These are much better quality than the castors that were supplied with a surprisingly expensive office chair. The OEM castors were made of cheap injection moulded plastic, and after a year of use, one shattered causing the chair to collapse dangerously.
I therefore purchased these as a replacement on the basis that they were described as "heavy duty". The first thing I noticed when they arrived was simply how heavy they were; nearly 3 times the weight of the original ones. Turning them over, it was clear that the entire frame was made of steel (fully galvanised) but there is a a matt black decorative plastic cover over the top to make them look discreet. The wheels also have a different arrangement to what most office chairs have; there is a single wheel in the centre, supported from both sides by a steel axle. The original wheels simply had plastic wheels supported on one end by the plastic frame.
The wheels are hard plastic, but turn freely even though they don't have a proper bearing. They are probably best used on hard floors, as the hard plastic may cause excess wear on carpets - however, they do roll perfectly acceptably on carpet.
They are a standard office chair fitting, so installing them was simply a matter of pulling off the old castors and pushing these ones on.
Summary: A suitable replacement for broken original office chair castors. They appear to be built to a dramatically higher level of quality than even the original wheels supplied with very expensive chairs.