CARBIDE DISSOLUTION AND PRECIPITATION PRO CESSES DURING HARDENING OF A HIGH-SPEED STEEL

The carbide dissolution during austenitization and the carbid precipit ation on cooling was investigated for high-speed tool steel 1.3343 in laboratory experiments and by vacuum hardening of samples with diamete rs up to 170 mm. During the austenitization process the carbides are d issolved with increasing temperature and holding time. At a holding ti me of about 200 s the maximum solubility of carbon in austenite is rea ched, so that a further extension of holding time has no influence on the remaining carbide contents. If the samples are slowly cooled after austenitization, the non-soluble contents of remaining carbides is in creased by prescipitation of new carbides. Premature carbide precipita tion occurs at non-soluble carbides or at other nucleation sides such as dislocations in the grains at high temperature (ca. 1100 degrees C) and at grain boundaries in case of lower temperatures (ca. 800 degree s C). They decrease the asqueched and the as-tempered hardness because the carbon contents bonded in the carbides cannot be used neither for a distortion and an increase of martensite hardness nor for a precipi tation of secondary carbides. The pre-eutectic carbide precipitation d oes not cause an embrittlement but is responsible for hardness. Simila r processes are going on during vacuum hardening of tools with large c ross sections.