What is quenching and tempering?

Quenching and tempering is a two-step heat treatment for non-alloy and alloy steels with enough carbon to harden. The steel is first hardened by austenitising and quenching. It is then tempered at a high temperature; for common quenched and tempered grades this is typically between about 540 and 680 °C. The result is less hard than steel that is hardened and given a low temper, but strong and tough. The condition follows the grade name: +QT is quenched and tempered, +A soft annealed, +N normalised.

What does quenching and tempering depend on?

The strength you reach depends on the grade, the tempering temperature and above all the section size. The thicker the material, the less deeply the quench takes effect, so in large sections the core stays softer than the surface. This hardenability differs by grade: an alloy steel such as 42CrMo4 still reaches high strength in a larger section, a non-alloy steel such as C45 far less.

What does the standard say about quenching and tempering?

The properties of quenched and tempered steels are set in the product standards, now the ISO 683 series, which in Europe has largely replaced the older EN 10083; drawings and certificates still carry both. These standards give tensile strength, yield strength and impact energy per range of ruling section: the same grade has a lower guaranteed strength in a thicker bar. A drawing that only says 42CrMo4 +QT therefore fixes only the minimum values for the ruling section of the material as supplied, tested at the location the standard prescribes; in the core of a thick bar the strength can be lower. A specified hardness or strength is only unambiguous with its test method and test location. The material certificate gives the measured values of the heat.

How does quenching and tempering work in practice?

The main choice is whether to buy quenched and tempered stock or to have the part treated after roughing. Bar in +QT is ordered as such and machined in one route, with no subcontracted step. Treating afterwards makes sense when the section in +QT does not reach the required strength, when the customer specifies it, or when roughing leaves a smaller section that hardens through better. The route then gains a trip to the heat treater, allowance for distortion and scale, a hardness check and finish machining to size. What that costs depends on quantity, the price per load, lead time and the extra setups.

What is quenching and tempering often confused with?

Quenching and tempering is not hardening in the narrow sense: after hardening the steel is tempered low and stays hard and less tough, while quenching and tempering uses a high temper for toughness. Stress relieving removes stress without materially changing strength, as long as it stays below the tempering temperature.

How do you see quenching and tempering in Blake?

If you have the part quenched and tempered after roughing, Blake carries that as a subcontract operation in the estimate, priced from your own subcontract library; if the price is missing, the line stays open. You enter and accept the setups before and after heat treatment per route. More on estimating.