51
troplating, the electroless nickel plate is completely uniform and will enter holes and crevices. The plate is dense and fairly hard at around 43 HRC. Other metals can also be electroless plated but nickel is most commonly used.
2
Anodizing
While aluminum can be electroplated (with difficulty), it is more common to treat it by anodizing. This process creates a very thin layer of aluminum oxide on the surface. The aluminum oxide coating is self-limiting in that it prevents atmospheric oxygen from further attacking the aluminum substrate in service. The anodized oxide coating is naturally colorless but dyes can be added to color the surface and provide a pleasing appearance in a variety of hues. This is a relatively inexpensive surface treatment with good corrosion resistance and negligible distortion. Titanium, magnesium, and zinc can also be anodized.
A variation on conventional anodizing of aluminum is so-called “hard anodizing.”
Since aluminum oxide is a ceramic material, it is naturally very hard and abrasion resistant. Hard anodizing provides a thicker (but not actually harder) coating than conventional anodizing and is often used to protect the relatively soft aluminum parts from wear in abrasive contact situations. The hardness of this surface treatment exceeds that of the hardest steel, and hard-anodized aluminum parts can be run against hardened steel though the somewhat abrasive aluminum oxide surface is not kind to the steel.
Plasma-Sprayed Coatings
A variety of very hard ceramic coatings can be applied to steel and other metal parts by a plasma-spray technique. The application temperatures are high, which limits the choice of substrate. The coatings as sprayed have a rough “orange-peel” surface finish which requires grinding or polishing to obtain a fine finish. The main advantage is a surface with extremely high hardness and chemical resistance. However, the ceramic coatings are brittle and subject to chipping under mechanical or thermal shock.
Chemical Coatings
The most common chemical treatments for metals range from a phosphoric acid wash on steel (or chromatic acid on aluminum) that provides limited and short-term oxidation resistance, to paints of various types designed to give more lasting corrosion protection. Paints are available in a large variety of formulations for different environments and substrates. One-part paints give somewhat less protection than two-part epoxy formulations, but all chemical coatings should be viewed as only temporary protection against corrosion, especially when used on corrosion-prone materials such as steel.
Baked enamel and porcelain finishes on steel have longer lives in terms of corrosion resistance, though they suffer from brittleness. New formulations of paints and protective coatings are continually being developed. The latest and best information will be obtained from vendors of these products.
52
MACHINE DESIGN -
An Integrated Approach
2.6
GENERAL PROPERTIES OF METALS
magnesium
6.5 (44.8)
The large variety of useful engineering materials can be confusing to the beginning 2
engineer. There is not space enough in this book to deal with the topic of material se-aluminum
lection in complete detail. Several references are provided in this chapter’s bibliogra-10.4 (71.8)
phy which the reader is encouraged to use. Tables of mechanical property data are also provided for a limited set of materials in Appendix A of this book. Figure 2-17 shows gray cast iron
the Young’s moduli for several engineering metals.
15 (104)
The following sections attempt to provide some general information and guidelines brass, bronze
for the engineer to help identify what types of materials might be suitable in a given 16 (110)
design situation. It is expected that the practicing engineer will rely heavily on the ex-titanium
pertise and help available from materials manufacturers in selecting the optimum ma-16.5 (114)
terial for each design. Many references are also published which list detailed property data for most engineering materials. Some of these references are listed in the bibli-ductile cast iron
ography to this chapter.
24 (166)
stainless steel
Cast Iron
27.5 (190)
Cast irons constitute a whole family of materials. Their main advantages are relatively steel
low cost and ease of fabrication. Some are weak in tension compared to steels but, like 30 (207)
most cast materials, have high compressive strengths. Their densities are slightly lower than steel at about 0.25 lb/in3 (6 920 kg/m3). Most cast irons do not exhibit a linear 0
10
20
30
stress-strain relationship below the elastic limit; they do not obey Hooke’s law. Their 0
70
140 210
modulus of elasticity E is estimated by drawing a line from the origin through a point on the curve at 1/4 the ultimate tensile strength and is in the range of 14–25 Mpsi (97–
172 MPa). Cast iron’s chemical composition differs from steel principally in its higher Young's Modulus E
carbon content, being between 2 and 4.5%. The large amount of carbon, present in some Mpsi (GPa)
cast irons as graphite, makes some of these alloys easy to pour as a casting liquid and also easy to machine as a solid. The most common means of fabrication is sand cast-F I G U R E 2 - 17
ing with subsequent machining operations. Cast irons are not easily welded, however.
Young's Moduli for
Various Metals
WHITE CAST IRON is a very hard and brittle material. It is difficult to machine and has limited uses, such as in linings for cement mixers where its hardness is needed.
GRAY CAST IRON is the most commonly used form of cast iron. Its graphite flakes give it its gray appearance and name. The ASTM grades gray cast iron into seven classes based on the minimum tensile strength in kpsi. Class 20 has a minimum tensile strength of 20 kpsi (138 MPa). The class numbers of 20, 25, 30, 35, 40, 50, and 60 then represent the tensile strength in kpsi. Cost increases with increasing tensile strength. This alloy is easy to pour, easy to machine, and offers good acoustical damping. This makes it the popular choice for machine frames, engine blocks, brake rotors and drums, etc.
The graphite flakes also give it good lubricity and wear resistance. Its relatively low tensile strength recommends against its use in situations where large bending or fatigue loads are present, though it is sometimes used in low-cost engine crankshafts. It runs reasonably well against steel if lubricated.
MALLEABLE CAST IRON has superior tensile strength to gray cast iron but does not wear as well. The tensile strength can range from 50 to 120 kpsi (345 to 827 MPa) depending on formulation. It is often used in parts where bending stresses are present.