HF-modified austenitic stainless steel cast piping has been in use in hydroprocessing reactor effluent high pressure loop for 35 years. Recent process modifications resulted in replacement of piping in a hydrocracking unit which was in service for 25 years. This paper reports on mechanical and metallurgical investigations of the replaced piping as well as samples removed from newer vintage HF-modified piping. The investigations show susceptibility to sensitization of HF-modified materials when exposed to the hydroprocessing operating temperature range. Loss of room temperature impact properties was also observed. Realistic flow tolerance of the HF-modified material was established by CTOD tests and a relationship is proposed between the CTOD values and Charpy V-notch values. The paper also describes three case histories where the failures resulted from the polythionic acid attack.

Subject
Materials, Piping, Carbides, Piping systems, Photomicrographs, Inspection, Flanges, Hydroprocessing, Intergranular cracking, Interfaces, Corrosion resistance, Ferrite, Austenite
Keywords:
Hydroprocessing, sensitization, ferrite, austenite, stress corrosion cracking, welding
© 1995 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1995
Association for Materials Protection and Performance (AMPP)
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