Introduction: Thermal Management in Welding
Preheat and post-weld heat treatment (PWHT) are critical thermal processes that control the metallurgical and mechanical properties of welds. Proper thermal management prevents cracking, reduces residual stress, and ensures that welded structures perform as designed throughout their service life.
This comprehensive guide covers the principles, procedures, and best practices for preheat, interpass temperature control, and post-weld heat treatment.
Understanding Preheat
Purpose of Preheat
Primary Functions
Slow cooling rate, reduce hardening, drive off moisture, reduce shrinkage stress, prevent cold cracking.
Benefits
Prevents hydrogen cracking, reduces residual stress, improves ductility, allows hydrogen escape, better fusion.
When Preheat is Required
Material Considerations
- Carbon content
- Carbon equivalent
- Alloy content
- Thickness
- Hardenability
Code Requirements
- AWS D1.1 tables
- ASME Section VIII
- API 1104
- Customer specifications
Determining Preheat Temperature
Carbon Equivalent Formula:
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
CE < 0.40: Room temperature | CE 0.40-0.60: 200-400°F | CE > 0.60: 400-600°F+
Preheat Methods
Flame Heating
Equipment
Oxy-fuel torches, propane torches, rosebuds for large areas, temperature indicators.
Advantages
Portable, no power needed, flexible, fast setup.
Disadvantages
Less uniform, fire hazard, skill dependent, weather sensitive.
Electrical Resistance Heating
Equipment
Ceramic pad heaters, flexible elements, temperature controllers, thermocouples.
Advantages
Uniform heating, precise control, safe, documented.
Disadvantages
Requires power, less portable, higher cost, setup time.
Induction Heating
Equipment
Induction coils, power supply, cooling system, temperature control.
Advantages
Very fast, uniform, efficient, controllable.
Disadvantages
Expensive, complex, specialized, limited portability.
Preheat Measurement and Control
Temperature Measurement
Methods
- Temperature Indicating Crayons: Inexpensive, simple to use, various temperatures
- Contact Pyrometers: Digital readout, thermocouple probe, accurate, portable
- Infrared Thermometers: Non-contact, fast, large areas, surface measurement
- Thermocouples: Continuous monitoring, recording capability, multiple points
Preheat Zone
Minimum Size
- 3 inches from joint
- Both sides
- Through thickness
- Uniform
Interpass Temperature
Definition and Importance
Interpass temperature is the temperature of the weld area immediately before the next pass is deposited.
Maximum Interpass
Prevents overheating, controls grain growth, maintains properties, code requirements.
Minimum Interpass
Prevents rapid cooling, maintains preheat benefit, code specified, material dependent.
Interpass Control
Typical Limits
- Carbon steel: 500°F max
- Low alloy: 400-500°F max
- Stainless: 300°F max
- Quenched/tempered: 200-300°F max
Post-Weld Heat Treatment (PWHT)
Purpose of PWHT
Stress Relief
Reduces residual stress, prevents stress corrosion, improves dimensional stability.
Tempering
Reduces hardness, improves toughness, modifies microstructure, required for some steels.
Stress Relief
Temperature Range
- Carbon steel: 1100-1250°F
- Low alloy: 1150-1350°F
- Time: 1 hour per inch (minimum 15 min)
1. Heat slowly (400°F/hr max) | 2. Soak at temperature | 3. Cool slowly (500°F/hr max) | 4. Air cool below 600°F
PWHT Methods
Furnace PWHT
Most common, uniform heating, controlled atmosphere, best quality.
Local PWHT
Field application, resistance heating, induction heating, limited area.
Special Considerations
Quenched and Tempered Steels
- Lower PWHT temperature
- Below original temper
- Maintain strength
- Careful control
Stainless Steels
Sensitization Risk
- Carbide precipitation
- 800-1600°F range
- Reduced corrosion resistance
- Solution anneal if needed
Dissimilar Metal Welds
- Different requirements
- Carbon migration
- Residual stress
- Service conditions
Code Requirements
AWS D1.1
Mandatory PWHT
Quenched/tempered steels, A709 Gr 100/100W, specific thickness, engineer approval.
Exemptions
Normalized steels, A709 Gr 36, 50, 50W, tubular structures, specific conditions.
ASME Section VIII
- P-number based requirements
- Thickness based requirements
- Group number considerations
- Exemptions listed
Documentation
Records Required
Preheat
- Method used
- Temperature
- Location
- Time
- Inspector
PWHT
- Procedure number
- Temperature chart
- Soak time
- Heating/cooling rates
- Calibration records
Conclusion
Proper preheat and post-weld heat treatment are essential for producing quality welds in many applications. Understanding when and how to apply these thermal processes ensures that welded structures have the required properties for safe, reliable service.
By following code requirements, using appropriate equipment, and maintaining proper documentation, fabricators can consistently achieve the metallurgical and mechanical properties needed for critical welded structures.
