The basic practice involves mounting and polishing sections and then etching with a suitable acid, sometimes aided by a small amount of electrical current. He showed that the optimum space-filling grain shape, with a minimum surface area and surface tension, is a polyhedron known as a tetrakaidecahedron, which has 14 faces, 24 corners, and 36 edges. Grain size usually is specified by a product engineer and is based on the application and wall thickness. To achieve a balance between the grain size and mechanical properties, proper process planning is critical. In the vast majority of cases, we merely determine the mean value of the planar grain size, rather than the distribution.
The most difficult process here is the etching procedure needed to reveal these prior boundaries. It describes the methods of revealing grain boundaries and of estimating the mean grain size of specimens with unimodal size distribution. In manual methods, it is essential to recommend the most efficient method for any measurement. In many cases, it is required that G be 5 or greater i. In this case, it may be possible to measure the low-carbon lath martensite packet size, which is a function of the prior-austenite grain size. For example, in body-centered cubic metals, such as Fe, Mo, and Cr, we have ferrite grains; in face-centered cubic metals, such as Al, Ni, Cu, and certain stainless steels, we have austenite grains. Again in metallurgy the particle size is the size of single particles, either spheres, spheroids or irregular in shape.
Note that these methods are applied on the polished surface of the specimen, that is, on a plane that cuts through the three-dimensional grains. Complications---Different Measures of Size Another complicating factor is the different measures of grain size. Complications--Grain Characteristics Grain size measurement is complicated by a number of factors. Certain elements, such as titanium and niobium, inhibit grain growth in stainless steels. There are other situations where Test Methods E 112 is not helpful and other standards have been developed.
The 1920 revision of Methods E 2 added details on performing the Jeffries planimetric measurement method. This consisted of two charts, one for twinned alloys, the other for non-twinned alloys; both charts had 17 pictures with grain sizes from 2 to 10. The higher the severity of cold work, the more time is taken to dissolve the grains into the solution and the less time is available for grain growth. Poor grain size control can cause surface defects from slight to severe. First, the three-dimensional size of the grains is not constant and the sectioning plane will cut through the grains at random.
However, it must be recognized that we are sampling grains with a range of sizes and shapes. The annealing temperature must be dropped at each cold work and anneal cycle, but not by a drastic amount. Proper process development requires a balanced approach. Harder took over this special subcommittee in 1936, with the idea of revising Classification E 19 and adding a non-carburizing method. Grain size is measured in transverse and longitudinal sections. They decided to adopt the McQuaid-Ehn carburizing test for evaluating the grain growth characteristics of steel, again with the aid of a comparison chart.
The 1930 revision of Methods E 2 witnessed the addition of Committee E-4's first standard chart, a grain size chart ten pictures for brass, i. The higher the annealing temperature, the faster the process of dissolving the cold-worked, elongated grains into solution and consequent grain growth. Therefore, the two key considerations in the process design are the amount of cold work and the annealing cycle temperature and time at temperature. Thus, these are planar rather than spatial measures of the grain size. However, for some alloys, because of their inherent structure of a mixture of small and large grains, automation is difficult to implement. The smaller drop in annealing temperature at each cycle minimizes the adverse impact on mechanical strength and hardness and thereby intermediate cold working capability.
Although grains are three-dimensional in shape, the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional plane, even in a sample with a perfectly consistent grain size. The simplest way to achieve and maintain small grain size is to use lower annealing temperatures while keeping it within the range allowed for the alloy. Cold work is achieved by either tensile force draw bench operations or compressive force pilger mill operations. A variety of planar grain size distribution methods have also been developed to estimate the number of grains per unit volume, N v, from which the average grain volume, V, can be calculated. Section C was chaired by Carl Samans American Optical Co. For stainless steels the initial annealing temperature may be about 1,950 degrees F, but toward the end of the process, a lower temperature, perhaps 1,850 degrees F, might be necessary to achieve the desired grain size.
Some of these alloys have an inherent mixed-grain structure, and control of the large grains requires close attention. Two basic approaches to measure grain size were being developed at that time. In some cases, such as heat clearance, grain size measurement is required. For low-carbon steel, the martensite forms in packets within the parent austenite grains. Although grains are three-dimensional in shape, the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional plane, even in a sample with a perfectly consistent grain size. Methods E 91 also had a short life, also being discontinued when Test Methods E 112 was adopted. Some of these requirements actually work against each other, making it extremely challenging to control them.
Hence, we must recognize the intent of the work being performed. Description This Standard specifies a micrographic method of determining apparent ferritic or austenitic grain size in steels. As the sidebar on grain structures demonstrates, it is easier to see the intergranular carbide phase if we use an etchant that darkens the grain boundary cementite. Although grains are three-dimensional in shape, the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional plane, even in a sample with a perfectly consistent grain size. For light walls, around 0.