A New Filler Metal for Grade 91 and 92 Joints
Over the past 10 years, the need for a new filler metal for DMWs has become even more pressing as new plants have been designed for higher efficiency and as advanced alloys, such as the higher-strength ferritic/martensitic Grade 91 and 92 alloys, have been developed for higher temperature/pressure operation. (See "Why new U.S. supercritical units should consider T/P92 piping" in POWER, April 2006.) Grade 91 is specially modified and heat-treated steel with 9% chromium, 1% molybdenum, and is vanadium enhanced. Grade 92 is similar to Grade 91, except that some of the molybdenum has been replaced with tungsten, resulting in even higher creep strength. These alloys have been the materials of choice for piping, tubing, and header retrofits and new installations for many cogeneration activities. They offer several advantages over conventional Cr-Mo steels in that they are often less expensive to install because their higher strength allows for lower material tonnage and fewer overall welding requirements due to the thinner cross sections required.
With the increasing use of Grade 91/92 steel, EPRI took another look at HFS6 to see if it could be reconstituted to avoid microfissuring and provide a new filler metal for weld joints between Grade 91/92 pipes and tubes and low-alloy ferritic or austenitic pipes and tubes.
Given the promising nature of the original filler metal, the research team used that metal’s composition as a starting point. More than 55 different chemical compositions of filler metals were manufactured and evaluated for microfissuring tendencies. The filler metals were produced through controlled additions of 16 different elements: carbon, silicon, manganese, phosphorus, sulfur, chromium, molybdenum, iron, vanadium, tungsten, copper, aluminum, cobalt, niobium, tin, and nickel.
Modifications to the baseline alloy composition eventually yielded an alloy that is virtually microfissure-free in the area where the weld is deposited. Like its predecessor, HFS6, the new filler metal, named EPRI P87 (P87), avoids the damage mechanisms that lead to failures in conventional filler metals. The thermal expansion of P87 is closer to that of low-alloy ferritic base metals, such as Grades 22, 91, and 92, than to traditional Inconel 625 and 309 stainless steel filler metals. This means that, as tubing is heated and expands, there is less difference in expansion between the filler metal and the base metal on the ferritic side of the joint, and therefore less stress on the welds. Because it contains less chromium, P87 also eliminates carbide formation and carbon migration, which have historically been shown to be detrimental in traditional DMWs.
In addition, P87 offers several advantages related to how the welding process is done. Welding requires post-weld heat treatment (PWHT), which is a standard tempering procedure of applying heat following the welding process in order to toughen the weld metal and the base metal affected by the welding. Current construction codes require PWHT at different temperatures for the hardenable ferritic materials, Grade 22 and Grade 91/92 steels. However, when two different steels are joined, the PWHT must be performed using the higher temperature of the two materials. If the lower-alloyed materials are heated to too high a temperature, it can weaken the base metal affected by the welding, and failures can occur.
Many studies have also shown that, at low stress levels (where piping and tubing normally operate), Grade 91 and 92 weldments will fail in the so-called Type IV location, which is an area of the base metal affected by the heat of welding. Research conducted by EPRI shows that P87 can be used, prior to making the final joint, to "butter the base metals," or to add metal to the end of the tube or pipe and thereby provide a protective buffer, allowing separate PWHT of each alloy at the optimum temperature. Once this step is performed, the final weld may then be made without PWHT.
The EPRI filler metal also allows this separate PWHT to be done at the factory, on many components at a time, rather than at the plant site, joint by joint. This capability can avoid the need for additional bracing that may be required during field PWHT to prevent distorting piping and can significantly reduce the time allotted for PWHT, thereby shortening the construction schedule.
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