Kwok Laser Surface Modification of Alloys for Corrosion and Erosion Resistance

1. Auflage 2012
ISBN: 978-0-85709-583-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 392 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

Corrosion and erosion processes often occur synergistically to cause serious damage to metal alloys. Laser surface modification techniques such as laser surface melting or alloying are being increasingly used to treat surfaces to prevent corrosion or repair corroded or damaged components. Laser surface modification of alloys for corrosion and erosion resistance reviews the wealth of recent research on these important techniques and their applications.After an introductory overview, part one reviews the use of laser surface melting and other techniques to improve the corrosion resistance of stainless and other steels as well as nickel-titanium and a range of other alloys. Part two covers the use of laser surface modification to prevent different types of erosion, including liquid impingement, slurry (solid particle) and electrical erosion as well as laser remanufacturing of damaged components.With its distinguished editor and international team of contributors, Laser surface modification of alloys for corrosion and erosion resistance is a standard reference for all those concerned with preventing corrosion and erosion damage in metallic components in sectors as diverse as energy production and electrical engineering. - Reviews recent research on the use of laser surface modification techniques, including the prevention of corrosion and repair of corroded or damaged components - Discusses the techniques for improving the corrosion resistance of steels, nickel-titanium and a range of alloys - Analyses the use of laser surface modification to prevent different types of erosion, including liquid impingement and laser remanufacturing of damaged components

Autoren/Hrsg.

Kwok, C T

Leseproben

2 Laser surface melting (LSM) to repair stress corrosion cracking (SCC) in weld metal
K. Shinozaki,     Hiroshima University, Japan T. Tokairin,     Babcock-Hitachi K.K., Japan Abstract:
This chapter discusses repairing stress corrosion cracking (SCC) in weld overlaid inconel 182 weld metal at nuclear power plants, using laser surface melting (LSM). The effects of microstructure, chemical composition and residual stress on the corrosion resistance of the weld metal undergoing LSM treatment are described, and theoretical models not only of Cr depletion zone at grain boundaries but also of residual stress analysis are discussed to identify why LSM treatment of welded metals improves corrosion resistance. Key words corrosion resistance laser surface melting Ni-base superalloys precipitation residual stress 2.1 Introduction
Intergranular cracking (IGC) and intergranular stress corrosion cracking (IGSCC) are common types of damage in nuclear power plants.1–6 The cladding materials used in some parts of nuclear power plants built several decades ago are usually nickel-based superalloys such as Inconel 182, and they are now suffering from IGC or IGSCC after long-term operation. For the safe working of these plants, it is important to find an efficient method to repair these components to prolong their life and reduce the costs associated with component replacement after long-term use. A practical repair method is urgently needed. The tungsten inert gas (TIG) welding process is one candidate method for this repair procedure. The alloy near the position where IGC or IGSCC has occurred is removed and then preparatory welding is carried out by TIG welding, as shown in Fig. 2.1(a). However, the length of time needed for this repair procedure, together with the high heat input, mean that the TIG welding technique has many disadvantages for this application. 2.1 Schematic illustration of repair method by TIG welding and LSM process. In recent years, laser beam processing has brought many technological and economic advantages, including its high precision, reliability, efficiency and productivity, and consequently it has attracted much attention as a new method. Laser surface melting (LSM) treatment is considered to be one of the most powerful surface modification techniques available to improve the surface properties of materials in cases of corrosion or wear, for example, as a result of homogenization and refinement of microstructures. LSM treatment can be applied to repair tubes degraded by IGC/IGSCC during normal operation of nuclear power plants, as shown in Fig. 2.1(b), since a laser beam can easily be directed to the failed parts through a beam transmission system such as an optical fiber. Moreover, much research on improvement of stress corrosion cracking (SCC) resistance by LSM treatment has been reported3–11 and results indicate that the IGC/IGSCC resistance of the base alloy Inconel 600 can be improved by LSM treatment; some research also refers to repairing procedures using LSM treatment, especially in the case of microstructure evolution and the relationship between the microstructure and the IGC/IGSCC susceptibility of Inconel 182 during the repairing procedure itself. However, it is obvious that the composition of an alloy has a large influence on its corrosion resistance. Alloys with different chemical compositions can have different corrosion behaviors. Commonly, the function of niobium in the nickel-based superalloy is used to stabilize carbon by formation of NbC, so that the formation of Cr carbides is avoided and the susceptibility of the material to IGC/IGSCC is lowered. For Inconel 600, the formation of NbC seemed to correlate with increased protection from IGC, where the suitable addition of Nb/C has a positive influence.12,13 For Inconel 182, the addition of Nb gives better IGC/IGSCC resistance compared with Inconel 600 with the same thermal treatment.13 However, the effect of the addition of Nb on the microstructure and IGC/IGSCC susceptibility of Inconel 182 after the LSM process has not yet been reported. In addition to the effects of service environment and weld structure, the presence of tensile residual stresses in the weld zone has been identified as a significant factor leading to the occurrence of IGSCC, since IGSCC mainly occurs in regions where the residual stresses due to manufacturing are highest; however, only a few papers about the effect of the residual stress of LSM treatment on SCC susceptibility are available.7 in this chapter, application of the LSM treatment method as a repair procedure for nickel-base superalloys (inconels 182 and 600) will be discussed. The factors which affect SCC susceptibility are studied from both a metallurgical and a mechanical viewpoint. Moreover, the effect of microstructure and residual stress on the SCC susceptibility of nickel-base superalloy are discussed separately. 2.2 Materials and experimental procedures
2.2.1 Materials used
Two kinds of inconel 182, containing 1.92% Nb (1.92% Nb inconel 182) and 1.10% nb (1.10% nb inconel 182), were clad onto a inconel 600 base plate by shielded metal arc welding (SMAW). The chemical composition of the alloys is shown in Table 2.1. Table 2.1 Chemical compositions of materials used (mass %) 2.2.2 Specimen preparation
The thermal cycle flow of the repair procedure is shown in Fig. 2.2. SMAW was used to clad inconel 182 and then stress relief (SR) treatment (898 K × 86.4 ks) was performed to reduce the residual stress. These two processes were used to simulate the actual multi-pass welding process used for repairing. Low temperature sensitization (LTS) treatment (773 K × 86.4 ks) was carried out to simulate the sensitization process after long-term operation. Next, LSM treatment was performed on the specimen surface before, finally, the LTS treatment was carried out again to verify the corrosion resistance of the material after the LSM treatment. 2.2 Thermal cycle flow of repairing procedure. An yttrium aluminum garnet (YAG) laser with 2 kW maximum power was used for the LSM treatment. Ar was used as the shielding gas to prevent oxidation of the melted region. Various parameters were tested (including combinations of laser power of 1.5 kW and a speed of 4.2 to 33.3 mm/s, with a defocus length of 0 or 10 mm; see Table 2.2) to find the optimum manufacturing conditions for repairing using LSM treatment. The average laser spot diameter was 0.5 mm?, whilst it was 4 mm? at the focal point and 10 mm at the defocus point. The lapping rate of the LSM treatment bead changed from approximately 25 to 75%; for a definition of the LSM treatment lapping rate; see Fig. 2.3. Table 2.2 Laser surface melting (LSM) conditions 2.3 Schematic illustration of lapping rate of LSM process. Figure 2.4shows a schematic illustration of the preparation of a specimen with SCC. To prepare the specimen for SCC sealing, SMAW was used to deposit SR and LTS treatments, then a tensile strain of 1% was loaded on the specimen surface, which was immersed into a caustic solution (K2S406 10 g/l and NaCl 1 g/l). After a few days, cracking (of 2–3 mm) was obtained and used to simulate the occurrence of SCC after long-term operation. An LSM sealing treatment was then performed on the specimen surface in the direction parallel to the surface cracks, as shown in Fig. 2.5. 2.4 Schematic illustration of SCC specimen preparation. 2.5 Sketch map of SCC sealing by LSM process. 2.2.3 Corrosion tests
Streicher test (modified ASTM G28) Tests for iGC and iGSCC were performed in a solution of 400 ml H2O, 233 ml H2SO4 and 50 g Fe2(SO4)3 using a Streicher test. The specimens were cut into small samples of size 40 mm × 8 mm × 5 mm (length × width × height), and put into the boiling solution for 86.4 ks. The samples were then bent to reveal the IGC/IGSCC for observation. After the Streicher test, the maximum IGC depths in two cross-sections were measured by optical microscopy to evaluate the IGC/IGSCC susceptibility of the samples. Open bend beam (OBB) test In addition, an open bend beam (OBB) test was used to evaluate the iGSCC resistance of the specimens. The specimen was cut into small samples of size 40 mm × 10 mm × 3 mm, and a tensile strain of 1% was loaded on the surface of the sample. The LSM treated surface of the specimen was immersed in a solution of...