V. Strain Hardening

If, prior to tension testing, a specimen of mild steel is loaded to a stress below the elastic limit and unloaded, the test diagram of the specimen will be no different from the tension test diagram of a specimen not subjected to preloading. If, however, a specimen is previously loaded to a stress above the yield point, the mechanical properties of the specimens being compared will be different.

Let a specimen of mild steel be stretched to a stress characterized by point on the tension test. If the tensile load is now removed, a line is obtained, which is very close to a straight line. The elastic part of the overall elongation of the specimen disappears and a permanent elongation is observed. If the specimen is immediately stretched again, precise measurements will show that its proportional limit is lowered and the yield point is raised. The reloading is represented by line. The yield point becomes approximately equal to the stress to which the specimen was first stretched. If the specimen is allowed to “rest” for some time after loading and then stretched, the proportional limit rises again, i. e., the material recovers its elasticity, and the yield point is raised to a still greater extent. The complete recovery of elastic properties requires a certain length of time which depends on the kind of material. An increase in strength and a loss in ductility because of pre-stretching beyond the yield point are termed work hardening or strain hardening. Strain hardening changes the mechanical qualities of a material and residual stresses are set up in the material. In some cases the phenomena of strain hardening is undesirable and should be controlled, while in other cases strain hardening is artificially produced.

When holes are punched in a sheet for rivets, the materia1 at the edge of a hole undergoes strain hardening and becomes stiffer. This promotes the formation of cracks.

To avoid the detrimental effect of strain hardening in this case, the strain-hardened portion of the material is removed, increasing the hole diameter by drilling. The effect of strain hardening can also be eliminated by annealing, i. e., by heating the material to a certain temperature, holding it at that temperature for some time and subsequently slowly cooling. In other cases, as already stated, strain hardening is artificially produced. For instance, chains of lifting machines are pre-stretched above the yield point to make them less ductile and to avoid large deformations during operation, which would prevent the entry of the chain links in their seats on the drum.

Strain hardening is responsible for the fact that a wire obtained by drawing has a considerably higher strength than the steel from which it is made.

Exsercises

1. Write 10 questions to each part of the text.

2. Write out of the text the sentences with the verbs in the Passive voice.

3. Translate any part of the text (1500 signs) in writing.

4. Retell part I.

5. Speak on «Temperature effect».