Graphitisation is a microstructural change which can occur in carbon or low alloy steels that are exposed to elevated temperatures, between approximately 425°C and 600°C, for long periods of time. It is most often encountered in boiler tubes and related equipment that have experienced service temperatures in this range for several thousand hours.
Exposure to such elevated temperatures for extended time periods results in the decomposition of pearlite within the steel into ferrite and carbon (graphite). This 'graphitisation' can lead to embrittlement of the steel. The effect is of most concern if the nodular graphite particles form in a continuous manner through a load-carrying member. Randomly distributed graphite particles tend to cause only moderate reductions in strength. Above around 550°C pearlite predominantly decomposes to spheroidal carbides and graphitisation becomes less of an issue.
Deoxidation practice during steelmaking has been found to have a significant effect on graphitisation. Aluminium deoxidised steels are the most susceptible, whereas silicon deoxidised or low aluminium steels are less susceptible. Steels that contain strong carbide stabilising elements such as chromium and molybdenum, however, are found to be much more resistant to graphitisation. Cr-Mo steels are specified for service at the higher temperatures of the graphitisation range.
Published data [1] on the graphitisation of an Al deoxidised 0.15%C 0.5%Mo cast steel is summarised below:
Exposure temperature
(°C) | Time for half conversion †
(hours) |
|---|
| 620 |
400 |
| 590 |
800 |
| 540 |
2500 |
| 470 |
20000 |
| 450* |
25000 |
| 430* |
>50000 |
* extrapolated data
† conversion of one half of the carbon in the steel to nodular graphite
For welded carbon steels, the most likely location for graphitisation to occur is in the heat affected zone (HAZ). Wilson [2] surveyed over 500 carbon and carbon-molybdenum welded steel specimens removed from various types of refining equipment and found evidence of graphitisation in a third of the samples. No graphitisation had occurred in the samples before at least 40000 hours exposure at temperatures between 450°C and 590°C. However, the formation of graphite along the edge of the HAZ, forming a plane of weakness, was said to be much more significant than the total degree of graphitisation.
References
- ASM Speciality Handbook: 'Carbon and alloy steels'. Publ: ASM International, USA, 1996.
- Wilson J G: 'Graphitisation of steel in petroleum refining equipment'. Welding Research Council Bulletin, January 1957, 32, pp.1-35.