Heat Treatment is a very important Process in developing a Tool steel die. The mechanical properties(hardness, toughness, wear resistence etc) are achieved only after proper heat treatment is done after machining. Normal procedures adapted in making a tool steel die are as under:

Rough machining,
Stress relieving,
Machining with grinding tolerances,
Heat treatment including tempering,
Final machining and grinding,
Production.
The main steps involved in Heat treatment are Quenching and Tempering.

Quenching

Heating	Deformation   cracking	Preheating
	Hardness,   Hotstrength	Higher quenching Temperature
	Toughness	Lower quenching Temperature
	Oxidation	Vacuum furnace
Cooling	Toughness	Rapid cooling,
	Deformation   cracking	Uniform quench, salt bath quench

Tempering

Hardness,Wear   resistance	Low temp. tempering, High temp. tempering
Hot strength	High temp tempering
Toughness	Higher temp tempering
Cracking	Uniform mild cooling
Stabilization/residual   stress	Multiple tempering

Common Quenching Problems

Problem	Possible Causes	Remedy
Distortion, Dimensional changes	Sharp comers, Notches, Thin fins Abrupt changes in section Improper marking	Well balanced design Chamfering Properly supported heating
Overheating Underheating	Failure to operate facilities Improper parts arrangement Inaccurate thermocouples	Appropriate arrangements Furnace control check
Decarburization	Improper furnace atmosphere	Controlled atmosphere Vacuum furnace Redesign removing amount
Improper cooling Non-uniform	Inappropriate quenching medium Failure to temper immediately	Less severe quench Careful operation about material temperature
microstructure	Inadequate spheroidized carbide Non-uniform carbide distribution	Spheroidizing annealing Normalize prior to quench

Common Problems in Hardening

Problem	Possible Causes	Remedy
Quenching temp. too low	Failure to operate facilities Improper parts arrangement Inaccurate thermocouples	Use recommended temperature Appropriate arrangements Furnace control check
Quenching temp.high	Failure to operate facilities Inaccurate thermocouples	Use recommended temperature Furnace control check
Inadequate cooling	Failure to begin cooling after extracting parts from furnace Inappropriate quenching medium Too much oxidation Improper quench bath temperature Improper agitation Improper interruption temp.	Improve facility layouts Employ more drastic quench Use of protecting coat, controlled atmosphere Oil 60 80 Water 80 Revie wprocedures andfacility Interrupt cooling at Ms+50
Tempering start temperature	Start tempering before material temperature reaches Ms.	Careful operation about material temperature
Decarburization	Improper furnace atmosphere	Controlled atmosphere Vacuum furnace Redesign removing amount

Comparison between several cooling methods

	Advantage	Disadvantage	Topics
Fan cool	Cheap Easy to operate Controllable of distortions	Inadequate cooling rate	Enhanced air flow Use of cool air
Salt bath	Uniform cooling Relatively high convection	Stuck into holes Water pollution Furnace pollution	Re-collecting salts from wasted water
Oil	Cheap Various cooling rate by selecting oil, temperature	Fire accident Local overcool	Step quenching Use of
Pressured Gas	Bright surface Clean	Expensive Difficult to control distortions Inadequate cooling rate for large parts	Increase pressure (max.20 bar) Enhance heat- exchange unit
Polymer	Various cooling rate by selecting concentration	Local overcool	Not in common for die quenching