Microstructural and Finite Element Analysis of Creep Failure in Dissimilar Weldment Between 9Cr and 2.25Cr Heat-Resistan
- PDF / 3,228,732 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 27 Downloads / 176 Views
TRODUCTION
DISSIMILAR welding between different kinds of heat-resistant steels is often performed to connect parts that are subjected to different conditions of heat and pressure. For example, in the boiler system of the coal-fired fossil power plant, the water wall is the part where water is evaporated to steam by heat of combustion from coal and the superheater is where steam from the water wall is heated. Then this hot, high-pressure steam turns turbines. Steam from the turbines is reheated in the reheater and turns the turbines again. Therefore, the steam conditions become increasingly severe from the water wall to the superheater/reheater. Heat-resistant steels that are of grades suitable for the temperature and pressure of steam are used at various positions in large-scale boiler components of power plants. For example, 2.25Cr heat-resistant steels are used for water-wall tubes in ultra-supercritical power plants, whereas 9Cr heat-resistant steels are used for boiler components, such as superheaters and reheaters,
HYUN JE SUNG is with the Technical Research Laboratories, POSCO, Pohang 37859, South Korea. JI HYUN MOON, MIN JI JANG, and HYOUNG SEOP KIM are with the Department of Materials Science and Engineering, POSTECH, Pohang 37673, South Korea. SUNG-JOON KIM is with the Graduate Institute of Ferrous Technology, POSTECH, Pohang 37673, South Korea. Contact e-mail: [email protected] Manuscript submitted October 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
which are exposed to high operating temperatures.[1] Therefore, dissimilar welding between different kinds of heat-resistant steels is always performed to connect parts and it is necessary to satisfy the service conditions as well as economic requirements due to the reduced use of expensive materials. Dissimilar welded parts generally form a heat-affected zone (HAZ) between the weld metal (WM) and base metal (BM) and show a rapidly solidified microstructure in the WM due to a severe thermal cycle during welding.[2] Therefore, to decrease the risk of brittle fracture, the welds are subjected to postweld heat treatment (PWHT). Unfortunately, during PWHT of such heterogeneous weld joints, interstitial C atoms migrate from low-Cr steel to high-Cr steel, so a carbon-depleted zone (CDZ) forms.[3–7] Tensile tests of dissimilar weldments between 2.25Cr1Mo and 9Cr1Mo steels showed that the deformation is initiated in the CDZ but that subsequent strain hardening prevents further deformation of the CDZ, so failure happens in the BM.[8] However, long-term creep tests with low applied stress at high temperature, at which strain hardening is negligible, produced fracture in the CDZ; this failure is defined as type IIIa cracking.[9,10] Previous studies[3–10] concentrated on the CDZ between 9Cr WM and 2.25Cr HAZ, because the chosen 9Cr filler metal has a similar chemical composition to 9Cr BM; therefore, only one CDZ appears between the 2.25Cr HAZ and the 9Cr WM. However, 9Cr is expensive, so in the present research, affordable 2.25Cr1Mo ER90S-B3 was evaluated instead as
Data Loading...
