Cold work, effect hardness

When lead is added to brass up to about 4%. improved machinabtlity results. The lead has practically no effect on tensile strength or hardness. However, for cold-worked materials, lead does lower ductility and shear strength. 

There are many commonly measured properties of metals which are known to change upon cold-working. The best known effect is an increase in hardness. Additionally, the resistivity increases and the thermal conductivity decreases the electronic work function is changed the X-ray diffraction pattern is broadened. 

It must be noted that cold working cannot be employed to increase the hardness, since its effect must be nullified by the rise in temperature. 

According to the systematic study by Ross (1967) of the effect of cold rolling on the hardness of yttrium and twelve of the lanthanide metals, the hardness of all metals except La, Nd, Er, Y, and perhaps Sm tends to increase rapidly during the initial 20-25 percent reduction. Further hardening does not occur until reductions of the order of 60 percent (see fig. 8.5). This is in agreement with the earlier work by Love (1960) on erbium and dysprosium shown in fig. 8.6. A comparison of the effect of 50 percent cold work by swaging (Love, 1959) or... 

Fig. 8.9. Effect of annealing temperature and rate of cooling from annealing temperature on the hardness of 30 percent cold worked yttrium. From Bohlander (1961).

Bata III Effect of cold work and aging time on hardness... 

---