Hot Worked Tool Steels Flashcards
What is secondary hardening and how does it improve the performance of hot work tool steels?
It is the introduction of Mo and W into the grain structure to produce M-carbides which reduce the softening occurring in an allow at high temperatures. Without these elements diffusion of the Fe-carbides occurs readily softening the alloy at high temps.
What are characteristics of hot worked tool steels?
*0.3-0.5wt% C
*High temperature (200-800 C) metal forming operations (hot forging, hot extrusion).
Has resistance to the following:
*Deformation at hot working temperatures
*Mechanical and thermal shock (this is helped by keeping the carbon content low)
*Wear and Erosion
*Deformation during heat treatment
*Heat checking
What is heat checking?
A sudden temperature fluctuation causing stress in an object or substance.
Originated due to stresses between the workpiece and defaults in die, transfer to the workpiece.
Cross section of tool steel, cracks forming at top oxide and propagating through.
How are hot worked tool steels divided into groups?
Chromium based [H11-H19]
Tungsten based [H20-H26]
Molybdenum based [H41-H43]
What are characteristics and applications of chromium based?
Contains chromium ~3.25%, and small amounts of V, W and Mb.
*High red hardness and hardenability
*Oil quenching is required when dimensional stability is not of prime importance. Tempering temperature for these steels varies from 550-675 C
Applications are :
*Hot dies for extrusion, forging, mandrels and punches
*Highly stressed structural parts of supersonic aircrafts
*Hot work steels
What are characteristics and applications of tungsten based?
Contains tungsten 9% and chromium 2-12%, and low carbon %.
*Resistance to high temperature softening
*Tempering temperature for these steels varies from 550-675 C.
Applications are:
*Punches and mandrels
*Extrusion dies for brass, steel and nickel alloys
What are characteristics and applications of molybdenum based?
Contains molybdenum 8% and chromium 4-12%, some tungsten and vanadium.
*High toughness and heat check resistance
*Tempering temperature for these steels varies from 500-650 C.
