In this article we will discuss about the features and principles of case-hardening of steels.
Features of Case-Hardening of Steels:
The case-hardening, where the chemical composition of steel surface is changed, has two important features:
1. The chemical composition of the steel is changed when it is in solid state.
2. The chemical composition changes only of the desired surface thickness, called the case. This results in changes in the microstructure of the case, and thus, the properties of this case can be made totally different from the interior core. The fatigue properties are also improved as the surface layers are under compressive stresses, while core is under tension.
For example, heavy duty gears, cams, etc., require the following two of the main properties:
(i) Very high surface hardness and wear resistance.
(ii) Fairly high impact resistance and toughness.
Such parts, if made from high carbon steels (as the hardness of the hardened steel depends mainly on the carbon of the steel), can be made to have high hardness by suitable hardening treatment, but their impact strength shall be poor. Low carbon steels, if used, provide high impact strengths, but very low surface hardness and wear resistance. Case-hardening can induce, both these properties simultaneously.
A low carbon steel if used to make the part, and then the case is enriched with say, carbon to make it high carbon. A proper heat treatment then, is able to impart high surface hardness and wear resistance, while the core retains its high impact strength of low carbon steel as it doesn’t really get hardened. This is the method of case-carburising.
The commonly used case-hardening methods are:
(iii) Cyaniding or carbonitriding.
Principles of Carbon- Hardening of Steels:
Case-hardening of steel consists of enriching the surface layers of steel, commonly with carbon, nitrogen, or both, or some other alloying elements. The steel surface is brought in contact with the solid, liquid, or gaseous medium which readily liberates the atoms of the element of interest, and the atoms are absorbed by the surface of the steel, thus, enriching it. If the absorbed element combines chemically with iron to form only a chemical compound, then the steel surface develops a thin film of this compound and further absorption almost stops.
The diffusion of the element inside the steel surface to have a certain thickness of the case with enriched element requires that this element should be able to form a solid solution with iron. More the solid solubility more is the enrichment of that element.
As carbon as well as nitrogen, the commonly used elements form solid solutions with iron, these elements can diffuse in steel theoretically at any temperature, but commonly the steel is made to absorb these elements only at high temperatures.
The three stages in the enrichment of the steel surface by the elements are:
1. Liberation of the saturating element in the atomic state as a result of the chemical reactions in the surrounding medium. It is only in the atomic state that an element can diffuse inside the steel lattice.
2. Contact of the steel surface with atoms of the element to absorb, i.e., to dissolve in the surface iron lattice.
3. Diffusion of this absorbed element to inside of the iron lattice.
The reasons for using high temperatures for case-hardening are:
1. The solubility of both carbon as well as nitrogen is more in the high temperature phase-gamma-iron than alpha iron. The maximum solid solubility of carbon in alpha-iron is 0.02% at 727°C, whereas at the same temperature gamma-iron can dissolve 0.77% carbon. This solid solubility also increases with the rise of temperature to become 2.11% carbon at 1147°C in gamma-iron.
2. The solid, liquid, or gaseous medium containing the enriching element is to be heated to high temperature to decompose to give enough concentration in atomic form of the element to be absorbed by the steel surface. For example, ammonia decomposes to give nitrogen atoms for enriching the steel during nitriding. The transport of diffusing element from the medium always takes place via a gas phase-even when the medium is solid or liquid.
3. Diffusion of interstitial solid solution forming elements like carbon, or nitrogen is much faster than the substitutional solid solution forming metallic elements, and the latter need much higher temperatures to be impregnated in the steel. The rate of diffusion of even carbon is exponentially increased with the rise of temperature (Fig. 8.1 a).
For example, the diffusion rate of carbon in iron is over 40% more if temperature is raised from 870°C to 925°C. Fig. 8 (b) illustrates the effect of temperature on case depth, while Fig. 8.2 (c) illustrates the effect of time at three constant temperatures. At a temperature, the case depth does not increase linearly with time but at a decreasing rate.
The use of very high temperatures for case- hardening has above advantages, but high temperatures decrease drastically the service life of retorts, muffles, and other furnace parts; even the life of electrical resistors is decreased drastically. Thus, the use of high frequency currents, or laser beams have promising fields of applications.
High temperatures have very adverse effect on the structure of the steel. The grains of the case as well as core may get coarsened. At high temperatures, the rate at which the atoms of the element are absorbed at the surface becomes much higher than the rate at which it can diffuse through the layers of the steel.
It leads to very high concentration of the element at the surface of the steel (Fig. 8.1 c). It may lead to the formation of the compounds such as carbides, nitrides, etc., which being extremely brittle make the surface to be highly brittle, with the possibility of even getting scaled off.
Even otherwise, the too steep a gradient of the element in the surface layers of the steel makes this thin case to have very much different properties than core, like high brittleness to get chip off in the initial use of service life. Thus, there is an upper limit of the temperature of case-hardening. The above mentioned effects on the structure of the steel, however, if developed, can be rectified easily in most cases.