PAPER READY TEMPERING OF STEEL

A STUDY ON HOW TO IMPROVING THE DUCTILITY OF LOCALLYMANUFACTURED STEEL RODS BY TEMPERING

H. A. Martin, O. A. Nunoo, A. B. C. Dadson

Department of Physics, KNUST, Kumasi

ABSTRACT

Heat treatment was conducted to improve the ductility of

locally manufactured mild steel rods (Fe-0.26 %C) obtained

from Ferro-Fabrik, Tema, using tempering. Microstructural

studies, hardness, impact and tensile tests were carried out

on the mild steel rod samples before and after heat treatment.

The samples were heated to 950℃ to austenitize them for 1

hour and rapidly quenched to room temperature in water to

produce martensites. They were then re-heated to the tempering

temperature of 500℃ for 1, 3 and 5 hours. Some of the

martensitic samples were not tempered after quenching. The as-

received samples (non-heat treated samples) had an average

hardness value of 2290.5 (Vickers Hardness 20) HV20. The

martensitic sample had an average hardness value of 5477.0

HV20. The samples tempered for 1 hour had an average hardness

value of 2735.7 HV20. The samples tempered for 3 hours had an

average hardness value of 2470.2 HV20 while the samples

tempered for 5 hours had an average hardness value of 2198.6

HV20. The as-received samples had an average impact value of

109.8 J. The martensitic sample had an average impact energy

value of 40.7 J. The samples tempered for 1 hour had an

average impact value of 114.3 J. The samples tempered for 3

hours had an average impact value of 139.8 J; while the

samples tempered for 5 hours had an average impact value of

141.1 J. From the tensile test the average percentage

elongation of the as-received samples was 30.8 %. The

martensitic sample had an average percentage elongation value

of 17.5 %. Whereas the samples tempered for 1, 3 and 5 hours

had an average percentage elongation of 25 %, 30.0 % and 32.5

% respectively. The as-received and the martensitic samples

had average percentage area reductions of 65.0 % and 42.5 %

respectively. The samples tempered for 1, 3 and 5 hours at 500oC had average percentage area reductions of 57.3 %, 64.0 % and

65.8 % respectively. The microstructure of the tempered

samples revealed the formation of even distribution of ferrite

and pearlite. The above results suggest that the ductility of

locally manufactured mild steel rods may be improved by

tempering.

INTRODUCTION

Mild steel rods are rolls of steel essentially made up of analloy of mild steel and carbon and other alloying elements(Lakhtin, 1977). The carbon content of steel is between 0.05 %and 1.2 %. The other elements may be controlled by impuritiesor other alloying elements used, such as Manganese, Chromium,Vanadium, and Tungsten (Ashby et al., 1992). Carbon and otherelements act as hardening agents, preventing dislocations inthe mild steel atom crystal lattice from sliding past oneanother. Varying the amount of alloy elements and the form oftheir presence in the steel controls the mechanical propertiessuch as the hardness, ductility and toughness.

Plain carbon steel contains carbon as the major alloyingelement. Carbon, a powerful alloying agent can give a varietyof strength and hardness by varying its composition in thesteel. It is in this regard that carbon steel can beclassified as low, medium and high carbon steel. It has beenshown that low carbon steel has carbon content up to 0.29 %;while those with lower and higher amounts are respectivelyclassified as low and medium carbon steel.

Heat treatment (tempering) of the locally manufactured mildsteel rod is done to achieve a desired result such ashardening or softening of the rod. Tempering is done, by firstheating to create a solid solution of mild steel and carbon ina process called Austenitizing. This is followed by rapidlycooling (quenching) in water to the critical temperature(Morrogh, 1948). The critical temperature is dependent on thecarbon content but as a general rule, lowers as the carboncontent increases (Rajan et al., 1988). This results in amartensitic microstructure that possesses super-saturatedcarbon content in a deformed body-centered cubic (BCC)crystalline structure, properly termed body-centeredtetragonal (BCT). This crystalline structure has a very highamount of internal stress (Gulyaev, 1980). Due to internalstress, quenched steel are extremely hard but brittle causingstress cracks on the surface (Higgins, 1995). The steel isthen tempered by heating between the ranges of 150–260 °C and370–650 °C. Tempering in the range of 260–370 °C is sometimesavoided to reduce temper brittling. The steel is then held attemperature 370–650 °C until the carbon trapped in themartensite diffuses to produce a chemical composition with thepotential to create either bainite or pearlite (a crystalstructure formed from a mixture of ferrite and cementite).Therefore, pearlites are formed depending on the duration andtemperature of the tempering process (Hassan, 2011).

However, such investigations have not been conducted on mildsteel rods by local manufacturers. The aim of this work is toinvestigate the process of improving upon the mechanical

properties such as ductility and toughness of the locallymanufactured mild steel rods by heat treatment for differentduration such as 1, 3 and 5 hours. Therefore, studies such asmicrostructures and mechanical properties (e.g. tensilestrength, impact strength and hardness) of the heat treatedlocally manufactured mild steel rods are reported in this workto show how ductile the mild steel could become after the heattreatment.

MATERIALS AND METHODS

Heat Treatment

A steel rod (dia: 19 mm) was obtained from a local shop inKumasi. It was manufactured by a local steel company based inTema. The long rod of mild steel was sectioned into severalsmall cylindrical samples of height 10 mm. These samples wereused for the micro-hardness tests and the microstructuralexaminations. Tensile testing specimens were also from thesteel rods. The short and long samples for the variousstandard tests were loaded into an electric furnace and heatedup to 950 °C for 1 hour then quenched rapidly in water to roomtemperature. These quenched specimens were grouped into four.One group remained as quenched samples (martensitic) and theremaining three were tempered for various times i.e. 1, 3 and5 hours at a temperature of 500 °C and cooled in air.

Grinding and Polishing of Mild Steel Rods

The cylindrical samples were prepared for micro-hardness testsand microstructural examinations by grinding the flat surfaceson different grades of silicon carbide paper starting fromgrade P240, P400, P600, and finally to P1000, with water being

used as a lubricant and extractor of heat resulting fromfriction between the sample and the grinding paper (Williams,2009). After the grinding, polishing was carried out on arotating disc covered with polishing cloth infused withAlumina suspension (0.1μ Al2O3 powder). The mild steel rodswere held against the rotating platen at a set speed of 430rpm until a mirror like finish was obtained on the surface.Then it was etched with 2 % of nital solution for 10s. Afterthe etching, the samples were washed in running water andalcohol and then dried in cold air with a blower. The etchedsamples were then placed on the sample stage of an opticalmicroscope for the microstructural work and image recording.

Chemical Compositional Analysis

Chemical composition analyses of the steel rods were performedon three samples at the Western Steel and Forging CompanyLimited, Tema. This was done using the optical emissionspectrometry (OES) to know the major chemical contents in themild steel.

Hardness Tests

The hardness values of the samples were determined using adigital Vickers Diamond Hardness (HV) testing machine. Thesamples were mounted in epoxy resin for effective handling andproper flatness. The surfaces were thoroughly polished beforesamples were tested. The load selector disc was turned to 20kgf and the diamond indenter was push into the sample for 1 to10 seconds. The two diagonals of the indentation left on thesurface of the material after removal of the load weremeasured using the incorporated microscope. The hardnessvalues were recorded in HV20.

Impact Tests

Samples of the mild steel were shaped into 75x10x10 mm usingthe milling machine while a 2 mm deep notch with an angle of45° created 25 mm away from one end. The sample was placed inthe anvil of an Izod impact test machine and the pendulum axeraised to a standard height then swung at the notch samplewhich fractured the sample, and the energy absorbed by thesample during the deformation was measured.

Tensile Tests

The tensile tests on the samples were conducted using an AveryTensile Testing machine. The test specimens were milled to theBritish Standard round test piece. From the tests, the modulusof elasticity, the ultimate tensile strength, percentageelongation and the percentage area reduction were determined.

RESULTS AND DISCUSSION

The results comprise chemical composition of the locallymanufactured mild steel samples, the mechanical properties,i.e. hardness, modulus of elasticity, tensile strength andpercent elongation obtained from samples which were heat-treated (tempered for different times) and those which werenot heat-treated as shown below.

Table 1: Mechanical properties of the mild steel sample.

SamplesAverage

Hardness /HV20

AverageModulus of

Elasticity /kN/mm2

AverageTensile

Strength /kN/mm2

As-received 2290.5 13.93 4.18

Martensitic 5477.0 45.87 8.03

Tempered for 1hour

2735.7 21.21 5.30

Tempered for 3hours

2470.2 15.43 4.63

Tempered for 5hours

2198.6 15.75 5.12

Chemical Composition

The purpose of analyzing the chemical composition of the mildsteel samples was to know the major chemical contents in themild steel. Table 1 show that the steel could be classified aslow carbon steel, since both AISI and SAE classified steelwith carbon content up to 0.29 % as low carbon steel. TheManganese content of 0.60-0.9 % and maximum Sulphur content of0.05 % justifies the steel to be a mild carbon steel(Lindberg, 1977).

Table 2: Approved elemental composition of low carbon steelobtained from Ghana Standard Board (GS 788-2:2008, Buildingand construction materials – steel for reinforcement ofconcrete – part 2: Ribbed bars)

Elements PercentagesDeviation (maximum allowed variation) percentages

C >0.25 +0.03

Si ≤0.60 +0.05

Mn ≤1.65 +0.06

P ≤0.05 +0.008

S >0.05 +0.010

Table 3: Major chemical composition of the mild steel sample investigated at Western Steel and Forging Company Limited at Tema using optical emission spectrometry (OES).

Elements Percentages

C 0.256

Si 0.272

Mn 0.837

P 0.03

S 0.059

Cu 0.222

From Table 3, the carbon content in the Ferro Fabrik sampleswas low, and so were the Phosphorus and Sulphur contents.Phosphorous and Sulphur are unwanted elements in steel becausethey eventually form phosphates and sulphides which cause thematerial to be hard and brittle; hence, the ability to bear ahigher load before fracture but poor in ductility. Manganeseplays a major function in steel because it improves thetensile properties such as ductility and toughness. The as-received samples showed a very low Manganese content inagreement with the range provided by Ghana Standard Board (seeTable 2). (GS 788-2-2008) - This explained why the localsamples (LS) were more brittle and harder than the importedsteel (IS) rods as shown in Table 4.

Table 4: Elemental composition of low carbon steel.

Elements C% Mn% Si% P% S%IS 1 0.20 0.78 0.03 0.021 0.019

IS 2 0.19 0.76 0.03 0.025 0.033LS 1 0.21 0.50 0.15 0.062 0.040LS 2 0.15 0.33 0.28 0.076 0.045

Microstructural Analysis

Fig1. Microstructure of as-received mild steel sample. The microstructure consists of ferrite (grey) and pearlite (dark) Mag: X500.

Fig 2. Microstructure of the quenched sample.

Fig. 3. Microstructure of the mild steel tempered at 500 oC for 1 hr. The microstructure consists of ferrite (grey) and pearlite(dark). X500.

Fig. 4. Microstructure of the sample tempered for 3 hours. Themicrostructure consists of ferrite (grey) and pearlite (dark) X500.

Fig.5. Microstructure of sample tempered for 5 hours. The microstructure consists of ferrite (grey) and pearlite (dark) X500.

The microstructure of the as-received mild steels revealed thepresence of pearlite and ferrite structures (Fig.1). Themicrostructure of the quenched samples in water showed highproportion of martensites with retained austenite of a needlelike cementite (Fig.2). The mild steel samples tempered for 1,3 and 5 hours revealed a microstructure consisting of ferrite(grey) and pearlite (dark). All the microstructurescorresponded to those of improved ductility in mild steels(Higgins, 1995).

0

40

80

120

160

Impact Energy /J

Fig.6. A bar chart showing the variation of Impact Energy withquenching and tempered times.

012002400360048006000

HARDNESS/(HV20)

Fig.7 A bar chart showing the variation of Hardness (HV20) with quenching and tempered times.

From the above bar charts, the as-received samples had anaverage hardness value of 2290.5 (HV20). The martensiticsample had an average hardness value of 5477.0 (HV20). Thesamples tempered for 1, 3 and 5 hours had an average hardnessvalues of 2735.7, 2470.2 and 2198.6 (HV20) respectively,showing a reduction of hardness with tempered period.

The as-received samples had an average impact value of 109.8J. The martensitic sample had an average impact energy valueof 40.7 J. The samples tempered for 1, 3 and 5 hours hadaverage impact energy values of 114.3, 139.8 and141.1 Jrespectively. The results indicated that increased temperedperiod caused an increased absorbed energy on impact due toimproved ductile phases introduced by tempering. The As-received sample had the lowest hardness values as compared tothose samples quenched in water and tempered. This may beattributed to the fact that oxidation of the carbon elementdepended on tempering (reheating) and time spent duringheating. The impact energy for the as-received mild steel was

generally lower than those of the tempered samples (Fig. 6).The samples tempered for 5 hours gave the highest impactstrength value and the sample quenched in water (martensite)gave the least impact strength. The increase in the impactenergy value and increase in hardness value were in conformitywith the standards of heat treatment research (Kempster,1976). The high hardness values and impact strength obtainedcould be attributed to the various microstructures obtained(see Micrographs in Fig 1.1- 1.5).

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

100020003000400050006000700080009000

As-received

Martensite

Tempered for 1hr

Tempered for 3hrs

Tempered for 5hrs

Strain

Stre

ss (

N/mm

²)

Fig. 8 A Graph of Stress against Stain for samples quenched and tempered for different times.

0

10

20

30

40

50

60

70

Percentage Elongation

Percentage area reduction

Perc

enta

ge

Elon

gati

on

(%)

Fig.9 Variation in Percentage Elongation and Percentage AreaReduction for quenched sample and samples tempered fordifferent times.

From Fig. 8, the yield strength and fracture strength for theas-received mild steel samples decreased while those temperedat different times increased. Toughness and ductility can befound by the area under the stress-strain graph up to thefailure point. This can easily be calculated by numericalintegration but as shown in the figure, it was clear that thesamples tempered for different times had larger areas underthe stress-strain graph than the as-received sample. Thereforesamples tempered for 5 hours showed the highest yield pointvalues as compared to the as-received sample.

The percentage elongation and percentage area reduction showedhow far the mild steel rods were elongated. Often ductility ischaracterized by the material's ability to be stretched. FromFig 9, the average percentage elongation of the as-receivedsamples was 30.8 %. The martensitic sample had an averagepercentage elongation value of 17.5 %. The samples tempered

for 1, 3 and 5 hours had an average percentage elongation of25.0 %, 30.0 % and 32.5 % respectively.

The as-received samples had an average percentage areareduction value of 65.0 %. The martensitic sample had anaverage percentage area reduction value of 42.5 %. The samplestempered for 1, 3 and 5 hours had average percentage areareduction values of 57.3 %, 64.0 % and 65.8 % respectively.The general increase in the ductility of the mild steel rodswith tempering was as a result of the rearrangement of thelaminated ferritic and pearlitic structures in the temperedsamples. This was as expected because the tempered structurehad fine carbide dispersion in the ferrite matrix (Degarmo etal 2003).

CONCLUSIONS

The general results obtained from the study showed that theductility of locally manufactured mild steel rods can beimproved by heat treatment (tempering) since the tempered mildsteel rods were found exhibiting higher level of ductility andtoughness. The locally manufactured mild steel rods used fromFerro Fabrik were found to be of low carbon steel as while asthe tempering processes depended largely on tempering time andtemperature to achieve ductility and toughness in a metal. Thesamples tempered for 5 hours were tougher and more ductilethan the as-received mild steel rods. The amount of manganesein the chemical composition must be increased. Localmanufacturers should incorporate tempering in theirmanufacturing processes to improve ductility.

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