8.2.Hardening Mechanisms 143 FIGURE 8.2.Schematic representation of a lamellar (plate-like)microstructure of steel called pearlite obtained by cooling a eutectoid iron-carbon alloy from austenite to below 727C.Pearlite is a mixture of a and Fe3C.Compare to Fig- ure 5.9. by primary a,as schematically depicted in Figure 8.3(a).The re- sulting steel is hard but still ductile due to the continuous and soft ferrite.The strength of hypoeutectoid steels initially in- creases with rising carbon content,but eventually levels off near the eutectoid composition. There are some more mechanisms that may further increase the hardness of hypoeutectoid steel.We learned in Section 5.3 that a large number of small particles pose an enhanced chance for blocking the moving dislocations.This causes an increase in strength compared to the action of only a few but large particles. The same is true for the number and size of pearlite domains or "colonies".The number of pearlite colonies can be increased by providing small austenitic grains to begin with on whose bound- aries the pearlite eventually nucleates.Specifically,the hardness a-coated grain boundaries pearlite cementite pearlite IP (a) (b) FiGURE 8.3.Schematic representation of(a)a hypoeutectoid microstruc- ture of steel at room temperature containing primary a and pearlite mi- croconstituents (the latter consisting of two phases,i.e.,a and Fe3C);(b) a hypereutectoid microstructure of steel.Note that the primary phases in both cases have "coated"the former grain boundaries of the austenite.8.2 • Hardening Mechanisms 143 by primary , as schematically depicted in Figure 8.3(a). The re￾sulting steel is hard but still ductile due to the continuous and soft ferrite. The strength of hypoeutectoid steels initially in￾creases with rising carbon content, but eventually levels off near the eutectoid composition. There are some more mechanisms that may further increase the hardness of hypoeutectoid steel. We learned in Section 5.3 that a large number of small particles pose an enhanced chance for blocking the moving dislocations. This causes an increase in strength compared to the action of only a few but large particles. The same is true for the number and size of pearlite domains or “colonies”. The number of pearlite colonies can be increased by providing small austenitic grains to begin with on whose bound￾aries the pearlite eventually nucleates. Specifically, the hardness   Fe3C FIGURE 8.2. Schematic representation of a lamellar (plate-like) microstructure of steel called pearlite obtained by cooling a eutectoid iron–carbon alloy from austenite to below 727°C. Pearlite is a mixture of  and Fe3C. Compare to Fig￾ure 5.9. ￾￾ ￾ ￾￾ ￾￾￾￾ ￾￾￾￾ ￾ ￾ ￾￾￾ ￾￾￾￾￾ ￾￾￾￾￾￾ ￾￾￾￾￾￾ ￾￾￾￾ ￾￾ ￾￾￾ ￾￾￾￾ ￾￾￾￾ ￾￾ ￾￾￾ ￾￾￾ ￾ ￾￾ ￾￾ ￾ ￾￾￾ ￾￾￾ ￾￾￾ ￾￾￾ ￾￾ cementite pearlite (b) ￾￾￾ ￾￾￾ ￾ ￾￾￾ ￾￾￾￾ ￾￾￾ -coated grain boundaries pearlite (a) ￾￾￾￾￾￾￾￾￾￾ ￾￾ ￾￾￾￾ ￾￾￾ ￾ ￾￾ ￾￾￾ ￾￾￾￾￾ ￾￾￾ ￾￾￾￾￾ ￾￾￾￾ ￾￾￾ ￾￾￾ ￾￾ ￾￾ ￾￾￾ ￾ ￾￾￾￾ ￾ ￾￾ ￾￾ ￾￾ ￾￾ FIGURE 8.3. Schematic representation of (a) a hypoeutectoid microstruc￾ture of steel at room temperature containing primary  and pearlite mi￾croconstituents (the latter consisting of two phases, i.e.,  and Fe3C); (b) a hypereutectoid microstructure of steel. Note that the primary phases in both cases have “coated” the former grain boundaries of the austenite