Q345 is a type of steel. It is a low-alloy steel (C < 0.2%), widely used in construction, bridges, vehicles, ships, pressure vessels, etc. The "Q" represents the yield strength of this material, and "345" indicates its yield value, which is approximately 345 MPa. The yield value decreases as the thickness of the material increases.
Q345 has good overall mechanical properties, acceptable low-temperature performance, and good plasticity and weldability. It is used for medium and low-pressure vessels, oil tanks, vehicles, cranes, mining machinery, power plants, bridges, and other structures subjected to dynamic loads, as well as mechanical parts, building structures, and general metal structural components. It is used in hot-rolled or normalized conditions and can be used in various structures in cold regions below -40℃.
Image Grade Classification
Q345 is classified into four grades: Q345A, Q345B, Q345C, Q345D, and Q345E. The main difference between them is their impact temperature.
Q345A grade: No impact testing;
Q345B grade: Impact test at 20°C (normal temperature);
Q345C grade: Impact test at 0°C;
Q345D grade: Impact test at -20°C;
Q345E grade: Impact test at -40°C.
The impact values vary depending on the impact temperature.
Image 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;
Image Comparison with 16Mn
Q345 steel is a replacement for several older steel grades, including 12MnV, 14MnNb, 18Nb, 16MnRE, and 16Mn, not just 16Mn steel. The chemical composition of 16Mn and Q345 is also different.
More importantly, the thickness grouping dimensions of the two steels based on their yield strength differ significantly, which will inevitably cause changes in the allowable stress of materials of certain thicknesses. Therefore, it is inappropriate to simply apply the allowable stress of 16Mn steel to Q345 steel; the allowable stress should be re-determined based on the new steel thickness grouping dimensions. The main component ratio of Q345 steel is basically the same as that of 16Mn steel, the difference being the addition of trace alloying elements V, Ti, and Nb. These small amounts of V, Ti, and Nb refine the grain structure, significantly improving the steel's toughness and overall mechanical properties.
This also allows for the production of thicker steel plates. Therefore, the overall mechanical properties of Q345 steel should be superior to those of 16Mn steel, especially its low-temperature performance, which 16Mn steel lacks. The allowable stress of Q345 steel is slightly higher than that of 16Mn steel.
Image
Image Performance Comparison
Q345D Seamless Steel Pipe Mechanical Properties:
Tensile Strength: 490-675 Nm; Yield Strength: ≥345 Nm; Elongation: ≥22%
Q345B Seamless Steel Pipe Mechanical Properties:
Tensile Strength: 490-675 Nm; Yield Strength: ≥345 Nm; Elongation: ≥21%
Q345A Seamless Steel Pipe Mechanical Properties:
Tensile Strength: 490-675 Nm; Yield Strength: ≥345 Nm; Elongation: ≥21%
Q345C Seamless Steel Pipe Mechanical Properties:
Tensile Strength: 490-675 Nm; Yield Strength: ≥345 Nm; Elongation: ≥22%
Q345E Seamless Steel Pipe Mechanical Properties:
Tensile Strength: 490-675 Nm; Yield Strength: ≥345 Nm; Elongation: ≥22%
Image Product Series
Comparison of Q345D steel with Q345A, B, and C steels. The low-temperature impact energy test temperature is low. It has good performance. The content of harmful substances P and S is lower than Q345A, B, and C.
The market price is higher than Q345A, B, and C.
Q345d definition: ① It consists of Q + number + quality grade symbol + deoxidation method symbol. Its steel grade is prefixed with "Q," representing the yield point of the steel. The following number indicates the yield point value in MPa. For example, Q235 represents carbon structural steel with a yield point (σs) of 235 MPa.
② If necessary, symbols indicating the quality grade and deoxidation method can be added after the steel grade. The quality grade symbols are A, B, C, and D.
Deoxidation method symbols: F indicates rimmed steel; b indicates semi-killed steel; Z indicates killed steel; TZ indicates special killed steel. Killed steel may not have a symbol, i.e., both Z and TZ may be omitted. For example, Q235-AF indicates grade A rimmed steel.
③ Carbon steel for special purposes, such as bridge steel and marine steel, basically adopts the designation method for carbon structural steel, but with an additional letter indicating the application added to the end of the steel grade.
Q345 (Low Alloy High Strength Steel) - Excerpt from online materials
Image: Material Introduction
1. The chemical composition of Q345 is shown in the table below (%):
Element
C≤
Mn
Si≤
P≤
S≤
Al≥
V
Nb
Ti
Content
0.2
1.0-1.6
0.55
0.035
0.035
0.015
0.02-0.15
0.015-0.06
0.02-0.2
2. The mechanical properties of Q345C are shown in the table below (%):
Mechanical property index
Elongation (%)
Test temperature 0℃
Tensile strength MPa
Yield point MPa≥
Value
δ5≥22
J≥34
σb (470-650)
σs (324-259)
Where the wall thickness is between 16-35mm, σs≥325Mpa; where the wall thickness is between 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
Calculated Ceq=0.49%, greater than 0.45%, indicating that the weldability of Q345 steel is not very good, and strict process measures need to be formulated during welding.
2.2 Common Problems in Welding Q345 Steel
2.2.1 Hardening Tendency of the Heat-Affected Zone
During the cooling process of welding Q345 steel, the heat-affected zone is prone to forming a quenched structure-martensite, which increases the hardness and decreases the plasticity near the weld. This results in post-weld cracking.
2.2.2 Cold Cracking Sensitivity
The main welding cracks in Q345 steel are cold cracks.
Welding Construction Process
Groove Preparation → Tack Welding → Preheating → Inner Edge Welding → Back Root Cleaning (Carbon Arc Gouging) → Outer Edge Welding → Inner Edge Welding → Self-Inspection/Special Inspection → Post-Weld Heat Treatment → Non-Destructive Testing (Weld Quality Grade 1 Qualified)
Selection of Welding Process Parameters
Based on the weldability analysis of Q345 steel, the following measures are formulated:
1. Selection of Welding Materials
Due to the high tendency of Q345 steel to cold crack, low-hydrogen welding materials should be selected. Considering the principle that the weld joint should have equal strength to the base metal, E5015 (J507) type welding electrodes are selected.
Chemical composition is shown in the table below (%):
Elements
C
Mn
Si
S
P
Cr
Mo
V
Ti
Content
0.071
1.11
0.53
0.009
0.016
0.02
0.01
0.01
0.01
Mechanical properties are shown in the table below:
Mechanical property indicators
σb (MPa)
σs (MPa)
δ5 (%)
Ψ (%)
AkvJ-30℃
Value
440
540
31
79
164 114 76
(Tensile strength should be greater than yield strength)
2. Bevel type: (Supply according to drawings and equipment)
3. Welding method: Manual arc welding (D).
4. Welding Current: To avoid coarse weld microstructure and reduced impact toughness, small-scale welding parameters must be used. Specific measures include: using small-diameter electrodes, narrow weld beads, thin weld layers, and a multi-layer, multi-pass welding process (welding sequence shown in Figure 1). The width of the weld bead should not exceed three times the electrode length, and the weld layer thickness should not exceed 5mm. For the first to third layers, use Ф3.2 electrodes with a welding current of 100-130A; for the fourth to sixth layers, use Ф4.0 electrodes with a welding current of 120-180A.
5. Preheating Temperature: Since the Ceq of Q345 steel is >0.45%, preheating is necessary before welding. The preheating temperature T0 = 100-150℃, and the interpass temperature Ti ≤ 400℃.
6. Post-Weld Heat Treatment Parameters: To reduce residual welding stress, decrease the hydrogen content in the weld, and improve the weld's microstructure and properties, post-weld heat treatment is required. The heat treatment temperature is 600-640℃, the holding time is 2 hours (for a plate thickness of 40mm), and the heating/cooling rate is 125℃/h.
On-site Welding Sequence
1. Preheating Before Welding
Before welding the flange plates, preheat them for 30 minutes before starting welding. Preheating, interpass temperature, and heat treatment are automatically controlled by a heat treatment temperature control cabinet using a far-infrared conveyor belt heating furnace. A microcomputer automatically sets and records the heating curves, and thermocouples measure the temperature. During preheating, the thermocouple measuring points are 15mm-20mm away from the bevel edge.
2. Welding
2.1 To prevent welding deformation, each column joint is welded symmetrically by two people, with the welding direction from the center outwards. When welding the inner edge (the bevel near the web), the first to third layers must use a small-scale operation, as this is the main cause of welding deformation. After welding the first to third layers, the back side is cleaned. After carbon arc gouging, the weld must be mechanically ground to remove surface carburization, exposing the metallic luster and preventing severe surface carbonization that could cause cracks. The outer weld should be completed in one pass, with the remaining inner weld completed last.
2.2 When welding the second layer, the welding direction should be opposite to the first layer, and so on. Each weld joint should be staggered by 15-20mm.
2.3 The welding current, welding speed, and number of weld layers should be consistent for both welders.
2.4 Welding should begin on the arc-starting plate and end on the arc-ending plate. After welding, cut off the weld and grind it clean.
3. Post-weld heat treatment: The weld joint should be heat-treated within 12 hours of completion. If heat treatment cannot be performed immediately, heat preservation and slow cooling measures should be taken. During heat treatment, two thermocouples should be used for temperature measurement, with the thermocouples spot-welded to the inside and outside of the weld joint.
4. Welding Inspection
According to the requirements of the *Code for Construction and Acceptance of Steel Structures*, ultrasonic testing was used to inspect weld joints, with an inspection rate of 100%.
On-site Technical Management
1. Detailed welding operation instructions were prepared.
2. Full-process control of the welding process is the core of ensuring quality.
During the welding of each column joint, a designated person should monitor the welding process. If the welder does not follow the operation instructions, welding should be stopped immediately. During the welding process, heat treatment personnel should monitor the interpass temperature throughout; if it exceeds the standard, the welder should be notified immediately to stop the welding.
3. Improving the quality awareness of construction personnel is key to implementing the welding process.
Before construction, a full briefing was conducted for all personnel, and construction process cards were issued. The briefing explained in detail the characteristics of the welding process and the necessity and key points of strictly controlling the on-site welding process.
Conclusion
Following this welding process measure, a total of 102 weld joints were welded on-site, and the first-pass yield rate of non-destructive testing reached 100%. Verified through actual construction, this welding process not only provides on-site guidance for welding Q345 steel, but also ensures welding quality.
