Heat treatment processes for alloy steel are more complex and critical than those for carbon steel. Due to the addition of alloying elements such as chromium, molybdenum, nickel, and vanadium, alloy steel has lower thermal conductivity and tends to form hardened microstructures, imposing stricter requirements on heating rate, cooling method, and holding time. The following are the most commonly used heat treatment methods for alloy steel in pipe fittings, flanges, and mechanical manufacturing:
1. Normalizing
Normalizing involves heating the steel above its critical temperature, holding, and then cooling in air. Purposes:
- Refine grains: eliminate coarse grains formed during forging or casting and homogenize the microstructure.
- Adjust hardness: for certain low-alloy steels, normalizing can achieve appropriate hardness and improve machinability.
Applications:
- Used in the manufacturing of pipe fittings made of A234 WPB (carbon steel) and some low-alloy steels.
- As a pre-heat treatment before quenching and tempering.
Note: For alloy steels with high chromium and molybdenum contents (such as P91), normalizing alone often cannot meet performance requirements and must be combined with tempering.
2. Quenching and Tempering (QT)
This combined process of quenching plus high-temperature tempering is the most important heat treatment method for alloy steel. Purposes:
- Achieve optimum comprehensive mechanical properties: endow the steel with high yield strength and tensile strength while maintaining good toughness and ductility.
- Improve fatigue resistance.
Applications:
- High-pressure flanges and forgings: e.g., A105 (usually normalized but QT-treated for high-pressure service), A182 F11/F22/F91.
- High-strength bolts and shaft parts.
- Thick-walled pipe fittings for high-pressure and impact loading conditions.
3. Annealing
Annealing for alloy steel generally refers to full annealing or isothermal annealing. Purposes:
- Soften the steel: reduce hardness to facilitate cold stamping or machining.
- Relieve internal stresses: prevent deformation or cracking of forgings or castings during storage.
- Improve microstructure: eliminate network carbides and prepare for subsequent quenching.
Applications:
- Forged blanks of alloy structural steels.
- Cold-formed pipe fittings (e.g., cold-bending elbows) to eliminate work hardening.
4. Post Weld Heat Treatment (PWHT)
PWHT is a mandatory process for welded alloy steel components, with stricter requirements than for carbon steel. Purposes:
- Relieve welding residual stresses: large welding stresses result from differences in thermal expansion coefficient and thermal conductivity of alloy steel, which must be relieved to avoid cracking.
- Improve the microstructure of the heat-affected zone (HAZ): prevent the formation of hard and brittle martensite near welds.
- Remove diffusible hydrogen: prevent hydrogen-induced cracking (delayed cracking).
Key parameters:
- Holding temperature: generally between 600°C and 700°C (varies by material; e.g., P91 steel requires a higher temperature).
- Heating/cooling rate: must be strictly controlled (normally no more than 200°C per hour) to avoid new cracks caused by excessive temperature differences.
Applications:
- Welding of all Cr-Mo steel pipe fittings.
- On-site installation of large-diameter thick-walled alloy steel pipelines.
5. Solution Annealing
This process is mainly applied to austenitic stainless steels (e.g., 304, 316L) and duplex stainless steels. Purposes:
- Dissolve carbides: redissolve carbides (e.g., chromium carbide) in the steel into the austenite matrix.
- Homogenize chemical composition.
- Rapid cooling: subsequent fast water cooling prevents carbide precipitation, thus ensuring corrosion resistance.
Applications:
- Final heat treatment of stainless steel pipe fittings (elbows, tees).
- Stainless steel components after welding.
6. Stabilization Treatment
This process is mainly used for stainless steels containing titanium (Ti) or niobium (Nb), such as 321 and 347. Purposes:Promote full combination of carbon with titanium/niobium to form stable carbides, thereby preventing chromium depletion at grain boundaries during subsequent welding or service and improving resistance to intergranular corrosion. Applications:
- Heat treatment of 321 stainless steel pipe fittings.
7. Temper Embrittlement Control
This is a risk specific to alloy steel. Certain alloy steels (especially those containing chromium, molybdenum, and manganese) may undergo embrittlement when tempered or slowly cooled in the temperature range of 350°C to 500°C. Preventive measures:
- Avoid prolonged holding within the temper embrittlement range.
- Adopt rapid cooling (e.g., oil cooling or water cooling) after tempering instead of furnace cooling.
Summary: Typical Heat Treatment for Different Alloy Steels
- Cr-Mo steels (e.g., 15CrMo, P91):Manufacturing: normalizing + tempering or quenching and tempering.Welding: strict PWHT is mandatory.
- Austenitic stainless steels (e.g., 304, 316L):Manufacturing/welding: solution annealing (water cooling).
- Duplex stainless steels (e.g., 2205):Manufacturing/welding: solution annealing (cooling rate controlled to ensure phase balance).
- Low-temperature steels (e.g., 3.5Ni):Manufacturing: quenching and tempering or normalizing + tempering (to guarantee low-temperature impact toughness).
