Evaluation of mechanical fitness for service of high temperature hydrogen attacked steels

Abstract

A novel approach is introduced in evaluating one of industry’s most complex corrosion phenomena, high temperature hydrogen attack (HTHA). Limited efforts have been made by industry to address this degradation mechanism and even less data are available to properly utilize their evaluation techniques. The predominant fracture mechanics based analysis of HTHA relies on Charpy V-Notch (CVN) impact test data. In the face of limited CVN test data, accurate results are shaky at best. The HTHA evaluation set forth in this paper combines the analysis tools of plastic collapse and both static and dynamic fracture mechanics. The first condition addresses plastic collapse. Upon treating the HTHA zone as “lost wall” thickness, a remaining wall calculation is performed using standard plastic collapse equations. The second condition evaluates the overall, or “averaged”, fracture toughness. The third and final condition separates the material’s cross-section into three distinct zones: 1) visually obvious HTHA zone, 2) unaffected “good wall”, and 3) the fracture toughness degradation zone (between the two preceding zones). HTHA depth in a material thickness is typically detected using ultrasonic (UT) testing equipment. Unfortunately, UT only provides a region of obvious HTHA damage and does not address the additional region suffering degradation. The fracture toughness degradation zone accounts for this area. These two degradation zones are considered brittle with the “good wall” treated as unaffected material. An unstable crack is considered initiated in the degraded zones and gets arrested in the “good wall”, affording its resistance through dynamic toughness. Material data (tensile, fracture toughness, and microhardness) from a retired heat exchanger is collected in order to evaluate its HTHA condition. The microhardness values allow for determining the fracture toughness degradation zone. Fracture toughness tests performed are the elastic-plastic technique of crack-tip opening displacement (CTOD). Due to the low operating stress state of this heat exchanger, the calculated critical crack lengths are extremely long and both fracture toughness conditions are satisfactory. The plastic collapse condition is found to be satisfactory. Further data is presented in this thesis to help address HTHA growth rates and other metallurgical aspects of a material affecting HTHA resistance.