Welding High-Strength Q&T Steels (S690–S960): Cooling Time Controls Everything

What makes these steels different

S690QL, S890QL and S960QL (EN 10025-6) achieve yield strengths of 690–960 MPa from a quenched and tempered microstructure, not from alloy richness — their carbon equivalents are surprisingly moderate. The weldability problem is therefore thermal, not chemical: the wrong cooling cycle either hardens the heat-affected zone (HAZ) into a crack-sensitive state or tempers it into a soft zone that becomes the weakest link.

The t8/5 concept

Weld cooling in these steels is specified by t8/5: the time the weld area takes to cool from 800 °C to 500 °C. This interval covers the phase-transformation range that decides HAZ hardness and toughness.

• Too short (very low heat input, no preheat on thick plate): hard, brittle HAZ, hydrogen crack risk.
• Too long (high heat input, excessive preheat/interpass): grain growth and over-tempering — HAZ softening and toughness loss.

Steel makers typically recommend a t8/5 window on the order of 5–20 seconds (narrower for S960), achieved by coordinating three variables together: preheat temperature, heat input and plate thickness. Procedures are qualified against this window, not against heat input alone.

Heat input and temperatures

Indicative practice (mill data sheets govern):

• Heat input commonly limited to roughly 0.5–1.5 kJ/mm, falling as grade level rises.
• Preheat is modest — often none to ~125 °C depending on thickness and combined joint thickness — because excessive preheat lengthens t8/5 and softens the HAZ.
• Interpass temperature is capped, frequently at 150–250 °C depending on grade; a joint that keeps climbing in temperature pass after pass drifts out of the window.

Filler: matched or undermatched?

Both philosophies are legitimate:

• Matching fillers (e.g. 690- or 890-class wires) are used where the joint must develop full parent strength, with strict hydrogen control (H5-class consumables, clean dry storage).
• Undermatching (soft) fillers are widely used for fillets and non-full-strength joints: the ductile deposit relaxes restraint stresses and is far more tolerant of fit-up and cracking risk, at an accepted strength reduction the designer must sign off.

Whichever is chosen, low hydrogen is non-negotiable — these grades crack on hydrogen levels that S355 would shrug off.

Shop discipline

• Follow a qualified WPS with the actual t8/5-relevant parameters recorded; do not let welders "speed up" by raising current — travel speed enters heat input directly.
• Preheat uniformly and measure; monitor interpass temperature between passes, not just at the start.
• No postweld heat treatment unless the maker approves it: a conventional stress-relief cycle will temper the whole component below its delivered strength.
• Edges from thermal cutting should be dressed, and tack welds made with the same preheat and consumables as the joint itself — undersized, unpreheated tacks are a classic crack origin.