Grain boundary engineering technology: Materials to reduce recovery boiler maintenance
By Pulp & Paper Canada
By Pulp & Paper Canada
It’s no secret that recovery boiler maintenance costs can be significant. This is why one newly formed company is responding to the industry need to develop technology that improves equipment life spa…
It’s no secret that recovery boiler maintenance costs can be significant. This is why one newly formed company is responding to the industry need to develop technology that improves equipment life span and reduces the need for frequent repairs.
In March 1999, Babcock & Wilcox Canada (B&W) created a new venture called Integran Technologies Inc. (ITI) with its co-shareholders, Ontario Power Generation and Nanometals, a company affiliated with Queen’s University, to develop and market advanced materials applications using Grain Boundary Engineering (GBE) and nanocrystalline technologies. One of the primary goals of this venture is to provide B&W and ITI’s customers with a better return on the investments they make in equipment and services. GBE, GBEST, Nanoplate, and Electrosleeve are trademarks for ITI’s patented technologies.
A key focus of this new technology is to improve the mechanical reliability of the lower recovery furnace — an area typically requiring frequent repairs. Historically, all boiler designs in Europe and North America have been susceptible to corrosion and cracking of 304L composite tubes. Although extensive studies by Oak Ridge National Laboratories (ORNL), the Pulp and Paper Research Institute of Canada (Paprican), and the Institute of Paper Science and Technology (IPST), have identified thermal fatigue and environmentally-assisted stress corrosion cracking as the potential mechanisms for cracking in 304L composite tubes, the exact mechanism is not fully understood . Nonetheless, because of these studies, it is now recognized that the mismatch of coefficients of thermal expansion between the carbon steel base and the 304L stainless steel cladding coupled with the low-yield strength of the cladding make 304L composite tubing an inferior choice for the lower recovery furnace environment at or below the primary air port elevation.
To avoid the possibility of tube failure, it is often necessary to repair or replace individual tubes or even sometimes replace the complete lower furnace. Not only is this maintenance costly, but it can also affect mill production. As a result, recovery boiler owners routinely inspect floors and air port openings for evidence of possible cracking. Cracks necessitate grinding and/or weld repair. Tube replacement is recommended when cracking penetrates the carbon steel wall.
ORNL studies have indicated that alloy 625 (A625) clad tubing is the material of choice from a mechanical design standpoint due to its high-yield strength and expansion coefficient similar to carbon steel . Welded overlay A625 has demonstrated excellent performance in the recovery boiler environment after more than 12 years’ service in smelt openings and as simulated in laboratory studies.
Metals and alloys used in industrial applications are made of crystals or grains that are groups of atoms arranged in a lattice structure. The crystals intersect each other at grain boundaries. Grain boundaries have unique properties that depend on the quality of fit between adjacent crystals. A better grain boundary fit enhances a material’s ability to resist sliding (creep), corrosion, fracture and solute precipitation. Grain boundaries where crystals fit together well are designated “special” boundaries.
GBE is the methodology by which the local grain boundary structure is characterized and the material processing variables are adjusted to create an optimized grain boundary structure that improves the performance of the material beyond that which would result from conventional processing techniques. Significant improvements in intergranular corrosion cracking resistance, creep resistance and fatigue resistance have been documented.
Typically, conventionally processed materials have a low percentage of “special” grain boundaries. However, variations in mechanical processing techniques and heat treatment can dramatically improve the crystal orientation and increase the frequency of “special” boundaries thereby providing enhanced properties. Advances in automated electron diffraction techniques have made it possible to rapidly assess the grain boundary crystal orientation. Samples are analyzed using a scanning electron microscope equipped with an automated electron backscattered diffraction system (orientation imaging microscopy, Fig. 1).
B&W with ITI have developed a process that shows promise to further optimize the performance of weld overlay alloy 625 tubes by implementing Grain Boundary Engineering Surface Treatment (GBEST). This specialized surface treatment results in a fine, closely aligned grain structure at the near surface of the tube. This structure, Fig. 2, is expected to enhance resistance to intergranular attack as well as enhancing the mechanical properties of A625 weld overlay.
Fatigue testing results, Fig. 3, demonstrate an improvement of at least 45% in the cycles to failure. Corrosion testing has also shown reduction (15%) in material loss under ASTM G28 test conditions. Also, the localized depth of attack was reduced significantly, Fig. 4. A significant benefit is that A625 in the GBEST condition is not susceptible to sensitization during heat treatments, which can dramatically affect normal 625 alloys. Testing regarding weldability and formability of this material has also demonstrated superior properties.
The first application of GBEST has been for primary air ports, in locations where recovery furnace wall cracking has been found. In addition to this new material, particular attention has been paid to membrane weld termination at the opening and the design of the air port casting that directs the airflow toward the char bed and protects the opening from rodding damage during cleaning.
Primary air ports formed out of GBEST material were installed in two recovery boilers in the fall of 2000. These openings consist of SA210 A1 2 3/8-in. diameter 0.200-in. minimum wall tubing weld overlaid with 0.070-in. of A625 and processed according to GBEST procedures. Inspection of one installation after six months’ service reveals no evidence of cracking, no cold side (caustic) corrosion and only some very localized hot side corrosion adjacent to the airport casting. B&W will be carefully monitoring the GBEST performance during the material development program as well as studying boiler operational variables, which may influence material performance.
The recovery boiler is a particularly harsh environment for tube material. Field tests have shown that local combustion processes can significantly affect tube surface operating temperature. Modification of the combustion and reduction processes to reduce tube surface temperature will also have an important effect on furnace life. Corrosion processes and thermally induced stresses can be significantly affected by the way that air and black liquor are introduced into the boiler. Field studies, along with numerical modelling of furnace processes, are used to better understand the environment to optimize future design and operation.
This focus on new materials and processes is combined with research into better understanding recovery boiler processes. When the knowledge from field testing is employed in combination with numerical modelling to optimize boiler processes, and this operating philosophy is combined with advanced materials, improvement in recovery furnace life is expected. Implementation of these technologies can result in long-term maintenance savings for boiler owners while providing the opportunity for higher production through improved availability. P&PC
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The authors thank Ron Ojanpera and Bryan Stone for their skillful assistance and advice in developing this article.
Keith C. Rivers and C. Malcolm MacKenzie are with Babcock & Wilcox, Cambridge, ON.