ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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Fig. 5 Stress-strain curves in tension at elevated temperature of wrought and composite

2014 aluminum alloy. (a) Wrought 2014-T6, 19.05 mm ( in.) bar. (b) Discontinuously

reinforced 2014 composite (15 vol% Al

, 0.5 h after T6). Source: Ref 4, 5

Fig. 6 Stress-strain curves in tension at various temperatures (30 min exposure) for 2024

aluminum sheet and plate in T3, T6, T81, and T86 conditions. Source: Ref 4

Fig. 7 Typical stress-strain curves in tension for wrought Ti-6Al-4V. (a) Annealed

extrusions. Static strain rate, after ½h exposure. (b) All product forms, solution treated

and aged (STA), longitudinal direction, after ½h exposure. Source: Ref 6

Deformation under tensile conditions is also governed to some extent by crystal structure. Face-centered cubic

(fcc) materials generally exhibit a gradual change in strength and ductility as a function of temperature. Such a

change in the strength of 304 austenitic stainless steel is illustrated in Fig. 1. Some body-centered cubic (bcc)

alloys, however, exhibit an abrupt change at the ductile-to-brittle transition temperature (~200 °C, or 390 °F, for

tungsten in Fig. 1), below which there is little plastic flow. In hexagonal close-packed (hcp) and bcc materials,

mechanical twinning can also occur during testing. However, twinning by itself contributes little to the overall

elongation; its primary role is to reorient previously unfavorable slip systems to positions in which they can be

activated.

There are exceptions to these generalizations, particularly at elevated temperatures. For example, at sufficiently

high temperatures, the grain boundaries in polycrystalline materials are weaker than the grain interiors, and

intergranular fracture occurs at relatively low elongation. In complex alloys, hot shortness, in which a liquid

phase forms at grain boundaries, or grain boundary precipitation can lead to low strength and/or ductility.

Diffusion processes are also involved in yield-point and strain-aging phenomena. Under certain combinations

of strain rate and temperature, interstitial atoms can be dragged along with dislocations, or dislocations can

alternately break away and be repinned, producing serrations in the stress-strain curves. This produces effects

such as discontinuous yielding and upper yield-strength behavior, which are a common occurrence in the

tension testing of low-carbon steels (see the article “Uniaxial Tension Testing” in this Volume).

Another effect that can be accelerated during high-temperature testing is strain aging. In strain aging, cold-

worked steels (especially rimmed or capped steels) undergo a loss in ductility while stored at room temperature.

This loss in ductility is typically attributed to precipitation and diffusion-controlled particle growth along the

slip planes. The precipitation occurs along slip lines because plastic deformation presumably causes local areas

of supersaturation along the slip lines (Ref 7). Strain aging is more pronounced in rimmed and capped steels

than in killed steels. Steels that are drastically deoxidized with aluminum or aluminum and titanium are

essentially nonaging (Ref 8).

Strain aging also causes a ductility drop (and a corresponding increase in hardness and strength) during high-

temperature tension testing of some steels. This effect on tensile strength is shown in Fig. 8 for a mild steel and

a stabilized (nonaging) steel. The increase in strength at elevated temperature is attributed to the acceleration of

precipitation in the grain boundaries, and at high temperature, the influence of strain on precipitation can be the

actual deformation occurring during the hot tension test (Ref 4). This effect is known as strain-age

embrittlement.

Fig. 8 Effect of testing temperature on tensile strength of ordinary mild steel and of

nonaging steel. The nonaging steel gives almost no indication of the “blue heat”

phenomenon. The ductility in a tension test of the nonaging steel in the “blue heat” region

is considerably higher than the ductility of ordinary aging mild steel sheet. Source: Ref 7

Hot tension testing is one of the simplest ways to distinguish aging steels from nonaging steels. This is shown

in Fig. 8 and 9. Aging steels develop an increase in strength and a decrease in ductility within the temperature

range of about 230 to 370 °C (450–700 °F). This strain-aging effect is also known as blue brittleness, because

the effect occurs in the blue-heat region. Tension testing in the blue-heat region is thus one way to identify

aging steels. Blue brittleness affects tensile strength and elongation values (Fig. 10a and b) but not yield

strength (Fig. 10c). Although the effect is more pronounced in rimmed or capped steels, strain aging is also

observed in killed steels (Fig. 11). Drastically killed steels are nonaging.

Fig. 9 Stress-strain curves of ordinary mild steel sheet and nonaging sheet tested at

various temperatures. The higher tensile strength and the “stepped” or “saw-toothed”

stress-strain curve of the ordinary sheet in the “blue heat” region are characteristic.

These features are absent in the nonaging sheet. Source: Ref 7

Fig. 10 Short-term elevated-temperature tensile properties of various normalized carbon

steels. (a) Tensile strength. (b) Elongation. (c) Yield strength. Source: Ref 9

Fig. 11 Effect of temperature on tensile strength and yield strength of structural carbon

steels. (a) Tensile strength. (b) Yield strength. Source: Ref 4

High-Temperature Creep in Structural Alloys. At higher temperatures (between 0.3 T

and 0.6 T

), metals and

ceramics are subject to thermally activated processes that can produce continuous plastic deformation (creep)

with the application of a constant stress. For metals, various mechanisms are used to explain creep deformation,

but all the mechanisms can fall into two basic categories: diffusional creep and dislocation creep (Ref 10).

In diffusional creep, diffusion of single atoms or ions, either by bulk transport (Nebarro-Herring creep) or by

grain-boundary transport (Coble creep) leads to Newtonian viscous flow. In this type of creep, steady-state

creep rates vary linearly. At low stresses, diffusional creep is seen only at very high temperatures in the hot

working region (T > 0.6 T

) and, thus, is not a factor in typical high-temperature structural applications.

For high-temperature structural applications (such as the examples in Table 1), dislocation creep mechanisms

are operative at intermediate and high stresses. These mechanisms include thermally activated processes, such

as multiple slip and cross slip, allowing stress relaxation and reductions in strength. In this temperature region,

creep rates are typically a nonlinear function of stress. They are either a power function or an exponential

function of stress (Ref 7).

Mechanical Testing for High-Temperature Structural Alloys. Maximum-use temperatures of structural

materials can depend on different design criteria, such as strength or graphitization/oxidation in steels (Table 2)

or creep rate and rupture strength (Table 3). When mechanical testing is performed for high-temperature

structural applications, the testing generally includes a combination of both short-term testing for tensile

properties and long-term testing of creep rate and rupture strength.

Table 2 Temperature limits of superheater tube materials covered in ASME Boiler

Codes

Maximum-use temperature

Oxidation/graphitization

criteria, metal surface

(a)

Strength criteria

metal midsection

Material

°C °F °C °F

SA-106 carbon steel

400–500 750–930 425 795

Ferritic alloy steels

0.5Cr-0.5Mo

550 1020 510 950

1.2Cr-0.5Mo

565 1050 560 1040

2.25Cr-1Mo

580 1075 595 1105

9Cr-1Mo

650 1200 650 1200

Austenitic stainless steel, Type 304H

760 1400 815 1500

(a) In the fired section, tube surface temperatures are typically 20–30 °C (35–55 °F) higher than the tube

midwall temperature. In a typical U.S. utility boiler, the maximum metal surface temperature is approximately

625 °C (1155 °F).

Table 3 Suggested maximum temperatures in petrochemical operations for continuous

service based on creep or rupture data

Maximum temperature

based on creep rate

Maximum

temperature

based on rupture

Material

°C °F °C °F

Carbon steel

450 850 540 1000

C-0.5Mo steel

510 950 595 1100

2¼Cr-1Mo steel

540 1000 650 1200

Type 304 stainless steel

595 1100 815 1500

Alloy-C-276 nickel-base alloy

650 1200 1040 1900

The test temperatures for short-term properties depend on the alloy and its typical maximum-use temperature

for application. For example, Fig. 12 is a summary of short-term and long-term properties of various low-alloy

steels with short-term properties up to about 540 °C (1000 °F). In contrast, short-term strength for austenitic

stainless steels (Fig. 13), martensitic stainless steels (Fig. 14), and superalloys (Fig. 15) are tested at higher

temperatures. Other examples of short-term strength at high temperatures are shown for nonferrous alloys (Fig.

16) and precipitation-hardening (PH) stainless steels (Fig. 17). The PH stainless steels have lower maximum-

use temperatures than other stainless steels due to a rapid drop in strength at about 425 °C (800 °F) (Fig. 18).

Alloy Heat treatment

1.0%Cr-0.5%Mo

Annealed at 845 °C (1550 °F)

0.5%Mo

Annealed at 845 °C (1550 °F)

Type 502

Annealed at 845 °C (1550 °F)

2.25%Cr-1.0%Mo

Annealed at 845 °C (1550 °F)

1.25%Cr-1.5%Mo

Annealed at 815 °C (1500 °F)

7.0%Cr-0.5%Mo

Annealed at 900 °C (1650 °F)

9.0%Cr-1.0%Mo

Annealed at 900 °C (1650 °F)

1.0%Cr-1.0%Mo-0.25%V

Normalized at 955 °C (1750 °F), tempered at 649 °C (1200 °F)

H11

Hardened at 1010 °C (1850 °F), tempered at 565 °C (1050 °F)

Fig. 12 Tensile, yield, rupture, and creep strengths of wrought alloy steels containing less

than 10% alloy

Alloy Annealed by rapid cooling from

304

1065 °C (1950 °F)

316

1065 °C (1950 °F)

347

1065°C (1950°F)

309

1095 °C (2000 °F)

310

1095 °C (2000 °F)

321 (stainless)

1010 °C (1850 °F)

Fig. 13 Tensile, yield, and rupture strengths of several stainless steels and higher-nickel

austenitic alloys

Alloy Heat treatment

430

Annealed

446

Annealed

403

Quenched from 870 °C (1600 °F), tempered at 621 °C (1150 °F)

410

Quenched from 955°C (1750°F), tempered at 593°C (1100°F)

431

Quenched from 1025 °C (1875 °F), tempered at 593 °C (1100 °F)

13%Cr-2% Ni-3%W (Greek Ascoloy)

Quenched from 955 °C (1750 °F), tempered at 593 °C (1100 °F)

422

Quenched from 1040 °C (1900 °F), tempered at 593 °C (1100 °F)

Fig. 14 Tensile, yield, rupture, and creep strengths for seven ferritic and martensitic

stainless steels

Alloy Heat treatment

19-9 DL-DX

Air cooled from 1010 °C (1850 °F), hot worked at 650 °C (1200 °F), air cooled

Hastelloy X

Water quenched from 1175 °C (2150 °F), air cooled

16-25-6

Water quenched from 1175 °C (2150 °F), hot worked at 790 °C (1450 °F), air cooled

from 649 °C (1200 °F)

Discaloy

Oil quenched from 980 °C (1800 °F), reheated to 720 °C (1325 °F), air cooled, reheated

to 650 °C (1200 °F), air cooled

A-286

Oil quenched from 900 °C (1650 °F), reheated to 720 °C (1325 °F), air cooled

Incoloy 901

Oil quenced from 1120 °C (2050 °F), reheated to 705 °C (1300 °F), air cooled

Unitemp 212

Rolled at 1010 °C (1850 °F), air cooled, reheated to 720 °C (1325 °F), air cooled

D-979 (vacuum

melted)

Oil quenched from 1010 °C (1850 °F), reheated to 843 °C (1550 °F), air cooled, reheated

to 705 °C (1300 °F), air cooled

Inconel “X” 550

Air cooled from 1177 °C (2150 °F), reheated to 871 °C (1600 °F), air cooled, reheated to

730 °C (1350 °F), air cooled

Inconel

Annealed at 900 °C (1650 °F)

Hastelloy R-235

Water quenched from 1205 °C (2200 °F)

Nimonic 80A

Air cooled from 1080 °C (1975 °F), reheated to 704 °C (1300 °F), air cooled

Inconel “X”

Air cooled from 1150 °C (2100 °F), reheated to 845 °C (1550 °F), air cooled, reheated to

704 °C (1300 °F), air cooled

Inconel 700

Air cooled from 1180 °C (2160 °F), reheated to 871 °C (1600 °F), air cooled

Udimet 500

Air cooled from 1080 °C (1975 °F), reheated to 845 °C (1550 °F), air cooled, reheated to

760 °C (1400 °F), air cooled

Nimonic 90

Air cooled from 1080 °C (1975 °F), reheated to 705 °C (1300 °F), air cooled

Unitemp 1753

Air cooled from 1175 °C (2150 °F), reheated to 900 °C (1650 °F), air cooled

Waspaloy

Air cooled from 1080 °C (1975 °F), reheated to 845 °C (1550 °F), air cooled, reheated to

760 °C (1400 °F), air cooled

René 41

Air cooled from 1177 °C (2150 °F), reheated to 900 °C (1650 °F), air cooled

Udimet 700

(vacuum melted)

Annealed at 1150 °C (2100 °F), air cooled, solution treated at 1080 °C (1975 °F), air

cooled, reheated to 815 °C (1500 °F), air cooled, reheated to 760 °C (1400 °F), air cooled

Fig. 15 Temperature versus tensile, yield, and rupture strengths of iron-nickel-

chromium-molybdenum and nickel-base alloys

Fig. 16 Comparison of short-time tensile strength for titanium alloys, three classes of

steel, and 2024-T86 aluminum alloy

Alloy Heat treatment

Finish hot worked from a maximum temperature of 980 °C (1800 °F), reheated to 932–954

°C (1710–1750 °F), water quenched, treated at -73 °C (-100 °F), and aged at 538 °C (1000

°F)

AM 355

Finish hot worked from a maximum temperature or 980 °C (1800 °F), reheated to 932–954

°C (1710–1750 °F), water quenched, treated at -73 °C (-100 °F), aged at 455 °C (850 °F)

17-7 PH

(TH1050)

Reheated to 760 °C (1400 °F), air cooled to 16 °C (60 °F) within 1 h, aged at 565 °C (1050

°F) for 90 min

15-7 PH Mo

(TH1050)

Reheated to 760 °C (1400 °F), air cooled to 16 °C (60 °F) within 1 h, aged at 565 °C (1050

°F) for 90 min

17-7 PH

(RH950)

Reheated to 954 °C (1750 °F) after solution annealing, cold treated at -73 °C (-100 °F), aged

at 510 °C (950 °F)

15-7 PH Mo

(RH950)

Reheated to 955 °C (1750 °F) after solution annealing, cold treated at -73 °C (-100 °F), aged

at 510 °C (950 °F)

17-4 PH

Aged at 480 °C (900 °F) after the solution anneal

AM 350

Solution annealed at 1038–1066 °C (1900–1950 °F), reheated to 932 °C (1710 °F), cooled in

air, treated at -73 °C (-100 °F), aged at 454 °C (850 °F)

Fig. 17 Short-time tensile, rupture, and creep properties of precipitation-hardening

stainless steels