AISI 4130-like specifications exist in many different forms
https://www.astmsteel.com/product/4130-steel-aisi-25crmo4-7218-scm430/There are very many Cr-Mo steel specifications; this one happens to have become popular (and therefore virtually commoditised) because it has adequate properties for a variety of applications, and doesn't have too many drawbacks.
It obviously varies in strength and formability depending on how it is heat treated. It responds to heat treatments and thermal cycles such that it may be softened, tempered to give high strength and reasonable ductility or turned into something that is hard and brittle. In the latter state it may suffer various forms of in-service cracking.
The reason it started to be used for manufacturing aircraft spaceframes is because it is very difficult to both create a sound fused weld and to have it cool too quickly, (such that the weldment is brittle) provided you use gas-welding, as per many early airframes. When even thin-walled tubes are gas welded, the HAZ is extensive and the weldment will usually cool slowly enough to avoid embrittlement. The weldment will be softer than the base material, but you would need to be working in a very draughty, cold workshop to embrittle a well-fused gas weld.
However when TIG welded cooling rates can easily be much higher, because the heat input is so much less. The recommendation for critical weldments is that they are at least stress-relieved, to avoid problems. However bike frames in 4130 are routinely TIG welded without such a stress-relief heat treatment, and it isn't that unusual to find that the weldment contains brittle regions (typically in the HAZ) that can cause in-service cracking.
Provided (as intended) the cooling rate isn't too high the main effect is that the weld and HAZ are softer than the base material; with butted tubes the loss of strength is mitigated and a satisfactory frame can be made.
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