In 1968 he habilitated and obtained the venia legendi [de] for Medieval and Modern History.
In 1971 Hammerstein was appointed professor in the course of the new Hessian Higher Education Act and in 1973 he was appointed to a newly established Extraordinary Office for Early Modern History at Frankfurt University.
His brother Reinhold was professor for musicology at the Heidelberg University, his brother Gerhard honorary professor for criminal law at the University of Freiburg.
[2] In 1999 Hammerstein published a book on the history of the Deutsche Forschungsgemeinschaft during the Weimar Republic and the Third Reich.
Ernst Klee called this book an "attempt at a clean slate", since Hammerstein described the work of the Nazi psychiatrist Robert Ritter as "general medical research", although he had divided the work of the Nazi psychiatrist into "full Gypsies", "Gypsy half-breeds" and "non-Gypsies" in his "expert opinions" in a racist manner.