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ASM Metals Handbook Volume 01 - Properties and Selection Irons, Steels, and High-Performance Allo

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of ±0.005 mm (±0.0002 in.) in parts such as crankshaft bearings, camshaft bearings, and
cylinder bores. Therefore, if a casting is properly designed, is cast under controlled conditions, and is allowed to cool sufficiently
in the mold before shakeout, and if proper machining practice is followed, extremely high dimensional stability can be obtained
in many applications without stress relieving. On the other hand, it is frequently economical to stress relieve complex castings
(other than engine blocks) that are produced in small quantities and that must be machined to precise dimensions.
Effect of Shakeout Practice
Shakeout practice may be influential in establishing the patterns of residual stresses in castings, because most residual stresses
are basically caused by differences in cooling rate, and thus in contraction behavior, between light and heavy sections.
In addition to its effect on residual stress, shakeout practice may influence both microstructure and hardness of gray iron
castings. If the iron is austenitic at the time the mold is dumped, higher hardness and residual stress may result. Many different
microstructures may be obtained; the austenite in thin sections may transform in the mold, whereas in heavier sections, which
cool more slowly, transformation may be delayed until air cooling after shakeout. In general, the effect of shakeout practice on
hardness is negligible in unalloyed irons. Alloy irons are the most sensitive. However, the martensitic irons that contain enough
alloying elements to reach full hardness with slow cooling in the mold will display little if any effect of shakeout practice.
Alloying to Modify As-Cast Properties
The term alloying as used here does not include inoculation because by definition the effect of inoculation on the mechanical
properties of an iron is greater than can be explained by the change in chemical composition.
Strength and hardness, resistance to heat and oxidation, resistance to corrosion, electrical and magnetic characteristics, and
section sensitivity can be changed by alloying, which can extend the application of gray iron into fields where costlier materials
have traditionally been used.
There has also been considerable replacement of unalloyed gray iron by alloy iron to meet increased service demands and to
provide increased safety factors.
The use of alloy iron often depends, in practice, on the relative production requirements in a given foundry. When the
applications requiring alloy iron in a given plant are a small fraction of total production, manufacturing policy may dictate the use
of unalloyed iron for all castings in order to achieve maximum uniformity of production practice. Continuous production of 450
to 1350 kg (1000 to 3000 lb) heats of alloy iron is usually needed for economical utilization.
Manganese, chromium, nickel, vanadium, and copper can also be used to strengthen cast irons. In many irons a combination of
elements will provide the greatest increase in strength.
To develop resistance to the softening effect of heat and protect against oxidation, chromium is the most effective element. It
stabilizes iron carbide and therefore prevents the breakdown of carbide at elevated temperatures; 1% Cr gives adequate protection
against oxidation up to about 760 °C (1400 °F) in many applications. For temperatures above 760 °C (1400 °F), the chromium
content should be greater than 15% for long-term protection against oxidation. This percentage of chromium suppresses the
formation of graphite and makes the alloy solidity as white cast iron.
For corrosion resistance, chromium, copper, and silicon are effective. Additions of 0.2 to 1.0% Cr decrease the corrosion rate
in seawater and weak acids. The corrosion resistance of iron to dilute acetic, sulfuric, and hydrochloric acids and to acid mine
water can be increased by the addition of 0.25 to 1.0% Cu. For sulfur and acid corrosion, 15 to 30% Cr is effective. Silicon
additions in the range of 14 to 15% give excellent corrosion resistance to sulfuric, nitric, and formic acids; however, both
high-chromium and high-silicon irons are white, and the high-silicon irons are extremely brittle.
The electrical and magnetic properties of cast iron can be modified slightly by minor additions of alloying elements, but a
major change in characteristics can be accomplished by the use of approximately 15% Ni or of nickel plus copper, which results
in an austenitic iron that is virtually nonmagnetic. The austenitic gray irons also have good resistance to oxidation and growth at
temperatures up to about 800 °C (1500 °F).
Molybdenum is an effective alloying addition for retaining strength in heavy sections. It is normally added in amounts of 0.5 to
1.0%, but the low end of this range applies chiefly when molybdenum is added in combination with other elements. In the casting
in thin sections, nickel is the most effective in combating the tendency to form chilled iron.
Base Irons. The selection of alloying elements to modify as-cast properties in gray iron depends to a large extent on the
composition and method of manufacture of the base iron. For example, a foundry producing a base iron containing 2.3% Si and
3.4% total carbon for automotive castings might add 0.5 to 1.0% Cr if required to make heavier castings with the same hardness
ASM Handbook,Volume 1 Gray Iron 01 Sep 2005
Copyright ASM International. All Rights Reserved. Page 58
and strength as the normal castings. However, a foundry producing a base iron with 1.7% Si and 3.1% C for a heavy casting
would add 0.5 to 0.8% Si to decrease hardness and chill when pouring this iron in light castings.
Depending on the strength desired in the final iron, the carbon equivalent of the base iron may vary from approximately 4.4%
for weak irons to 3.0% for high-strength irons. The method of producing the base iron will affect mechanical properties and the
alloy additions to be made, because factors such as type and percentage of raw materials in the metal charge, amount of
superheat, and cooling rate of the iron after pouring all affect the properties. The base iron used for alloying will vary
considerably from foundry to foundry, as will the alloying elements selected to give the desired mechanical properties. However,
parts produced from different base irons and alloy additions can have the same properties and performance in service.
Heat Treatment
Gray iron, like steel, can be hardened by rapid cooling or quenching from a suitable elevated temperature. The quenched iron
may be tempered by reheating in the range from 150 to 650 °C (300 to 1200 °F) to increase toughness and relieve stresses. The
quenching medium may be water, oil, hot salt, or air, depending on composition and section size. Heating may be done in a
furnace for hardening throughout the cross section, or it may be localized as by induction or flame so that only the volume heated
above the transformation temperature is hardened. In the range of composition of the most commonly used unalloyed gray iron
castings, that is, about 1.8 to 2.5% Si and 3.0 to 3.5% TC, the transformation range is about 760 to 845 °C (1400 to 1550 °F). The
higher temperature must be exceeded in order to harden the iron during quenching. The proper temperature for hardening
depends primarily on silicon content, not carbon content; silicon raises the critical temperature.
During the heating of unalloyed gray iron for hardening, graphitization of the matrix frequently begins as the temperature
approaches 600 to 650 °C (1100 to 1200 °F) and may be entirely completed at a temperature of 730 to 760 °C (1350 to 1400 °F).
This latter range is used for maximum softening. The changes in combined carbon content and hardness that occur upon heating
and quenching of both alloyed and unalloyed gray iron are shown in Table 25 .
Table 25 Hardness of quenched samples of gray iron
Condition
Plain cast iron Cr-Ni-Mo