Position of the Grade
Q690E is a low alloy high strength structural steel plate specified in GB/T 1591-2018. It combines a 690 MPa minimum yield strength class with the E quality grade, which requires Charpy impact energy of at least 47 J at minus 40 degrees Celsius. The combination places the grade in the group used for heavy machinery and engineering structures where high static load and repeated stress occur at the same time, often in cold climates.
Compared with a 355 MPa structural plate, the grade allows roughly a forty percent reduction in section area for the same load if stiffness and stability are not the governing criteria. In practice the designer recalculates stiffness and buckling, then uses the strength increase to reduce weight or to increase the machine's working capacity.
Chemical Basis and Production Route
The steel is built on a low carbon base with manganese as the strengthening element and microalloying additions of niobium, vanadium or titanium for grain refinement. Carbon is capped at 0.18 percent and phosphorus and sulphur are restricted, which keeps the toughness at low temperature within reach and limits the risk of cracking in a welded fabrication.
| Property | Value for the grade |
|---|---|
| Minimum yield strength | 690 MPa in the thin plate bands |
| Tensile strength | 770 to 940 MPa |
| Minimum elongation | 14 percent |
| Impact energy at minus 40 degrees Celsius | 47 J minimum |
| Maximum carbon | 0.18 percent |
| Maximum manganese | 2.00 percent |
Two production routes deliver the grade. Thermomechanical rolling achieves the properties through a controlled rolling schedule with accelerated cooling, which produces a fine grained structure without a separate quench. Quenching and tempering achieves the same class through a reheat, quench and temper cycle. The two routes give slightly different through thickness behaviour and different fabrication sensitivities, so the delivery condition is stated on the purchase order and on the certificate.
Fatigue Behaviour in Service
Heavy machinery structures rarely fail from a single overload. They fail from fatigue at a detail where a fluctuating stress concentrates, and the design of the detail matters more than the nominal strength of the plate. Using a higher strength grade allows the designer to reduce stress range in the base metal, but the fatigue strength of a welded joint does not increase in proportion to the plate strength, because the notch at the weld toe dominates.
For that reason Q690E is used in a structure that is detailed for fatigue: smooth transitions at thickness changes, welds placed away from peak stress regions, weld toes dressed where necessary, and a controlled surface condition adopted so that no crack initiation site is introduced by handling or cutting. In this way the benefit of the higher strength class is realised in the base metal while the joints are protected by geometry rather than by material alone.
Applications and Supply Range
The grade is used for crane and lifting equipment structures, mining and earthmoving machine frames, drilling rig substructures, heavy transport frames, pressure loaded structures in industrial plants, and offshore handling equipment. It is also found in steel structures where the design is governed by the need to minimise dead load, such as long span trusses and mobile bridging equipment.
Plate is supplied with full heat number traceability, chemical analysis, tensile and impact results, and dimensional records. Thickness coverage runs from thin plate through heavy sections, and additional requirements that can be ordered include ultrasonic examination, through thickness Z quality, restricted carbon equivalent for a specific welding procedure, and third party inspection.
Processing and Quality Control
Cutting is performed by plasma, laser or flame process. Thermal cut edges on heavy plate are preheated, and edges that will sit in a fatigue sensitive region are ground to remove the hardened layer and the cut striations. Cold forming uses large radii, and when forming is carried out the direction of rolling relative to the bend line is recorded, because the toughness of the plate is direction dependent.
Quality control at the mill includes analysis per heat, testing of tensile and impact specimens from each test unit, ultrasonic inspection where ordered, and dimensional verification. At the fabricator, incoming inspection typically covers hardness, dimensions and certificate verification against the order, followed by welding procedure qualification and non destructive examination of the completed joints.
Frequently Asked Questions
Q: What makes grade E different from grade D?
A: The impact test temperature. Grade E is tested at minus 40 degrees Celsius and grade D at minus 20 degrees Celsius, both with a minimum absorbed energy of 47 J.
Q: How is the strength achieved?
A: Through a low carbon microalloyed composition combined with thermomechanical rolling or with quenching and tempering.
Q: What is the tensile range?
A: 770 to 940 MPa, with a minimum elongation of 14 percent.
Q: Does a stronger plate always give a stronger welded joint?
A: No. The fatigue strength of a welded detail is governed mainly by geometry and notch condition, so joint design has to be adapted when a higher strength plate is used.
Q: Is post weld heat treatment required?
A: It depends on the restraint, the thickness and the design code. Where it is applied, the temperature is chosen so that the base metal strength is preserved.
Q: What documentation is issued with the plate?
A: A mill test certificate with chemical analysis, tensile and impact results, heat traceability and dimensional data, plus optional ultrasonic and third party inspection reports.
