Tin Bronze Overlay Welding Process for Spherical Bearing Housings

Literature Overview

This paper by Zhang Xin and Zhou Dajie, published in the Welding journal in 2008, documents a practical overlay welding solution for spherical bearing housings (ball-and-socket joints) in bucket wheel products. The specific challenge involves depositing QSn6.5-0.1 tin bronze onto a ZG35SiMnMo cast steel substrate, a combination known for severe metallurgical incompatibility. The authors propose a high-current, high-speed GTAW (Tungsten Inert Gas Welding) process that effectively resolves the classic problems of overlay delamination, cracking, and porosity.

Metallurgical Challenges

The ZG35SiMnMo cast steel substrate presents several difficulties for copper alloy overlay:

High sulfur and phosphorus content promotes hot cracking at the weld fusion zone
Significant difference in thermal expansion coefficient between steel (approximately 12 x 10⁻⁶/K) and tin bronze (approximately 17-18 x 10⁻⁶/K)
Poor wetting of bronze on steel surface due to oxide formation
High melting point of steel compared to bronze promotes dilution of the overlay layer

The QSn6.5-0.1 tin bronze (6.5% Sn, 0.1% P) was selected for its excellent wear resistance, low friction coefficient, and compatibility with the bearing application. The phosphorus addition improves fluidity and castability but can promote hot cracking in thick sections.

Process Development and Parameters

The key innovation is the adoption of high-current, high-speed GTAW parameters, which differ significantly from conventional overlay welding practice:

Process Parameter	Conventional Approach	Developed Approach	Effect
Welding current	80-120 A	180-250 A	Deeper penetration, better fusion
Travel speed	50-80 mm/min	150-250 mm/min	Reduced heat input per unit length
Arc length	2-4 mm	1-2 mm	More concentrated heat
Shielding gas flow	10-15 L/min	15-20 L/min	Enhanced protection
Preheat temperature	200-300°C	150-200°C	Reduced residual stress

The high-current, high-speed combination achieves several beneficial effects simultaneously:

The high current ensures adequate penetration into the steel substrate, creating a strong metallurgical bond rather than a weak mechanical attachment
The high travel speed limits the total heat input, reducing the grain growth in the heat-affected zone and minimizing the size of the dilution zone
The rapid solidification rate promotes a fine-grained structure in the overlay, improving its mechanical properties
The concentrated heat input reduces the area susceptible to hot cracking

Defect Analysis and Countermeasures

The paper addresses three primary defect modes:

Delamination: Caused by insufficient penetration and oxide films at the interface. Countermeasure: mechanical cleaning of the substrate surface to bare metal, followed by immediate welding within 15 minutes to prevent re-oxidation. The high current ensures penetration depth exceeds 0.5 mm into the steel.
Hot cracking: Resulting from sulfur and phosphorus segregation at grain boundaries during solidification. Countermeasure: the high travel speed reduces the time spent in the critical temperature range (600-800°C), and the rapid cooling rate limits the growth of brittle intermetallic phases.
Porosity: Caused by hydrogen absorption from the atmosphere or moisture contamination. Countermeasure: increased shielding gas flow rate and flowmeter verification, along with preheating to 150-200°C to drive off adsorbed moisture.

Engineering Practice Integration

For the spherical bearing housing application in bucket wheel excavators, the overlay must withstand cyclic loading, abrasive contact with rock and soil, and occasional impact. The tin bronze overlay provides:

Coefficient of friction of 0.08-0.12 against steel, significantly lower than bare steel-on-steel contact
Wear life exceeding 5000 operating hours under typical conditions
Resistance to galling and seizure during cyclic rotation
Self-lubricating properties due to tin oxide film formation

The weld sequence for the spherical housing typically involves:

First pass: single V-groove preparation with 30° included angle, root gap of 1-2 mm
Overlay passes: 3-5 layers with stringer beads, each layer thickness controlled at 1.5-2.0 mm
Final pass: capping layer with full coverage of the previous pass to ensure uniform surface finish

Post-weld inspection includes visual examination (VT) of the entire overlay surface, ultrasonic testing (UT) for subsurface porosity and delamination, and hardness testing (HV 5) to verify the bronze composition has not been excessively diluted by the steel substrate. Acceptable hardness range is 80-120 HV for QSn6.5-0.1.

Study Insights

This paper demonstrates that process parameter optimization can overcome fundamental metallurgical incompatibilities between dissimilar materials. The high-current, high-speed approach is counterintuitive from a conventional welding perspective, where lower heat input is typically achieved by reducing current. However, by simultaneously increasing both current and speed, the net heat input per unit length is actually reduced while maintaining the penetration depth necessary for a strong metallurgical bond. This is a valuable principle applicable to other challenging overlay welding applications, including hardfacing deposits on high-carbon steels and copper alloy overlays on low-carbon steel substrates.