Study on phase transformation and microstructural evolution in a super bainitic steel
Date Issued
2011
Date
2011
Author(s)
Chang, Hsiao-Tzu
Abstract
In recent years, a novel high carbon high silicon alloy has been developed by Bhadeshia et al. Different from general high strength steels, in which grain refinement is limited, a bulk material with a nano structure is achieved by simple heat treatments. This steel has excellent strength and high toughness after isothermal transformation at low temperature for several days. It can be use as a prospective armor material and is named as “super bainite”.
The super bainite microstructure consists of a mixture of the nano-scaled sub-unit bainite structure and fine austenite films. The major alloying element, silicon, suppresses precipitation of brittle cementite from the austenite, and the other alloyed elements, namely manganese, chromium, and vanadium, enhance the hardenability.
After hot rolling, the banded structure is often observed in this steel. Experimental results have shown that this banded structure, containing different phases, is caused by elemental segregation, one example being manganese, which is segregated because of its low diffusion coefficient in solid. The way to eliminate the banded structure is to homogenize the alloy at high temperature. The resulting low temperature bainite steel is uniform in both microstructure and hardness.
The main purpose of this research was to investigate the formation of low-temperature bainite under different isothermal heat treatment conditions (isothermal temperature and holding time). The results revealed that martensite is the major structure and leads to the high strength in the early stage. As the isothermal heat treatment continues, the amount of bainite rises, and it suppress the formation of martensite, which causes the strength to fall. The strength increases again when the bainite structure achieves saturation.
The transformation temperature also greatly influences the bainite structure. At higher transformation temperatures (e.g., 300oC), the bainite sheaves grow faster but thicker; the isothermal heat treatment at lower transformation temperatures (e.g., 150oC) can produce long and thin sheaves with superior hardness, but the growth rate is excessively slow. As a consequence, isothermal transformation at 250oC appears to be the proper condition for super bainite transformation. Completion of the bainite reaction requires only about 48 h, and the matrix of nano-scaled bainitic ferrite plates with carbon-enriched retained austenite can be obtained.
Although super bainite has excellent strength, the transformation time is too long for application in actual industrial production. Therefore, another goal of this work was to study two-step isothermal heat treatments, which could accelerate the bainitic transformation. First, specimens were isothermally heated at high temperature (300oC) for a very short time to nucleate the bainite sheaves and then isothermally heated at low temperature (200oC) to grow a finer bainite structure. The results showed that two-step isothermal heat treatment accelerated the formation of bainite structure. The process of first heating at 300 oC for 4 h and then cooling to 200 oC for the remainder of the time give rise to shorter transformation time and high final strength.
A high silicon content is added to suppress the precipitation of carbides in super bainitic steel; therefore, the thermal stability of high-carbon super bainite is also discussed. The specimens were tempered at 400-600oC for different holding times to observe the evolution of carbides. The results revealed that the amount and size of carbides were influenced greatly by tempering temperature and time. Larger and more massive carbides form at higher tempering temperatures or longer tempering times. However, in all experimental parameters, the observed carbides are identified as cementite, which is the stable carbide form in tempered bainitic steels.
Previous research indicated that super bainitic steel possessed excellent strength and great fracture toughness (KIC). However, the weak performance in impact toughness was observed in this research. The present study indicates that super bainitic steel has poor CVN impact toughness (about 6.5 ±0.3 J), which may be caused by the carbon pinning effect on dislocations in bainite sheaves. In addition, TEM revealed the α’-martensite structure in fracture surfaces after CVN impact tests, which might be one of the reasons for the low impact absorbed energy.
Subjects
low temperature bainite
high-carbon high-silicon alloy steel
banded structure
isothermal transformation
tempering
TEM
fracture toughness
Type
thesis
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