Investigations of structural transformation within metal (austenite chromium-manganese steel) at the external surface of steam superheating tubes

Abstract

The elemental composition of an altered layer at the external surface of a steam superheating tube of grade DI59 steel is investigated after long-term operation. It is shown that the layer is located between a scale and a matrix and depleted by silicon, manganese, copper, and chromium with the maximum oxidizer affinity, enriched by iron and nickel to 90%, and mainly composed of the α-Fe phase (ferrite) with the ferromagnetic properties. The layer formed as a result of selective oxidation and diffusion from the matrix into the metal scale with the less standard free energy of the formation of sulfides and oxides. A magnetic ferrite meter is used in the experimental investigation of the layer evolution by testing grade DI59 steel for heat resistance in air environment at temperatures of 585, 650, and 700°С for 15 × 10³ h; creep at a temperature of 750°С and a stress of 60 МPа; and long-term strength at temperatures of 700 and 750°С and stresses of from 30 tо 80 МPа. Specimens for tests are made of tubes under as-received conditions. The relationship between the ferrite phase content in the surface metal layer and the temperature and time of test is determined. The dependence is developed to evaluate the equivalent temperature for operation of the external surface of steam superheating tubes using data of magnetic ferritometry. It is shown that operation temperatures that are determined by the ferrite phase content and the σ phase concentration in the metal structure of steam superheating tubes with the significant operating time are close. It is proposed to use magnetic ferritometry for revelation of thermal nonuniformity and worst tubes of steam superheaters of HPP boilers.