Also denoted by QT or Q/T, these steels are weldable, but care must be taken to adhere to established procedures at all times, as they often have high carbon contents, and thus high hardenability, leading to a hard heat-affected-zone (HAZ) and susceptibility to cracking. The BS EN 10277-5:2008 (old BS 970 Part 3) steels are good examples of these types of steel. Conversely, some Q&T steels have been developed with very low carbon contents and excellent weldability.
There is nearly always a heat input limit recommended by the manufacturer for the welding of Q&T steels, as there can be problems with softening of the HAZ when the cooling rate of the weld is slower than that of the quenching process.
Care needs to be taken in the selection of suitable welding consumables, particularly where there is a requirement to match the parent material properties. With some Q&T steels this can be very difficult to achieve, particularly if the weld metal needs to match the ultimate tensile strength of the steel, rather than the yield strength.
Increasing thickness and strength of Q&T steels can increase the susceptibility of the steel to fabrication hydrogen cracking (also known as cold cracking), caused by the interaction of the diffusible hydrogen content of the weld, a susceptible microstructure and the tensile stresses applied to the weld, whether residual or applied. Precautions to avoid fabrication hydrogen cracking may include using an undermatching filler metal with respect to the parent material strength, increasing the preheat used or increasing the heat input used.
Thin sections of Q&T steels do not present any significant welding problems. It is easier to match the parent and weld metal properties than for thick, high strength Q&T steels, and the HAZ properties are maintained when the cooling rate is great enough. The preheat and interpass temperatures must be limited, and the heat input kept below the steel supplier's recommended value to avoid HAZ softening.
Many steel suppliers have information about welding their Q&T steels, giving recommended values of heat input, preheat and usually some guidance on which consumables to use (e.g. [1,2]).
Postweld heat treatments are usually carried out with controlled heating and cooling rates, and the holding temperature should be kept below the original parent material tempering temperature to avoid softening.
1. Quenched and Tempered High Strength Steels (RQT 501), British Steel, P6A
2. Welding Instructions for Weldox and Hardox steel plate, Svenskt Stål
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