Chill specimen structure tells iron properties; part 3 of 3.

Talk to any gray iron melter and he will agree that taking chill tests every quarter or half hour is as important to his furnace's production as taking a position fix is to a ship's captain. Both the chill test and the position fix act as guides. Like the position fix tells the captain exactly where his ship is sailing, the chill test tells the metallurgist the precise condition of his melt .

Assuming that wedge or chill tests are run according to ASTM A 367-60 sampling preparation specifications, it is important to pour all samples at consistent pouring temperatures because the amount of chill is governed by the pouring temperature. it is also important to pour test samples from at least a five pound, graphite-free refractory-lined ladle to avoid excessive iron cooling. Chill Measurement

Wedge test molds may range up to 1.25 x 2 x 6 in. and chill test molds up to 13/16 x 2.5 x 5 in.; the higher the iron's carbon equivalent, the smaller the wedge or chill specimen. The test specimen, must be broken so the fracture is straight and through the long axis of the wedge, making it possible to "read" the white-to-dark gray shades in the fracture. The area from the apex of the wedge to the end of the white area is designated as "clear chill." From the end of the clear chill to the area where the last spot of white iron is visible is called the "mottled zone." The "total chill" is measured from the junction of the gray fracture to the first indication of chilled iron.

Figure 1 illustrates what an experienced iron melter can interpret from a properly executed chill test. The drawings simulate what the polished fracture surfaces of wedge and chill test specimens would reveal if viewed under a microscope.

At the top of each sample, Type A graphite flakes are present. This type of graphite will normally precipitate during solidification if equilibrium conditions, established by a slow rate of solidification, take place. This type of graphite structure is the ideal that melters strive for through inoculation. Under ideal conditions (rare in a commercial environment) in a perfectly designed casting in which the rate of solidification is very slow, inoculation would theoretically be unnecessary. Therefore, inoculation is required to provide sufficient nuclei to causea 4.3% carbon equivalent gray iron to solidify more rapidly to achieve optimal mechanical properties when cast. Undercooling

As the test specimens taper down in size, some undercooling takes place and disrupts the equilibrium conditions of solidification as evidenced by the presence of Type B, or rosette, graphite. This undercooling causes a drop in tensile strength.

As the rate of solidification increases, a greater degree of undercooling occurs. Proper inoculation will minimize the formation of B and D graphites and will result in castings having more uniform properties as well as improved machinability.

In the bottom or white portions of the test specimens shown in Fig. 1, the solidification rate is so rapid there is insufficient time for carbon to precipitate as graphite. This causes the formation of a form of iron carbide known as cementite. Although it has a tensile strength of approximately 40,000 psi, it is highly undesirable because it makes brittle castings that are almost impossible to machine. A skilled melter can gauge from the degree of chill in a chill specimen just how much inoculation is necessary to eliminate cementite in a final gray iron casting.

Figure 2 schematically illustrates the structural changes that take place during undercooling. The test is of a specimen of 4.5% carbon equivalent, post-inoculated ductile iron. It signifies serious undercooling as the rate of solidification rises.

Chill testing is an important step in the microstructural control of cast iron. It helps determine the need for and the amount of inoculation necessary. it is an equally important tool for both gray and ductile irons.