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 Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals or tough thermoplastics subjected to a constant tensile stress in a corrosive environment, especially at elevated temperature (in the case of metals). than Type 304L stainless steel stainless steel: see steel.
Any of a family of alloy steels usually containing 10–30% chromium. The presence of chromium, together with low carbon content, gives remarkable resistance to corrosion and heat. under typical boiler operating conditions.
A nonmagnetic solid solution of ferric carbide or carbon in iron, used in making corrosion-resistant steel.
[After Sir William Chandler Roberts-Austen (1843-1902), British metallurgist. 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.
This study attempts to clarify whether salt mixtures enriched in sulfur, potassium, and chlorine could cause stress corrosion cracking (SCC SCC - strongly connected component ) of composite floor tubes at normal boiler operating temperatures.
Materials and methods
Test materials were AISI AISI American Iron and Steel Institute
AISI African Information Society Initiative
AISI Alberta Initiative for School Improvement (Canada)
AISI As I See It
AISI American International Supply, Inc (Oakland, CA) 304L-type stainless steel and nickel Alloy Noun 1. nickel alloy - an alloy whose main constituent is nickel
alloy, metal - a mixture containing two or more metallic elements or metallic and nonmetallic elements usually fused together or dissolving into each other when molten; "brass 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 (Scalable Sampling Rate) See AAC.
SSR - Scalable Sampling Rate ) 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 Molten salt may refer to:
Stress corrosion results
Under air exposure, the Type 304L austenitic stainless steel specimens fractured purely by plastic deformation plastic deformation,
n any irreversible deformation of tissues. . 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]
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 VTT Technical Research Centre of Finland
VTT Valtion Teknillinen Tutkimuskeskus (Finnish: Technical Research Centre of Finland)
VTT Vélo Tout Terrain (French: mountain bike; aka ATB or MTB) Manufacturing Technology, P.O. Box 1703, FIN-02044 VTT, Espoo, Finland. Address corresspondence to Makipaa by email at firstname.lastname@example.org