CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Grade B Steel Overlay Repair
Literature Overview
This study by Chang Xia, Zhang Xiaobin, Cheng Li, and Zhang Kailin from the School of Materials Science and Engineering at Chongqing University of Technology was published in 2014 in "Hot Working Technology" (热加工工艺). The research investigates the microstructural evolution and mechanical properties of overlay weld repair on Grade B steel, a low-carbon low-alloy steel widely used in pressure vessel fabrication according to GB 713 and ASME SA-516 standards.
Technical Background
Grade B steel (equivalent to SA-516 Gr.70 or Q345R) is the most commonly used base material for pressure vessels due to its good combination of strength, toughness, and weldability. During fabrication, damage to the pressure boundary (such as gouges, cracks, or machining overshot) requires repair by overlay welding. The repair must restore the mechanical properties of the pressure boundary while maintaining compliance with applicable codes (GB/T 150, ASME VIII Div.1).
The repair welding typically uses a matching consumable (such as E7018 electrode or ER70S-6 wire) to maintain the strength and toughness characteristics of the base material. The challenge is that the overlay weld metal and HAZ may have different microstructures from the base plate due to the different thermal cycles experienced.
Microstructural Analysis
The study examines the microstructure of the overlay repair zone at different locations:
| Zone | Microstructure | Hardness (HV) | Tensile Strength (MPa) |
|---|---|---|---|
| Base metal (as-received) | Ferrite + Pearlite | 180-210 | 515-620 |
| Base metal (normalized) | Fine ferrite + Pearlite | 170-200 | 485-585 |
| Overlay weld metal | Acicular ferrite + Grain boundary ferrite | 190-230 | 550-650 |
| HAZ (coarse grain) | Widmanstätten ferrite + Bainite | 200-250 | 530-620 |
| HAZ (fine grain) | Fine ferrite + Pearlite | 180-220 | 520-600 |
The overlay weld metal typically exhibits acicular ferrite microstructure when using appropriate consumables and heat input, which provides excellent toughness properties. However, if the heat input is too high, the weld metal may develop coarse grain structures with reduced toughness.
Mechanical Property Evaluation
The study evaluates the mechanical properties of the repair zone through standardized tests:
- Tensile testing: The overlay weld metal typically exceeds the minimum tensile strength requirement of the base material, providing a safety margin for the repair.
- Hardness mapping: A Vickers hardness traverse across the repair zone reveals the hardness distribution. The maximum hardness typically occurs in the HAZ due to grain coarsening and possible martensitic transformation. The hardness should not exceed 350 HV (per ASME VIII Div.1 limits for SA-516) to ensure adequate ductility.
- Impact testing: Charpy V-notch impact tests at the repair temperature (typically -20°C or the minimum design metal temperature) verify that the repair maintains adequate toughness. The minimum required absorbed energy is typically 47 J (35 ft-lb) per ASME VIII Div.1.
- Fracture mechanics testing: For critical repairs, fracture toughness (KIC) testing may be required to ensure the repair can arrest crack propagation.
Heat Input Control and Its Effects
The welding heat input is the most critical parameter affecting the repair quality:
| Heat Input (kJ/mm) | HAZ Width (mm) | HAZ Hardness (HV) | Impact Energy (J) |
|---|---|---|---|
| 0.5-1.0 | 2-4 | 220-260 | 45-60 |
| 1.0-2.0 | 4-8 | 200-240 | 50-70 |
| 2.0-3.0 | 8-15 | 180-220 | 55-80 |
| 3.0-4.0 | 15-25 | 170-200 | 50-75 |
The optimal heat input range of 1.5-2.5 kJ/mm provides a balance between adequate weld penetration and controlled HAZ refinement. Too low heat input produces incomplete fusion and high residual stresses, while too high heat input causes excessive grain coarsening and potential toughness degradation.
Engineering Practice and Code Compliance
The repair procedure must comply with the applicable pressure vessel code:
- ASME VIII Div.1 UW-30: Specifies requirements for repair of pressure boundaries, including weld procedure qualification, welder qualification, and inspection requirements.
- GB/T 150.4: Chinese standard specifying repair requirements for pressure vessels, including acceptable defect sizes, repair procedures, and post-repair testing.
- NB/T 47014: Welding procedure qualification standard requiring demonstration of mechanical properties for the repair procedure.
Key practical considerations include:
- Defect characterization and acceptance criteria determination before repair
- Selection of repair method (GTAW for root, SMAW or GMAW for fill)
- Preheating temperature (typically 100-150°C for Grade B steel to prevent cold cracking)
- Interpass temperature control (maximum 250°C)
- Post-weld heat treatment (PWHT) when required by the code (typically for thicknesses >25 mm)
- Non-destructive examination of the completed repair (RT or UT per code requirements)
Study Insights and Reflections
This research provides valuable data on the metallurgical behavior of Grade B steel repairs that can be directly applied to engineering practice. The key finding is that the repair weld metal, when properly controlled, can achieve mechanical properties equal to or better than the base material, confirming that code-compliant repairs can restore the pressure boundary to its original integrity. The study also highlights the importance of heat input control in maintaining HAZ toughness, which is often the critical factor in determining repair acceptance. Engineers should always verify that the repair procedure has been qualified per the applicable code and that the welder performing the repair is certified for the specific procedure being used.