Metallographic Characterization of an NBS Spectrometric Low-Alloy Steel Standard

The spectrometric standard steel designated NBS Low-Alloy Steel 461 was investigated by means of electron probe microanalysis and quantitative metallographic techniques employing a digital computer. Electron probe microanalysis showed the steel to be homogeneous in nickel and iron at two to four microns of spatial resolution. The average of all determinations agreed with the certified values for these elements. Inclusions in the steel were identified, classified as to size and shape, and counted. Mean free path data on the inclusions were calculated. The ASTM ferrite grain size number was deduced as 13.5 for the steel in the unetched condition. From the mean free paths in ferrite and pearlite, it was found that the steel is structurally homogeneous at a five micron level. It is concluded that the homogeneity level corresponds closely to the grain size of the material. It is further concluded that NBS-461 steel is sufficiently homogeneous that any present microanalytical technique can be carried out with little chance of inaccuracy duo to inhomogeneity.