Quenched and Tempered Steels
Heat treatment, temper embrittlement, and cracking tendency when
AI generated Quenched and tempered steels get a combination of high strength and good through and – i.e., . Typical representatives are 42CrMo4 and 34CrNiMo6. When , they are demanding because the hardens and is prone to cold cracking without preheating.
is a two-stage heat treatment: the steel is – austenitized and – and then . The hard, brittle transforms into the tough : finely dispersed carbides in a ferritic matrix. It is exactly this combination of high strength and good that makes quenched and tempered steels the material for shafts, axles, gear parts, crankshafts, and highly stressed bolts.
Typical representatives are () and (). The carbon content is between 0.2 and 0.6%, plus , , and . and improve hardenability – they ensure that even thicker cross-sections harden through to the core – and also lowers the , i.e., the temperature below which the steel becomes brittle.
When , all this hardening knowledge catches up with you. The is locally brought to hardening temperature during and cools so quickly due to the cold mass of the component that it forms fresh – it "hardens up." This hard, brittle rim next to the weld is the perfect starting point for , especially if from moist also comes into play. The three ingredients for cold cracking – , tensile stress, hydrogen – are all present when quenched and tempered steels.
Add to that : if the welding heat brings the already tempered structure into the critical tempering range, the steel can become brittle – an effect that the quenched and tempered steel painstakingly avoided during manufacturing and that you inadvertently bring back when . That's why the same iron rule applies when quenched and tempered steels as with : never weld cold, always preheat and cool in a controlled manner. The depends on carbon content, plate thickness, and alloy – a demands significantly more than a plain .