Application and Analysis of Cobalt-Based Alloy Surfacing

Literature Overview

This paper by Yu Jianping and colleagues from Lanzhou Refining and Chemical Machinery Factory, published in Welding Technology (Vol. 29, No. 3, 2000), addresses the practical challenges encountered during cobalt-based alloy surfacing on refinery equipment. The study is grounded in extensive industrial experience, examining the cracking phenomena that occur when different surfacing methods are applied to cobalt-based alloys, analyzing the crack formation mechanisms from multiple perspectives, and proposing preventive measures. The authors emphasize the critical importance of strict adherence to surfacing procedures, proper surfacing sequence, and control of harmful elements in both the surfacing material and the base metal.

Core Technical Findings

The paper documents that cobalt-based alloy surfacing is prone to cracking, and the type and severity of cracking depend on the surfacing method employed. The authors identify multiple crack formation mechanisms and propose a systematic set of preventive measures. The key conclusion is that cracking can be effectively controlled by strictly following the surfacing procedure, maintaining the correct surfacing sequence, and controlling harmful elements in both the surfacing material and the base metal.

Surfacing Methods and Associated Cracking Risks

Crack Formation Mechanisms

Cobalt-based alloys, such as Stellite 6, Stellite 21, and similar compositions, are known for their excellent high-temperature strength, corrosion resistance, and wear resistance. However, they are also susceptible to cracking during and after surfacing due to several metallurgical factors.

Hot Cracking

Hot cracking occurs during solidification when the alloy is in a low-ductility temperature range. In cobalt-based alloys, the wide solidification range and the tendency for sulfur and phosphorus to segregate at grain boundaries create conditions favorable for hot cracking. The solidification shrinkage stress, combined with the restraint imposed by the base metal, can exceed the limited ductility of the solidifying alloy, causing intergranular cracking. The authors note that submerged arc surfacing, which involves higher heat input and slower cooling rates, is particularly prone to hot cracking because the prolonged time in the critical temperature range allows more time for crack initiation and propagation.

Cold Cracking

Cold cracking in cobalt-based surfacing deposits is primarily caused by hydrogen embrittlement. Hydrogen from moisture in the flux, from the base metal surface, or from the atmosphere can dissolve in the molten metal and remain trapped during solidification. As the deposit cools, the hydrogen diffuses to regions of high triaxial stress, typically at the weld toe or at the interface between the deposit and the base metal. If the hydrogen concentration exceeds a critical threshold, microvoids nucleate and coalesce into cracks. Shielded metal arc surfacing, which uses flux-cored or coated electrodes, has a higher risk of hydrogen pickup compared to gas-shielded methods.

Reheat Cracking

Reheat cracking, also known as stress-relief cracking, occurs during post-weld heat treatment. In cobalt-based alloys, carbide precipitation at grain boundaries during PWHT reduces the grain boundary cohesion. If the residual stress from the welding process is not fully relieved, the reduced grain boundary strength can lead to intergranular cracking during PWHT. This is particularly problematic for thick-section components where the PWHT temperature is higher and the cooling rate is slower, promoting more extensive carbide precipitation.

Preventive Measures

The authors propose a multi-faceted approach to prevent cracking:

1. Surfacing procedure compliance: Strict adherence to the recommended surfacing procedure, including preheating temperature, interpass temperature, and cooling rate, is essential. Preheating to 200-300°C is typically required to reduce thermal gradients and residual stresses.
2. Surfacing sequence: The sequence of surfacing passes must be carefully planned to minimize restraint and to allow stress relief between passes. A build-up sequence that starts from the center and moves outward, or a sequence that alternates between different areas of the component, can help distribute the thermal stress more evenly.
3. Harmful element control: The sulfur and phosphorus content in both the surfacing material and the base metal must be controlled. The surfacing material should be selected with low S and P content, and the base metal surface should be cleaned to remove any contaminants that could introduce these elements into the weld pool.
4. Hydrogen control: For methods susceptible to cold cracking, low-hydrogen consumables should be used, and the base metal surface should be thoroughly cleaned to remove moisture and oil. Preheating also helps to reduce hydrogen pickup by keeping the base metal above the dew point.
5. PWHT optimization: If PWHT is required, the temperature and time should be carefully controlled to avoid excessive carbide precipitation. In some cases, a two-stage PWHT with a lower temperature soak followed by a higher temperature soak can help to relieve stress without promoting reheat cracking.

Engineering Practice Implications

Cobalt-based alloy surfacing is widely used in the oil refining and chemical processing industries for repairing and upgrading components exposed to high-temperature, high-pressure, and corrosive environments. Typical applications include pump shafts, valve seats, mixing blades, and furnace tubes. The cracking issues documented in this paper are a common source of quality failures in these applications, and the preventive measures proposed are directly applicable to production environments.

A practical consideration is that the surfacing procedure must be adapted to the specific component geometry and base metal composition. For example, surfacing a small-diameter shaft requires different preheating and cooling strategies than surfacing a large-diameter pump housing. The authors' emphasis on surfacing sequence is particularly relevant for complex geometries where access is limited and the thermal cycle must be carefully managed.

Key Questions and Reflections

The paper raises the question of whether alternative surfacing methods, such as laser cladding or cold spray, could reduce the cracking susceptibility of cobalt-based alloys. Laser cladding, with its lower heat input and faster cooling rates, has been shown in other studies to produce crack-free cobalt-based deposits. However, the equipment cost and process complexity may not be justified for all applications. The authors' focus on conventional surfacing methods is pragmatic, but the limitations of these methods in terms of cracking control should be acknowledged.

Another reflection is that the cracking prevention measures proposed are essentially conservative—strict procedure adherence, careful sequence planning, and element control. While these measures are effective, they also add to the process cost and complexity. In high-volume production environments, the balance between crack prevention and process efficiency must be carefully managed.

Study Insights and Outlook

This paper provides a valuable practical guide to cobalt-based alloy surfacing, grounded in real-world industrial experience. The systematic analysis of crack formation mechanisms and the corresponding preventive measures offer a clear framework for engineers to apply in their own surfacing operations. The emphasis on surfacing sequence and harmful element control is particularly insightful, as these factors are often overlooked in favor of more obvious parameters such as current and travel speed. The study also highlights the importance of considering the entire process chain—from material selection to PWHT—in the design of a reliable surfacing procedure. Future work should explore the integration of advanced characterization techniques, such as in-situ temperature monitoring and residual stress measurement, to provide real-time feedback on the surfacing process and enable adaptive process control.