What is Graphitization in Carbon steel, low alloy steel?


Graphitization?

Graphitization is a change in the microstructure of certain carbon steels and 0.5 Moly steels. During Graphitization, these changes takes place in certain grades of Carbon steel and 0.5 Moly steels during exposure to long term operating temperature of 800°F to 1100°F or 427°C to 593°C range. This results in the loss of mechanical strength, ductility and, or creep resistance.

The first picture shows the unaffected base metal of a carbon steel having pearlite and ferrite microstructure. When this material is exposed to high temperature above 800-degree Fahrenheit, the graphitization takes places as visible by the black area in the second picture.

Watch this YouTube video for full classroom training on API 571 Damage Mechanism- Graphitization.

Which materials are being affected by the Graphitization

It happen is  certains grades of Carbon steel or C S material

 and 0.5 Molybdenum steels such as S A 182 F 1 grade and S A 204 grade A, B and C etc.

Critical Factors for the Graphitization

The primary factor affect graphitization are the:

a. Chemistry of base metal

b. Stress, temperature and time exposure

It Affect Carbon steel and carbon molybdenum steels upto 1.0% Moly. Addition of 0.7% chromium effectively eliminate the graphitization effect in steel.

Temperature as being the primary factor control the rate of the graphitization. Below 800 degree Fahrenheit ( or 427 degree Celsius), the rate is extremely slow.  A rise in temperature increase the rate of the graphitization.

Types of the graphitization

The graphitization is divided into two parts.

First one, random graphitization

Second, Chains or local planes of Concentrated Graphitization. This form of graphitization can result in a significant reduction in load bearing capability while increasing the potential for brittle fracture along this plane.

Chains or local planes of concentrated graphite nodules which can be further classified into:

  • a. Weld heat affected zone (HAZ) graphitization, also called EYEBROW graphitization. This is most frequently found in the heat-affected zone adjacent to welds in a narrow band, corresponding to the low temperature edge of the heat affected zone. In multipass welded butt joints, these zones overlap each other, covering the entire cross-section. Graphite nodules can form at the low temperature edge of these heat affected zones, resulting in a band of weak graphite extending across the section. Because of its appearance, this graphite formation within heat-affected zones is called eyebrow graphitization.

b. Non-weld graphitization, a form of localized graphitization which also occurs in chain-like form. It sometimes occurs along planes of localized yielding in steel. in a chain-like manner it experienced significant plastic deformation as a result of cold working operations or bending.

Timeline for occurrences of Graphitization

Severe heat-affected zone graphitization can develop in as little as five years at service temperatures above 1000°F (538°C). Very slight graphitization would be expected to be found after 30 to 40 years at 850°F (454°C).

The graph here shows the relationship of various heat affected zone graphitization with service temperature and service hours.

Affected Units or Equipment’s in Refinery

  1. Primarily hot-wall piping and equipment in the FCC, catalytic reforming and coker units.
  2. Reactors and piping of fluid catalytic cracking units, as well as with carbon steel furnace tubes in a thermal cracking unit and the failure of seal welds at the bottom tube sheet of a vertical waste heat boiler in a fluid catalytic cracker.
  3. Economizer tubing, steam piping and other equipment that operates in the range of temperatures of 850°F to 1025°F (441°C to 552°C) is more likely to suffer graphitization.

Appearance or Morphology of Damage

As graphitization damage occurs at microstructure level, it requires microscope for the observation.

Advanced stages of damage related to loss in creep strength may include micro-fissuring/micro-void formation, subsurface cracking or surface connected cracking.

Prevention / Mitigation of Graphitization

Graphitization can be prevented By using chromium containing low alloy steels for long-term operation above 800°F (427°C).

Inspection and Monitoring

  1. Evidence of graphitization is most effectively evaluated through removal of full thickness samples  for examination using metallographic techniques. Damage may occur mid-wall so that field replicas may be inadequate.
  2. Advanced stages of damage related to loss in strength include surface breaking cracks or creep deformation that may be difficult to detect.

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