Despite its versatility, designers must consider:
Corrosion Resistance: It is not a stainless steel. It will rust in humid or corrosive environments and requires protective coatings (paint, plating) for such service.
Weldability: As discussed, it is poor. Welded designs should be avoided or require strict, costly procedures.
Not for Extreme Temperatures: While Mo improves high-temp strength, it is unsuitable for sustained service above ~450-500°C (creep becomes an issue) or for cryogenic applications (toughness drops significantly below -20°C).
Machining in Hardened State: Requires appropriate tooling and processes, adding cost and time.
Overkill for Simple Parts: For non-critical, lightly loaded parts, a cheaper plain carbon steel (like 1045) is more economical.
Understanding these constraints ensures AISI 4140 is selected appropriately, maximizing its benefits while mitigating its weaknesses in the final application.