Hardfacing Process Improvement for Waste Heat Boiler Tube Sheets: Elimination of Arc-Transition Cracking

Literature Overview

The paper by Liu Baoxiang, published in China Chemical Equipment (Vol. 11, No. 1, 2009, pp. 43-44), addresses a specific and practically important welding challenge: cracking defects occurring at arc transition zones during the hardfacing of waste heat boiler tube sheets. Waste heat boiler tube sheets are critical structural components that connect boiler tubes to the pressure vessel shell, and they are frequently subjected to thermal cycling, pressure loading, and corrosive environments from flue gases. The hardfacing overlay on tube sheets serves to protect against corrosion and erosion, but the welding process must be carefully controlled to avoid defects that compromise structural integrity.

Core Technical Viewpoints

The central problem identified in this paper is the occurrence of cracks at the arc transition area, which is the region where the welding arc initiates or terminates during the hardfacing process. This is a well-known but often underappreciated defect location in overlay welding. At the arc transition zone, the thermal cycle is highly non-uniform: the start of the arc produces a rapidly heating, low-temperature region that can trap hydrogen and promote cold cracking, while the end of the arc produces a rapidly cooling zone where residual stresses are concentrated. These combined factors create a favorable environment for crack initiation and propagation.

The proposed solution involves process modifications to the hardfacing procedure. While the paper does not provide exhaustive detail on every parameter, the key improvements typically include: optimizing the preheating temperature to reduce thermal gradients, implementing proper interpass temperature control to prevent excessive cooling rates, using back-strap or backing bar techniques to ensure full penetration and reduce backside cracking, and applying post-weld heat treatment to relieve residual stresses and reduce hydrogen content. The transition layer concept is also discussed, which involves depositing a metallurgically compatible intermediate layer between the base metal and the final corrosion-resistant overlay to reduce dilution effects and minimize cracking susceptibility.

Defect Analysis and Process Correction

The root cause analysis follows a systematic approach that aligns with FMEA (Failure Mode and Effects Analysis) principles. The failure mode is cracking at the arc transition zone. The potential causes include: excessive cooling rate leading to hard and brittle microstructures in the heat-affected zone; hydrogen diffusion from the base metal or welding consumables into the rapidly cooling weld metal; and residual stress concentration due to the non-uniform thermal input at the arc boundaries. The effects are loss of structural integrity, potential leak paths in pressure-containing components, and shortened service life of the boiler tube sheet.

A critical aspect of the solution is the transition layer concept. Waste heat boiler tube sheets are typically fabricated from low-alloy steels such as 15CrMo or similar grades that may contain elevated carbon and alloying elements. When a high-alloy corrosion-resistant hardfacing alloy is deposited directly onto such a base metal, the dilution effect can produce a weld metal with an unfavorable composition that is prone to cracking. By introducing a transition layer with an intermediate composition, the compositional gradient between the base metal and the final overlay is reduced, which minimizes cracking susceptibility and improves metallurgical bonding.

Engineering Practice and Quality Control

In engineering practice, the hardfacing of waste heat boiler tube sheets must comply with applicable standards such as ASME Section IX, API 579, and relevant Chinese national standards (GB/T 985, JB/T 4709). The welding procedure specification (WPS) must be qualified through appropriate testing, including visual inspection, magnetic particle testing (MT), and possibly radiographic testing (RT) or ultrasonic testing (UT) depending on the criticality of the component. The hardness of the hardfacing overlay should be verified by surface hardness testing, and the metallurgical examination should confirm proper fusion and absence of defects at the base metal-weld interface.

Post-weld heat treatment (PWHT) is often essential for hardfacing applications on boiler tube sheets. The PWHT parameters must be carefully selected to relieve residual stresses without adversely affecting the hardfacing microstructure. For high-alloy hardfacing alloys, the PWHT temperature is typically limited to avoid excessive grain growth or precipitation coarsening that would reduce the corrosion resistance or hardness of the overlay. A typical PWHT cycle for a hardfacing overlay on a low-alloy steel tube sheet might involve heating to 550-650 degrees Celsius at a controlled rate, holding for a period proportional to the thickness, and then cooling in the furnace at a controlled rate to avoid thermal shock.

The quality control plan for hardfacing tube sheets should include the following elements: visual inspection of all welds for surface defects; MT of all hardfacing welds to detect surface and near-surface cracks; hardness survey to confirm that the overlay meets specification requirements; and, for critical applications, macroscopic metallographic examination of a test coupon welded under identical conditions to verify soundness at the base metal-weld interface. Any detected defects must be repaired and re-inspected according to the applicable code requirements.

Study Insights and Practical Recommendations

The most important lesson from this paper is that arc transition zones are high-risk areas in hardfacing welding and require specific attention in procedure design. Many welding engineers focus on the main weld bead characteristics while overlooking the start and end points, which are often the weakest links in the weld sequence. A practical recommendation is to implement a systematic approach to arc transition management, which includes: using a low-hydrogen flux-cored wire or a well-shielded electrode to minimize hydrogen pickup; preheating the entire tube sheet to a uniform temperature to reduce thermal gradients; welding in a sequence that allows each pass to be deposited onto a warm base rather than a cold one; and terminating each arc with a proper crater fill technique to avoid crater cracks.

Furthermore, the concept of a transition layer should be considered as a standard practice for hardfacing applications on high-carbon or high-alloy base metals. The transition layer acts as a metallurgical buffer that reduces the compositional mismatch between the base metal and the overlay, thereby minimizing cracking susceptibility. The selection of the transition layer material should be based on dilution calculations and metallurgical compatibility analysis, ensuring that the transition layer composition is intermediate between the base metal and the final overlay.

In summary, this paper provides a practical and focused solution to a common welding problem in waste heat boiler tube sheet hardfacing. The key takeaway is that arc transition cracking can be effectively prevented through a combination of process optimization, transition layer implementation, and proper post-weld heat treatment. Engineers responsible for hardfacing procedures on pressure-containing components should incorporate these lessons into their welding procedure specifications and quality control plans to ensure reliable and durable hardfacing overlays.