Advantages And Disadvantages Of Cold Working 6v2j6k

Abstract: Cold working is the plastic deformation of metals below the recrystallization temperature. In most cases, such cold forming is done at room temperature. The major cold-working operations can be classified basically as squeezing, bending, shearing and drawing.

Cold working is the plastic deformation of metals below the recrystallization temperature. In most cases of manufacturing, such cold forming is done at room temperature. Sometimes, however, the working may be done at elevated temperatures that will provide increased ductility and reduced strength, but will be below the recrystallization temperature. When compared to hot working, cold-working processes have certain distinct advantages:

• • • • • •

No heating required Better surface finish obtained Superior dimension control Better reproducibility and interchangeability of parts Improved strength properties Directional properties can be minimized

Some disadvantages associated with cold-working processes include:

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Higher forces required for deformation Heavier and more powerful equipment required Less ductility available Metal surfaces must be clean and scale-free Strain hardening occurs (may require intermediate anneals) Imparted directional properties may be detrimental May produce undesirable residual stresses Table 1: The major cold-working operations

The major cold-working operations can be classified basically under the headings of squeezing, bending, shearing and drawing, as follows. Squeezing Processes Most of the cold-working squeezing processes have identical hot-working counterparts or are extensions of

them. The primary reasons for deforming cold rather than hot are to obtain better dimensional accuracy and surface finish. In many cases, the equipment is basically the same, except that it must be more powerful. Cold Rolling Cold rolling s for by far the greatest tonnage of cold-worked products. Sheets, strip, bars and rods are cold rolled to obtain products that have smooth surfaces and accurate dimensions. Swaging Swaging basically is a process for reducing the diameter, tapering, or pointing round bars or tubes by external hammering. A useful extension of the process involves the formation of internal cavities. A shaped mandrel is inserted inside a tube and the tube is than collapsed around it by swaging. Cold Forging Extremely large quantities of products are made by cold forging, in which the metal is squeezed into a dive cavity that imparts the desired shape. Cold heading is used for making enlarged sections on the ends of rod or wire, such as the heads on bolts, nails, rivets and other fasteners. Sizing Sizing involves squeezing areas of forgings or ductile castings to a desired thickness. It is used principally on basses and flats, with only enough deformation to bring the region to a desired dimension. Extrusion Extrusion is a bulk deformation process where a billet, generally cylindrical, is placed in a chamber and forced through a die. The die opening can be round to produce a cylindrical product, or the opening can have a variety of shapes. Typical extrusion products are shown in Figure 1. Because of large reductions imparted during the extrusion process, most extrusion processes are performed hot in order to reduce the flow strength of the metal. Cold extrusion can occur but it is usually one step in a multi step cold forging operation.

Figure 1: Schematic of extrusion processes: a-direct or forward extrusion; b-indirect or reverse extrusion; c-impact extrusion and d-hydrostatic extrusion.

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5 Benefits of Cold Work in Steels Here are 5 benefits of cold working of steels that make a difference to your machining operations. 1.

Increased strength

2.

Improved surface finish

Controlled dimensional tolerance and concentricity 3. 4.

Improved straightness

5.

Improved machinability.

Increased Strength It is widely known that cold working strain changes the properties of most metals. When as rolled steel bars are cold worked by cold drawing through a die, a significant increase in yield and tensile strength is obtained. At the same time, The reduction in area and percent elongation are reduced. The graph below shows the effect of cold drawing on the tensile properties of 1 inch round diameter steel bars.

Mechanical Properties % Change resulting from % Cold Work

There are two important lessons in this graph: 1) As strength properties increase, ductility measures decrease; 2) Up to about 15% cold reduction, yield strength increases at a much greater rate than tensile strength. The first 5% of cold work results in the greatest increase in strength. Improved Surface Finish Hot rolled steel bars are finished at high temperatures, and so the surface has a hard abrasive scale made up of various oxides of Iron. This scale is hard and abrasive ranging from 270 – 1030 DPH (Vickers) microhardness depending on the type of oxide (s) formed. In order to cold draw the bars, cold finshers typically remove the sacle byshot blasting or acid pickling. This results in the removal of the hard abrasive scale. By pulling the bars though the die, the surface finish is also improved, with Cold Drawn bars typically running 50 microinches maximum and modern equipment typically working at 25-30 micro inches. Compare this to a roughness height of 250 or more for hot rolled bars. Controlled Dimensions Because the bars are cold reduced at room temperature by pulling through an oil lubricated die, the dimensional conformance of the steel is much more easily controlled. Typical tolerances for cold drawn 1 ” low carbon steel bars are +0.000″/ – 0.002″. this compares favorably to +/0.010 for hot rolled steel of the same chemistry and diameter. Concentricity is improved by the cold drawing operation. Improved Straightness The straightness of hot roll bars is generally 1/4″ max deviation in any 5 foot length. In cold drawn bars,

depending on size and grade this deviation can be held to as little as 1/16″ in 10 feet. Please see our post here for a more complete discussion of bar straightness. Improved Machinability Improved machinability is really the synergistic result of all of the above improvements made by cold work (cold drawing). Higher yield to tensile ratio means the tool has less work to do to move the metal in the workpiece to its ultimate strength when it will separate as a chip. This translates into less force on the tool and greater tool life and productivity. Not putting hard abrasive scale and oxides into your cutting fluids nor on to your tool because the bar has been cleaned results in longer uptime and less maintenance for tools, workholding, and machines. More tightly controlled dimensions and concentricity means that the bars can be run at higher speeds without creating harmful vibrations and chatter. Finer tolerances can be held by your equipment when bars are sized properly going into the machine. Similarly, improved straightness results in less runout and permits higher speeds in production. Bottom line: Hot roll bars may be cheaper by the pound, but machining them will cost your company a lot more because they lack the benefits of cold drawing:  Increased strength  Improved surface finish  Controlled dimensional tolerance  Improved straightness  Improved machinability.

Graph and data: AISI Cold Finished Steel Bar Handbook, 1968. (Out of print)