Chapter 2

MATERIALS AND PROCESSES

53

 

 

NODULAR (DUCTILE) CAST IRON has the highest tensile strength of the cast irons, ranging from about 70 to 135 kpsi (480 to 930 MPa). The name nodular comes from the fact that its graphite particles are spheroidal in shape. Ductile cast iron has a higher 2

modulus of elasticity (about 25 Mpsi {172 GPa}) than gray cast iron and exhibits a linear stress-strain curve. It is tougher, stronger, more ductile, and less porous than gray cast iron. It is the cast iron of choice for fatigue-loaded parts such as crankshafts, pistons, and cams.

 

Cast Steels

Cast steel is similar to wrought steel in terms of its chemical content, i.e., it has much less carbon than cast iron. The mechanical properties of cast steel are superior to cast iron but inferior to wrought steel. Its principal advantage is ease of fabrication by sand or investment (lost wax) casting. Cast steel is classed according to its carbon content into low carbon (< 0.2%), medium carbon (0.2–0.5%) and high carbon (> 0.5%). Alloy cast steels are also made containing other elements for high strength and heat resistance. The tensile strengths of cast steel alloys range from about 65 to 200 kpsi (450

to 1380 MPa).

 

Wrought Steels

The term “wrought” refers to all processes that manipulate the shape of the material without melting it. Hot rolling and cold rolling are the two most common methods used though many variants exist, such as wire drawing, deep drawing, extrusion, and cold-heading. The common denominator is a deliberate yielding of the material to change its shape either at room or at elevated temperatures.

HOT-ROLLED STEEL is produced by forcing hot billets of steel through sets of rollers or dies which progressively change their shape into I-beams, channel sections, angle irons, flats, squares, rounds, tubes, sheets, plates, etc. The surface finish of hot-rolled shapes is rough due to oxidation at the elevated temperatures. The mechanical properties are also relatively low because the material ends up in an annealed or normalized state unless deliberately heat-treated later. This is the typical choice for low-carbon structural steel members used for building- and machine-frame construction. Hot-rolled material is also used for machine parts that will be subjected to extensive machining (gears, cams, etc.) where the initial finish of the stock is irrelevant and uniform, non-cold-worked material properties are desired in advance of a planned heat treatment. A wide variety of alloys and carbon contents are available in hot-rolled form.

COLD-ROLLED STEEL is produced from billets or hot-rolled shapes. The shape is brought to final form and size by rolling between hardened steel rollers or drawing through dies at room temperature. The rolls or dies burnish the surface and cold work the material, increasing its strength and reducing its ductility as was described in the section on mechanical forming and hardening above. The result is a material with good surface finish and accurate dimensions compared to hot-rolled material. Its strength and hardness are increased at the expense of significant built-in strains, which can later be released during machining, welding, or heat treating, then causing distortion. Cold-rolled shapes commonly available are sheets, strips, plates, round and rectangular bars, tubes, etc. Structural shapes such as I-beams are typically available only as hot rolled.

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54

MACHINE DESIGN -

An Integrated Approach

 

 

Steel Numbering Systems

 

Several steel numbering systems are in general use. The ASTM, AISI, and SAE* have 2

devised codes to define the alloying elements and carbon content of steels. Table 2-5

 

 

lists some of the AISI/SAE designations for commonly used steel alloys. The first two digits indicate the principal alloying elements. The last two digits indicate the amount of carbon present, expressed in hundredths of a percent. ASTM and the SAE have developed a new Unified Numbering System for all metal alloys, which uses the prefix UNS followed by a letter and a 5-digit number. The letter defines the alloy category, F for cast iron, G for carbon and low-alloy steels, K for special-purpose steels, S for stainless steels, and T for tool steels. For the G series, the numbers are the same as the AISI/SAE designations in Table 2-5 with a trailing zero added. For example, SAE 4340

 

becomes UNS G43400. See reference 2 for more information on metal numbering systems. We will use the AISI/SAE designations for steels.

 

* ASTM is the American Society

PLAIN CARBON STEEL is designated by a first digit of 1 and a second digit of 0, for Testing and Materials, AISI is

the American Iron and Steel

since no alloys other than carbon are present. The low-carbon steels are those numbered Institute, and SAE is the Society of

AISI 1005 to 1030, medium-carbon from 1035 to 1055, and high-carbon from 1060 to Automotive Engineers. AISI and

SAE both use the same designa-

1095. The AISI 11xx series adds sulphur, principally to improve machinability. These tions for steels.

are called free-machining steels and are not considered alloy steels as the sulphur does Table 2-5

AISI/SAE Designations of Steel Alloys

A partial list - other alloys are available - consult the manufacturers Type

AISI/SAE Series

Principal Alloying Elements

 

 

Carbon Steels

Plain

10xx

Carbon

Free-cutting

11xx

Carbon plus Sulphur (resulphurized)

 

 

Alloy Steels

 

 

Manganese

13xx

1.75% Manganese

 

15xx

1.00 to 1.65% Manganese

Nickel

23xx

3.50% Nickel

 

25xx

5.00% Nickel

Nickel-Chrome

31xx

1.25% Nickel and 0.65 or 0.80% Chromium

33xx

3.50% Nickel and 1.55% Chromium

Molybdenum

40xx

0.25% Molybdenum

44xx

0.40 or 0.52% Molybdenum

Chrome-Moly

41xx

0.95% Chromium and 0.20% Molybdenum

Nickel-Chrome-Moly

43xx

1.82% Nickel, 0.50 or 0.80% Chromium, and 0.25% Molybdenum

47xx

1.45% Nickel, 0.45% Chromium, and 0.20 or 0.35% Molybdenum

Nickel-Moly

46xx

0.82 or 1.82% Nickel and 0.25% Molybdenum

48xx

3.50% Nickel and 0.25% Molybdenum

Chrome

50xx

0.27 to 0.65% Chromium

 

51xx

0.80 to 1.05% Chromium

 

52xx

 

1.45% Chromium

Chrome-Vanadium

61xx

0.60 to 0.95% Chromium and 0.10 to 0.15% Vanadium minimum

 

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Image 154