Welding of commonly used steel pipe metal materials

1. What is weldability? Try to describe the weldability of carbon steel.

Weldability refers to the ability of materials to be welded into components according to specified design requirements under limited construction conditions and meet the predetermined service requirements. Weldability is affected by four factors: material, welding method, component type, and use requirements. Carbon steel is based on iron, an iron-carbon alloy, with carbon as an alloying element. The mass fraction of carbon does not exceed 1%. In addition, the mass fraction of manganese does not exceed 1.2%, and the mass fraction of silicon does not exceed 0.5%. The latter two are not alloying elements. Other elements such as Ni, Cr, Cu, etc. are controlled within the limit of residual amount and are not used as alloying elements. Impurity elements such as S, P, O, N, etc. are strictly restricted according to different types and grades of steel. Therefore, the weldability of carbon steel mainly depends on the carbon content. As the carbon content increases, the weldability gradually deteriorates, among which low-carbon steel has the best weldability.

2. What is carbon equivalent? How to calculate the carbon equivalent of carbon steel?

The content of alloying elements (including carbon) in steel converted into equivalent carbon content according to their effects is called the carbon equivalent of the steel, which can be used as a reference indicator for evaluating the weldability of steel.

In addition to C, the elements in carbon steel are mainly Mn and Si. As their content increases, the weldability deteriorates, but their effect is not as strong as carbon.

As the carbon equivalent value increases, the weldability of the steel will deteriorate. When the CE value is greater than 0.4% to 0.6%, the sensitivity of cold cracks will increase, and a series of process measures such as preheating, post-heating, and welding with low-hydrogen welding materials are required during welding.

3. What are the limitations of using carbon equivalent values to evaluate the weldability of steel?

The carbon equivalent value can only be used to evaluate the weldability of steel in a general and relative manner within a certain range. This is because:

1) If the carbon equivalent values of two sheets of steel are equal, but the carbon content is different, the steel with a higher carbon content is more likely to produce hardened structures during welding, and its crack tendency is greater than that of the steel with a lower carbon content, and its weldability is poorer. Therefore, when the carbon equivalent values of steels are equal, it cannot be considered that the weldability is the same.

2) The calculated carbon equivalent value only expresses the effect of chemical composition on weldability and does not take into account that different cooling rates can produce different structures. When the cooling rate is fast, hardened structures are easily produced, and weldability will deteriorate.

3) In addition to chemical composition and cooling rate, the factors that affect the weld metal structure and thus the weldability include parameters such as the maximum heating temperature and the dwell time at high temperatures in the welding cycle, which are not expressed in the carbon equivalent value calculation formula.

Therefore, the calculation formula of carbon equivalent value can only evaluate the weldability of steel in a general and relative manner within a certain range of steel grades, and cannot be used as an accurate evaluation index.

4. Describe the weldability of low-carbon steel.

Because low carbon steel has low carbon content, and low manganese and silicon content, it usually does not produce severe hardening or quenching structure due to welding. The plasticity and impact toughness of the joint after low-carbon steel welding are good. During welding, it is generally not necessary to preheat and control the interlayer temperature and post-heat. It is not necessary to use heat treatment to improve the structure after welding. No special process measures are required for the entire welding process, and the weldability is excellent.

However, in a few cases, difficulties may occur during welding:

1) The converter steel produced by the old smelting method has a high nitrogen content and a high impurity content, which leads to high cold brittleness, increased aging sensitivity, reduced quality of welded joints, and poor weldability.

2) The boiling steel is not completely deoxidized, has a high oxygen content, and impurities such as P are unevenly distributed. The content in some areas will exceed the standard, the aging sensitivity and cold brittleness sensitivity are high, and the tendency of hot cracks is also increased.

3) Using welding rods that do not meet the quality requirements will cause the carbon and sulfur content in the weld metal to be too high, which will lead to cracks. For example, when a factory uses acid welding rods to weld Q235-A steel, the carbon content of ferromanganese in the welding rod coating is too high, which will cause thermal cracks in the weld.

4) Some welding methods will reduce the quality of low-carbon steel welded joints. For example, electro-slag welding, due to the large line energy, will make the grains in the coarse grain area of the welding heat-affected zone grow very coarse, causing a serious decrease in impact toughness. After welding, it is necessary to perform normalizing treatment to refine the grains to improve the impact toughness.

In short, low-carbon steel is the steel with the best weldability and the easiest to weld. All welding methods can be applied to the welding of low-carbon steel.

5. How to correctly select welding materials when welding low-carbon steel?

⑴ Selection of manual arc welding electrodes The average tensile strength of the commonly used low-carbon steel Q235 is 417.5MPa. According to the principle of equal strength, the matching welding rod should be the E43 series.

⑵ Matching and selection of submerged arc welding wire and flux Matching and selection of welding wire and flux for low carbon steel submerged arc welding.

⑶ Selection of CO2 welding wire The solid welding wire is selected from two grades: H08Mn2Si and H08Mn2SiA. The strength of the deposited metal after welding is relatively high. The grades of flux-cored welding wire are YJ502-1, YJ506-2, YJ506-3, and YJ506-4.

⑷ Matching and selection of electro-slag welding wire and flux The molten pool temperature of electro-slag welding is lower than that of submerged arc welding, so the reduction effect of silicon and manganese in the flux is weak, and welding wire with higher manganese and silicon content should be selected. H10Mn2, H10MnSi welding wire with flux HJ360, or H10MnSi welding wire with flux HJ431 is often used.

6. How to weld low-carbon steel at low temperatures?

When welding low-carbon steel structures under severe winter conditions, the crack tendency increases due to the rapid cooling rate of the welded joint, especially when the first weld of thick and large structures is prone to cracking. Therefore, the following process measures must be taken:

1) Preheat before welding, and strictly keep the interlayer temperature not lower than the preheating temperature during welding.

2) Use low hydrogen or ultra-low hydrogen welding materials.

3) Increase the welding current during positioning welding, slow down the welding speed, appropriately increase the cross-sectional area and length of the positioning weld, and preheat if necessary.

4) The entire weld should be welded continuously as much as possible to avoid interruption.

5) The arc should not be struck on the parent material other than the groove surface, and the arc pit should be filled when the arc is extinguished.

6) Try not to bend, correct, and assemble weldments under low temperature conditions.

Preheating temperature for low temperature welding of various metal structures. Preheating temperature for low temperature welding of pipelines and pressure vessels.