Submerged Arc Welding Preparation of Carbide Steel Bimetallic Composites

Overview of the Study

This literature investigates the use of submerged arc welding (SAW) to fabricate bimetallic composites consisting of a carbide-reinforced overlay layer on a structural steel substrate. The approach leverages the high deposition rate and deep penetration characteristics of SAW to achieve thick, uniform carbide layers with controlled distribution. The study examines how welding parameters, flux composition, and wire-powder combination affect the microstructure, hardness, and wear resistance of the resulting composite material.

Core Technical Points

Process Configuration and Parameters

The SAW process for bimetallic composite fabrication typically employs a single-wire or dual-wire configuration with a specially formulated flux that contains carbide particles. In some configurations, a separate powder feeder is used to introduce additional carbide into the weld pool, creating a three-component system of wire, flux, and powder.

The high current density and deep penetration of SAW create a large weld pool, which allows carbide particles to be entrained and distributed throughout the overlay thickness rather than being concentrated at the surface. This is a significant advantage over gas-shielded processes where carbide particles tend to float to the top of the weld pool due to buoyancy.

Carbide Distribution and Microstructure

The study reveals that carbide distribution is highly dependent on the interplay between thermal gradients and fluid flow within the weld pool. At lower travel speeds, the larger weld pool promotes more uniform carbide distribution through enhanced fluid convection. However, excessively low travel speeds increase heat input and may lead to excessive grain coarsening in the substrate heat-affected zone.

Microstructural analysis shows three distinct zones: a dilution zone near the substrate interface where carbide content is minimal, a transition zone with increasing carbide concentration, and a top zone with the highest carbide density. The carbide particles themselves undergo partial dissolution and re-precipitation during solidification, resulting in a mixture of primary carbides (retained from the original particles) and secondary carbides (precipitated during cooling).

Hardness Profile

The hardness profile across the overlay thickness is a critical performance indicator. Typical values show a gradient from 45-55 HRC near the substrate interface to 65-75 HRC at the surface. This gradient is actually beneficial for many applications, as it provides a tough base for the wear-resistant surface while maintaining adequate load-bearing capacity.

Standards and Specifications

Applicable Standards

The fabrication of SAW-produced bimetallic composites falls under several standards depending on the application. For pressure vessel components, NB/T 47014 and ASME IX govern the qualification of welding procedures and operators. For wear-resistant overlays specifically, ASTM A263 provides general requirements for weld-overlay deposits, while ASTM A264 covers hard-facing welding electrodes.

Qualification Testing

Procedure qualification for SAW bimetallic composites requires mechanical testing of the overlay layer, including tensile testing, hardness mapping, and wear testing. The dilution zone must also be tested for impact energy to ensure that the heat-affected zone of the substrate does not suffer excessive embrittlement. For pressure vessel applications, intergranular corrosion testing may be required if the overlay contains austenitic stainless steel components.

Common Defects and Countermeasures

Defect Analysis

The most common defects in SAW bimetallic composites include carbide agglomeration, lack of fusion at the overlay-substrate interface, porosity, and cracking. Carbide agglomeration occurs when particles cluster together, creating localized regions of excessive hardness that are prone to cracking under impact loading. Lack of fusion is often caused by insufficient preheating or excessive travel speed.

Process Control Measures

To minimize defects, the study recommends a systematic approach to process control. Flux must be dried at 300°C for at least 2 hours before use, and moisture content should be monitored regularly. The wire feed mechanism must be calibrated to ensure consistent wire feed speed, as variations of more than ±2% can significantly affect carbide distribution.

Preheating is critical for thick-section substrates. For carbon steel substrates thicker than 25 mm, a preheating temperature of at least 150°C is recommended to reduce cooling rates and prevent hydrogen-induced cracking. The interpass temperature should be maintained between 150-250°C to avoid excessive thermal cycling.

Engineering Practice Applications

Wear-Resistant Components

SAW bimetallic composites find extensive application in mining, cement, and power generation industries. Typical applications include ball mill liners, conveyor rollers, crusher jaws, and pump impellers. The high deposition rate of SAW makes it particularly suitable for large-area overlay applications where gas-shielded processes would be impractical.

For ball mill liners, the typical overlay thickness is 15-30 mm, achieved through multiple SAW passes. The carbide content is usually in the range of 20-40 wt%, providing hardness values of 60-75 HRC. The expected service life in a cement grinding application is 3-5 times that of plain steel liners.

Pressure Vessel Applications

In pressure vessel fabrication, SAW bimetallic composites are used for reactor internals, heat exchanger tube sheets, and distillation column trays. The substrate provides the structural strength required for pressure containment, while the overlay provides corrosion resistance and wear resistance.

For hydrogenation reactor internals, the overlay layer typically consists of a nickel-based alloy with carbide reinforcement. The welding procedure must be qualified according to NB/T 47014, and the overlay must pass intergranular corrosion testing according to ASTM A263. The dilution zone must be tested for impact energy at the minimum service temperature.

Study Insights and Implications

The study demonstrates that SAW is an excellent process for fabricating thick bimetallic composites with controlled carbide distribution. The key to success lies in optimizing the interplay between welding parameters, flux composition, and carbide particle characteristics. Engineers must carefully balance hardness requirements against toughness requirements, as excessive carbide content can lead to brittle failure under impact loading.

One important observation is that the hardness profile across the overlay thickness is not uniform but follows a predictable gradient. This gradient can be exploited in design by placing the highest hardness zone at the wear surface and the lowest hardness zone near the substrate interface. For applications requiring uniform hardness throughout the overlay thickness, multiple passes with varying carbide content may be necessary.

The economic advantage of SAW for bimetallic composites is substantial. The high deposition rate, low gas consumption, and minimal shielding requirements make it the most cost-effective process for thick overlay applications. However, the process is less suitable for thin overlays or applications requiring precise geometric control, where gas-shielded processes or laser cladding may be more appropriate.

In conclusion, this literature provides a comprehensive guide to SAW-based bimetallic composite fabrication, covering process parameters, microstructure control, defect prevention, and engineering applications. The key takeaway for practicing engineers is that successful fabrication requires a holistic approach that considers not only the welding parameters but also the material characteristics, service conditions, and quality requirements.