Preheat reduces the cooling rate of the weld zone to prevent hydrogen-induced cracking in the heat-affected zone (HAZ). The minimum preheat temperature depends on the base metal composition, the material thickness, the hydrogen level of the welding consumable, and the restraint of the joint. Skipping preheat when it’s required is one of the most consequential shortcuts in welding because the resulting cracks may not appear for hours or days after the weld is complete.
For common structural steels (A36, A992) in thicknesses under 3/4 inch, using low-hydrogen electrodes, preheat above freezing is all that’s needed. Once material gets thicker, preheat temperatures rise rapidly. Chrome-moly alloys, high-carbon steels, and quenched-and-tempered steels all require careful preheat management at much lower thicknesses.
Why Preheat Matters
Three things must be present simultaneously for hydrogen-induced cracking:
- Hydrogen in the weld zone (from moisture in electrodes, flux, or contamination)
- A susceptible microstructure (hard martensite formed by rapid cooling)
- Tensile stress (residual stress from welding shrinkage, or applied loads)
Preheat addresses factor #2 by slowing the cooling rate. A slower cooling rate produces softer, more ductile microstructures (bainite or ferrite-pearlite) instead of hard martensite. Preheat also helps hydrogen diffuse out of the weld zone before the metal cools to the temperature where cracking occurs (typically below 300F).
Preheat Charts by Material
Carbon Steel (AWS D1.1 Table 3.3)
| Steel Group (Examples) | Thickness | Preheat (Low-H Process) | Preheat (Non-Low-H) |
|---|---|---|---|
| Group I (A36, A53, A500, A992) | Up to 3/4 in | 32F (0C) | 32F (0C) |
| Group I | 3/4 to 1-1/2 in | 150F (66C) | 200F (93C) |
| Group I | 1-1/2 to 2-1/2 in | 225F (107C) | 300F (149C) |
| Group I | Over 2-1/2 in | 300F (149C) | 400F (204C) |
| Group II (A572 Gr 50, A588) | Up to 3/4 in | 32F (0C) | 32F (0C) |
| Group II | 3/4 to 1-1/2 in | 150F (66C) | 200F (93C) |
| Group II | 1-1/2 to 2-1/2 in | 225F (107C) | 300F (149C) |
| Group II | Over 2-1/2 in | 300F (149C) | 400F (204C) |
| Group III (A514, A517 Q&T) | Up to 3/4 in | 50F (10C) | Not permitted |
| Group III | 3/4 to 1-1/2 in | 125F (52C) | Not permitted |
| Group III | 1-1/2 to 2-1/2 in | 175F (79C) | Not permitted |
| Group III | Over 2-1/2 in | 225F (107C) | Not permitted |
Low-hydrogen processes include: SMAW with E7018 (and other low-hydrogen electrodes), GMAW, FCAW, SAW. Non-low-hydrogen includes: SMAW with E6010, E6013, E7024. Note that Group III (quenched-and-tempered) steels require low-hydrogen processes only.
Chrome-Moly Steels (ASME B31.3 / AWS D10.8)
Chrome-moly alloys used in high-temperature piping and pressure vessel service require careful preheat:
| Material | P-Number | Typical Preheat |
|---|---|---|
| 1/2 Cr - 1/2 Mo (A335 P1) | P-3 | 250-400F (depending on thickness) |
| 1-1/4 Cr - 1/2 Mo (A335 P11) | P-4 | 300-400F |
| 2-1/4 Cr - 1 Mo (A335 P22) | P-5A | 400-500F |
| 5 Cr - 1/2 Mo (A335 P5) | P-5B | 400-500F |
| 9 Cr - 1 Mo (A335 P9, P91) | P-5B | 400-500F (P91 may require 500F min) |
Chrome-moly steels are highly susceptible to hydrogen cracking. Preheat is mandatory regardless of thickness for most chrome-moly grades.
Carbon Equivalent Method
When the specific steel group isn’t known or falls outside standard tables, the carbon equivalent (CE) provides a preheat guideline based on the chemical composition:
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 (IIW formula)
| Carbon Equivalent | Suggested Minimum Preheat | Weldability |
|---|---|---|
| CE < 0.35 | None required (above freezing) | Good |
| CE 0.35 - 0.45 | 200-400F | Fair, preheat recommended |
| CE 0.45 - 0.60 | 400-600F | Poor, preheat required |
| CE > 0.60 | 500F+ (consult metallurgist) | Very poor weldability |
The CE calculation uses the actual chemistry from the material test report (MTR), not the specification maximum. A36 steel can have a CE as low as 0.25 or as high as 0.45 depending on the heat.
Measuring Preheat Temperature
Temperature-Indicating Crayons (Tempsticks)
Tempsticks are the most common field method. Each crayon melts at a specific temperature. Draw a mark on the base metal near the joint; when the mark melts, the surface has reached that temperature.
Advantages: Cheap ($3-5 each), no batteries, no calibration, available in many temperature increments (50F, 100F, 150F, etc.)
Limitations: Provides a go/no-go indication at one temperature only. Doesn’t show the actual temperature. May leave residue that contaminates the weld if used too close to the joint.
Contact Pyrometers (Thermocouple Surface Probes)
A thermocouple probe pressed against the base metal surface gives a direct digital readout of temperature.
Advantages: Exact temperature reading, fast response, can measure interpass temperature
Limitations: Requires calibration, probe must make good surface contact, reads surface temperature only
Infrared (IR) Thermometers
Non-contact temperature measurement using infrared sensors.
Advantages: Fast, no contact required, can scan large areas quickly
Limitations: Emissivity setting must match the surface (shiny metal gives false low readings). Affected by ambient temperature and reflected radiation. Not accurate on rust, scale, or uneven surfaces without emissivity correction.
Where to Measure
AWS D1.1 specifies measuring 3 inches from the joint on the side opposite the heat source. For thick material (over 3 inches), the measurement distance equals the material thickness but not exceeding 3 inches.
Why 3 inches on the opposite side? Measuring directly at the joint gives you the surface temperature where the heat source was, which may be hotter than the through-thickness temperature. Measuring 3 inches away on the opposite side confirms that heat has soaked through the material, not just warmed the surface.
Preheat Application Methods
Oxy-Fuel Heating Torch (Rosebud)
The most common method. A multi-orifice rosebud tip on an oxy-fuel setup provides broad, even heating.
Procedure:
- Start heating 6 inches from the joint and work outward, then back toward the joint
- Heat both sides of the joint equally
- Use a sweeping motion; don’t hold the flame in one spot
- Verify temperature on the opposite side from the heat source
- Allow time for heat to soak through (especially on thick material)
Electric Resistance Heating
Ceramic pad heaters or resistance heating blankets wrapped around pipe or placed on plate. Powered by a controller that maintains a set temperature.
Best for: Pipe preheat, PWHT, maintaining preheat on long welds, situations where open flame is prohibited
Induction Heating
Electromagnetic induction coils wrapped around pipe or placed near the joint. Heats the metal directly without flame.
Best for: Pipe preheat (fast, uniform), field applications, repetitive work
Maintaining Preheat During Welding
Preheat is not a one-time event. The minimum preheat temperature must be maintained throughout the welding operation. If the workpiece cools below the minimum between passes or during a break, it must be reheated before welding resumes.
Practical Methods
- Insulating blankets: Wrap the preheated area in ceramic fiber blankets to retain heat. This is especially effective on pipe and thick plate
- Interpass monitoring: Use a contact pyrometer to check temperature before each pass. If it’s dropped below the minimum, reheat before welding
- Continuous heating: For large or thick components, keep the heating source running at a reduced level during welding to maintain temperature
Interpass Temperature Maximum
Most WPSs specify a maximum interpass temperature in addition to the minimum preheat. For carbon steel, the maximum is typically 600F. For quenched-and-tempered steels (A514, A517), the maximum may be 400F to avoid softening the base metal.
Exceeding interpass temperature degrades mechanical properties by:
- Reducing toughness (too much grain growth)
- Softening the HAZ in quenched-and-tempered steels
- Reducing tensile strength
Special Situations
Cold Weather Welding
Welding in ambient temperatures below 32F (0C) requires preheat on all materials, regardless of thickness. AWS D1.1 prohibits welding when the ambient temperature is below 0F unless the base metal temperature is at least 70F at the start of welding.
In practice, cold weather means:
- Higher preheat requirements (more fuel, more time)
- Faster cooling rates (heat escapes through cold base metal and cold air)
- Condensation risk (moisture on cold steel surfaces)
- Electrode moisture pickup (keep electrodes in rod ovens)
Dissimilar Thickness Joints
When joining materials of different thickness, apply preheat based on the thicker member. The thicker material has a faster cooling rate due to greater heat sink mass, so it governs the preheat requirement.
Repair Welding on Unknown Steel
If the base metal is unknown (field repair on old equipment, unidentified steel):
- Assume a CE of 0.40-0.45 (conservative but reasonable for older structural and machine steels)
- Preheat to a minimum of 200-300F
- Use low-hydrogen electrodes (E7018)
- Maintain preheat until welding is complete
- Allow slow cooling (insulating blankets)
Preheat is cheap insurance. A few dollars in fuel and ten minutes of heating prevents cracks that cost hundreds in NDE, repair, and lost time. When the WPS says preheat, heat it. When you’re not sure, heat it anyway. The downside of unnecessary preheat is a few extra minutes. The downside of skipping necessary preheat is a cracked weld you might not find until it fails.