Is the ASTM A335 stainless

A335 material classification analysis
1. Material properties of A335
Definition of alloy steel: A335 is a seamless ferritic alloy steel pipe for high temperature use under ASTM standard, mainly adding chromium (Cr), molybdenum (Mo) and other elements to enhance the high temperature performance, for example, A335P5 contains Cr 4.0~6.0%, Mo 0.45~0.65%.
Differences with stainless steel:
Compositional differences: Stainless steel is dominated by high chromium (≥10.5%) and nickel (Ni), with emphasis on corrosion resistance; while A335 is centred on high-temperature strength, and the total amount of alloying elements is usually less than 5%.
Application scenarios: A335 is used for high temperature and high pressure pipelines (e.g. boilers, petrochemicals), while stainless steel is more suitable for corrosive environments (e.g. chemical containers, food equipment).
2. Contradictions in search results
Misnaming of some grades:
Some web pages (e.g. 34) describe grades such as A335P91, P5 as 'stainless steel', but their technical standard (ASTM A335) is clearly categorised as an alloy steel, which may be a commercial nomenclature confusion or a misplacing of processing.
For example, the composition of A335P91 Cr 8.0 ~ 9.5%, Mo 0.85 ~ 1.05%, although the chromium content is high, but not up to the corrosion resistance threshold of stainless steel (need to be ≥ 10.5% Cr), still belongs to the category of alloy steel.
3. Authoritative standards and classification basis
ASTM and ASME standards:
ASTM A335 standard all grades (such as P5, P9, P11, etc.) are defined as ferritic alloy steel, does not contain austenitic or duplex stainless steel category.
ASME SA335 (equivalent to ASTM A335) likewise applies it to high-temperature piping, not to corrosion-resistant scenarios.
4. Summary
A335 is not a stainless steel in the traditional sense, but a ferritic alloy steel orientated towards high temperature performance.