the most severely corroding boiler identified austenitic stainless steel as sufficiently resistant to the new corrosive conditions, and discussions with Sandvik AB, a Swedish tube manufacturer, led to the suggestion that coextruded tubes be used for water wall service in kraft recovery boilers. Replacement of carbon steel by coextruded tubes has solved most of the corrosion problems experienced by carbon steel wall tubes, however, these tubes have not been problem-free. Beginning in early 1995, a multidisciplinary research program funded by the US Department of Energy was established to investigate the cause of cracking in coextruded tubes and to develop improved materials for use in water walls and floors of kraft recovery boilers. One portion of that program, a state-of-the-art review of public- and private-domain documents related to coextruded tube cracking in kraft recovery boilers is reported here. Sources of information that were consulted for this review include the following: tube manufacturers, boiler manufacturers, public-domain literature, companies operating kraft recovery boilers, consultants and failure analysis laboratories, and failure analyses conducted specifically for this project. Much of the information contained in this report involves cracking problems experienced in recovery boiler floors and those aspects of spout and air-port-opening cracking not readily attributable to thermal fatigue. 61 refs

Fatigue cracking of coextruded 304L/CS tubes

The mechanical and thermal fatigue of authentic stainless steels was examined for the maximum temperature range expected in coextruded floor tubes of recovery boilers to determine the likelihood that the cracking in the 304L stainless steel cladding could be fatigue related. The microstructures and cracking patterns of fatigue-tested specimens were compared to features observed in cracked cladding and significant differences were found which suggested that fatigue was not the most likely cause for failure. Biaxial thermal fatigue testing of coextruded tubes and panels was performed to gather more evidence of cracking patterns. Here, transient thermal stresses were imposed by rapidly heating the tubing surface with lamps. In spite of high surface temperatures, no cracks were produced in the 304L stainless steel cladding, and this observation was interpreted as evidence that cracking must be corrosion related

Investigations using smooth and notched specimens into validity of caustic cracking susceptibility diagram

Susceptibility to caustic cracking at different temperatures and caustic concentrations, as predicated by the caustic cracking susceptibility diagram, has been examined by stress corrosion cracking tests using smooth specimens (by slow strain rate testing) and notched specimens (by cylindrical notch tensile testing). Intergranular fracture, as established by scanning electron microscopy, was taken as the confirmatory evidence of caustic cracking. The results generated using notched specimens largely have been consistent with the prediction of the susceptibility diagram

Circumventing Practical Difficulties in Determination of Threshold Stress Intensity for Stress Corrosion Cracking of Narrow Regions of Welded Structures

Determination of the threshold stress intensity for stress corrosion cracking (KIscc) of narrow areas such as weld and heat-affected zone (HAZ) of a weldment is a nontrivial task because of the requirements of large specimens in testing by the traditional techniques and the difficulty of restricting crack propagation to narrow regions in such specimens. This article describes a successful application of the circumferential notch tensile (CNT) technique to determine the KIscc of narrow regions of the weld and HAZ. Also, the microstructure of the HAZ of the manual metal arc-welded steel was simulated over a relatively small length of specimens and its KIscc in a hot caustic solution was determined successfully. Intergranular stress corrosion cracking was confirmed with a scanning electron microscope

Analysis of composite tube cracking in recovery boiler floors

Cracking of co-extruded (generally identified as composite) floor tubes in kraft black liquor recovery boilers was first observed in Scandinavia, but this problem has now been found in many North American boilers. In most cases, cracking in the outer 304L stainless steel has not progressed into the carbon steel, but the potential for such crack propagation is a cause of concern. A multidimensional study has been initiated to characterize the cracking seen in composite floor tubes, to measure the residual stresses resulting from composite tube fabrication, and to predict the stresses in tubes under operating conditions. The characterization studies include review of available reports and documents on composite tube cracking, metallographic examination of a substantial number of cracked tubes, and evaluation of the dislocation structure in cracked tubes. Neutron and X-ray diffraction are being used to determine the residual stresses in composite tubes from two major manufacturers, and finite element analysis is being used to predict the stresses in the tubes during normal operation and under conditions where thermal fluctuations occur

Status Report on Studies of Recovery Boiler Composite Floor Tube Cracking

Cracking of the stainless steel layer of co-extruded 304L stainless steel/SA210 Gd A 1 carbon steel black liquor recovery boiler floor tubes has been identified as one of the most serious material problems in the pulp and paper industry. A DOE-funded study was initiated in 1995 with the goal of determining the cause of and possible solutions to this cracking problem. These studies have characterized tube cracking as well as the chemical and thermal environment and stress state of floor tubes. Investigations of possible cracking mechanisms indicate that stress corrosion cracking rather than thermal fatigue is a more likely cause of crack initiation. The cracking mechanism appears to require the presence of hydrated sodium sulfide and is most likely active during shut-downs and/or start-ups. Based on these results and operating experience, certain alloys appear to be more resistant than others to cracking in the floor environment, and certain operating practices appear to significantly lessen the likelihood of cracking. This report is the latest in a series of progress reports presented on this project