Thursday, October 11, 2007

61st Annual Technical Meeting - Indian Institute of Metals (IIM)

The Indian Institute of Metals (IIM), which is the largest professional organization for metallurgists in India with over 8000 members from R&D laboratories, academia and industries, is organizing a technical meet at Nehru Centre, Mumbai during November 13-16, 2007. This Annual Technical Meet is being organized under the Chairmanship of Dr. S. Banerjee, Director, Bhabha Atomic Research Centre (BARC), who is the current President of the IIM. On this occasion, there will be a special publication on “Stainless Steels”.


Mr. V.K. Bafna and Mr. Paresh Haribhakti of TCR Engineering Services, India are presenting a paper on, “Metallurgical Investigation of blister formation on Hydrocracker Reactors of Refinery”  at this technical meet.


The Hydrocracker Reactors represents amongst the critical component of the Refinery and Petrochemical industries both in terms of cost and operation point of view. The Reactor vessel used for the hydrogenation service are made from of low alloy steels having chromium and molybdenum being typical weld overlaid with stainless steels on the inner surface of the vessel in order to protect the low alloy steel against corrosive environments such H2S and other corrosion.


One of the Refineries in the Middle East country faced the problem of blister formation on the Six Hydrocracker Reactors. There were total 102 blisters  formed inside surface of the Reactor and were formed all over the reactors surface having the small bulging with diameter of about 1" to 3".


The Hydrocracker operates at 380-445OC temperature and 112-158 bars pressure during service. The feed for the Reactors are VGO residue and hydrogen which is received from hydrogen plant.


The Metallurgical root cause investigation was undertaken both at site with visual, in-situ Metallography, hardness and ultrasonic testing. Cut samples from the blister were removed and studied for detailed metallurgical analysis in the laboratory to find out the reason as well fitness for future service of the weld overlay.


The results of filed Metallography showed presence of martensite layer at the interface with Inter-granular cracking a typical signature of hydrogen de-bonding. The laboratory testing further establish the presence of hydrogen in the weld overlay samples with the help of simple tensile testing on both as received condition and after hydrogen removal heat treatment. For future fitness of these weld overlays extensive Metallography studies were undertaken to find out the mechanism of crack formation and role of harmful phases like sigma and carbides.   With the use of quantitative Metallography the % age of these phases were evaluated to judge the overall integrity of Reactors and valuable inputs were provided for repair weld of these Reactors.


The reason for de-bonding was identified due to accumulation of hydrogen at the interface between alloy steel and stainless steel weld overlay region. Under the operating temperature and pressure hydrogen diffusion through weld overlay is inevitable. Nevertheless, slow cooling of the Reactor would facilitate hydrogen removal during shutdown. While on other hand, fast cooling will retain diffused hydrogen. Also, it can cause de-bonding between base metal and weld overlay.   This was confirmed at the Refinery was attacked in the war and shutdown was not done as per the recommended practice. The trapped hydrogen in the reactor over the time period it migrated and got accumulated at the interface and created de-bonding.