Principal defects in steel due to heat treatment and measures for their elimination:
Defect # 1. Overheating:
Coarse grained micro- structure and fracture. Reduced ductility and specially reduced impact strength
Causes:
Heating for longer periods at temperatures considerably above the normal values.
ADVERTISEMENTS:
Prevention:
i. For slight overheating, use normal annealing and normalising.
ii. For considerable overheating, use double annealing or normalising; first annealing or normalising at temperature of AC3 + 100°C to 150°C, second at normal temperature.
Defect # 2. Burning Grain Boundaries:
They have:
ADVERTISEMENTS:
i. Regions enriched in carbon —1st stage of burning.
ii. Non-oxidised cavities and blow holes— 2nd stage.
iii. Iron oxide inclusions—3rd stage. Stone-like fracture and low ductility.
Causes:
ADVERTISEMENTS:
Heating for a long period at high temperature in an oxidising atmosphere of temperature near the melting point.
Prevention:
1st Stage—use of homogenising followed by double annealing.
2nd Stage—use forging followed by annealing.
ADVERTISEMENTS:
3rd Stage—defects can’t be remedied.
Defect # 3. Oxidation:
Thick layer of scale on the surface of steel articles.
Causes:
Oxidising atmosphere in the heating furnace.
ADVERTISEMENTS:
Prevention:
i. Heat in furnaces with reducing, neutral or protective atmosphere.
ii. Heat in boxes with used carburising agent or cast iron chips.
iii. Heat in molten salt baths.
Defect # 4. Decarburisation:
Loss of carbon in the surface layers of the article. Lower hardness after quenching, lower fatigue limit.
Causes:
Oxidising atmosphere in the heating furnace.
Prevention:
Measures of avoiding same as for Serial Number 3.
Correction:
Machine off the decarburised layer if the machining allowance permits.
Defect # 5. Excessive Hardness of Hot Worked Annealed Steel:
Causes:
Excessive cooling rate for ordinary annealing or insufficient holding time for isothermal annealing.
Correction:
Repeat annealing with cooking at specified rate.
Defect # 6. Black Fracture:
For graphite inclusion in the steel.
Causes:
Excessive heating time and slow cooling after annealing.
Correction:
Heat the steel to a high temperature and forge it thoroughly.
Defect # 7. Quenching Cracks:
Usually appear at the grain boundaries and are zig-zag in form. The cracks may be either external or internal.
Causes:
(i) Internal stresses due to volume changes at low temperatures, caused by martensitic transformation.
(ii) The martensitic transformation is not simultaneous throughout the article being hardened.
Prevention:
This defect can’t be corrected. It may be prevented by:
i. Avoiding sharp projections, sharp-corners and sudden transitions from thick to thin section in designing articles.
ii. The articles should be free from stresses before being hardened; stresses should be relieved by annealing.
iii. Heat to minimum stable temperature for hardening.
iv. Cool slowly in the martensitic range (use interrupted quenching, oil quenching and martempering).
v. Use austempering.
vi. Temper immediately after quenching.
Defect # 8. Deformation and Volume Changes after Hardening:
Causes:
The volume of the steel increases due to the martensitic transformation. The higher the hardenability of the steel, the more severe will be the deformation due to hardening.
Prevention:
i. Use alloy steels that are only slightly deformed by hardening.
ii. Cool slowly in the martensitic range.
iii. Use surface hardening when possible.
Defect # 9. Warping:
Assymmetrical deformation of article in quenching.
Causes:
i. Volume changes in cooling (or heating).
ii. Non-uniform heating or cooling of the article.
iii. Internal stresses in the article before heating.
iv. The article is lowered into the quenching bath in an inclined position.
Prevention:
a. Use alloy steels that are only slightly deformed by hardening.
b. Cool slowly in the martensitic range.
c. Use surface hardening when possible.
Correction:
i. Anneal, normalise or temper at a high temperature before hardening.
ii. Heat uniformly for hardening.
iii. Quench as uniformly as possible in hardening.
iv. Hold the article in the proper position when lowering it into the quenching bath.
v. Use special quenching jigs. The defect may be corrected by straightening or grinding.
Defect # 10. Insufficient Hardness after Quenching:
Causes:
i. Hardening temperature is too low.
ii. Holding time is insufficient at the hardening temperature.
iii. Cooling rate is too low.
Correction:
The defect may be corrected by normalising or annealing followed by hardening with the proper specified procedure.
Defect # 11. Soft Spots:
Zones on the surface of hardened articles with lower hardness.
Causes:
i. Vapour accumulates (forming bubbles) on the surface of the quenched articles reducing the cooling rate at these places.
ii. Localised decarburisation.
iii. Inhomogeneity of initial structure during solidification of steel.
Prevention:
i. Using a more effective cooling medium.
ii. Obtaining a more homogeneous structure employing annealing or normalising before hardening.
iii. Protecting against decarburisation in heating.
Defect # 12. Excessive Hardness after Tempering:
Causes:
Low temperature or insufficient holding time in tempering.
Correction:
Corrected by a second tempering with proper temperature and holding time.
Defect # 13. Insufficient Hardness after Tempering:
Causes:
Tempering temperature too high.
Correction:
Anneal, re-harden and temper at normal temperature.
Defect # 14. Corrosion-Pitting or Groove Type Destruction on the Surface of Article:
Causes:
Heating in the molten salt baths:
i. High content of sulphuric salts,
ii. Bath has become rich in oxygen or iron oxides,
iii. Heating in flame furnaces—non uniform scale formation.
Prevention:
i. Carefully control salt composition.
ii. Deoxidise the bath.
iii. Eliminate the oxidising atmosphere when heating in flame furnaces.