S3/2012 appears to be telling us that the part of the shaft in question was air spec 16NCD13 in the old shaft and air spec 32CDV13 for the new shaft. It is not clear to me what the construction method is for the entire shaft assembly or what the materials are for the other components of the welded assembly.
16NCD13 (old) is an alloy steel presented as a case hardening spec. Tensile strengths of up to 1380 MPa can be achieved. This material has less carbon and less other alloying elements than the newer shaft. Case hardening adds carbon to the surface of the component and typically gives hardnesses of around 700HV/58Rc. Once heat treatment is complete, core and surface properties are different.
32CDV13 (new) is an alloy steel presented as a nitriding spec. Tensile strengths of up to 1350 MPa can be achieved. Nitriding converts certain alloying elements (such as Vanadium) at the surface to nitrides and typically gives similar hardnesses of around 700HV/58Rc. Vanadium is key to the nitriding properties (to spot a nitriding steel, look for V and Ti) but higher levels of chromium and molybdenum also contribute. Once heat treatment is complete, core and surface properties are different.
To most people, 700 Hardness Vickers (about 58 Rockwell C) gives the impression of glassy hard. This surface layer is much harder and stronger than the core. Hardness and tensile strength aren't everything though and a balance needs to be struck between strength and ductility. There is no point in having a very strong component if it cannot absorb the flexing and shock loads of the application. Making things stronger can appear so simple but it is usually a nightmare of conflicting requirements and properties.
How these materials respond to welding is likely to be under intense scrutiny at the moment. Welding different alloys together can be problematic. Different carbon content in each component would be one area of concern. It would be interesting to know the order of processes in the manufacture of these shafts since that might reveal the properties at the time of welding.
JimF
(Automotive background. Spec'd case hardening and nitriding steels for engine components in the past.)