annealing heat treatment.pdf

7/27/2019 Annealing heat treatment.pdf

1/5

austenite again causes a certain volume increase, and in the third tempering stage the

progressive decomposition of martensite leads to the volume decrease.

For high-alloy tool steels (e.g., 210CrW46, curve 3 of Figure 6.49), a stabilization of

austenite is evident, so that the effect of the volume increase (due to austenitebainite or

austenitemartensite transformation) takes place only at higher temperatures. In most cases,

as can be seen from Figure 6.49, a reduction in length, i.e., a volume decrease, can be found

after tempering.

It should be noted that the changes in length shown in Figure 6.49 represent only the order

of magnitude of the expected changes, because the actual value depends in each case on the

specific heat treatment conditions. The austenitizing temperature, which determines the

amount of carbon dissolved and the amount of retained austenite, has a strong influence

on expected volume changes.

6.2 ANNEALING PROCESSES

6.2.1 STRESS-RELIEF ANNEALING

Stress-relief annealing is an annealing process below the transformation temperature Ac1,with subsequent slow cooling, the aim of which is to reduce the internal residual stresses in a

workpiece without intentionally changing its structure and mechanical properties.

Residual stresses in a workpiece may be caused by

1. Thermal factors (e.g., thermal stresses caused by temperature gradients within the

workpiece during heating or cooling)

2. Mechanical factors (e.g., cold-working)

3. Metallurgical factors (e.g., transformation of the microstructure)

In processes that involve heat, residual stresses are usually caused by the simultaneous

existence of thermal and transformational stresses (e.g., during the solidification of liquid

metals, hot forming, hardening, or welding). Thermal stresses are always directly propor-

tional to the existing temperature gradient, which further depends on the cross-sectional size

and on the heating or cooling rate.In workpieces made of steel, for the above reasons, local residual stresses may amount to

between about 10 N/mm2 and values close to the yield strength at room temperature. The

consequences of residual stresses may include

1. Dimensional changes and warpage of the workpiece

2. Formation of macroscopic and microscopic cracks

3. Asymmetric rotation of shafts

4. Impairment of the fatigue strength of engineering components

2

3

4651

Tempering temperature, C

Change

inlength,%

0

1: DIN 105WCr6

2: DIN 40CrMOV21.14

3: DIN 210CrW46

4: DIN X100CrMoV5.15: DIN 50NiCr13

6: DIN 165CrMoV460.15

0.100.05

0

0.05

0.10

0.15

100 200 300 400 500 600

FIGURE 6.49 Change in length of different steels during tempering as a function of tempering tem-

perature. (Designation of steels according to DIN.) (From H.J. Eckstein (Ed.), Technologie der Warme-

behandlung von Stahl, 2nd ed., VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1987.)

2006 by Taylor & Francis Group, LLC.


2/5

Residual stresses in a workpiece can be reduced only by a plastic deformation in the

microstructure. This requires that the yield strength of the material be lowered below the

value of the residual stresses. The more the yield strength is lowered, the greater the plastic

deformation and correspondingly the greater the possibility or reducing the residual stresses.

The yield strength and the ultimate tensile strength of the steel both decrease with increasing

temperature, as shown in Figure 6.50 for a low-carbon unalloyed steel. Because of this, stress-

relief annealing means a through-heating process at a correspondingly high temperature. For

plain carbon and low-alloy steels this temperature is usually between 450 and 6508C (842 and

12008F), whereas for hot-working tool steels and high-speed steels it is between 600 and

7508C (1112 and 13828F). This treatment will not cause any phase changes, but recrystalliza-

tion may take place. Tools and machine components that are to be subjected to stress-relief

annealing should be left with a machining allowance sufficient to compensate for any warping

resulting from stress relief.

When dealing with hardened and tempered steel, the temperature of stress-relief annealing

should be about 258C (778F) below that used for tempering. If the tempering temperature was

quite low, after stress-relief annealing quite a high level of residual stresses will remain. In

some other cases, for instance with a gray iron, the maximum temperature of the stress-reliefannealing should be limited because of possible strength loss. Therefore gray iron must not be

stress-relief annealed above 5508C (10228F).

In the heat treatment of metals, quenching or rapid cooling is the cause of the greatest

residual stresses. A high level of residual stress is generally to be expected with workpieces that

have a large cross section, are quenched at a high cooling rate, and are made of a steel of low

hardenability. In such a case high-temperature gradients will arise on the one side, and on the

other side structural transformations will occur at different points of the cross section at

different temperatures and different times. In contrast to heat treatment processes with con-

tinuous cooling, processes with IT (e.g., austempering) result in a low level of residual stresses.

To activate plastic deformations, the local residual stresses must be above the yield

strength of the material. Because of this fact, steels that have a high yield strength at elevated

temperatures can withstand higher levels of residual stress than those that have a low yield

strength at elevated temperatures.

A

1000

Yieldstrengthandultimatetensile

strength,

MPa

Elongation,

%

Temperature, C

800

600

400

200

200 100 100 200 300 400 500 600

Rm

sso

ssu

0

0

10

20

30

40

50

0

FIGURE 6.50 Change in some mechanical properties of low-carbon unalloyed steel with increasing

temperature, according to Christen. A, Elongation; Rm, ultimate tensile strength; sso, upper yield

strength; ssu, lower yield strength. (From H.J. Eckstein (Ed.), Technologie der Warmebehandlung von

Stahl, 2nd ed., VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1987.)



3/5

The level of yield strength at elevated temperatures depends on the alloying elements in

the steel. Figure 6.51 shows the increase in yield strength at temperatures of 3005508C (572

10228F) when 0.5% of each element was added to an unalloyed steel. It can be seen from this

diagram that additions of Mo and V are most effective in increasing the yield strength at

elevated temperatures.

To reduce residual stresses in a workpiece by stress-relief annealing, a temperature must

be reached above the temperature corresponding to the yield strength that is adequate to the

maximum of the residual stresses present. In other words, every level of residual stress in a

workpiece corresponds to a yield strength that in turn depends on temperature. In addition to

temperature, soaking time also has an influence on the effect of stress-relief annealing, i.e., onthe reduction of residual stresses, as shown in Figure 6.52.

The relation between temperature and soaking time during stress-relief annealing can be

described by Hollomons parameter:

P T(C log t) (6:33)

where Pis Hollomons parameter (heat treatment processes with the same Hollomon para-

meter value have the same effect), Cis the HollomonJaffe constant, Tis temperature (K),

and t is time (h).

The HollomonJaffe constant can be calculated as

C 21:3 (5:8% carbon) (6:34)

Figure 6.53 shows (according to LarsonMiller method) calculated values of the yield strength

at elevated temperatures (for 0.2% strain) for three grades of alloyed structural steels for

hardening and tempering (designations according to DIN). Using this diagram, the abscissa

ofwhichrepresents theactualHollomonparameterP, knowing thetemperature andtimeof the

stress-relief annealing, one can read off the level of residual stresses that will remain in the

workpiece after this annealing process, i.e., the level up to which the residual stresses will be

reduced by this stress-relief annealing. If, for instance, for DIN 24CrMoV5.5 steel, a

Mo

V

Ti

Cu

Mn

Cr

Ni0

20

40

60

80

100

300 350 400 450 500 550

Temperature, C

120

Increaseoftheyieldstrength

,N/mm2

FIGURE 6.51 Increase in yield strength at elevated temperatures when 0.5% of each alloying element

indicated is added to an unalloyed steel. (From G. Spur and T. Stoferle (Eds.), Handbuch der Fertigung-stechnik, Vol. 4/2, Warmebehandeln, Carl Hanser, Munich, 1987.)



4/5

1 h

10 h

10

20

30

40

50

60

70

80

90

100

Reductionofresidualstresses,%

Temperature, C

200 300 400 500 600 700

24 h

48 h

FIGURE 6.52 Effect of soaking time (at different temperatures) of stress-relief annealing on the

reduction of residual stresses for hardening and tempering steels. (From G. Spur and T. Stoferle

(Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser, Munich, 1987.)

P=T(20+log t)103

Temperature T, C

Hollomon's parameter P

Holdingtime,

t

30CrMoN

iV5.11

a

b

400

200

80

6050

40

30

20

10(a) h

(b)

15

0.1

5

10

20

16 17 18 19 20 21 22 23

100

700650550 600

700650550 600

700650550 600

700650550 600

Yieldstrengthorminimum

residualstress

afterstress-reliefannea

ling,

N/mm2

24C

rMoV

5.5

28NiCrM

o7.4

FIGURE 6.53 Yield strength at elevated temperatures (for 0.2% strain) calculated according to the

LarsonMiller method for three grades of alloyed structural steels for hardening and tempering

(designations according to DIN). (a) Calculated values and (b) experimentally obtained values. (From

G. Spur and T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser,

Munich, 1987.)



5/5

temperature of 6008C (11128F)and a soaking timeof 10h arechosen for stress-reliefannealing,

the residual stresses will, after this annealing, be reduced to a maximum of 70 N/mm2. Higher

temperatures and longer times of annealing may reduce residual stresses to lower levels, as can

be seen from Figure 6.53.

As in all heat treatment processes where Hollomons parameter is involved, selection of a

higher temperature may dramatically shorten the soaking time and contribute substantially to

the economy of the annealing process.

Dealing with structural steels for hardening and tempering, the stress-relief process and the

tempering process can be performed simultaneously as one operation, because Hollomons

parameter is also applicable to tempering. In such a case the stress-relief diagram may be used

in combination with the tempering diagram to optimize both the hardness and the level of

reduced residual stresses.

The residual stress level after stress-relief annealing will be maintained only if the cool-

down from the annealing temperature is controlled and slow enough that no new internal

stresses arise. New stresses that may be induced during cooling depend on the cooling rate, on

the cross-sectional size of the workpiece, and on the composition of the steel. Figure 6.54

shows the effect of cooling rate and cross-sectional diameter of forgings made of a CrMoNiVsteel on the level of tangential residual stresses after stress-relief annealing.

A general conclusion about stress-relief annealing is the following: In the temperature

range 4506508C (84212008F), the yield strength of unalloyed and low-alloyed steels is

lowered so much that a great deal of residual stress may be reduced by plastic deformation.

The influence of the steel composition on the level of residual stresses after annealing can be

considerable. While unalloyed and low-alloy steels with Ni, Mn, and Cr after stress-relief

annealing above 5008C (9328F) may get the residual stresses reduced to a low level, steels

alloyed with Mo or Mo V will retain a much higher level of the residual stresses after stress-

relief annealing at the same temperature because of their much higher yield strength at

elevated temperature.

6.2.2 NORMALIZING

Normalizing or normalizing annealing is a heat treatment process consisting of austenitizing

at temperatures of 30808C (861768F) above the Ac3 transformation temperature (for

00

20

40

60

80

100

120

10 20 30 40 50 60 80Tangentia

lresidualstresses,

N/mm2

Average cooling rate to 400, C/h

Dia

m.=

1000

mm

800

mm

600

mm

400mm

200m

m

FIGURE 6.54 Tangential residual stresses in a CrMoNiV alloy steel depending on the cooling rate and

cross-section diameter. (From G. Spur and T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2,

Warmebehandeln, Carl Hanser, Munich, 1987.)


annealing heat treatment.pdf

Documents