Susceptibility of composite floor-tube materials to stress corrosion cracking during recovery boiler operations.

Application: Laboratory data indicate that nickel-based Alloy 825 is more resistant to stress corrosion cracking than Type 304L stainless steel under typical boiler operating conditions.

Austenitic stainless steel cladding on composite floor tubes is prone to cracking, leading to higher maintenance costs and increased downtime for recovery boilers. Most analyses suggest that the cracking mechanism is environmentally assisted, although pure thermal fatigue failure can occur.

Study objectives

This study attempts to clarify whether salt mixtures enriched in sulfur, potassium, and chlorine could cause stress corrosion cracking (SCC) of composite floor tubes at normal boiler operating temperatures.

Materials and methods

Test materials were AISI 304L-type stainless steel and nickel Alloy 825. The AISI 304L was chosen because it is commonly used as a composite tube material. The high-nickel material was selected to represent newer composite tubes with greater corrosion resistance, e.g., extruded Sanicro 38 and welded overlay of Incoloy 825.

High-temperature stress corrosion tests were performed in a laboratory furnace using the slow strain rate (SSR) test method, where test specimens are gradually strained to failure. Figure 1 illustrates the test setup, with a loading frame placed inside the furnace.

[FIGURE 1 OMITTED]

Strain tests were conducted in conditions simulating the floor-tube environment in a recovery boiler (300 [degrees] C in synthetic alkaline polysulfide-chloride salt mixtures). The amount of melt was adjusted so that just the lower part of the specimen was covered. The thin portion that failed during the experiment was covered with only a film of molten salt. Reference strain tests were also carried out in air, with no synthetic smelt.

Stress corrosion results

Under air exposure, the Type 304L austenitic stainless steel specimens fractured purely by plastic deformation. The 304L specimens exposed to the salt mixtures fractured without substantial plastic deformation, showing fractographic features typical of transgranular SCC. In contrast, the Alloy 825 specimens fractured by plastic deformation under all test conditions.

Figure 2a shows the fracture-surface morphology of the Alloy 825 specimen exposed to the salt mixture. The figure shows dimples typical of ductile fracture, with only superficial cracking, in contrast, the fracture surface of the 304L specimen in Fig 2b shows fanlike morphological features typical of transgranular SCC cracking of austenitic stainless steel and no evidence of ductile fracture.

[FIGURE 2 OMITTED]

Summary

Austenitic Type 304L stainless steel is susceptible to stress corrosion cracking in the presence of alkali polysulfide-chloride mixtures at 280-300 [degrees] C. In contrast, Alloy 825 showed only superficial cracking under the same test conditions. However, the results suggest that even high-nickel alloys are susceptible to cracking during severe thermal transients.

Given the critical performance requirements for materials in high-pressure boilers, the authors recommend further study using lower strain rates over a wider temperature range.

Makipaa, senior research scientist, Oksa, research scientist, and Pohjanne, senior research scientist, are affiliated with VTT Manufacturing Technology, P.O. Box 1703, FIN-02044 VTT, Espoo, Finland. Address corresspondence to Makipaa by email at [e-mail]