Working of Metals
In this article, heat treatment and the working of metals will be discussed. The working of metals can be thought of as metallurgical processes that change the properties of metals.
Heat Treatment
Heat treatment of large carbon steel components is done to take advantage of crystalline defects and their effects, and thus obtain certain desirable properties or conditions.
During manufacture, by varying the rate of cooling (quenching) of the metal, grain size and grain patterns are controlled. Grain characteristics are controlled to produce different levels of hardness and tensile strength. Generally, the faster a metal is cooled, the smaller the grain sizes will be. This will make the metal harder. As hardness and tensile strength increase in heat-treated steel, toughness, and ductility decrease.
The cooling rate used in quenching depends on the method of cooling and the size of the metal. Uniform cooling is important to prevent distortion. Typically, steel components are quenched in oil, water, or an emulsion of the two.
Welding can induce internal stresses that will remain in the material after welding is complete. In stainless steels, such as type 304, the crystal lattice is face-centred cubic (austenite). During high temperature welding, some surrounding metal may be elevated to between 500°F (260°C) and 1000°F (537°C). In this temperature region, the austenite is transformed into a body-centred cubic lattice structure (bainite). When the metal has cooled, regions surrounding the weld contain some original austenite and some newly formed bainite. A problem arises because the "packing factor" (PF = volume of atoms/volume of unit cell) is not the same for FCC crystals as for BCC crystals.
The bainite that has been formed occupies more space than the original austenite lattice. This elongation of the material causes residual compressive and tensile stresses in the material. Welding stresses can be minimised using heat sink welding, which results in lower metal temperatures, or, by annealing.
Annealing is another common heat-treating process for carbon steel components. During annealing, the component is heated slowly to an elevated temperature and held there for a long period of time, then cooled. The annealing process is done to obtain the following effects:
a. To soften the steel and improve ductility.
b. To relieve internal stresses caused by previous processes such as heat treatment, welding, or machining.
c. To refine the grain structure.
Cold and Hot Working
Plastic deformation which is carried out in a temperature region and over a time interval such that the strain hardening is not relieved, is called cold work. Considerable knowledge on the structure of the cold-worked state has been obtained. In the early stages of plastic deformation, slip is essentially on primary glide planes and the dislocations form coplanar arrays. As deformation proceeds, cross slip takes place. The cold-worked structure forms high dislocation density regions that soon develop into networks. The grain size decreases with strain at low deformation but soon reaches a fixed size. Cold working decreases ductility.
Hot working refers to the process where metals are deformed above their recrystallisation temperature and strain hardening does not occur. Hot working is usually performed at elevated temperatures. Lead, however, is hot-worked at room temperature because of its low melting temperature. At the other extreme, molybdenum is cold-worked even at relatively high temperatures because of its high recrystallisation temperature.
The resistance of metals to plastic deformation generally reduces at higher temperatures. For this reason, larger massive sections are always worked hot by forging, rolling, or extrusion.
Summary
The important information in this section is summarised below:
Effects of Heat Treatment on Metal Properties Summary
- Quenching
Varying the rate of cooling (quenching) of the metal controls grain size and grain patterns.
Grain characteristics are controlled to produce different levels of hardness and tensile strength.
Hardness and tensile strength increase in heat-treated steel; toughness and ductility decrease. - Welding
Produces residual compressive and tensile stresses.
Stresses are minimised by using heat sink welding and annealing. - Annealing
Softens steel and improves ductility.
Relieves internal stresses caused by previous processes.
Refines grain structure.