274 Steels: Metallurgy and Applicationsindicates that ferrite makes a significant contribution to the yield strength even
when there is 90% pearlite in the microstructure.
Gladman et al. 42 also generated an equation for the impact transition temper-
ature of high-carbon steels and this is shown below:
1
27J ITr (*C)--- f~[-46- l l.5d-~]
1 1
+ (1 - fa)[-335 + 5.6So g - 13.3p-g + 3.48 x 106t]+ 48.7% Si + 762~/%N
where d = ferrite grain (mm)
p = pearlite colony size (mm)
t = cementite plate thickness (mm)This equation again emphasizes the importance of a fine grain size in producing
a low impact transition temperature and it should be noted that the coefficient for
the pearlite colony size p (13.3) is of a similar order to that of the ferrite grain
size d (11.5).Rail steels
Up until the 1970s, rails for passenger and freight trains were regarded as
relatively simple undemanding products and the specifications had changed very
little for a number of decades. However, investment in railway systems, the
advent of high-speed passenger trains and the requirement for longer life track
imposed a demand for rails of high quality, greater strength and tighter geometric
tolerances. Therefore there have been major innovations in the past 20 years in
terms of method of manufacture, degree of inspection and range of products.
Typically, rail steel is produced in large BOS vessels and is vacuum
degassed prior to being continuously cast into large blooms. Vacuum degassing,
coupled with ladle trimming facilities, permits very tight control over chemical
composition. After casting, the blooms are placed in insulated boxes, whilst
still at a temperature of about 600C, and are cooled at a rate of IC per
hour for a period of three to five days. This treatment, coupled with prior
vacuum degassing, reduces the hydrogen level in the finished rail to about
0.5 ppm, thereby reducing substantially the susceptibility to hydrogen cracking.
The blooms are then reheated and rolled directly to the finished rail profile. The
rail produced from each bloom is hot sawn to specific lengths prior to passage
through a rotary stamping machine en route to the cooling areas. Depending upon
the properties required, the rails are either cooled normally in air or subjected
to enhanced cooling for the development of high strength. On cooling to room
temperature, the rails are passed through a roller-straightener machine which
subjects the section to a number of severe bending reversals and emerge with
a very high degree of straightness. Finally, the rails pass through a series of
ultrasonic, eddy current and laser inspection stations which monitor non-metallic
inclusions, external defects and the flatness of the running surface. The final