Precipitation Behavior of Boron in High Strength Steel
1976 Volume 62 Issue 14 Pages 1851-1858
Details
1976 Volume 62 Issue 14 Pages 1851-1858
The hardenability of boron steel is affected by the precipitation behavior of boron. Precipitates in the boron containing steel were studied by extraction replica technique at successive manufacturing steps from hot rolling to heat treatment.
When steels are hot-rolled after heating at 1300°C for 1hr, changes of boron precipitates with the cooling rate after hot rolling are as follows:
1) When steels are water quenched (cooling rate of 90000°C/min) after hot rolling, no precipitates can be observed onγ grain boundaries.
2) When steels are air cooled (cooling rate of 300°C/min) after hot rolling, boron nitride precipitates on γ grain boundary region.
3) When steels are furnace cooled (cooling rate of 0.67°C/min) after hot rolling, AIN grows radially from BN which has precipitated before the nucleation of AIN. AIN formed on BN in furnace cooling is dendritic.
By reheating, at a rate of 15°C/min, a specimen which has been water quenched after hot rolling (above mentioned as 1), the following boron precipitates appear depending upon the temperature range:
1) Below 880°C, M23(CB)6 precipitates and coagulates on prior grain boundaries.
2) Above 900°C and below about 1000°C, BN appears on prior grain boundary region which has been previously occupied by M23(C, B)6. M23(C, B)6 is no longer observed at this stage.
3) Heating around 1000°C, AIN which is not dendritic but acicular grows radially from BN. The size of BN diminishes with growth of acicular AIN.
The formation of AIN on BN is interpreted as a transit from the non-equilibrium to equilibrium state in Al-B-N system in steels.
According to PEASE, BN (hexagonal close packed) has the same crystal structure as that of graphite. Because BN in steels was revealed to have spherulite morphology such as spherical graphite, it is considered that [00.1] axes of BN are arranged in radial fashion and (00.1) planes are perpendicular to them, by an analogy to the crystal arrangement of spherical graphite. As the growth direction of AIN is [00.1]AIN, it is considered that an orientation relationship of (00.1)BN//(00.1)AIN is satisfied when MN grows on BN.
