Surface Decarburization

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

Legend Surface decarburization Internal decarburization Carbon... 

T. Forged tubesheet cracked with surface decarburation tubes blistered. 

A 960 516 331 2.28 26 1.5 NPS Schedule 80 nozzle was broken off a catalytic reformer outlet line during a shut down. Metallography indicated surface decarburization and intergranular cracking with bubbles. Cr content was 1.09 percent. 

B 977 525 354 2.44 14 Blistering was detected with ultrasonic examination in catalytic reformer piping. Metallography indicated surface decarburization and blistering at non-metallic inclusions, with intergranular cracks growing from the blisters. Cr content was 1.10 percent. 

At elevated temperatures, molecular hydrogen dissociates into the atomic form, which can readily enter and diffuse through the steel. Under these conditions, the diffusion of hydrogen in steel is more rapid. As discussed in Section 4, Forms of High Temperature Hydrogen Attack, hydrogen may react with the carbon in the steel to cause either surface decarburization or... 

The combination of high temperature and low hydrogen partial pressure favors surface decarburization without internal decarburization and Assuring. The combination of low temperature, but above 430°F (221°C), and high hydrogen partial pressure favors internal decarburization and Assuring. These mechanisms are described more fully below. 

The broken-line curves at the top of Figure 1 represent the tendencies for surface decarburization of steels while they are in contact with hydrogen. The solid-line curves represent the tendencies for steels to decarburize internally with resultant Assuring and cracking created by methane formation. 

Surface decarburization does not produce Assures. In this respect, it is similar to decarburization created by the exposure of steel to certain other gases, such as air, oxygen, or carbon dioxide. The usual effects of surface decarburization are a slight, localized reduction in strength and hardness and an increase in ductility. Because these effects are usually small, there is often much less concern with surface decarburization than there is with internal decarburization. 

In cases where surface decarburization predominates over internal attack, the actual values of pressure-temperature combinations have not been extensively studied but the limits deAned by Naumann8 probably give the most accurate trends. Naumann s work, which is based on 100-hour tests, indicates decarburization tendencies however, long-time exposures have indicated lower operating limits. 

Legend Satisfactory Hydrogen attack Surface decarburization — and Assuring ... 

Mayos, M., Segalini, S. Putignani, M. (1987). Electromagnetic nondestructive evaluation of surface decarburization on steels, Memoires et Etudes Scientifiques de la Revue de Metallurgie, France, 85, 85-96. 

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