Quenching and tempering (QT) of steel

Hardening by quenching

The steel is heated above Ac3 until it is fully austenitic and then cooled fast enough to avoid ferrite and pearlite. The austenite transforms to martensite, which is very hard and strong but brittle in the as-quenched state.

How deep the martensite reaches depends on the steel's hardenability and the section size. Plain carbon steels harden only to a shallow depth in thicker sections. Alloying with chromium, molybdenum, manganese, nickel or boron raises hardenability so that larger sections harden through. Bolt steels are chosen so the threaded section reaches a mostly martensitic core before tempering.

Quench media

The quenchant is matched to the grade and the part. A faster quench hardens deeper but raises distortion and the risk of quench cracking.

• Water and brine: the most severe quench, used for plain carbon and low-hardenability steels.
• Polymer solutions: severity adjustable by concentration, between water and oil.
• Oil: slower and gentler, used for alloy steels such as 4140 / 42CrMo4 to limit distortion and cracking.
• Salt baths and gas: used for martempering, austempering and vacuum hardening of tool and high-alloy steels.

Tempering: the hardness and toughness trade-off

Tempering reheats the quenched steel to a temperature below Ac1, holds it and cools it. It relieves quenching stress and lets carbon in the martensite form fine carbides.

As tempering temperature rises, hardness and tensile strength fall and toughness and ductility rise. Low-temperature tempering, about 150–200 °C, keeps high hardness for wear parts and tools. Fasteners and shafts are tempered much higher to obtain toughness.

Tempering between about 260 °C and 370 °C is generally avoided because toughness drops there (tempered martensite embrittlement). Some alloy steels also lose toughness if held or slowly cooled in a higher range, roughly 375–575 °C (temper embrittlement).

Why bolts are quenched and tempered

High-strength bolts need a high tensile and yield strength together with enough ductility and toughness to yield, not snap, when overloaded. Quenching and tempering is the only common heat treatment that gives both in medium-carbon and alloy steels.

Bolt standards control the route as well as the result. ISO 898-1 requires property classes 8.8 and above to be quenched and tempered, sets a minimum tempering temperature for each class and requires enough hardenability for an approximately 90 % martensite core before tempering. Class 8.8 has a minimum tensile strength of 800 MPa up to M16 and 830 MPa above.

ASTM A193 Grade B7 is a chromium–molybdenum alloy steel, quenched and tempered at no less than 1100 °F (593 °C), with a minimum tensile strength of 125 ksi (860 MPa) up to 2½ in diameter. ASTM F3125 Grade A325 bolts (120 ksi, 830 MPa, from ½ to 1 in) and ASTM A449 bolts are tempered at no less than 800 °F (427 °C).

A high minimum tempering temperature prevents a supplier from reaching the strength by under-tempering, which would leave the bolt hard but brittle.