Analysis of Mechanical Properties of EN 24 Steel after Austempering and Martempering

JoEAM (2014) 1-8 © STM Journals 2014. All Rights Reserved

Journal of Experimental & Applied Mechanics ISSN: 2230-9845 (online), ISSN: 2321-516X (print)

Volume 5, Issue 3

www.stmjournals.com

Analysis of Mechanical Properties of EN 24 Steel after

Austempering and Martempering

Tridib Kumar Mahata* Manipal Institute of technology, Manipal, Karnataka-576104

Abstract The mechanical properties of steel decide its applicability for a particular condition.

Heat treatment processes are commonly used to enhance the required properties of steel.

The present work aims at experimentally investigating the effects of normalizing, austempering and timed quenching on AISI 4340 steel. The material was machined to

ASTM standards and then different tests like microstructure analysis, hardness test,

impact test, and were carried out after the heat treatment processes. All the tests were carried out in the mechanical workshops and laboratories under the supervision of their

respective faculty, at the MIT campus, Manipal. It was found that normalized steel was least hard and while timed quenched steel was hardest. The ASTM grain size affirmed

these results by displaying a similar trend. An increase in brittleness was observed with

the increase in hardness, with the timed quenched specimen displaying the least impact strength. Heat treatment is necessary to obtain the required mechanical and physical

properties for a material to make it suitable for fabrication. For this reason, the knowledge of heat treatment is necessary to suitably condition the material for the

subsequent stages of manufacture, or in the case of semi-finished component to impart the

desired mechanical and physical properties such as increased strength, toughness and wear resistance. Heat treatment is also resorted to relieve internal stresses and to soften

hard metals in order to improve machinability. The outstanding advantage of steel as an

engineering material is due to its versatility. Its properties can be controlled and changed at will by the heat treatment. Thus, if steel is to be formed into some intricate shape, it

can be made very soft and ductile by the heat treatment, if, on the other hand, it is to resist water, it can be heat treated to a very hard, wear-resisting condition.

Keywords: Austempering, martempering, tensile, steel

*Author for Correspondence E-mail:

INTRODUCTION Heat treatment is a combination of heating and

cooling operations carried out on a metal or

alloy in the solid state so as to produce a

particular microstructure and hence the desired

properties. The objective of heat treatment is

to improve the mechanical properties like

tensile strength, impact strength, to improve

machinability, to improve Hardness, to modify

magnetic and electrical properties, to refine the

grains, to produce hard surfaces and soft

interiors, to relieve internal stresses and

residual stress, to overcome the effects of

strain hardening and restore ductile properties.

Applications on EN24 Steel: is used in

components subject to high stress and with a

large cross section. AISI 4340 alloy steel is

mainly used in power transmission gears and

shafts, aircraft landing gear and other

structural parts. It is also used in heavy-duty

axles, spindles, pins, studs, collets, bolts,

couplings, sprockets, torsion bars, connecting

rods, crow bars, conveyor parts etc. EN24 steel

is a high tensile alloy steel renowned for its

wear resistance properties and also where high

strength properties are required.

EN 24 Equivalents

BS970: 1955 EN24

BS970/PD970: 1970 onwards 817M40

European 34CrNiMo6

Werkstoff No. 1.6582

US SAE (AISI) 4340

Mechanical properties of EN 24 Steel Tridib Kumar Mahata


Basic Heat Treatment Processes

Annealing

It is a heat treatment wherein a material is

altered, causing changes in its properties such

as strength and hardness.

It the process of heating solid metal to high

temperatures and cooling it slowly so that its

particles arrange into a defined lattice (Figure

1).

Fig. 1 Annealing.

Normalizing

Normalizing is similar to annealing except that

the cooling is done at a faster rate. Instead of

furnace cooling, air cooling is done.

Normalized is accomplished by heating at

approximately 400

C above the upper critical

temperature lines, holding it at that

temperature for a short time and then cooling it

in still air.

Thus we can see the range of temperature

shown by the gray shaded region in the figure

given below.

Normalizing is a technique used to provide

uniformity in grain size and composition

throughout an alloy.

The term is often used for ferrous alloys that

have been austenitized and then cooled in open

air.

Normalizing not only produces pearlite, but

also bainite and sometimes martensite, which

gives harder and stronger steel, but with less

ductility for the same composition than full

annealing (Figure 2).

Fig. 2 : Normalizing.

Austempering

Austempering is heat treatment that is applied

to ferrous metals, most notably steel and

ductile iron. In steel it produces a bainite

microstructure whereas in cast irons it

produces a structure of acicular ferrite and

high carbon, stabilized austenite known as

ausferrite. It is primarily used to improve

mechanical properties or reduce/eliminate

distortion. Austempering is defined by both

the process and the resultant

microstructure(Figure 3).

Fig. 3: Austempering.

Martempering

Martempering involves heating the steel to the

austenitizing temperature followed by

quenching in a constant temperature both

maintained above Ms point. The temperature

of the bath is between 280˚C -350˚C. As soon

as this temperature is attained, steel is

withdrawn and cooled in air. Martempering

results in minimum internal stresses, distortion

and improves mechanical properties as

Journal of Experimental & Applied Mechanics

Volume 5, Issue 3

ISSN: 2230-9845 (online), ISSN: 2321-516X (print)


compared to conventional quenching and

tempering [1–15]. The resultant microstructure

of Martempered steel is martensite(Figure 4).

Fig. 4: Martempering.

METHODOLOGY Tests To Be Conducted

Ultimate tensile strength (UTS), often

shortened to tensile strength (TS) or ultimate

strength, is the maximum stress that a material

can withstand while being stretched or pulled

before failing or breaking. Tensile strength is

the opposite of compressive strength and the

values can be quite different. Hardness is the

parameter to measure of how resistant solid

matter is to various kinds of permanent shape

change when a force is applied.

Impact test: A test designed to give

information on how a specimen of a known

material will respond to a suddenly applied

stress, e.g. shock. The test ascertains whether

the material is tough or brittle. A notched test

piece is normally employed and the two

methods in general use are either the Izod or

Izod test [9–15].

Microstructure Analysis: (pre and post heat

treatment): analysing the structure of a

prepared surface or thin foil of material as

revealed by a microscope above 200×

magnification.

Preparation of Specimens and Conducting

Tests

Impact Specimen

Toughness is the ability of a material to

absorb energy and plastically deform

without fracturing. Material toughness is

the amount of energy per volume that a

material can absorb before failure.

Phase Machinng: The sample for Impact

test (Izod) (Figure 5) is of the following

dimensions (ASTM):

10 mm x 10 mm x 75 mm.

V-notch of radius 1 mm.

Procedure for production of sample

specimen:

A square block of 45 mm x 45 mm x

75 mm is cut from the stepped specimen.

Using a band saw, cuboidal shaped

specimens of 12 mm x 12 mm x 77 mm

are cut.

Next, the specimen is filed to give the final

dimension finish of 10 mm x 10 mm x 75

mm.

The specimen is then placed in the fixture

of the shaper and using the Izod tool a

notch is cut into the specimen to the depth

of 5 mm.

The sample is ready for conducting Izod

Impact test.

Problems faced during machining

operations:

Perfect V-notch was not possible due to

uncertainty in depth.

Worn out blades and improper alignment

sometimes lead to oblique cutting.

Phase 2: Pre- heat treatment tests:

The Izod Test (Figures 6 and 7) was

conducted in the TMA Pai Polytechnic

institute.

The Specimen is placed with its notch

facing away from the striker.

Fig. 5: Impact Test Standard Specimen.



Fig. 6: Schematic Diagram of Izod Test Apparatus.

a.

b.

c.

Fig. 7(a): Izod Test Apparatus (b) Specimen Placed on the Work Centre (c) Schematic Diagram of

Izod Test Apparatus.

Consolidated Steps of Preparation of

Impact Test Specimen

1. ¾ inch dia EN24 rod was procured.

2. Rod was cut into 80 mm pieces using band

saw.

3. Facing was done to reduce the length to

75 mm.

4. Drilling was done at one end in order to

hold the work piece with the tail stock.

5. Turning was done to reduce the dia to

14.14 mm.

6. Shaping was carried out to obtain 10x10

mm square cross section.

7. V notch was made on the square work

piece using shaper machine.

Tensile Specimen Phase 1: Machining

Samples were prepared as per ASTM

standards in the machine shop. The following

steps were undertaken on the procured

material (Figure 8).

1. Rod was cut into 30 mm pieces using band

saw.

2. Facing was done to reduce the length to 27

mm.

3. Drilling was done at one end in order to

hold the work piece with the tail stock.

4. Turning was done to reduce the dia to 7

mm.


Volume 5, Issue 3



5. Turning was done for the gauge length of

16.05 mm to reduce the dia to 4.53 mm.

6. Filleting was done to obtain 0.8 mm

radius.

7. Filing was performed on the fillet.

8. Chamfering was done at the edges of

radius.

Fig. 8: Machining Tensile Specimens.

Phase 2: Test on Tensometer

First the specimen is checked once for

dimensions using a ‘Digital Vernier

Calliper’.

It is then clamped in the Tensometer

(Figure 9). and then load is applied till it

fails. The corresponding value of the

elongation and ultimate strength is

obtained. (Figures 10–14)

Fig. 9: Tensometer

Fig. 10: Broken Tensile Specimen.

Fig. 11: Digital Vernier Calliper.

Fig. 12: Prepared Tensile Specimen.

Fig. 13: Broken Tensile Specimen.

Fig. 14: Standard Tensile Specimen.

Table 1: Dimensionsof Standard Tensile

Specimen.

Symbol Dimension (mm)

A 17.65

D 4.53

G 16.05

R 0.80

F 7.00



RESULTS The results otained in the conducted experimental research are presented in the tabulates form.

Impact Test Izod test was conducted and the following results were obtained (as shown in Table 2):

Table 2: Impact Test Result.

Condition Reading 1 Reading 2 Average Reading

As bought 60 J 110 J 85 J

Normalized 56 J - 56 J

Austempered 24 J 28 J 26 J

Martempered 24 J 24 J 24

From the above values, we infer that

The toughness value for AISI 4340 was

the most at 110 J.

The toughness value for AISI 4340 was

the least at 24 J.

We can deduce that AISI 4340 timed

quenched specimen has the least impact

energy or the impact energy absorbed by

the time quenched specimen is the least.

(Figure 15).


Volume 5, Issue 3



Fig. 15: Graph Showing Result.

CONCLUSIONS On conducting the pre-heat treatment tests,

i.e. the as bought condition, the hardness

was found to be of a value which is lesser

than values when the specimens are heat

treated. As expected, timed quenching

increased the hardness by a considerable

amount. The percentage increase in the

hardness after martempered condition

with respect to the as bought condition is

20%.

The trend in the properties concerning the

Izod charpy impact test is that it is a

decreasing function in a non linear

fashion. This is because the surface

hardness increases and the interior of the

specimen becomes more brittle.

The conclusion drawn from the tensile test

on the specimens in the as bought

condition approximately matched with the

standard ductile graphs under similar

conditions. Unfortunately tests under any

of the heat treated specimens could not be

conducted due to unforeseen

circumstances which involved the failure

of the holder of the Tensometer.

The ASTM grain size number is an

indication of the number of grains per sq.

inch. A higher ASTM grain size number

indicates a higher number and smaller size

of grains as compared to a smaller ASTM

number i.e. a high ASTM number

indicates more refine grains which lead to

better hardness.

As far as microstructure is concerned,

Martempered specimen has increased

grain size as compare to Austempered.

Martempered specimen is found to be rich

in density over surface compare to

Austempered specimen. Normalized

specimen found to have decreased in grain

size.

Martempered specimen was the hardest

amongst all the specimens. Martensite is

hardest because of conversion of austenite

to martensite structure. Normalized

specimen found to be less hardened.

Austempered specimen hardness is less

and closer to martempered specimen.

REFERENCES 1. Vijay GS, Annealing-Material Science &

Metallurgy 2001.

2. Sydney H. Avner, Introduction to Physical

Metallurgy, Singapore: Mc Graw-Hill;

1974.

3. Prabhudev K.H.,. Handbook of Heat

Treatment of Steels, Bangalore: Tata Mc-

Graw-Hill Publishing; 1988

4. Romesh C. Sharma, Principles of Heat

Treatment of Steels, India: New Age

International Limited; 1996

5. Rajan T.V., Sharma C.P., Sharma A, Heat

Treatment, New Delhi: Prentic Hall of

India Pvt Ltd.; 1999.

6. Rajan T. V., Sharma C. P., Ashok Sharma,

Heat Treatment Principles and

Techniques, New Delhi: Prentice Hall of

India Private Limited; 2001.

7. Totten G. E. Steel Heat Treatment:

Metallurgy and Technologies. NewYork:

Taylors & Francies Group; 2007.

8. Garima Sharma, Sundararaman M.,

Prabhu N., et al. Sliding Wear Resistance

of Iron Aluminides, Indian Academy of

Sciences, Mater. Sci. 2003; 26(3): 311–

314p.

0

10

20

30

40

50

60

70

80

AS BOUGHT NORMALIZED AUSTEMPERED MARTEMPERED

TRIAL 1

TRIAL 2

AVERAGE



9. Nga D.H.L., Cho a K.S., Wong a M.L., et

al. Study of Microstructure, Mechanical

Properties and Magnetization Process in

Low Carbon Steel Bars. Materials

Science and Engineering. 2003; A358:

186–198p.

10. Mitra P.K., Paul S., Chatterjee S.,

Treatment, Structure, Corrosion,

Correlation of AISI 8640 Steel” , IE (I)

Journal MM. 2004; 85.

11. Ashassi-Sorkhabi H, Seifzadeh D, The

inhibition of steel corrosion in

hydrochloric acid solution by juice of

Prunus cerasus, International Journal of

Electrochemical Science. 2006; 1:92–98p.

12. Ayo Samuel Afolabi, Corrosion and Stress

Corrosion Behaviors of Low and Medium

Carbon Steels in Agro-Fluid Media,

Leonardo Electronic Journal of Practices

and Technologies. 2007; 10: 55–66p.

13. Shanbhag A.V, Venkatesha T.V, Prabhu

R. A., et al. Corrosion inhibition of mild

steel in acidic medium using hydrazide

derivatives, J Appl Electrochem . 2008;

38:279–287p.

14. Rainer Menig, Volker Schulze, Otmar

Vohringer, Effect of Short-Time Annealing

on Fatigue Strength of Shot Peened AISI

4140 in a Quenched and Tempered

Material State, Institutfür Werkstoffkunde

I, Universität Karlsruhe (TH), Karlsruhe,

Germany

15. Qamar S.Z., Effect of heat treatment on

mechanical properties of H11 tool Steel,

Journal of Achievements in Materials and

Manufacturing Engineering. 2009. 35(2).

Analysis of Mechanical Properties of EN 24 Steel after Austempering and Martempering

Documents

Transcript of Analysis of Mechanical Properties of EN 24 Steel after Austempering and Martempering