When considering the use of valve components manufactured from high-performance alloys for demanding applications, the issue of “lock up” or galling is an important consideration. Stainless steels typically suffer from this phenomenon, related to the nature of their oxide layer.
The standard galling resistance test (ASTM G98) is prone to variability, associated with misalignment of test pieces and non-uniform forces applied across the test piece surface. The test method has subsequently been revised to overcome these concerns, using newly designed annular test specimens. The test method was originally developed for, and validated with, stainless steels. However, the performance of super duplex stainless steels and other high-strength alloys (including copper and nickel alloys) has been tested. As these alloys are more resistant to galling failure, the equipment used to perform the tests has had to be uprated in order to accommodate their increased load carrying and reduced tendency to galling.
The test determines a Galling50 (G50) value in order to characterise the relative resistance of materials. This statistical approach considers the load at which a greater than 50% chance of galling occurring is predicted. An overview of the revised test protocol is provided, together with a review of its merits against alternative test methods. Further insight into the failure mechanisms can be seen from the real-time reporting of sample loads and torque during the test process. Very few other references to this type of testing exist, reflecting the relatively expensive and time-consuming programme needed to generate a statically-robust body of data. However, the inherit variability of the galling phenomenon requires this level of rigour to generate useful results.
Various combinations of alloys have been tested, including Ferralium 255-SD50 super duplex stainless steel, either as part of a self-couple (against itself) or in combination with dissimilar alloys. It has been demonstrated that super duplex stainless steels provide superior resistance to galling when used in combination, partly explaining their widespread use in a variety of valve trim applications, particularly for the most demanding of applications. A selection of customer applications are showcased, demonstrating the selection criteria for materials in demanding applications.
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