The Germans, from memory, made excellent rolled, face-hardened steel which had a much harder surface than cast steel & they preferred welding to rivets flying around inside their tanks.
Is the lack of access to such techniques the reason why the French resorted to casting the turret front instead of adopting a wholly-welded turret?
Not necessarily. Many different nations adopted different methodologies to produce a tank turret. The quickest way often was to cast the turret - the British, the Soviets and the Americans took that route often with their heavier tanks. Casting offered advantages of speed and the ability to shape the turret. One only has to think of the Churchill, the JS series and the US heavies from the M26 onwards. Casting however required expertise in making cast armour - something the French lacked in 1944-45. The French were experts in making tanks until they were defeated. After that, they were forced to wait out the war, unable to design or build any heavy armour until liberated.
Another alternative war was a composite system - mixing cast components with the flat plate and welding it together. The British were masters at this and the late model Churchills had cast wide walls and plate turret roofs. The Americans used it to a lesser extent in their M4 Sherman. It was often forced on a manufacturer out of expediency but usually worked rather well.
The Germans were seeking heavier armour and speed of manufacture. Flat plate, welded together, often with interlocking plates became their trademark as the war progressed with the Panther and Tiger II being the most obvious methodology. The British copied this method with their Centurion hull, with the upper and lower glacis interlocking at the nose and sides. The Germans carried the concept over into their Sdkfz 250 and 251 APCs and their SWS half-tracks. It worked, why not?