Common Riveting and Welding Methods and Case Studies in Mechanical Equipment Maintenance and Modification

Repair welding——The Art of Bringing Equipment Back to Life
Throughout the extended lifecycle of mechanical equipment, wear, corrosion, fatigue cracking, and even accidental damage are inevitable. Direct replacement of entire units or major components often proves prohibitively costly and time-consuming. In such instances, masterful riveting and welding repair and modification techniques become the key means to restore equipment performance, extend service life, and even facilitate functional upgrades. Unlike new-build manufacturing, repair welding confronts unique challenges including unknown materials, structural constraints, and demanding on-site conditions. This guide systematically outlines common riveting and welding methods employed in mechanical equipment repair. Drawing upon real-world case studies, it provides a set of proven, practical strategies for effective implementation.

Part One: Core Challenges in Repair Welding and Pre-Treatment Principles
Four Core Challenges

Material weldability unknown: Older equipment may utilise obsolete steel grades with high carbon equivalent, resulting in poor weldability.

High restraint stress: Localised repairs prevent thermal stresses from being freely released, making cracking highly likely.

Complete removal of defects: Should crack ends or fatigue sources remain uncleared, recurrence is inevitable following repair.

Deformation control: When welding on assembled precision equipment, deformation control demands extremely high standards.

Four-Step Diagnostic Method Before Repair“

Step One: Historical and Operating Conditions Investigation: Understand the equipment's service environment (load, temperature, medium) and the damage progression.

Step Two: Material Identification: Conduct on-site material analysis using a spectrometer to determine the base material composition.

Step Three: Precise Defect Detection: Employ dye penetrant testing (PT) and ultrasonic testing (UT) to determine crack orientation and depth.

Step Four: Formulating the Repair Plan: Based on the above information, select the welding method and consumables, and establish the welding sequence and heat treatment plan.

Part Two: Detailed Explanation of Six Common Maintenance Welding Methods
Shielded Metal Arc Welding (SMAW)

Applicable scenarios: On-site emergency repairs, confined spaces, thick and large components.

Key points of technique:

Welding rod selection: For unknown steel grades, employ low-hydrogen alkaline welding rods (such as J507), which exhibit favourable metallurgical properties and superior crack resistance.

Key technical points: Employ low current, narrow-pass welding and segmented skip welding to minimise heat input and stress. For extended cracks, weld from both ends towards the centre.

Case Study: A 300mm-long crack developed in the frame of a large mining crusher. Repaired using J507 electrodes with a U-shaped groove, preheated to 120°C, employing segmental back-welding with post-weld heat preservation and controlled cooling. The equipment has operated normally since the repair.

Gas Metal Arc Welding (GMAW/MAG) and Gas Tungsten Arc Welding (GTAW/TIG)

GMAW (MIG/MAG): Suitable for rapid repairs on medium-thickness steel and stainless steel. Solid-core welding wire offers high efficiency, whilst flux-cored welding wire (FCAW) produces minimal spatter and superior bead formation, making it more suitable for maintenance work.

GTAW (TIG): Suitable for precision components, thin-walled parts, dissimilar steel repairs, and aluminium/titanium alloy repairs. Concentrated heat input results in minimal distortion.

Case Study: Repair of surface corrosion pitting on a paper-making drying cylinder. Employing TIG cold welding (extremely low heat input), point-by-point build-up welding was performed using compatible filler material. Post-repair grinding restored both dimensions and surface finish, thereby avoiding the need for complete cylinder replacement.

Oxygen-acetylene welding (OFW) and brazing

Suitable applications: Cast iron component repair, thin-walled tubing, and small parts sensitive to heat input.

Key technique: Adjust the flame to a neutral or slightly carbonising flame. For cast iron welding repairs, preheat the entire section to 600–700°C (hot welding) or employ cold welding with nickel-based electrodes.

Case Study: Localised scratches on the cast iron guideways of an antique machine tool. Repaired using the oxy-acetylene hot welding method with cast iron welding rod. Post-weld heat retention followed by furnace cooling. Precision restored through scraping after repair.

Surfacing and Surface Repair

Purpose: To restore dimensions and impart special properties such as wear resistance and corrosion resistance to the surface.

Methods: Manual arc surfacing, flux-cored wire self-shielded surfacing, plasma arc surfacing (PAW).

Case Study: Wear on vertical mill rollers in a cement plant. Automated build-up welding was performed using open-arc self-shielded flux-cored wire, employing high-chromium cast iron series filler material. Post-repair wear resistance exceeded 90% of new roller performance, with costs amounting to merely 30% of new roller procurement expenses.

Cold welding and snap-fit processes

Cold welding (no heat input): Utilising polymer composite materials (such as metal repair compounds) or micro-arc cold welding equipment, this method is suitable for repairing casting defects, leaks, and similar issues without risk of deformation.

Interlocking (mechanical reinforcement): For cracks in load-bearing sections, during welding, “wave-shaped keys” or “reinforcement blocks” are machined to embed and mechanically interlock, significantly enhancing the repair strength.

On-site machining and online repair technology

Online cutting/grooving: Welding grooves are machined in situ on equipment using portable milling apparatus.

Narrow-gap welding: For thick-walled components such as large shafts, a narrow and deep groove is opened, significantly reducing the weld volume and deformation.

Part Three: Comprehensive Analysis of Typical Maintenance Cases
Case Study: Repair of Severe Surface Scratching on the Plunger of the Main Cylinder in a 10,000-Ton Hydraulic Press

Fault Diagnosis: The plunger (material: 45 steel) exhibits multiple axial scratches reaching 2mm in depth along its surface, resulting from seal failure. The total affected length is approximately 1 metre.

Core challenges: ① Ensuring hardness (HRC 45-50) and surface finish (Ra 0.4) post-repair; ② Preventing welding distortion of the cylindrical component; ③ Achieving a robust bond between the repair layer and base material, eliminating risk of spalling.

Remediation Plan and Implementation:

Step One: Pre-treatment: Remove the fatigue layer by turning and machine a shallow U-shaped groove. Clean and inspect for defects to confirm no other flaws are present.

Step Two: Welding Method Selection: Employ oscillating TIG automatic welding, which concentrates heat, produces aesthetically pleasing welds, and facilitates automation.

Step Three: Welding Material Selection: Select ER50-6 welding wire, which has a composition similar to the base material but higher hardenability. Post-weld surface hardening is then performed to achieve the required hardness.

Step Four: Process Control: Position the plunger horizontally on the roller stand and rotate it at a constant speed. Secure the welding torch and perform multi-layer, multi-pass welding. Strictly control the interpass temperature.

Step Five: Post-welding treatment: First, perform stress-relief annealing. Subsequently, employ medium-frequency induction hardening equipment to surface-harden the overlay weld. Finally, precision-grind on a large grinding machine to achieve the specified dimensions and surface finish.

Result: The repair cost amounted to merely 20% of that for a newly manufactured plunger, with the project duration reduced by 60%. Following the repair, the plunger's operational performance fully met all required standards.

Part IV: Safety and Quality Assurance in Repair Welding
Safety First: Particularly during emergency repairs, power, hydraulic and pneumatic systems must be shut down, with lockout/tagout (LOTO) procedures implemented. For container-type equipment, thorough cleaning and gas testing must be conducted.

Records and Traceability: Establish comprehensive maintenance welding documentation, including diagnostic data, process cards, welding consumable batch numbers, and operator details, to provide a basis for subsequent maintenance.

Verification and Acceptance: Following repairs, corresponding non-destructive testing (UT/MT/PT) and dimensional/functional testing must be conducted. The item may only be put into service upon successful completion of these tests.

reach a verdict
Mechanical Equipment Maintenance WeldingIt is a comprehensive discipline integrating materials science, process technology and practical expertise. Successful repairs not only yield substantial cost savings and minimise downtime, but also provide invaluable insights for equipment enhancement and preventive maintenance through failure mode analysis. Mastering scientific diagnostic methodologies, flexibly applying diverse welding techniques, and rigorously adhering to safety and quality standards constitute the core competencies enabling every maintenance engineer and technician to restore equipment to operational life and deliver exceptional value.