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| Strain Rate Sensitivity | Theoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n = 0. The metal should continue to flow at the same flow stress, once that stress is reached. However, an additional phenomenon occurs during deformation, especially at elevated temperatures: Strain rate sensitivity. |
| What is Strain Rate? | Strain rate in forming is directly related to speed of deformation v. Deformation speed v = velocity of the ram or other movement of the equipment. Strain rate is defined: (strain rate)=V/h, where = true strain rate; and h = instantaneous height of workpiece being deformed. |
| Evaluation of Strain Rate | In most practical operations, valuation of strain rate is complicated by- Workpart geometry. Variations in strain rate in different regions of the part. Strain rate can reach 1000 s-1 or more for some metal forming operations. |
| Effect of Strain Rate on Flow Stress | Flow stress is a function of temperature. At hot working temperatures, flow stress also depends on strain rate- As strain rate increases, resistance to deformation increases. This effect is known as strain‑rate sensitivity. |
| Strain Rate Sensitivity Equation | Yf=C(strain rate)^m, where C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‑rate sensitivity exponent. |
| Observations about Strain Rate Sensitivity | Increasing temperature decreases C and increases m- At room temperature, effect of strain rate is almost negligible. Flow curve is a good representation of material behavior. As temperature increases, strain rate becomes increasingly important in determining flow stress. |
| Friction in Metal Forming | In most metal forming processes, friction is undesirable: Metal flow is retarded. Forces and power are increased. Tooling wears faster. Friction and tool wear are more severe in hot working. |
| Lubrication in Metal Forming | Metalworking lubricants are applied to tool‑work interface in many forming operations to reduce harmful effects of friction. Benefits: Reduced sticking, forces, power, tool wear. Better surface finish. Removes heat from the tooling. |
| Considerations in Choosing a Lubricant | Type of forming process (rolling, forging, sheet metal drawing, etc.). Hot working or cold working. Work material. Chemical reactivity with tool and work metals. Ease of application. Cost. |
| Bulk deformation- | Rolling. Forging. Extrusion. Wire and bar drawing. |
| Sheet metalworking- | Bending. Deep drawing. Cutting. Miscellaneous processes. |
