Heat Treatment Of Alloys: Rajesh Kumar S Ap/aero C. Abdul Hakeem College Of Engg And Tech

Heat treatment of alloys Rajesh Kumar S AP/Aero C. Abdul Hakeem College of Engg and Tech

HEAT TREATMENT OF ALUMINIUM ALLOY

• Wrought form of aluminium-pure form- AL1100 • High Degree of corrosion resistance but low strength

• Aluminium becomes stronger (i) Rolled (ii) Heat treated Formed (or) Cold Worked T4- Solution Heat Treated; H-strain Hardened O- Soft or Annealed; T6-Solution Heat Treated and then artificially aged

HEAT TREATMENT OF ALUMINIUM ALLOY

• Alclad Aluminium • Pure clad • The sheets consists of an aluminium alloy core coated with a layer of pure aluminium • The pure aluminium coating give protection to core against corrosive agents, scratching or other forms of abrasion.

Two types of HEAT TREATMENT to ALUMINIUM ALLOY

• (i) Solution heat treatment –(Al2017, Al2024) • (ii) Precipitation Heat treatment (Al2014, Al7075) requires both types

solution heat treatment

• 825 F to 980 F • RULE: Temperature must be controlled within a very narrow range (±10 F) • Low temp: max. strength may not be obtained • Excessive temp: danger of melting, the low melting constituents. Process: (i) Soaking: time at temperature, soaking time: measured as the time the coldest metal reaches the minimum desired temperature range (i) Quenching:-cold water, Hot water, Spray quenching, Lag b/w soaking and Quenching (10 sec)

Precipitation hardening

• 3 steps: • Solution heat treatment • Quenching • Precipitation Heat Treatment

Precipitation hardening Aluminium Magnesium Nickel Titanium Some steels and stainless steels

Aging: Alloys can be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging. SHT: involves formation of a single phase solid solution via quenching PHT: involves the addition of impurity particles to increase material’s strength.

HEAT TREATMENT OF magnesium ALLOY

• 95% of Mg used in aircraft construction is in cast form • Heat treatment is similar to Al alloy (i) Solution heat treatment (ii) Precipitation heat treatment

• Change in properties is negligible when allowed to age naturally at room tempratures.

solution heat treatment of MAGNESIUM ALLOY

• Magnesium alloy casting-SHT • To improve tensile strength, ductility and shock resistance • T4• T6• Solution heat treatment • Solution heat treatment + ARTIFICIAL aging • Artificial aging is necessary to develop full properties. • 730 F to 780 F • Soaking time: 10 to18 hr……………….(longer than 18 hrs…more than 2inch thick) • NEVER heat a magnesium alloy in a salt bath- explosion • Fire hazard exists in SHT of magnesium alloy (protective atm of SO2)

precipitation heat treatment of MAGNESIUM ALLOY

• To increase hardness and yield strength • Relieve stresses • Stabilize alloys • Amount of ductility reduced • Corrosion resistance also improves • Lower temp range than SHT (325 F to 500 F) • Soaking time 4 to 18 hrs

HEAT TREATMENT OF TITANIUM ALLOY PURPOSE • • • •

Relief of internal stresses Annealing – to provide maximum ductility Thermal Hardening - to improve strength Case hardening

Heat treatment of TITANIUM ALLOY

• STRESS RELIEVING • To remove stress concentration – 650 F to 1000 F • Time at temperature: (few minute to an hour or more for heavier sections) • Scales or discoloration removed by pickling in acid solution. 10 to 20% HNO3 and 1 to 3% hydrofluoric acid

• FULL ANNEALING • Toughness, Ductility at room temperature, dimensional and structural stability, Improved machinability. • 1200 F to 1650 F • Time at temperature: 16 min to several hours • Scales or discoloration : Use of caustic descaling –sodium hydride salt bath.

• THERMAL HARDENING • • • •

Unalloyed titanium cannot be heat treated. Most of alloys strengthed by heat treatment Sacrificing ductility Water quenching from 1450 F followed by reheating to 900 F , 8 hours

Heat treatment of TITANIUM ALLOY

• CASE HARDENING• Nitriding, carburizing and carbonitriding • Wear Resistance case of 0.0001 to 0.0002 inch

Melting point of ALLOYs

• • • • •

Carbon – 3540 C Titanium – 1650 C Aluminium – 660 C Magnesium – 650 C Mild steel – 1370 C

Soaking Temperature

Time at temperature Soaking Time

AL-SHT AL-PHT

825 F to 980 F

10 min to 12 hours

Mg-SHT Mg-PHT

730 F to 780 F 325 F to 500 F

10 hours to 18 hours 4 hours to 18 hourss

Ti- SHT Ti-PHT