A hybrid, austenitic stainless steel has been developed for heater tubes, forgings, bends, fittings and welding consumables, which has higher strength than TP 316 and TP 347 in API 530 up to 1200°F (650°C). This allows for thinner wall tubes that provide optimum heat transfer and stress reduction and results in significant savings in material, fabrication and operational costs. 16Crl 1Ni2.5MoCb has the attributes of many of the higher alloyed materials and in terms of life expectancy, creep rupture strength is an order of magnitude higher than TP 347.
The 2.5% minimum molybdenum makes the material ideally suited for high naphthenic acid applications. In the as-welded condition the material offers high immunity to sensitization thus eliminating the need for stabilization annealing both before and after welding. The high resistance to polythionic acid eliminates the need for a soda ash wash during shutdowns, which provides added reliability, and reduces operation downtime during turnarounds. 16Crl 1Ni2.5MoCb is predicted to have higher creep rupture strength than comparable austenitic alloys I and can be readily formed and fabricated in a similar way to TP 347.
Some 600 metric tons of wrought 16Crl 1Ni2.5MoCb tubes and fittings were required for twelve heaters, for a minimum 100,000 hrs service life for a new refinery in Alberta, Canada. The objective was to develop a low cost stainless steel that would be sufficiently robust to resist high sulfur naphthenic acid crude, and polythionic acids attack. TP 316 and TP 347 were selected for the investigation due to extensive experience in similar service. Columbium containing grades of stainless steel were chosen for study due to the higher carbon stability provided compared to the titanium grades.
Severe demands will be put on the material selected, as it will have to withstand the following process conditions. The bitumen being processed is high in naphthenic acid, with a Total Acid Number (TAN) up to 4.0mg KOH/g with various sulfur concentration levels. Naphthenic acid corrosivity is a complex issue and appears to be a function of several parameters: naphthenic acid chain length (molecular weight), vapour vs. liquid phase, true boiling point, velocity, temperature, sulfur type and level. Stainless steels with molybdenum content above 2.5% appears to provide good resistance against attack by naphthenic acids. Experience has shown that TP 316, with nominal molybdenum content of 2.1%, is often insufficient to resist this type of corrosion 2.
The heater tubes have been designed to operate up to 1200°F (650°C), which is within the sensitization range for most stainless steels, i.e. between 900-1500°F (480-816°C). Heating in this range can result in the precipitation of a continuous band of fine chromium carbides at the grain boundaries, depleting the adjacent regions in chromium. Sensitization can be delayed by lowering the carbon content, but because the design codes and conditions require a minimum of 0.04% carbon for service temperatures greater than 1000°F (538°C), this is not an option. Alternatively, stabilization of the carbon with either titanium or columbium, followed by stabilization annealing at 1650°F (900°C), is another solution. But as this temperature is within the sigmatization range of most stainless steels, a serious loss of toughness could occur. Therefore the development of the 16Crl 1Ni2.5MoCb material eliminates the need for stabilization annealing heat treatment without lowering the carbon content.
Polythionic acids can form in the heater tubes during shutdowns when the interaction of sulfurous compounds, oxygen and moisture occur. Stainless steels will corrode and crack inte
