11145 An Approach to Determine the Initiation of Carburization in a 304H Stainless Steel Piping Under Petrochemical Environment

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Product Number: 51300-11145-SG

ISBN: 2011 11145 CP

Author: Jeffrey Xie, Marek Crawford, Lorrie Davies, David Eisenhawer, Randy Saunders and Les Benum

Publication Date: 2011

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Carburization is a common degradation mechanism in carbonaceous gas streams at elevated temperatures in petrochemical industries. The crossover piping system between the convection section and radiant section of ethane cracking furnaces is fabricated from 304H stainless steel (SS), and is normally protected by a chromium oxide layer. Carburization (and potentially metal dusting) forms once degradation of the oxide layer occurs during the ethane cracking/decoking process.

A thorough metallurgical analysis demonstrated that some components of the crossover piping system in one of our plants suffered from carburization and metal dusting. This paper describes a methodology developed to determine the initiation point of carburization based on metallurgical analysis and theoretical modeling. The predicted initiation point of carburization, using this approach, was correlated with a major process parameter change in the plant operation, which greatly reduced the protection of the aged 304H SS from carburization and metal dusting.

Fick’s Second Law of Diffusion was applied to describe carbon diffusion kinetics into the bulk pipe steel, the heat-affected zone (HAZ) and the weld. It has been established through metallurgical analysis that depth of carburization of this particular pipe section followed a power-law relationship with carburization times (in the unit of years). These relationships can be used to ascertain the initiation point of carburization for the HAZ and the bulk steel, thus providing important information for life assessment of this crossover piping system.

Key Words: carburization, 304H stainless steel, crossover piping, diffusion, metal dusting.