The malleable irons are a family of cast alloys — consisting primarily of iron, carbon, and silicon — which are cast as hard, brittle white iron and then rendered tough and ductile through a controlled heat conversion process. Because of their unique metallurgical structure, they possess a wide range of desirable engineering properties including strength, toughness, ductility, resistance to corrosion, machinability, and castability.
Three principal types of malleable iron are in wide use in this country: ferritic, pearlitic, and alloy malleable iron. A fourth type, called cupola malleable because of the method of manufacture, is also produced but only in small tonnages. Most important from the standpoint of production volume and use is standard malleable iron, which has a ferrite matrix. Pearlitic malleable, which, as the name implies, has a pearlitic matrix, is being produced in ever-increasing quantities. Alloy malleable iron is basically a specialty type iron with higher strength and corrosion resistance, finding primary use in railroad parts.
By far the largest tonnage of malleable castings normally is consumed by the automotive industry. The railroad, agricultural implement, electrical line hardware, pipe fittings, and detachable chain industries, and most other basic industries use standard and pearlitic malleable castings.
Advantages
Important attributes of malleable iron can be summarized as follows:
1. Malleable iron can be produced with a high yield strength, which is the static mechanical property upon which most mechanical design is based.
2. Ferritic and pearlitic malleable irons have a high ratio of yield strength to tensile strength. This means that the engineer can design to high applied strength values in service for materials of construction, concomitant with good machinability and low production cost for the final part.
3. Pearlitic malleable irons can be produced to a wide range of mechanical properties through carefully controlled heat treatments.
4. Malleable irons have a high modulus of elasticity and a low coefficient of thermal expansion, compared with the nonferrous metals.
5. Malleable and pearlitic malleable irons exhibit a low nil ductility transition temperature for brittle fracture.
6. Compared with steel, malleable and pearlitic malleable irons have considerably better damping capacity, which makes operation of moving components less prone to noise because of resonant vibration.
7. Pearlitic malleable irons exhibit good wear resistance and can be selectively hardened by flame, induction, or the carbonitriding process.
8. Pearlitic malleable irons will take a high-quality finish. Honed surfaces with 2 to 3 |in. finish at hardness values of 197 to 207 Bhn have been reported.
9. The uniformity of properties from surface to center is excellent, particularly in oil-quenched and tempered pearlitic malleable iron.
10. All malleable irons are substantially free from residual stresses as a result of long heat treatments at high temperatures.
11. Pearlitic malleable iron provides the properties of medium to high carbon steel coupled with a machinability rating unequaled for a material of similar hardness.
With respect to mechanical properties, minimum specification values are generally exceeded by a comfortable margin in the better-controlled malleable foundries.
Brinell hardness of ferritic malleable irons varies from about 110 to 145. Pearlitic malleable and alloyed malleable iron grades have higher values, ranging usually between 160 and 280 Bhn. Both hardness and tensile strength increase with combined carbon content.
Since the final properties of malleable iron castings are the result of thermal treatments, section thickness has no appreciable effect on strength. Therefore, mechanical properties will be essentially the same throughout the entire cross section.
Manufacture
The manufacture of malleable iron castings is fundamentally a two-phase operation. Phase one consists of producing the white iron castings, and the second phase involves the controlled heat treatment of these castings to obtain the desired finished product.
Structurally, malleable iron castings consist essentially of carbon-free iron (ferrite) and uniformly dispersed nodules of temper carbon. This combination of soft, ductile ferrite and nodular temper carbon accounts for the desirable mechanical properties of malleable iron. In pearlitic malleable iron the matrix is essentially pearlitic, resembling that of a medium-carbon steel.
Properties
Effect of Temperature
Studies of the behavior of ferritic malleable iron at both high and lower temperatures demonstrate, in general, that this material is well suited to applications in a temperature range from -51 to 649°C.
Low-temperature investigations have been concerned primarily with impact resistance and notch sensitivity; high-temperature studies have focused principally on tensile strength, yield point, elongation, stress rupture, and creep behavior.
Results of research have indicated a high level of performance at elevated temperatures, equal or superior to other ferritic materials for which data are available, particularly at 427°C. Strength at 538°C is adequate for many applications and strength is retained even at 649°C. No evidence was found in any of the investigations of changes in structure or performance during the test periods, which extended from 1 to over 2000 hr.
Surface Hardness
Many structural parts require high surface hardness backed up by a strong, tough core. In steel components this can be accomplished by car-burizing or nitriding after machining, followed by a suitable heat treatment. In pearlitic malleable iron the combined carbon content is adequate for production of high surface hardnesses through quenching after either induction or flame heating. Many parts are preferentially hardened on wearing surfaces.
