Chemical plants like natural gas-based fertilizer industries operating at high temperature and handling corrosive and hazardous substances at higher pressure are subjected to ageing and its equipment and piping tend to degrade over time. There are many areas which are prone to aging but can go unnoticed, leading to safety hazards for the plant and people. Replacement of whole plant and even sub-systems are not always economically feasible which give rise to the need of assessment of systems and its components for risk associated with failure. With a view to assess the same, it calls for understanding the mechanism of degradation so that remaining life assessment of equipment can be carried out more precisely.
Materials degrades with passage of time due to many factors, out of which following major mechanisms are creep, microstructural degradation, high temperature fatigue, carburization, hydrogen damage and embrittlement, graphitization, thermal shock, liquid erosion, dual phase erosion, corrosion, erosion-corrosion mechanism, liquid metal embrittlement, Stress corrosion cracking, caustic gauging, metal dusting etc
Equipment and piping under high temperature and stress are subjected to progressive degradation called creep. Creep is one of the most important degradation mechanisms which involves time-dependent deformation which can lead to rupture of the material. The failure of materials by creep damage depends on temperature and the extent of stress the material is subjected to. The degradation once starts gradually progresses and develops microcracks and propagates through grain boundaries leading to rupture.
High temperature equipment like catalyst tube in fired furnace, Boiler, tubes and headers in steam superheater, piping inside waste heat section of duct, tubes in BFW preheater, reactor, catalyst basket screen of Ammonia reactor, tubes TTS weld of heat exchangers handling hot process gas, intercoolers in syn gas compressor system, gas turbine components etc are always subjected to continuous exposure of high temperature where stresses like internal pressure centrifugal force, static load are associated causing the combine degradation of creep.
Fig-1: Stages of damage by creep
At higher temperatures, creep can occur uniformly or with localized deformation having symptoms of large plastic strains, local wall thinning, elongation etc. The combination of lower and higher temp and stresses at higher or lower level produces different types of fractures and characterization of this cracks under various conditions can be known from characteristic map developed by Ashby, Mohamed and Langdon.
Qualitative approach of evaluating defects like creep void, microcracks can be done with Neubauer and Wedel based model. This model helps understand damage stages in high temperature steam network with the help of in-situ metallography. Collecting replica in in-situ metallography is easy method with damaging the component, but not suitable for many metals like some Cr-Mo steel, where voids may get created during polishing and etching.
The degree of spheroidization of carbides in bainitic and pearlitic structure tells about extent of thermal exposure and consequential damage.
Sometime, measurement of hardness is required to substantiate loss of tensile strength and assessment of remaining life in material under creep damage mechanism.
This type of damage mechanism is caused by other damaging processes like creep, for which continual degradation in metallographic structures are seen. Most predominantly the metal structures with grains gradually degraded with precipitation in grain boundary, developing micro cracks and sometimes trans-granular cracks are being propagated leading failure.
This is the aging phenomena, when certain type of carbon steel and low alloy steel are subjected to a specific range of temp (440 degC to 760deg C), carbide phases become unstable and start to form cluster resulting in loss of strength. The higher the temp close to higher end the degradation is faster even can happen in hours, however the process significantly slows down with the fall in temperature.
Although, the threat to failure due to spherodization is quite less, except for a few pressure vessels or piping operating in the susceptible temperature range or in a fired furnace where conditions tend to fluctuate widely which may include furnace catalyst tubes, exothermic reactors (e.g. Ammonia converter), and other pressure vessel with fluctuating operating temperatures which are exposed to uncontrolled exothermic reaction with possibilities of runaway reaction.
Designing these pressure vessels and choosing the materials for construction is tricky thing; as some steels are more susceptible to this degradation, including fine-grained normalized steels. Therefore, choosing the correct ASTM grain size range is key factor for ensuring long life of the components.
Likelihood of spheroidization damage can be evaluated with standard hardness testing and can be confirmed by metallography.
Softening and Decarburization
It is process of depleting carbons in carbon and low alloy steel when subjected to high temperatures in a oxidizing or reducing environment. It generally happens metals are heated beyond 700 deg C when the metal reacts with gases containing oxygen and hydrogen. The carbons which resides in the form of metal carbides, gets removed in the form of CO, CO2 or CH4. The depletion of carbon results in softening of matal, primarily on the external surface where decarburizing gas reacts with metal.
The steps of decarburization mechanism can be three distinct events: the reaction at the steel surface, the interstitial diffusion of carbon atoms and the disbanding of carbides within the steel. Decarburization can be utilized for softening of metal where it is desired.
Embrittlement and carburization
This phenomenon is reverse of Softening and decarburization as described above. This can happen in several different ways.
Austenitic stainless steels: Sigma phase formation in high temperature or in critical temperature range (approximately 565 to 980 °C) which causes loss of ductility and induce embrittlement.
Ferritic stainless steels: Embrittlement is observed when held at or cooled over in the temperature range 550 to 400 °C. If service conditions are close to this temperature range, metallographic checks are recommended after extended exposure to avoid unexpected rupture.
In addition to the embrittlement, carburization can lead to brittle structure it is exposed to a high temperature carburizing atmosphere for long time.
Fig2 . shows extensive carbide formation in grain boundary of SS321 pipe in flue gas service.
Hydrogen damage can occur in carbon steels under hydrogen atmosphere particularly in high temp and pressure, results in diffusion of atomic hydrogen to form methane reacting with carbon present in iron carbide.
Hydrogen damage or embrittlement can be categorized into two broad categories HAC (hydrogen assisted cracking) and HIC hydrogen induced cracking.
It has been revealed that steels with lower UTS (less than 140KSI) or hardness less than 32 HRC are not susceptible to hydrogen embrittlement phenomenon. Steels like precipitation hardened steel (like 17-4PH) severe loss of ductility is observed when exposed high pressure hydrogen.
Hydrogen damages are most common in carbon steel, aluminium and titanium, however austempered steels are resistant to embrittlement caused by hydrogen. Austenitic stainless steel, High nickel alloys, beryllium copper are ale resistant to hydrogen embrittlement.
The following three parameters are considered responsible for hydrogen damage:
Presence and partial pressure of hydrogen
Pressure or stress
When assisted by a concentration gradient where there is significantly more hydrogen outside the metal than inside, hydrogen diffusion can occur even at lower temperatures. Hydrogen damage mechanism is consisting of following:
Internal pressure within the metal: Hydrogen atoms get absorbed in surface and get diffused and may recombine to form hydrogen molecules creating pressure within the metal. This pressure ultimately led to opening of cracks when metal lose ductility and toughness. (HIC)
Metal hydride formation: Hydrogen reacts with metal to form brittle hydrides which allows cracks to propagate.
Hydrogen enhanced decohesion: Hydrogen can induce decohesion where the strength of the atomic bonds of the parent material are reduced.
Hydrogen enhanced localised plasticity (HELP): The mechanism of HELP was first proposed by C. Beachem. The process illustrates enhancement of mobility of dislocations in the presence of hydrogen atoms, results in propagation of defects /cracks even at lower stresses required.
Hydrogen enhanced dislocation emission: This process tells about the enhancement of dislocation due to adsorption of hydrogen onto metal surface. Hydrogen enhances the mobility of dislocations in FCC, BCC and HCP ordered and disordered materials. It occurs for all dislocation types including partial and perfect lattice dislocations and grain boundary dislocations.
Graphitization: The process of microstructural degradation takes place in ferritic steels if is exposed to high temperature for long time. Since graphite phase is more stable the phases gradually change from cementite to pearlite and ultimately to graphite. During the service graphite particles tend to align and form the crack. With the development of Cr-Mo Steel this degradation could be avoided.
To be concluded in second part
DISLOCATION MOBILITY AND HYDROGEN – A BRIEF REVIEW by I. M. Robertson and H. K. Birnbaum (University of Illinois)
Image courtesy: American Chemical Industries