Two Strength Tiers, One Toughness Grade
Both grades carry the D suffix, which in the Chinese structural steel system means Charpy V-notch impact testing at -20 C, and both are supplied in the low-alloy high-strength family. Q690D is a recognised grade of the high-strength low-alloy structural steel series, while Q960D belongs to the ultra-high-strength group supplied as quenched and tempered plate under dedicated high-strength plate specifications rather than the general structural series. The strength step between them is 270 MPa of minimum yield, roughly 39 % more capacity per unit of section area at the lower end of the thickness range.
Side-by-Side Comparison
| Property | Q690D | Q960D |
|---|---|---|
| Minimum yield strength (up to 16 mm) | 690 MPa | 960 MPa |
| Impact quality grade | D, tested at -20 C | D, tested at -20 C |
| Typical delivery condition | Thermo-mechanical rolled or quenched and tempered | Quenched and tempered |
| Alloying approach | Microalloyed, moderate alloy content | Higher alloy content for hardenability and toughness |
| Weldability | Weldable with qualified procedures | Requires tighter preheat, hydrogen and heat input control |
| Typical relative material cost | Baseline | Substantially higher per tonne |
The table shows the structural trade: the higher tier buys a large reduction in section area and weight, and pays for it in stricter process control and a higher price per tonne.
Where Q690D Is the Right Answer
Q690D occupies the band between ordinary structural steel and ultra-high-strength plate. It suits members that must be stronger than conventional grades but do not justify the processing discipline of a 960 MPa material: wind turbine tower flanges, heavy truck frames, large-span bridge truss members, mining equipment frames and supports, and general heavy machinery. Its production route is well established, welding procedures are widely qualified, and the risk of a fabrication problem is lower. For most heavy engineering, it delivers the better balance of strength, robustness and cost.
Where Q960D Earns Its Premium
Q960D becomes worthwhile when weight itself is the design constraint. Because yielding starts at 960 MPa, a member carrying the same load can be built with a substantially smaller cross-section, and structures that were previously built from Q690D can shed both area and self-weight. The classic applications are large-tonnage crane booms, hydraulic supports for mining, pressure hulls and other weight-critical equipment, and machines where reduced mass improves mobility or energy consumption. In these cases the extra material cost is offset by reduced weight, lower loads on adjacent components and cheaper handling.
Fabrication Reality Check
Higher strength always costs fabrication freedom. Q960D demands a qualified welding procedure with controlled preheat, low-hydrogen consumables handled to strict exposure limits, and documented limits on heat input and interpass temperature to protect the heat-affected zone. Restraint and fit-up tolerances are tighter because the material has less capacity to yield and redistribute stress. Cold forming radii are larger, and thermal cutting edges in tension zones may need grinding. Mechanical cutting, drilling and machining become more demanding. Post-weld heat treatment is generally avoided for quenched and tempered plate because it reduces strength, so residual stress is managed through welding sequence and design instead. For plate and welds in primary members, ultrasonic testing is standard practice.
Design Checks and Cost Logic
Two points decide the economics. First, stiffness does not increase with yield strength: Young's modulus stays around 200 GPa, so deflection, vibration and buckling of slender members are unchanged and may force the section back to a larger size, erasing part of the weight saving. Second, the decision should compare fabricated cost rather than material price. The higher-priced plate may be cheaper in place if it removes tonnes of steel, simplifies lifting and reduces foundation loads; conversely, it is a losing choice if the fabricator cannot qualify the welding procedure cost-effectively or if the weight saving is small.
Frequently Asked Questions
Q: Do Q960D and Q690D have the same impact test temperature?
A: Yes. Both carry the D quality grade and are impact tested at -20 C; the difference between them is the strength level, not the impact grade.
Q: How much weight can Q960D save compared with Q690D?
A: Minimum yield strength is 960 MPa against 690 MPa, an increase of about 39 %. For members governed by strength rather than stiffness, that typically allows a reduction of roughly one third in section area, which translates into a similar order of weight saving.
Q: Is Q960D harder to weld than Q690D?
A: Yes. It requires tighter control of preheat, interpass temperature, heat input and consumable hydrogen content, and the welding procedure must be qualified with impact testing of the heat-affected zone at -20 C.
Q: Does the higher strength grade give a stiffer structure?
A: No. Young's modulus is about 200 GPa for both grades, so stiffness and deflection are essentially unchanged; only the stress at which yielding begins is raised.
Q: When should Q690D be preferred?
A: When strength above conventional structural steel is needed but weight is not the dominant constraint, so that mature welding procedures, lower material cost and simpler fabrication are more valuable than the last increment of strength.