Steels_ Metallurgy and Applications, Third Edition

Engineering steels 277

Flash butt welding is used to join lengths of rail into continuous track and
Preston 43 has commented on the effects of this process on the hardness of
enhanced cooled rail. In flash butt welding, the rail ends are heated to high
temperatures by an electric arc and then squeezed together by a hydraulic force
of 40-60 tonne. The rail ends fuse, excess material is extruded from the joint
and material in the heat-affected zone cools through the transformation range at
the rate of about 1.3"C/s. In contrast, the head of mill-hardened rail cools at a rate
of about 2.5~ and after flash butt welding, the hardness of enhanced cooled
0.8% C 0.9% Mn steel falls to about 300 BHN compared with 350-370 BHN
in the mill-hardened condition. However, this can be avoided by increasing the
alloy content of the steel such that a hardness of about 370 BHN is developed
under a natural cooling rate of 1.3"C/s after welding.

Wear resistance of rails

British Steel has carried out extensive laboratory wear tests on rail steels at its
Swinden Technology Centre in Rotherham. This involves the rotation of discs
of railway wheel and rail materials, under a controlled contact stress and with a
controlled amount of slip between the two discs. Wear is determined by weight
loss on the rail test disc and expressed in terms of mg/m of slip. In this test, a
very clear relationship has been established between wear rate and hardness and
this relationship is shown for a range of rail steel grades in Figure 3.41. 45 Thus

150

..~ ._o. 100

(9

t,,,,

so

(^0 0 0)
0 O0
o ~o
0
o %o
~
o 0 0
0
o ,,,I I
220 240 260
O O
Rail type
o BSll Normal grade
o Wear-resisting grade A
9 Wear-resisting grade B
9 110 kg/mm 21% Cr
A Enhanced I% Cr
& 9
9
A 9 AA~~ AA
Q
" ,i
9 ,'r :.=:
I,: I I , I I I I
280 300 320 340 360 380 400
Hardness (Hv)
Figure 3.41 Effect of hardness on wear rate in laboratory tests (After British Steep s )