Engineering steels 279applied stress or abrasion. This special grade of steel is made in electric arc
furnaces but is rolled to rail in the same type of mill as that employed for the
pearlific grades. The mechanical properties 45 of 14% Mn rail material are given
below:
TS (N/ram 2) YS (N/mm 2) Elong. (%)
818-973 355-386 40-60The low YS/TS ratio illustrated above is indicative of the high rate of work
hardening and material with an initial hardness of 180-210 B HN develops a
hardness of over 400 BHN, and to an appreciable depth, after a short period of
service. The steel can be welded, using suitably modified techniques, by either
the thermit or flash butt welding processes.
14% Mn rails are used traditionally in railway points and crossings and in other
situations where the extended service life justifies their higher cost compared with
pearlitic grades.
At one time, it was postulated that the high rate of work hardening in this
steel (also known as Hadfields Manganese Steel) was due to the formation of
strain-induced martensite. However, it is now known that the hardening effect
is associated with the formation of stacking faults in the austenitic structure and
strain-induced martensite will only form in decarburized material or in steels of
a lower alloy content.
Micro-alloy forging steels
Up until the late 1940s, the engineering steels that were used for automotive
engine and transmission parts were based largely on compositions containing
substantial amounts of nickel and molybdenum. The philosophy that prevailed
was that these components were subjected to arduous service conditions that
required high levels of strength and toughness and that this combination of
properties was best achieved in Ni-Mo grades. However, during the 1950s,
there was the realization that many of the steels were over-alloyed with regard
to the hardenability requirements of the components and that the specified levels
of strength could be achieved by steels of leaner composition. In the 1960s,
the emerging technology of fracture mechanics provided greater knowedge on
the level of toughness required in engineering components and indicated that
satisfactory performance could be provided by steel compositions which gave
lower impact energy values than the traditional Ni-Mo grades. These factors,
coupled with major advances in heat treatment technology, led the way to the
gradual substitution of the Ni-Mo grades by cheaper steels involving additions
of manganese, chromium and boron.
By the 1970s, the opportunities for alloy reduction and substitution had largely
been exhausted but competition in the automotive industry maintained the impetus
for further cost reduction. Attention therefore turned to potential savings in
manufacturing costs and particularly in the area of heat treatment. Tradition-
ally, components such as crankshafts and connecting rods are cooled to room
temperature after the forging operation, only to be reheated to a temperature