Cold-bending Q690E is a high-risk, high-precision operation due to its ultra-high strength (≥690 MPa yield) and quenched & tempered microstructure. The primary risks are cracking, springback, and loss of mechanical properties. Unlike lower-grade steels, there is minimal margin for error.
Here are the critical key points, structured by process phase:
Core Philosophy: The goal is to induce plastic deformation without exceeding the material's ductility limit or inducing brittle fracture. For Q690E, this window is narrow.
1. Pre-Bending Preparation & Design (The Most Critical Phase)
1.1 Minimum Bend Radius (R): This is the non-negotiable rule.
Consult the Steel Mill's Datasheet: This is mandatory. As a conservative guideline, the minimum inside bend radius is typically ≥ 5 to 8 times the material thickness (t). For example, for a 10mm thick Q690E plate, R should be ≥ 50-80mm.
Rationale: A smaller radius imposes extreme local tensile strain on the outer fibers, leading to micro-cracking or outright rupture. The bend radius for Q690E is significantly larger than for Q550E or Q620E.
1.2 Edge and Surface Conditioning:
Machine All Edges in the Bend Zone: Sheared or flame-cut edges are strictly prohibited for bending. They are work-hardened, micro-cracked, and act as potent stress concentrators. A minimum of 2-3 times the material thickness must be machined (milled or ground) away from all edges within the bend area to create a smooth, rounded finish.
Surface Inspection: Remove all scratches, gouges, and defects from both sides of the bend line. These can become crack initiation sites.
1.3 Material Verification & Directionality:
Verify the material certificate and ensure it meets Q690E specs, paying attention to the actual yield strength and elongation.
Note the rolling direction. Bending transverse to the rolling direction is generally preferred as it may offer slightly better ductility. Bending parallel to the rolling direction may require an even larger radius.
2. During the Bending Process
2.1 Tooling & Equipment:
Use High-Rigidity, Powerful Presses: Standard press brakes may be inadequate. Requires machinery with high tonnage capacity and precision control.
Tooling Material & Condition: Use premium, hardened tool steels for punches and dies. Surfaces must be polished and free of dents to prevent marring the Q690E surface.
Die & Punch Radii: Must conform to or exceed the specified minimum bend radius. The V-die opening should be carefully selected (often 8-12 times t) to allow smooth material flow without excessive force.
2.2 Bending Speed and Control:
Use Very Slow, Controlled Bending Speeds. Hydraulic press brakes with programmable speed control are essential.
Rationale: High speeds generate localized adiabatic heat, can cause unpredictable material behavior, and increase the risk of shear cracking.
2.3 Lubrication:
Apply a high-performance lubricant between the tooling and the steel to minimize friction and galling, ensuring uniform strain distribution.
2.4 "Coining" and Bottom Bending:
Avoid or Extremely Limit Coining (excessive pressure at the bottom of the stroke). The extreme localized pressure can cause subsurface damage and work-hardening, increasing brittleness.
Air bending is preferred, but the springback will be significant and must be accounted for.
3. Post-Bending Considerations
3.1 Springback Management:
Springback is severe due to the high yield strength. The bend angle must be over-bent by a calculated amount (often 5-15 degrees more than the desired final angle). This requires experience and/or trial bends on test coupons from the same batch.
DO NOT attempt to "correct" the angle by re-bending in the opposite direction. This adds damaging reverse plasticity.
3.2 Stress Relieving (May be Required):
For critical components or severe bends, localized low-temperature stress relief might be specified.
Temperature: Must be below the original tempering temperature of the Q690E (typically 600-650°C) to avoid softening. A range of 550-600°C is common, followed by controlled cooling.
Purpose: Reduces residual bending stresses that could impair fatigue performance or promote stress corrosion cracking.
3.3 Non-Destructive Testing (NDT):
Mandatory for all bends in critical components.
100% Visual Inspection followed by Magnetic Particle Testing (MT) or Dye Penetrant Testing (PT) on the outer tensile surface of the bend to detect any micro-cracking.
3.4 Avoid Straightening:
If a part is over-bent, do not attempt cold straightening. The additional plastic strain will almost certainly cause cracking. The part should be scrapped.
4. Critical "DO NOTs" for Q690E Cold Bending
DO NOT deviate from the mill's specified minimum bend radius.
DO NOT bend with unprepared sheared or flame-cut edges.
DO NOT use high bending speeds.
DO NOT apply local heat (flame or induction) to assist bending unless it is a fully qualified and controlled hot-forming procedure. Uncontrolled heat will destroy the Q&T microstructure.
DO NOT assume its behavior is similar to any other steel. Treat it as a unique, high-performance material.
Summary: The Mandatory Protocol
Qualify: Perform trial bends on test coupons from the exact same batch of Q690E to determine springback and verify the chosen radius.
Prepare: Machine edges, clean surfaces, verify material.
Execute: Use correct, robust tooling. Bend slowly with lubrication to the over-bent angle.
Inspect & Treat: Perform MT/PT on all bends. Apply stress relief if specified by design.
Document: Record heat numbers, bend parameters, and inspection results for full traceability.
Conclusion: Cold-bending Q690E is feasible but demands military-grade precision and planning. Success hinges on respecting its material limits, flawless preparation, and controlled execution. The cost of failure (cracking) is high, making consultation with the steel producer and experienced fabricators specializing in ultra-high-strength steels an essential first step. For many critical applications, designers may opt to avoid cold-bending Q690E altogether, choosing welded assemblies or hot-formed components instead.