1. Introduction

Due to its high strength, excellent toughness, and good castability, ductile iron is widely used in key components of the ferrous metal casting industry, such as engineering machinery parts, automotive components, and pipeline valves. However, during the casting process, defects like blowholes, shrinkage cavities, cracks, sand inclusions, and slag inclusions are prone to occur, affected by factors including molding sand quality, pouring temperature, and process parameters. Direct scrapping of defective castings leads to raw material waste, increased production costs, and extended production cycles. As professional welding and heat treatment engineers, it is necessary to adopt scientific repair processes to realize the resource utilization of defective castings, while ensuring that the mechanical properties and metallographic structure of the repaired area match those of the base material. Based on the actual production of ferrous metal foundries, this report systematically elaborates on the process flow and core process control points for defect repair of ductile iron castings.

2. Classification of Common Defects in Ductile Iron and Judgment Standards

2.1 Main Defect Types and Characteristics

        1. Blowholes: Circular or elliptical holes with smooth inner walls on the surface or inside, mainly caused by poor air permeability of molding sand, incomplete gas discharge during pouring, or high gas content in molten iron;

        2. Shrinkage Cavities/Shrinkage Porosity: Irregular holes with dendritic crystals in thick sections of castings, resulting from insufficient feeding during the solidification and volume shrinkage of molten iron;

        3. Cracks: Divided into hot cracks (formed during solidification, with tortuous paths and oxidized surfaces) and cold cracks (formed when cooled to room temperature, with straight paths and non-oxidized surfaces), mostly caused by stress concentration and uneven cooling rate;

        4. Sand Inclusions: Defects containing sand particles with irregular shapes, originating from sand falling off and incomplete cleaning of the mold cavity;

        5. Slag Inclusions: Defects containing non-metallic inclusions (e.g., oxides, sulfides) with gray or black appearances, caused by insufficient purity of molten iron and poor slag blocking effect of the gating system.

2.2 Defect Judgment Standards

In accordance with GB/T 9444-2020 Magnetic Particle Testing of Steel Castings, GB/T 9443-2020 Penetrant Testing of Steel Castings, and the factory's internal control standards, the defect judgment shall meet the following requirements:

• · Surface Defects: The diameter of surface blowholes and sand inclusions shall be ≤ 3mm, the area of a single defect ≤ 10mm², and the distance between defects on the same plane ≥ 50mm;

• · Internal Defects: Ultrasonic testing (UT) shall show that the equivalent size of internal defects is ≤ φ2mm, and crack-like defects are not allowed;

• · Key Load-Bearing Components (e.g., crankshafts, valve bodies): Additional mechanical property requirements shall be met for defect repair: the tensile strength after repair ≥ 500MPa, and the impact toughness (-20℃) ≥ 15J/cm².

3. Core Process Flow for Defect Repair of Ductile Iron

The repair process follows the five-step principle of "Defect Detection → Defect Cleaning → Welding Repair → Post-weld Heat Treatment → Quality Inspection". Each process is interrelated, and the process parameters must be strictly controlled.

3.1 Process 1: Defect Detection and Marking

1. Selection of Detection Methods:

o Surface Defects: Visual testing (VT) is used for preliminary screening, followed by penetrant testing (PT, with color penetrant) to confirm the defect boundary and ensure no hidden surface cracks;

o Internal Defects: For thick castings (wall thickness ≥ 50mm) or key components, ultrasonic testing (UT, with probe frequency of 2.5-5MHz) is adopted to determine the depth, shape, and distribution of defects.

o Defect Marking: Mark the center position, boundary range, and depth of defects on the casting surface with a marker pen. The marking lines shall be clear and continuous to facilitate subsequent cleaning operations.

3.2 Process 2: Defect Cleaning (Core Control Point 1)

The core goal of defect cleaning is to completely remove defective structures and prevent residual defects from causing welding failure. The specific requirements are as follows:

1. Selection of Cleaning Methods:

o Small Surface Defects (depth ≤ 5mm): Use an angle grinder (equipped with φ100mm resin grinding wheel) for grinding. The grinding direction shall be perpendicular to the extension direction of defects to avoid base material damage caused by excessive grinding;

o Deep Internal Defects (depth ＞ 5mm): First, use a pneumatic chisel (air pressure of 0.6-0.8MPa) for preliminary cleaning, then use a CNC milling machine or electric grinder for fine trimming to ensure no sharp corners at the defect bottom (fillet radius ≥ 3mm);

o Crack-like Defects: First, drill stop holes (diameter φ5-8mm, depth 2-3mm beyond the crack end) at both ends of the crack to prevent crack propagation during cleaning, then clean along the crack path to form a "V"-shaped or "U"-shaped groove (groove angle 60°-80°, root face thickness 2-3mm).

o Quality Requirements After Cleaning:

o The surface roughness of the groove shall be Ra ≤ 12.5μm, with no scale, oil stains, sand particles, or other impurities. Alcohol wiping can be used for inspection;

o After cleaning, PT or UT testing shall be conducted again to confirm that defects are completely removed. If there are still residues, repeat the cleaning until the test is qualified.

3.3 Process 3: Welding Repair (Core Control Point 2)

Welding of ductile iron is prone to the formation of white cast iron structure, cold cracks, and blowholes. High-quality welding shall be achieved through material matching, parameter control, and preheating insulation:

1. Selection of Welding Materials:

o Nickel-based electrodes (e.g., ENiFe-CI, EZNiFe-1) are preferred, with nickel content ≥ 50%, which can effectively inhibit the formation of white cast iron structure and ensure that the weld toughness matches the ductile iron base material;

o For non-critical components with low load-bearing requirements, iron-nickel-based electrodes (e.g., EZNiCu-1) can be used to reduce costs, but the welding process must be strictly controlled.

0. Welding Equipment and Parameters:

o Use a DC manual arc welding machine (e.g., ZX7-400) with DC reverse polarity (electrode connected to positive pole);

o Welding parameters shall be adjusted according to the electrode diameter and defect thickness (see Table 1):

o Pretreatment Before Welding:

o Preheating Temperature: Determine according to the casting wall thickness: preheat to 200-250℃ when wall thickness ≤ 20mm; 250-350℃ when wall thickness is 20-50mm; 350-400℃ when wall thickness ＞ 50mm;

o Preheating Range: Take the defect as the center and extend 100-150mm outward. Use a crawler-type heating plate for heating, with a temperature uniformity error ≤ ±15℃. Insulate for 30-60min after preheating to ensure uniform temperature inside and outside the casting.

0. Welding Process Control:

o Welding Sequence: Adopt the "segmental backstep welding method", i.e., weld from the center of the defect to the edge. Control the length of each weld segment to 50-80mm to avoid local overheating caused by continuous welding;

o Interpass Temperature: Maintain within the range of preheating temperature ±50℃. If the temperature is too low, reheat to prevent cold crack formation;

o Weld Formation: After welding each layer, clean the slag with a steel wire brush. Check for no blowholes or slag inclusions before welding the next layer. The final weld shall be 2-3mm higher than the casting surface (reserving grinding allowance for subsequent operations), with smooth transition and no defects such as undercut or incomplete fusion.

3.4 Process 4: Post-weld Heat Treatment (Core Control Point 3)

The core functions of post-weld heat treatment are to eliminate welding internal stress, improve the structure of weld and heat-affected zone (HAZ), and avoid white cast iron structure. The specific process is as follows:

1. Selection of Heat Treatment Types:

o Routine Defect Repair: Stress relief annealing is adopted, applicable to welds without white cast iron structure;

o Presence of White Cast Iron Structure: First perform softening annealing (to eliminate white cast iron), then stress relief annealing.

o Heat Treatment Parameters:

o Softening Annealing: Heat to 850-900℃, hold for 1.5-2h (calculate 1h holding time per 25mm wall thickness), cool with the furnace to below 500℃, then air cool;

o Stress Relief Annealing: Heat to 550-600℃, hold for 2-3h, cool slowly with the furnace (cooling rate ≤ 50℃/h) to below 300℃, then air cool;

o Temperature Control: Use a thermocouple to monitor the casting temperature in real time. Control the heating rate ≤ 100℃/h to avoid casting deformation caused by rapid temperature rise.

0. Inspection After Heat Treatment:

o Visual Inspection: No deformation, cracks, or scale peeling on the casting surface;

o Hardness Testing: Use a Brinell hardness tester (HBW) to test the hardness of the weld and HAZ. The hardness value shall be ≤ 220HBW, and the hardness difference from the base material (usually 180-220HBW) shall be ≤ 30HBW.

3.5 Process 5: Quality Inspection

1. Visual Inspection: The weld surface shall be flat, with no defects such as blowholes, cracks, or slag inclusions. After grinding, it shall be flush with the casting surface and have a smooth transition;

o Non-Destructive Testing:

o Surface Testing: Conduct PT testing on the weld and its surrounding 20mm area. No linear defects (cracks) or circular defects (blowholes, slag inclusions) are allowed;

o Internal Testing: For key components, UT testing shall be conducted. No internal defects with equivalent size ＞ φ2mm are allowed in the weld;

o Mechanical Property Sampling Testing: Extract 1-2 repaired castings per batch, process them into standard tensile specimens (GB/T 228.1) and impact specimens (GB/T 229), and test the tensile strength, yield strength, and impact toughness, which shall meet the performance requirements of the base material;

o Metallographic Inspection: Sample and inspect the metallographic structure of the weld and HAZ. The weld shall have no white cast iron structure, the spheroidization grade of the ductile iron base material shall be ≥ Grade 4 (GB/T 9441), and the pearlite content shall be consistent with the base material (usually 30%-50%).

4. Precautions for Repair Process and Safety Requirements

4.1 Process Precautions

· After repairing crack-like defects, conduct aging treatment (placing at room temperature for 24h) before non-destructive testing to avoid delayed cracks;

· After repairing thick castings, conduct natural aging (placing for 72h) to eliminate residual stress;

· Adjust the welding materials and heat treatment parameters for different grades of ductile iron (e.g., QT450-10, QT600-3). For example, QT600-3 requires higher-strength nickel-based electrodes (e.g., ENiCrFe-3).

4.2 Safety Requirements

· During welding operations, wear a welding helmet (shade number 10-12), heat-resistant gloves, and flame-retardant work clothes. Ensure good ventilation in the operation area and equip with fire extinguishers;

· During heat treatment operations, wear high-temperature gloves to avoid touching high-temperature castings. Set up safety warning lines around the furnace;

· During grinding and pneumatic chiseling operations, wear dust masks and goggles to prevent harm from dust and debris.

5. Conclusion

The defect repair of ductile iron castings shall follow the principles of "accurate detection, thorough cleaning, matched welding, proper heat treatment, and strict inspection". The core control points include: no residual requirements after defect cleaning, preheating and interpass temperature control during welding, and parameter matching in post-weld heat treatment. Through the process described in this report, the mechanical properties and metallographic structure of the repaired area can be highly consistent with the base material, significantly reducing the casting scrap rate and providing technical support for ferrous metal foundries to save production costs and improve production efficiency. In practical application, the process parameters shall be flexibly adjusted according to the casting material, defect type, and application scenario to ensure stable and reliable repair quality.