Carburization | Materials And Corrosion Control

Damage Description

·         Three conditions must be satisfied for carburization to occur:

o   Exposure to a carburizing environment or carbonaceous material.

o   Temperature high enough to allow diffusion of carbon into the metal [typically above 1100°F (593°C)].

o   Susceptible material.

·         Conditions favoring carburization include a high gas phase carbon activity (hydrocarbons, coke, gases rich in CO, CO₂, methane, ethane) and low oxygen potential (minimal O₂ or steam).

·         Initially, carbon diffuses into the component at a high rate and then tapers off as the depth of carburization increases.

·         In carbon steels and low alloy steels, carbon reacts to form a hard, brittle structure at the surface that may crack or spall upon cooling.

·         300 Series SS are more resistant than carbon steel and the low alloy steels due to higher chromium and nickel content.

·         Carburization can result in the loss of high temperature creep ductility, loss of ambient temperature mechanical properties (specifically toughness/ductility), loss of weldability, and corrosion resistance.

Affected Materials

·         Fired heater tubes are the most common type of equipment susceptible to carburization in the environments mentioned earlier.

·         Coke deposits are a source of carbon that may promote carburization, particularly during decoke cycles where temperatures exceed the normal operating temperatures, accelerating the carburization.

·         Carburization is sometimes found in heater tubes in catalytic reformers and coker units or other heaters where steam/air decoking is performed.

·         Carburization is also encountered in ethylene pyrolysis and steam reformer furnaces. Significant carburization occurs during decoking cycles.

Control Methodology

·         Reduce the carbon activity of the environment through lower temperatures and higher oxygen/sulfur partial pressures. Sulfur inhibits carburization and is often added in the process stream in small amounts in steam/gas cracking in olefin and thermal hydrodealkylation units.

Monitoring Techniques

·         Inspection techniques based on determining increased levels of ferromagnetism (magnetic permeability) are also useful for alloys that are paramagnetic when initially installed (austenitic alloys). However, surface oxides may interfere with the results.

·         In the advanced stages of carburization where cracking has initiated, RT, UT and some magnetic techniques may be used.

·         The depth of carburization can be confirmed by metallography.

·         Carburization can be confirmed by substantial increases in hardness and loss in ductility.

·         In a more advanced stage, there may be a volumetric increase in the affected component.

·         A change (increase) in the level of ferromagnetism can occur in some alloys.

·         Carburization results in the formation of metal carbides depleting the surrounding matrix of the carbide forming element.

Inspection Frequency

KPIs

Reference Resources (Standards/GIs/BPs)