Alloy steel refers to steel based on carbon steel, to which one or more alloying elements (such as chromium, molybdenum, nickel, vanadium, etc.) are specially added to improve its properties, including strength, toughness, corrosion resistance, and heat resistance. In the manufacture of pipe fittings, alloy steel is commonly used in extreme working conditions involving high temperature, high pressure, or low temperature. Below is a detailed classification of alloy steel material grades, mainly in accordance with international standards (ASTM) and Chinese national standards (GB).
I. Classification by Alloy Element Content
Low Alloy Steel
Total alloy content: < 5%.Features: High strength, good toughness, relatively low cost.Applications: High-pressure pipelines, low-temperature pipelines, bridge structures.Typical grades: 16Mn (Q345), 15CrMo.
Medium Alloy Steel
Total alloy content: 5% – 10%.Applications: Steam turbine blades, gears, high-strength bolts.
High Alloy Steel
Total alloy content: > 10%.Features: Excellent corrosion resistance or heat resistance.Applications: Chemical industry, nuclear power, ultra-supercritical power generation.Typical grades: Stainless steel (e.g., 304, 316), heat-resistant steel (e.g., 12Cr1MoV).
II. Common Specific Material Grades in Pipeline Engineering (ASTM Standards)
This is the most widely used classification in the international petrochemical and power industries.
1. Chromium-Molybdenum Steel – the main material for high temperature and high pressure service
This type of steel exhibits excellent resistance to high-temperature creep and is widely used in main steam pipelines for petroleum refining and power station boilers.
2. Low Temperature Steel
This type of steel ensures no cold brittleness (i.e., maintains high toughness) at extremely low temperatures by reducing carbon content and adding nickel.
3. High-Strength Low-Alloy Steel (HSLA)
Mainly used to improve the pressure-bearing capacity of pipelines, reduce wall thickness, and lighten weight.
- A234 WPS (A516 Gr. 70): Commonly used for manufacturing large-diameter welded pipe fittings, with higher strength than ordinary carbon steel (WPB).
- A860 WPHY series:WPHY 42 / 52 / 60 / 65 / 70: The numbers represent the minimum yield strength in ksi.Applications: Elbows and tees for oil and gas transmission pipelines (long-distance pipelines). For example, WPHY 65 or 70 is commonly used in the West-East Gas Pipeline Project.
III. Special Characteristics of Alloy Steel Manufacturing and Heat Treatment
Compared with carbon steel, alloy steel has extremely strict requirements for heat treatment during manufacturing, which is critical to ensuring performance.
Normalizing + Tempering
This is the standard heat treatment process for most alloy steels (e.g., 15CrMo, P91).Purpose: Refine grains, relieve internal stress, and achieve good comprehensive mechanical properties.
Post Weld Heat Treatment (PWHT)
Alloy steel generally has poorer weldability than carbon steel (prone to cracking).High-temperature tempering must be performed after welding to relieve welding residual stress and prevent delayed cracking.
Preheating
Before welding alloy steel (especially high Cr-Mo steel), the base metal must be preheated (usually above 200°C–300°C) to slow down the cooling rate.
Core Tips
- For high-temperature service, the first choice is the chromium-molybdenum (Cr-Mo) series.
- For extremely low-temperature service, the first choice is nickel steel (3.5Ni).
- For high-pressure service at room temperature, the first choice is high-strength low-alloy steel (WPHY series).
