Mechanical and intergranular corrosion properties of auto TIG welded TP347H-T91 dissimilar weldments

Abstract

Introduction

The Biggest challenges coal-fired power plants are facing are improving their energy efficiencies and reducing their environmental impact due to CO2 and NOX emissions, can be attained by maximizing the steam pressure and temperature. However, the operating temperatures and pressures in boiler materials were restricted due to limited weldability of essential dissimilar joints between Austenitic Stainless Steel (ASS) and Creep-Enhanced Ferritic Steel (CEFS) due to carbon migration and sensitization. Present research work provides enhanced metallurgical and mechanical properties of weldment by optimizing the welding process and welding parameters.

Aim

The aim of the manuscript is to explores the welding compatibility of TP347H (stabilized ASS) with solution annealed T91 (CEFS), by Auto TIG welding technique, such that carbon migration at the fusion line of SA213 T91 and sensitization in the heat affected zone of TP347H can be minimized.

Methods

Influence of Auto TIG welding parameters on the mechanical, microstructure, and corrosion properties are studied and discussed. Carbon migration, mechanical and corrosion properties for lower (0.792 KJ/mm) and higher (1.124 KJ/mm) heat inputs weldments were compared. Investigation on mechanical properties test such as tensile, hardness, and root bend performed as per American Society of Mechanical Engineers (ASME) Section IX. Optical microscopy and Analytical Scanning Electron Microscope (SEM) coupled with Energy Dispersive Spectroscopy (EDS) have been performed to investigate carbon migration and sensitization behaviour. Sensitization susceptibility further validated by American Society for Testing and Materials (ASTM) A 262 Practice E.

Results

Investigation on tensile, hardness, and root bend tests revealed that mechanical properties for lower heat input weldment were superior. Restricted carbon migration due to optimized heat input improves mechanical properties and this was evidenced by satisfactory results of the bend test which showed no opening at the fusion line. A noticeable improvement in Ultimate Tensile Strength (UTS) 666 and 680N/mm2 (compared to 578 and 561N/mm2 for 1.124 KJ/mm heat input) and an acceptable hardness of 307 HV (compared to 399 HV for 1.124 KJ/mm heat input) were observed at T91 and weld metal interface. No fissures on bend surface and no Cr23C6 precipitation at grain boundary after performing ASTM A 262 Practice E and SEM-EDS respectively on heat affected zone (TP347H) of lower heat input weldment which shows higher resistance to intergranular corrosion (Sensitization).

Conclusion

Optimizing the welding parameters and decreasing the heat input mitigate the carbon migration at T91 fusion zone and, meeting the requirements for satisfactory mechanical properties in materials used for power generation applications. Further optimized welding heat input have lesser Cr23C6 precipitation in the heat-affected zone of TP347H, making the heat affected zone less susceptible to sensitization. Heat input must be controlled to (1) avoid intergranular corrosion failure or sensitization and (2) improve the mechanical properties of the weldment where requirement of post weld heat treatment is mandatory in the application (as recommended in the ASME Section 1, PW 39.1).