Hardfacing Welding Process for Heat Exchanger Tube Sheets

Literature Overview

The paper by Shi Liang (2009), published in Welding Technology (Vol. 38, No. 12, pp. 68–69), addresses a practical manufacturing challenge encountered in the production of heat exchanger tube sheets for an oil slurry steam generator at a petrochemical company. Tube sheets are critical structural components that must withstand high-temperature and high-pressure service while maintaining dimensional accuracy for tube insertion and sealing. The paper focuses on two dominant failure modes during hardfacing: excessive welding distortion and cracking in the overlay deposit. The author proposes a systematic approach combining pre-setback (counter-deformation) techniques with a carefully sequenced welding strategy to mitigate these issues.

Core Technical Analysis

Distortion Mechanism and Counter-Deformation Strategy

Hardfacing welding on tube sheets introduces significant thermal gradients because the overlay material is typically deposited in a localized area while the bulk of the tube sheet remains relatively cool. The resulting differential thermal contraction generates residual stresses that manifest as angular or transverse distortion. The counter-deformation method involves deliberately pre-bending or pre-setting the tube sheet in the opposite direction of the expected distortion before welding begins.

Parameter	Typical Value	Rationale
Pre-setback angle	0.3–1.5°	Compensates for post-weld angular distortion
Pre-setback measurement location	Diagonal or edge	Ensures uniform compensation
Welding layer thickness	3–8 mm	Balances wear resistance and thermal input
Interpass temperature	≤ 200 °C	Controls HAZ microstructure and stress
Cooling rate	Controlled by back-plate or insulation	Prevents martensitic transformation cracking

The counter-deformation amount must be calibrated through trial welds or finite element simulation. Over-compensation is as detrimental as under-compensation, as it introduces reverse residual stresses that may compromise the flatness tolerance required for gasket seating and tube-to-tubesheet joint integrity.

Welding Sequence Optimization

The welding sequence is a critical lever for distortion control. A symmetric, multi-pass sequence that progresses from the center outward, or alternates between opposing quadrants, minimizes cumulative angular distortion. The paper advocates a zigzag or spiral pattern that ensures thermal input is distributed evenly across the tube sheet diameter.

Divide the overlay area into equal segments or quadrants.
Weld in a balanced sequence: complete one pass in segment A, then immediately weld the diametrically opposite segment B, before returning to the next segment.
Maintain low heat input per pass to limit the thermal cycle amplitude and reduce the driving force for distortion.
Monitor in-process distortion using dial gauges or laser displacement sensors mounted at predetermined points, and adjust the counter-deformation in real time if necessary.

Cracking Prevention

Cracking in hardfacing deposits on tube sheets can be classified into hot cracks (solidification cracks) and cold cracks (hydrogen-induced or delayed cracks). The following measures are recommended:

Preheat and interpass temperature control: A preheat of 150–250 °C is typically applied to reduce the cooling rate below the threshold for martensite formation in the HAZ and deposit.
Low-hydrogen consumables: Use of low-hydrogen flux-cored or solid wires limits diffusible hydrogen levels.
Post-weld stress relief: A localized stress relief heat treatment at 600–650 °C for 2–4 hours per 25 mm of thickness can reduce residual stresses to below the cracking threshold.
Dilution control: Limiting the dilution ratio to below 30% ensures that the overlay alloy retains its intended microstructure and mechanical properties.

Engineering Practice Integration

In the context of API 660 (Shell-and-Tube Heat Exchangers) and ASME Section VIII Division 1 requirements, tube sheet fabrication must satisfy strict flatness tolerances, typically ±0.15% of the tube sheet diameter or ±1.5 mm, whichever is less. The hardfacing process described in this paper is particularly relevant for tube sheets that require corrosion-resistant or wear-resistant overlays, such as those used in sour service or abrasive slurry applications.

The PDCA cycle is well-suited for implementing this technology: Plan the counter-deformation and welding sequence based on FEA and trial welds; Do the welding with real-time distortion monitoring; Check the as-welded distortion and overlay quality via MT/PT and hardness mapping; Act by refining the counter-deformation parameters for subsequent production runs.

Study Insights and Reflections

This paper, though concise, captures a fundamental principle in welding engineering: distortion control is not merely a post-weld correction exercise but must be embedded into the process design from the outset. The counter-deformation approach is a form of "inherent residual stress management" that reduces the need for expensive and time-consuming post-weld straightening. The welding sequence strategy is equally important, as it directly governs the thermal history and residual stress distribution.

One area where further investigation would be valuable is the quantitative relationship between counter-deformation angle and the actual distortion produced, as a function of tube sheet material (carbon steel vs. low-alloy steel), overlay alloy type, and welding parameters. A parametric study using response surface methodology could provide a predictive model that engineers could use to optimize counter-deformation without extensive trial-and-error.

In summary, the hardfacing welding of heat exchanger tube sheets requires a holistic approach that integrates material selection, process parameter optimization, counter-deformation, and welding sequence design to achieve dimensional accuracy, crack-free deposits, and long-term service reliability in demanding petrochemical applications.