Cassells, 22, and Scrimgeour clocked six minutes and 29.63 seconds to finish ahead of Denmark and China.
The British duo won the title in the non-Olympic class in France last year.
Meanwhile Ireland's Olympic silver medallist Paul O'Donovan, 22, won his lightweight single sculls semi-final.
The Skibbereen man, who clinched Olympic silver in the lightweight double sculls in Rio with his brother Gary, produced a strong late burst to come from fourth at the halfway stage to win his semi-final in 6:51.71.
Slovenian Rajko Hrvat finished second in 6:52.31 with Germany's Konstantin Steinhuebel clinching the third qualifying spot for Saturday's final.
The Rotterdam championships features non-Olympic classes.
Irish duo Mark O'Donovan and Shane O'Driscoll will compete against Cassells and Scrimgeour in Saturday's lightweight pairs final after finishing second in their semi-final behind the French boat.
The British pair were the fastest qualifiers for final, ahead of the Danes (6:30.12) and France (6:30.56), while the Irish duo next quickest in 6:32.18.
After winning gold in their class at last year's World Championships in France, Cassells and Scrimgeour retained the European title in Brandenburg in May.
Coleraine rower Joel Cassells and Great Britain team-mate Sam Scrimgeour remain on course to defend their lightweight pairs world title after winning their semi-final in Rotterdam on Thursday.