Alloy pipes are tubular with higher percentages, than standard carbon steel pipes, of alloying elements as Molybdenum (Mo), Chromium (Cr), Nickel, etc. Actually, the ASTM A335 covers “low-alloy” steel pipes, i.e. pipes that have a total amount of alloying elements below 5%. The addition of higher percentages of alloying elements (example Nickel and Chromium) transforms the steel into higher alloys, like stainless steel, duplex, up to super-alloyed materials like Inconel, Hastelloy, Monel, etc.

Alloy steel pipes are used in the energy industry for high temperature and very low-temperature service (cryogenic), or for applications with very high pressures.

ASTM A335 alloy steel pipes fit ASTM A234 WPx series buttweld fittings (WP5, WP9, WP91) and A182 Fx forged fittings and flanges (A182 F5, F9, F11, F22, F91). All these materials have similar chemical and mechanical properties and can be joined or welded.

The addition of Molybdenum (“Moly”) increases the strength of the steel and its elastic limit, enhance the steel resistance to wear, its impact qualities, and the hardenability. It also improves the resistance to softening, makes chromium steel less prone to embrittlement and prevents pitting.

Chromium, a key element also for stainless steel alloys, prevents steel oxidation at elevated temperatures and increases the resistance of steel to corrosion. It enhances the tensile, yield, and hardness properties of low-alloy pipes at room temperatures.
Other alloying elements, present in various degrees in pipes of all grades are:

Aluminum: decreases oxygen from steelmaking

Boron: used to produce fine grain size and enhance steel hardness

Cobalt: used to enhance the steel’s heat and wear-resistance

Manganese: gives better steel hardenability

Nickel: Enhances toughness, hardenability and impact strength at low temperatures

Silicon: decreases oxygen, enhances hardenability and toughness

Titanium: prevents precipitation of chromium carbide

Tungsten: refines steel grain size and enhance the steel hardness, especially at high temperatures

Vanadium: gives steel enhanced fatigue resistance

As mentioned, low-alloy steels have a total amount of alloying elements below 5%; high alloy steel has a higher percentage of these elements.

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