and dimentional stability.
Cold work tool steels are normally used at room temperatures featuring good wear resistance, toughness
and dimentional stability.
and dimentional stability.
HEAT
TREATMENT DETAILS
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.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
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 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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