PPA/T/84

Effect of heat treatment on toughness and strength properties of C-Mn steel

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Effect of heat-treatment on toughness and

strength properties of C-Mn steel.

Kohd b. Harun, Goh Kian Seong and Jasmin Baba

Abstract

The strength and toughness of the heat-treated and tempered

C-Kn are studied. Two type of heat-treatments have been

carried out with the specimens in an argon gas. The variation

in the fracture surfaces of the heat-treated and tempered

specimens with impact test temperatures also is discussed.

+ Fakulti Kejuruteraan Jentera,

L'niversiti Teknoloci Malaysia.

Effect of heat-treatment on toughness and

strength properties of C-Mn steel

Introduction

C-Mn steel is tr.e most common and economic material used

in structural components such as bridges, building, ships,

venicles, boilers and pipelines. The steel has a medium re-

sistance to sulphide stress corrosion, hydrogen induced

croocing and stress corrosion cracking. It is not suitable

to use under wet Co and pitting. More recent steel making

developments are introducing low carbon products manufactured

using controlled rolling and cooling techniques. According

to chemistry requirement, increasing of toughness can be

achieved by lowering the carbon and sulphur levels with some

additions of nicroalloying elements for grain refinement such 1 2

as No one V; and sulphide shape control (Ca) ' .

Charpy V-notch impact is used for comparing the influence

of alloy studies and heat-treatment on toughness. It is used

for quality control and material acceptance purpose. The

notcn tougnness or tendency for brittle failure is determined

:jy t.ne transition temperature. The transition-temperature

can nelp engineer to select a material which has a sufficient

notcn toughness wnen subjected tc severe service conditions.

Tne :,oter:ol witn the lowest transition temperature is to be

preferred oecause the lower this transition temperature, the

greater the fracture toughness of the material. In mild steel,

the change in transition tenonrature can oe produced by changes 3

in microstructure or the chemical compostion sucn as C and Mn . Grain size also nas d strong effect on transition temperature.

The mecnanical oroperties of a quenched and tempered steel

may be altered oy changing tne tempering temperature. In tne

case of SME 4340 steel, hardness and tensile properties are

reduced with increasing of tempering temperature. The tensile

properties of annealed and normalised steels are controlled

by the ferrite and the amount, shape and distribution of

Cementite. The strength of ferrite depend on the amount of

alloying element in solid solution and the ferrite grain size.

The strenth depends on the amount of cementite either as

pearlite or as spherodite, in which the tensile strength of

the spherodized structure are difference compared with a pearlite

structure.

The oest combination of strength and ductility is obtained

in steel whicn has neen quenched to a fully nartensite structure

and then tempered. In this study, the works have been carried

out to investigate the effect of heat-treatments on the tough-

ness anc strength properties of the carbon steel.

Material and Procedure

The chemical composition of tne material used in this study

is given in table 1.

Table 1

element C

% weight 0.17

Mn

0.79

Si

0.12

P

0.04

Cr

0.09

Mo

0.02

Ni

0.09

Cu

0.32

Fe

Balance

Two pieces of as-received s t e e l s has been homogenized a t 1423K

for lOh in argon gas environment. After being homogenized, two

types of heat treatments were ca r r i ed out with the specimans in an

argon gas» They were type A; aus ten i t i zed a t 980 C for 2H and

furnace cooling t o roon temperature, and heated to 760 C for

3h, water quenched. Type B, was aus ten i t i zed a t 980 C for 2h,

furnace cooled to 760 c and maintain a t 760 C for 3h, water

quenched.

Both types of specimen were tempered a t 200 C and 500 C

for l h . The Charpy V-notch impact t e s t have been ca r r i ed out

on the specimens a t severa l temperatures, ranging from - 200 C

to + 150 c . These f rac tu re surfaces were subsequently

J

examined by scanning e l ec t ron microscope and the t r a n s i t i o n

temperature ( f t - l b ) was determined. The t e n s i l e t e s t s were

ca r r i ed out using a 10-T zwick Universal Test ing machine.

The hardness was measured using a Vickers Microhardness Tester

with a load of lOOg.

Results and discussion

( i ) Microstrueture and fractouraphy

The op t i ca l r.icrograpns of the as - rece ived and hea t -

t rea ted specimens were shown in f igure 1. The micro-

s t ruc tu re showing the presence of the ferrite—marten-

s i t e mixed s t r u c t u r e s . The morphology fea tures of the

s t ruc tu re s observed by o p t i c a l microscope were s l i gh t l y

changed by the tempering.

Scanning micrographs of the f r ac tu re surface a t d i f f e r e n t

temperature t e s t s were shown in f igures 2 and 3 . In

f igure 2/\, tne neat- t reatment of type A shown dimpled

rupture a t the temperatures of 50 C, 124 C and 191 C.

Some inclus ions can be seen within dimples. The

inclusions have been iden t i f i ed as Mn S. The f rac ture

exnioi ts cleavage facets and dimples a t the temperature

of - 15 C. Tne f r a c t u r e mechanism was B r i t t l e cleavage

a t temperature of - 116 C and - 167 c . In f igure 2B,

the fractcgraph showed two mecnanisms of f rac tu re a t

temperature of - 116 C.

In tne heat- t reatment of type B.as shown in f igure 3A,

two mechanism of f r ac tu re occured a t the temperature of -

129 C, meanwhile in f igure 3B, the two mechanisms of

f rac ture occured a t the temperature of - 133 C.

( i i ) Hardness and s t r eng th

Heat-treated samples A and B have a higher hardness;

93.0VHN and 83.2 VHW, r e s p e c t i v e l y . After tempering at

4 -

500 C and 200 C, lh; the hardness of the sample B was

reduced t o 67.6 VHN and 60.6 VHN re spec t ive ly . Mean-

whi le , the hardness of t h e sample A was reduced t o

61.5 and 62.8 VHN, r e s p e c t i v e l y . The as-received

sample has a hardness of 64.6 VHN.

Graph l has shown the e f fec t of heat- t reatments on the

s t r e s s - s t r a i n curves . The t e n s i l e s t rengths of the

hea t - t r ea t ed specimens A. and B were 598,98 and 592.03 _2

Nram respec t ive ly . The t e n s i l e s t reng th of A was _2

reduced to 388.83 and 365.28 Nmm af te r tempering a t

temperatures of 200 C and 500 c for lh, r e spec t ive ly .

Meanwhile, the t e n s i l e s t r e n g t n of B was reduced to -2

3 72.56 and 363.17 Nmm a f t e r tempering a t temperatures

of 200 C and 500 C, for lh , r e spec t ive ly . The s t r a i n

was higher a t tempering temperature of 500 C, lh

compared witn tempering temr-erature of 200 lh for both

samples of A and 3 . The s t r a i n values of sample A and B

as a r e s u l t of tempering at 500 C, In was 45.64% and

43.55%, r e spec t i ve ly .

The s t r a i n values of sample n and B before tempering

were 13.97 and 12.75%, r e s p e c t i v e l y . The s t reng th and

d u c t i l i t y of A was b e t t e r than t h a t of B.

( i i i ) The t r a n s i t i o n temperature

The e f fec t of he^ t - t rea tments on the energy- t rans i t ion

temperature was shown ingraph 2 . The tempered samples

n. and B a t 200 C, Ih has a t r a n s i t i o n temperatures of —

57 C and - 121 C, r e s p e c t i v e l y . . The tempered samples

of A and B a t 500 C, lh had a t r a n s i t i o n temperature of

- 148 and - 152 C, r e s p e c t i v e l y . The t r a n s i t i o n

temperature of B was s l i g h t l y lower than tha t of A.

Conclusion

Heat-treatment and tempering of C-Mn s t e e l can effect to

the toughness and s t r eng th . After the heat treatments and

5

tempering the microstructure consists of a Fcrrite matrix and

cenentite changed their size and distribution. Combination of

good strength and toughness had be obtained in the C-Mn steel

after heat-treatment A and tempering at temperature of 500 C,

lh.

Acknowledgements

The author would l ike to thank Mrs. Zaiton Selamat and Mr.

Krishnamoorthy, for their experimental help. He also thanks Mr.

Samad Solnae, Dean, Faculty of Mechanical Engineering, University

Teknologi Malaysia and Dr. Abd. Ghaffar Rarali, Head of Nuclear

Technology Programme, Nuclear Energy Unit for the ir cooperations

in this work.

References

1. C.L. Jones : in 'Xanufacturing, f ab r i ca t ion , and operation

of p i p e l i n e s ' , 2; 1982, Glasgow, Scot t i sh

Association of meta l s .

2 . R.J. Gr^y, V..R. Irving and R. Saker : in Proc. Cor.f. on

•SteelTiakinc, c a s t i n g , and r o l l i n g of p ipe l ine

sceels ir. -iSC , 3; 1932, Glasgow, Scot t i sh

Association cf n e t a l s .

3 . J.A. Pinebolt and :..J. Ha r r i s , J r . : Trans. Am. Soc. Met.

Vol. 43 , pp. 1175 - 1214.

4. H.J. French, : Trans. .Metall. Soc. AIHE, Vol. 206, p. 770,

1956.

?c /r

ft/

"ig. i : (a) as - received Sample.

(b) Heat treatment A.

(c) HT A + tempering at 200°C, lh.

(d) HT A + tempering at 500°C, lh.

(e) HT 3.

(f) HT B + tempering at 20C°C, lh.

(q) HT 3 •+ tempering at 500°c, lh.

F i g . 2n: SEK f rac tog raphs of HT(A) and tempered 200 C, ih

r.pecimeiis t e s t e d a t tern: er . i tur .T. o f : -

F i g . 2B: SEM f r a c t o g r a p h s of HT (a) p lus tempered a t 500 C, lh

.specimens tested at temperatures oi : -

(c) -H5°c and {(-•) -"iB?

F i g . 3&: SEK f r a c t o g r a p h s of HT la) p lus tempecud a t 200"c , l h .

Specimens t i s t e d a t t empera tu res o f : -

(a>

and

12 A°C

( e )

la) 52 C ( c ) - 1 1 4 C Id) -Ul

ftgssspaa**

F i g . 3B: SEM f r ac tog raphs of HT (B> p lus tempered a t 500 C, I n ,

specimens t e s t e d a t t empera tures o f : -

( a ) . 51 (b) 124 Z (c ) •109°C

(d) -166°C arid.- (e ) - l 9 2 ° c

3KA1-H I.* STRESS-STRAIN CURVE CF HEAT-TREATED AND TE..**ERED C-Mn STEEL

A i s o l u t i o n - t r e a t e d a t 980 C, 2 h , ! F C , t o 760°C, m a i n t a i n e d a t 760°C 3 h , WQ (B)

• S o l u t i o n - t r e a t e d a t 980 C, 2h , FC t o room t e m p e r a t u r e , Heated a t

•760°C, 3h , WQ (A)

t emper ing a t 200 c , l h , WQ

t emper ing a t 200°C, l h , WQ

+ t emper ing a t 500°c , lh WQ

temper ing a t 500°c , lh WQ

80-

60

• 0 .

20

S t r a i n (%)

ZOO -160 •«20 •80

I Tempera ture ( ° c ) '

~ i -j ' i ' " " d i i ' ^ n l ^

—V O 4 0 SO

EFFECT OF HEAT TREATMENT ON THE ENERGY-TRANSITION-TEMPERATURE

CURVES FOR Mn-C STEEL