Comparative Galling Resistance of Engineering Alloys

When considering the use of fasteners made from materials engineered for extreme environments in safety critical applications the issue of ‘pick up/lock up’ or galling is an important consideration. Stainless steels typically suffer from this phenomenon, which is related to the nature of their oxide layer.

Historically the standard test (ASTM G98) used two plain discs specimens for one-off tests to determine a Threshold Galling Stress to characterise the performance of a material combination. This method has been revised recently to adopt annular test specimens (Figure 1) with a central alignment pin to improve test alignment and repeatability (Figure 2) together with adopting a replicate test method (to recognise that this type of failure is a statistical process). The test determines a Galling50 (G50) value in order to characterise the relative resistance of materials.

The test method was originally developed and validated with stainless steels as shown in Figure 3, but NCT has asked to apply this test method to assess the relative performance of other engineering alloys (such as super duplex stainless steels as well as copper and nickel alloys). These materials are more resistant to this type of failure, so 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.

If you would like to use or find about more about this test technique please call or email Paul Tweedale on 01925 843476 or This email address is being protected from spambots. You need JavaScript enabled to view it..