This paper will discuss the results of an investigation into cracking that occurred on alloy 800H pigtails from a steam reformer furnace. Typically, cracks on pigtails occur circumferentially due to high bending stresses in the bends, or at welded connections. However, in present samples, longitudinal cracks were found on the pigtails. Metallurgical evaluation revealed that the pigtails had cracked from both ID and OD as a result of intergranular stress oxidation cracking. This failure mechanism results from the combination of intergranular creep crack growth from repeated loading and high temperature oxidation. Our metallurgical observations are consistent with a common failure mechanism reported for alloy 800H.
BACKGROUND
Modern refineries are large consumers of hydrogen, which is needed for Hydrotreating and Hydrocracking Process Units. While a large portion of hydrogen is produced in Catalytic Reforming Units, additional hydrogen is often manufactured on site. The steam reforming process is commonly used to generate hydrogen in the refinery. The heart of the process is the fired furnace where a high temperature reaction of steam and hydrocarbon produce a hydrogen rich stream. The high temperature effluent stream is cooled and post processed to remove CO and CO2 by-products. The high temperature shift gas reaction takes place in the range of 800-900°C (1472-1652°F) in catalyst-filled tubes, typically fabricated in cast stainless steel. The cast tubes in most furnaces belong to the heat resistant cast stainless steel family and rely on large fractions of carbides to help give high temperature creep resistance. These high alloys generally belong to the HK and HP families (nominally 25Cr-20 Ni and 25Cr-35 Ni respectively). Due to large amount of thermal expansion that takes place from cold to hot positions, the furnace tubes are allowed to freely 'hang' inside the furnace casing. To accommodate the physical expansion, the furnace tubes are connected to inlet/outlet headers by more ductile, wrought tubing alloys, with Alloy 800H (25Cr-32 Ni) and its derivatives being most common. These inlet/outlet connectors are generally known as 'pigtails', although in the present case, are called hairpins.
This paper reviews the failures of several outlet Hairpin tubes (i.e. pigtails) on a furnace after approximately seven years service. The tubing in question was Alloy 800H (ASTM B407, UNS N 08810), chemical composition 19-23% Cr, 30-35% Ni, 0.06-0.10%C, and specified grain size of ASTM 5 or coarser. An initial failure occurred onstream, and during shutdown testing and inspection, other hairpins were found with through wall fissures. On particular interest was why the pigtails failed in well less time than their design life of 100,000 hrs, and in a location not commonly reported. The furnace was constructed in 1990 but had seen only approximately seven years (60,000 hrs) of on-stream service from 1990 to failures in 2001. Figure 1 shows the failure location and the orientation of the crack in the unit. The furnace was designed to operate at an Outlet Temperature of 840°C (1544°F) max and pressure of 2480kPa. Under simple hoop stress only, the pigtails calculated life should have been 100,000 hrs minimum. This paper examines why Alloy 800H may fail prematurely despite appearing to have sufficient life by creep mechanism only.
METALLOGRAPHY AND SCANNING ELECTRON MICROSCOPY
A cross-sectional sample in transverse direction was prepared for metallography. The optical micrograph at low magnification shown in Figure 2 shows that cracks were initiated both at ID and OD. The damage from the OD surface extends to greater depths than from the ID surface.
The microstructure of the ID surface shown i
