1 INTRODUCTION

Generally, low carbon steels are delivered in as

rolled, annealed or normalized condition and, for a

given chemical composition, their mechanical

characteristics depend on their microstructure. At

room these steels consists of ferrite and pearlite. The

mechanical characteristics of ferrite-pearlite

microstructure are strongly influenced by the ferrite

grain size [1]. A series of methods are applied in the

industry in order to refine the ferrite-pearlite

microstructure: modification of the chemical

composition [2], normalizing [3], plastic

deformation by controlled rolling [4], rapid cooling

[5] , warm working [6,7].

Situated between the cold and hot working,

the warm-working process corresponds to a

temperature range in which, after the plastic

deformation, the material is partially strain hardened

and partially recrystallized [8]. In several works

have been studied the microstructure development

and mechanical behavior during forging [9],

stamping [10], rolling[11,12],

caliber rolling and

drawing [13] at warm temperatures. For industrial

applications, warm working is very attractive

because it offers certain advantages. Thus, compared

to cold working, it requires lower deformation

forces, can be applied to a broader range of steels,

allows for higher deformation ratios, generates a

more uniform deformation across the transversal

section and leads to a less strained microstructure

[14].Compared to hot working, it leads to a finer

microstructure with superior mechanical properties,

better surface quality and better dimensional control,

lower material losses due to decarburization and

oxidation.

Attempts to apply thermochemical treatments

ABSTRACT: Nitriding is usually applied to alloyed steels with the scope of increasing their surface hardness

and wear resistance. Warm working has been found to produce a fine-grained microstructure, which makes

possible further treatment of low carbon steels. In combination with a low temperature thermochemical

treatment, such as nitriding, warm working can be used to produce machine parts with a though core and with

a hard, wear resistant surface layer. This paper presents a study of mechanical and structural properties of

AISI 1015 carbon steel nitrided after warm rolling. The rolling was performed in the following conditions:

temperature 670 – 550 oC, rolling speed 1.39 s

-1 and deformation ratio 36.4%. After rolling, the samples were

reheated to 550 oC for a duration varying from a few minutes to 10 hours. The microstructural changes were

assessed by light microscopy and quantitative microscopy analysis. Warm rolled samples were ion nitrided at

510-520 oC in dissociated ammonia. The microstructure was analyzed by scanning electron microscopy and

the mechanical properties were evaluated by tensile testing, surface hardness and friction coefficient

measurements. Prior application of warm rolling makes possible (in the sense that is a viable solution) the ion

nitriding of low carbon steels in order to produce machine parts with improved mechanical properties in the

core (due to warm rolling) and longer service life (due to ion nitriding).

Key words: carbon steels, warm rolling, nitriding, microstructure, mechanical properties.

Mechanical and structural properties of AISI 1015 carbon steel nitrided

after warm rolling

C. Medrea1, G. Negrea

2

1Department of Physics, Chemistry and Materials Technology, Technological Education Institute of

Pireaus,250 , Thivon &P. Ralli Street, 12244 Aegaleo, Greece URL: www .teipir .gr e-mail: medrea@ internet.gr;

2Faculty of Materials Science and Engineering, Technical University of Cluj-Napoca, Muncii Avenue 103-

105, 3400 Cluj-Napoca URL: www.utcluj.ro e-mail:Gavril.Negrea@ispm.utcluj.ro

b

b/2

y

x

Investigated

surfaceRolling direction

to some carbon steels, previously subjected to warm

working, have resulted in superior results as

compared to those subjected to hot working [15].

However, desirable results can only be achieved if

the heat-treating regime is properly defined. In the

case of warm rolled steel products, the heating

temperature must be limited below the pearlitic

reaction temperature (Ac1) and the soaking time

needs to be established such as to preserve the fine

microstructure produced by warm working. The

present study focuses on nitriding a low carbon steel

after warm rolling and the evaluation of the final

mechanical properties and microstructure.

2 EXPERIMENTAL DETAILS

The AISI 1015 carbon steel bars were warm rolled

in the following conditions: the temperature at the

beginning and at the end of rolling - 670 o

C and

550 oC, respectively, the rolling speed 1.39 s

-1 and

the deformation ratio 36.4%. A number of 12

samples were cut from warm rolled steel and were

heated in an electrical laboratory furnace. Each

sample was heated separately and then cooled in still

air. The heating temperature and soaking time for

each sample are given in table 1.

Table 1. Parameters of the heat treatment applied to AISI 1015

steel after warm rolling.

Sample No. Temperature[oC] Time [min.]

1 1 2 3 3 5 4 10 5 15 6 20 7 30 8 60 9 120

10 180 11 300 12

550

600

Microstructural changes that took place during the

heat treatment were assessed by light microscopy.

Quantitative microscopy analyses were also

performed by using an automated image analyzer

type Epiquand. The analyses were made in two

directions: parallel (x) and perpendicular (y) to the

rolling direction as shown in fig. 1. The field of

investigation had dimensions 4x4 mm. The

quantitative microscopy analysis was based on the

premise that inclusions are uniformly distributed,

have the same geometrical shape and differ only in

size. Based on Cavalieri-Aker principle [16] and by

using statistical analysis of results [17], the

frequency histograms were determined. As a result

of this study it was possible to define the nitriding

regime that can be suitable to warm worked

products.

Fig. 1. Schematic draw of a plastically deformed sample and

location of the specimen cut for microscopic analysis.

.

For determination of mechanical properties,

standardized samples were machined from warm

rolled steel and then subjected to ion nitriding in

dissociated ammonia for 10 h at 510-520 oC by

using a Nitrion 10 type equipment. Tensile

characteristics, surface hardness and dry friction

behavior were evaluated. Friction coefficient was

determined by using a home made ring-on-block

tribometer. The block (10x10x10 mm) was made

from warm rolled steel and the ring from gray cast

iron. The tests were performed under dry friction by

using a constant normal force (Fn=181,85N) and

recording the variation of the friction force as a

function of time. After 3 … 4 sec. the friction force

has stabilized (Ff max). The friction coefficient was

calculated by the relationship:

Fn

Ff max!" (1)

The fracture surface of the samples subjected to

tensile testing was investigated by scanning electron

microscopy (SEM).

3 RESULTS AND DISCUSSIONS

After warm rolling the microstructure of the steel

consists of flattened and partially strain hardened

ferrite grains and very fine pearlite particles

distributed in lines parallel to the rolling direction

(Fig. 2 a). Figs. 2 and 3 show the microstructure and

the distribution curves, respectively, for significant

soaking times. By heating to 550 oC, the

microstructure is completely recrystallized after 10

min and displays well defined grain boundaries (fig.

2 b ). The ferrite has a fine grain size, close to the

Fig. 2. The microstructure of samples warm rolled (a) and

after different soaking times at 550 oC ( b - 10 min , c - 2 h ,

d – 10 h). initial grain size, with uniformly distributed grains

(fig. 3 a, doted line). In the rolling direction the

dispersion is larger and the average grain size values

are shifted to the right. The maximum grain

dimension is situated in the range [45.2 – 90.5] "m.

Immediately after recrystallization, the ferrite is very

fine, with slightly elongated grains in the rolling

direction and uniformly distributed in the

microstructure. The pearlite grain size is little

affected by the soaking time (fig. 3 b doted line).

Increasing of the soaking time to 2 hours does not

affect essentially the ferrite grain size (fig. 2 c). The

microstructure appears fine and homogenous. In the

rolling direction the distribution curve shifts slightly

to the right and the maximum frequency increases

from (8.0 … 11.3) "m to (11.3 … 16.0) "m (fig. 3 a,

continuous line). Large and medium size grains

grow very slightly on the expense of fine grains and

lead to a homogenous microstructure. The pearlite is

distributed in lines in the form a spheroidal

separations (fig. 2 c). The distribution curves shift

slightly to the left and the dispersion degree

decreases (fig. 3 b, continuous line).Increasing of the

soaking time to 10 hours leads only to a slight

increase of the ferrite grain size (fig. 2 d ). It can be

noticed a decrease of the frequency values in the

very fine grain classes. The distribution curves

remain in the same grain size field but the central

parts of the curves shift slightly to the right. The

dispersion degree of the grain size decreases further

(fig. 3 a, dashed line). The pearlite particles have a

spheroidal shape and the line distribution is

preserved (fig.2 d). The dispersion of the distribution

curves decreases significantly indicating a

dimensional leveling of the particles (fig. 3 b,

dashed line). The stability of the microstructure to

heating after warm rolling allows for

thermochemical treatment by nitriding of steel.

Fig. 3.Grain size distribution curves of the ferrite (a ) and

pearlite (b) after heating to 550 oC with different soaking times. (Parallel to the rolling direction)

Table 2 shows the mechanical properties of the ion

nitrided samples after warm rolling.

Table2. Mechanical properties of samples after warm-rolling

and in normalized condition.

AISI1015Yield

strength

[MPa]

Tensile

strength

[MPa]

Elongation

[%]

Reduction

in area

[%] HV5

Friction

coefficient

!

Warm

rolled 359 493 24,7 59 188 0,342

Nitrated

after warm

rolling 398 538 18,6 49,5 304 0,206

The application of nitriding after warm rolling

determines a slight increase of the strength (10% for

yield strength and 20% for tensile strength,

respectively). The surface hardness increases

significantly (62%), while the friction coefficient

decreases much (by 40%). The significant

modifications of the hardness and friction

coefficients lead to the improvement of the wear

resistance of the parts treated in this way and to the

increase of their lifetime.

Figure 4 shows the fracture surfaces of warm rolled

0

5

10

15

181.0

12

8.0

90

.5

64.0

45.2

32

.0

22.6

16

.0

11.3

8.0

5.5

4.0

2.8

2.0

10 min

2 h

10 h

Fre

qu

en

cy, %

Grain size, "m

(a)

0

5

10

15

181.0

128.0

90.5

64.0

45.2

32.0

22.6

16.0

11.3

8.0

5.5

4.0

2.8

2.0

10 min

2 h

10 hF

rqu

en

cy, %

Grain size, "m

(b)

100 "m

100 "m

100 "m

100 "m

and nitrided samples subjected to tensile testing.

Due to nitriding, the fracture surface losses the cone-

and-cup aspect. A clear delimitation appears

between the white layer (8 !m) and the diffusion

zone (0.2 mm). There is a good link between the

diffusion zone and the core (Fig. 4a). In the white

layer, the fracture surface is rather smooth indicating

a brittle fracture due the fine and very hard nitrides

(Fig. 4b). In the diffusion zone, the fracture surface

presents shallow hollows. In the core (Fig. 4c), with

fine hollows which preserves the aspect and

dimensions of those obtained by simple warm

rolling (Fig. 4d). The similarity of the two images

(Fig. 4c and 4b) confirms the stability during

nitriding of the microstructure obtained by warm

rolling.

Fig. 4. SEM micrographs showing the morphology of the

fracture surfaces of nitride after warm-rolling (a,b,c) and warm

rolled (d) samples subjected to tensile testing.

4 CONCLUSIONS

Warm rolling determines the refining of the ferrite-

pearlite microstructure and, implicitly, an

improvement of mechanical properties. This

microstructure is stable to later heating up to 550 oC

with long soaking time. The stability of the

microstructure makes possible the thermal treatment

of the steel in this temperature range.

Nitriding of the steel after warm rolling gives a hard

surface layer which presents a significant

improvement of wear resistance. In the same time,

the core preserves its fine grain microstructure with

improved tensile properties as compared to untreated

pieces. Nitriding of steel after warm rolling leads to

increased wear resistance of parts made from low

carbon steel and implicitly to increasing their

lifetime.

REFERENCES

1. I. Hrisulakis, D. Pandelis, Science and Technology of

Metallic Materials, Papasotiriu Publishing House, Athens

(1996), 427.

2. G. Vermesan, Guide for Heat Treatments, Dacia

Publishing House, Cluj-Napoca (1987), 167 (in

Romanian).

3. T. Dulamita, et. al.: Heat Treating Technology, Editura

Didactica si Pedagogica, Bucharest (1982), 108 (in

Romanian).

4. A.J. DeArdo, C.I. Garcia, E.J. Palmiere, Thermo-

mechanical Processing of Steels," Handbook of Heat Treating , ASM International (1991), 237.

5. A. Canonico, Accelerated cooling for carbon steels for

pressure vessels, Welding Journal, No. 9 (1961) 400.

6. D. B. Santos, R.K. Bruzszek, P.C.M. Rodriguez and

E.V. Pereloma, Formation of Ultra-fine Ferrite

Microstructure in a Warm Rolled and Annealed C-Mn

Steel, Materials Science and Engineering, 346 (2003)

189-195.

7. A. Haldar, R.K. Ray, Microstructural and Textural

development in an extra low carbon steel during warm

rolling, Materials Science and Engineering A, 391 Is.1-

2 (2005) 402-407.

8. D.N. Hawkins, A.A. Shuttleworth,,The effect of warm

Rolling on the Structure and Properties of Low Carbon

Steel,, Journal of Mechanical Working Technology, 2 Is.

4 (1979) 333-345.

9. M. Garaway, I.P. Gobbo, Warm forging, Journal de l’electrothermie, No. 14 ( 1986) 52.

10. R.D.H. Lester, Literature Review on Perspectives in

Forging and Stamping, Metalurgia and Metal Forming,

12 (1972) 12.

11. S. Serajzadeh, Modeling the Warm Rolling of a Low

Carbon Steel, Materials Science and Engineering A, 371

(2004) 318-323.

12. C. Medrea-Bichtas, I. Chicinas, S. Domsa, Study on

Warm Rolling of AISI 1015 Carbon Steel, Zeitschrift für Metallkunde, 93 No. 6 (2002) 554-558.

13. S. Torizuka, E. Muramatsu, S.V.S. Narayana Murty, K.

Nagai ,Microstructure Evolution and Strength-reduction

in Area Balance of Ultrafine-grained Steels Produced by

Warm Caliber Rolling, Scripta Materialia, 55 Is.8

(2006) 751-754.

14. M.C. Chaturredi, Y. Han ,Effect of Cold and Warm

Working on the Microstructure and Mechanical

Properties of Alloy 718 , Materials Science and

Engineering A, 156 (1992) 53-59

15. A.J. DeArdo, C.L. Garcia, E.J. Palmiere, Heat Treating ,

ASM Handbook, 4 (1991) 237.

16. R.T. De Hoff : Quantitative Microscopy, Masson,

(1972)34

17. K. Kostaki, G. Kostaki, H. Kostaki, Statistical

methods for analysis of experimental results, Statistics,

Kostaki Publishing, Athens, (2002), 61-148(in greek).

50 "m

5 "m 5 "m

10 "m