Measuring the Residual Ferrite Content of

The ferrite content of mg-sized specimens can be determined using a vibrating sample magnetometer ABSTRACT. Electron beam welds, laser beam welds and rapidly solidified stainless steel alloys have small physical dimensions and very fine microstructures. These char-acteristics prevent accurate measure-ments of the residual ferrite by conven-tional techniques. To overcome these dif-ficulties, a new technique for measuring ferrite is applicable to specimens that weigh as little as 5 mg and are less than 0.5 mm (0.02 in.) thick. This technique uses a vibrating sample magnetometer to mea-sure the magnetic properties of a small specimen removed from the weld. This measurement can then be used to calcu-late the residual ferrite content of the specimen if the saturation magnetization of the ferrite is known. The saturation magnetization of the ferrite depends on its Fe content, and a method was developed (using thermodynamically created dia-grams) to predict the Fe content of there-sidual ferrite from the nominal alloy com-position. The saturation magnetization of the residual ferrite was further determined as a function of its Fe content by measur-ing the magnetic properties of fully ferritic stainless steel specimens containing be-tween 50 and 80 wt- % Fe. This allowed the vibrating sample magnetometer to be "calibrated " for a wide range of stainless steel alloy compositions. The results of this study were used to show that the Magne-Gage (and other ferrite-measurement de· vices) must incorporate the saturation magnetization of ferrite in order to accu-rately determine the residual ferrite con-tent of stainless steel alloys. Without this correction, a magnetic measurement of residual ferrite may be in error by as much as 25 % from the true value. /. W. ELMER is the joining Croup Leader