Hardening Mechanisms of Steels

An external pressure (stress) that is exerted on a material will cause its thickness to decrease. A shear stress is applied parallel to the surface of a material, and may cause the sliding of atomic layers over one another. The resultant deformation in the size/shape of the material is referred to as strain, related to the bonding scheme of the atoms comprising the solid. For example, a rubbery material will exhibit a greater strain than a covalently bound solid such as diamond. Since steels contain similar atoms, most will behave similarly as a result of an applied stress. If a stress causes a material to bend, the resultant flex is referred to as shear strain. For small shear stresses, steel deforms elastically, involving no permanent displacement of atoms. The deformation vanishes when shear stress is removed. However, for a large shear stress, steel will deform plastically, involving the permanent displacement of atoms, known as slip. 

The introduction of other metals into the lattice through alloy formation introduces alien crystallites that will also impede slip in steel crystals. In particular, if the alloying agent is carbon, hard crystallites of iron carbide may form, changing the microstructure to impede slip. By comparison, austenite usually does not contain iron carbide, and is quite susceptible to slip. 

The fcc-bct conversion, known as the Bain transformation, is a diffusionless process. That is, unlike the previous high-temperature conversions we saw earlier e.g., austenite to ferrite), martensite can form at temperatures significantly below room 

Equation 9 quantifies the effects of dopant concentrations on the temperature required for the onset of martensite formation, Mg. The greatest effects are seen for the austenite-forming elements of C, Mn, and Ni where even small concentrations result in a sharp decrease in Mg. Whereas pure y-iron may be converted to martensite at temperatures in excess of 500°C, hypereutectoid steel is not transformed to martensite until a temperature of ca. 160°C is reached during quenching. At carbon concentrations above 0.7%, martensite is still being formed at temperatures well below 0°C. Hence, high-C steels must be quenched in low-temperature media e.g., dry ice/acetone, liquid nitrogen) to ensure full conversion of austenite to martensite. 

We have seen that only certain transition metals will form stable carbides as a relevant digression, let us consider the chemical rationale behind such reactivity. The general trend for increasing carbide-forming ability of transition metals is 

In this section, we will describe the primary techniques that may be used to strengthen a metal. It should be noted that these methods are applicable to all metal classes, not just the iron alloys predominantly described herein. 

As we have seen, it is not simply the carbon concentration, but rather the microstructure of Fe-C alloys that affects its physical properties. The size of the individual microcrystals (or grains) that comprise these aggregates greatly influences many properties of the bulk crystal. Both optical microscopy and X-ray diffraction are used to 

We are all familiar with the picture of a blacksmith withdrawing red-hot iron from a furnace and hammering it into the desired shape. Although these early laborers were 

The phases of austenite, pearlite, and ferrite are relatively soft hence, the observed high hardness of steels is obtained through processing of these materials. For instance, hypoeutectoid steel may be heated to form austenite and then slowly cooled so the cementite/ferrite phases may be worked into desired shapes. If the material is re-austenized and quickly quenched to room temperature, a very hard phase known as martensite is formed. Some of the remaining pearlite and ferrite phases (if present) would still remain in the matrix. Hence, only when the steel has been heated to temperatures sufficiently high to convert all of the ferrite into austenite, that quenching will result in pure martensitic steel. It should be noted that the martensite phase does not appear in the above Fe-C phase diagram since it is a non-equilibrium phase. 

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