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Heat Treatment

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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