Cold bending Q690E is a high-risk, high-precision operation because of the material's ultra-high strength, at least 690 MPa yield, and its quenched and tempered microstructure. The primary risks are cracking, springback, and loss of mechanical properties. Unlike lower-grade steels, there is minimal margin for error, so the process must follow a strict protocol from preparation through inspection.
What Is Q690E Steel?
Q690E is an ultra-high strength quenched and tempered structural steel defined by the Chinese standard GB/T 16270, with a minimum yield strength of 690 MPa in the thinner thickness range. The E designation indicates Charpy V-notch impact testing at -40 degrees Celsius. The quenched and tempered microstructure gives the steel its high strength and toughness, but it also narrows the window for plastic deformation: the goal of cold bending is to induce plastic deformation without exceeding the ductility limit or inducing brittle fracture, and for Q690E this window is narrow.
Pre-Bending Preparation and Design
The most critical phase is preparation. The minimum inside bend radius must be respected as a non-negotiable rule: as a conservative guideline, it is typically at least 5-8 times the material thickness, so a 10 mm thick Q690E plate requires a radius of at least 50-80 mm. A smaller radius imposes extreme local tensile strain on the outer fibers, leading to micro-cracking or rupture, and the bend radius for Q690E is significantly larger than for Q550E or Q620E. All edges in the bend zone must be machined: sheared or flame-cut edges are work-hardened, micro-cracked, and act as stress concentrators, so a minimum of 2-3 times the material thickness must be milled or ground away to create a smooth, rounded finish. Surface scratches, gouges, and defects on both sides of the bend line must be removed because they become crack initiation sites. The material certificate should be verified, and the rolling direction noted: bending transverse to the rolling direction is generally preferred because it may offer better ductility, while bending parallel to the rolling direction may require an even larger radius.
Bending Process Control
During bending, high-rigidity, powerful presses are required because standard press brakes may be inadequate. Premium hardened tool steel punches and dies with polished, dent-free surfaces should be used, and the die and punch radii must conform to or exceed the specified minimum bend radius. The V-die opening is often selected at 8-12 times the material thickness to allow smooth material flow without excessive force. Bending speeds must be very slow and controlled, because high speeds generate localized adiabatic heat, can cause unpredictable material behavior, and increase the risk of shear cracking. A high-performance lubricant should be applied between the tooling and the steel to minimize friction and galling. Coining, or excessive pressure at the bottom of the stroke, should be avoided or extremely limited because the localized pressure can cause subsurface damage and work-hardening; air bending is preferred, with the understanding that springback will be significant and must be accounted for.
Springback and Post-Bending Treatment
Springback is severe because of the high yield strength, so the bend angle must be over-bent by a calculated amount, often 5-15 degrees more than the desired final angle, determined by experience or trial bends on test coupons from the same batch. The angle must not be corrected by re-bending in the opposite direction, which adds damaging reverse plasticity. For critical components or severe bends, localized low-temperature stress relief may be specified: the temperature must be below the original tempering temperature of Q690E, typically 600-650 degrees Celsius, so a range of 550-600 degrees Celsius is common, followed by controlled cooling. Non-destructive testing is mandatory for all bends in critical components: 100% visual inspection followed by magnetic particle testing or dye penetrant testing on the outer tensile surface of the bend to detect micro-cracking. If a part is over-bent, it must not be cold straightened; the additional plastic strain will almost certainly cause cracking, and the part should be scrapped.
Critical Prohibitions and Qualification
The mandatory protocol for Q690E cold bending is: qualify by performing trial bends on test coupons from the exact same batch to determine springback and verify the chosen radius; prepare by machining edges, cleaning surfaces, and verifying material; execute with correct, robust tooling, bending slowly with lubrication to the over-bent angle; inspect and treat with magnetic particle or dye penetrant testing on all bends and stress relief if specified by design; and document heat numbers, bend parameters, and inspection results for full traceability. The critical prohibitions are: never deviate from the specified minimum bend radius, never bend with unprepared sheared or flame-cut edges, never use high bending speeds, never apply local heat such as flame or induction to assist bending unless it is a fully qualified hot-forming procedure, and never assume the behavior is similar to any other steel. For many critical applications, designers may avoid cold bending Q690E altogether, choosing welded assemblies or hot-formed components instead.
Frequently Asked Questions
What is the minimum bend radius for Q690E? s a conservative guideline, the minimum inside bend radius is typically at least 5-8 times the material thickness. For a 10 mm thick plate, the radius should be at least 50-80 mm, and the mill datasheet should always be consulted.
Why must sheared or flame-cut edges be machined before bending? Sheared and flame-cut edges are work-hardened and micro-cracked, and they act as stress concentrators. A minimum of 2-3 times the material thickness should be machined away from the edges in the bend zone to create a smooth, rounded finish.
How much springback should be expected? Springback is severe because of the high yield strength, and the bend angle must be over-bent by a calculated amount, often 5-15 degrees more than the desired final angle, based on experience or trial bends on test coupons from the same batch.
What temperature is used for stress relief after bending? Low-temperature stress relief is typically performed at 550-600 degrees Celsius, below the original tempering temperature of about 600-650 degrees Celsius, followed by controlled cooling, to avoid softening the quenched and tempered microstructure.
What inspection is required after bending? Non-destructive testing is mandatory for critical components: 100% visual inspection followed by magnetic particle testing or dye penetrant testing on the outer tensile surface of the bend to detect any micro-cracking.
Can local heating be used to assist bending? No. Local heat such as flame or induction must not be applied to assist bending unless it is part of a fully qualified and controlled hot-forming procedure, because uncontrolled heat will destroy the quenched and tempered microstructure.