EXPERIMENTAL ANALYSI S ON FORMABILITY OF …onset of localized necking. The concept of Forming Limit Diagram (FLD) was introduced by Keeler and Back ofen and Goodwin [5]. Keeler et

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International Journal of Civil Engineering and Technology (IJCIET)Volume 8, Issue 7, July 2017, pp.

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EXPERIMENTAL ANALYSI

FORMABILITY OF ASS30

VARYING PARAMETERS

Assistant Professor

MLR Institute of Technology, Hyderabad, Telangana,

Assistant Professor


Assistant Professor


Assistant Professor

Vardhaman College of Engineering,

Professor

Institute of Aeronautical Engineering

ABSTRACT

Since few decades the ASS304 material has become a work horse material for

many of the industrial equipments and their

Nuclear, Marine applications and most widely in Automobile Sector. ASS 304 is used

in nuclear industries especially to Manufacture heat exchangers. Sheet Metal products

produced by this material will entrain exce

resistance, light weight, high Tensile strength etc. As understood from the various

studys that the Forming Limit diagrams

the formability behavior of materials,

experiments were carried out and major strains and minor strains were measured for

the ASS304 material. In ASS, austenite will

microstructures, either due to change in temperature or due to change in

Therefore the present investigation studies the effect of strain rate on formability at

different temperatures and punch speeds.

Key words: Formability, Stretch Forming, Forming Limit diagram(FLD).

IJCIET/index.asp 993 [email protected]

International Journal of Civil Engineering and Technology (IJCIET) 2017, pp. 993–1002, Article ID: IJCIET_08_07_106

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6308 and ISSN Online: 0976-6316

Scopus Indexed

EXPERIMENTAL ANALYSIS ON

FORMABILITY OF ASS304 SHEET METAL AT

VARYING PARAMETERS

P. Raghavendra

Assistant Professor, Department of Mechanical Engineering

MLR Institute of Technology, Hyderabad, Telangana, India

P. Kezia



J. Gangadhar

stant Professor, Department of Mechanical Engineering


S. M. Gangadhar Reddy


rdhaman College of Engineering, Hyderabad, Telangana, India

Dr. D. Govardhan

Professor, Department of Mechanical Engineering

Aeronautical Engineering, Hyderabad, Telangana,


many of the industrial equipments and their applications, such as in Thermal, Defense,



produced by this material will entrain exceptional properties such as corrosion


studys that the Forming Limit diagrams (FLDs) are very widely used to understand

the formability behavior of materials, Inorder to construct FLDs stretch forming


the ASS304 material. In ASS, austenite will be converted into some other

microstructures, either due to change in temperature or due to change in


different temperatures and punch speeds.

Formability, Stretch Forming, Forming Limit diagram(FLD).

[email protected]

http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=8&IType=7

S ON

4 SHEET METAL AT

, Department of Mechanical Engineering

India


India


India


Hyderabad, Telangana, India

, Hyderabad, Telangana, India


applications, such as in Thermal, Defense,



ptional properties such as corrosion


) are very widely used to understand

stretch forming


be converted into some other

microstructures, either due to change in temperature or due to change in strain rate.


Formability, Stretch Forming, Forming Limit diagram(FLD)..

Experimental Analysis on Formability of ASS304 Sheet Metal at Varying Parameters


Cite this Article: P. Raghavendra, P.

and Dr. D. Govardhan, Experimental Analysis on Formability of ASS304 Sheet Metal

at Varying Parameters. International Journal of Civil Engineering an

8(7), 2017, pp. 993–1002.

http://www.iaeme.com/IJCIET/issues.

1. INTRODUCTION

Metal forming is one of the most important process in manufacturing of a large variety of

products. In the recent practical cost conscious world, owing for relatively low cost, high

productivity, enhanced mechanical properties, flexible operations, considerable material

saving and greater control over technical and aesthetic parameters, hence many expensive

cast, rolled and forged parts have been replacing with sheet metal parts. The objects and

articles that we use in our daily life are man

some raw material through the

Forming is the process of obtaining the required shape and size on the raw ma

subjecting the material to plastic deformation by applying

force, bending or shear force or the combinations of these forces through various dies and

tooling.

In sheet metal forming a flat thin sheet metal blank is

tensile loads into a three-dimensional shape, often without significant changes in sheet

thickness. It involves conversion of flat thin sheet metal blanks into parts of required shape

and size. The process is carried out

of surface area to thickness [1]. In this the residual stresses in the material will cause the sheet

to spring back slightly after the deformation. Due to this elastic recovery, it is necessary t

consider for achievement of the desired shape and size [2]. Friction conditions at the tool

metal interface are very important and controlled by press conditions, lubrication, tool

material and surface condition [3]. These processes are extensively used

simple to complex shapes and producing large number of variety of

industries like food, beverages, automobile, thermal, marine, aerospace, defence, nuclear and

other sheet forming applications.

Figure 1 Some parts Manufactured by Sheet Metal Forming Operations

1.1. Stretch Forming Process

In stretching operation, the sheet metal is clamped around its edges and stretched over a die or

form blocks which moves upward, downward depending on the particular machine as sho

below in Fig.2. Aluminum skins for the Boeing 767 and 757 fuselages, for example are made

by stretch forming using a blank under a tensile force as

strength metals at room temperature is a little difficult, because of the large deformation and



P. Raghavendra, P. Kezia, J. Gangadhar, S. M. Gangadhar Reddy

Experimental Analysis on Formability of ASS304 Sheet Metal

. International Journal of Civil Engineering an

ET/issues.asp?JType=IJCIET&VType=8&IType=7



nhanced mechanical properties, flexible operations, considerable material



icles that we use in our daily life are man-made, engineered parts, which are obtained from

some raw material through the various manufacturing process as shown in figure 1.Metal

Forming is the process of obtaining the required shape and size on the raw ma

subjecting the material to plastic deformation by applying the tensile force, compressive


In sheet metal forming a flat thin sheet metal blank is subjected to plastic deformation by

dimensional shape, often without significant changes in sheet


and size. The process is carried out on the plane of the sheet by tensile forces with high ratio


to spring back slightly after the deformation. Due to this elastic recovery, it is necessary t

the desired shape and size [2]. Friction conditions at the tool


material and surface condition [3]. These processes are extensively used for manufacturing of

simple to complex shapes and producing large number of variety of components for various


other sheet forming applications.

arts Manufactured by Sheet Metal Forming Operations

Stretch Forming Process


form blocks which moves upward, downward depending on the particular machine as sho

. Aluminum skins for the Boeing 767 and 757 fuselages, for example are made

by stretch forming using a blank under a tensile force as high as 9MN. Stretching of high



[email protected]

Kezia, J. Gangadhar, S. M. Gangadhar Reddy

Experimental Analysis on Formability of ASS304 Sheet Metal

. International Journal of Civil Engineering and Technology,

asp?JType=IJCIET&VType=8&IType=7



nhanced mechanical properties, flexible operations, considerable material



made, engineered parts, which are obtained from

various manufacturing process as shown in figure 1.Metal

Forming is the process of obtaining the required shape and size on the raw material by

the tensile force, compressive


subjected to plastic deformation by

dimensional shape, often without significant changes in sheet


on the plane of the sheet by tensile forces with high ratio


to spring back slightly after the deformation. Due to this elastic recovery, it is necessary to

the desired shape and size [2]. Friction conditions at the tool-


for manufacturing of

components for various


arts Manufactured by Sheet Metal Forming Operations


form blocks which moves upward, downward depending on the particular machine as shown

. Aluminum skins for the Boeing 767 and 757 fuselages, for example are made

high as 9MN. Stretching of high


P. Raghavendra, P. Kezia, J. Gangadhar, S. M. Gangadhar Reddy and Dr. D. Govardhan

http://www.iaeme.com/IJCIET/index.asp 995 [email protected]

high flow stresses of the materials. Whereas stretching at elevated temperatures leads to the

decrease in flow stresses, relieves residual stresses and made the deformation easier. It allows

deeper drawing and more stretching in the final products. [4].

Figure 2 Punch and Die assembly in Stretch Forming process

1.2. Austenitic Stainless Steel Grade 304 (ASS 304)

Austenitic stainless steels are an exceptional class of materials, which have been used to

construct cars since the mid-1930s. Still, despite continuing growth, the production of

stainless steel amounts to only about 2.5% of the annual production of carbon steels.

Although still rare in the car industry, they offer superior strength associated with good

formability and exceptional work-hardening ability due to deformation-induced phase

transformation. Strength levels from 800 to 2000MPa may be reached with cold formed

commercially available stainless steel sheets due to deformation-induced phase transformation

to martensite during or before forming.

Nickel-based austenitic steels are classified as 300 series. The most common of these is

grade 304. When 18% chromium and 8% nickel are added to convert all the ferrite to

austenite and thus crystal structure of austenite remains stable over all temperatures. The

chromium content would impart corrosion and oxidation resistance to steel. The chromium

present in the alloy reacts with oxygen and forms a passive layer of Cr2O3 on the surface of

the metal, which protects the steel against corrosive environments.

2. LITERATURE SURVEY ON FORMING LIMIT DIAGRAMS AND

ASS 304 MATERIAL

The present workis mainly focused on understanding the formability behavior of ASS304

material with the help of Forming limit Diagram by conducting the stretch forming

experiments. An extensive literature study has been carried out to analyze the previous work

done by different research people in the area of formability of sheet metal forming. In

industrial sheet metal forming operations involving thin sheets, formability is limited by the

onset of localized necking. The concept of Forming Limit Diagram (FLD) was introduced by

Keeler and Back ofen and Goodwin [5]. Keeler et al. determined strains on the right hand side

of the FLD and Goodwin extended the FLD by including negative minor strains. Further

experimental investigations of the FLD were conducted for example by Ahmadi et al. [6].it

has proved to be a useful tool to represent conditions for the onset of necking and evaluate

formability of sheet metals. However, experimental determination of FLDs seems to be easy,

but would be time consuming and expensive.



One of the first theoretical models to predict forming limits of a sheet metal was proposed

by Hill [7], where it was assumed that the existence of a zero-extension direction is a

necessary condition for localized necking. Hills approach dictated that localized necking can

only occur when the ratio of the minor strain to the major strain is less than or equal to zero,

which corresponds to the left-hand side of the FLD. Keeler and others conducted extensive

forming limit tests and measurements for a wide variety of sheet metals, with focus on

different steel grades. They found that necking does occur in a sheet metal even when it is

under biaxial stretching with positive minor strains (the strain ratio is larger than zero),

contradicting Hills conclusion. To resolve the discrepancy between the Hills theory and

experimental observations of the right-hand side of FLD[8]

Sailaja et al carried his research on the magnesium ZE41 rare earth alloy which has a

potential application in automotive & aero.The results indicates that the microstructure 0f

ZE41 alloy consists of α-Mg matrix which forms the main body of the grain along the

secondary phases. consisting of randomly distributed β Phase at the grain boundaries were

observed with a magnification of X200 respectively and in this way material properties will

change due to effect of temperature by the SEM ANALYSIS. [9]

Marciniak and Kuczynski et al. introduced the concept of an initial imperfection to

account for localized necking in biaxial stretched sheet metals [10].

Ghosh et al. (1977) reported that the limit strain, tensile instability and necking in sheet

metals were strongly influenced by the strain hardening and strain rate hardening .The limit

strain increases when the strain hardening and strain rate hardening exponent increases [11]

Ozturk and Lee (2005) reported the lubrication effect on FLD in the conventional dome

test. The lubrication between punch and blank reduces the frictional forces, improves strain

distribution, and delays localized thinning [12].

Singh et al. and jayahari et al carried out investigations on Drawability of ASS-304

material at elevated temperatures. In deep drawing experiments they found that there was a

significant improvement in the formability by increasing temperature. As the load–

displacement curve predicted by increasing temperature there was a decrease in the load

during forming due to a decrease in the mean flow stresses [13].

Gupta et al. in his investigations on ASS304 in the temperature range of 550 to 650°c

showed that a dynamic strain aging phenomenon occurring at a strain rate of 0.0001s-1 this

would be the primary cause for brittle fracture in the specimen. Thus at lower temperatures

ASS304 undergoes ductile fracture, whereas at higher temperatures, micro cleavages are

observed due to either strain hardening or dynamic strain aging[14].

Husaini et al. recently carried out his research work on development of FLD s for ASS316

at 3000c and deep drawing experiments to find LDR at higher temperatures. He concluded

that maximum LDR at 3000 c was 2.47 and thickness variation in cup was found to be less

than 0.3mm and this was the maximum formability at this temperature. FLC developed from

stretching experiments intersects the major strain line approximately at 0.3, which is very

close to the work hardening exponent of the material at300◦c.FLC intersects the major strain

line approximately at 0.3, which is very close to the work hardening exponent of the material

at 300◦c.[15].

lot of research work has been carried out on the various materials such as ASS 304,

ASS316, EDD STEELS, Aluminium alloys and Titanium alloys to evaluate the material

properties and to study the formability behavior by carrying different experiments on the

equipments such as Hydraulic Press, Universal Tensile Testing Machine and the outcomes of

the investigations have been studied for understanding the formability behavior of the

materials.



From the background work it is observed that the considerable amount of research work

has been already carried out in the area of formability of sheet metal forming of different

materials such as ASS304,ASS316,EDD STEELS and many other materials. Since there has

been no work carried out on the development of forming limit diagrams on ASS 304 metal by

the process of stretch forming experiments so far, the current work emphasizes on the stretch

forming experiments based on heckers simplified technique. To plot the forming limit

diagrams for the fore mentioned material at room temperature as well as 150◦c accompanied

with two different punch speeds namely 30mm/sec and 50mm/sec by taking the blanks of

different sizes (110mm×110 mm,110mm×100 mm, 110mm×90 so on up to

110mm×20mm).To discuss the formability behavior from the data obtained at different

speeds and different temperature.

3. EXPERIMENTAL DETAILS AND PROCEDURE

Work base relies on stretch forming experiments on 1mm ASS304 specimens of different

sizes for excessive plastic deformation up to necking and failure at room temperature and

1500 c The experimental setup required for conducting experiments includes electro chemical

etching machine to mark circle grids of 5 mm diameter on the specimens, a power source

(Hydraulic Power source) to apply different punch speeds and to apply required blank holding

force to restrict the material to flow inside the lock bead at the die and draw bead interface ,

heating system for heating blank and die (two induction heaters) with a 3 phase induction

motor , data acquisition system for observing the change in parameters while experimentation

is going on and temperature measuring device (contact type digital thermocouple) to measure

the temperature of the blank. Major strains and minor strains have to be calculated on the

stretched specimens very near to fracture by using travelling microscope with 0.01mm least

count and the measured data values will be saved in the excel sheet and the same is used to

develop forming limit diagram. Attempts are made to evaluate the influence of various

parameters such as major strain, minor strain, speed and temperature on the formability of

ASS304 material.

4. RAW MATERIAL

The chemical composition of ASS304 sheets used in the present investigations was analyzed

by a spectrometer. These chemical composition results are shown in Table 1 and mechanical

properties in Table 2.

Table 1 Chemical composition of ASS304 steel sheets (in weight percent).

Element C Si Mn Co Fe Cr Cu Ni Mo others

Weight

%

0.0025 0.410 1.14 0.210 70.78 18.40 0.18 8.19 0.360 0.305

Table 2 Mechanical Properties of ASS 304 at room temperature

UTS,(M Pa) YS, (M Pa) STRAIN AT YS ELG in %

570 275.6483 0.0222 41.9387

4.1. Die, Blank Holder and Punch

Punch and the dies were designed using the standard procedure as used by Ravi Kumar et

al[16] in his research work, in such a way that these tools can be used successfully. Die and

punch assemblies were designed and fabricated depending on the thickness of the sheets. A

48.6mm diameter hemispherical punch was fabricated on a numerically controlled lathe by



generating NC code. Fig. 3. shows the punch-die assembly and all the components fabricated

for punch stretching experiments. A draw bead of 70mm diameter was provided on the dies to

restrict the material flow from outside. Sufficient blank holding pressure was applied using

the upper die to clamp the material in the draw bead.

Figure 3 Schematic diagram of the punch-die assembly for punch stretching experiments.

4.2. Forming Limit Diagram by Punch Stretch Forming

The forming limit diagrams were determined by following the Heckers simplified technique

[Hecker (1975)]. In this method, the experimental procedure mainly involves three stages -

grid marking the sheet samples, punch stretching the grid marked samples to failure or onset

of localized necking and measurement of strains.

The major and minor strains e1 and e2 of the deformed ellipses lying in safe, necked and

fractured regions were determined by measuring the major and minor diameters of the ellipses

in both longitudinal and transverse directions of the sample using a travelling microscope with

0.01mm least count.

Strain is calculated from the following formula

Major strain =major axis length – original circle dia

original circle dia× 100

Minor strain =major axis length – original circle dia

original circle dia× 100

The major and minor strains were plotted against each other. The forming limit curve was

drawn clearly demarcating the safe limiting strains from the unsafe zone containing the

necked and fractured ellipses. Because of overlap of necked and safe strains in the critical

region, bands were plotted.

5. RESULTS AND DISCUSSIONS

After the experimentations major strain and minor strain values are calculated for safe and

break specimens at different temperatures. The specimens in safe condition and break

condition are shown in the Fig 4.1 and 4.2.

P. Raghavendra, P. Kezia, J. Gangadhar, S. M. Gangadh


Figure 4 (a) and (b) Specimens of all

5.1. Forming Limit Diagrams

Figure 5 Forming limit diagram of ASS304 at Room Temp with 30 mm/min Punch Speed.



Specimens of all different sizes (110mmx110mm to 110mmx20mm) drawn at

safe and break condition.

iagrams

Forming limit diagram of ASS304 at Room Temp with 30 mm/min Punch Speed.

ar Reddy and Dr. D. Govardhan

[email protected]

110mmx20mm) drawn at

Forming limit diagram of ASS304 at Room Temp with 30 mm/min Punch Speed.



Figure 6 Forming limit diagram of ASS304 at Room Temp with 50 mm/min

Figure 7 Forming limit diagram of ASS304 at 150°c with 30 mm/min Punch Speed.

Figure 8 Forming limit diagram of ASS304 at 1

The forming limit diagrams of ASS304 sheets were obtained by conducting punch

stretching experiments on specimens of different widths. Because of the large scatter in the

measured strains with varying blank width and also due to the overlap of some points

maximum safe strains and the strain in the portions where necking has just started), it is



Forming limit diagram of ASS304 at Room Temp with 50 mm/min

Forming limit diagram of ASS304 at 150°c with 30 mm/min Punch Speed.

Forming limit diagram of ASS304 at 150°c with 50 mm/min Punch Speed

The forming limit diagrams of ASS304 sheets were obtained by conducting punch


measured strains with varying blank width and also due to the overlap of some points



[email protected]

Forming limit diagram of ASS304 at Room Temp with 50 mm/min punch speed.

Forming limit diagram of ASS304 at 150°c with 30 mm/min Punch Speed.

50°c with 50 mm/min Punch Speed

The forming limit diagrams of ASS304 sheets were obtained by conducting punch-


measured strains with varying blank width and also due to the overlap of some points (the




difficult to draw a very precise curve indicates the onset of failure. Therefore, it is more

appropriate to show the forming limit diagram by a band rather than as a line. Fig.5 to Fig.8

show the forming limit diagrams. The area below the lower line of the band is the safe

working zone for the sheets for all possible combination of strains. Above the upper line of

the band, the sheet metal is certain to fail by necking/fracture. The area within the band

represents the critical region where the sheet is likely to develop the necking/onset of failure.

The forming limit diagram was constructed using strain values obtained from the specimens

with varying width and some of the data points were omitted for clarity.

The Flds that are plotted for room temperature shows that the intersection of lines on the

Fld (which is said to be plain strain condition)value is 0.32. When comparing the value at

intersection with n value obtained at room temperature while finding the mechanical

properties of ASS 304 by jayahari et al (38). it was almost same at room temperature. The

variation of Flds obtained at room temperature at two different speeds does not show any

significant effect. Due to the

higher load requirement for the deformation of material at room temperature the strains

are not uniformly distributed and a lot of scatter was seen in plotting the values at room

temperature.

At 150°c the intersection of Fld showed a significant variation when compared to room

temperature. This is because unlike room temperature the value of intersection I.e. plain strain

condition is not equal to the work hardening coefficient. At 150°c due to very low mean

stresses are required to deform the material the plain strain condition value is slightly

decreased. Since ass 304 is unstable, due to increase in the speed it is observed that the

portion of austenite phase will convert into martensite.that leads to brittle fracture.

6. CONCLUSIONS

In this study formability limit diagram of ASS 304 was constructed at different temperatures

(i.e RT, 150°c, ) and deformation speeds (i.e.30mm/min & 50mm/min). Because of presence

of nickel and chromium, austenite phase is stable at room temperature. This austenite phase is

unstable according to TTT (Time-Temperature-Transformation) diagram. So, due to influence

of temperature and punch speed it will convert into martensite. Generally the intersection

points of lines (plain strain condition) on FLD is approximately equals to work hardening

exponent but it was understood from this study that at 150 0 c, fracture lines are meeting the

major strain axis at a slightly different point and the neck region appears approximately in

10% below range of the fracture line. Since at higher punch speed tendency of austenite

converting into martensite will be more. So it was observed in FLD that for higher punch

speed bi-axial tension and tension –compression region lines are having a downward trend

i.e., fracture is pre-dominant. At higher speeds the value of major strains are appeared to be

more for the same temperature.

So the deformation of ASS304 material has to be carried out at lower temperatures as

possible. At room temperature it has good formability due to nickel and chromium alloy.

At150°c the formability is decreased due to lower and lower mean flow stresses.

REFERENCES

[1] SeropeKalpakjian, Steven R. Schmid Manufacturing Processes for Engineering

Materials, Pearson Education, 2008.

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[3] A. ErmanTekkaya - Sheet Metal Forming: Fundamentals by ASM- 2012



[4] J. Gronostajski, A. Matuszak, A. Niechajowicz, Z. Zimniak: “The system for sheet metal

forming design of complex parts”, J. of Materials Processing Tech., (2004) Vol. 157–158,

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[5] Serope Kalpakjian, Steven R. Schmid Manufacturing Processes for Engineering

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[14] Jayahari L, Sasidhar PV, Prudvi Reddy P, Balu Naik B, Gupta A K, Singh S K.

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setup at elevated temperatures using LS-DYNA. J King Saud UnivEng Sci 2012.

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Documents

EXPERIMENTAL ANALYSI S ON FORMABILITY OF …onset of localized necking. The concept of Forming Limit Diagram (FLD) was introduced by Keeler and Back ofen and Goodwin [5]. Keeler et