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The effect of salt bath cementation on mechanical behaviorof hot-rolled and cold-drawn SAE 8620 and 16MnCr5 steels

_Ilyas Yegen, Metin Usta*

Gebze Institute of Technology, Department of Materials Science and Engineering, No: 101, 41400 Gebze, Turkey

a r t i c l e i n f o

Article history:

Received 4 June 2010

Received in revised form

22 July 2010

Accepted 24 July 2010

Keywords:

Cementation

SAE 8620 steel

16MnCr5 steel

Mechanical behavior

a b s t r a c t

In this study, the effect of salt bath cementation on mechanical behavior of SAE 8620 and 16MnCr5cementation steels, which are widely used in industry, was investigated. The experiments were carried

out with hot rolled and cold rolled specimens. The cementation processes were performed in NaCN salt

bath at 920 C temperature for 1, 2, 3 and 4 h. Abrasive wear tests of specimens were conducted with

Wolfram Carbide (WC) ball for 1 h. After cementation processes, a martensite phase on the surface of

specimens was conrmed by X-ray diffraction analysis. After cementation processes carried out with

different times, a different surface hardness and effective cementation depth values were obtained.

Experimental results showed that an effective cementation depth increased with increasing the

cementation time. Wear tests showed that the wear resistance of specimens increased by the cemen-

tation processes. Experimental results revealed that the surface hardness of specimen affects the wear

resistance of specimens.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Steels are most useful engineering materials because of theirhigh mechanical properties, machining, rolling, forging capabilityand lower cost than the alternative materials. Steels are widely

used for production of machine parts subjected to abrasive wear,automotive industry, construction industry and the other engi-neering areas. Wear is the most common problem for steels used inthese engineering areas. Therefore, production of wear resistance

steels came into prominence[1e5].As a consequence of wear, the shape, dimension and the surface

structure of machine parts changes. The result of these changes themachine parts are not be able to do their functions. The materials

lost, exchanging the worn parts, the expensing time and the cost for

maintenance- repair of machines, human resource etc. cause a bigeconomical lost for world industry[1e5].

Machine components such as shaft, gears and cams often require

a very hard surface that can resist wear and a soft, tough core thatcan withstand the impact stress that occurs during operation. Anestablished method for production of such a combination of hardcase and soft, tough core is case hardening of steels through carbu-

rizing and quenching called cementation. This procedure involves

rst the addition of carbon to the surface of a low-carbon steel (%

C< 0.25) to produce a composite consisting of high carbon steel caseand low-carbon steel core. During subsequent quenching the highcarbon austenitic surface layer transforms to martensite. Themicrostructure of the core region is determined by the carbon

content and by the base hardenability of the steel. If steel has lowhardenability, the core may transform to ferrite and small amount ofpearlite, depending on the quenching rate. If the steel has high

hardenability, the core may transform to martensite [1e4].Cementation is a remarkable method of enhancing the surface

properties of shafts, gears, high stressed machine parts to obtainvery high surface hardness, fatigue resistance and wear resistance.

Tempered martensite is the dominant microstructure constituentof properly carburized steel. However, the martensite changes in

morphology, amount, and properties as a function of distance fromthe surface. Other microstructural constituents may also be present

and signicantly affect the performance of carburized parts. Theseother microstructural components include retained austenite,massive carbides, prior austenite, grain boundaries carbides,

phosphorus segregation and surface oxides. Core microstructuresdepending on hardenability may consist of tempered martensite,bainite, or ferrite and pearlite. The case and core microstructuresaffect residual stresses levels occurred in the microstructures,

fatigue resistance, rolling-contact fatigue, hardness and wearresistance of steels. After carburizing processes, the carbon contentthat is normally maximum level at the surface decreases with

* Corresponding author. Tel.: 90 262 605 17 82; fax: 90 262 6538490.

E-mail address:[email protected](M. Usta).

Contents lists available at ScienceDirect

Vacuum

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c om / l o c a t e / v a c u u m

0042-207X/$e see front matter 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.vacuum.2010.07.013

Vacuum 85 (2010) 390e396
mailto:[email protected]://www.sciencedirect.com/science/journal/0042207Xhttp://www.elsevier.com/locate/vacuumhttp://dx.doi.org/10.1016/j.vacuum.2010.07.013http://dx.doi.org/10.1016/j.vacuum.2010.07.013http://dx.doi.org/10.1016/j.vacuum.2010.07.013http://dx.doi.org/10.1016/j.vacuum.2010.07.013http://dx.doi.org/10.1016/j.vacuum.2010.07.013http://dx.doi.org/10.1016/j.vacuum.2010.07.013http://www.elsevier.com/locate/vacuumhttp://www.sciencedirect.com/science/journal/0042207Xmailto:[email protected]

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distance into the specimen. Hardness level in the microstructurealso changes with carbon contents of the specimens. The effectivecase depth is inuenced by carburizing time, temperature, carbon

content of carburizing atmospheres, chemical composition of steel,product shape and size, and the rate of cooling [1e4].

Case depth of carburized steel is a function of carburizing timeand the available carbon potential at the surface. When prolonged

carburizing times are used for deep case depths, a high carbonpotential produces a high surface-carbon content, which may thusresult in excessive retained austenite or free carbides. These twomicrostructural elements both have adverse effects on the distri-

bution of residual stress in the case-hardened part. Consequently,a high carbon potential may be suitable for short carburizing timesbut not for prolonged carburizing[1e8].

Carburizing steels for case hardening usually have base-carbon

contents of about 0.2%, with the carbon content of the carburizedlayer generally being controlled at between 0.8 and 1% C. However,surface carbon is often limited to 0.8% because too high a carboncontent can result in retained austenite and brittle martensite.

Retained austenite reduces the hardness of the carburized surface[1e8].

The aimof this study is to investigate the mechanical behavior ofhot rolled and cold-drawn SAE 8620 and 16MnCr5 cementationsteels exposed to carburizing processes. SAE 8620 and 16MnCr5

cementation steels have high mechanical and hardenability prop-erties and the lower cost than the other cementation steels.Therefore, SAE 8620 and 16MnCr5 cementation steels are widely

used in industry. Although these steels are widely used, there is notenough study that compares the carburizing properties and wearresistances of both steels in the same study. Most of the studieswere carried out with SAE 8620 steel, and most of those studies

widely focused on fatigue performance, retained austenite andresidual stresses affected by carburizing practices [1e16]. There-fore, this study is important to compare the carburizing and wear

properties of SAE 8620 and 16MnCr5 steels at the same conditions.

2. Experimental studies

The experiments were carried out with hotrolled andcold rolled

specimens. Hot-rolled 22 mm. diameterspecimens were colddrawnto the 20 mm. diameter. Afterthe cold-drawnprocess for twoquality

Table 1

The chemical composition of the SAE 8620 and 16MnCr5 test specimens (written in bold) and the standard chemical analysis of SAE 8620 and 16MnCr5 cementation steels.

Chemical Analysis (wt.%)

C Si Mn P S Cr Mo Ni

DIN 16MnCr5 0.14e0.19 0e0.40 1.00e1.30 0e0.035 0e0.035 0.80e1.10 e e

16MnCr5 Test Specimen 0.16 0.26 1.13 0.01 0.009 0.95 0.03 0.12

SAE 8620 0.17e0.23 0e0.40 0.65e0.95 0e0.035 0e0.035 0.40e0.70 0.15e0.25 0.40e0.70

8620 Test Specimen 0.19 0.25 0.85 0.01 0.004 0.54 0.19 0.46

Fig. 1. X-ray analysis of test specimens a) Hot-rolled SAE 8620 specimens b) Cold-drawn SAE 8620 specimens c) Hot-rolled 16MnCr5 specimes d) Cold-drawn 16MnCr5 specimens.

_I. Yegen, M. Usta / Vacuum 85 (2010) 390e396 391

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steels (SAE 8620 and 16MnCr5), the hot rolled (22 mm. diameter)and the cold-drawn (20 mm. diameter) specimens were obtained.

The chemical compositions of the SAE 8620 and 16MnCr5 testspecimens and the standard chemical analysis of SAE 8620 and16MnCr5 cementation steels are given inTable 1.

The cementation processes were carried out in NaCN salt bath

called Durferrit at 920 C temperature for 1, 2, 3 and 4 h. Duringcarburizing process, the carbon potential of salt bath was % 1 C.Specimens were processed to the direct quenched from carburizingtemperature. The specimens quenched into salt bath named AS 160

were tempered 180 C for 2 h.X-ray diffraction analyses were performed for determination of

the phases on the surfaces after the cementation processes. Theanalysis with Cu Ka radiation source has 1541 Angstron wavelength

and the scanning angles (2q) ranged 2qfrom 20 to 100.The microhardness values were measured by the Vickers

hardness method with the load of 20 g.Wear experiments were conducted with wolfram carbide (WC)

ball for 1 h with the load of 10 N. Prior to the experiment, thesamples were cleaned with alcohol and the mass of the sampleswas measured gravimetrically with 104 mg sensitivity. Then, the

wear rates of the specimens were calculated according to the

equation below.

Wa G

d$M$S

mm3=Nm

(1)

Where Wa: wear rate (mm3/Nm),G: weight lost (mg),M: load (N),

S: wear distance (m), d density (g/cm3)

3. Results and discussions

3.1. X-ray diffraction analysis

The XRD analysis of carburized and non-carburized specimensare given inFig. 1. InFig. 1,the specimens that were not carburized

named as Base Specimen, and the carburized specimens weremarked by 1 h, 2 h, 3 h, 4 h according to carburizing times.

The result of X-ray analysis showed that only iron phase wasdetermined on the non-carburized specimen surfaces as expected.Martensite phase was determined on the carburized specimen

surfaces as expected. Determination of martensite phase oncarburized surfaces proved that the cementation processes wereeffectively carried out. In addition to the martensite phase austenitephase (retained austenite), oxide phases and iron carbide phase

were determined on the carburized specimen surfaces.Increasing the carboncontent of the austenite also depresses the

martensite start (Ms) temperature and the martensite nish (Mf)temperature, which leads the difculties in converting whole

austenite to martensite[2e4,6,7].

In steels, austenite is stable at temperatures above the A3 andAcm phase lines. On cooling from such temperatures, it becomesunstable and decomposes to some new constituent such as ferrite,pearlite, or bainite depending on the chemical composition of the

steel and the rate of cooling. These resulting products are referredas high temperature transformation products. In low-carbon steels,these transformations take place at temperatures between the A 3and about 400 C. Martensite is formed by quenching at a rate

above cooling rate. If Mf temperature is lowered below thequenching temperature, austenite does not transform tomartensite. Steelwhich contains above % 0.8 C must be quenchedtosubzero temperatures to form all martensite microstructure

[2e4,6e16].Increasing the carbon content on the carburized specimen

surfaces leads to carbon atoms combined with iron to form iron T

able

2

Surfacehardness,effectivecementationdepthsa

ndwearrateofcarburizedandnon-carburizeds

pecimens.

Carburizing

condition

Surfacehardness(HV)

CoreHardnes(HV)

EffectiveCaseDepth(mm)

WearRate(mm3/Nm)

1hour2hours3hours4hoursBaseSpecimen1hour2hours3hours4ho

ursBaseSpecimen1hour2hours3hours4

hoursBaseSpecimen1hour2hours3hours

4hoursBaseSpecimen

HotRolled

SAE8620

808

848

793

697

250

442

445

442

446

240

0.8

1

1.1

1.6

e

6.4

5.8

7

7.7

16.8

ColdDrawn

SAE8620

804

787

813

833

290

435

446

442

446

250

0.8

0.9

1

1.4

e

6.4

7.2

6.3

6

14.8

HotRolled

16MnCr5

639

830

767

773

268

456

461

458

455

265

0.9

1

1.1

1.3

e

7.9

6

6.7

6.8

16.2

ColdDrawn

16MnCr5

800

845

813

822

310

463

460

460

462

270

0.8

0.9

1.1

1.2

e

6.5

5.6

6.4

6.2

14.2

_I. Yegen, M. Usta / Vacuum 85 (2010) 390e396392

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carbide. When carbon content on surfaces increases, more carbonatoms combine with iron atoms and form iron carbide phases[2e4].

In the oxidation process, oxygen atoms, released by thegasemetal reactions that take place during carburizing, are adsor-bed onto the metallic surface. From there, the oxygen atoms diffuseinward along grain and subgrain boundaries and into the lattice.

They can chemically combine with available substitutionalelements that have high oxidation potential and form oxides. Incommercial case hardening steels, the depths at which the oxides

are detected by conventional optical microscopy are typically less

than 25mm. Deeper cases will produce deeper penetrating oxides.When oxidation takes place, the amounts of alloying elements arereduced due to the chemically combining of oxygen to the alloying

elements such as Mn, Cr providing hardenability of steels. Thisreducing causes the non-martensitic transformation in steels.A consequence of forming oxides phases or increasing the amountof oxide phases leads to non-martensitic transformation, resulting

in reducing the amount of martensite and increasing the amount ofretained austenite[17,18].

The chemical compositions of test specimens given Table 1havealloying elements that have high oxidation potential such as Mn, Cr,

Si, Ni. Due to the chemical compositions of specimens, oxidesphaseswere determined on allof the carburized specimen surfaces.

X-ray analysis inFig. 1showed that the specimens that have the

highest intensity of oxide phases peaks, have the highest intensity

of austenite phase peaks and the lowest intensity of martensitephase peaks. X-ray analysis revealed that the oxides phases lead tonon-martensitic transformation by reducing the amount of alloying

elements providing hardenability.In hot-rolled carburized SAE 8620 specimens, the specimen

carburized for 4 h has the highest intensity of oxides phase peaksand has the highest intensity of austenite phase peaks. The highest

intensity of oxides phases and austenite phase caused the lowestintensity of martensite phase in hot-rolled carburized SAE 8620specimens. The specimen carburized for 2 h has the lowest inten-

sity of oxides phase peaks and has the lowest intensity of austenite

phase peaks. The lowest intensity of oxides phases and austenitephase caused the highest intensity of martensite phase in hot-rol-led carburized SAE 8620 specimens.

In cold-drawn carburized SAE 8620 specimens, the specimencarburized for 2 h has the highest intensity of oxides phase peaksand has the highest intensity of austenite phase peaks. The highestintensity of oxides phases and austenite phase caused the lowest

intensity of martensite phase peaks in cold-drawn carburized SAE8620 specimens. The specimen carburized for 4 h has the lowestintensity of oxides phases and has the lowest intensity of austenitephase. The lowest intensity of oxides phases and austenite phase

caused the highest intensity of martensite phase in cold-drawncarburized SAE 8620 specimens.

These results are valid for 16MnCr5 hot-rolled and cold-drawn

carburized specimens. In hot-rolled carburized 16MnCr5

Hot Rollled SAE 8620

0

100

200

300

400

500

600

700

800

900

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2

Distance from surface (mm)

)VH(

ssendraH

Base Specimen

1 hour

2 hours

3 hours

4 hours

Cold Drawn SAE 8620

0

100

200

300

400

500

600

700

800

900

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2


)VH(ssendraH

Base Specimen

1 hour

2 hours

3 hours

4 hours

Hot Rolled 16MnCr5

0

100

200

300

400

500

600

700

800

900

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2


)VH(ssendraH

Base Specimen

1 hour

2 hours

3 hours

4 hours

a b

dc Cold Drawn 16MnCr5

0

100

200

300

400

500

600

700

800

900

1000

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2


)VH(ssendraH

Base Specimen

1 hour

2 hours

3 hours

4 hours

Fig. 2. The hardness proles and the effective cementation depths of specimens a) Hot-rolled SAE 8620 Specimens b) Cold-drawn SAE 8620 specimens c) Hot-rolled 16MnCr5specimens d) Cold-drawn 16MnCr5 specimens.


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specimens, the specimen carburized for 2 h has the lowest intensityof austenite phase and the highest intensity of martensite phase. In

cold-drawn 16MnCr5 specimens, the specimen carburized for 2 hhas the lowest intensity of austenite phase and the highest inten-

sity of martensite phase.In comparison of the cold-drawn with the hot-rolled carburized

specimens, the cold-drawn specimens have the higher intensity of

martensite phase rather than hot-rolled specimens, because oftheir different diameters leading to probable different cooling rate.

The chemical composition of 16MnCr5 specimens have higheramount of Mn, Cr alloying elements, forming oxide phases, than

SAE 8620 specimens. Therefore, highest intensity of oxide phaseswas obtained in 16MnCr5 specimens.

In addition to the chemical composition of 16MnCr5 specimens,SAE 8620 specimens have Mo and Ni alloying elements that provide

hardenabilityof steels. Therefore, the highest intensityof martensite

phases was obtained in SAE 8620 specimens.

3.2. The hardness and the effective cementation depth results

The hardness proles and the effective cementation depths aregiven inFig. 2. The SEM pictures show the microhardness values of

hot-rolled carburized SAE 8620 and 16MnCr5 for 2 h given inFigs. 3 and 4. The SEM photos were taken for each hardness valuesfrom surface to the core. After that, each SEM photos was combinedand Figures 3, 4 were obtained. Surface hardness, effective

cementation depths and wear rate of carburized and non-carbu-rized specimens given inTable 2.

The hardness tests showed that the hardnesses of carburizedspecimens decreased from the surface to the core depending on the

carbon pro

le that reaches maximum content at surface and

decreases in the core. The core hardness of carburized specimensincreased by the quench process performed after the carburizing.The chemical composition of carburized specimens consists of Mn,

Cr, Ni, Mo alloying elements that provides the hardenability.Therefore, the specimens couldbe hardened by the quench process.

Effectivecementation depths increased dependingon the carbondiffusionthat increased withtime. While carburizing timeincreases,the carbon atoms can diffuse deeper distances from the surface. As

a consequence of increasing carbon diffusion, the effective cemen-tation depth increases as well.

The hardness test showed that surface hardnesses of carburizedspecimens varied depending on the intensity of phases obtained by

X-ray diffraction analysis. In all of the steel quality groups (SAE8620-16MnCr5 hot rolled-cold drawn) the specimens that have thehighest intensity of martensite phase peaks have the highestsurface hardnesses. The specimens that have the highest intensity

of oxide phases peaks, causing the highest intensity of austenite

phase peaks, have the lowest surface hardnesses. When oxidationoccurs, the amounts of alloying elements are reduced due to thechemically combining of oxygen to the alloying elements such as

Mn, Cr that providing the hardenability of steels. This reducingcauses the non-martensitic transformation in steels. This results indecreasing the surface hardness by reducing the amount of

martensite and increasing the amount of retained austenite.In cold-drawn materials, the degree of the plastic deformation

on the surfaces is greater than the one on the core. Therefore, thedislocation density of surfaces is greater than the cores in cold-

drawn materials. In addition, the dislocation density is greater incold-drawn materials than in the hot-rolled materials. Thus, thesurface hardnesses of cold-drawn materials are greater than thecore hardnesses and also hot-rolled materials. As shown in Table 2,

the surface hardnesses of cold-drawn non-carburized specimens

Fig. 4. The SEM micrograph shows the microhardnesses of carburized 16MnCr5

specimen carburized for 2 h.Fig. 3. The SEM micrograph shows the microhardnesses of carburized SAE 8620

specimen carburized for 2 h.


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are greater than core hardnesses, and hot-rolled non-carburizedspecimens by the greater dislocation density forming on thesurfaces.

It can be expected that the surface hardnesses of cold-drawncarburized specimens are greater than the hot-rolled carburizedspecimens. However, the hardness tests showed that there were nodistinct differences between the cold-drawn and hot-rolled speci-

mens. This results from the carburizing process carried out at 920Cin austenite phase zone which is higher than the recrystallizationtemperature. The carburizing process carried out above the recrys-

tallization temperature softened the cold-drawn specimens by

eliminating the dislocation density and rearranging the dislocationarray. Therefore, there is no distinct difference determined on thesurface hardnesses of the cold-drawn and hot-rolled specimens.

The chemical composition of 16MnCr5 specimens have moreMn, Cr alloying elements, providing solid solution hardening.Therefore, 16MnCr5 specimens are harder than SAE 8620 speci-mens due the solid solution hardening.

3.3. Abrasive wear tests

Hardness is the most important factor that inuences wearresistance of materials. As a result of increasing hardness, thewear resistance of materials increases. It is concluded that thewear resistance of low-carbon steel is increased by the cemen-

tation process[5,19,20].

The wear rates of carburized and non-carburized test specimensare given inTable 2andFig. 5.

As shown inTable 2, the wear rate of test specimens decreased

after the cementation processes. The abrasive wear test resultsshowed that the wear rate of materials varies with the specimensurface hardnesses. The specimens which have greater surfacehardnesses are worn less than the others. As shown inTable 2,the

highest wear rates or the highest material loses are obtained innon-carburized specimens.

The carburizing process carried out above the recrystallization

temperature softened the cold-drawn specimens by eliminating

the dislocation density and rearranging the dislocation array.Therefore, there is no distinct difference determined on the surfacehardnesses. Thus, there are no distinct wear rate differences

determined between hot rolled and cold-drawn carburized speci-mens. The differences were found in non-carburized specimens. Asa result of increasing the surface hardness by cold-drawn process,cold-drawn non-carburized specimens have lesser wear rate than

the hot-rolled non-carburized specimens.

4. Conclusions

Martensite phase was determined on all of the carburizedspecimen surfacesby X-ray diffractionanalysis. Determinationofmartensite phase on carburized surfaces is the sign of cemen-

tation processes carried out effectively.

Hot Rolled SAE 8620

251 HV

808 HV848 HV

793 HV697 HV

0,0

2,0

4,0

6,0

8,0

10,0

12,0

14,0

16,0

18,0

0 1 2 3 4 5

Carburizing time (hour)

mm(etarraeW

3

01x)mN/

5-

Base Specimen

1 hour

2 hours

3 hours

4 hours

Cold Drawn SAE 8620

290 HV

804 HV

787 HV

813 HV833 HV

0,0

2,0

4,0

6,0

8,0

10,0

12,0

14,0

16,0

0 1 2 3 4 5


mm(etaRraeW

3

01x)mN/

5-

Base Specimen

1 hour

2 hours

3 hours

4 hours

Hot Rolled 16MnCr5

268 HV

639 HV

830 HV767 HV 773 HV

0,0

2,0

4,0

6,0

8,0

10,0

12,0

14,0

16,0

18,0

0 1 2 3 4 5


mm(etaRraeW

3

01x)mN/

5-

Base Specimen

1 hour

2 hours

3 hours

4 hours

Cold Drawn 16MnCr5

310 HV

800 HV

845 HV

813 HV 822 HV

0,0

2,0

4,0

6,0

8,0

10,0

12,0

14,0

16,0

0 1 2 3 4 5


mm(etaRraeW

3

01x)mN/

5-

Base Specimen

1 hour

2 hours

3 hours

4 hours

a b

c d

Fig. 5. The wear rates of test specimens a) Hot-rolled SAE 8620 Specimens b) Cold-drawn SAE 8620 specimens c) Hot-rolled 16MnCr5 specimens d) Cold-drawn 16MnCr5specimens.


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Martensite phase, austenite phase and oxide phases weredetermined on all of the carburized specimen surfaces.

The surface and the core hardnesses of carburized specimenswere increasedby the cementation which consists of carburizingand quenching processes. The hardness test showed that thesurfacehardnessesof carburized specimens varied dependingon

the intensity of phases obtained by X-ray diffraction analysis. Effective cementation depths increased depending on the

carbon diffusion that increased with time as expected.

The abrasive wear test results showed that the wear rate ofspecimens varied depending on the specimensurface hardnesses.

The abrasive wear test results showed that cold-drawn non-carburized specimens have lesser wear rate than the hot-rolled

non-carburized specimens.

Acknowledgments

The authors thank Mr. Adem Sen for running the X-ray diffrac-tometer, Mr. O. Faruk Deniz for helping hardness measurements,Mr. Ahmet Nazm for helping with SEM study at Gebze Institute ofTechnology and the company of Akelik Iron and Steel for their

support during this study.

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