High Speed Tool Steels Flashcards
What is the application of the HSS?
Mill cutters operating at high speeds.
What contributes to the materials hardness?
W and/or Mo provide carbide formation and “red hardness” (secondary hardness).
V provides increased abrasion resistance, reduced oxidation and increased hardness.
Co provides high temperature hardening.
What are characteristics of high speed tool steels?
*Used for very high cutting rates on very hard materials
*Resists softening in the ‘red heat’ temperature range which s very important for these steels
*Great wear resistance, high hardness and edge retention
What two groups are high speed tool steels divided into? And what are the generalities?
Molybdenum HSTS [Grade M], contains Mo, W, Cr, V and sometimes Co
Tungsten HSTS [GRADE T], contains high W, with Cr, V and some Co
Generally:
*W and/or Mo provide carbide formation and “red hardness” (secondary hardness)
*V provides increased abrasion resistance, reduced oxidation and increased hardness.
*Co provides high temperature hardening.
What are heat treatments for HSTS?
Austenitizing: at 1250-1290 C, need careful control to avoid partial melting. The aim is to try dissolve as much of the carbide as possible so that after quenching and tempering, maximum hardness is obtained.
Must not be held too long at temperature otherwise significant coarsening of the crystal structure will occur, adversly affecting the properties.
How are HSTS tempered?
Heating to below the melting point then cooling in air.
Depending on time and temperature, the following can occur:
*~270 C martensite loses its tetragonality and becomes more cubic and the epsilon carbide begins to form
*~300-400 C the epsilon carbide starts to be replaced by cementite
*~470-570 C some cementite is replaced by “M”2C precipitation, leading to secondary hardening.
*~above 600 C, “M”2C and any remaining cementite dissolve and are replaced by “M”6C and “M”23C.
Results in a rapid increase in hardness, particularly between ~620-650 C
