4 results listed
Advanced High Strength Steels (AHSS) grades are currently the fastest growing materials in the
automotive industry. AHSS offer a great potential for the development of automobile bodies with their
mechanical properties of high formability and strength. Dual phase (DP) steels are increasingly
popular AHSS grade in the automotive industry due to the excellent combination of high strength
and good formability. The intercritical annealing of DP steel is generally two-stage heat treatment.
DP steel sheet is annealed in the intercritical temperature range to produce ferrite and
austenite and followed by accelerated cooling to transform the austenite phase to martensite. This
study aims to examine effect of a cyclic intercritical annealing below and above A1 temperature on
DP steel microstructure with comparison to the conventional intercritical annealing. The
specimens were conventional intercritically annealed at different temperatures between 720 °C and
760°C. In cyclic intercritical annealing experiments, the temperature is cycled between 630 °C and an
intercritical annealing temperature without any isothermal holding. After the conventional and
cyclic intercritical annealing, the samples were investigated using light microscope and scanning
electron microscope. Experimental results showed that a refinement of martensite and ferrite dual phase
microstructure compare to conventional intercritical annealing.
International Iron & Steel Symposium
UDCS
Ayşe Kavruk
Oğuz Gürkan Bilir
Özge Ararat
Ahmet Efe Gezmişoğlu
Influence of different austenitization conditions on retained austenite quantity and composition investigated
on SAE 52100 steel. SAE 52100 steels are being widely used as bearing component regarding to their
hardness and wear resistance. Traditional production cycle includes hot rolling, spheroidization, cold rolling,
quenching from two phase region and tempering steps. This cycle results with martensitic matrix and
undissolved course carbides. Course carbides maintain adequate wear performance but due to course
carbides providing appropriate nucleation sites for crack propagation and growth this structure shows poor
fatigue behavior. Previous studies showed better carbide size and distribution with experimental two step
hardening cycle. This cycle results with considerable increase at retained austenite fraction. It’s well known
that mechanically unstable retained austenite improves fatigue resistance in this type of steel.
Transformation retained austenite to martensite caused by deformation eliminates local stresses and retards
crack propagation and growth. XRD studies are carried on to determining retained austenite quantity with
various austenitizing conditions. Computational simulations with Thermo-Calc and Dictra are used for
predicting carbon concentration gradient of austenite during austenitization which determines retained
austenite fraction at the end. A good correlation between XRD data and simulation results was obtained.
International Iron & Steel Symposium
UDCS
Ersoy Erişir
Oğuz Gürkan Bilir
Ahmet Efe Gezmişoğlu
Co-Cr and 316L are the commonly used alloys in biomedical industry. However, both 316L and mostly used
Co-Cr alloys as F-562, L-605 contain large portions of Ni as alloying element. Ni may cause allergic and
toxic impacts on human health. In recent years, N-alloyed Ni free stainless steels seemed as good candidate
for replacing Ni contained metals. However, liquid steel has a limited N solubility at atmospheric pressure.
Thus, high pressured melting techniques are required. In this study, N alloying of ferritic 430 stainless steel
sheet was performed in solid state. Nitrogen absorption of stainless steel was performed in a furnace filled
with nitrogen gas with atmospheric pressure at 1200°C. Solid state N absorption was applied in four different
time periods as 4, 8, 12, 24 hours. Microstructure after absorption process was investigated by Scanning
Electron Microscope and Light Microscope. Fully austenitic structure was observed in surface of specimens.
However, duplex microstructure of ferrite and austenite was seen in the center of specimen. Vickers hardness
was carried out to evaluate of phases. It was concluded that optimum microstructure is achieved by 4 hours
of N absorption.
International Iron & Steel Symposium
UDCS
Oğuz Gürkan Bilir
İsmail GÜVENİR
Ersoy Erişir
Finer martensitic and ferritic microstructures are demanded to obtain high strength and toughness for dual
phase steels. In this study, reducing of prior austenite grain size selected as an effective method to reduce
the size of dual phase microstructure. The effect of prior austenite grain size on microstructure of medium
carbon dual phase steel produced by intercritical annealing were investigated for different austenitizing
conditions. Annealing experiments were performed to fully austenitizate the steel at different austenitizing
temperatures (900, 950, 1000ᵒC) and times (5, 10, 15 min) and followed by quenching. Finally, as-quenched
martensitic microstructures were intercritical annealed at 760ᵒC for 15 min to attain martensitic and ferritic
dual phase steel. The microstructures were analyzed using light microscopy and image analysis methods. It
is concluded that grain size of dual phase steel is reducing by decreasing of the austenitizing temperature
and the holding time.
International Iron & Steel Symposium
UDCS
Ersoy Erişir
Özge Ararat
Oğuz Gürkan Bilir