Changes to the microstructure of joints of welded iron superalloys IN 519 and H39WM during operation

Abstract

Steam reforming is one method of producing hydrogen. The endothermic decomposition process of mixtures consisting of methane and water vapour under pressures of up to 4.0 MPa is conducted at a temperature of up to 780°C in austenite pipes cast centrifugally and filled with nickel catalytic agent. The results of microstructure tests on the welded joints of centrifugally cast catalytic pipes, made of second generation alloy IN 519 and third generation alloy H39WM, are presented in this article. The tests were conducted on butt welds drawn from the area of the substrate inlet of an approx. 500°C temperature, and from the area of maximum speed of pipe creep. The temperature of the pipe's external wall at the outlet of the products approaches 900°C under the operating conditions of ammonia reformers when internal pressure reaches up to 4.0 MPa. Long-lasting operational results in phase and structural changes depend on the local temperature of the pipe material. These changes affect the mechanical properties of the pipe material and contribute to initiation of the creep process, which manifests itself in macroscopic aspect as a local increase in the diameter of the pipes. The extreme stage of the high-temperature degradation of the microstructure reveals itself as macro-cracks leading to a loss of pipe leak-tightness.