Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes

Abstract

A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M 23 C 6 carbides transformed to M 7 C 3 at the inner wall region and NbC carbides partially transformed to the chromium-rich g-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M 23 C 6 /M 7 C 3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response.

Detecção De Baixos Níveis Carburização Em Tubos De Reforma Usando Um Sensor Magnético

“…A carburização promove a formação massiva de carbetos de cromo na região próxima à parede interna mudando a microestrutura do aço, criando novos carbetos e favorecendo a coalescência dos já existentes (6). Dita precipitação leva ao empobrecimento de cromo na matriz tornando-a ferromagnética (7,8). Observa-se na Tabela 4 os valores simulados da densidade de campo magnético do corpo de prova C com e sem carburização.…”

“…O fenômeno de carburização acarreta a redução da ductilidade da liga, o aumento da tensão interna na camada carburizada afetando negativamente a resistência à fluência e aos ciclos térmicos (6,7). Dependendo do nível de carburização, a vida útil das colunas pode diminuir ou ainda apresentar falhas prematuras.…”

The Relationship Between Magnetism and Microstructure of Ethylene Pyrolysis Furnace Tubes after a Long-term Service

“…Many researchers have investigated the magnetism of the pyrolysis furnace tube after service, but most of them only take into account carburization on the impact of the magnetism change (Stevens et al, 2003; Silva et al, 2008; Khodamorad & Haghshenas, 2012; McLeod et al, 2015). Only a few researchers pay attention to the oxidation and precipitated phases (Miodownik, 1982; Mamiya et al, 2016) effects on magnetic properties of the furnace tubes.…”