GRAlN STRUCTURE TYPES
The measurement of grain size, whether by the chart comparison method or by manual or automated measurement methods, is complicated by the different types of grain structures encountered and by the etched appearance of the grains. For example, as shown in Figure A, we may have ferrite grains in a non- heat treated or non-hardenable body-centered cubic (bcc) metal or alloy. These do not contain annealing twins, but could contain deformation twins, and second-phase constituents may be present. The example shown is ferrite in a low-carbon sheet steel; carbides are present. This specimen was etched with nital and not all of the grain boundaries are visible; those that are visible are variable in darkness and width. These factors are a minor nuisance for manual rating and a significant problem for automatic rating.
Figure B depicts a single phase austenitic alloy that contains annealing twins. Like the previous micrograph, it shows the boundaries as dark lines, a so-called "flat etch." The austenitic alloy shown, L605, illustrates a common problem with such alloys, they are very difficult to etch so that all of the grain boundaries are visible. This makes it very difficult to measure the grain size with a high degree of precision. Also, when rating grain size the twin boundaries must be ignored, which is not easy, especially by image analysis. Not all austenitic alloys will exhibit annealing twins, aluminum alloys rarely are twinned.
Austenitic alloys may also be etched with reagents that produce grain contrast or color variations as a function of their crystallographic orientation. Figure C shows the twinned austenitic grain structure of cartridge brass that was etched producing grains with different contrast in black and white. Note that unlike the flat etched L605 specimen, all of the grains are revealed. This structure is easy to rate by the comparison method if the grain size chart depicts grains etched in the same manner. This condition is virtually impossible to measure by automatic image analysis, however. Again, twins are present but the coloration or contrast varies within the grains.
To measure twinned austenitic grain structures by image analysis, we need to either suppress the etching of twins or be able to identify and ignore them. At the same time, all of the grain boundaries must be revealed and be identifiable. The best solution is to use an etchant that reveals only the grain boundaries. To illustrate this, the next micrograph, Figure D, shows AlSl 316L stainless steel electrolytically etched with 60% nitric acid in water (Pt cathode, 0.8 V dc, 45 s). The grain boundaries are almost completely revealed but no twins are visible. The accompanying micrograph, Figure E, shows a tint etched view of this specimen at the same magnification where the twins are visible.
In dealing with carbon and alloy steels, the steelmaker generally performs a test known as the McQuaid- Ehn test, to determine if the steel is inherently fine grained. A specimen is carburized at 1700"F for 8 h and furnace cooled. The excess carbon in the carburized case precipitates during cooling as cementite in the austenite grain boundaries present at the end of the carburizing cycle. The specimen is cut and polished so that the case structure is revealed. Generally, nital is used as the etchant and a comparison chart rating is made where the Test Methods E 112 chart exhibits the same contrast. Such a structure, Figure F, is not very good if actual measurements are made, especially if image analysis is employed. The alternative is to darken the grain boundary cementite films. A number of etchants will darken cementite, the one used here, Figure G, was Beraha's sodium molybdate tint etch but the familiar alkaline sodium picrate etch works well also. Etched in this way, the grain structure shows up much more clearly and image analysis could be used.
Once an alloy steel part is heat treated, only etching can be used to try to reveal the prior-austenite grain boundaries, that is, the austenite boundaries present when the part was soaked at the austenitizing temperature. While many etchants have been developed for this purpose, such work is fraught with difficulty. One of the most successful prior-austenite grain boundary etchants is a saturated aqueous solution of picric acid containing a wetting agent, several of which have been used. This etch is sensitive to phosphorus segregated to the prior-austenite grain boundaries and will not work otherwise. Figure H illustrates a fairly successful effort with a quenched and tempered experimental alloy steel. This type of etch rarely, if ever, yields an etch quality adequate for image analysis and is usually accompanied by substantial pitting.