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.
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Learn about the alloy steel pipes specification ASTM A335 that covers “chrome-moly” seamless pipes with remarkable resistance to corrosion and good tensile strength at high-temperature service. Generally, ASTM A335 P11, P22, and P91 pipes are used in power generation and in downstream oil and gas, P5 and P9 grades are for refinery applications.
ALLOY PIPE A335
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.
The types of alloy steel covered by the ASTM A335 – ASME SA335 specification are designed with a “P” prefix, from P5 to P92. Grades P11/P22 and P91/92 are typically found in power stations, whereas grade P5 and P9 are more common for application in the petrochemical industry. Grades P9, P91 are, in the list, the more expensive (a P91 seamless pipe may cost approx 5€ per kg.).
|A335 Low-Alloy Pipe||UNS Number||Yield Strength ksi||Tensile Strength ksi||Elongation %||Rockwell||Brinell|
- Transverse/longitudinal: tension and flattening, hardness, bend tests- for material which has been heat-treated in batch furnaces, these tests shall be made on the 5% of the pipes from each heat lot number. For smaller lots, one pipe at a minimum has to be tested
- ASTM A335 Gr. P91 shall have a hardness of 250 HB / 265 HV (25 HRC)
- Hydro testing: shall be applied to every length of pipe
- The non-destructive electric test is optional
|1/8 to 1 1/2 / DN 6 to 40||1/64 (0.015)||0.4||1/64 (0.015)||0.4|
|Over 1 1/2 to 4 / DN 40 to 100||1/32 (0.031)||0.79||1/32 (0.031)||0.79|
|Over 4 to 8 / DN 100 to 200||1/16 (0.062)||1.59||1/32 (0.031)||0.79|
|Over 8 to 12 / DN 200 to 300||3/32 (0.093)||2.38||1/32 (0.031)||0.79|
|Over 12 / > DN 300||+/- 1% of the specified outside diameter|
The tolerances in WT, in %, from specified are:
- 1/8 to 2 1/2 [6 to 65] incl., all t/D ratios: over 20%, under 12.5%
- Above 2 1/2 , t/D < or = 5%: over 22.5%, under 12.5%
- Above 2 1/2 , t/D > 5%: over 15%, under 12.5%
t = Specified Wall Thickness; D = Specified Outside Diameter)
Chrome Moly Pipes: Werkstoff vs EN vs ASTM
|1.5415||16Mo3||A335 Grade P1|
|1.7335||13CrMo4-5||A335 Grade P11, P12|
|1.7380||10CrMo9-10||A335 Grade P22|
|1.7362||X11CrMo5||A335 Grade P5|
|A335 Grade P9|
|1.4903||X10CrMoVNb9-1||A335 Grade P91|