Acta Metallurgica Slovaca, Vol. 20, 2014, No. 1, p.

71-81 71

FORMABILITY OF AUSTENITIC STAINLESS STEEL 316 SHEET IN DYNAMIC

STRAIN AGING REGIME

Syed Mujahed Hussaini1), Swadesh Kumar Singh2)*, Amit Kumar Gupta1)

1)
Mechanical Engineering Department, BITS-Pilani, Hyderabad campus, India
2)
Mechanical Engineering Department, GRIET, Bachupally, Hyderabad, India

Received: 14.08.2013
Accepted: 01.10.2013

*Corresponding author: e-mail:swadeshsingh@griet.ac.in, Tel.:+91(40)64601921, Mechanical

Engineering Departmet, GRIET, Bachupally, Hyderabad, 500090, India

Abstract
Dynamic strain aging region for austenitic stainless steel 316 was investigated from room
temperature to 650°C at constant strain rates of 1x10 -2, 1x10-3 and 1x10-4 sec-1. Characteristics
indicators of serrated plastic flow were observed in the temperature range of 400°C to 600°C at
these strain rates. Strain rate sensitivity of the material is found to be negative in this region.
Study of fracture surface of tensile test specimen by scanning electron microscope revealed that
ductility in this region decreased. The limiting drawing ratio of sheet metal is the indicator of
formability in deep drawing. LDR of the sheet metal was estimated by performing the deep
drawing of different diameter blanks in finite element simulation software LS-DYNA in DSA
temperature region. It was observed that in DSA region, formability of sheet metal decreased.
These simulations are validated and compared with experimental results.

Keywords: Dynamic Strain Aging, Serration, Strain rate sensitivity, limiting drawing ratio,

1 Introduction
Discontinuous plastic flow in metals referred to as dynamic strain aging (DSA). It has been
reported and various physical models, micro-mechanisms have been proposed in an attempt to
explain this phenomenon [1–2]. The occurrence of DSA during plastic deformation is a well-
known phenomenon in metallic materials. It is attributed to the interaction of solute atoms with
moving dislocations [3, 4]. It generally observed when the deformation temperature is high
enough to permit short range diffusion of solute atoms to dislocation cores. This strong elastic
interaction result in a temporary arrest of the dislocation in the slip plane. This DSA always
appears during the plastic deformation process of metallic materials under certain temperatures
and strain rates. It was found in recent years that the influence of DSA phenomenon on the
mechanical behavior of materials should not be ignored [5–7]. Most of the literature shows that
the tensile strength [8, 9] and fatigue strength [10, 11] of the material are increased by DSA and
ductility and rupture toughness [12] are also influenced.
Typical macroscopic features of DSA include serrated flow behavior, sharp yield points,
maxima in the work hardening temperature plot, negative strain-rate sensitivity. Both interstitial
and substitutional elements are the responsible for such effects in an appropriate range of strain-
rate and temperature. Mobile dislocations move by successive jerks between ‘forest’ obstacles,
i.e. other dislocations piercing their slip plane. Solute atoms diffuse to and age mobile
dislocations while they are temporarily arrested at these obstacles. This mechanism leads to

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negative strain rate sensitivity (SRS) of the flow stress in a range of strain rates where the two
types of defects have comparable mobility [13, 14]. No material particle deforms at a plastic
strain rate from within this interval. If the imposed strain rate falls into this range, plastic flow is
heterogeneous, with plastic strain rate highly localized in narrow bands, either static or
propagating in a continuous or discontinuous manner.
Austenitic stainless steels are particularly suited to the study of DSA mechanisms because of the
large temperature range over which serrated flow can occur [15, 16]. Certain solute atom
concentrations and at some specific strain rates over which serrated flow may be observed [17,
18]. The temperature range for serrated flow indicates a thermally activated behavior. Tensile
tests of an AISI310 type austenitic stainless steel have shown [19] that serrated yielding is
observed within a certain range of strain rates and temperatures, where the dynamic strain
hardening is high. Both the activation volume and the activation energy for plastic deformation
are a function of temperature, and increase with an increase in test temperature. The austenitic
stainless steel as the structural components in petrochemical industry, power plants and nuclear
reactors is usually served under a load with long period of time at a high temperature region, and
it is influenced by DSA during deformation.
The present work tried to study the DSA phenomenon in austenitic stainless steel 316. The
serrated flow behavior of the material is one of the evident features of DSA phenomenon and
also this region is characterized by negative strain rate sensitivity. The temperature range and
strain rate where serrations can observed in flow stress diagram were investigated. The strain
rate sensitivity index which is evident of DSA was characterized in this range. This non-classic
mechanism takes place at higher temperatures and causes more drastic changes in ductility.
Nature of fracture of the tensile specimens was analyzed by Scanning electron microscope
(SEM).
Deep drawing is one of the important but very complex forming process where the sheet metal
blank is subjected to the different types of stresses. The limiting drawing ratio (LDR) is
commonly used to measure the formability of sheet metal in deep drawing. The LDR is defined
as the ratio of the maximum blank diameter to the cup diameter which can produce in a single
stroke without fracture. In this study the deep drawing simulations were performed in the
explicit finite element code LS-Dyna on the different diameter blanks and found the LDR of
ASS 316 sheets in and below the DSA region. Effect of DSA on the formability was analyzed.

2 Experimental material and methods

Material used in this study is ASS 316 sheet of 1.0 mm thick. Chemical composition of ASS316
sheet metal blank is listed in Table 1. This alloy contains 2.42% molybdenum which enhances
the corrosion resistance. This steel is more resistant to general corrosion, pitting and crevice
corrosion than the conventional chromium-nickel austenitic stainless steels such as Alloy 304.
Resistance to corrosion in the presence of chloride or other halide ions is enhanced by higher
chromium (Cr) and molybdenum (Mo) content. This alloy also offers higher creep, stress-to-
rupture and tensile strength at elevated temperatures. It is primarily austenitic phase and small
quantities of ferrite may be present. Due to the presence of these phases it has excellent
toughness besides high strength. These combinations of properties provide the excellent
formability to the material.

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Table 1 Chemical composition of ASS 316

Element Fe Cr Ni Mo Si Mn Cu Co C
Composition (Wt. %) 67.69 16.63 10.85 2.42 1.28 0.38 0.21 0.21 0.018

Tensile testing was conducted on Universal testing machine (UTM) made by MCS. It is
electronic screw driven machines with precision screw, column construction and completely
controlled by computer. It has variable speed drive. For conducting test at higher temperature,
this machine is attached with a special type split furnace. It is having uniform distribution of
heating coils, which are arranged in three zones to achieve temperature up to 1200° C with ±
1°C accuracy. Temperature control and measurement is done by thermocouple. Tests were
conducted in air by maintaining constant strain rate of 1x10 -2, 1x10-3 and 1x10-4 sec-1 in the
temperature region from 50 to 650°C. Flat tensile specimen blanks were cut from the flat sheets.
The tensile specimens having 30 mm gauge length and 6.4 mm gauge width according to ASTM
standard E8M (Sheet type sub-size specimen) were cut from the blanks. Tensile properties were
evaluated and draw the true stress and true strain diagram to find the temperature range where
the serration occurs, which is characterized as DSA. The fracture surfaces of tensile failed
specimens were examined by scanning electron microscope to study the nature of fracture in
DSA region.
The deep drawing experiments were carried out on the experimental test rig. This test rig is
specially designed for deep drawing operations which can be performed at elevated
temperatures. Complete punch and die setup is made with Inconel-600 to prevent the materials
to change dimensions at higher temperatures. An induction furnace was developed to heat the
blank to maximum temperature of 400°C. Two sets of furnaces were installed on a 20 ton
hydraulic press. One furnace is utilized to heat the blank and another is attached to the lower die
in order to prevent the blank from becoming cold before actual drawing starts. A continuous
coolant supply is provided for the heaters of the die and blank to prevent overheating. The
temperatures are recorded by using pyrometer which is a non-contact temperature detecting
instrument. This works on the principle of capturing the wavelength of the radiation that is
emitted by the material. A data acquisition system which is connected to the press obtains input
parameters like punch travel, load applied on the blank, blank holding pressure during deep
drawing. These are fed to the computer where it directly plots outputs like variation of load with
displacement and blank holding pressure.

3 Finite element simulation

Finite element methods have been extensively used in forming operations to optimize various
process variables in order to produce defect free parts. The input models like die, blank, blank
holder and punch were constructed in pre-processor dynaform. After the surface was created,
fine meshing was generated on the surface of the tool components and the blank. This gives
automatically the nodes. Fine meshing is done on the blank to obtain accurate results. The
complete tooling model of the pre-processor is shown in Fig. 1.
The blank and the tool components were meshed using Belytschko- Tsay shell elements as it
takes less computational time, around 30–50% less than others [20]. Material properties were
measured at different temperatures with the universal testing machine (UTM) which is coupled
with the furnace and given as input to Dynaform to run the simulation. Friction in deep drawing
under warm conditions can be reduced by using molycote as lubricant, which is calculated by

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Singh et al [21] at various temperatures. Simulations were repeatedly performed by changing the
size of the blank and keeping the other parameters constant. The blank holding force and punch
speed were similar to those used in the experiments.
Barlat’s yield criteria are chosen as the material model in simulations. This criteria incorporates
the effect of both normal and planar anisotropy in the yielding behavior of the material. This
model was developed by Borliate and Lian [22] for modeling the sheets with anisotropic
materials under plain strain conditions. This material model allows use of Lankford parameter in
0°, 45° and 90° to the rolling direction for the defining of anisotropy. Anisotropic yield criterion
for plane stress is defined in equation (1). The anisotropic material constants a, c, h and p are
obtain through Lankford parameters [22].

Fig.1 Construction of tooling in pre-processor

 =a|K1 +K 2 |m +a|K1 -K 2 |m +c|2k 2 |m =2 Y m (1)

Where σy is the yield stress and Ki are given by

 x  h y (2)
K1 
2 (2)
and
 x  h y (3)
K2   P 2 xy
2

4 Results and discussion

4.1 The DSA phenomenon
After conducting the tensile test as per ASTM standard true stress vs true strain graphs are
constructed to find the serration. Serrations in the stress–strain curve were observed at lower
strain rates. For the entire set of tests, serrations were observed only over a limited range of
strain-rate and temperature, as summarized in Table 2.
DSA serrations on the stress–strain curves are well-defined at the testing of lower strain rate and
temperatures of 400°C to 600°C, while at the lower temperature they are absent in the whole
range of the strain rates. Fig. 2 provides a graphical summary of the results of tensile test carried
at lower strain rate 1x10-4 sec-1. It is clearly seen that the serrated flow appears at certain
temperature at this strain rate.

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Table 2 Serration in flow stress curve for ASS 316 at different strain rate and temperatures
Strain Temperature (°C)
Rate
50 100 150 200 250 300 350 400 450 500 550 600 650
-2
10 X X X X X X X X X

10-3 X X X X X X X X

10-4 X X X X X X X X

Fig.2 Flow stress curve of ASS316 at constant strain rate of 1x10-4 sec-1

The strain-rate sensitivity index (m) is considered to be the one of the parameter that can
characterize DSA phenomena. The flow stress equation that describes plastic behavior is usually
written as equation (4) where ‘σ’ is the flow stress, ‘K’ is a material constant, '  ' is the strain
rate and ‘m’ is the strain-rate sensitivity index of the flow stress. The m-value is a function of
the forming parameters, such as the strain rate and the temperature. The most convenient method
of measuring ‘m’ is a uniaxial tensile test at a particular constant temperature and at different
strain rates. The simplest method is reflected in the relationship between the flow stress (σ) and
the strain rate (  ).

  K m (4)

The ‘m’ value is usually calculated from the logarithmic plot of the flow stress vs strain rate.
Fig. 3 shows the graph at DSA temperature range covering the true strains of 0.2. The ‘m’
values were calculated from the slopes of the graph. The negative value of ‘m’ for the
temperature from 400°C to 600°C evident the occurrence of DSA. Decreasing strain rate
sensitivity with increasing strain in the DSA regime has been reported in low carbon steel [23]
and subsequently analyzed in detail by McCormick [24]. The appearance of negative strain-rate
sensitivity coincides with the appearance of serrations in the stress–strain curves, again
suggesting solute induced dynamic strain aging.

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Fig.3 True stress Vs True strain rate graph on log scale

The occurrence of the serration is linked to a bounded region of negative strain-rate dependence
of the flow stress, which may be explained by DSA resulting from diffusion of solute atoms to
dislocations temporarily arrested at obstacles in the slip path [27]. It clearly indicates that within
the temperature range where serrated flow occurs, the strain rate sensitivity of the flow stress
becomes negative. This serrated flow to originate from the pinning of dislocations by solute
atoms of the alloy. These are attributing the occurrence of DSA in ASS 316.

4.2 Fracture study

SEM photographs of the fracture surface of tensile specimens at 300°C and 550°C are shown in
Fig. 4 at different magnifications. There is a difference in the fracture surface. Fracture usually
occurs under single load or tearing. This is shown by depressions in the microstructure called
dimples, which occur from micro void emergence in places of high local plastic deformation.
The observed micro voids can be generated from non-metallic inclusions. For example, voids on
the surface may be initiated by carbide inclusions or nucleates at precipitates. At 300°C, the
large dimples developed at non-metallic inclusions can be seen. Under increased strain, micro
voids grow, coalescence, until rupture occurs, thus dimples fracture. Dimple size and shape
depends on the type of loading and extent of micro void emergence. When a material is put
under uniaxial tensile loading, equi-axed dimples appear which have complete rims. Under a
shear loading the dimples are elongated, the rims of the dimples are not complete and the
dimples are in the same direction as the loading. Oval dimples occur when a large void intersects
a smaller subsurface void the dimples form an oval shape and exhibit complete rims.
Fracture surface at 550°C consists of small dimples and flat areas looking like semi-cleavage.
The flat area of fracture occurs without significant plastic deformation. These are also looking
like quasi cleavages. Cleavage results from high stress along three axes with a high rate of
deformation. Characteristics of cleavage are cleavage steps, feather markings, herringbone
structure, tongues and micro-twins, Wallner lines and quasi-cleavage. A cleavage step is a step
on a cleavage facet joining two parallel cleavage fractures. Feather markings are very fine, fan
like markings on a cleavage fracture. Quasi cleavage is a fracture mode resembling cleavage
because of its planar facets but where the fracture facets are not specific well-defined planes.
This difference of fracture surface is in agreement with variation of nature of fracture at these
conditions. At non DSA region of 300°C fracture occurred through severe plastic deformation

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results in large dimples which show that fracture is ductile. Whereas in DSA region at 550°C
fractured surface had small dimples and quasi-cleavage which shows fracture is less ductile.
With this it can be observed that in DSA region ductility of the material decreases.

Fig.4 SEM images of fracture surface of 300°C and 550°C (a) 1000X at 550°C (b) 10, 000X
at 550°C (c) 1000X at 300°C (d) 10, 000X at 300°C

4.3 Formability study

Fig. 5 shows the drawn cup from the blanks of 74 mm diameter by LS Dyna at 300°C

Fig.5 Drawn cup at 300°C from ø74 mm blank and FLD by FEM

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temperature and forming limit diagram (FLD) of the cup. FLD is a graph between the minor
strain and major strain of the sheet metal. Possibility of fracture in cup can analyze and
compared with the forming limit curve (FLC), which appears in FLD. It shows that strain in the
cup is below FLC and drawing is in the safe zone. There is no indication of fracture in the cup
walls. Thickness of the cup at punch corner is reduced to 0.88 mm without necking. At this
temperature when 75 mm blank was drawn, the strain in the cup crosses the FLC as shown in
Fig. 6, which is not safe. The thickness at punch corner is reduced to less than 0.2 mm which
indicates the fracture. Fracture occurred at the punch corner due to excessive strain. At 300°C
temperature maximum of 74 mm diameter blank can draw into 30 mm cup without fracture
hence LDR is 2.47.

Fig.6 Fracture in the cup drawn from ø75 mm blank and FLD at 300°C

Finite element simulations were performed on the blank of same diameter but at higher
temperature of 400°C. As the temperature increases, these higher sizes of the blanks can be
deep drawn safely but here it fractured in the cup. At this temperature maximum of 72 mm
diameter blank can be deep draw safely as shown in Fig. 7.

Fig.7 Deep drawn cup from ø72 mm blank at 400°C

This temperature is in the DSA region which makes the blank to fracture during deep drawing.
It leads to fracture at punch corner. In the DSA region LDR is reduced. At the elevated
temperatures, cups are drawn from the higher size of the blanks without fracture but here due to

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occurrence of DSA, LDR decreased. It shows that in the DSA region formability of the sheet
metal decreased. Experimentally drawn cups from 72 mm, 74 mm and 76 mm diameter blanks at
300°C temperature are shown in Fig. 8.
Maximum of 74 mm blanks are drawn into cups at this temperature. Next higher size of the
blank of 76 mm fractured at punch corner. So the limiting drawing ratio at this temperature is
2.47. These experiments confirm the FE predictions.

Fig.8 Experimentally drawn cups at 300°C

Fig. 9 shown the cups drawn from the same size of the blanks but at higher temperature 400°C.
Here the cups are drawn only from 72 mm blank whereas 74 mm blank fractured during
drawing. LDR of the material is decreased to 2.4. But from the previous work it was investigated
that as the temperature increased, LDR of sheet metal increases [25]. At the temperature of
400°C, ASS 316 has undergone DSA phenomena. During DSA phenomena temporary arrest of
the dislocation in the slip plane due to the interaction of solute atoms with moving dislocations
in the material, which results in the increase of the strength of the sheet. During this phase as the
force to draw the cup gradually increased. Once the dislocation escaped from the solute atoms,
the strength of the material decreased but punch force has remained higher, which leads to
localize fracture initiations in the material. This makes the sheet material fractured before
complete forming which leads to brittle fracture. At the higher size of the blank, force to draw
the cup is higher and this force for 74 mm blanks fractured the cup before complete drawing.
Limiting drawing ratio of ASS 316 decreased in DSA region. This shows that formability of this
material decrease in DSA region.

Fig.9 Experimentally drawn cups at 400°C

5 Conclusions
In this study, DSA region of austenitic stainless steel 316 had been investigated by uniaxial
tensile test. Occurrence of serrated plastic flow and negative strain rate sensitivity of the flow
stress curve indicate the presence of DSA in the temperature range of 400°C to 600°C. Forming
of quasi cleavages on the fractured surface of the tensile test specimen in the DSA region were

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observed. In the DSA region ductility of the material decreased. Deep drawing of the sheet
blanks were simulated in LS Dyna and measure the LDR below and in DSA region. In the DSA
region LDR was decreased and it concluded that the formability of sheet metal decreased in the
DSA region. Simulations were in good agreement with the experimental results.

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Acknowledgement
The author would like to acknowledge the financial support given by All India Council of
Technical Education (AICTE), Government of India, Research Promotional Scheme
(RPS)20/AICTE/RIFD/RPS(POLICY-III)99/2012-13.

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