2.4 T EMPERATURE E FFECTS (B OTH H IGH AND L OW )

The test data used in design and engineering decisions should always be obtained under conditions that simulate those of actual service. A number of engineered structures, such as aircraft, space vehicles, gas turbines, and nuclear power plants, are required to oper-ate under temperatures as low as
130°C (
200°F) or as high as 1250°C (2300°F). To cover these extremes, the designer must consider both the short- and long-range effects of temperature on the mechanical and physical properties of the material being con-sidered. From a manufacturing viewpoint, the effects of temperature are equally im-portant. Numerous manufacturing processes involve heat, and the elevated temperature and processing may alter the material properties in both favorable and unfavorable ways. A material can often be processed successfully, or economically, only because heat-ing or coolheat-ing can be used to change its properties.

Elevated temperatures can be quite useful in modifying the strength and ductili-ty of a material. Figure 2-30 summarizes the results of tensile tests conducted over a wide range of temperatures using a medium-carbon steel. Similar effects are presented for magnesium in Figure 2-31. As expected, an increase in temperature will typically in-duce a decrease in strength and hardness and an increase in elongation. For manufac-turing operations such as metalforming, heating to elevated temperature may be extremely attractive because the material is now both weaker and more ductile.

FIGURE 2-29 Fatigue fracture of AISI type 304 stainless steel viewed in a scanning electron microscope at 810X. Well-defined striations are visible.

(From “Interpretation of SEM Fractographs,” Metals Handbook, Vol. 9, 8th ed., ASM International, Materials Park, OH, 1970, p. 70.)

120

200 400 600 800 1000 1200 1400 FIGURE 2-30 The effects of

temperature on the tensile properties of a medium-carbon steel.

50

FIGURE 2-32 The effects of temperature and strain rate on the tensile strength of copper. (From A. Nadai and M. J.

Manjoine, Journal of Applied Mechanics, Vol. 8, 1941, p. A82, courtesy of ASME.)

70

FIGURE 2-33 The effect of temperature on the impact properties of two low-carbon steels.

Figure 2-32 shows the combined effects of temperature and strain rate (speed of testing) on the ultimate tensile strength of copper. For a given temperature, the rate of deformation can also have a strong influence on mechanical properties. Room-temper-ature standard-rate tensile test data will be of little value if the application involves a ma-terial being hot-rolled at speeds of 1300 m/min (5000 ft/min).

The effect of temperature on impact properties became the subject of intense study in the 1940s when the increased use of welded-steel construction led to catastrophic fail-ures of ships and other structfail-ures while operating in cold environments. Welding pro-duces a monolithic (single-piece) product where cracks can propagate through a joint and continue on to other sections of the structure! Figure 2-33 shows the effect of de-creasing temperature on the impact properties of two low-carbon steels. Although simi-lar in form, the two curves are significantly different. The steel indicated by the solid line becomes brittle (requires very little energy to fracture) at temperatures below 4°C (25°F) while the other steel retains good fracture resistance down to 26°C (15°F).

The temperature at which the response goes from high energy absorption to low energy absorption is known as the ductile-to-brittle transition temperature. While all steels tend to exhibit this transition, the temperature at which it occurs varies with carbon content and alloy. Special caution should be taken, therefore, when selecting steels for low-tem-perature applications.

Elongation in 2 in. (%)

Temperature (°F)

Psi*10³ Temperature (°C)

50

FIGURE 2-31 The effects of temperature on the tensile properties of magnesium.

Figure 2-34 shows the ductile-to-brittle transition temperature for steel salvaged from the Titanic compared to currently used ship plate material. While both are quality materials for their era, the Titanic steel has a much higher transition temperature and is generally more brittle. Recalling that the water temperature at the time the Titanic struck the iceberg was 2°C, the results show that the steel would have been quite brit-tle. Two curves are provided for each material, reflecting specimens in different orien-tation with respect to the direction of product rolling. Here we see that processing features can further affect the properties and performance of a material.

CREEP

Long-term exposure to elevated temperatures can also lead to failure by a phenomenon known as creep. If a tensile-type specimen is subjected to a constant load at elevated temperature, it will elongate continuously until rupture occurs, even though the applied stress is below the yield strength of the material at the temperature of testing. While the rate of elongation is often quite small, creep can be an important consideration when designing equipment such as steam or gas turbines, power plant boilers, and other de-vices that operate under loads or pressures for long periods of time at high temperature.

If a test specimen is subjected to conditions of fixed load and fixed elevated tem-perature, an elongation-versus-time plot can be generated, similar to the one shown in Figure 2-35. The curve contains three distinct stages: a short-lived initial stage, a rather long second stage where the elongation rate is somewhat linear, and a short-lived third stage leading to fracture. Two significant pieces of engineering data are obtained from this curve: the rate of elongation in the second stage, or creep rate, and the total elapsed

Test temperature (°C) Longitudinal

Longitudinal Modern hull plate

Transverse

Transverse Titanic plate

Absorbed energy (J)

350

300

250

200

150

100

-100 -50 0 50 100 150 200

50 0 FIGURE 2-34 Longitudinal

and transverse notch toughness impact data: steel from the Titanic versus modern steel plate, with both longitudinal and transverse specimens.

(Courtesy I&SM, September 1999, p. 33, Iron and Steel Society, Warrendale, PA.)

Time, t 0

Strain, 

A

Fracture

Slope =d

dt = creep rate

o

  

FIGURE 2-35 Creep curve for a single specimen at a fixed temperature, showing the three stages of creep and reported creep rate. Note the nonzero strain at time zero due to the initial application of the load.

Stress

FIGURE 2-36 Stress–rupture diagram of solution-annealed Incoloy alloy 800 (Fe–Ni–Cr alloy). (Courtesy of Huntington Alloy Products Division, The International Nickel Company, Inc., Toronto, Canada.)

time to rupture.These results are unique to the material being tested and the specific con-ditions of the test. Tests conducted at higher temperatures or with higher applied loads would exhibit higher creep rates and shorter rupture times.

When creep behavior is a concern, multiple tests are conducted over a range of temperatures and stresses, and the rupture time data are collected into a single stress–

rupture diagram, like the one shown in Figure 2-36. This simple engineering tool pro-vides an overall picture of material performance at elevated temperature. In a similar manner, creep-rate data can also be plotted to show the effects of temperature and stress.

Figure 2-37 presents a creep-rate diagram for a high-temperature nickel-based alloy.

■ 2.5 M

, F

,

W

While it is common to assume that the various “-ability” terms also refer to specific ma-terial properties, they actually refer to the way a mama-terial responds to specific process-ing techniques. As a result, they can be quite nebulous. Machinability, for example, depends not only on the material being machined but also on the specific machining process; the conditions of that process, such as cutting speed; and the aspects of that process that are of greatest interest. Machinability ratings are generally based on rela-tive tool life. In certain applications, however, we may be more interested in how easy a metal is to cut, or how it performs under high-speed machining, and less interested in the tool life or the resulting surface finish. For other applications, surface finish or the for-mation of fine chips may be the most desirable feature. As a result, the term machin-ability may mean different things to different people, and it frequently involves multiple properties of a material interacting with the conditions of a process.

In a similar manner, malleability, workability, and formability all refer to a materi-al’s suitability for plastic deformation processing. Since a material often behaves differently at different temperatures, a material with good “hot formability” may have poor defor-mation characteristics at room temperature. Furthermore, materials that flow nicely at low deformation speeds may behave in a brittle manner when loaded at rapid rates. Forma-bility, therefore, needs to be evaluated for a specific combination of material, process, and process conditions. The results cannot be extrapolated or transferred to other processes or process conditions. Likewise, the weldability of a material may also depend on the spe-cific welding or joining process and the spespe-cific process parameters.