Q345 is a type of steel. It is a low-alloy steel (C < 0.2%) widely used in buildings, bridges, vehicles, ships, and pressure vessels. The "Q" represents the material's yield strength, and the "345" designation refers to its yield value, which is approximately 345 MPa. This value decreases with increasing thickness.
Q345 offers excellent overall mechanical properties, acceptable low-temperature performance, and good plasticity and weldability. It is used in structures subject to dynamic loads, such as medium- and low-pressure vessels, oil tanks, vehicles, cranes, mining machinery, power plants, and bridges, as well as mechanical parts, building structures, and general metal structural parts. It can be used in hot-rolled or normalized conditions and can be used in various structures in regions colder than -40°C.
Grade Classification
Q345 is categorized by grade as Q345A, Q345B, Q345C, Q345D, and Q345E. These grades primarily represent differences in impact temperatures. Q345A grade is not subjected to shock testing; Q345B grade is subjected to shock testing at room temperature (20°C); Q345C grade is subjected to shock testing at 0°C; Q345D grade is subjected to shock testing at -20°C; and Q345E grade is subjected to shock testing at -40°C.
The shock values vary at different shock temperatures. Chemical Composition
Q345A: C ≤ 0.20, Mn ≤ 1.7, Si ≤ 0.55, P ≤ 0.045, S ≤ 0.045, V 0.02-0.15;
Q345B: C ≤ 0.20, Mn ≤ 1.7, Si ≤ 0.55, P ≤ 0.040, S ≤ 0.040, V 0.02-0.15;
Q345C: C ≤ 0.20, Mn ≤ 1.7, Si ≤ 0.55, P ≤ 0.035, S ≤ 0.035, V 0.02-0.15, Al ≥ 0.015;
Q345D: C ≤ 0.20, Mn ≤ 1.7, Si ≤ 0.55, P ≤ 0.030, S ≤ 0.030, V 0.02-0.15, Al ≥ 0.015;
Q345E: C ≤ 0.20, Mn ≤ 1.7, Si ≤ 0.55, P ≤ 0.025, S ≤ 0.025, V 0.02-0.15, Al ≥ 0.015;
Comparison with 16Mn
Q345 steel replaces several older steel grades, including 12MnV, 14MnNb, 18Nb, 16MnRE, and 16Mn, rather than simply replacing 16Mn. 16Mn and Q345 differ in chemical composition.
More importantly, the thickness groups of the two steels, based on yield strength, differ significantly, which inevitably results in changes in the allowable stresses of certain thicknesses. Therefore, it is inappropriate to simply apply the allowable stress of 16Mn steel to Q345 steel. Instead, the allowable stress should be redefined based on the new steel thickness groups. The ratio of the main elements in Q345 steel is essentially the same as that in 16Mn steel, differing only in the addition of trace alloying elements such as V, Ti, and Nb. These small amounts of V, Ti, and Nb refine the grain size, significantly increasing the steel's toughness and overall mechanical properties.
This also allows for thicker steel plates. Therefore, Q345 steel's overall mechanical properties should be superior to those of 16Mn steel, particularly its low-temperature performance, which 16Mn steel lacks. [Official account @Mechanical Knowledge Network: Sharing knowledge, spreading value.] The allowable stress of Q345 steel is slightly higher than that of 16Mn steel.
Performance Comparison
Q345D Seamless Pipe Mechanical Properties:
Tensile Strength: 490-675°F, Yield Strength: ≥345°F, Elongation: ≥22°F
Q345B Seamless Pipe Mechanical Properties:
Tensile Strength: 490-675°F, Yield Strength: ≥345°F, Elongation: ≥21°F
Q345A Seamless Pipe Mechanical Properties:
Tensile Strength: 490-675°F, Yield Strength: ≥345°F, Elongation: ≥21°F
Q345C Seamless Pipe Mechanical Properties:
Tensile Strength: 490-675°F, Yield Strength: ≥345°F, Elongation: ≥22°F
Q345E Seamless Pipe Mechanical Properties:
Tensile Strength: 490-675°F, Yield Strength: ≥345°F, Elongation: ≥22°F
Product Series
Q345D steel has a lower low-temperature impact energy test temperature than Q345A, B, and C steels. It offers superior performance. The amount of hazardous materials, P and S, is lower than that of Q345A, B, and C.
The market price is higher than that of Q345A, B, and C.
Definition of Q345d: ① It consists of Q + a number + a quality grade symbol + a deoxidation method symbol. The "Q" prefix in the steel number represents the steel's yield point, and the following number represents the yield point value in MPa. For example, Q235 indicates a carbon structural steel with a yield point (σs) of 235 MPa.
② If necessary, the steel number may be followed by a symbol indicating the quality grade and deoxidation method. The quality grade symbols are A, B, C, and D. Official account @Mechanical Knowledge Network: Sharing knowledge, spreading value.
Deoxidation method symbol: F indicates rimmed steel; b indicates semi-killed steel; Z indicates killed steel; and TZ indicates special killed steel. Killed steel may be omitted; both Z and TZ are optional. For example, Q235-AF indicates Grade A rimmed steel. ③ Carbon steel for specialized applications, such as bridge steel and shipbuilding steel, is generally designated as carbon structural steel, but with a letter indicating the intended use appended to the end of the steel grade.
Q345 (low-alloy high-strength steel) - Excerpt from online sources 2. Welding Characteristics of Q345 Steel
2.1 Calculation of Carbon Equivalent (Ceq)
Ceq = C + Mn/6 + Ni/15 + Cu/15 + Cr/5 + Mo/5 + V/5
The calculated Ceq = 0.49%. This value, exceeding 0.45%, indicates that the weldability of Q345 steel is not very good, requiring strict welding procedures.
2.2 Common Problems in Welding Q345 Steel
2.2.1 Hardening Tendency of the Heat-Affected Zone
During the cooling process of welding, the heat-affected zone (HAZ) of Q345 steel tends to form a quenched structure-martensite-which increases the hardness and decreases the plasticity of the area near the weld. This results in cracks after welding.
2.2.2 Cold Crack Susceptibility
Welding cracks in Q345 steel are primarily cold cracks. Welding Process
Groove Preparation → Tack Welding → Preheating → Internal Welding → Back Cleaning (Carbon Arc Gouging) → External Welding → Internal Welding → Self-Inspection/Special Inspection → Post-Weld Heat Treatment → Non-Destructive Testing (Weld Quality Level 1 Pass)
Welding Parameter Selection
Based on an analysis of the weldability of Q345 steel, the following measures were formulated:
1. Welding Material Selection
Due to the high cold cracking tendency of Q345 steel, low-hydrogen welding materials should be selected. Furthermore, considering the principle that the weld joint should be as strong as the parent material, E5015 (J507) welding electrodes were selected.
Q345D seamless pipe mechanical properties: tensile strength: 490-675 yield strength: ≥345 elongation: ≥22Q345B seamless pipe mechanical properties: tensile strength: 490-675 yield strength: ≥345 elongation: ≥21Q345A seamless pipe mechanical properties: tensile strength: 490-675 yield strength: ≥345 elongation: ≥21Q345C seamless pipe mechanical properties: tensile strength: 490-675 yield strength: ≥345 elongation: ≥22Q345E seamless pipe mechanical properties: tensile strength: 490-675 yield strength: ≥345 elongation: ≥22
