Page 77 - Shigley's Mechanical Engineering Design

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52 Mechanical Engineering Design
elements course, it is a good class project (many hands make light work) to study the
method and report to the class.
For low-alloy steels, the multiplication method of Grossman 11 and Field 12 is
explained in the Standard Handbook of Machine Design (Secs. 29.6 and 33.6).
Modern Steels and Their Properties Handbook explains how to predict the Jominy
curve by the method of Grossman and Field from a ladle analysis and grain size. 13
Bethlehem Steel has developed a circular plastic slide rule that is convenient to the purpose.

2–15 Alloy Steels

Although a plain carbon steel is an alloy of iron and carbon with small amounts of
manganese, silicon, sulfur, and phosphorus, the term alloy steel is applied when one or
more elements other than carbon are introduced in sufficient quantities to modify its
properties substantially. The alloy steels not only possess more desirable physical
properties but also permit a greater latitude in the heat-treating process.
Chromium
The addition of chromium results in the formation of various carbides of chromium that
are very hard, yet the resulting steel is more ductile than a steel of the same hardness pro-
duced by a simple increase in carbon content. Chromium also refines the grain structure
so that these two combined effects result in both increased toughness and increased hard-
ness. The addition of chromium increases the critical range of temperatures and moves
the eutectoid point to the left. Chromium is thus a very useful alloying element.
Nickel
The addition of nickel to steel also causes the eutectoid point to move to the left and
increases the critical range of temperatures. Nickel is soluble in ferrite and does not
form carbides or oxides. This increases the strength without decreasing the ductility.
Case hardening of nickel steels results in a better core than can be obtained with plain
carbon steels. Chromium is frequently used in combination with nickel to obtain the
toughness and ductility provided by the nickel and the wear resistance and hardness
contributed by the chromium.

Manganese
Manganese is added to all steels as a deoxidizing and desulfurizing agent, but if the sul-
fur content is low and the manganese content is over 1 percent, the steel is classified as a
manganese alloy. Manganese dissolves in the ferrite and also forms carbides. It causes
the eutectoid point to move to the left and lowers the critical range of temperatures. It
increases the time required for transformation so that oil quenching becomes practicable.
Silicon
Silicon is added to all steels as a deoxidizing agent. When added to very-low-carbon
steels, it produces a brittle material with a low hysteresis loss and a high magnetic
permeability. The principal use of silicon is with other alloying elements, such as
manganese, chromium, and vanadium, to stabilize the carbides.

11 M. A. Grossman, AIME, February 1942.
12 J. Field, Metals Progress, March 1943.
13 Modern Steels and Their Properties, 7th ed., Handbook 2757, Bethlehem Steel, 1972, pp. 46–50.