The carbon equivalent (Ceq) is the single most important parameter for assessing the weldability of Q390B and its risk of cold cracking.
1. Carbon Equivalent (Ceq) Range for Q390B
The Ceq for Q390B is not a fixed single value, but a defined maximum allowable limit set by the standard. Its typical/actual range in commercially produced steel is derived from its chemical composition limits in GB/T 1591-2018.
Some producers may aim for the lower end (~0.44-0.46%) to improve weldability, while others push toward the limit to ensure strength targets are met with less costly alloying.
Key Point: When procuring Q390B, the actual Ceq value must be stated on the Mill Test Certificate. Do not rely on the typical range; use the provided value for your welding procedure calculations.
2. How Ceq Affects Cold Cracking Susceptibility
Cold cracking (also called hydrogen-induced cracking or delayed cracking) is a major risk when welding high-strength, low-alloy steels like Q390B. The Ceq is a direct predictor of this risk through the following mechanism:
The Link: Ceq → Hardenability → Microstructure → Susceptibility
High Ceq → High Hardenability:
The Ceq formula approximates the steel's ability to form hard phases upon rapid cooling. Each element in the formula contributes to shifting the Time-Temperature-Transformation (TTT) diagram to the right, meaning martensite forms even at slower cooling rates.
Rapid Cooling → Formation of Hard Martensite:
During welding, the Heat-Affected Zone (HAZ) is heated to high temperatures and then cooled rapidly by the surrounding cold base metal. If the Ceq is high, the HAZ will transform into hard, brittle martensite rather than softer ferrite/pearlite.
Martensite + Hydrogen + Stress = Cold Cracking:
This is the classic "three-factor" model for cold cracking:
Susceptible Microstructure (Martensite): Provided by the high Ceq. Martensite has low toughness and is highly prone to cracking.
Hydrogen (H): Introduced from moisture in welding flux, shielding gas, or plate surface contaminants.
Tensile Stress: Inevitable from thermal shrinkage and joint restraint.
Hydrogen atoms diffuse and accumulate in the stressed martensitic HAZ. When a critical combination is reached, a brittle crack propagates, often hours or days after welding ("delayed" cracking).
Practical Implications for Welding Q390B
Because Q390B's Ceq (0.44-0.49%) is significantly higher than that of mild steel (e.g., ~0.35% for Q235) and moderately higher than Q355B, strict welding procedures are non-negotiable.
| Ceq (IIW) Range | Welding Procedure Implication for Q390B |
|---|---|
| ~0.44 - 0.46% | Moderate Risk. Preheating is mandatory for most thicknesses (>10-15mm). Strict control of hydrogen sources required. |
| ~0.47 - 0.49% | High Risk. Higher preheat/interpass temperatures are required. May require the use of ultra-low hydrogen electrodes (<5ml/100g deposited metal). Post-weld heat treatment (PWHT) may be necessary for thick, highly restrained joints. |
Required Mitigation Measures:
Preheat & Interpass Temperature Control: This is the primary control. Preheating slows the cooling rate, allowing the HAZ to transform into softer, more ductile microstructures (e.g., bainite) instead of martensite. The required preheat temperature (e.g., 80°C - 150°C) is directly determined by the actual Ceq and plate thickness.
Ultra-Low Hydrogen Practice:
Use low-hydrogen (H4) or very low-hydrogen (H5, H10) classified welding consumables.
Strictly bake electrodes according to manufacturer specs.
Keep workpieces perfectly dry (no moisture, ice, rust).
Optimal Heat Input: Maintain heat input within a qualified range. Too low promotes fast cooling and martensite. Too high can excessively coarsen the HAZ grains, reducing toughness.
Post-Weld Heat Treatment (PWHT): For thick sections (often >30-40mm) or highly restrained joints, a stress relief bake (e.g., 550-600°C) is used to temper martensite, diffuse out hydrogen, and relieve stresses.
Welding Procedure Qualification (WPQR): A procedure qualification test must include hardness traverses across the HAZ to verify it does not exceed safe limits (typically 350 HV10) and that no cracks are present.
Summary
The Carbon Equivalent (Ceq) of Q390B, typically 0.44-0.49%, is a direct quantitative measure of its tendency to form hard, crack-sensitive martensite in the weld HAZ. This elevated Ceq compared to lower-strength grades fundamentally increases its susceptibility to cold cracking. Consequently, welding Q390B demands a rigorously controlled, low-hydrogen procedure with mandatory preheating, with parameters tailored to the specific Ceq value and thickness. Ignoring this relationship is the most common cause of catastrophic weld failures in high-strength steel structures.