Finish blast

When he d finished blasting every chicken he could see, he walked over to the nearest corpse. And it started running away from him, head flopping down its chest on the end of a flaccid strip of skin. He stared at it disbelievingly he d always thought that was an urban myth. Then another of the corpses sprinted for freedom. Moyo pushed his sleeves up and summoned a larger bolt of white fire. 

This industry sector includes pig iron manufacture manufacture of ferro-alloys from iron ore and from iron and steel scr< converting pig iron, scr iron, and scrap steel into steel hot rolling and cold finishing. Blast furnaces and by-product (or beehive) coke ovens are also included under this category, although these are almost nonexistent in the United States today. The complex and interdependent operations involved in a steel industry around the world can be listed as follows ... 

Steel Shot and Grit. Steel shot and grit are also widely used ia grit blasting and abrasive finishing. In 1989, 220, 196 metric tons of metallic abrasives were produced in the United States with a combined value of 89.55 million (20). 

In addition to then use in bonded and coated products, both natural and manufactured abrasive grains are used loose in such operations as polishing, buffing, lapping, pressure blasting, and barrel finishing. AH of these operations are characterized by very low metal removal rates and are used to improve the surface quaUty of the workpiece. 

Application of protective paints consists of surface preparation of steel, priming coat and finishing coats. Wherever possible, steel should be blast-cleaned before painting. Primers thoroughly wet the metal to promote adhesion of finishing paints and carry inhibitive pigments. For example, red lead oxide will minimize the spread of rust on metal surfaces. The total thickness of fmishing coats must be at least 0.125 mm for adequate protection and life. Four coats of paint usually are necessary to achieve this. 

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

Surface finish is increasingly referred to as surface cleanliness . This can be misleading because the standards refer to the appearance of the blasted steel and do not deal with chemical contamination. Site tests for assessing the level of soluble salts on freshly blast-cleaned surfaces, and which allow the semi-quantitative determination of the chlorides, soluble sulphates and soluble iron salts, are urgently needed. 

In BS 4232, Surface Finish of Blast Cleaned Steel for Painting, first quality corresponds to SA3,2nd to SA2-5 and 3rd toSA2of the much more extensive Swedish Standards Commission s SIS 055 900-1962, RusI Grades for Steel Surfaces and Preparation Grades Prior to Protective Coaling, Stockholm (1962) see also BS 7079 Part A1 (1989)... 

Note The above systems are for application to steel blast-cleaned to a near-while finish (Second Quality of BS 4232 1967) and immediately shop-primed before fabrication. The shop primer must be thoroughly cleaned and degreased at the lime of painting. 

Figure 19.60 illustrates a steel surface which should have been blast cleaned to a high standard before being coated at works with a zinc-rich epoxy primer and on site with two-pack intermediate and finishing coats. After exposure for 18 months in a marine environment, flaking millscale from beneath the paint was observed, and a survey showed that the paint... 

The inspector s working standard for the surface finish of blast-cleaned steel is now the new British Standard . In the special circumstances where absolute freedom from soluble contaminants such as ferrous sulphate is necessary, the specification should include reference to a test for removal of such residual salts, e.g. the potassium ferricyanide test . The area to be sampled should be wetted with a fine spray of distilled water and the paper held against it. The development of blue spots on the paper indicates the presence of ferrous salts on the surface. 

It should be recognized that surface finish can be more than just a cosmetic standard. It also affects product quality, mold or die cost, and delivery time of tools and/or products. The surface can be used not only to enhance clarity for the sake of appearance but to hide surface defects such as sink and parting marks. The Society of Plastics Engi-neers/Society of Plastics Industries standards range from a No. 1 mirror finish to a No. 6 grit blast finish. A mold finish comparison kit consisting of six hardened tool steel pieces and... 

Presplitting. A technique of blasting which gives accurate finished contours (see p. 148). 

The manufacture of Portland concrete consists of three basic steps—crushing, burning, and finish grinding. As noted earlier, Portland cement contains about 60% lime, 25% silicates, and 5% alumina with the remainder being iron oxides and gypsum. Most cement plants are located near limestone (CaCOs) quarries since this is the major source of lime. Lime may also come from oyster shells, chalk, and a type of clay called marl. The silicates and alumina are derived from clay, silicon sand, shale, and blast-furnace slag. 

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