Session: CS-07-04 Recent Developments in ASME Codes and Standards-4

Paper Number: 123202

123202 - Mastering Critical Factors Affecting Toughness Degradation Is Key to Effective Use of Duplex Stainless Steel in Pressure Vessels 

The great number of built pressure vessels using duplex (austenitic-ferritic) stainless steel alloys clearly demonstrate their position as the preferred alloy type for many branches of the process industry. The group of duplex stainless steel (DSS) alloys offers very high corrosion resistance which most often eliminate both corrosion allowance and protective coatings. In addition, their high allowable design stresses can reduce wall thicknesses which altogether lowering the material consumption. Moreover, high durability minimizes maintenance and repair needs and calls for long and reliable service life, and hence low life cycle cost.

The most used DSS alloys in pressure vessels are S31803/S32205, S32304 and S32750. These alloys can be designed according to both ASME Section VIII Division 1 and Division 2, where the latter allowing higher design stresses which typically can be economical to use when the internal pressure or hydrostatic pressure is high. Whereas Division 1 for many cases show satisfactory results and offers a straightforward design route. Over the last 20 years leaner (low nickel) alloyed DSSs, e.g., S32101, have been developed to offer a corrosion resistance comparable to type 304 and 316. The main objective is to utilize the high strength advantage with DSS but without over-specifying in less critical environment. However, these leaner DSS alloys are only qualified for Division 1 design.

Would it then be motivated to approve Division 2 also for these leaner DSS alloys which could enable further structural optimization and hence improve cost-effectiveness?

This paper will elaborate on this proposal starting from the limiting design criteria for DSS alloys:  First, by considering the maximum operating temperature for a vessel avoiding critical level of embrittling phases, and secondly, by considering the transition from ductile to brittle behavior at lower temperatures. These limiting factors are addressed in the ASME code, but arguably somewhat limited, and significantly different compared to the European pressure vessel design standard EN 13445. This toughness degradation at both high and low temperatures is most critical for the fabricated vessel, e.g., for shaped and welded components, especially for the leaner low nickel containing DSS alloys at low temperatures, and for the higher alloyed DSS grades at elevated temperatures.

A review based on predominantly European research projects will be discussed considering the consequences of both low and high temperature exposure for both parent material and weldment of DSS alloys. Fracture mechanics and Charpy impact test investigations for assessing, correlating, and validating transition temperatures are covered. In addition, heat treatment experiments at different temperatures and soaking times to simulate elevated temperature use, where the degree of embrittlement is assessed with Charpy impact tests. The collected findings are compared with the existing guidelines in the European pressure vessel standard and the ASME standard, and an attempt is made to propose the applicability range for sound performance of different types of DSS alloys.

Concluding remarks to the proposition of having the leaner DSS alloys qualified according to ASME Section VIII Division 2 will be presented with a brief view on the market needs for such vessel design.

Presenting Author: Claes Tigerstrand Outokumpu Stainless AB

Presenting Author Biography: Mr. Tigerstrand holds a master’s degree in mechanical engineering and 25 years of experience in technical marketing of stainless steels, particularly related to duplex and structural engineering. He has worked in several different managerial positions for Outokumpu covering R&D, technical & quality, and sales & marketing.

Standards development has always been an important part of the work and Mr. Tigerstrand has been active in both pressure vessel and civil engineering related standards committee work in Europe and in a supporting role for standards development work in United States.

Currently Mr. Tigerstrand is holding a position as global technical manager heavy industry for business line Advanced Materials. With special focus on the process industry and storage tanks and pressure vessel applications. He is based in Avesta, Sweden.

Authors:

Claes Tigerstrand Outokumpu Stainless AB
Johan Pilhagen Outokumpu Stainless AB
Jan Y Jonsson Outokumpu Stainless AB