1. Chapter 7

2. §7.4 Cold Working and Aging of Building Steels the definitions of cold working and aging principium of cold working and aging strengthening significances of cold working of steels and aging strengthening

3. Cold Working Cold working refers to the processes of cool drawing and cool rolling treatment at normal temperature. Cold Working Strengthening It refers to the phenomenon that strength and rigidity of steel are improved while the plasticity and toughness are reduced by cold working.

4. Aging In the process of cold-drawing, the longer it is drawing, the better the strength is, and the worse the plasticity and toughness. This phenomenon is called aging. It is classified into two kinds: natural aging and manmade aging.

5. Aging 1.natural Aging  Expose the cold-drawn steels at the normal temperature for 15 -20 days.  The strength and rigidity of are improved while the plasticity and toughness are reduced. 2.the treatment of manmade aging  Heat the cold-drawn steels to 100-20 ℃, and keep them at this temperature for some time.  This can accelerate the aging.

6. Aging sensitivity The greater the properties changes for aging, the worse the plasticity and toughness are. As a result, the structure of dynamic load demands high sensitivity. Explanation Some steels can also undergo aging after a long time even if they are not processed only with less apparent effects.

7. Principium of cold working strengthening plasticity As a result of the plasticity transmutation, by the outer force, the metal atoms of crystal in the flexibility phase deviates the balanced position, which leads to the transmutation. However, transmutation will disappear as long as the outer force is cancelled. When the outer force continues increasing until the crystal lattice transmutation is beyond the limit of flexibility transmutation, the crystal lattice will slip and the eternal transmutation results.

8. Principium of cold working strengthening Owing to the aberration of the crystal grains and the debris on the slide plane, the crystals have difficulties in continuing its sliding and the steels can resist transmutation better. rigidity. All of these efforts bring about the improvement of the limit of yielding and rigidity. The crystals have slided nearly all of the possible sliding areas, so the plasticity reduces.

9. Principium of aging strengthening The solubility of the melted atoms in N and O in high temperature will decreases if the temperature drops. The atoms aren't totally separated as soon as the temperature drops, but diffuse gradually into the defective parts in the storage, and form particles in the form of solid which prevent the crystal grains from sliding so as to resist the plasticity transmutation better.

10. σ ε OO1O1 B D1D1 C D C1C1 K1K1 K Ageing of without cold-drawing yield point B Cold-drawing  yield point: σs become bigger  plasticity and toughness decrease. Ageing  σ s, σ ν become bigger  plasticity and toughness decrease

11. Cold Working and aging of Building Steels 1.It improves the strength, and the yield strength can raise 20-30% after cold working. Tensile- strength can also be improved after aging. 2.It helps to use fewer steels.  Steels with less aging sensitivity should be adopted to avoid the over brittleness in the steel structure under dynamic load or in constantly medium- temperature condition, which causes the breaking.