8-12 EFFECT OF ALLOYING ELEMENTS

8-12 EFFECT OF ALLOYING ELEMENTS

Alloying elements greatly affect the equilibrium diagram and alter the rate at which transformation reactions occur. The iron—carbon diagram may be profoundly altered; therefore its application should be limited to plain-carbon and low-alloy steels. For high- alloy steels it is necessary to consider the iron—chromium, iron—chromium—nickel, and iron—chromium--carbon systems.

Alloying elements can be divided into two classes: (1) those like nickel, manganese, copper, cobalt, carbon, and nitrogen that stabilize austenite. And (2) those like chromium, molybdenum, silicon, tungsten, vanadium, tin, columbium, and titanium that stabilize ferrite. These latter are called ferrite stabilizers or carbide-forming elements. since they easily combine with carbon to form carbides, whereas the first are called austenite formers. Silicon, although a ferrite former, is not a carbide-forming element but, on the contrary, it is a very effective graphitizing agent. Confusion may arise from the fact that these two groups of elements increase the stability of austenite with respect to the temperature (M s ) of spontaneous martènsite transformation. This is due to the ability of elements such as chromium, nickel, manganese, and molybdenum to form solid solutions with both austenite and ferrite. The elements, however, differ in their solubility in austenite or ferrite. Nickel, which stabilizes austenite, dissolves in austenite in all proportions but, inferrite, it dissolves only up to 25 to 30%. Chromium, a ferrite former, has only a limited solubility in austenite, up to 12.8%, but dissolves in all proportions in ferrite. In the presence of carbon the solubility of chromium in austenite increases and, for 0.5% carbon, the solubility of chromium, in austenite increases to 20%. Silicon is soluble in austenite only up to 2.1%; in ferrite it dissolves up to 18%.

The effect of nickel is to lower the critical transformation temperature, whereas the effect of chromium is to raise it. Both these elements shift the S curve ro the right, thus delaying the transformation of austenite to pearlite. This permits the formation of The effect of nickel is to lower the critical transformation temperature, whereas the effect of chromium is to raise it. Both these elements shift the S curve ro the right, thus delaying the transformation of austenite to pearlite. This permits the formation of

This principle is used in improving the hardenability of low-alloy engineering steels (AISI steels), which contain at least 0.3% C. Because of the presence of alloying elements such as nickel, chromium, manganese, or molybdenum, a complete transformation of austenite to martensite can be achieved in heavy cross sections, even in oil quenching. This makes it possible to achieve maximum strength in large forgings and to reduce the adverse effects of thermal stresses arising on very rapid cooling. Low-alloy, high-strength structural steels containing only up to 0.25% are not hardened to martensite. The purpose of the alloying elements is to improve the general overall properties such as corrosion resistance and durability.

A 1 Chromium steel

A 1 Carbon steel 727 o C

o C A 1 Nickel steel

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Time, log scale

FIGURE 8-23 Effect of alloying elements on the Bain curve. (E. C. Bain, Alloying Elements in Steel, American Society for Metals, Metals Park, Ohio, 1939.)

With an increasing amount of alloying elements it is possible to control the basic microstructure of steels and, accordingly, to modify the properties. For example, on increasing the manganese content up to 12% while maintaining the carbon content of 1.1%, austenite becomes metastable at room temperature. Thus the austenitic microstructure results; such steel is referred to as austenitic-manganese steel. It exhibits exceptional hardening at the surface when subjected to impact or heavy cold work because austenite. as a result of its instability, transforms spontaneously to wear-resistant martensite, whereas the inner core of the steel remains austenitic. The combination of an extremely hard surface with a relatively soft but tough core makes the steel particularly suitable for jaw crushers, grinders, rails, and the teeth of power shovels, which require high abrasive resistance and high hardness as well as high impact resistance. Because of this extensive work hardening, austenitic manganese steel is very difficult to machine and is generally used in the form of castings.

Another example is a very important group of high-alloy steels known as stainless steels and heat-resistant steels.