9.5 - Heat Treatment Flashcards
Factors affecting plain carbon steel hardness?
Amount of carbon present
Speed at which the steel is cooled from hardening temp
Hardening temp for medium carbon steels with 0.8% carbon?
Bright red heat
Hardening temp for medium carbon steels with above 0.8% carbon?
Dull cherry red heat
Heat treatment for low carbon?
<0.3% carbon - negligible
Heat treatment for medium carbon?
- 3-0.5% - tougher - oil
0. 5-0.9% - harder - water (oil for toughening)
Heat treatment for high carbon?
0.9-1.3% - very hard (oil)
What is underheating?
Temp of steel doesn’t reach critical temp, won’t harden
What is overheating?
If critical temp exceeded, grain growth will occur and the steel will be weakened, and cooling rate will be slowed (reducing hardness)
Causes of cracking?
Sharp corners
Sudden changes of section
Screw threads
Holes too near the edge of a component
Causes of distortion?
Lack of balance in shape of component
Lack of uniform cooling
Change in grain structure of steel causes shrinkage
What is tempering?
Further heat treatment (after quenching) that greatly increases the toughness of a hardened steel at the expense of hardness
Reheats the steel, and quenching again
How to temper?
After quenching, clean component and heat up slowly to see colour, then quench immediately
Temping edge tools?
Pale straw, 220C
Temping tools?
Medium straw, 230C
Temping twist drills?
Dark straw, 240C
