  Very dense and/or hot hadronic substance (e.g. the one with energy density
greatly exceeding that of a normal nucleus) transforms itself into a
subhadronic substance which obeys macroscopic classical physics, in particular
suffers phase transitions. The most popular Single Phase Transition Model
(SPTM) assumes that the new phase is the Quark Gluon Plasma (QGP) consisting of
deconfined, chiral symmetric, pointlike "current" quarks q and gluons g of
Quantum Chromodynamics (QCD). This paper is devoted to another, Double Phase
Transition Model (DPTM) according to which hadronic substance (H) and QGP
transform one into another via an intermediate phase consisting of deconfined
constituent massive quarks Q which for brevity sake we call also equivalently
valons (Q, valonic phase) with broken chiral symmetry (plus pions as Goldstone
particles). I. e. we consider the phase transformation chain H-Q-QGP instead of
usually assumed H-QGP. The phase transition H-Q is the Hagedorn one and
corresponds to the Hagedorn temperature. Connection with the relativistic heavy
ion collision is discussed. H-Q transformation may take place even at low (e.g.
Dubna) energies.
