Alloying Elements in Steel
Alloying elements are added to steel for the following purposes:
1. To increase hardenability
2. To increase strength at ordinary temperatures
3. To improve high-temperature properties
4. To improve toughness
5. To increase wear-resistance
6. To increase corrosion-resistance.
When alloys are added in the right amount and in the correct combination, it can be said that the steel will be improved in one or more of the above-mentioned characteristics. It is important, however, to have the right amount and the right combination of alloying elements. Merely adding more alloy does not necessarily improve the steel proportionately; it may even have a harmful effect. Two alloys when added to a steel are more effective than the same amount of a single alloy, if in combination, they enhance a property of the steel. The effect of a single element depends upon the effects of other elements, and this must be taken into account when evaluating specific compositions of steels.
To simplify the discussion to follow, each major alloying element will be discussed separately; however, the reader must not lose sight of the fact that these elements usually act in combination with other alloying elements.
Carbon. Carbon is the most important and effective alloying element in steel. Each small increase in the carbon content increases the hardness and tensile strength of the steel in the “as rolled” or “normalized” condition. When the carbon content exceeds.85 percent, the resulting increase in hardness and strength is less than in the lower carbon ranges; however, the wear-resistance continues to increase above this carbon percentage.
The presence of carbon accounts for the ability to harden steel by heat treatment. For plain carbon steel the maximum hardness that can be attained by heat treatment increases with increased carbon content until the carbon content is about.60 percent. Above this carbon content the rate of increase in hardness is very small. The effect of alloying elements is to lower the carbon content at which the maximum hardness occurs.
Only a very small percentage of carbon can be dissolved in ferrite; however, all of the carbon present in steel will dissolve in the austenite, which, it will be remembered, normally exists at a higher temperature. At room temperature the carbon is combined with iron to form the very hard and brittle iron carbide (Fe3C), called cement - ite. Cementite strengthens the steel, increases its hardness, and increases its wear-resistance in the unheat-treated condition. Manganese. Next to carbon, manganese is probably the most impor* tant alloy that is added to steel because it combines with the sulfur to form manganese sulfide. Sulfur, when not combined with manganese, is very harmful, causing hot shortness in steel. When combined with manganese, the sulfur is harmless. Therefore, manganese is an essential ingredient in all steels; for the purpose of combining with sulfur, in quantities ranging from.40 to 1.00 percent. When present above this amount, manganese is considered to be an alloying element; it also acts to deoxidize steel.
When present as an alloying element, manganese contributes to the strength and toughness of steel, and it greatly increases its hardenability. in very large amounts (12 to 15 percent) the steel will remain austenitic at room temperature. Austenitic manganese steel is an extremely tough alloy that is used for applications requiring severe impact and abrasion resistance, such as for power shovel blades. High manganese content adversely affects the weldability of steel by increasing its crack sensitivity.
Silicon. Silicon is one of the principal deoxidizers used in steel. It is a very important element in the metallurgy of gray cast iron. In steel it dissolves in the ferrite increasing its strength and toughness. Steel having less than. 10 percent carbon and about 3 percent silicon has excellent magnetic properties and is used in the cores and poles of electrical machinery. A steel containing 1 to 2 percent silicon is used for structural applications requiring a high yield point.
Nickel. Nickel is another very important alloying element in steel. When present in appreciable amounts it improves the toughness and impact resistance of the steel, particularly at lower temperatures. It contributes to easier and more foolproof heat treatment of steel thereby reducing costly heat-treating failures. It dissolves in the ferrite and strengthens it. It causes more pearlite to form and the pearlite formed is finer; therefore, it is a stronger and tougher pearlite. Nickel steels are particularly suitable for case hardening. Other than making steels more hardenable, the presence of nickel in steel causes no difficulty in welding.
Chromium. Chromium increases hardenability and abrasion resist - ance. When present in quantities in excess of 4 percent, the corrosion resistance of steel is improved. It is one of the most effective alloys in promoting hardenability. High-chromium steels are air- hardening. Chromium forms a very stable carbide that has exceptional wear-resistance. It also promotes carburization of the steel.
The presence of chromium in steels presents problems in welding, however, as the increased hardenability can cause cracking in and adjacent to the weld joint. Steels containing 5 to 6 percent chromium can be welded only by using special techniques.
Molybdenum. Molybdenum, manganese, and chromium have a greater effect on hardenability than any other commonly used alloying element. Molybdenum has a powerful effect in increasing the high-temperature strength of steel and it retards grain growth at temperatures above the upper critical temperatures. Quench - hardened molybdenum steel is fine grained and very tough at all hardness levels. It is used as an alloy in many grades of high-speed tool steels.
Vanadium. Vanadium is used to inhibit grain growth in steel at elevated temperatures, thereby causing the steel to be fine grained at room temperature, adding to its strength and toughness. It improves the hardenability of medium carbon steels when present in amounts of.04 to.05 percent. Above this content, hardenability decreases when the steel is heated to normal hardening temperature; however, if a higher hardening temperature is used, the hardenability is increased. In high-speed steels used for cutting tools, vanadium is an essential alloying element to improve hardenability and to obtain a fine-grain size.
Tungsten. Tungsten is used in high-speed tool steels to promote the retention of the hardness, obtained through heat treatment, at high temperatures. It forms an extremely hard carbide that is very wear - resistant.
Cobalt. Cobalt is an unusual element because it decreases the hardenability of steel. It strengthens the ferrite and it is used in some high-speed steels to increase resistance to abrasion at high temperatures.
Boron. Boron is used in steel for only one purpose, to increase the hardenability of steels having less than.60 percent carbon content. It is effective when used in quantities of only a few thousandths of a percent. Perhaps, for this reason, the degree of effectiveness of boron steels is sometimes rather unpredictable.
Titanium. Titanium has a very strong tendency to form carbides and to reduce the ability of the steel to be hardened by heat treatment. It is used as a deoxidizer and in deep-drawing steels to prevent age hardening. It is also used for this purpose in stainless steels and in heat-resisting steels, to increase their strength.
Aluminum is used principally as a deoxidizer in steel although it also promotes a fine austenitic grain size. Copper is sometimes added to steel to improve its resistance to atmospheric corrosion. Lead is added to some steels to improve their machinability. While sulfur is normally considered to be an impurity in steel, it is sometimes intentionally added, along with the required amount of manganese to form manganese sulfide. This is done to improve the machinability of the steel. Phosphorus is considered to be an impurity.