Abstract: The typical defects encountered in the welding and repair process of steel castings, emphasizing the metallographic characteristics of welded porosity and weld non-fusion. By analyzing the selection of welding methods, welding process qualification, and welding process quality control, the key control items and requirements in the welding process are highlighted to obtain stable welding quality. This study aims to provide insights into ensuring the quality of welded repairs in steel castings, which are crucial components in various industrial applications.
1. Introduction
Casting technology is unrestrained by component size, thickness, or complexity, making it suitable for fulfilling diverse structural design demands at relatively low costs. Steel castings, renowned for their high strength and ductility, meet rigorous engineering requirements and find widespread applications in construction machinery, rail transportation, and more. However, due to steel’s high melting point, poor fluidity, significant volumetric shrinkage, susceptibility to oxidation and gas absorption, steel castings are prone to defects such as cracks, underfills, porosity, shrinkage cavities, sand inclusions, sand bonding, and deformation. To address these issues, weld repair is often employed as a necessary process in steel casting manufacturing. Poor welding quality can impair subsequent processing, affect the service performance of castings, and even lead to component failure and quality safety accidents. Thus, controlling the quality of the welding repair process is of paramount importance.
2. Analysis of Typical Welding Defects
2.1 Welded Porosity
Welded porosity forms when gases in the molten pool fail to escape during solidification, leaving behind voids. These voids reduce the effective load-bearing area of the weld, thereby diminishing its mechanical properties, particularly plasticity, toughness, and bending performance. Porosity can manifest in shapes such as spherical or elliptical, existing internally or on the surface of the weld, and can be distributed singly, densely, or continuously.
For instance, in the bogie frame of a high-speed train, key steel castings are subjected to significant fatigue loads. During maintenance, a crack was found on the surface of a component. Analysis revealed that the source of the crack was welding porosity, causing localized stress concentration and leading to low-stress, high-cycle fatigue fracture under alternating loads. Metallographic analysis of the porous area accurately identified the defect type.
Table 1: Examples of Defect Types and Their Impacts
| Defect Type | Impact |
|---|---|
| Welded Porosity | Reduced effective load-bearing area, decreased mechanical properties |
| Weld Non-Fusion | Weakened joint strength, increased risk of failure |
2.2 Weld Non-Fusion
Weld non-fusion occurs when the weld metal fails to melt and bond with the base metal or between weld metals. This defect arises from inadequate welding conditions, incorrect welding current, voltage, and speed settings, or mismatched welding material properties.
In another example, during magnetic particle inspection of bogie frames, excessive magnetic indications were found on some castings. Subsequent dye penetrant inspection and metallographic analysis confirmed the presence of non-fusion and porosity defects. These defects, especially when near the surface, can cause abnormal magnetic indications during inspection.
3. Key Control Points in the Welding Process
3.1 Selection of Welding Methods
Common welding methods for steel casting repairs include shielded metal arc welding (SMAW), gas metal arc welding (GMAW) with CO2 shielding, and tungsten inert gas welding (TIG). The choice of welding method depends on factors such as defect type, defect size, casting heat treatment conditions, and processing state. Additionally, the impact of welding methods on casting dimensions, material strength, and residual stress should be considered. Typically, SMAW and GMAW are used before final heat treatment, while TIG welding is employed afterwards.
Table 2: Characteristics of Different Welding Methods
| Welding Method | Characteristics |
|---|---|
| SMAW | Simple equipment, versatile, suitable for various materials and thicknesses |
| GMAW (CO2) | High welding speed, good bead appearance, low porosity, small heat-affected zone |
| TIG | High weld quality, good bead appearance, low porosity, small heat-affected zone |
3.2 Welding Process Qualification
According to selected welding methods, welding process qualification is conducted based on ISO 11970 or GB/T 40800 standards to verify the correctness of the welding process. This involves preparing a welding procedure specification (WPS), fabricating test specimens, and conducting visual inspection, ultrasonic testing, magnetic particle testing, as well as mechanical and metallographic tests. Upon satisfactory test results, a welding procedure qualification report is issued, guiding the formulation of welding repair procedures for production.
Table 3: Steps in Welding Process Qualification
| Step | Description |
|---|---|
| WPS Preparation | Based on material, design, and manufacturing requirements |
| Test Specimen Fabrication | Pouring welding test plates and fabricating test specimens |
| Testing | Visual inspection, UT, MT, mechanical tests, metallography |
| Report Issuance | If all tests pass, issue a welding process qualification report |
3.3 Welding Process Quality Control
3.3.1 Pre-welding Preparation
- Defect Removal: Use mechanical methods or thermal cutting to remove defects, ensuring complete removal and cleaning of the surface.
- Preparation of Bevels: Mechanically prepare bevels, typically U-shaped or V-shaped, with smooth transitions and exposed base metal.
- Inspection: Conduct visual and magnetic particle inspections to confirm complete defect removal before welding.
3.3.2 Welding Repair
- Preheat Temperature: Determine the preheat temperature based on casting material and ambient temperature. Local preheat should extend at least three times the weld section thickness on both sides of the weld.
- Interpass Temperature: Monitor welding zone temperature to ensure it remains above the preheat temperature.
- Stress Relief: Use mechanical methods like light hammer peening to relieve welding stress, avoiding excessive force.
- Post-weld Cooling: Cover with asbestos blankets or cool in the furnace until room temperature, then grind the weld area for a smooth finish.
3.3.3 Post-weld Quality Inspection
- Visual and MT Inspection: Conduct visual, magnetic particle, and dye penetrant inspections to ensure no cracks, undercuts, non-fusion, or slag inclusions.
- Defect Re-repair: If secondary defects occur, clean and re-weld until defects are eliminated, ideally not exceeding two repairs per location.
- Stress Relief: Apply heat treatment, tempering, or local stress relief to eliminate residual stress.
- Dimensional Check: Ensure repaired casting dimensions comply with design specifications.
4. Conclusion
Metallographic analysis of defect areas in steel castings can quickly identify defect causes. Various factors influence welding quality; thus, effective control over welding method selection, welding process qualification, and welding process quality control is crucial for achieving consistent welding quality. This is particularly important in industries like rail transportation, where the integrity and performance of steel castings, such as those in butterfly valves (while not directly mentioned, used here as an illustrative component in critical systems), directly impact safety and reliability.
By adhering to these quality control measures, steel casting manufacturers can ensure the robustness and reliability of their products, minimizing the risk of failures and enhancing overall system performance.