Chromoly Preheat and PWHT Guide: Temperature Charts & Procedures

Preheat and post-weld heat treatment (PWHT) are the two thermal controls that make chromoly welding work. Preheat slows cooling during welding to reduce HAZ hardness and allow hydrogen diffusion. PWHT tempers the hardened HAZ after welding and reduces residual stress. Skipping either one on thick or heat-treated chromoly dramatically increases the probability of cracking in the HAZ.

This guide covers both 4130 and 4140 chromoly steel with specific temperature requirements, hold times, and heating/cooling rate limits. The principles apply to other Cr-Mo steels in the 41xx series as well.

Why Chromoly Needs Thermal Management

The chromium (0.80-1.10%) and molybdenum (0.15-0.25%) in 41xx steels increase hardenability. This means martensite forms at slower cooling rates than in plain carbon steel. A 1/4 inch section of 1030 carbon steel might need a water quench to form martensite. The same section of 4130 forms martensite just from air cooling after welding.

Martensite in the HAZ is the root cause of all chromoly welding problems:

• It’s hard (35-60 HRC depending on carbon content and cooling rate)
• It’s brittle in the as-formed condition
• It has high residual stress from the volume change during transformation
• It traps hydrogen, which drives delayed cracking

Preheat and PWHT address these problems at different points in the thermal cycle:

Preheat (before and during welding):

• Reduces the cooling rate so less martensite forms
• Gives hydrogen time to diffuse out of the weld zone during welding
• Reduces the thermal gradient (and resulting stress) across the joint

PWHT (after welding):

• Tempers any martensite that did form, converting it to tempered martensite (ductile, tough)
• Reduces residual stress by allowing the steel to yield at elevated temperature
• Provides additional hydrogen diffusion time at temperature

Preheat Temperature Charts

4130 Chromoly

4140 Chromoly

Cold Weather Adjustments

When ambient temperature is below 50F:

• Add 50F to all preheat requirements
• Thin-wall 4130 tubing that normally needs no preheat should be preheated to 200F
• Shield the work from wind, which accelerates cooling
• Allow extra time for preheat to equalize through the thickness

Preheat Methods

Oxy-Fuel Rosebud Torch

The most common shop method. Use a multi-orifice rosebud tip, not a single cutting/welding tip. Apply heat broadly, moving the flame constantly to avoid localized hotspots. On tubing, rotate the flame around the circumference.

Advantages: Fast, portable, readily available Limitations: Uneven heating on complex shapes, requires operator skill, creates thermal gradients

Electric Resistance Heating Pads

Flexible ceramic heating elements that wrap around the workpiece. Connected to a temperature controller that ramps and holds at the target temperature. These provide the most uniform, controlled preheat short of a furnace.

Advantages: Precise control, uniform heating, programmable ramp rates Limitations: Expensive, limited to shapes the pads can wrap around, slower heating rate

Induction Heating

Coils wrapped around the workpiece generate eddy currents that heat the steel from within. Very fast, very uniform, and precisely controlled. Common in pipeline and industrial applications.

Advantages: Fastest, most uniform, excellent control Limitations: Most expensive equipment, limited portability, requires specific coil configurations

Temperature Verification

Regardless of heating method, verify temperature using:

• Temperature crayons (Tempilstik): Keep a range on hand (200F, 300F, 400F, 500F, 600F)
• Contact thermocouples: K-type probe with digital readout, most accurate
• Infrared thermometer: Quick spot checks, less accurate on shiny surfaces

Measure on the opposite side from the heat source to confirm through-thickness temperature.

PWHT Options for Chromoly

Option 1: Stress Relief (Most Common)

Temperature: 1100-1275F (593-691C) Hold time: 1 hour per inch of thickness, 1 hour minimum Heating rate: 200-400F per hour (no shock heating) Cooling rate: 100F per hour maximum (furnace cool) down to 600F, then air cool

Stress relief is the standard PWHT for most 4130 and 4140 welding. It tempers martensite in the HAZ to tempered martensite, which retains significant strength while gaining ductility and toughness. It also reduces residual welding stress by 70-80%.

Choose the PWHT temperature based on the desired final hardness and the original material condition. If the part needs to maintain high hardness for service (like a Q&T shaft), stress relieve at the lower end (1100F). If maximum ductility and toughness are the priority, use the higher end (1200-1275F).

Option 2: Normalizing

Temperature: 1575-1650F (857-899C) for 4130, 1525-1600F (830-871C) for 4140 Hold time: 1 hour per inch of thickness Cooling: Air cool in still air

Normalizing is a more complete heat treatment than stress relief. It heats the steel above the upper critical temperature, dissolving the as-welded microstructure entirely. On cooling, a new, uniform fine-grained structure forms. The result is a base metal, HAZ, and weld that all have similar microstructure and properties.

When to normalize instead of stress relieve:

• Aerospace applications where uniform properties are required
• When the part will receive a subsequent Q&T treatment
• When the as-welded HAZ has extremely high hardness that stress relief alone can’t adequately reduce
• When multiple repair cycles have degraded the local microstructure

Caution: Normalizing resets any prior Q&T heat treatment in the entire piece, not just the weld zone. The part reverts to normalized properties (97 ksi tensile for 4130). If Q&T properties are needed, a full Q&T cycle must follow normalizing.

Option 3: Full Q&T (Quench and Temper)

Austenitize: 1525-1575F for 4130, 1500-1550F for 4140 Quench: Oil quench (water quench risks cracking) Temper: Per target hardness (see 4140 tempering table in the 4140 article)

Full Q&T is used when the welded component must return to its original heat-treated strength. This is the only way to achieve full property matching between the base metal, HAZ, and weld deposit. It requires a furnace, quench tank, and precise temperature control.

When to use full Q&T:

• Aerospace and military components with property specifications
• Heavily loaded shafts, gears, and structural members
• Any application where the original Q&T properties are required

Hydrogen Bake-Out

Even with low-hydrogen consumables, some diffusible hydrogen enters the weld zone. On chromoly, where the HAZ is susceptible to hydrogen cracking, a post-weld hold at elevated temperature allows hydrogen to diffuse out before the part cools.

Hydrogen bake-out procedure:

1. After the last weld pass, maintain preheat temperature for the soak time
2. Soak time: 1 hour per inch of thickness (minimum 1 hour)
3. This can be done as a separate step or as the first stage of PWHT
4. If PWHT will follow, maintain preheat temperature until the part transfers to the PWHT furnace

If PWHT is not immediate (the part must cool before furnace is available), the hydrogen bake-out at preheat temperature is critical. It’s the safety margin between welding and the cooling transient where hydrogen cracking occurs.

Post-Weld Hardness Testing

After PWHT, hardness testing verifies that the treatment achieved the target. Hardness readings in the HAZ indicate whether the martensite was adequately tempered.

Acceptable HAZ hardness after PWHT:

• 4130 (normalized base): 25-35 HRC
• 4130 (Q&T base): Varies with original temper, typically 28-40 HRC
• 4140 (normalized base): 28-38 HRC
• 4140 (Q&T base): Varies, typically 30-45 HRC

Hardness testing methods:

• Portable Rockwell: Most common, direct C-scale reading
• Portable Leeb (rebound): Fast, suitable for field work
• Portable Brinell: Good for rough surfaces
• Cross-section microhardness: Most detailed, shows hardness gradient across HAZ (lab test)

If HAZ hardness exceeds the target range after PWHT, the stress relief temperature was too low or the hold time was too short. Re-treat at a higher temperature or longer hold time.

Localized PWHT for Large Assemblies

When the welded assembly is too large for a furnace (like a race car chassis), localized PWHT using resistance heating pads or oxy-fuel torches is an option.

Procedure:

1. Apply heating pads or torches to heat a band 3 times the joint width (minimum 6 inches total) centered on the weld
2. Ramp to the PWHT temperature at 200-400F per hour
3. Hold for the specified time
4. Wrap the heated zone in ceramic fiber blanket and allow to cool naturally

Localized PWHT is less effective than furnace treatment because temperature gradients exist between the heated zone and the surrounding cold metal. These gradients create their own stresses. For critical applications, furnace PWHT is always preferred.

For 4130 tubing applications with ER70S-2 filler on thin wall, PWHT is often not required. For 4140 repairs, PWHT is almost always mandatory. The material condition, filler selection, and application requirements determine which PWHT option is appropriate.