Why Q355B Welds Well
Q355B is a low alloy structural steel defined in GB/T 1591 with a minimum yield strength of 355 MPa for thickness up to 16 mm, a tensile strength range of about 470 to 630 MPa and a minimum elongation of 22 percent at that thickness. The B suffix denotes a Charpy impact requirement for the 20 degree Celsius temperature class. Its weldability comes from a moderate carbon level together with a controlled alloy addition, which limits the hardenability of the heat affected zone and therefore the risk of cold cracking.
The chemical limits in GB/T 1591 for Q355B are a maximum carbon of 0.24 percent, silicon up to 0.55 percent and manganese up to 1.60 percent, with phosphorus and sulfur each capped at 0.035 percent. The standard also caps the carbon equivalent, and that single value, more than the carbon figure alone, decides how much preheat a given section needs.
Chemistry and the Heat Affected Zone
Carbon is the element most responsible for hardening the heat affected zone. At the 0.24 percent ceiling the zone can still harden under fast cooling in thick sections, which is why heavy plates are usually produced with carbon well below the maximum. Manganese at 1.00 to 1.60 percent raises strength in the base metal and improves weld metal toughness, offsetting some of the loss caused by other elements. Micro-alloying with niobium, vanadium or titanium provides strength by precipitation and, at the low levels used, contributes little to hardening.
Phosphorus and sulfur are restricted because they promote cold and hot cracking respectively. Sulfur forms low melting inclusions that can open during solidification, and phosphorus raises the ductile to brittle transition temperature of the heat affected zone. Steel purchased against a mill certificate that shows both elements inside the cap and a reported carbon equivalent is the practical prerequisite for a crack free weld.
Welding Processes Used with Q355B
| Process | Consumable class | Typical use |
|---|---|---|
| Shielded metal arc, SMAW | Electrodes per AWS A5.1 or an equivalent covered electrode classification | Site work, small batches, difficult access |
| Gas metal arc, GMAW | Solid wire per AWS A5.18 or EN ISO 14341 | Shop fabrication of thin to medium sections |
| Flux cored arc, FCAW | Flux cored wire per AWS A5.20 | Medium and heavy plate, higher deposition rate |
| Submerged arc, SAW | Wire and flux combination matched to the base metal | Long straight seams on heavy plate |
Consumables should match or slightly under-match the base metal strength, because a matching or over-matching deposit in a restrained joint raises the risk of cracking without adding capacity. For Q355B the usual choice is a consumable with a minimum yield of 355 MPa and adequate impact energy at the service temperature.
Heat Input, Preheat and Interpass Control
Heat input is the product of arc voltage, current and a thermal efficiency factor divided by travel speed, and it is normally recorded in kilojoules per millimetre on the welding procedure. Excessive heat input coarsens the heat affected zone and lowers toughness; too little heat input raises the cooling rate and hardness. A procedure qualified by testing, in the manner of ISO 15614 or ASME Section IX, fixes a working range for both.
Preheat is decided by carbon equivalent and section thickness. Thin sections welded at normal ambient temperature generally need no preheat, while thick plates and cold or damp conditions call for preheat and for control of the interpass temperature. Moisture on the joint surface is a direct source of hydrogen and must be removed by drying before welding. Consumables that have absorbed moisture are a common and avoidable cause of cracking.
Distortion, Inspection and Common Defects
Q355B has a yield to tensile ratio that leaves limited plastic reserve, so restrained joints and thick sections need attention to welding sequence. Balanced welding, back step technique and adequate tacking reduce angular distortion. Where a full penetration joint is required, back gouging to sound metal before the second side is welded avoids slag entrapment.
Inspection practice follows the joint class rather than the steel grade. Visual inspection and dimensional checks are routine, ultrasonic or radiographic testing is applied to critical joints, and hardness surveys of the heat affected zone are useful where a hard zone would be unacceptable in service. For structures exposed to low temperature, impact testing of the procedure qualification at the design temperature confirms that the toughness of the joint matches the parent metal class.
Frequently Asked Questions
Q: What is the carbon limit for Q355B?
A: GB/T 1591 caps carbon at 0.24 percent for Q355B, with silicon up to 0.55 percent, manganese up to 1.60 percent and phosphorus and sulfur each at 0.035 percent maximum.
Q: Does Q355B need preheating?
A: Not for thin sections welded in normal conditions. Preheat is decided by carbon equivalent, thickness and ambient temperature, and thick or damp joints should be preheated and dried before welding.
Q: Which filler metal matches Q355B?
A: A consumable with a minimum yield of about 355 MPa, such as a solid wire per AWS A5.18 or a flux cored wire per AWS A5.20, selected together with the required impact energy.
Q: What is the yield strength of Q355B?
A: At least 355 MPa for thickness up to 16 mm, with a tensile range of roughly 470 to 630 MPa and minimum elongation of 22 percent at that thickness.
Q: What causes cracking in Q355B welds?
A: Hydrogen from moisture or damp consumables, excessive restraint, an inappropriate cooling rate and heat affected zone hardening. Control of preheat, consumable storage and heat input addresses all four.
Q: Is a welding procedure qualification required?
A: Yes for structural work. The procedure is qualified by test in the manner of ISO 15614 or ASME Section IX, and the qualified range for heat input and preheat must be followed in production.