Information Part I

Enabling Objectives

1.7Unaided, the participant will correctly describe different surface conditions.part i

Describe Different Surface ConditionsThere are a variety of material attributes leading to surface conditions which can affect the efficacy of a visual examination, these include: cleanliness, color, condition, geometry, size, temperature, surface texture, type of material and any surface coatings present.

Cleanliness of test surface

The component or part to he tested must he adequately cleaned prior to inspection. A dirty surface will make the surface finish appear different, obstructs visual assessment of a surface and could mask defects. The cleaning method most suitable to use depends on various factors including the properties of the test-piece, the contaminants to he removed, skill required, access and cost.

Examples: a large fixed component will not be able to be dipped in a tank of solvent; hut could be cleaned by brush. Aluminum will react with alkalis, so basic cleaning agents cannot be used with aluminum components. Certain solvents will dissolve the test-piece or may cause corrosion. Abrasive cleaning may damage the surface of the-test piece.

Methods of CleaningThe surfaces may have adherent materials or surface contamination requiring different methods for removal. The main cleaning methods are as follows:

1.Dry abrasive blasting

2.Wet blasting

3.Wire brushing

4.Grinding

5.Scraping

6.Needle gunning

7.Flame cleaning

8.Paint stripper

9.Vapor degreasing

10.Solvent cleaning

11.Detergent cleaning

Dry Abrasive Blasting

Abrasive blasting either dry or wet, is carried out with a concentrated stream of small abrasive particles. The abrasive can be either metallic or mineral and includes sand, slag, steel or chilled iron grit and shot, slag and bead.

These are blasted at the surface to remove surface scale, rust or paint that is adherent to the surface. Grease and oil must be removed prior to blasting.

The action of abrasive blasting results in a clean surface which may, depending upon the abrasive used, produce a rough surface finish which may he required for adhesion purposes if a subsequent coating is to be used. When used on steel, some abrasives (especially shot) plastically deform and work harden the surface.

Small surface-breaking defects may he peened over and hidden from view. Wet blasting Wet blasting methods are good for removing chlorides from surfaces and are good for the removal of toxic coatings, e.g. red lead paint films, because they do not create dust.

However, all wet blasting methods have similar disadvantages over dry abrasive blasting, including: the availability and drainage of water, the production and disposal of sludge (particularly with abrasive injection), the extra cost of supplying and mixing a substrate inhibitor and the problem of drying a large surface area.

High-pressure Pure Water Blasting

Operates at pressures up to 35,000 psi (pounds per square inch) which can be extremely dangerous. The advantages of this method are as follows:

Simple to operate

Highly flexible and mobile in use

Suitable for removing soluble contamination

Will remove mill scale at high pressures

High-pressure Water Pus Abrasive InjectionOperates at pressures up to 20,000 psi which can be extremely dangerous. The advantages of this method are the same as for high-pressure pure water blasting, but will also remove firmly held contamination and will leave a surface profile.

Low-pressure Water Plus Abrasive Injection

Operates at 100 psi. It is claimed that this technique is very controllable and will remove one coat of paint of a multi-coat system if required. Disadvantages include high cost and low efficiency.

Steam Cleaning (with or without abrasive injection)Operates at approximately 100 psi. This method is ideal for surfaces contaminated with oil, grease, etc.. Disadvantages include high cost and low efficiency.

Air Blasting with Water InjectionWater with or without an inhibitor is injected into an air/abrasive stream.

Wire BrushingHand and power wire brushing is an effective method of removing the majority of the less adherent materials but not as effective as abrasive blasting for scaled and painted surfaces.Grinding

The action of grinding removes the surface metal/material in a localized area and is useful for spot dressing. Full surface grinding is when all the surface is dressed with machine grinding producing a uniform surface compared to hand grinding which tends to produce undulations.

Belt or disc sanding is a less severe method than grinding for surface dressing.

Some materials, i.e. those that can be hardened, are susceptible to grinding cracks caused by the heat generated by the friction and the subsequent rapid cooling related to area effects.Scraping

Hand or power tool cleaning with scrapers remove lightly adherent material from the surface without significant metal removal, except when employed on soft materials.Needle Gunning

The needle gun consists of numerous air-operated, reciprocating needles that are used to clean areas difficult to reach by other methods, welds and rivet heads.Flame Cleaning

The method of applying an oxyacetylene flame to the steel surface to be cleaned is an efficient method of removing rust, mill scale and other contamination. The effectiveness of the process is due to a combination of factors: Differential expansion The mill scale, on contact with the intense heat, expands at a faster rate than the steel to which it is attached and flakes off.

Dehydration Rust is a combination of iron oxide and moisture. As the moisture is rapidly driven off, the rust is dehydrated and converted to a dry powder which can then be removed by wire brushing.

Heat penetration The heat from the flame penetrates all the surface irregularities and removes all traces of moisture, oil, grease etc. The flame cleaning of any form of fastener, e.g. rivets or bolts, should be avoided as a loss of mechanical strength may be caused.Paint strippers

One of the most effective methods of removing paint coatings without mechanical damage to the base material is paint stripper or paint remover. The painted surface is softened and may then be removed by either scraping or washing. Paint strippers are solvents and blends of solvents specially formulated to remove different types of paint. They do not clean dirt, scale, grease etc. effectively.

Vapor degreasing

True vapor degreasing is the immersion of the component for cleaning in a solvent vapor of 1, 1, 1 trichloroethane or similar. This is regarded as the most effective method for the removal of grease, oil and semi -adherent surface debris. This method requires specialist equipment and is most useful for components in a factory house situation. It requires a special tank/heater and condensation tubes to prevent the vapor from spilling out of the tank.Solvent Cleaning

Solvents which are capable of breaking down grease and oil-based surface contamination are very effective in removing lightly adherent surface contamination. Application by immersion, brushing, or wiping must be thorough and may require repeated applications on heavy deposits. No special equipment is required, although good ventilation is essential for safety. This method can be used on large or small test-pieces and can be employed in or out of doors. Care must be used to ensure that the solvent does not react with the test material.Detergent CleaningDetergents are either alkaline or acid based and are generally used to remove light surface contamination. Grease or oil-based deposits, especially heavy deposits, are very difficult to remove. Detergent cleaning is a relatively safe method of cleaning.Surface Profile and FinishThis section deals with the shape of a surface and its texture. Defects can be difficult to identify in a rough surface and rough surfaces can cause problems with magnification. The roughness of a surface is governed by the peak to trough height, the density of texture, mainly peaks and troughs and shape of the undulations. The peak to trough height (amplitude) or appearance of a surface may be assessed by a number of methods, including the use of a surface profile needle gauge, surface replica tape or a surface comparator.

Figure 1

Shape of a Surface

Amplitudes are often measured on blast surfaces prior to coating application, but for most other surface assessments a judgment is made by eye, sometimes with the aid of a comparator. Comparators are available for a variety of surface textures including machining and blasting.

Replication is a method of copying the surface condition of components for analysis remote from the test item. Methods of replication range from plaster casting to cellulose acetate films, the latter being used for metallographic examinations.

Areas

A common method for grading a surface texture, especially on machined or ground components, is to use the arithmetical mean deviation from a reference line drawn midway between the peaks to trough height. The values, measured in microns, are suffixed with the R? parameter. This measurement was previously known as the CLA or Center Line Average.

Figure 2A

Figure 2B

Grading a Surface TextureAnother method is to use R(, which is the 10 point height parameter, which is defined as the average distance between the five highest peaks and the five deepest valleys within the sample length, measured from a line parallel to the reference line.Surface FinishThe surface finish of a test item immediately after processing will typically be as follows:As CastCast surfaces vary from a poor to excellent finish. The degree of fine detail that can be detected is dependent upon method of casting and condition of the moulds or masters. Surface roughness ranges from about 1.0 ? m R? on die castings to 25 (m R( for sand castings.

Sand casting - generally rough surface finish.

Shell mould - smooth, good surface finish.

Die casting - smooth, very good finish.

Investment casting - depends on the master, but can be a very good finish.

Hot workedHot worked surfaces invariably have an oxide layer and, in some instances, grain boundary penetration of oxide which is part of the high-temperature scaling mechanism. Examination for fine detail on such surfaces is not possible without dressing (grinding, sanding etc.) either a small area or the whole surface, in order to remove the oxides. Surface roughness on hot rolled surfaces is between 12.5 to 25 (m R(, and 3.2 to 12.5 (m for R( forged material.

Hot rolled - scaled finish, surface quality depends upon the finishing temperature.

Hot rolled de-scaled - surface condition depends upon roll surface condition.

Cold rolled - generally very good surface finish.

Hot forged open scaled, moderate surface finish.

Closed die forging - scaled good surface finish.

Cold forged (cold heading) - usually very good.

Machined

Machining introduces plastic deformation into the machined surface which tends to smear the surface layer, so masking fine detail. Machined surfaces vary from 0.05 to 1.6 (m R( Rough machined - coarse surface finish.

Fine - good surface finish.

Polished - very good/excellent surface finish.

Mirror excellent surface finish.

Sawn

The roughness of a sawn surface falls within the range of 3.2 to 25 (m R(. Circular - rough surface finish.

Bandsaw - fine surface finish.

GroundDepends upon the grit used for grinding; fine to coarse.

Polished

The surface produced may he by mechanical methods, fixed abrasion, slurry/paste, or electropolished. Excellent surface finish, typically from 0.02 to 1.0 (m R(.

Blasted

Finish depends on the hardness of surface, abrasive characteristics (density, size and shape etc.), and angle of impingement, but amplitudes vary from a few microns to greater than 100 (m.Temperature

The temperature of objects for visual inspection would normally be at room temperature (approximately 20C). However, there are instances where this is not possible and may create dimensional problems and possibly a health hazard. Objects with temperatures below freezing or above about 60C can inflict damage on unprotected hands thereby producing a health risk. Dimensional stability is also affected since calibration is usually at 20C and a component temperature that varies from this will produce inaccuracies in both component and measuring equipment.

Visual inspection of very hot objects from a distance is a frequent event in manufacturing to observe the condition of plant and product. Furnace hearth and ladle lining conditions are examined between casts, soaking pit and reheating furnace every shift for refractory damage or burner problems. During the manufacturing process, inspection of ingots takes place above 900C, and of hot rolled and forged products between 960C and 1 250C. In-service inspection of engines, hollers etc. takes place at the elevated working temperature.

Surface Coating

The visual examination of a surface coating is a subject by itself where specialist knowledge would be required. Within the scope of this unit, surface coatings are relevant because they interfere with the visual examination of the metal substrate. There are many types of coating such as organic paints and metallic coatings. Anodized surfaces may also be encountered; this is a very thin surface finish, 1-2 microns, consisting of an oxidized layer which can easily he destroyed.Surfaces should be visually examined prior to coating for surface defects and also for the correct finish for coating adhesion and coating durability purposes. It is worth noting that the degree of surface preparation often governs the service life of the coating.

Sometimes the visual examination of a coated surface is a requirement in order to detect and measure corrosion. To determine the full extent of corrosion the surface coating must be removed in the vicinity of the problem area. It must not be assumed that organic paints can all be removed with a solvent. There are many types of paint and some can only be successfully removed with abrasive blasting or heating, scraping, and power brushing.ColorA strong contrast of color and patterns such as black and white, red and blue, or red and green should be avoided as this may cause physiological problems and interfere with perception.

The color of a surface is considered in terms of its three chief qualities: hue, lightness, and saturation.HueColors are divided into groups having the same hue, i.e. into reds, yellows, greens, blues, purples etc.. In ordinary speech this quality is often vaguely called color.LightnessThe lightness of a color is determined by the proportion of light which it reflects, irrespective of hue and saturation. Corresponding terms used are value and reflectance value.

SaturationColors of similar hue and lightness may differ in colorfulness or intensity of color. This quality, termed saturation, may be defined as the intensity of any particular hue when compared with a neutral grey of similar lightness, the spectrum colors being the most intense or of highest saturation. The terms chroma and intensity are also used in a similar sense.The Munsell systemThe Munsell system of color coding shows in a convenient manner the relationship between the three chief qualities of color. The circular band represents the hues in proper order, the vertical axis is the scale of value and the paths protruding outwards from the center represent the degree of chroma which increases in intensity in the direction of the arrow.

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