4140 steel and 4140H steel are both Cr-Mo alloy structural steels from the same family, with essentially the same composition.
H stands for Hardenability. 4140H has mandatory requirements to guarantee hardenability, which is the core difference.Therefore, 4140H can be considered a version of 4140 with guaranteed hardenability.
Compared to 4140, 4140H represents more stable and controllable hardenability, but also implies stricter quality control and higher cost.It is more suitable for large components requiring high consistency in overall performance after heat treatment.