15. - 17. 5. 2013, Brno, Czech Republic, EU MICROSTRUCTURAL CHARACTERIZATION OF MEDIUM CARBON DUAL PHASE STEELS AFTER INTERMEDIATE QUENCHING Ersoy ERİŞİR 1 , Serap GÜMÜŞ 1 , Oğuz Gürkan BİLİR 1 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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15. - 17. 5. 2013, Brno, Czech Republic, EU
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.
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.
15. - 17. 5. 2013, Brno, Czech Republic, EU
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.
15. - 17. 5. 2013, Brno, Czech Republic, EU
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.
15. - 17. 5. 2013, Brno, Czech Republic, EU
REFERENCES
[1] OLIVER, S., JONES, T.B. and FOURLARIS, G. Dual Phase versus Trip Strip Steels: Microstructural Changes as
a Consequence of Quasi-Static and Dynamic Tensile Testing. Materials Characterization, 2007, Vol. 58, Issue: 4,
page 390–400.
[2] SPEICH, G. R. Dual Phase Steel ASM Handbook Volume 1: Properties and Selection: Irons, Steels and High