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Introduction
Heat Treatment is a very important process in developing a tool steel die. The mechanical properties (hardness, toughness, wear resistence ect) are achieved only after proper heat treatment is done after machining. Normal procedures adapted in making 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
Hardness, Hot strength Toughness Oxidation |
Preheating
Higher quenching temperature.
Lower quenching temp. Vacuum furnace |
| Cooling | Toughness
Deformation cracking |
Rapid cooling
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 |
| Over heating Under heating | 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 | Inappropriate quenching medium Failure to temper immediately | Less sever quench Careful operation about Material temperature |
| Microstructure | Inadequate spheroidized carbide Non-uniform carbide distribution | Spheroidizing annealing Normalize prior to quench |
Common Problem in Harding :
| 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. too 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 Review procedures and facility 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 in to 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 semi-hot oil |
| 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 |
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