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.
