55
not improve the mechanical properties and also makes it brittle. The ultimate tensile strength of plain carbon steel can vary from about 60 to 150 kpsi (414 to 1 034 MPa) AISI #
depending on heat treatment.
2
4340
ALLOY STEELS have various elements added in small quantities to improve the material’s strength, hardenability, temperature resistance, corrosion resistance, and other 4140
properties. Any level of carbon can be combined with these alloying elements. Chromium is added to improve strength, ductility, toughness, wear resistance, and harden-1095
ability. Nickel is added to improve strength without loss of ductility, and it also enhances case hardenability. Molybdenum, used in combination with nickel and/or chromium, adds hardness, reduces brittleness, and increases toughness. Many other alloys in vari-6150
ous combinations, as shown in Table 2-5, are used to achieve specific properties. Spe-cialty steel manufacturers are the best source of information and assistance for the 9255
engineer trying to find the best material for any application. The ultimate tensile strength of alloy steels can vary from about 80 to 300 kpsi (550 to 2 070 MPa), depending on 3140
its alloying elements and heat treatment. Appendix A contains tables of mechanical property data for a selection of carbon and alloy steels. Figure 2-18 shows approximate 1050
ultimate tensile strengths of some normalized carbon and alloy steels and Figure 2-19
shows engineering stress-strain curves from tensile tests of three steels.
1040
TOOL STEELS are medium- to high-carbon alloy steels especially formulated to give very high hardness in combination with wear resistance and sufficient toughness 1030
to resist the shock loads experienced in service as cutting tools, dies and molds. There is a very large variety of tool steels available. Refer to the bibliography and to manu-1118
facturers’ literature for more information.
1020
STAINLESS STEELS are alloy steels containing at least 10% chromium and offer much improved corrosion resistance over plain or alloy steels, though their name should not 1015
be taken too literally. Stainless steels will stain and corrode (slowly) in severe environments such as seawater. Some stainless-steel alloys have improved resistance to high Kpsi 0
100
200
MPa 0
700
1400
tensile strength
F I G U R E 2 - 18
Approximate Ultimate
Tensile Strengths of
Some Normalized Steels
F I G U R E 2 - 19
Tensile Test Stress-Strain Curves of Three Steel Alloys (From Fig. 5.16, p. 160, in N. E. Dowling, Mechanical Behavior of Materials, Prentice-Hall, Englewood Cliffs, N.J., 1993, with permission)
56
MACHINE DESIGN -
An Integrated Approach
temperature. There are four types of stainless steel, called martensitic, ferritic, aus-tenitic, and precipitation hardening.
2
Martensitic stainless steel contains 11.5 to 15% Cr and 0.15 to 1.2% C, is magnetic, can be hardened by heat treatment, and is commonly used for cutlery. Ferritic stainless steel has over 16% Cr and a low carbon content, is magnetic, soft, and ductile, but is not heat treatable though its strength can be increased modestly by cold working. It is used for deep-drawn parts such as cookware and has better corrosion resistance than the martensitic SS. The ferritic and martensitic stainless steels are both called 400
series stainless steel.
Austenitic stainless steel is alloyed with 17 to 25% chromium and 10 to 20% nickel.
It has better corrosion resistance due to the nickel, is nonmagnetic, and has excellent ductility and toughness. It cannot be hardened except by cold working. It is classed as 300 series stainless steel.
Precipitation-hardening stainless steels are designated by their alloy percentages followed by the letters PH, as in 17-4 PH which contains 17% chromium and 4% nickel.
These alloys offer high strength and high temperature and corrosion resistance.
The 300 series stainless steels are very weldable but the 400 series are less so. All grades of stainless steel have poorer heat conductivity than regular steel and many of the stainless alloys are difficult to machine. All stainless steels are significantly more alloy
expensive than regular steel. See Appendix A for mechanical property data.
7075-T6
Aluminum
2014-T6
Aluminum is the most widely used nonferrous metal, being second only to steel in world consumption. Aluminum is produced in both “pure” and alloyed forms. Aluminum is commercially available up to 99.8% pure. The most common alloying elements are 2024-T4
copper, silicon, magnesium, manganese, and zinc, in varying amounts up to about 5%.
The principal advantages of aluminum are its low density, good strength-to-weight ratio (SWR), ductility, excellent workability, castability, and weldability, corrosion resis-6061-T6
tance, high conductivity, and reasonable cost. Compared to steel it is 1/3 as dense (0.10
lb/in3 versus 0.28 lb/in3), about 1/3 as stiff (E = 10.3 Mpsi {71 GPa} versus 30 Mpsi 6063-T6
{207 GPa}), and generally less strong. If you compare the strengths of low-carbon steel and pure aluminum, the steel is about three times as strong. Thus the specific strength 1100-H18
is approximately the same in that comparison. However, pure aluminum is seldom used in engineering applications. It is too soft and weak. Pure aluminum’s principal advantages are its bright finish and good corrosion resistance. It is used mainly in decorative applications.
1100-0
The aluminum alloys have significantly greater strengths than pure aluminum and are used extensively in engineering, with the aircraft and automotive industries among 0 kpsi
50
100
the largest users. The higher-strength aluminum alloys have tensile strengths in the 70
0 MPa 345
690
to 90 kpsi (480 to 620 MPa) range, and yield strengths about twice that of mild steel.
tensile strength
They compare favorably to medium-carbon steels in specific strength. Aluminum com-petes successfully with steel in some applications, though few materials can beat steel F I G U R E 2 - 20
if very high strength is needed. See Figure 2-20 for tensile strengths of some aluminum alloys. Figure 2-21 shows tensile-test engineering stress-strain curves for three Ultimate Tensile
Strengths of Some
aluminum alloys. Aluminum’s strength is reduced at low temperatures as well as at Aluminum Alloys
elevated temperatures.