Heat Treating

Heat treating processes for steel:

Annealing:

Full Annealing:
- Heat above upper critical (Ac3)
- Slow furnace cool
- Softens for machining
- Relieves stress

Process Annealing:
- Below lower critical (Ac1)
- Relieves work hardening
- Cold-worked materials
- Faster than full annealing

Spheroidize Annealing:
- Prolonged heating near Ac1
- Forms spheroidal carbides
- Maximum softness
- For high-carbon steels

Normalizing:

Process:
- Heat 100-200F above Ac3
- Air cool (faster than annealing)
- Refines grain structure
- Uniform properties

Applications:
- After forging/casting
- Before hardening
- Improve uniformity
- Refine coarse structure

Hardening:

Process:
- Heat to austenitizing temperature
- Soak for through-heating
- Quench rapidly
- Forms martensite

Quench Media:
- Water: Fastest, most severe
- Oil: Moderate, less distortion
- Polymer: Adjustable severity
- Air/gas: Slowest, least distortion

Factors:
- Carbon content (minimum 0.3%)
- Alloy content (hardenability)
- Section size
- Quench severity

Tempering:

Purpose:
- Reduce brittleness of martensite
- Adjust hardness and toughness
- Trade-off: more tempering = lower hardness, higher toughness

Temperature Ranges:
- Low (300-400F): Maximum hardness, spring temper
- Medium (500-800F): Balance hardness/toughness
- High (900-1200F): Maximum toughness, lower hardness

Critical:
- Must follow hardening
- Never skip tempering on hardened parts
- Multiple tempers may be required

Heat treating aluminum and other alloys:

Aluminum Heat Treatment:

Solution Treatment:
- Heat to solvus temperature
- Dissolve precipitates
- Quench to retain supersaturation
- Sets up for aging

Temperatures:
- 6061: 985-1000F
- 7075: 870-900F
- Critical to reach but not exceed

Quenching:
- Water quench (typical)
- Speed critical for properties
- Delay reduces properties
- Distortion consideration

Aging:

Natural Aging (T4):
- Room temperature
- Days to weeks
- Some alloys self-harden
- Lower strength than artificial

Artificial Aging (T6):
- Elevated temperature (300-400F)
- Controlled time (hours)
- Maximum strength
- Most common treatment

Over-Aging:
- Higher temperature or longer time
- Reduces strength
- Improves stress corrosion resistance
- T7 tempers

Other Alloys:

Titanium:
- Solution treatment and aging
- Stress relief critical (900-1000F)
- Atmosphere protection required
- Alpha-beta treatments

Nickel Alloys:
- Solution treatment
- Precipitation hardening
- Stress relief
- Complex multi-step treatments

Copper Alloys:
- Annealing for formability
- Precipitation hardening (beryllium copper)
- Stress relief

Stainless Steel:

Austenitic (300 series):
- Annealing (1900-2050F, water quench)
- Solution treating
- Stress relief (carefully)
- Cannot be hardened by heat treatment

Martensitic (400 series):
- Hardening similar to carbon steel
- Higher temperatures
- Temper to desired hardness

Precipitation Hardening (17-4 PH):
- Solution treat and age
- Multiple condition options
- High strength achievable

Ensuring heat treatment quality:

Temperature Control:

Furnace Types:
- Batch furnaces
- Continuous furnaces
- Vacuum furnaces
- Salt bath

Temperature Accuracy:
- Controller accuracy
- Uniformity surveys (AMS 2750)
- Thermocouple placement
- Calibration requirements

Documentation:
- Time-temperature charts
- Load records
- Material identification
- Traceability

Atmosphere Control:

Purpose:
- Prevent oxidation/decarburization
- Enable surface treatments
- Protect reactive metals

Types:
- Endothermic (generated gas)
- Nitrogen-based
- Vacuum
- Salt bath (inherent protection)

Carbon Potential:
- Neutral for hardening
- Carburizing for surface hardening
- Decarburization must be prevented

Quenching:

Variables:
- Media selection
- Temperature
- Agitation
- Part orientation

Issues:
- Distortion
- Cracking
- Soft spots
- Residual stress

Quality Verification:

Hardness Testing:
- Most common verification
- Multiple locations
- Surface and core (if applicable)
- Per specification requirements

Microstructure:
- Metallographic examination
- Grain size
- Phase verification
- Decarburization check

Mechanical Testing:
- When required by specification
- Tensile, impact testing
- Coupons or actual parts

Specifications:

Industry Standards:
- AMS 2750 (pyrometry)
- AMS 2759 (steel heat treating)
- AMS 2770/2771 (aluminum)
- ASTM standards

Quality Systems:
- Nadcap (aerospace)
- CQI-9 (automotive)
- ISO 17025 (laboratory)
- Customer specifications

Careers in heat treatment:

Heat Treat Technician:
Operate heat treat equipment:
- Furnace operation
- Load handling
- Process monitoring
- $40,000-$60,000

Heat Treat Operator:
Advanced operations:
- Complex treatments
- Quality verification
- Equipment maintenance
- $45,000-$70,000

Heat Treat Engineer:
Process engineering:
- Procedure development
- Problem solving
- Process optimization
- $70,000-$105,000

Metallurgist:
Materials expertise:
- Material selection
- Failure analysis
- Process development
- $75,000-$115,000

Skills Development:

Fundamentals:
- Heat treatment principles
- Equipment operation
- Safety procedures
- Basic metallurgy

Intermediate:
- Multiple alloy systems
- Quality verification
- Troubleshooting
- Specification interpretation

Advanced:
- Procedure development
- Failure analysis
- Process optimization
- Specification writing

Training:

Industry:
- ASM Heat Treating Society
- MTI (Metal Treating Institute)
- Equipment manufacturer training

Certifications:
- MTI certifications
- Nadcap requirements
- Internal certifications

Work Environments:

Commercial Heat Treaters:
- High variety
- Customer interaction
- Production focus

In-House Operations:
- Product-specific
- Integrated manufacturing
- Process control focus

Aerospace:
- Stringent specifications
- Nadcap requirements
- Documentation intensive

Heat treating expertise ensures materials perform as designed.