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MICROSTRUCTURAL CHARACTERIZATION OF MEDIUM CARBON DUAL PHASE STEELS

AFTER INTERMEDIATE QUENCHING

Ersoy ERİŞİR1, Serap GÜMÜŞ1, Oğuz Gürkan BİLİR1

1Department of Metallurgical and Materials Engineering, Kocaeli University, 41380-Kocaeli-Turkey

Abstract

Multi-phase microstructure gain importance increasingly in new generation steels. Dual phase (DP) steels

can be produced by the quenching from a temperature in intercritical temperatures in the phase region of

austenite and ferrite. The final microstructure is composed of martensite and ferrite. Microstructure before

the annealing at intercritical temperature can include martensite, ferrite, pearlite or austenite. The initial

microstructure affects the final morphology strongly and the annealing temperature affects the amount of

ferrite and martensite phases. Thus, the mechanical properties of dual phase steel depend on the

morphology and phase distribution. In the intermediate quenching method, the initial microstructure is fully

martensitic and final microstructure is fine and randomly scattered at former martensite boundaries in a

ferritic matrix. This type of microstructure is called fibrous microstructure. There are some difficulties in

selecting the correct etching solutions to obtain a sharply different contrast for the microstructure like this. On

this study, hot rolled medium carbon steel was employed and intermediate quenching method was applied.

Dual phase steel showed randomly distributed martensite laths in ferritic matrix in the microstructure. On the

other hand, not only martensite and ferrite but also bainite, and pearlite were observed. The work presented

here demonstrates the effect of the different etchings on the microstructure. Nital and Le Pera solutions were

applied in order to overcome the difficulties in distinguishing the different phases. Light microscope and

scanning electron microscope were carried out observing the microstructures. The Le Pera etchant was

more effective to distinguish the martensite and ferrite. On the other hand, distinguishing the bainite, retained

austenite and pearlite from martensite, same etchant was not favorable.

Keywords: microstructural characterization, dual phase steel, medium carbon steel, intermediate quenching, phase transformation

1. INTRODUCTION

Dual phase microstructures have enormous advantages over the conventional steels with respect to good

combination of high strength, good machinability and high toughness [1, 2]. Dual phase microstructures

consist of ferrite and martensite phases as a ferrite matrix together with distributed martensite phase

commonly up to %15 volume fractions. The main application area of dual phase steels is in general limited to

the hot rolled sheets for automotive industry. Thus, many research papers are related to the low carbon

steels containing relative small amounts of martensite. In the automotive industry, carbon amount is

restricted by the machinability and weldability properties which inversely proportional to the carbon content

and amount of martensite [1-15]. However, recent studies about dual phase steels with increased amount of

martensite gained importance to strength and wear resistance. Medium carbon dual phase steels can be

used for applications in mineral and mining processes which do not require any welding operation [16, 17].

Even the obtaining a dual phase microstructure from medium carbon steels is an important challenge, there

are some difficulties. Calculating volume fractions of different phases is not as easy as low carbon steels and

also it made increase possibility of forming retained austenite, pearlite and bainite. It may be sometimes

difficult separating this phases. Therefore, because of the expectations about the differences between

microstructures of low carbon and medium carbon steels, etching solutions may differ with respect to

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characterizing the different microstructures. In this study, different etching solutions applied to medium

carbon hot rolled steel to analyze the microstructure of medium carbon dual phase steels.

2. MATERIAL AND EXPERIMENTAL

Chemical composition of investigated steel is given in Table 1. The intermediate quenching experiments

were carried out in a Bähr DIL805 plastodilatometer in order to investigate the phase transformations in the

investigated steel. The standard specimen size of 5 mm diameter and 10 mm length was used for the

dilatometer experiments. The samples were annealed at 1100 ºC for 5 min and followed by gas quenching

with a cooling rate of 130 K/s to obtain fully martensitic initial microstructure. After martensitic

transformation, specimens were annealed respectively at 725, 730, 740 and 750°C for 15 min and finally gas

quenched again. The samples were metallographically prepared. After sample preparation, samples were

finally etched with Nital and Le Pera. Nital etchant contains 3% nitric acid in ethyl alcohol. Le Pera etchant is

prepared as a mixture of 1% metabisulfide in distilled water and 4% picric acid in ethyl alcohol. Light

microscope and scanning electron microscope investigations were carried out.

Table 1 Chemical composition of steel.

3. RESULTS

Dual phase microstructure of annealed sample is given in Fig. 1. The samples for SEM are Nital etched in

order to etching ferrite, pearlite and bainite while it leaves martensite unetched state. In Fig. 1, SEM

micrograph reveals the martensite islands in a ferritic matrix as well as a third phase (mixture of

bainite/pearlite) at ferrite-martensite phase boundaries. It was very hard to identify the nano-sized F/P with

light microscope as dark areas in Fig. 2. However, martensite islands (brown) and ferrite (white) can be

distinguished using light microscope.

Fig. 1 SEM micrograph of steel annealed at 730°C and intermediate quenched, M: martensite, F: ferrite, B/P:

bainite/pearlite.

Chemical composition (%weight)

C Si Mn P S Cr Mo Ni V W Cu

0,368 0,279 0,865 0,0195 0,0063 0,0413 0,0168 0,0769 0,0021 0,0310 0,0597

Al Co Nb Ti B Zr As Pb Sn Ca

0,0472 0,0076 0,0033 0,0034 0,0012 0,0025 0,0133 0,0050 0,0055 0,0031

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Fig. 2 LM micrograph of steel annealed at 730°C and intermediate quenched, M: martensite, F: ferrite, B/P:

bainite/perlite.

3.1 Le Pera Etching

Le Pera etchant allow the distinction of ferrite (brown/blue), bainite (dark brown/black), martensite and

retained austenite (white). Fig. 3 shows the microstructures of steels annealed at 730 and 740°C after

etching with Le Pera. The third phase (dark brown/black) can be noticed in the microstructure which can be

pearlite and/or bainite. There are two possibilities that can explain the formation of pearlite/bainite. First

thought may be that pearlite/ bainite is formed before decomposition of austenite and ferrite to dual phase

microstructure. Second is the pearlite/bainite is formed during quenching from annealing temperature.

According to first thought, one may think that pearlite/bainite is already existed in martensitic initial structure

or it was formed as a result of annealing the initial martensite structure.

Fig. 3 LaPera etched microstructure of steel annealed at (a) 730°C and (b) 740°C, and followed by

intermediate quenching.

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3.2 Nital Etching

Nital etchant can be alternative where Le Pera etchant cannot produce desired tinted effect in microstructural

constituents. For example, SEM investigations are required the topographic contrast for low alloyed steels in

which phases do not have enough difference in the chemical compositions to produce the material contrast

in SEM. In such a case, Nital can etch ferrite, pearlite and bainite while martensite remains unetched

condition. Figs. 4 and 5 show micrographs of LM and SEM after Nital etching. In Fig. 4, dual phase

microstructure can be seen with the third phase as in Le Pera etching in Fig. 3. However, it can be said that

Nital is better to recognize the Widmanstätten type ferrite (white). Monotype coaxial ferrite is seen in Le Pera

etched samples in Fig. 3. However, according to intermediate quenching, there must be fibrous

microstructure consist of martensite and ferrite which can be seen in Nital etched samples. This is due to

Nital has a better performance on obtaining grain boundaries. However, in point of the calculating phase

distributions, separating the Widmanstätten ferrite and martensite from fibrous complex microstructure is not

easy. La Pera etching may be useful to fulfill this difficulty for the image analysis studies. SEM micrographs

are given in Fig 5. Ferrite seems darker than martensite on the contrary LM micrographs after Nital etching.

a)

b)

c)

d)

Fig. 4 LM micrographs of steels annealed at a) 725, b) 730, c) 740, and d) 750°C, and followed by

intermediate quenching; etchant: Nital.

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a)

b)

c)

d)

Fig. 5 LM Micrographs of steels annealed at a) 725, b) 730, c) 740, and d) 750°C, and followed by

intermediate quenching; etchant: Nital.

4. DISCUSSION AND CONCLUSION

The advantages and disadvantages of applying Le Pera and Nital etchants to medium carbon dual phase

steels were demonstrated. Martensite seemed lighter (light brown) and ferrite seemed darker colors

(brown/blue) in Le Pera etched samples, while martensite is brown and ferrite is white in Nital etched

samples. A third phase (bainite/pearlite) is also seen in black color and nearly same contrast in Le Pera and

Nital. The third phase may be obtained due to quenching product from annealing temperature. As a result of

microstructural investigations with different etchants, Nital is favorable to understand all microstructure, while

Le Pera is not as good as Nital for obtaining grain boundaries and Widmanstätten ferrite. Therefore, it is

concluded that Le Pera is better for using in image analysis for phase distributions for medium carbon dual

phase steels.

ACKNOWLEDGEMENT

The authors are grateful to Yapı-Tek Çelik Sanayi A.Ş., Kocaeli, Turkey for the supplement of the alloy and to Dr. U. Prahl for access to a precision dilatometer.

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