ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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Fig. 18 General comparison of the hot-strength characteristics of austenitic, martensitic,

and ferritic stainless steels with those of low-carbon unalloyed steel and semiaustenitic

precipitation and transformation-hardening steels

When short-term tests are considered at high-temperature, the effect of testing time must be considered.

Because creep occurs continuously over time, a longer test in the creep region results in lower strength values.

This is shown in Fig. 19 for various mechanical properties of H11 die steel. Testing at higher temperatures also

increases strain-rate effects because slower strain rates allow more time for creep to occur (Fig. 4).

Fig. 19 Effect of time on high-temperature mechanical properties of H11 die steel

The time-dependent properties of high-temperature structural alloys are determined by a variety of methods, as

discussed in more detail in the Section “Creep and Stress-Relaxation Testing” in this Volume. Many of these

test methods in the creep region involve long-term testing. However, short-term tension tests on universal

testing machines (UTMs) can also be used in the evaluation of creep deformation or stress relaxation. (See the

article “Stress-Relaxation Testing” in this Volume.)

Hot Working Range. At higher temperatures in the hot working range (T > 0.6 T

), mechanical behavior is

different from plastic deformation at cold and warm working temperatures (where the change in microstructure

is largely a distortion in the grains). In the hot working regime, creep and work softening can occur from self-

diffusion (diffusion creep), dynamic recovery, and dynamic recrystallization. These high-temperature

mechanisms are only briefly described here as a general reference on the overall effects of temperature on

mechanical properties. More detailed coverage appears in the Section “Testing for Deformation Processes” in

this Volume.

Static Recrystallization. When work-hardened alloys are heated, at some point a temperature level is reached

where the atoms rearrange to form an entirely new set of crystals. This process, where the stored energy

(produced by previous working at cold or warm temperatures) is released by migration of the grain boundaries,

is known as recrystallization. The process is distinct from recovery, and the starting temperature for

recrystallization depends on the amount of prior plastic deformation. At higher levels of working, more strain

energy is stored in the crystal structure; therefore, a lower temperature initiates recrystallization.

Dynamic Recovery. In some cases, increased plastic strain results in a decrease in the necessary stress for

continued deformation. This effect, known as work softening, occurs in the hot working range, as shown

schematically in Fig. 20. The two main mechanisms of work softening in the hot working regime are dynamic

recovery and dynamic recrystallization.

Fig. 20 Typical flow curves for metals deformed at cold working temperatures (A, low

strain rate; B, high strain rate) and at hot working temperatures (C, D). Strain hardening

persists to large strains for curve A. The flow stress maximum and flow softening in curve

B arise from deformation heating. The steady-state flow stress exhibited by curve C is

typical of metals that dynamically recover. The flow stress maximum and flow softening

in curve D may result from a number of metallurgical processes. Source: Ref 11

In dynamic recovery, the dislocations obtained during previous working become unstable upon further working.

The relative amount of softening depends on the ratio of the yield strength to the applied stress and the ratio of

strength from previous work hardening to the yield strength (Ref 11). For example, as noted in Ref 11, the bcc

and hcp metals have relatively high yield strengths in the annealed condition, so the amount of strain hardening

is typically a smaller percentage of overall strength than for fcc metals. Thus, the amount of strain softening for

fcc metals tends to be greater than that for hcp or bcc metals (Ref 11).

Dynamic Recrystallization. In contrast to dynamic recovery, dynamic recrystallization involves the motion of

grain boundaries and annihilation of large numbers of dislocations in a single event, thereby producing new

strain-free grains. Dynamic recrystallization is also distinct from static recrystallization (Ref 12). In dynamic

recrystallization, the process occurs during the deformation process, thus facilitating working. In contrast, static

recrystallization is a purely kinetic process where a fixed amount of stored energy (dependent on cold work) is

released by thermally activated dislocation recovery and migration of the grain boundaries.

Dynamic recrystallization is largely limited to the fcc metals (Ref 11). For example, Fig. 21 shows true stress-

strain curves for HY-100 steel tested in compression at 1000 °C (1832 °F) and different strain rates (Ref 13).

There is a peak on the curves at strain rates of 0.01 s

-1

and above, which is usually an indication of dynamic

recrystallization. The amount of strain required to trigger recrystallization during deformation varies with

temperatures and strain rates.

Fig. 21 True stress-strain curves for HY-100 steel tested in compression of 1000 °C (1832

°F) and various strain rates. Source: Ref 13

References cited in this section

1. W.D. Nix and J.C. Gibeling, Mechanisms of Time-Dependent Flow and Fracture of Metals, Flow and

Fracture at Elevated Temperatures, R. Raj, Ed., American Society for Metals, 1985, p 2

2. J.D. Wittenberger and M.V. Nathal, Elevated/Low Temperature Tension Testing, Mechanical Testing,

Vol 8, Metals Handbook, ASM International, 1985, p 36

3. W.F. Brown, Jr., Ed., Aerospace Structural Metals Handbook, Metals and Ceramic Information Center,

Columbus, OH, 1982

4. Structural Alloys Handbook, Metals and Ceramic Information Center, Columbus, OH, 1973

5. D. Zhao, “Deformation and Fracture in Al

Particle-Reinforced Aluminum Alloy 2014,” Ph.D. thesis,

Worcester Polytechnic Institute, 1990

6. MIL-HDBK 5, 1991

7. Metals Handbook, American Society for Metals, 1948, p 441

8. The Making, Shaping, and Treating of Steel, United States Steel, 1957, p 822–823

9. F.T. Sisco, Properties, Vol 2, The Alloys of Iron and Carbon, McGraw-Hill, 1937, p 431

10. R. Viswanathan, Damage Mechanisms and Life Assessment of High Temperature Components, ASM

International, 1989, p 62

11. S.L. Semiatin and J.J. Jonas, Formability and Workability of Metals: Plastic Instability and Flow

Localization, American Society for Metals, 1984, p 1–5, 93–97

12. Y.V.R.K. Prasad and S. Sasidhara, Hot Working Guide: A Compendium of Processing Maps, ASM

International, 1997, p 9

13. M. Thirukkonda, D. Zhao, and A.T. Male, Materials Modeling Effort for HY-100 Steel, NCEMT

Technical Report, TR No. 96–027, Johnstown, PA, March, 1996

Hot Tension and Compression Testing

Dan Zhao, Johnson Controls, Inc.; Steve Lampman, ASM International

Hot Tension Testing

Tension testing is a very common mechanical test for the evaluation of service properties. Most materials

laboratories have tensile-testing machines. When compression-testing equipment is not available, or when a

metalworking process involves mainly tensile stresses, the tension test can also be used for processing

properties. More information on typical tension testing at room temperature is provided in the article “Uniaxial

Tension Testing” in this Volume.

At high temperatures, the procedures and specimens of the tension test are basically the same as room-

temperature testing. The key differences are the heating apparatus, accurate measurement of specimen

temperature, and suitable instruments for measuring strain at high temperature. ASTM E 21 is the prevailing

U.S. standard for high-temperature tensile testing (Ref 14).

General Characteristics. A typical high-temperature mechanical test setup is shown in Fig. 22. The system is the

same as that used at room temperature, except for the high-temperature capabilities, including the furnace,

cooling system, grips, and extensometer. In this system, the grips are inside the chamber but partly protected by

refractory from heating elements. Heating elements are positioned around a tensile specimen. Thermocouple

and extensometer edges touch the specimen.

Fig. 22 A typical high-temperature mechanical testing system

Most tensile specimens are cylindrical. Specimens with rectangular cross sections can also be used. The

specimen ends can be machined into smooth cylindrical or screw heads. The cylindrical head is usually used

with split-ring types of grips, which provide for quick removal of the specimen if quenching outside the furnace

immediately after testing is specified.

Universal joints are necessary to align the loading train. In any case, the maximum bending strain should not

exceed 10% of the axial strain. Strain gages can be used to examine bending strains of the gage length. If the

maximum (or minimum) bending strain is within the limit at room temperature, the alignment should be fine at

elevated temperatures. Care must be taken to ensure that the alignment of the loading train is maintained when

attaching the furnace and its accessories. The alignment needs to be tested periodically.

Heating methods for high-temperature mechanical testing include vacuum or environmental furnaces, induction

heating, and resistance heating of the specimen. Vacuum furnaces are expensive and have high maintenance

costs. The furnace has to be mounted on the machine permanently, making it inconvenient if another type of

heating device is to be used. The heating element is expensive and oxidizes easily. The furnace can only be

opened at relatively lower temperatures to avoid oxidation. Quenching has to be performed with an inert gas,

such as helium.

Environmental chambers are less expensive (Fig. 23). An environmental chamber has a circulation system to

maintain uniform temperature inside the furnace. Inert gas can flow through the chamber to keep the specimen

from oxidizing. Temperature inside the chamber can be kept to ±1 °C (2 °F), about the nominal testing

temperature. However, the maximum temperature of an environmental chamber is usually 550 °C (1000 °F),

while that of a vacuum furnace can be as high as 2500 °C (4500 °F). The chamber can either be mounted on the

machine or rolled in and out on a cart.

Fig. 23 Environmental chamber for elevated-temperature mechanical testing

A split furnance is also cost effective and easy to use (Fig. 24). When not in use, it can be swung to the side.

The split furnance shown in Fig. 24 has only one heating zone. More sophisticated split furnance have three

heating zones for better temperature control. Heating rate is also programmable.

Fig. 24 Split furnace for high-temperature mechanical testing

Induction-heating systems allow fast heating rates (Fig. 25). Specimens can reach testing temperatures within

seconds. Induction heating heats up the outer layer of the specimen first. Furnances with a lower frequency

have better penetration capability. Coupling of the heating coil and the specimen also plays an important role in

heating efficiency. The interior of the specimen is heated through conduction. With the rapid heating rate, the

temperature is often overshot and nonuniform heating often occurs.

Fig. 25 Induction-heating furnace for high-temperature mechanical testing

Direct resistant heating is used in Gleeble machines with electric current going through the specimen (Ref 15).

Advanced Gleeble testing systems, as shown in Fig. 26, are capable of rapid heating rates up to 10,000 °C/s

(20,000 °F/s) (Ref 16). Grips with high thermal conductivity also allow rapid cooling rates up to 10,000°C/s

(20,000°F/s) at the specimen surface.

Fig. 26 A Gleeble 3800 testing system. Source: Ref 12

Temperature Measurement. Thermocouples are the most common method for temperature measurement in hot

tension tests. Because tensile specimens usually have longer gage length than compression specimens, more

than one thermocouple may be needed to monitor the temperature along the gage length of the specimen. It is

necessary to shield the thermocouple unless the difference in indicated temperature from an unshielded bead

and a bead inserted in a hole in the specimen has been shown to be less than one half the variation listed below

(Ref 14):

Temperature Variation

Up to including 1800 °F (1000 °C)

±5 °F (3 °C)

Above 1800 °F (1000 °C)

±10 °F (6 °C)

Thermocouples need to be calibrated as specified in Ref 10. Thermocouple wire exposed to a hot zone should

be cut off after each test, and a new bead should be formed for subsequent tests. During the entire test,

temperature variation should not exceed the ranges indicated previously for the entire test, temperature

variation should not exceed the ranges indicated previously for the entire gage length.

When testing with high heating rates, a thermocouple welded on the test specimen can provide a more accurate

temperature reading (Ref 15). A thermocouple touching the specimen surface usually takes more time to reach

the same reading as a thermocouple welded on the specimen. However, welding produces a heating affected

zone and must be conducted carefully to minimize this effect. Drilling a hole in the gage length of a specimen

for insertion of a thermocouple is not recommended, especially for specimens with small diameters. The hole

may cause premature necking and failure, and lower the ultimate tensile strength.

Strain-Measurement. The simplest method for strain measurement is to take the crosshead displacement as the

deformation of the specimen reduced section. However, this assumes that the rest of the loading train does not

deform during testing and introduce error only at large plastic strains. Young's modulus cannot be determined

in this way, and 0.2% offset yield strength would not be accurate. To accurately measure strains, strain gages or

extensometers must be employed.

Strain gages can be used up to 600 °C (1112 °F) (Ref 2). Several extensometers are commercially available for

strain measurement at high temperature: clip-on, water cooled, air cooled, and noncontact extensometers. Clip-

on extensometers can be used up to 200 °C (392 °F); they are simple easy to use, and provide accurate readings.

Water-cooled (Fig. 27) and air-cooled (Fig. 28) extensometers can be used at higher temperatures, up to 500 °C

(930 °F) and 2500 °C (4500 °F), respectively.

Fig. 27 Water-cooled extensometer used up to 500 °C (932 °F)

Fig. 28 Air-cooled extensometer used at temperatures up to 2500 °C (4532 °F)

Water- and air-cooled extensometers are contact extensometers that use rods touching the specimen; the rods

transmits the relative motion of the specimen to a sensing device, usually a linear variable differential

transformer (LVDT). Some capacitive extensometers have high resolution and extremely low contact force.

The contact rods are made of various materials, ranging from nickel-base superalloys to ceramics, for different

temperature ranges.

The extensometer should be attached very carefully because it may affect alignment (Ref 17). Attaching

extensometers on opposite sides and averaging the reading may reduce the error. Some of the commercial

extensometers are designed to attach to both sides of the specimen. Whenever feasible, extensometers should be

attached directly to the reduced section (Ref 14). Stress-strain data may not be useful beyond the maximum-

load point due to necking.

Noncontact extensometers include laser interferometers, optical extensometers, and video extensometers. These

methods use more sophisticated instrumentation and are more expensive, but they are becoming more common.

Laser extensometers, which allow faster sampling rates than optical extensometers, are used for measuring