“Quenched and tempered” (Q&T) steels for welded structures and pressure vessels provide enhanced hardness and strength properties compared to “regular” steels while maintaining adequate ductility, weldability and toughness.
Steel manufacturers obtain these properties by using low alloy chemistries, heat treatment and controlled rolling.
The regular structural grades of steel are designated by the specification, grade and impact strength (when required); trade names are not usually specified. AS/NZS 1554 Part 1 covers the welding of these materials.
In comparison Q&T steels are often specified and marketed by tradenames. The applications for these materials can be quite specialised and manufacturers may offer a range of steels tailored to particular applications. Fabricators need to be aware that not all types may meet a specification/grade. AS/NZS 1554 Part 4 covers the welding of high strength quenched and tempered steels.
Key points to bear in mind when qualifying procedures for welding Q&T steels:
- Q&T steels are generally designed for one of 2 types of application: structural or wear (abrasion) resistance.
- Structural grades are specified by the strength and impact properties, while the wear resistance grades by the hardness e.g. a “500” wear resistance grade refers to a typical hardness of 500 on the Brinell scale.
- The weldability of the wear resistance grades may not be as good as that of the structural grades.
- Not all types will meet the “prequalified” parent metal requirements of AS/NZS 1554.4.
Issues can arise when drawings or job specifications call up a Q&T material by a trade name and a steel merchant supplies an “equivalent” material. Because there is often subtle differences in the chemistry and manufacturing process between proprietary products there can be different requirements for preheat, and control of inter-run temperature and arc energy.
While AS/NZS 1554 Part 1 sets out preheat inter-run temperature and arc energy requirements for all grades of regular structural steel, AS/NZS 1554.4 has a different approach. Preheating, maximum inter-run temperature, and maximum arc energy are as specified by the material manufacturer; the Standard does have requirements but only for use should the manufacturer’s recommendations not be available.
Fabricators need to take this into account when establishing welding procedures: if the manufacturer’s recommendations are not followed, or a material does not have a specification and grade, additional testing is required and the advantages of the prequalified route for WPS qualification do not apply. When qualifying procedures obtain the manufacturer’s technical information for the brand of steel being used; do not assume it is going to be the same as that from another manufacturer.
AS/NZS 1554 notes (Ref. 4.1.1) that in NZ where the specification requires fabrication to NZS 3404.1 approval of welding procedures is required by the principal prior to the commencement of welding. Especially with Q&T steels the fabricator should establish, at the earliest stage, the designer’s requirements for items such as qualification of consumables, steel types and any additional testing requirements (ref. Appendix D Matters for Resolution). For critical work, relying on the prequalified approach may not provide adequate assurance of the suitability of a WPS.
With these steels materials traceability in the workshop becomes important; mixing Q&T grades with regular steels can lead to expensive mistakes. Be aware that AS/NZS 1554 requires that “all welding and related activities are managed under a suitable quality system” and “this should generally comply with the requirements of AS/NZS ISO 3834 series”. Following the AS/NZS ISO 3834 principles will avoid the cost and frustration of correcting non-conforming welding procedures and the wrong grades of materials being fabricated.