Weldability of Commonly Used Metal Materials

1. What is weldability? Discuss the weldability of carbon steel. Weldability refers to a material's ability to be welded into components meeting specified design requirements under given construction conditions and to satisfy intended service requirements. Weldability is influenced by four factors: material, welding method, component type, and service requirements. Carbon steel is an iron-carbon alloy based on iron, with carbon as the primary alloying element, whose mass fraction does not exceed 1%. Additionally, the mass fractions of manganese and silicon do not exceed 1.2% and 0.5%, respectively; these two elements are not considered alloying elements. Other elements such as Ni, Cr, and Cu are kept within residual limits and are not used as alloying elements. Impurity elements like S, P, O, and N are strictly controlled according to steel grade and type. Therefore, the weldability of carbon steel mainly depends on its carbon content. As carbon content increases, weldability gradually deteriorates, with low-carbon steel exhibiting the best weldability.

| Carbon Content (%) | Typical Hardness | Typical Applications | Weldability |
|--------------------|------------------|------------------------|-------------|
| Low-carbon steel   | ≤0.15            | 60 HRB                 | Special sheets and profiles, thin sheets, strips, welding wires | Excellent |
| 0.15–0.25          | 90 HRB           | Structural sections, plates, rods | Good        |
| Medium-carbon steel| 0.25–0.60        | 25 HRC                 | Machine parts and tools | Moderate (preheating and post-heating required; low-hydrogen welding methods recommended) |
| High-carbon steel  | ≥0.60            | 40 HRC                 | Springs, molds, rails | Poor (preheating and post-heating required; low-hydrogen welding materials mandatory) |

2. What is carbon equivalent? How is the carbon equivalent calculated for carbon steel? The carbon equivalent is defined as the equivalent carbon content obtained by converting the contents of alloying elements (including carbon) in steel into their respective effects on hardenability. It serves as a reference indicator for assessing steel weldability. In addition to carbon, the main alloying elements in carbon steel are Mn and Si. Increasing their contents reduces weldability, although their effects are less pronounced than those of carbon. The International Institute of Welding recommends the following carbon equivalent formula:

As the carbon equivalent value increases, steel weldability deteriorates. When the CE value exceeds 0.4%–0.6%, susceptibility to cold cracking increases, requiring preheating, post-heating, and use of low-hydrogen welding materials during welding, along with other process measures.

3. What are the limitations of using carbon equivalent to evaluate steel weldability? The carbon equivalent can only provide a general, relative assessment of steel weldability within certain limits. This is because: (1) Two steels may have equal carbon equivalent values but different carbon contents. Steel with higher carbon content tends to form hardened microstructures during welding, resulting in greater crack sensitivity compared to lower carbon steel, thus poorer weldability. Therefore, equal carbon equivalent values do not necessarily imply identical weldability. (2) The carbon equivalent calculation reflects only the influence of chemical composition on weldability and does not account for cooling rates, which affect microstructure formation. Faster cooling promotes martensitic structures, reducing weldability. (3) Factors affecting weld metal microstructure—such as peak temperature and dwell time at high temperatures during the welding cycle—are not included in the carbon equivalent formula. Thus, the carbon equivalent formula provides only a general, relative evaluation of weldability within specific steel types and cannot serve as an accurate assessment criterion.

4. Discuss the weldability of low-carbon steel. Due to its low carbon content and relatively low levels of Mn and Si, low-carbon steel generally does not develop severe hardened or quenched microstructures during welding. After welding, the joint retains good ductility and impact toughness. Typically, no preheating, interpass temperature control, or post-weld heat treatment is required. No special process measures are needed throughout the welding process, making low-carbon steel highly weldable. However, in some cases, welding difficulties may arise: 1) Converter steel produced using old smelting methods has high nitrogen content and a higher level of impurities, resulting in increased cold brittleness and susceptibility to aging, which reduces weld joint quality and worsens weldability. 2) Killed steel is incompletely deoxidized, leading to relatively high oxygen content and uneven distribution of impurities such as phosphorus (P), with localized concentrations exceeding allowable limits; this increases sensitivity to aging and cold brittleness, as well as the tendency for hot cracking. 3) Using substandard welding electrodes can result in excessive carbon and sulfur content in the weld metal, causing cracks. For example, when an acid electrode was used to weld Q235-A steel at one plant, the high carbon content in the ferromanganese coating caused hot cracking in the weld. 4) Certain welding methods can degrade the quality of low-carbon steel weld joints. For instance, submerged arc welding, due to its high heat input, produces very coarse grains in the coarse grain zone of the heat-affected zone, significantly reducing impact toughness. Therefore, normalizing treatment must be performed after welding to refine the grain structure and improve impact toughness. In summary, low-carbon steel is among the most weldable and easiest-to-weld types of steel, and all welding methods are suitable for welding low-carbon steel.

5 How to properly select welding materials when welding low-carbon steel?  
(1) Selection of SMAW electrodes  
The average tensile strength of commonly used low-carbon steel Q235 is 417.5 MPa. According to the principle of equal strength, the matching electrode should be from the E43 series. The following table shows electrode selection for manual arc welding of various grades of low-carbon steel:

| Steel Grade | General Structures | Dynamic Loads, Complex or Thick Plate Structures, Pressure Vessels, Boiler Components, Low-Temperature Welding |
|-------------|--------------------|------------------------------------------------------------------------------------------------------------------|
| Q235        | E4313, E4303, E4301, E4320, E4311 | E4316, E4315 (E5016, E5015) |
| Q255        | Generally no preheating | Generally no preheating |
| Q275        | E4316, E4315 | E5016, E5015 |
| Thick plate structures, preheat above 150°C | 08, 10, 15, 20 | E4303, E4301, E4320, E4311 | E4316, E4315 (E5016, E5015) | Generally no preheating |
| 25          | E4316, E4315 | E5016, E5015 | Thick plate structures, preheat above 150°C |
| 20g, 22g    | E4303, E4301 | E4316, E4315 (E5016, E5015) | Thick plate structures, preheat 100–150°C |
| 20R         | E4303, E4301 | E4316, E4315 (E5016, E5015) | Generally no preheating |

Note: Electrode models in parentheses indicate acceptable substitutes.

(2) Matching selection of wire and flux for submerged arc welding  
For submerged arc welding of low-carbon steel, proper matching between wire and flux is essential.

(3) Selection of CO₂ welding wire  
Solid wires are typically selected from grades H08Mn2Si and H08Mn2SiA, which produce slightly higher strength in deposited metal. Flux-cored wires include YJ502-1, YJ506-2, YJ506-3, and YJ506-4.

(4) Matching selection of wire and flux for electroslag welding  
In electroslag welding, the molten pool temperature is lower than in submerged arc welding, so the reduction effect of silicon and manganese in the flux is weaker. Therefore, wires with higher manganese and silicon content should be selected. Common combinations include H10Mn2 or H10MnSi wire paired with flux HJ360, or H10MnSi wire paired with flux HJ431.

6 How to weld low-carbon steel at low temperatures?  
When welding low-carbon steel structures in severe winter conditions, the rapid cooling of the weld joint increases the risk of cracking, especially in thick sections where the first pass is prone to cracking. To prevent this, the following process measures must be taken: 1) Preheat before welding and strictly maintain interpass temperature not below the preheat temperature during welding. 2) Use low-hydrogen or ultra-low-hydrogen welding materials. 3) Increase the welding current and slow down the welding speed during tack welding, appropriately increase the cross-sectional area and length of the tack welds, and preheat when necessary.  
4) The entire weld should be completed in one continuous pass whenever possible to avoid interruptions.  
5) Do not strike an arc on base metal outside the groove face; backfill the crater completely when ending the arc.  
6) Avoid bending plates, correcting, and assembling welded components under low-temperature conditions as much as possible.