Alloy Steel Fittings

Reading the Chrome-Moly Grades

Chrome-moly grades are named by their chromium and molybdenum content, and the grade choice is driven by design temperature and by hydrogen partial pressure. WP11 is 1-1/4Cr-1/2Mo and is the most widely used grade, covering refinery piping up to about 550 degrees Celsius. WP22 is 2-1/4Cr-1Mo, with higher creep strength at elevated temperature and better resistance to hydrogen attack, used in hydrocracker and hydrotreater circuits. WP5 is 5Cr-1/2Mo and WP9 is 9Cr-1Mo, both used in higher temperature and more corrosive refinery service. WP91, which is 9Cr-1Mo-V, has substantially higher creep strength than WP22 and allows thinner walls at the same temperature and pressure, which is why it dominates modern high energy piping in power plants. The addition of vanadium and niobium in WP91 is what provides the improved creep performance, and it also makes the material more sensitive to welding heat input.

Why PMI Matters More Here

Alloy steel fittings look and weigh almost the same as carbon steel fittings, and a grade substitution between WP11 and carbon steel WPB, or between WP22 and WP11, would not be visible on the rack. The consequence, however, is serious: an incorrect grade in a high temperature hydrogen line can fail by hydrogen attack or by creep rupture at temperatures well below the design condition, and the failure can be catastrophic. For this reason, positive material identification is not optional on chrome-moly fittings. X-ray fluorescence or optical emission spectrometry is used to measure the chromium and molybdenum content of every fitting, and the readings are recorded on the mill certificate. Some specifications also require hardness testing and, for thick sections, full heat treatment records. When ordering alloy steel fittings, state the requirement for PMI explicitly, and confirm the acceptance criteria for the measured chemistry.