Centrifugally cast HF-modified austenitic-ferritic stainless steel piping has been used in hydroprocessing reactor circuit for about 40 years. Over the years, there have been a few reported cases of cracking incidences. One resulted in a leak upon start up, after a planned shut down. The others were non-penetrating.
Room temperature Charpy V-notch impact values of this alloy are significantly reduced once the material is subjected to service conditions. Crack tip opening displacement (CTOD) tests were performed on service exposed, as well as unused HFmodified piping to study resistance to crack propagation. Base metal, weld metal, and heat affected zone were tested. Some samples were subjected to a hydrogen charging treatment prior to testing. The flaw tolerance of HF-modified piping materials was estimated with the aid of the PREFIS(1) software. The lower bound Charpy - CTOD correlation was used to compute limiting flaw size versus Charpy V-notch energy.
(1) Materials Properties Council, Joint Industry Project on Fitness-for-Service
The reactor circuit piping in hydroprocessing units is critical to their successful operation. Appropriate material selection is crucial because of the severe operating conditions that include high pressure hydrogen, hydrogen sulfide (H2S), chlorides, nitrogen compounds, and other corrosives at temperatures in the range of 600 to 8500F (315 to 4540C). These conditions dictate the use of stainless steels. Fabrication and long term metallurgical stability problems preclude the use of straight chromium stainless steels such as 400 series stainless, often leading to the use of Cr-Ni austenitic stainless steels.
A serious shortcoming of austenitic stainless steels, however, is their susceptibility to chloride and polythionic acid stress corrosion cracking.1 In addition, some grades of stainless steels, such as the commonly used TP321 and TP347, may suffer from welding related cracking in the thicker sections required for this service.
Clearly, alternate materials that are less susceptible to stress corrosion cracking and have better weldability in many conditions, at competitive cost, would be welcome. Available information in existence 40 years ago indicated that duplex austenitic-ferritic stainless steels merit consideration because of the beneficial influence of ferrite in an austenitic matrix on the properties of the Cr-Ni stainless steels.2
The important benefits include:
1. Increased resistance to sensitization.
2. Increased resistance to polythionic acid and chloride stress corrosion cracking.
3. Excellent weldability in new and aged conditions.
4. Higher allowable ASME B 31.3 Code design strength than competitive austenitic
stainless steels. This is particularly true for the newest variation of HF- modified alloy,
identified as A351 CE 20N.
It should be noted that duplex stainless steels lose a considerable portion of their Charpy V-notch toughness when exposed to temperatures above 6000F (3150C). 3,4 The degree of embrittlement relates to composition and ferrite content : the higher the ferrite content, the greater the embrittlement.
Piping made from the HF- modified alloy is not available in wrought form in large diameters and thick walls, commonly employed in hydroprocessing units. Forging difficulties also preclude manufacturing at an economic cost, the smaller diameter pipe with the desired low ferrite content. Because of these manufacturing difficulties, cast pipes and fittings are usually utilized.5 The usual method of production is by centrifugal casting, although some sta
