Q690 High-Strength Steel Cutting: Why Standard Parameters Don't Apply
Q690 has a minimum yield strength of 690 MPa. Its carbon equivalent is higher than common mild grades, which means cutting with standard flame parameters can increase the risk of edge cracking. Process review — including preheat where applicable — is done before production.
What Makes Q690 Different
Q690 is a quenched and tempered high-strength structural steel. Minimum yield strength is 690 MPa. Its carbon equivalent is higher than common structural grades, which makes the steel more sensitive to rapid thermal cycling.
Rapid heating followed by rapid cooling — which can happen during flame cutting without process control — can create conditions that increase the risk of hydrogen-induced cracking along the cut edge. Exact chemistry and cutting conditions depend on the specific standard, subgrade, thickness, and project specification.
Why Standard Cutting Parameters Need Review
Running Q690 with the same oxygen pressure, gas flow, and cutting parameters used for a mild grade at the same thickness can increase the risk of one of two problems: edge cracking (which may not appear immediately after cutting), or a heat-affected zone (HAZ) that is wider or harder than required.
Neither problem is always visible immediately after cutting. Edge cracks can appear during cooling. HAZ hardness is only measurable with instruments. This is why Q690 orders receive a process review before production starts — not a check after delivery.
Preheat Considerations
Preheat slows the cooling rate after cutting, reducing the thermal gradient that can cause cracking. Whether preheat is needed — and at what temperature — depends on grade, thickness, project specification, and process review.
At YUNCUT, Q690 orders are flagged for preheat confirmation at quotation stage. Any preheat requirement and dwell time are reviewed before production begins. This adds setup time and substantially reduces post-cutting edge crack risk — it does not remove the need for downstream inspection.
Heat Affected Zone Control
The goal for Q690 HAZ is to keep the hardened zone as narrow as possible while maintaining cut edge integrity. Adjusted cutting parameters help achieve this by reducing peak temperature at the cut face.
For parts requiring subsequent welding, the HAZ from cutting must be compatible with the welding procedure. If your application has specific HAZ requirements (hardness limit, width limit), state them at quotation stage.
Laser vs Flame for Q690
Q690 up to 55mm can be laser cut with adjusted parameters, subject to grade-specific process review. Laser typically produces a narrower HAZ than flame, which is often an advantage for high-strength steel. Surface quality is controlled according to material, thickness and project requirements.
Above 55mm, flame cutting — with process review for preheat — is the applicable option at YUNCUT. For thicker Q690 sections, confirm part geometry and any post-cut requirements before the order is placed. Complex shapes or tight hole patterns in thick Q690 require pre-production discussion.
Common Q690 Applications at YUNCUT
Excavator boom and arm sections. Crane boom chords and lacing plates. Bridge structural components in seismic zones. Mining equipment primary frames. In all of these applications, the cutting process affects downstream welding and structural performance — which is why pre-production confirmation is standard practice.