Hardfacing Process for Heat Exchanger Tube Sheets

Literature Overview and Engineering Context

Heat exchanger tube sheets serve as the critical interface between the tube side and shell side, and in corrosive service environments, they frequently require hardfacing or cladding of stainless steel, nickel-based alloy, or duplex steel overlay layers. The hardfacing process for tube sheets presents unique challenges due to the large surface area, the presence of tube holes that must remain dimensionally accurate, and the requirement for uniform overlay thickness across the entire surface. This study note examines the process design, quality control, and inspection requirements for tube sheet hardfacing as discussed in the literature.

Process Selection and Parameter Optimization

The choice of hardfacing process for heat exchanger tube sheets depends on the required overlay thickness, the alloy system, and the production volume. For overlay thicknesses of 1–3 mm, submerged arc welding (SAW) is the preferred process due to its high deposition rate and excellent protection against atmospheric contamination. For thinner overlays of 0.5–1.5 mm or for repair applications, gas tungsten arc welding (GTAW) or hot-wire TIG offers superior control and minimal dilution.

Process	Typical Deposit Rate (kg/h)	Dilution (% Base Metal)	Suitable Overlay Thickness (mm)	Alloy Systems
SAW	15–30	5–15	2–6	304, 316, 321, Inconel 625
GTAW	1–3	2–8	0.5–2	316L, Inconel 625, Monel 400
Hot-Wire TIG	3–8	3–10	1–4	304, 316, 347
PTA Powder Cladding	5–15	1–5	1–3	Inconel 625, Hastelloy C276

For tube sheets in hydrogenation reactor service, the overlay material is typically Inconel 625 or Hastelloy C276, applied to a carbon steel or low-alloy steel base plate. The literature recommends a two-pass SAW process: the first pass with a transition flux and wire combination to establish good wetting and bonding, followed by a second pass with the final alloy composition. The welding current is typically set at 450–650 A with a voltage of 28–35 V, and the travel speed is controlled at 200–350 mm/min to achieve a bead height of 3–5 mm per pass.

Tube Hole Protection and Dimensional Control

One of the most critical aspects of tube sheet hardfacing is the protection of tube holes during the overlay process. Molten metal flow into tube holes can cause blockage, dimensional distortion, or contamination that compromises tube installation. The literature describes several protection methods:

Mechanical plug protection: Each tube hole is plugged with a tapered steel or brass plug that fits snugly, preventing molten metal ingress while allowing thermal expansion.
Welding sequence optimization: A spiral or systematic zigzag welding pattern is used to minimize thermal distortion and ensure uniform heat distribution across the tube sheet surface.
Post-weld hole reaming: After hardfacing and stress relief, all tube holes are reamed to the specified diameter and tolerance (typically H8 or H9 per ISO 286) to compensate for any thermal distortion.

The welding sequence is particularly important for large tube sheets exceeding 600 mm in diameter. A center-outward spiral pattern is recommended to minimize edge cracking and to ensure that the final weld passes cross-tie the earlier beads, improving inter-bead bonding.

Quality Inspection and Acceptance Criteria

The literature outlines a comprehensive inspection regime for tube sheet hardfacing, aligned with ASME VIII Div.1, GB/T 150, and API 934 requirements.

Inspection Method	Coverage	Acceptance Criteria	Standard Reference
Visual Examination (VT)	100%	No undercut > 0.5 mm, no cracks, no porosity clusters	NB/T 47013.1
Magnetic Particle Testing (MT)	100% of overlay surface	No linear indications > 3 mm	NB/T 47013.4
Ultrasonic Testing (UT)	100% of bond line	No delamination > 20% of area	NB/T 47013.2
Hardness Test	3 points per 500 mm²	Within ±5 HRC of specification	ASTM E18
Chemical Composition	1 coupon per heat	Within alloy specification	ASTM E415
Intergranular Corrosion Test	1 coupon per heat	No intergranular attack per ASTM A263	ASTM A263 / A264

The intergranular corrosion test is particularly important for austenitic stainless steel overlays, as the sensitization temperature range of 450–850°C can be reached during the welding thermal cycle. The literature recommends using low-carbon grades (304L, 316L) or stabilized grades (321, 347) for the overlay material to mitigate sensitization risk.

Engineering Practice Integration

In practice, the hardfacing of heat exchanger tube sheets requires close coordination between the welding engineer, the pressure vessel designer, and the quality assurance team. The welding procedure specification (WPS) must be qualified in accordance with NB/T 47014 or ASME IX, with specific attention to the qualification of the bond line and the overlay layer separately. The procedure qualification test requires metallographic examination of the bond line for intergranular attack, hardness profiling across the overlay thickness, and mechanical property testing of the overlay.

A common engineering challenge is the management of welding distortion in thin tube sheets (less than 1.5 times the tube outer diameter in thickness). Pre-fabrication of the tube sheet with a slightly larger thickness allowance, followed by machining after hardfacing and stress relief, is a practical approach that ensures dimensional accuracy.

Study Reflections

The hardfacing of heat exchanger tube sheets is a process that demands precision at every stage, from material selection to final inspection. The literature reinforces that the bond line is the weakest link and must be treated with the same rigor as the base metal weld joints. The engineer must balance deposition rate against metallurgical quality, and production efficiency against inspection completeness. A well-designed hardfacing procedure for tube sheets not only ensures corrosion resistance but also guarantees the long-term mechanical integrity of the heat exchanger under thermal cycling and pressure loading.