Concept in Definition ABC

When we speak of deformation we refer to the movement relationship between grains and other discontinuities of the metal (at the microstructural level of the material). When the deformation is even greater, cracks develop and grow and eventually become a through fault, at which point they become clearly visible.

Essential parameters

The most relevant parameters that come into play are: temperature, the loads and the material, since the yield stress value depends on it. However, it is also valid to clarify that failure times are reduced if there is an increase in stress due to material thinning due to corrosion. Also, time to failure is non-linear with temperature and load increase, for example a 15°C or 15% load increase can cut life by half or more.

There are tabulated values ​​in the literature about the temperature limits for some materials, however, considers that all metals and their alloys are to a greater or lesser extent susceptible to this mechanism of degradation. Working above this stated temperature can contribute to creep deformation and subsequent cracking.

Process

The creep is a mechanism that develops over time and can cause the total rupture of the component subjected to the load. However, the development of the mechanism occurs in three instances. First, creep resistance increases due to deformation. In a second instance, the speed of deformation is constant, while it grows rapidly in the last stage, leading to irreparable damage such as material breakage.

To prevent the development and propagation of the mechanism, API 571 suggests continuous inspection and monitoring. For example, minimize the temperature to which the material is subjected and monitor it (in the case of furnace tubes, in direct contact with the fire, their tube skin temperature must be monitored). Along the same lines, it is suggested to anticipate and avoid stress concentrations during the design Y manufacturing (for example, in heaters minimizing hot spots and localized overheating, verify that there is no deviation of flame) and select less susceptible materials in the working temperature range, as well as carry out post-treatment welding. On the other hand, more ductile materials will be more resistant.

Regarding different parameters that are suggested as indicated for the monitoring of the mechanism, we have: the formation of cracks and changes in the microstructure of the material, review the existence of buckling, deformations in general and/or blistering In addition, as an inspection activity, it is suggested to monitor the thickness of the material, for example, in tubes of heaters and ovens, in their elbows, etc.

To identify the mechanism, it is important to know at what stage of development it is, since, for example, In initial states, when the deformation is at the microstructural level, it is only possible to detect it through a microscope scanning electronics. While, as fissures (micro-fissures) and later cracks are formed, they can be searched visually, with some specialized technique for this purpose or with metallography. When the exposition to the load and the temperature has increased considerably, bulges and a series of deformations are observed.

In general, the type of equipment most affected by this mechanism are fire heater tubes, such as tube supports and other internal components of furnaces. It is also in the kit critical, steam tubes in boilers and catalytic reactors (exposed to high temperatures).

When a component has been exposed to severe conditions and there is creep damage, it is irreversible. In many of these cases, the remaining life of the component can be evaluated by following the methodology of API 579-1 and/or ASME FFS-1.

Bibliography

American Petroleum Institute (Wash.). (2011). Damage Mechanisms Affecting Fixed Equipment in the Refining Industry: API Recommended Practice 571. American Petroleum Institute.