Here are 5 benefits of cold working of steels that make a difference to your machining operations.
- Increased strength
- Improved surface finish
- Controlled dimensional tolerance and concentricity
- Improved straightness
- Improved machinability.
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.
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 by shot 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.
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.
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 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:
Graph and data: AISI Cold Finished Steel Bar Handbook, 1968. (Out of print)