Steels_ Metallurgy and Applications, Third Edition

276 Steels: Metallurgy and Applications

Table 3.24 BSI 1 wear-resisting grade A/UIC 860-0 Grade 90A

C% Si% Mn% TS min. Elong. min.

(N/mm 2) 5.65~So

0.65-0.8 0.1-0.5 0.8-1.3 880 8

Table 3.25 BSl l wear-resisting grade B/UIC 860-0 Grade 90B

C% Si% Mn% TS min. Elong. min.

(N/mm 2) 5.65~/So

0.55-0.75 0.1-0.5 1.3-1.7 880 8

High-strength grades
For the extremely arduous service conditions encountered in tightly curved track
and under very high axle loads, even higher strengths are required which demand
further refinement of the pearlitic structure. Up until 1985, most European
railmakers produced such material by adding up to 1% Cr to the basic C-Mn
composition and this increased the hardness of the rail head from 280 BHN
to approximately 330 BHN with an improvement factor of about two in wear
resistance. However, the gradual replacement of bolted track by welded track
brought about the requirement for an adequate level of weldability, a property
not readily satisfied with the high hardenability introduced by the addition
of 1% Cr. Attention therefore turned to the use of accelerated cooling from
the austenite range, rather than high-alloy additions, for the depression of the
pearlite transformation and the development of high-strength rails. A number of
railmakers have now introduced in-line cooling for the head hardening of rails
and detailed accounts of the computerized facility installed by British Steel at
Workington are given in publications by Preston 43 and Hodgson and Preston. 44
Brief details are as follows.
The rail leaves the finishing stands at a temperature of about 10(~~ and is
stood head up. The rail passes under temperature monitors and then into a 55-m-
long cooling train where it is sprayed with water over all surfaces. The outgoing
rail leaves the cooling station at dull red heat and the main transformation to
pearlite takes place in still air. Obviously the rate of cooling has to be balanced
very finely so as to provide sufficiently high rates of cooling in the centre of
the rail head so as to depress the transformation of austenite to pearlite without
cooling the surface of the rail at a rate which would lead to the formation of
bainite or martensite. During cooling, the rail is driven through the cooling station
by rollers which also maintain straightness in the rail. On leaving the cooling
station, the rail is finally turned on its side and passes down the cooling banks
for finishing in the normal manner. Using this facility, the hardness developed in
the head of a 0.8% C 0.9% Mn rail is controlled to within a fairly narrow range
of about 350-370 BHN. 44