How can one ensure that there are no cracks when welding SG295 steel?
In the manufacturing of gas cylinders, welding is the second most crucial process after stamping. Whether it is circumferential welding or longitudinal welding, if cracks occur, the entire gas cylinder will be directly scrapped and may even lead to serious safety accidents.
To ensure there are no cracks in the welding, one cannot merely focus on the welder's skills and the welding materials. The chemical composition of the base material (SG295 steel coil) itself is the first genetic factor determining the welding quality. The strict limitations in the JIS G3116 standard on C (carbon), P (phosphorus), and S (sulfur) essentially cut off the "spark" of welding cracks at the source.
Low carbon content (C ≤ 0.20%) - Prevents "quench hardening" and "cold cracking"
Carbon is the most effective element determining the strength of steel, but it is also the number one enemy of welding performance.
Academic mechanism:
Control of carbon equivalent (Ceq): During welding, the weld seam and the heat-affected zone (HAZ) undergo rapid heating and cooling. If the carbon content is too high, during the cooling process, the austenite will transform into high-carbon martensite. This is a very hard and brittle structure with severely distorted internal lattice and extremely high internal stress.
Formation of cold cracks (delayed cracks): This high-carbon martensite structure is extremely sensitive to hydrogen (which comes from the flux of the welding rod or moisture in the air). Hydrogen atoms diffuse into the martensite lattice and accumulate at areas of high internal stress concentration, causing the grain boundaries to crack. These cracks usually do not appear until several hours or even days after welding, hence they are called "delayed cracks", which are highly concealed and destructive.
Product advantages:
The standard requires C to be less than or equal to 0.20%, but the typical value you provided is 0.16 - 0.18%.
What does this mean? It means that your material has a lower carbon content. Even without using the costly preheating and post-heating treatments, the tendency for the heat-affected zone to form brittle martensite is significantly reduced. For gas cylinder manufacturers, this lowers their stringent requirements for the welding preheating temperature, not only saving energy, but more importantly, reducing the risk of latent cracks caused by uneven temperature control at the site.
Low phosphorus (P) content - Eliminating "cold brittleness" and "grain boundary weakening"
Phosphorus is usually regarded as a harmful impurity in steel, coming from iron ore and the smelting process.
Academic mechanism:
Inhomogeneity and grain boundary embrittlement: During solidification, phosphorus atoms are prone to undergo microscopic inhomogeneity, concentrating at the grain boundaries. This inhomogeneity itself reduces the binding energy at the grain boundaries, causing the steel to become brittle (cold brittleness).
Welding thermal influence: Under the high-temperature cycle of welding, the phosphorus accumulated at the grain boundaries will further promote the formation of brittle second phases. This causes a sharp decline in the toughness of the heat-affected zone. When the weld metal cools and contracts, generating tensile stress, the stress cannot be released through plastic deformation and can only crack along the brittle grain boundaries, forming liquefying cracks in the welding heat-affected zone.
Product advantages:
The standard requires that P should be less than or equal to 0.030%. However, the typical value you provided is less than or equal to 0.020%.
What does this mean? It means that the purity of the material grain boundaries is higher. The heat-affected zone at the weld edge still maintains good toughness, which can effectively absorb the welding stress instead of cracking "cracklingly" like glass.
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