**Unexplained Cracking After Heat Treatment?**
**Unveiling the Hidden Traps of Welding Reheat Cracking**
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### **01 Definition**
> Certain high-strength steels and high-temperature alloys containing precipitation-strengthening elements (e.g., Al, Ti, Nb), such as low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation-strengthened superalloys, and some austenitic stainless steels, may develop cracks during post-weld heat treatment (PWHT) or high-temperature service, even if no cracks were present after welding. These cracks are termed **Stress Relief Cracking (SR Cracking)**. In engineering, cracks formed during PWHT or long-term service (e.g., at 500–600°C) are collectively referred to as **Reheat Cracking**.
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### **02 Key Features of Reheat Cracking**
> (1) Occurs only in materials with precipitation-strengthening elements. Carbon steels and solid-solution strengthened alloys are generally immune.
> (2) Exhibits a **temperature-sensitive range** (e.g., 500–700°C for low-alloy steels; 700–900°C for austenitic stainless steels and superalloys).
> (3) Cracks initiate and propagate along **coarse-grained boundaries in the heat-affected zone (HAZ)**, following intergranular paths near the fusion line.
> (4) Requires high residual stresses and stress concentration in welded regions.
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### **03 Factors Influencing Reheat Cracking**
> (1) **Carbide-forming elements** (Cr, Mo, V, Ti, Nb) increase susceptibility. For example, vanadium (V) significantly raises SR cracking risk in pearlitic heat-resistant steels.
> (2) Heating rate and dwell time within the sensitive temperature range.
> (3) **Coarse grain size** in HAZ enhances cracking tendency.
> (4) **Welding methods**: High heat input processes (e.g., submerged arc welding) promote grain coarsening, increasing reheat cracking risk in grain-sensitive steels.
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### **04 Prevention Measures**
> (1) Use **low-strength matching welding materials**.
> (2) Apply preheating or post-heating to control cooling rates.
> (3) Avoid prolonged dwell in the sensitive temperature range.
> (4) Reduce residual stresses and mitigate stress concentration.
> (5) Perform **stabilization heat treatment** for specific alloys (e.g., Incoloy 800HT for service temperatures ≥538°C).
> (6) Conduct **non-destructive testing (NDT)** after heat treatment for susceptible materials.
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### **05 Reheat Cracking-Sensitive Materials**
> 15MnVR, 15MnNbR, 18MnMoNbR, 13MnMoNbR, 07MnCrMoVR, 07MnNiMoVDR, 17Cr1Mo1V, and Japanese CF-62 steel.
> **Note**: Reheat cracks are insidious and may lead to sudden failures. Risk assessment and mitigation plans are critical during design, fabrication, and inspection of pressure vessels.
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### **06 Mechanisms of Reheat Cracking**
> Reheat cracking originates from **microcrack nucleation** at grain boundaries due to preferential sliding during stress relaxation. Two dominant theories:
>
> **1. Grain Boundary Embrittlement Theory**
> Impurity segregation (e.g., P, S) weakens grain boundaries. Carbide/nitride precipitation (Cr, Mo, V, Nb) strengthens the matrix, concentrating stress at brittle boundaries.
>
> **2. Intragranular Strengthening Theory (Wedge Cracking)**
> Dispersed precipitates (e.g., Cr, V carbides) hinder intragranular deformation, forcing stress relaxation to occur at grain boundaries.
>
> **3. Creep Rupture Theory**
> Accumulated creep damage during stress relaxation accelerates intergranular cracking.
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### **07 Reheat Cracking Sensitivity Formula**
> The susceptibility to reheat cracking depends on chemical composition and residual stress. Use the following formula for evaluation:
>
> **△G = Cr + 3.3Mo + 8.1V + 10C – 2**
>
> - **△G < 1.5**: Low sensitivity
> - **1.5 < △G < 2**: Moderate sensitivity
> - **△G > 2**: High sensitivity
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### **08 Expert Analysis**
> **1.** Reheat cracking occurs in constrained welded joints during PWHT or high-temperature service. Historically observed in austenitic stainless steels in power plants, it is linked to **intragranular precipitates** that strengthen the matrix, shifting strain to weakened grain boundaries. The exact mechanisms remain partially unresolved.
>
> **2.** Coarse grains, poor weld design (e.g., excessive shrinkage), and aggressive welding techniques exacerbate cracking.
>
> **Two Types of Reheat Cracking**:
> 1) **Stress Relief Cracking (SR Cracking)**: Occurs during PWHT.
> 2) **Long-Term Service Cracking**: Develops during high-temperature operation.
>
> **Key Characteristics**:
> - Limited to materials with precipitation-strengthening elements (Ti, Al).
> - Temperature-dependent, with distinct sensitive ranges.
> - Intergranular propagation in HAZ coarse-grained regions.
> - Requires residual stress and stress concentration.
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