Chapter 17 Flashcards
Plasticity?
The ability to flow as solids without deterioration of their properties.
Deformation processes
All processing done in solid state. Required forces are often high.
deformation may be
Bull flow, simple shearing, or compound bending
Independent variables
Aspects of a process over which the engineer or operator has direct control
Some independent variables
Starting material, starting geometry, tool or die geometry, lubrications, starting temp, speed of operation, amount of deformation
Dependent variables
Consequences of the independent variable selection.
Some dependent variables
Force or power requirements, material properties of the product, exit or final temp, surface finish and dimensional precision, nature of the material flow, anisotrope and residual stresses.
Gaining information on the independent dependent relationship, using three ways:
1) experience: requires longtime exposure to a process. 2) experiment: can be time consuming and costly. Size and speed of deformation are often reduced. 3) Process modeling: high speed computing and mathematical models. Models are used to compute précipitons for dependent outcomes.
Increase in temperature brings about
A decrease in the rate of strain hardening and strength, with an increase in ductility.
Hot working
Recristallisation occurs simultaneously with the deformation
Hot working is defined as
The plastic deformation of metals at a temperature above the recristallisation temperature
Hot working positive characteristics
Does not require excessively high forces, elevated temps promote diffusion, pores can be welded shut, and the recrystallization improves the final properties.
Hot working negative characteristics
High temps may promote undesirable reactions, tolerances are poorer distortion can occur from no uniform cooling.
Minimum temperature for hot working
Recrystallization determines this
If a cast metal is reheat with a sufficient amount of prior deformation it will produce strain free grains
This recrystallization is follow by 1) grain growth, 2) additional deformation and recrystallization, or 3) a drop in temperature that will terminate diffusion and feee the recristallises structure. By replacing the initial structure with a new one, it is possible to rodoirs and increase in strength and ductility.
Stringers
Impurities that flow along with the base metal or fracture into rows or fragments that are aligned in the direction of working. These impurities do not recristallise and they retain their distorted shape
Heat loss in hot working
Heat is lost through the workpiece surfaces, with a majority of the loss occurring where the workpiece is in contact with lower temperature tooling.
Residual stress
Nonuniform cooling from hot working. Associated with this is warping distortion and cracking
Cold working
Plastic deformation of metals below the recristallisation temp
Advantages of cold working
1) no heating required. 2) better surface finish. 3) superior dimensional control because the tooling sets dimensions at room temp. 4)products possess better reproducibility and interchangeability. 5) strength, fatigue, and wear properties are all improved through statin hardening. 6) Directional properties can be imparted. 7) contamination problems are minimized.
Disadvantages of cold working:
1) higher forces required. 2) heavier and more powerful equipment required. 3) less ductility is available. 4) metal surfaces must be clean and scale free. 5) intermediate anneals may be required to compensate for the loss of ductility. 6) imparted directional properties may be detrimental. 7) undesirable residual stress may be produced.
Metal properties of cold working
Cold working alters the structure of the metal, thus changing its mechanical properties.
Spring back
A cold working phenomenon. When a metal is deformed by the application of a load, part of the deformation is elastic. When the load is removed, it will return its original size and shape.
Elastic spring back
The decrease in strain
Cold working properties,
Material must be clean, and scale can be removed by pickling, a process in which the metal is dipped in acid and then washed. A light cold rolling may be used to ensure uniform starting thickness and a smooth surface.
Yield point phenomenon
Can be removed by a light cold rolling pass
Yield point runout
After loading to the upper yield point, the material exhibits this, wherein the material can statin up to several percent without any additional force being required.
Annealing heat treatment
To maximize starting ductility, this treatment is applied to a metal prior to cold working.
Intermediate anneals
If the required amount of deformation exceeds the fracture limit, this may be necessary to restore ductility.
Anisotropy of cold working
Related to the distorted crystal structure
Residual stresses
Associated with cold working. They can be quite harmful.
