Alloying and Its Effects on the Critical Temperature, Hardness and Tensile Strength

Alloying elements have significant effect on the iron-iron carbide equilibrium diagram. The addition of some of these alloying elements will widen the temperature range through which austenite (g -iron) is stable while other elements will constrict the temperature range. What this means is that some elements will raise and some elements will lower the critical tempearture of steel.

Manganese, cobalt, and nickel increase the temperature range through which austenite is stable. This also means that the lower critical temperature of steel will be lowered by these alloying elements. Other alloying elements that lower the critical temperature of steel are carbon, copper and zinc. The alloying elements that are used to reduce the critical temperature are highly soluble in the gamma iron (austenite). Figure 1 shows the effect of manganese on the critical temperature of steel.

Figure 1. The effect of alloying with manganese on the critical temperature of steel and austenite (g -iron) phase transformation zone on the iron-iron carbide diagram..

Alloys such as aluminum, chromiuim, molybdenum, phosphorus, silicon, tungsten tend to form solid solutions with alpha iron (ferrite). This constricts the temperature region through which gamma iron (austenite) is stable. As shown in Figure 2, chromium at different percentages constricts the critical temperature range which results in a marked reduction of the region where austenite is stable.

Figure 2. Effect of alloying with chromium on the critical temperature of steel and austenite (g -iron) phase transformation zone on the iron-iron carbide diagram.

The elements shown in Figure 3 have the greatest solubility in ferrite and also influence the hardenability of iron when in the presence of carbon. With a slight increase in the carbon content, they respond markedly to heat treating, because carbon acts as a ferrite strengthener. As indicated in Figure 3, Phosphorus will improve the hardness of the ferrite significantly by adding only a very small percentage of Phosphorus, while Chromium will not strengthen the ferrite that well even at very high percentage of Chromium addition to the steel

Figure 3. The effect of various alloying elements on the hardness of steel.

Figure 4 shows the effect of furnace cooling vs. air cooling on the tensile strength of steel for three different percentages of carbon in the presence of chromium. As this figure indicates, furnace cooling has very little effect on the tensile strength of the material. The addition of chromium does not change the tensile strength properties when the steel is cooled in the furnace. If the same steels are air cooled at the same rate, the slope of the curves increases significantly which means that a slight increase in the chromium content increases the strength drastically when air cooling is applied.

Figure 4. Effect of different percentages of carbon on the tensile strength of steel in the presence of chromium.

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Last update: March 15, 1999

By: Serdar Z. Elgun