When added to iron, various elements (such as chromium, molybdenum, tungsten, titanium and niobium) form highly stable alloy carbides. In most cases, these carbides are harder than iron carbide, resulting in strengthening of the metal matrix by forming interstitial compounds. Other elements, such as manganese, improve the stability of the other alloy carbides even though they are not a strong carbide former. Still other elements (such as silicon, nickel, aluminum and cobalt) do not alloy with carbon and, in fact, tend to add to the instability of the metal matrix.
As most of us know, care must be taken to prevent being too high in austenitizing temperature or staying too long at temperature due to a concern over grain growth and subsequent mechanical-property effects, such as loss of strength and toughness. Some elements – chromium being a prime example – tend to contribute to the rate of grain growth, particularly at high temperatures.
Other elements such as vanadium, niobium, titanium and aluminum tend to “pin” the grain boundaries and make them less likely to grow large grains. Vanadium is reportedly the most potent of these grain-refining elements. As little as 0.1% will inhibit grain growth by forming finely dispersed carbides and nitrides that are relatively insoluble at high temperatures and act as barriers to grain growth.
Eutectoid Point Effects
The process of alloying steel shifts the eutectoid point toward the left on the iron-iron carbide phase diagram (that is, it diminishes the solubility of carbon in austenite). In other words, an alloy steel will be pearlitic even though it contains less than 0.8% carbon. Thus, low-alloy steels can contain less carbon than plain-carbon steels of similar characteristics and uses.
Ferrite stabilizers such as chromium, tungsten, molybdenum and titanium raise the eutectoid temperature as well as the Ac3 temperature. By contrast, austenite stabilizers such as nickel and manganese lower the eutectoid temperature.
For example, the addition of 2.5% manganese to a steel containing 0.65% carbon will give it a completely pearlitic structure in the normalized condition, along with a reduction in the eutectoid temperature to about 700°C (1300°F).
In another case, a high-speed steel may contain only 0.7% carbon, but its microstructure exhibits large amounts of free carbide due to the shifting of the eutectoid point far to the left by the effects of alloying elements present. At the same time, the eutectoid temperature in the high-speed steel is raised to approximately 850°C (1560°F).
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