Sep 21, 2026
Why Do Cr-Mo Steel Welds Crack? Causes and Prevention
Learn why Cr-Mo steel welds crack, including hydrogen, hard HAZ, residual stress, wrong consumables and PWHT problems. Discover practical ways to prevent Cr-Mo welding cracks.
Why Do Cr-Mo Steel Welds Crack?
Cr-Mo steel welds can crack even when the welding wire appears to meet the required strength and AWS classification.
The reason is that Cr-Mo steel welding cracks are usually caused by several factors working together, rather than by the filler metal alone.
Common causes include:
- Hydrogen-assisted cracking
- Excessive HAZ hardness
- Insufficient or inconsistent preheat
- Incorrect welding consumable selection
- High residual stress and joint restraint
- Incorrect PWHT
- Poor control of welding heat input
This is particularly important for Cr-Mo grades such as P11, P22, P5 and P91, which are widely used in boilers, pressure vessels, refineries and high-temperature piping.
For buyers and welding contractors, the key question is not simply:
“Which welding wire is stronger?”
The more important question is:
“Does the consumable, welding procedure and heat-treatment process match the actual Cr-Mo steel and service requirements?”
1. Hydrogen Can Cause Delayed Cracking
Hydrogen-assisted cracking is one of the most important welding risks for Cr-Mo steels.
The problem usually involves three conditions:
Hydrogen + susceptible microstructure + tensile stress
When Cr-Mo steel cools too quickly after welding, the HAZ can develop a hard microstructure that is susceptible to cracking. If diffusible hydrogen is also present, cracks may develop during cooling or several hours after welding.
This creates a common field problem:
The weld passes visual inspection immediately, but cracks appear later.
Hydrogen can enter the welding area through:
- Moisture
- Contaminated joint surfaces
- Poorly stored consumables
- Improper electrode handling
- Humid working conditions
How to Reduce Hydrogen Cracking
Depending on the applicable specification and WPS:
- Keep consumables dry and properly stored
- Clean the joint before welding
- Control preheat and interpass temperature
- Use suitable low-hydrogen consumables where required
- Follow the qualified welding procedure
- Apply postheat or hydrogen removal treatment when specified
For some Cr-Mo repair applications, the condition of the base material itself may also need to be considered, especially where the component has been exposed to hydrogen-containing service.
Customer pain point: A supplier who only provides tensile strength data is not giving the complete picture. For Cr-Mo applications, chemistry, hydrogen control, welding procedure and heat treatment can all affect weld performance.
2. A Hard HAZ Increases Cracking Risk
Cr-Mo steels contain alloying elements that increase hardenability.
If the weld cools too rapidly, the HAZ may develop a hard microstructure that is more susceptible to cracking.
This becomes increasingly important as the alloy system becomes more complex.
For example, P91 is a highly alloyed Cr-Mo steel with a carefully controlled microstructure. Its welding procedure requires much tighter control than ordinary carbon steel.
Therefore, simply choosing a welding wire with higher tensile strength does not solve the problem.
The complete welding cycle matters:
Preheat → Welding Heat Input → Interpass Control → Cooling → PWHT
If one part of this sequence is poorly controlled, cracking risk can increase.
3. Insufficient Preheat Can Increase Cracking Risk
Preheat is one of the most important controls when welding many Cr-Mo steels.
Proper preheat can:
- Reduce the cooling rate
- Reduce the formation of excessively hard HAZ structures
- Give hydrogen more time to diffuse out
- Reduce thermal gradients
- Reduce the risk of hydrogen-assisted cracking
However, there is no single preheat temperature that should be applied to every Cr-Mo grade.
P11, P22, P5 and P91 may have different requirements depending on:
- Material thickness
- Carbon equivalent
- Welding process
- Joint restraint
- Consumable
- Heat input
- Applicable code
- Qualified WPS/PQR
Therefore, copying a preheat temperature from another project can create unnecessary risk.
The actual requirement should come from the applicable code, project specification and qualified WPS.
Do you know,
What Are Cr-Mo Welding Consumables? Grades, Materials and Applications
Please visit,
4. The Wrong Welding Consumable Can Create Problems
One of the most common purchasing mistakes is selecting a welding wire based mainly on tensile strength.
Cr-Mo filler metals are designed around specific alloy systems.
Base Material | Typical Alloy System | Common Matching Wire |
P11 / T11 | 1.25Cr-0.5Mo | ER80S-B2 |
P22 / T22 | 2.25Cr-1Mo | ER90S-B3 |
P5 | 5Cr-0.5Mo | ER80S-B6 |
P91 / T91 | 9Cr-Mo-V-Nb | ER90S-B9 / B91* |
*Check the applicable AWS edition and project specification. ER90S-B91 replaced ER90S-B9 in AWS A5.28:2020, although B9 remains widely used in product specifications and industry searches.
The important lesson is:
Similar tensile strength does not mean two Cr-Mo welding wires are interchangeable.
For example, ER80S-B2 and ER80S-B6 belong to different alloy systems and are intended for different base materials.
Know more of,
ER80S-B6 vs ER80S-B2: Key Differences and Applications
The same principle applies when comparing B3 and B9/B91 for P22 and P91.
A correct consumable should be selected based on base material, alloy system, welding process, AWS classification and qualified welding procedure, not simply price or tensile strength.
5. High Restraint Can Increase Cracking Risk
Cr-Mo welding becomes more challenging when the joint has high restraint.
Examples include:
- Thick-wall pipe
- Pressure vessel components
- Large nozzles
- Complex joints
- Heavy sections
- Components where uniform preheating is difficult
Welding creates thermal expansion and contraction. When the joint cannot freely move, residual stress can become significant.
This means the same consumable and welding process may behave differently on a small test coupon and a large, highly restrained production component.
For difficult joints, cracking prevention should therefore consider joint design, welding sequence, preheat, heat input and cooling, rather than focusing only on filler metal.
6. Cracks Can Also Occur During PWHT
Not every Cr-Mo welding crack is a hydrogen crack.
Some cracking can occur during or after post-weld heat treatment (PWHT).
Reheat cracking, also called stress-relief cracking, is different from hydrogen-induced cold cracking. It is associated with metallurgical and stress conditions in susceptible steels and welded joints.
This creates another frustrating situation for contractors:
The weld looks acceptable after welding, but cracks appear during or after PWHT.
When this happens, changing the welding wire immediately may not solve the problem.
The investigation should consider:
- Base material chemistry
- Weld metal chemistry
- HAZ condition
- Welding heat input
- Joint restraint
- PWHT temperature
- Holding time
- Heating and cooling rates
- Actual WPS/PQR compliance
For high-alloy Cr-Mo steels such as P91, PWHT is an especially important part of controlling the final welded microstructure.
7. When Did the Crack Appear?
One practical way to investigate Cr-Mo welding cracks is to consider when the crack appeared.
Crack Timing | What to Investigate |
During welding | Joint condition, restraint, heat input, material and procedure |
Shortly after welding | Hydrogen, HAZ hardness, preheat and cooling |
Hours after welding | Delayed hydrogen-assisted cracking |
During PWHT | Reheat cracking, stress and metallurgical condition |
After service begins | Service temperature, stress, material condition and welding history |
The timing does not prove the cause by itself, but it can help narrow the investigation.
This is particularly useful for purchasing and quality teams because the correct response is not always:
“Replace the welding wire.”
A cracked weld should be investigated systematically before changing materials or suppliers.
8. How Can Cr-Mo Welding Cracks Be Prevented?
A practical prevention strategy should cover the entire welding process.
Before Welding
Check:
- Correct base material
- Material certificate
- Required AWS classification
- Consumable storage
- Joint cleanliness
- Preheat requirements
- WPS/PQR
- Project specification
During Welding
Control:
- Preheat
- Interpass temperature
- Heat input
- Welding sequence
- Consumable handling
- Hydrogen exposure
- Cooling conditions
After Welding
Verify:
- Required postheat
- PWHT temperature
- Holding time
- Heating and cooling requirements
- Hardness where specified
- NDE requirements
- Material and consumable traceability
The goal is not simply to select a “strong” welding wire.
The goal is to maintain metallurgical compatibility and process control from material preparation through final heat treatment.
Know more of,
How to select welding consumables for Cr-Mo steel?
9. What Should Buyers Ask a Cr-Mo Welding Wire Supplier?
Before placing an order, buyers should confirm more than price and tensile strength.
Ask the supplier:
- What AWS classification is specified?
- Which Cr-Mo base materials is the wire intended for?
- Is it suitable for the required welding process?
- What diameters are available?
- What chemical composition is guaranteed?
- What mechanical properties are guaranteed?
- Can you provide TDS and MTC/CoA?
- Is batch traceability available?
- What storage conditions are recommended?
- Does the product match the project WPS/PQR?
This is particularly important for P11, P22, P5 and P91 projects, where repairing a cracked weld can cost significantly more than the original consumable.
For industrial buyers, documentation and traceability are part of product selection, not an afterthought.
Cr-Mo Welding Crack Prevention Checklist
Before changing the welding wire, check:
1. Base material
Is the material actually P11, P22, P5, P91 or another grade?
2. Consumable classification
Does the filler metal match the required alloy system?
3. Hydrogen control
Were the consumables dry and properly handled?
4. Preheat
Was the required preheat achieved throughout the joint?
5. Interpass temperature
Was it maintained within the WPS range?
6. Heat input
Was the actual welding procedure followed?
7. Joint restraint
Could residual stress be contributing to cracking?
8. PWHT
Was the specified heat treatment correctly performed?
9. Crack timing
Did the crack appear during welding, after cooling or during PWHT?
Final Takeaway
Cr-Mo steel welding cracks are rarely caused by one factor alone.
Hydrogen, HAZ hardness, insufficient preheat, incorrect consumable selection, residual stress and PWHT conditions can all contribute to cracking.
For buyers, the safest approach is to match the:
Base Material → AWS Classification → Welding Process → WPS/PQR → Preheat → PWHT
rather than selecting a welding wire based only on tensile strength or price.
For P11, P22, P5 and P91 applications, the right consumable is only one part of the solution. The welding procedure and heat-treatment process must also be properly controlled.
GD WELD supplies specialty welding consumables for Cr-Mo and other demanding industrial applications. Buyers can provide the base material, AWS classification, diameter and application to identify a suitable welding consumable for their project.
