The effect of PWHT on the microstructural evolution, carbides formation and mechanical properties of a Nb containing martensitic heat resistance steel used in gas turbine

Abstract

The purpose of this study was to investigate post weld heat treatment (PWHT) of a heat resistant alloy steel used in gas turbines, in terms of microstructural evaluation, carbide phases formation and mechanical properties. EN10302.2002.X10CrMoVNb9-1(DIN1.4903) plates were welded by shielded metal arc welding process using an AWS E9015-B9 electrode, which is a cellulosic coated electrode. PWHTs were conducted at 700 and 740 °C for 4 and 8 h. Optical microscopy, Field Emission Scanning Electron Microscopy (FE-SEM) equipped with Electron Diffraction Spectroscopy (EDS), and X-Ray Diffraction (XRD) analyses were used to study the microstructure of the samples and to perform a thorough analysis. Uniaxial tensile tests, Charpy impact tests and Vickers microhardness were used to assess the mechanical properties of the welded joints. To define the fracture mode, the surfaces of the fractured samples were analyzed by FESEM after tensile and Charpy impact tests. XRD results showed that, in the weld metal, the ferrite phase is formed together with the iron and chromium carbides in the post weld heat treated cases. Whereas mostly martensite is formed in the as-welded condition. It was observed that increasing the PWHT soaking time and temperature increased the amount of ferrite. Results of microstructural observations and phase analyses showed that the microstructure of the weld joints contain ferrite phase together with retained austenite and chromium carbides, as well as niobium carbides. With increasing the temperature and soaking time of the PWHT, the amount of ferrite was increased at the expense of martensite. Assessing the tensile tests and Charpy tests results showed that performing the PWHT decreased yield strength and increased elongation to fracture in the joints. Ductility increased from 10.5% in as-welded condition, to 15% after PWHT at 740 °C for 8h. As such, the yield strength decreased from 600 MPa to 540 MPa after this PWHT process. On the other hand, toughness was improved with increasing the temperature and soaking time of the heat treatment. As well, performing heat treatment removed hardness gradient in these welded joints. Finally, fracture mode analyses by FESEM showed that by performing this specific PWHT process, the fracture mode has changed from brittle to ductile one making it an attractive procedure to pursue industrially.