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| Galvanic Action |
| Corrosion induced by the action of an electric current caused to flow when two metals of dissimilar composition are connected electrically by an electrolyte. The metal higher in the galvanic series will act as cathode, while the other will act as anode and will therefore corrode. |
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| Galvanising |
| Galvanising is the process in which corrosion protection is provided to a steel product by coating it in zinc. This can be achieved either through a hot dip process, or electrolytically. In the former, the steel, typically a cold reduced strip passes through a series of heat treatments in which it is annealed and the surface is cleaned prior to the strip being passed continuously into a bath of molten zinc at about 440C. The coating mass is controlled as a result of the strip then passed through a set of air (or nitrogen) knives which force surplus zinc back into the bath. The action of these knives is controlled by a meter which is set according to the coating mass actually required. The alternative method of coating is to do it electrolytically; in this case the feedstock is an annealed and temper rolled cold reduced product. Generally speaking, the coating masses so applied are much lighter than by hot dipping. Other steel products such as wire and sections may also be hot dip galvanised, often at a later stage in the production route. It should be remembered that hot dip galvanising alters the properties of the steel and therefore if steels are to be post-galvanised as part of the manufacturing process this should be stated at the time of ordering. |
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| Galvanneal |
| A variation of the continuous hot dip galvanising process. Following the solidification of the strip on exit form the galvanising bath it is passed through another furnace in which the iron and zinc are caused to alloy (8-12% iron) more strongly than can be achieved purely by dipping. This product is used in the automotive industry where good corrosion resistance is combined with much improved weldability compared with a pure zinc coating. |
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| Gamma Iron |
| The form of iron (also called austenite) which exists between 910C and 1392C in which the iron atoms are arranged in a face centred cubic (fcc) crystalline pattern, ie an atom at each corner of a cube, and a further atom in the centre of each face. See also austenite. |
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| Gas Carburising |
| A method of case-hardening a steel in which carbon, from a gaseous atmosphere, is diffused into the surface of the steel while it is at high temperature in the austenitic condition. |
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| Gauge Length |
| A length on the parallel-sided central portion of a tensile test specimen which is used as the basis for determining the strain and total elongation of the specimen. |
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| Gauge Plate |
| In the trade this is also known as 'ground flat stock'. It is a tool steel usually containing 0.9% C, 1.2% Mn, 0.5% Cr. It is supplied, in the annealed condition, in short lengths, of rectangular cross-section strip with faces ground flat to close tolerances. It is used in the manufacture of gauges, dies, templates etc. |
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| Grain Size Control |
| A term used to describe methods for controlling the grain size of steel during its manufacture. |
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| Grain Size Measurement |
| The measurement of the grain size of a metal by observation of a metallographically prepared sample under a microscope. There are several methods available for quantifying the observations. |
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| Graphitising |
| The partial, or complete conversion of the combined carbon which is in the form of iron carbide, Fe3C (cementite) uncombined carbon (graphite) and ferrite. This reaction can take place in steel or in cast iron. In the latter it often occurs at solidification. In the solid form graphitisation requires extended heat treatment times at sub-critical temperature. |
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| Grey Iron |
| A common form of cast iron so-called because the fracture surface of the iron has a grey appearance due to the graphite flakes present in the microstructure. |
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| Grinding |
| The removal of material from a work-piece surface using a grinding wheel or abrasive belt. |
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| Grinding Cracks |
| The formation of shallow surface cracks on a ground work-piece. Normally caused by the generation of excessive heat when an incorrect grinding procedure has been used. Most commonly seen on hardened steel. |
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| Grit |
| An angular steel abrasive used in the blast cleaning process. |
