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.
GRADE
| Standard | Low-Alloy Steel Grade |
| ASTM A335 | P1 P2 P5 P5b P5c P9 P11 P12 P15 P21 P22 P91 P92 |
CHEMICAL COMPOSITION
| Element | C | Mn | Si | P | S | Cr | Mo |
| P1 | 0.10~0.20% | 0.30~0.80% | 0.10~0.50% | 0.025% | 0.025% | - | 0.44~0.65% |
| P2 | 0.10~0.20% | 0.30~0.61% | 0.10~0.30% | 0.025% | 0.025% | 0.50~0.81% | 0.44~0.65% |
| P5 | 0.15% | 0.30~0.60% | 0.50% | 0.025% | 0.025% | 4.0~6.0% | 0.44~0.65% |
| P5b | 0.15% | 0.30~0.60% | 1.00~2.00% | 0.025% | 0.025% | 4.0~6.0% | 0.44~0.65% |
| P5c | 0.12% | 0.30~0.60% | 0.50% | 0.025% | 0.025% | 4.0~6.0% | 0.44~0.65% |
| P9 | 0.15% | 0.30~0.60% | 0.50~0.10% | 0.025% | 0.025% | 8.0~10.0% | 0.44~0.65% |
| P11 | 0.05~0.15% | 0.30~0.61% | 0.50~0.10% | 0.025% | 0.025% | 1.0~1.5% | 0.44~0.65% |
| P12 | 0.05~0.15% | 0.30~0.60% | 0.50% | 0.025% | 0.025% | 0.8~1.25% | 0.44~0.65% |
| P15 | 0.05~0.15% | 0.30~0.60% | 1.15~1.65% | 0.025% | 0.025% | - | 0.44~0.65% |
| P21 | 0.05~0.15% | 0.30~0.60% | 0.50% | 0.025% | 0.025% | 2.65~3.35% | 0.80~1.60% |
| P22 | 0.05~0.15% | 0.30~0.60% | 0.50% | 0.025% | 0.025% | 1.9~2.6% | 0.87~1.13% |
| P91 | 0.08~0.12% | 0.30~0.60% | 0.20~0.50% | 0.02% | 0.01% | 8.0~9.0% | 0.85~1.05% |
| P92 | 0.07~0.13% | 0.30~0.60% | 0.50% | 0.02% | 0.01% | 8.5~9.5% | 0.30~0.60% |
ALLOYING ELEMENTS
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.
