The Use of Vanadium Modified Materials for Reactor Fabrication

Abstract

Vanadium modified 2 1/4Cr-1Mo and 3Cr-1Mo alloys used for the fabrication of hydroprocessing reactors offer a number of important advantages over the corresponding conventional alloys. These include increased resistance to hydrogen attack, a lower susceptibility to temper embrittlement, increased resistance to weld overlay disbonding and higher strength resulting in thinner and lighter reactors. Since the first vanadium modified 3Cr-1Mo reactors first went into service in the early 1990’s, vanadium modified alloys have gained acceptance and today more than one hundred and forty vanadium modified reactors and pressure vessels have been placed in service and are operating in severe process environments. Despite the excellent benefits of these materials, they also exhibit less desirable characteristics such as reduced weldability, higher hardnesses in the base metal, weld metal and heat affected zones and the need for higher post weld heat treatment (PWHT) temperatures. Additionally, these materials have a reduced notch toughness at lower temperatures especially in the as welded condition and require intermediate stress relieving (ISR) in lieu of dehydrogenation treatment (DHT) in restrained and highly stressed joints such as nozzle to shell and head welds. These materials also require extra care and effort to be taken during fabrication. The paper presents a serious weld metal cracking problem that occurred with vanadium modified materials during the installation of a nozzle in a restrained and highly stressed weld when only DHT was performed instead of the more beneficial ISR. This fabrication problem is provided as a typical example of problems that can occur during fabrication with vanadium modified materials, and points out that additional care must be taken during fabrication when using these materials. The paper identifies the main causes for the cracking using information based upon mechanical, metallurgical and stress analyses and suggests steps that may be taken to circumvent similar reoccurrences.