Thermal treatment of metals.

The treatment of metals is now an elaborated science of its own. The heating and the quenching are designed to bring about the arrangement of the crystalline structure in the desired way, to give the particular properties required. For example, steels containing a high percentage of carbon may be quenched after being heated to ahigh temperature to make them hard, and then moderately tempered to restore toughness, both these qualities being required in a steel to be used for cutting other metals as in machining. No steel is used today that has not been tested. The rigorous nature of the test depends upon the purpose for which the steel is to be used. The simplest examinations are those made with magnifying glass or a microscope. Spectroscopic or chemical analysis, X-ray examination and magnetic or electrical tests may be applied. Specimens are subjected to physical tests for toughness, strength and hardness, all quantities that may be altered by the heat treatment. The testing of materials is now carried out scientifically by special machines designed for the purpose, able to duplicate the effect of forces likely to be encountered by the steel, but compressing the wear and strain of years into a few minutes.

10. Six basic factors are involved in casting processes:

Mould Cavity. A mould cavity, having the desired shape and size, must be produced with due allowance for shrinkage of the solidifying metal. Any complexity of shape desired in the finished casting must exist in the cavity. The mould material must be able to reproduce the desired detail and also must have a refractory character. Either a new mould must be prepared for each casting (expendable moulds), or the mould must be made from a material that can withstand being used for repeated castings.

Melting Process.A melting process must be capable of providing molten material not only at the proper temperature, but also in the desired quantity, with an acceptable quality, and within a reasonable cost.

Pouring Techniques.A pouring technique must be devised to introduce the molten metal into the mould. The molten metal can then completely fill the cavity, producing a quality casting that is fully dense and free of defects.

Solidification Process.Provision must be made so that the mould will not cause too much restraint to the shrinkage that accompanies the cooling of solidified metal. Otherwise, the casting will crack when it is still hot and its strength is low. In addition, the design of the casting must be such that solidification and solidification shrinkage can occur without producing internal porosity or voids.

Mould Removal. It must be possible to remove the casting from the mould(i.e., mould removal). When the metal is poured into moulds that are broken apart and destroyed after each casting is made, there is no serious difficulty

Cleaning, Finishing and Inspection. After the casting is removed from the mould, various cleaning, finishing, and inspection operations may need to be performed. Extraneous material that is attached where the metal entered the cavity, excesses at mould parting lines, and mould material that is attached to the casting surface must all be removed.

Date: 2015-12-24; view: 873