1. Overview
Welding preheat is the process of heating the base metal to a specified temperature before welding begins. Preheat serves several critical metallurgical functions: it slows the cooling rate of the weld and heat-affected zone (HAZ), reduces the risk of hydrogen-induced cracking, minimizes residual stresses, and improves weldability of hardenable steels.
Pipeline Welding
API 1104
Cross-country pipeline construction and in-service welding with specific preheat requirements.
Process Piping
ASME B31.3
Preheat and PWHT requirements based on P-number, thickness, and service conditions.
Structural
AWS D1.1
Preheat for structural steel welding based on steel category and thickness.
Pressure Vessels
ASME VIII
Preheat and PWHT for vessels, columns, and heat exchangers per Division 1.
Why preheat? The three conditions for hydrogen-induced cracking are: susceptible microstructure (martensite), sufficient hydrogen, and tensile stress. Preheat addresses the first two by slowing the cooling rate (reducing martensite formation) and allowing hydrogen to diffuse out of the weld zone before it becomes trapped.
2. Carbon Equivalent Calculations
Carbon equivalent (CE) is a single number that represents the combined effect of all alloying elements on the hardenability (and therefore crack susceptibility) of steel. Higher CE values indicate greater hardenability and higher risk of hydrogen-induced cracking.
CE (IIW) Formula
IIW Carbon Equivalent (CE):
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Where all values are weight percent from the MTR
(Material Test Report / mill certificate).
Interpretation:
CE < 0.35: Low hardenability, generally no preheat
CE 0.35-0.45: Moderate, preheat 100-200°F
CE 0.45-0.60: High, preheat 200-400°F
CE > 0.60: Very high, preheat 400-600°F + PWHT
Best suited for: Carbon steels with C > 0.12%
(structural steels, older pipe specifications)
Pcm (Ito-Bessyo) Formula
Ito-Bessyo Critical Metal Parameter (P_cm):
P_cm = C + Si/30 + Mn/20 + Cu/20 + Ni/60
+ Cr/20 + Mo/15 + V/10 + 5B
Interpretation:
P_cm < 0.20: Low susceptibility
P_cm 0.20-0.25: Moderate susceptibility
P_cm > 0.25: High susceptibility
Best suited for: Modern low-carbon microalloyed steels
(API 5L X60, X65, X70, X80) with C < 0.12%
The P_cm formula gives more weight to carbon and is
more appropriate for modern pipeline steels where
carbon content has been reduced through TMCP processing.
Which Formula to Use?
| Steel Type | Carbon Content | Recommended Formula |
|---|---|---|
| Older carbon steels (A106, A53) | C > 0.18% | CE (IIW) |
| Structural steels (A36, A572) | C = 0.12-0.25% | CE (IIW) |
| Modern pipeline (X52-X65) | C = 0.08-0.12% | P_cm or CE (IIW) |
| High-strength pipeline (X70-X80) | C < 0.08% | P_cm |
| Alloy steels (P11, P22) | Varies | CE (IIW) + code-specific rules |
Example Calculation
Given MTR for API 5L X65 pipe:
C = 0.07, Mn = 1.45, Si = 0.25
Cr = 0.05, Mo = 0.02, V = 0.04
Ni = 0.03, Cu = 0.02, B = 0.0001
CE (IIW):
= 0.07 + 1.45/6 + (0.05+0.02+0.04)/5 + (0.03+0.02)/15
= 0.07 + 0.242 + 0.022 + 0.003
= 0.337
P_cm:
= 0.07 + 0.25/30 + 1.45/20 + 0.02/20 + 0.03/60
+ 0.05/20 + 0.02/15 + 0.04/10 + 5(0.0001)
= 0.07 + 0.008 + 0.073 + 0.001 + 0.001
+ 0.003 + 0.001 + 0.004 + 0.001
= 0.162
This X65 has low hardenability by both measures.
3. Preheat Requirements by Code
ASME B31.3 Preheat Requirements
| P-Number | Material | Minimum Preheat (°F) | Conditions |
|---|---|---|---|
| P-1 | Carbon steel (A106, A53) | 50°F (t ≤ 1") 200°F (t > 1") |
Min. interpass: same as preheat |
| P-3 | Alloy steel (1/2Cr-1/2Mo) | 250°F | All thicknesses |
| P-4 | 1-1/4Cr-1/2Mo (P11) | 300°F | All thicknesses |
| P-5A | 2-1/4Cr-1Mo (P22) | 400°F | All thicknesses |
| P-8 | Austenitic SS (304, 316) | 50°F | No preheat required (except ambient min.) |
Wall Thickness Effect
Thickness-Dependent Preheat (AWS D1.1 Approach):
Thicker sections cool faster because there is more
mass to conduct heat away from the weld. This increases
the risk of martensite formation.
General guidelines for carbon steel (CE = 0.40):
t ≤ 3/4": 50°F minimum (ambient)
t = 3/4" - 1-1/2": 150°F minimum
t = 1-1/2" - 2-1/2": 225°F minimum
t > 2-1/2": 300°F minimum
Higher CE values shift all thresholds downward
(thinner material needs preheat).
The combined effect of CE and thickness determines
the required cooling time (t8/5) to avoid martensite.
4. Hydrogen-Induced Cracking
Hydrogen-induced cracking (HIC), also called cold cracking, delayed cracking, or underbead cracking, is the most common and most dangerous type of weld cracking in carbon and low-alloy steels. It occurs hours to days after welding is complete.
Three Required Conditions
HIC Triad (All Three Must Be Present):
1. Susceptible microstructure:
Martensite or upper bainite in the HAZ
Formed when cooling rate is too fast
Controlled by: PREHEAT (slows cooling)
2. Diffusible hydrogen:
H enters weld pool from moisture, contamination,
flux, or shielding gas impurities
Controlled by: Low-hydrogen processes (SMAW E70xx-H4,
GTAW, GMAW), dry electrodes, clean joint
3. Tensile stress:
Residual welding stress + external restraint
Present in virtually all welds
Reduced by: Joint design, weld sequence, PWHT
Remove ANY one condition and HIC cannot occur.
Preheat primarily addresses conditions 1 and 2.
Hydrogen Sources
| Source | Hydrogen Level | Prevention |
|---|---|---|
| Cellulosic electrodes (E6010) | 30-60 mL/100g | Use for root pass only, low-H fill/cap |
| Low-hydrogen electrodes (E7018) | 4-8 mL/100g | Store in heated rod ovens (250-300°F) |
| GTAW / GMAW | < 4 mL/100g | Inherently low-hydrogen processes |
| Moisture on base metal | Variable (high) | Preheat drives off surface moisture |
| Rust, scale, contamination | Variable | Clean joint preparation (grinding, brushing) |
Cooling Rate and t8/5
Critical Cooling Time (t_8/5):
t_8/5 = Time to cool from 800°C to 500°C (seconds)
(1,472°F to 932°F)
This temperature range is where austenite transforms
to martensite, bainite, or ferrite/pearlite.
Fast cooling (short t_8/5) → Martensite (hard, brittle)
Slow cooling (long t_8/5) → Ferrite/pearlite (soft, ductile)
Preheat effect on t_8/5:
No preheat (70°F): t_8/5 = 5-10 sec (typical)
Preheat 200°F: t_8/5 = 10-20 sec
Preheat 400°F: t_8/5 = 20-40 sec
Preheat 600°F: t_8/5 = 40-80 sec
Target t_8/5 depends on steel composition (CE/P_cm)
and desired maximum HAZ hardness (typically < 350 HV).
5. Post-Weld Heat Treatment (PWHT)
PWHT is a controlled thermal cycle applied after welding to relieve residual stresses, temper hard microstructures in the HAZ, and improve ductility and toughness. PWHT is typically required for thicker materials, higher-alloy steels, and services where stress corrosion cracking is a concern.
PWHT Requirements by Material
| Material | PWHT Temp (°F) | Hold Time | Trigger |
|---|---|---|---|
| Carbon steel (P-1) | 1,100-1,200 | 1 hr/inch, 15 min minimum | t > 3/4" (B31.3), sour service |
| C-1/2Mo (P-3) | 1,100-1,200 | 1 hr/inch, 15 min minimum | All thicknesses (typical) |
| 1-1/4Cr-1/2Mo (P-4) | 1,250-1,350 | 1 hr/inch, 15 min minimum | All thicknesses |
| 2-1/4Cr-1Mo (P-5A) | 1,300-1,400 | 1 hr/inch, 15 min minimum | All thicknesses |
| Austenitic SS (P-8) | Not required | N/A | PWHT can cause sensitization |
PWHT Procedure
Typical PWHT Cycle:
1. Heating rate: ≤ 400°F/hr ÷ (t/inch)
Maximum: 400°F/hr for t ≤ 1"
Example: 2" thick = 200°F/hr maximum
2. Hold temperature: Per code table (above)
Uniformity: ±25°F across heated band
3. Hold time: 1 hr per inch of thickness
Minimum: 15 minutes
Example: 1.5" thick = 1.5 hours
4. Cooling rate: ≤ 400°F/hr ÷ (t/inch)
Cool under insulation to 600°F
Below 600°F: may cool in still air
Heated Band Width:
Minimum = 3 × wall thickness on each side of weld
Plus the weld width itself
Soak band must achieve uniform temperature
6. API 1104 Pipeline Requirements
API 1104 governs welding of pipelines and related facilities. It specifies preheat requirements based on the qualified welding procedure specification (WPS) and material properties.
API 1104 Preheat Guidelines
API 1104 Section 7.11 - Preheat:
The preheat temperature specified in the qualified
WPS shall be maintained during welding. Preheat
temperature is typically determined by:
1. Steel grade and carbon equivalent
2. Wall thickness
3. Welding process (hydrogen level)
4. Ambient temperature and wind conditions
Typical Pipeline Preheat Temperatures:
X42-X52, CE < 0.35: 50-100°F (dew point +)
X52-X65, CE 0.35-0.42: 100-200°F
X65-X80, CE > 0.42: 200-300°F
Heavy wall (> 1"): Add 50-100°F
In-Service Welding (flowing product):
Product flow removes heat rapidly
Preheat must overcome heat sink effect
Higher preheat required (typically 300-400°F)
Minimum wall thickness check required first
In-Service Welding Considerations
| Factor | Effect | Mitigation |
|---|---|---|
| Flowing product | Rapid heat removal, fast cooling | Higher preheat, higher heat input |
| Burn-through risk | Thin wall at temperature may fail | Minimum wall thickness 0.250" (API 2201) |
| Product decomposition | Hydrogen from hydrocarbon breakdown | Control heat input to limit inner wall temp |
| Restraint | Pipeline cannot move to relieve stress | Temper bead technique, hydrogen bake-out |
Temper Bead Welding
Temper bead welding is a specialized technique used for in-service repairs and situations where PWHT is impractical. Subsequent weld passes are deposited with controlled heat input to temper the HAZ of previous passes, reducing hardness without a separate PWHT cycle.
Temper Bead Technique:
1. First layer: Deposited directly on base metal
- Creates hard HAZ in base metal
- Controlled bead placement and heat input
2. Second layer: Overlaps first layer by 50%
- Heat from second layer tempers HAZ of first
- Reduces hardness to acceptable levels (< 350 HV)
3. Subsequent layers: Normal welding
- HAZ refinement continues
Requirements:
- Qualified WPS with specific heat input ranges
- Controlled bead placement and overlap
- Preheat maintained throughout
- Maximum interpass temperature controlled
- Hardness verification on procedure qualification
7. Practical Considerations
Preheat Methods
| Method | Temperature Range | Application |
|---|---|---|
| Oxy-fuel torch | Up to 600°F | Field pipeline welding, small areas |
| Electric resistance (ceramic pad) | Up to 1,400°F | PWHT, controlled preheat, shop |
| Induction heating | Up to 1,400°F | Pipeline PWHT, fast heating |
| Propane ring burner | Up to 500°F | Field pipeline preheat, large diameter |
Temperature Measurement
Preheat Temperature Verification:
Measurement location:
- 2 inches from weld edge (minimum)
- On the face opposite the heat source
- Both sides of the joint
Methods:
- Temperature-indicating crayons (Tempilstik)
Quick, inexpensive, ±1% accuracy
- Contact pyrometer (thermocouple)
More accurate, provides continuous reading
- Infrared pyrometer
Non-contact, fast, affected by emissivity
Timing:
- Verify preheat immediately before welding
- Maintain throughout welding (interpass)
- Do not allow temperature to drop below minimum
- Maximum interpass: typically 500-600°F (varies)
Electrode Storage and Handling
Low-hydrogen electrode storage is critical for preventing hydrogen-induced cracking. Electrodes that have absorbed moisture must be re-dried or discarded.
Rod oven requirements: Low-hydrogen electrodes (E7018, E8018, E9018) must be stored in heated holding ovens at 250-300°F after removal from the hermetically sealed container. Electrodes exposed to ambient conditions for more than 4 hours (E7018) or 2 hours (E8018/E9018) must be re-dried at 600-800°F for 1-2 hours or discarded. AWS A5.1 and A5.5 provide specific re-drying requirements.
Cold Weather Welding
- Minimum ambient temperature for welding: 0°F (most codes)
- When ambient is below 32°F, heat all weld joints to at least 50°F regardless of CE
- Remove moisture, ice, and frost from the joint area before preheat
- Wind shields required to prevent rapid cooling of the weld
- Increase preheat by 50-100°F above normal when ambient is below 32°F
- Consider post-heat (hydrogen bake-out) at 400-600°F for 1-2 hours
Ready to determine preheat requirements?
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