On the effect of heat treatment on the forming properties of pre-deformed low carbon steel

1. On the e€ect of heat treatment on the forming properties of
pre-deformed low carbon steel
A.M. ALASKARI1 AND S.E. ORABY2
Department of Mechanical Production Technology, College of Technological Studies,
PAAET, P.O. Box 42325 Shuwaikh 70654, Kuwait, Tel: +965 2314846 Fax: +965
4832761. 1Aalaskari@gmail.com, 2se.oraby@paaet.edu.kw
ABSTRACT
Strain hardening or cold work hardening usually accompanies most forming
manufacturing processes as well as many other processes. This inevitable consequence,
in a common functional sense, is undesirable from the design viewpoint. Therefore,
attempts are always in progress to enhance the predeformed material in such a way to
reduce this hardening e€ect. Annealing heat treatment is a familiar and e€ective
technique that is frequently employed. In the current study, two annealing methods,
used separately to investigate their e€ect on predeformed materials, are stress relief at
610 C
and process annealing at 710 C
. Both techniques have enhanced the resulting
hardening stress to di€erent degrees, depending on the degree of deformation and the
value of true strain. Stress relief annealing tends to have the best outcome at moderate
deformation levels from 0.12 to 0.16 predeformation strain. However, little gain is
obtained at higher deformation levels especially at moderate to high true strain values.
Process annealing is found to produce better qualitative and quantitative results than
those obtained by stress relief annealing. Stress reduction has been achieved with process
annealing within a wider domain of predeformation and true strains.
Keywords: forming; predeformation; process annealing; stress relief annealing.
INTRODUCTION
In many forming applications, a steel product is usually cold worked in a way
that it is plastically deformed beyond the yield point, and at much below the re-
crystallization temperature (Kulas 2004). As a result, an increase in both
strength and hardness with ductility reduction is usually expected (Callister
2007), and usually called work hardening and strain hardening. This may a€ect
the performance of the manufactured product sometimes leading to
unaccounted mechanical failure and, at the same time, may jeopardize work
safety environment (Tang et al. 1999, Lee et al. 2004, Fan et al. 2005).
Strain hardening behavior by cold work can be described by the conventional
relationship:
Kuwait J. Sci. Eng. 34 (2B) pp. 193-206, 2007

2. ˆ k…o ‡ a†n …1†
where o is the e€ective strain prior to cold working, a is the e€ective strain due
to subsequent plastic deformation, is the e€ective stress, n is the strain
hardening exponent and k is the strength coecient. Both k and n have long
been thought to be predeformation independent. However, in a relevant
research by the authors (Alaskari Oraby 2006), k and n were found likely to
be predeformation-dependent the deformation strain range from 0.05 - 0.1.
Accordingly, a more practical relation is proposed as:
ˆ k:n …2†
where is the e€ective strain. Around such a forming domain, di€erent
behavior of forming parameters is noticed and a true stress compensation factor
is proposed. However, the current study investigates the feasibility of applying
di€erent types of annealing on the predeformed cold worked material. Gain
obtained is justi®ed by the recovery extent (reduction) of the developed true
stress as a result of this work hardening.
Annealing is usually a subsequent recovery process after deformation (Tale€ et
al. 2001, Davis et al. 2002). Annealing in metallurgy and material science is a heat
treatment that alters the microstructure of a material, causing changes in its
properties such as strength and hardness. There are several phases in the annealing
process, with the ®rst being the recovery phase, which results in softening of the
metal through removal of crystal defects and the resulting internal stresses. The
second phase is recrystallization, where new grains nucleate and grow to replace
those deformed by internal stresses. If annealing is allowed to continue once
recrystallization has been completed, grain growth will occur, causing the
microstructure to coarsen which may cause the metal to have less than satisfactory
mechanical properties (Fang et al. 1995, Hsiao Huang 2002).
Stress relief annealing is usually used to reduce residual stresses in large
castings, welded and cold-formed parts. Such parts tend to have stresses due to
thermal cycling or work hardening. Parts are heated to temperatures of up to
600 -- 650oC, held for an extended time (about 1 hour or more) and then slowly
cooled in still air (Fang et al. 1995, Perez-Prado et al. 2001).
Process Annealing is usually used to treat work-hardened parts out of low-carbon
steels ( 0.25% carbon). This allows the parts to be soft enough to undergo further
cold working without fracturing. Process annealing is done by raising the
temperature to about 700 oC which is just below the Ferrite-Austenite region
(727oC). This temperature is maintained long enough to allow recrystallization of
the ferrite phase, followed by cooling in still air. Since the material stays in the same
194 A.M. Alaskari and S.E. Oraby

3. phase throughout the process, the only change that occurs is the size, shape and
distribution of the grain structure. This process is cheaper than either full annealing
or normalizing since the material is not heated to a very high temperature or cooled
in a furnace (Martyin et al. 2001, Hsiao Huang 2002).
Process annealing usually takes more time and uses more facilities and,
consequently has a higher manufacturing cost. Therefore, the designer must
justify the selected annealing methods according to the functional
manufacturing objectives and constraints imposed. One of our objectives is to
assist designers and decision makers taking the practical decision through the
o€ered numerical database charts that are produced in forms of empirical
equations together with 3-D plots and contours.
EXPERIMENTAL SETUP
Long ¯at low carbon steel specimens were prepared for tension testing
according to BS 18 with con®gurations of 25, 100, 300 and 25 mm for width,
gauge length, total length and radius of shoulder, respectively. The common
chemical composition by weight of this low carbon steel was 0.16% C, 0.21% Si,
0.54% Mn, 0.009% P and 0.022% S. Testing operations were carried on a
moderate capacity Hoytom Di-10-Cp/MH 1491 universal tensile testing
machine. Output was digitally in-process collected using compatible Hoytom PC
software. Testing procedures and conditions were as indicated in Table 1. More
details about experimental procedures and the selection of predeformation
strain levels are found in a previous work (Alaskari Oraby 2006).
Table 1: Experimental design and specimen conditions
Predeformation Level
# Set Seq.
Number of Specimens Conditions
SetI:Conventional(zeropredeformation) 5
SetII:PredeformedwithoutHeatTreatment 3 3 3 3
SetIII:Predeformedwith610oCStressRelief 3 3 3 3
Set IV: Predeformed with 710 oC Process
Annealing
3 3 3 3
Zerostrain
0.08Eng.Strain
(8mmPredeformed)
0.12Eng.Strain
(12mmPredeformed)
0.16Eng.Strain
(16mmPredeformed)
0.20Eng.Strain
(20mmPredeformed)
195On the e€ect of heat treatment on the forming properties of pre-deformed low carbon steel

4. EXPERIMENTAL RESULTSAND ANALYSIS
Mechanicalpropertiesand behaviorof theoriginal material(Set I)
Five specimens were tested until fracture to obtain the mechanical and forming
properties of the low carbon steel. For each specimen, the onset of elastic-plastic
zone was selected to be at 0.5% strain ratio.
While strain hardening exponent n is extracted from the result graph at the
ultimate tensile stress, the strength coecient k can be determined using any
data point within the elastic-plastic zone. Accordingly, the general strain
hardening relationship was found to have the form:
ˆ 707:30:2218: …3†
Mechanicalpropertiesand behaviorof the predeformedmaterial(Set II)
As indicated in Table 1, four levels of predeformation were prepared using at
least three specimens each. The predeformation levels were 8, 12, 16 and 20 mm
corresponding to 0.08, 0.12, 0.16 and 0.2 engineering strain respectively. Then,
for each level, specimens were tensile tested until fracture independently and the
average results were considered.
Testing results for this set of experiments have led to the following general
equations (Alaskari Oraby 2006):
8 ˆ 665:89:0:1552; …4†
12 ˆ 640:71:0:1212; …5†
16 ˆ 611:49:0:0853; …6†
20 ˆ 578:96:0:0603: …7†
As indicated by Eqs. 4-7, at higher predeformation level, lower values of k
and n were obtained. As predeformation level increased, yield strength gradually
increased with a corresponding decrease in ductility. Also, the higher the
predeformation level, the closer the magnitude of the yield and ultimate tensile
strengths. In other words, the curve beyond the yield point in the stress-strain
graph became more and more ¯attened as the predeformation level increased.
The e€ect of various predeformation levels within the forming true strain
domain on the values of the emerged true stresses is shown in Fig. 1. For the
196 A.M. Alaskari and S.E. Oraby

5. sake of comparison, data and results of the original undeformed material are
superimposed. For all predeformation levels at a given strain, the true stress
increased due to work hardening from preceding deformation. However, the
rate of increase of true stress seems to be positively a€ected, to di€erent extents,
by predeformation level and negatively by true strain.
At strain ratio above 0.1, the predeformation levels of 16 and 20 mm seemed
to have almost similar in¯uence on true stress suggesting that the true stress was
no longer a€ected by any further predeformation. An interpretation is that, at
those high predeformation levels, the material approached the full plastic region
and this should be considered during the forming process. In Alaskari Oraby
(2006), a correction factor was proposed by which the di€erence between
original stress and that at any predeformation level and true strain was
determined.
Figure 1:The e€ect of predeformation on the material behavior
E€ect of stress reliefannealing (610o
C)on predeformedmaterial(Set III)
Another set of 12 specimens were prepared for predeformation testing. These
specimens were heated in a furnace up to 610oC. To maintain temperature
homogeneity, the specimens were retained at this temperature for almost an
hour before switching o€ the furnace. The specimens were left to gradually cool
down to room temperature in still air. Tensile testing was then preformed until
fracture; data was collected and processed.
General equations for this testing were as follows:
8ÿsr ˆ 686:91:0:1884; …8†
12ÿsr ˆ 675:54:0:1653; …9†
197On the e€ect of heat treatment on the forming properties of pre-deformed low carbon steel

6. 16ÿsr ˆ 687:98:0:1487;and …10†
20ÿsr ˆ 693:51:0:1223: …11†
Between the untreated set (Eqs. 4-7), and the current set (Eqs. 8-11), the latter
have higher values of k and n. Treatment up to 610oC represents the process of
stress relief which, as expected, leads to a stress reduction of the predeformed
material. Experimental data for this annealed set, together with conditions of
original data of material without predeformation or treatment, are plotted in
Fig. 2. Quantitative assessment of the e€ect of the annealing process may be
deduced by comparing the results plotted in Fig. 2 to the corresponding
untreated results in Fig. 1. For each level of the untreated predeformed material
and, within the entire region of true strain (0.02-0.15) (Fig. 1), true stress is
found to be higher in the stress-relived material compared to the original
undeformed material. Increment ranges were found to be: (6.82-22.16%), (9.6-
34.28%), (11.9-47.47%) and (11.2-53.97%) for 8, 12, 16 and 20 mm
predeformation levels, respectively.
The corresponding values for the stress relieved specimens (Fig. 2) were found
to be: (3.35-10.67%), (6.3-19.14%), (11.73-29.47%) and (18.4- 44.71%).
Figure 2:The e€ect of stress relief on the material behavior
Therefore, higher true stress reduction improvement was obtained at the
lowest true strain (0.02) with stress relief heat treatment. The improvement was
found to be 51.85%, 44.16%, 37.9% and 17.6% for 8, 12, 16 and 20 mm
predeformation strain levels, respectively. However, at the maximum true strain
of 0.15, the corresponding improvement factors were found to be 50.88%,
34.38%, 2.17% and -64.3%. Although an undesirable outcome was observed at
the extreme region of maximum predeformation strain level of 20 mm, stress
relief heat treatment generally showed a very prospective trend of behavior
198 A.M. Alaskari and S.E. Oraby

7. homogeneity since, at any given strain level; entire results plots indicated a
regular pattern represented by parallel trends.
In Fig. 3ab, the e€ect of stress relief heat treatment is depicted as a function
of predeformation strain level and true strain. Figure 3a produces values of a
quantitative trend while Fig. 3b shows data contours that represent stress gain
(stress reduction). Gain here is considered as the di€erence between stress values
before and after treatment. These results may produce a functional and
operational requirement for the establishment of a numerical database to
predict the bene®ts of applying stress relief annealing.
As shown in Fig. 3b, two regions (AB) separated by a zero contour border
line can be distinguished. Below the zero line in region A, stress relief heat
treatment leads to stress reduction with increasing values toward the medium-
predeformation-low-true strain region. In the low true strain range (0.02-0.06),
stress relief heat treatment reduces true stress at all predeformation strain levels
with the highest gain at moderate predeformation strain levels of 0.12-0.16.
However, at true strain between 0.06 and 0.15, stress relief seems to lead to a
contradictory in¯uence at predeformation levels above 0.16 predeformation
strain (16-20 mm, region B). However, as discussed earlier, at such late
deformation stage, the gain behind annealing seems to be restricted to increasing
material homogeneity. Additionally, at this advanced deformation stage,
material may approaches the plastic region at which a complete strain hardening
is usually the predominant factor such that stress relief heat treatment will have
little e€ect.
E€ect of processannealing 710o
C onpredeformedmaterials
As shown in Table 1, twelve specimens were prepared, predeformed and then
annealed to a temperature of 710oC. Process annealing of 710oC was maintained
for an hour before leaving specimens to gradually cool down to room
temperature under still air conditions. Each specimen was tested until fracture.
Testing was performed randomly for the twelve specimens. After data
processing, the following set of general equations was established:
8ÿpro ˆ 686:31:0:21; …12†
12ÿpro ˆ 664:87:0:198; …13†
16ÿpro ˆ 657:82:0:188;and …14†
20ÿpro ˆ 634:66:0:177: …15†
199On the e€ect of heat treatment on the forming properties of pre-deformed low carbon steel

8. As indicated by Eqs. 12-15, values of k and n were changed due to process
annealing so that the true stress of the predeformed material was closer to the
original (Figure 4). Therefore, the experimental results for all predeformation
strain levels were very close, revealing a constant higher gain than that obtained
by stress relief annealing as shown in Fig. 2.
Figure 3: Properties improvement gain due to stress relief annealing process
200 A.M. Alaskari and S.E. Oraby

9. Figure 4: E€ect of process annealing HT on the material behavior
In the true strain range form 0.02-0.15 the percentage of stress changes
between original and predeformed material was: (1.6 to -0.8%), (1.5 to -1.7%),
(6.16 to -0.8%) and (6.9 to -2.3%) for 8, 12, 16 and 20 mm predeformation
levels, respectively. In comparison to stress values for untreated material, the
ratio of stress reduction within the same true strain range as a result of process
annealing were: (76.5-103.6%), (84.4-105.2%), (47.87-101.69%) and (62.3-
104.26%) for 8, 12, 16 and 20 mm predeformation levels, respectively. Thus, a
great improvement was attained in the form of true stress reduction of the
material due to process annealing at 710oC.
Comparison between stress values of untreated and process annealed
materials all predeformation levels is qualitatively shown as a 3-D surface in
Fig. 5a, together with a quantitative contour plot in Fig. 5b. Three separate
zones can be recognized as high predeformation-low true strain (A), moderate
predeformation-moderate true strain (B) and low predeformation-high true
strain (C). The gain due to process annealing is the highest in region A while it is
drastically decreased at low predeformation-high true strain region (C). The
selection of the appropriate heat treatment type is usually determined according
to both functional and economical considerations taken into account the gain
obtained, Fig. 5(b).
ComparisonbetweenStressRelief and ProcessAnnealing
The use of the appropriate annealing cycle can save both time and money and
therefore is based on an objective and functional basis. Quantitative comparison
between the two applied techniques is presented in Fig. 6a. Figure 6b shows a
contour plot by which numerical di€erence value can be determined at given
level of predeformation and true strain within the tested operational domain.
Firstly, process annealing to 710oC essentially takes a longer time but produces
better material improvement than that obtained by stress relief treatment.
201On the e€ect of heat treatment on the forming properties of pre-deformed low carbon steel

10. Figure 5:Properties improvement gain due to process annealing
Di€erences between stress values with the two methods are shown in Fig. 6a.
Generally, the process annealing method is more bene®cial at higher
predeformation levels, while Fig. 6b illustrates that little e€ect is noticed at
lower predeformation levels especially at higher true strain values.
To get a physical and feasible indicator of the gain obtained, an improvement
gain ratio (GR) for process annealing is proposed here as:
202 A.M. Alaskari and S.E. Oraby

11. Gain Ratio…GR† ˆ
…Stress Relief Reductionÿ Process Annealing Reduction†
Original Stress
2 100: …16†
Figure 6: Gain di€erence between stress relief and process annealing
At a given true strain, process annealing had an increasing positive e€ect
(reduction in true stress). For instance, at the lowest true strain of 0.02, the gain
ratio was found to have values of 9%, 16%, 23.3% and 37.79% for
203On the e€ect of heat treatment on the forming properties of pre-deformed low carbon steel

12. predeformation levels of 8, 12, 16 and 20 mm, respectively. Corresponding
values at the highest true strain of 0.15 were 6.6%, 12.4%, 6.66% and 32.4%.
Therefore, it can be concluded that process annealing is most bene®cial at the
highest predeformation level where the GR is about four times better than that
at the lowest predeformation level (8 mm).
However, at given a predeformation level, gain ratio (GR) seemed to decrease
as true strain increased to reach a minimum value at the highest true strain of
0.15. Within the proposed range of true strain, the gain ratio range was (9-
6.6%), (16-12.4%), (23.3-16.66%) and (37.7932.4%) for 8, 12, 16 and 20 mm
predeformation levels, respectively. While gain ratio lost a constant amount of
about 25% of its original value at 8, 12 and 16 mm predeformation levels, its
variation at 20 mm predeformation was only about 14%.
CONCLUSIONS
The main objective of the current research was to evaluate the e€ect of
annealing heat treatments on the stresses developed as a result of deformation.
To achieve this goal, four main stages were sequentially accomplished:
development of the investigation plan; predeformation without heat treatment;
predeformation with stress relief annealing; and predeformation with process
annealing as well as investigation of the original undeformed untreated material.
Predeformation of a material is generally found to increase true stress but
with lower rates at higher true strain. Also, true stress is positively a€ected by
the applied predeformation level.
In this study, true stress has been improved (reduced) after the use of stress
relief annealing up to 610
C with the best performance at predeformation strain
range of 0.12 to 0.16 at low true strain. However, stress relief annealing was not
bene®cial in the high predeformation-high true strain region. In comparison,
process annealing up to 710
C improved the predeformed material in a wide
spectrum of predeformation levels and true strain resulting in a better overall
performance than that obtained by stress relief annealing.
A formability database approach can be achieved to help the designer to take
a decision prior, during and after the forming process. To make this possible,
functional assessment of all available options should be produced in an easy
interpreted ways such as equations, plots and contours. Within the current work
many of these facilities were extensively produced and analyzed.
ACKNOWLEDGEMENTS
This research was supported by a contract grant (TS-05-03) from the Public
204 A.M. Alaskari and S.E. Oraby

13. Authority of Applied Education and Training (PAAET), Kuwait. The authors
wish to express their deep gratitude to the departmental sta€, Technological
Studies College and PAAET higher education research administrations for
whatever assistance produced. Special thanks for Mr. Al-Bannai for his
technical assistance in specimen preparation and setup procedures.
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Submitted : 21/6/2006
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205On the e€ect of heat treatment on the forming properties of pre-deformed low carbon steel

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206 A.M. Alaskari and S.E. Oraby