Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 143

Nb(CN) will precipitate at the austenite-ferfitr interface during transforma-
tion (interphase precipitation) which leads to substantial strengthening. On the
other hand, on reheating to a typical normalizing temperature of 920~ very
little Nb(CN) will dissolve and therefore virtually no precipitation strengthening
can take place. However, the undissolved particles will act as pinning agents,
restricting austenite grain growth and leading to the formation of a fine ferfitr
grain size. Therefor the reheating temperature controls the potential for precip-
itation strengthening and the strength increases progressively as the temperature
is raised from 920 to 1250~
As indicated in Figure 2.3(b), vanadium dissolves more easily than niobium
and complete solution of V4C 3 would be expected to occur in commercial grades
of structural steel at typical normalizing temperatures, e.g. 920~ Slightly higher
temperatures are required for the solution of VN which can act as a grain-refining

0.15 (a) .025C .05C

: /

NbC / / / .IOC

900 1000 1100 1200 1300

ta 0.2

=3

Temperature (~

04-cb~ oosN/ / /

/ _ I

VN. .01N/ ; I

.. i//II
0.3

/ / ./ ,,,

o.I. // ./' /

.j~---.~ o ~ i I I I

9()0 1000 11 O0 1200 1300

Temperature (~

Figure 2.3 Solubility of NbC and VN in austenite at various temperatures (After Irvine
et al. 5)