Welding cast steel - which rods?

Are you sure it is cast steel and not cast iron If it is defintely steel then try some 309L rods.

I googled that by the way, but did work with a welder many moons ago.

These are Critall windows? I've always assumed they were fabricated from simple hot-rolled steel sections, welded and then galvanised. In which case, they are mild steel, but watch out for fume from the zinc.

I was surprised to learn they made aluminium windows in the 50's.

Google some more:

Maybe they're different nowadays but these are un-galvanised cast steel

- hence the need to weld-in a few new sections. According to Wikipedia "In 1939, Crittall built its first galvanising plant ..." - this house was finished in 1939 so just missed out. Rather unfortunate timing because there are a LOT of steel casements here.

How were they made, then? Sand casting? Doesn't sound right to me.

Irrespective of how yours were made, I'd bet that they are a fairly low carbon steel, so you have the usual choice of "mild steel" rods for MMA. Useful that you don't have the zinc to worry about.

Might be some form of die casting, but could be sand

TBH do you think that sounds likely, with that sort of section? And being steel, the casting temperature is significantly higher than cast iron, multiplying thermal expansion/contraction issues.

You could probably just about use ductile cast iron in that geometry, especially if you stove enamelled it to get rid of the "roughness". Think old fashioned gas cooker pan supports.

My money would still be on hot rolled mild steel section. Perhaps brazed rather than welded some time pre-1940?

Arc welding dates from the 1880s but IIRC did not really become mainstream until the wartime Liberty ships. Apparently the Balmoral (1948) has both welds and rivets

Yup. I am not going to defend my surmises as assertions of fact here! :-)

Acetlyne welding has been around since forever. lead soldering too.

IIRC my late parents Crittall (sp?) windows were as you describe, with welded joints

Oxy acetyline goes back to WWI at least:

Wiki

"Oxy-fuel welding (commonly called oxyacetylene welding, oxy welding, or gas welding in the United States) and oxy-fuel cutting are processes that use fuel gases (or liquid fuels such as gasoline) and oxygen to weld or cut metals. French engineers Edmond Fouché and Charles Picard became the first to develop oxygen-acetylene welding in 1903. Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material (e.g. steel) in a room environment. A common propane/air flame burns at about 2,250 K (1,980 °C; 3,590 °F), a propane/oxygen flame burns at about 2,526 K (2,253 °C;

During the early 20th century, before the development and availability of coated arc welding electrodes in the late 1920s that were capable of making sound welds in steel, oxy-acetylene welding was the only process capable of making welds of exceptionally high quality in virtually all metals in commercial use at the time. These included not only carbon steel but also alloy steels, cast iron, aluminium, and magnesium. In recent decades it has been superseded in almost all industrial uses by various arc welding methods offering greater speed and, in the case of gas tungsten arc welding, the capability of welding very reactive metals such as titanium. Oxy-acetylene welding is still used for metal-based artwork and in smaller home-based shops, as well as situations where accessing electricity (e.g., via an extension cord or portable generator) would present difficulties. The oxy-acetylene (and other oxy-fuel gas mixtures) welding torch remains a mainstay heat source for manual brazing and braze welding, as well as metal forming, preparation, and localized heat treating. In addition, oxy-fuel cutting is still widely used, both in heavy industry and light industrial and repair operations. "

I think that is a very fair summary...

Indeed. But it is a fairly skilled process, perhaps less suited to mass production. Whereas if making large numbers of standard windows, cutting bits to size and shape then brazing or silver soldering in a suitable jig still requires some skill, but it's relatively easy to train people for that. And can be done with an air / town gas mix, even just propane with a suitable torch.