Selection of stainless steel in petrochemical industry

The role of the main elements in austenitic stainless steel, the steel's structure, mechanical properties, corrosion resistance, high temperature performance, low temperature performance, etc. are analyzed, and the principles of reasonable selection in the design of petrochemical equipment are explained.

Carburization is extremely uneven
The problem with the undetectable carburization of ethylene furnace tubes and some other equipment is its unpredictability and unevenness. Mathematical models developed based on operating experience can be somewhat helpful in predicting the overall carburization rate. Unfortunately, none of the existing models is absolutely accurate or satisfactory. The main reason is uneven carburization. Temperature itself is not a decisive factor.
Many years ago it was thought that the surface condition of the pipe was an important factor, but it is no longer the only factor at play as these pipes all start out in the same conditions and are exposed to the same conditions over the years. This non-uniformity also brings about a problem, which is that the degree of carburization cannot be measured. Carburization makes normally non-magnetic forged (rolled) and cast heat-resistant alloys become magnetic. For many years this magnetic permeability has been used to measure the degree of carburization. The measuring equipment used ranges from hand-held magnets to the most technologically advanced multi-frequency eddy current meters today. Although the eddy current meter can help determine not only the carburization rate but also the carburization pattern, due to the variability discussed previously, caution must be taken when making conclusions. There are other benefits to conducting such an investigation for radiant heat zone coils. Carburization patterns can reveal patterns of uneven combustion that might otherwise go undetected. They also facilitate different metallurgical treatments of furnace tube alloys. For example, for multi-tube coils and steam inlet pipes, low-cost, low-nickel improved HP alloy can be used, and the steam outlet pipe can use higher-cost but more durable 35Cr-45Ni alloy.
Metal powdering
Metal dusting is considered a common form of carburization in some refining and petrochemical production processes. Although there are some similarities in their properties, it differs from traditional carburizing in many ways. It loses metal at an extremely fast rate, while normal carburizing does not lose metal; the depth of carburization before metal loss is usually shallow, but very intense. It is often associated with carbon monoxide-rich CO/CO2/H2 gas streams associated with reforming, synthesis gas, partial oxygen or other production processes. Metal powdering usually occurs in the temperature range of 900~1600 (482~871°C), and the reaction rate is highest at 1300~1350 (704~832°C). This reaction can be very rapid, and the consequences can be disastrous. The damage it causes is in the form of round pits with a powdery surface. Metal dusting is simply unpredictable and most stainless steels and heat-resistant alloys will be attacked. Hot aluminizing using gas diffusion is effective and can be controlled when adding steam, sulfur or ammonia. However, the most positive solution is to reduce the CO partial pressure and adjust the gas composition.
Nitriding
Nitriding is very similar to carburizing and occurs when chromium and other elements combine with nitrogen to form brittle nitrides. Nitriding usually occurs when carbon steel, low carbon steel, low alloy steel and stainless steel are in a high temperature nitrogen-containing environment. As in the case of carburizing, high-nickel alloys are highly resistant to nitriding attack due to the low solubility of nitrogen in nickel. Alloy 600, which contains 72% nickel, is often used in the heat treatment industry and is occasionally used in the oil refining and petrochemical industries at temperatures above 650°C (343°C) and when it contains ammonia. Compared with Alloy 800H and cast improved HP, Alloy 600 has very few applications in the refining and petrochemical industries due to economic factors and lower strength. These alloys are not immune to corrosion, but they are more cost-effective. Although nitrogen molecules are very stable, sufficient dissociation occurs at the combustion temperature, allowing nitriding of 800H or HP type furnace tubes at metal temperatures of 1850 (1010°C) or higher.