What are the differences in the performance of Q345A, Q345B, Q345C, Q345D and Q345E steel?
Q345 is made of steel. It is a low-alloy steel (C<0.2%), which is widely used in construction, bridges, vehicles, ships, pressure vessels, etc. Q represents the yield strength of this material, and the following 345 refers to the yield value of this material, which is about 345MPa. and will decrease its yield value as the thickness of the material increases.
Q345 has good comprehensive mechanical properties, acceptable low-temperature performance, good plasticity and weldability, and is used as a structure, mechanical parts, building structure, general metal structural parts that bear dynamic loads such as medium and low pressure containers, oil tanks, vehicles, cranes, mining machinery, power stations, bridges, etc., and can be used in a hot rolled or normalized state, and can be used for various structures in cold areas below -40 °C.
Level classification
Q345 can be divided into Q345A, Q345B, Q345C, Q345D, Q345E according to grade. What they represent, mainly the temperature of the impact, is different.
- Q345A级,是不做冲击;
- Q345B, 20 degrees normal temperature impact;
- Q345C grade, is 0 degree impact;
- Q345D class, is -20 degree impact;
- Q345E, is -40 degree shock.
The value of the impact varies depending on the impact temperature.
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。
Contrast 16Mn
Q345 steel is a replacement for the old grades of 12MnV, 14MnNb, 18Nb, 16MnRE, 16Mn and other steel grades, rather than just replacing 16Mn steel. In terms of chemical composition, 16Mn is also different from Q345.
More importantly, there is a large difference in the size of the thickness grouping of the two steels according to the different yield strengths, which will inevitably cause changes in the allowable stress of some thicknesses of the material. Therefore, it is not appropriate to simply apply the allowable stress of 16Mn steel to Q345 steel, but to re-determine the allowable stress according to the new steel thickness grouping size.
The proportion of the main components of Q345 steel is basically the same as that of 16Mn steel, except that V, Ti and Nb trace alloying elements are added. A small amount of V, Ti and Nb alloy elements can refine the grains, greatly improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel.
Because of this, the thickness of the steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be better than that of 16Mn steel, especially its low-temperature performance is not available in 16Mn steel. 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 Yield Strength: ≥345 Elongation: ≥22
Q345B seamless pipe mechanical properties:
Tensile Strength: 490-675 Yield Strength: ≥345 Elongation: ≥21
Q345A seamless pipe mechanical properties:
Tensile Strength: 490-675 Yield Strength: ≥345 Elongation: ≥21
Q345C seamless pipe mechanical properties:
Tensile Strength: 490-675 Yield Strength: ≥345 Elongation: ≥22
Q345E seamless pipe mechanical properties:
Tensile Strength: 490-675 Yield Strength: ≥345 Elongation: ≥22
Product range
Q345D steel compared to Q345A, B, C steel. The test temperature of low-temperature impact energy is low. Good performance. The amount of P,S containing harmful materials is lower than that of Q345A,B,C.
The market price is higher than that of Q345A, B, and C.
Q345d definition: (1) It is composed of Q + number + quality grade symbol + deoxidation method symbol. Its steel number is crowned with "Q", which represents the yield point of the steel, and the following number indicates the yield point value, and the unit is MPa, for example, Q235 represents the yield point (σs) of carbon structural steel with 235MPa.
(2) If necessary, the symbol indicating the quality grade and deoxidation method can be marked after the steel number. The quality grade symbols are A, B, C, D.
Deoxidation method symbol: F represents boiling steel, b represents semi-calm steel: Z represents calm steel, TZ represents special calm steel, and calm steel can not be marked with symbols, that is, Z and TZ can not be marked. For example, Q235-AF indicates A-grade boiling steel.
(3) Carbon steel for special purposes, such as bridge steel, marine steel, etc., basically adopts the representation method of carbon structural steel, but the letter indicating the purpose is attached to the end of the steel number.
Q345 (low-alloy high-strength steel) online excerpt related information
Introduction to the material
1. The chemical composition of Q345 is as follows (%):
2.Q345C mechanical properties are as follows (%):
Mechanical performance indicators
Elongation (%)
The test temperature is 0°C
Tensile strength MPa
Surrender Point MPa
numeric value
≥22
≥34
470-650
324-259
When the wall thickness is between 16 and 35 mm, σs ≥ 325Mpa, and when the wall thickness is between 35 and 50 mm, σs ≥ 295 Mpa
2. Welding features of Q345 steel
2.1 Calculation of carbon equivalent (Ceq).
Ceq=C+Mn/6+Ni/15+Cu/15+Cr/5+Mo/5+V/5
Calculate Ceq = 0.49%, greater than 0.45%, it can be seen that the welding performance of Q345 steel is not very good, and strict process measures need to be formulated when welding.
2.2 Q345 steel is prone to problems when welding
2.2.1 Hardening tendency of the heat-affected zone
During the welding and cooling process of Q345 steel, the heat-affected zone is easy to form quenching structure-martensite, which increases the hardness of the proximal seam zone and decreases the plasticity. As a result, cracks occur after welding.
2.2.2 Cold crack susceptibility
The weld cracks of Q345 steel are mainly cold cracks.
Welding construction process
Bevel preparation→ spot solid welding→ preheating→ inside mouth welding→ back root cleaning (carbon arc gouging) → outer mouth welding, → inside mouth welding→ self-inspection/special inspection→ post-welding heat treatment→ non-destructive inspection (weld quality first-class qualified)
Selection of welding process parameters
Through the weldability analysis of Q345 steel, the following measures are formulated:
1. Selection of welding materials
Due to the large tendency of cold cracks of Q345 steel, low-hydrogen welding materials should be selected, and E5015 (J507) electrodes should be selected considering the principle that the welded joints should be as strong as the base metal.
The chemical composition is shown in the following table (%):
element
C
Minutes
Yes
S
P
Cr
With
V
In
content
0.071
1.11
0.53
0.009
0.016
0.02
0.01
0.01
0.01
The mechanical properties are shown in the following table:
σb/Mpa
σs/Mpa
D5(%)
Ψ（%）
Akv/J-30℃
440
540
31
79
164/114/76
(Tensile strength should be greater than yield)
2. Bevel form: (according to drawings and equipment supply)
3. Welding method: manual arc welding (D) is adopted.
4. Welding current: In order to avoid the coarse structure of the weld and the reduction of impact toughness, it is necessary to adopt small standard welding. The specific measures are: the selection of small diameter electrode, narrow weld bead, thin weld layer, multi-layer multi-channel welding process (the welding sequence is shown in Figure 1). The width of the weld bead is not more than 3 times that of the welding rod, and the thickness of the weld layer is not more than 5mm. The first layer to the third layer uses Ф3.2 welding electrode with welding current of 100-130A, and the fourth to sixth layer uses Ф4.0 welding electrode with welding current of 120-180A.
5. Preheating temperature: Since the Ceq > of Q345 steel is 0.45%, it should be preheated before welding, the preheating temperature T0=100-150°C, and the interlayer temperature Ti≤400°C.
6. Post-weld heat treatment parameters: In order to reduce the residual stress of welding, reduce the hydrogen content in the weld, and improve the metal structure and properties of the weld, the weld should be heat treated after welding. The heat treatment temperature is: 600-640 °C, the constant temperature time is 2 hours (when the plate thickness is 40mm), and the temperature rise and fall speed is 125 °C/h.
On-site welding sequence
1. Preheat before welding
Before the flange plate is welded, the flange plate is first preheated, and the welding starts after 30 minutes of constant temperature. The preheating, interlayer temperature and heat treatment of welding are automatically controlled by the heat treatment temperature control cabinet, and the far-infrared crawler heating furnace is adopted, the microcomputer automatically sets the curve and records the curve, and the thermocouple measures the temperature. When preheating, the measuring point of the thermocouple is 15mm-20mm away from the edge of the groove.
2. Soldering
2.1 In order to prevent welding deformation, each column joint adopts two-person symmetrical welding, and the welding direction is welded from the middle to both sides. When welding the inner mouth (the inner mouth is a groove close to the web), the first to third layers must be operated using a small specification, because its welding is the main cause of welding deformation. After the welding of one to three layers, the back side is cleaned. After the use of carbon arc gouging to clean the roots, the weld must be mechanically polished, and the surface of the weld must be cleaned for carburizing, exposing the metallic luster, and preventing cracks caused by serious carbonization of the surface layer. The outer mouth welding should be completed at one time, and the remaining part of the inner mouth should be welded at the end.
2.2 When welding the second layer, the welding direction should be opposite to the direction of the first layer, and so on. Each layer of welded joints should be staggered by 15-20mm.
2.3 The welding current, welding speed and number of welding layers should be consistent between the two welders when welding.
2.4 In the welding, the welding should start from the arc striking plate and end on the arc closing plate. After the welding is completed, cut off and sanded clean.
3. Post-weld heat treatment: Heat treatment should be carried out within 12 hours after the weld welding is completed. If heat treatment cannot be carried out in time, heat preservation and slow cooling measures should be taken. When heat treatment, two thermocouples should be used to measure the temperature, and the thermocouples should be spot welded on the inside and outside of the weld.
4. Welding inspection
According to the requirements of the "Code for Construction and Acceptance of Steel Structure Engineering", the weld is inspected by ultrasonic flaw detection method, and the inspection ratio is 100%.
On-site technical management
1. Prepare detailed welding construction work instructions.
2. Whole-process control of the welding process is the core to ensure quality.
When welding each column joint, there should be a special person to monitor the welding process, and if the welder does not follow the work instructions, the welding should be terminated immediately. During the welding process, the heat treatment personnel should monitor the interlayer temperature throughout the process, and if the standard is exceeded, the welder should be notified immediately to stop.
3. Improving the quality awareness of construction personnel is the key to implementing the welding process
Before construction, all staff should be made to make a disclosure and a construction process card should be issued. In the disclosure, the characteristics of the welding process and the necessity and control points of strict control of the on-site welding process are explained in detail.
conclusion
According to the construction of this welding process measure, there are 102 welded joints on site, and the pass rate of one pass rate is 100% after non-destructive inspection. After the verification of actual construction, this welding process measure can not only guide the welding of Q345 steel on site, but also ensure the welding quality.