danuarly 1948
Corrosion A new malleable austenitic cast iron and also recent developments in the Ni-Resist irons are described.
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bg Mars 6, Fontana
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I-RESISTcast irons are well known and widely used in the chemical industry because of their excellent corrosion resistance as compared to ordinary cast iron. A new development which resulted in a malleable cast iron, with essentially the chemical composition of Ni-Resist, should expand the use of this type of material. The malleable material shows an elongation of about 5% as compared to less than 1% for the nonmalleable material, and also higher strength for the former. This improvement in ductility, or decrease in brittleness, is a desirable property. This malleable iron was developed by the American Cast Iron Pipe Company. Most engineers or persons working with materials of construction are familiar with the common forms of cast ironnamely, gray cast iron, white cast iron, and malleable cast iron. Gray cast iron is the most common variety and it is called gray iron because of its characteristic gray fracture. Gray iron contains free carbon in the form of graphite flakes. White iron is often chilled in the mold or rapidly cooled, so that the fracture is white and none of the carbon is present as free graphite. Malleable iron is white iron that has been heated a t an elevated temperature to form free nodular graphite. The white iron is hard and very brittle. The gray iron is usually comparatively soft. and brittle. The malleable cast iron is soft and, as the name implies, quite ductile. The well known Ni-Resist is a gray iron. It is available in several grades and types, but all types contain a large percentage of nickel which makes the material austenitic or nonmagnetic (similar to 18-8 stainless steel in this respect). The composition range for Type 1 Ni-Resist is as follows: 3.0% max. carbon, 13.5 to 17.5% nickel, 1.75 to 2.5070 chromium, 5.5 to 7.5% copper, 1.0to 2.5% silicon, and 1.0 to 1.5% manganese. Some of the other grades are copper-free for applications wherein copper contamination is undesirable. The desired composition for the malleable austenitic iron is as follows: 2.3y0 carbon, 15.oY0 nickel, 3.0% chromium, 6,0% copper, 2.0% silicon, 1.25% manganese, 0.10% phosphorus, and 0.12% sulfur. This composition is a slight modification of the regular Type 1 Ni-Resist described above, but the malleable material is processed and heat-treated to
produce the desired malleable structure instead of the gray iron structure. In the manufacture of austenitic malleable iron bolts, the metal is cast into split metal molds which chill the metal and result in a white iron. These castings are then heated for 2 hours a t 1800" F. which decomposes most of the combined carbon to give the soft, tough, malleable structure consisting of globules of graphite in an austenitic matrix. The material in this condition is readily machinable. The austenitic malleable iron has a tensile strength of approximately 70,000 pounds per square inch, yield strength of approximately 50,000 pounds per square inch, elongation around 5oJ0, and a Brinell hardness of 180 to 200. These properties can be compared with a tensile strength of 40,000 pounds per square inch and less than 1% elongation for the usual sand cast gray material. TABLEI. CORROSION TESTSON MALLEABLE AND GRAY AUSTENITIC CASTIRONS Solution 1% Has04
0.5% HCI
:?$ E 10% HCI
(Room!temperature) Corrosion Rate, Mila per Year Gray Malleable 4 5 8 8 10 2 3 3 3 4 3 4 4 4 2 8 9 13
In order to study the comparative corrosion resistance of malleable austenitic iron and gray austenitic iron, cast rods of both materials were obtained and tested in the Corrosion Research Laboratory a t The Ohio State University by William C. Leslie and Elwood Mead. The results obtained are shown in Table I. The tests in sulfuric acid were run for three 48-
TABLE11. COMPOSITION RANGESOF NI-RESISTIRONS Name Total carbon Si1icon Manganese Nickel Copper Chromium Mean coef. of thermal expansion per F. (70-400'F.) X 10-6
Type la High Type 1 Strength 2 . 8 0 max. 3.00 max. 1.50-2.75 1 .OO-2.50 1.00-1.50 1 .OO-1.50 13.50-17.50 13.50-7.50 5.50-7.50 5.50-7.50 1.75-2.50 1.75-2.50
Type2 20 % Ntakel 3 . 0 0 max. 1.00-2.50 0 . SO-1.50 18.00-22.00 0.50 max. 1 ,75-2.50
Type2a High Strength 2.80 max. 1.50-2.75 0.80-1.50 18.00-22.00 0.50 max., 1.75-2.50
T e2b Eat Resistant 3.00 max. 1 .OO-2.50 0.80-1.50 18.00-22.00 0.50 max. 3,OO-6.00
T30% ype8 Heat Type & Stain 4 Nickel Resistant 2.75 max. 2.60 max. 1 ,oo-2.00 5 .OO-6 .OO 0.40-0.80 0.40-0.80 28.00-32.00 29.00-32.00 0.50 max. 0.50 max. 2.50-3.50 4.5-5.5
Type 5 Minovar 2 . 4 0 max. 1 .oo-2.oo 0.40-0.80 34.00-36.00
10.7
10.4
10.4
10.4
5.3
2.8
10.7
~
89 A
7.8
0.50 max. 0.10 max.
C o r r'osi o n hour periods. The tests in hydrochloric acid were run for one 48-hour period followed by one 120-hour period. The results listed are cumulative for the entire time of exposure. All tests are of the static immersion type. The results of these tests indicate no essential differences in the corrosion resistance of these two materials to sulfuric acid, sodium chloride, and dilute hydrochloric acid. I n the stronger hydrochloric acid, the gray austenitic iron shows some superiority. The corrosion rates for the gray iron may be a little low because the specimens were somewhat porous and accordingly the corrosion products could not all be removed (resulting in lolTer weight loss). However, for practical purposes it appears safe to assume that there is no essential difference in the corrosion resistance of gray and malleable austenitic irons. The important conclusion is that the malleableizing treatment results in higher strength and ductility with little or no loss in corrosion resistance. The more dense structure of the malleable material may have practical advantages. At present the chief use for austenitic malleable iron consists of bolts for pipe joints, but the characteristics of this material should result in many other applications, particularly in the chemical plants. I n some cases the malleable bolts have replaced bronze andstainless steel. Austenitic irons are widely used in the manufacture and/or processing of soaps, petroleum, caustic, paper, plastics, and rayon, and in other chemical industries. These materials are also considerably superior to ordinary cast iron in heat resistance or applications involving elevated temperatures. Not all types of castings would be amenable to chill casting and malleableizing. H i g h nickel Ni-Rssist irons A comparatively recent development in the austenitic cast irons involves increasing the nickel content from 15 to 30% and also to 35%. Type 3 Ni-Resist contains 30% nickel. This material has a lower coefficient of thermal expansion than the Type 1 Ni-Resist: 10.7 X 10-6 and 5.3 X 10-6 per O F., respectively. Because of this difference the 3Oy0 nickel iron is less susceptible to cracking because of thermal shock. I n addition, the increased nickel content results in better corrosion resistance to caustic solutions, sulfuric acid, and other chemicals. Type 3 Ni-Resist is used for filter press plates and frames, pumps, valves and fittings, and cast kettles. The Type 5 Ni-Resist, also known as Minovar, contains 35% nickel and i t has a thermal expansion coefficient of 2.8 x 10-6 per O F. This material is useful where changes in dimensions with temperature should be kept to a minimum, such as accurately machined assemblies including molds for plastics and glass. The Ni-Resist with best heat and corrosion resistance is the Type 4 which has the highest alloy content of all. Type 4 contains approximately 3070 nickel, 5% silicon, and 5% chromium. This alloy is used where a relatively cheap material and minimum contamination of product are desired. The Type 4 material can be used at higher temperatures and shows considerably better corrosion resistance to combustion products, particularly for high sulfur fuels, than the other grades of Ni-Resist. T o complete the picture from the composition standpoint, the compositions of the various grades of Ni-Resist are listed in Table 11. 90A
