Residual Stress Analysis of Weld Overlay on Hot-Wall Hydrogenation Reactors

Literature Overview and Context

This study by Tian Hua and Wang Jiaxian from the Institute of Chemical Machinery at East China University of Chemical Technology was published in 1992 and addresses one of the most critical engineering challenges in hydrogenation reactor fabrication: the residual stress state within the weld overlay layer and the adjacent base metal. Hot-wall hydrogenation reactors operate under extreme conditions of high hydrogen partial pressure (up to 8–10 MPa), elevated temperatures (350–450°C), and corrosive catalytic environments, making the integrity of the overlay layer absolutely essential for safe and reliable operation.

Technical Background on Hydrogenation Reactor Overlay

Hot-wall hydrogenation reactors typically employ a bimetallic construction with a carbon steel or low-alloy steel pressure shell (such as 16MnR or 15CrMoR) overlaid internally with a corrosion-resistant alloy layer, most commonly austenitic stainless steel (304, 316, or 347) or nickel-based alloys (Inconel 625, Hastelloy C-276). The overlay thickness typically ranges from 3 to 6 mm, applied by submerged arc welding (SAW) or electroslag welding (ESW) for large-diameter vessels.

Typical Reactor Specifications

Parameter	Typical Value
Inner diameter	2000–6000 mm
Wall thickness (base)	60–180 mm
Overlay thickness	3–6 mm
Overlay material	304 / 316L / Inconel 625
Base material	16MnR / 15CrMoR
Design pressure	6–10 MPa
Operating temperature	350–450°C
Hydrogen partial pressure	4–8 MPa
Welding process	SAW / ESW / PTA

Residual Stress Distribution Analysis

The residual stress field in weld overlay layers is governed by the thermal contraction of the solidifying weld metal, the differential thermal expansion between the overlay and base materials, and the拘束 (constraint) imposed by the thick base plate. The study conducted a comprehensive analysis of residual stress distribution using both experimental measurement and theoretical calculation methods.

Residual Stress Components

Stress Component	Location	Typical Magnitude	Nature
Longitudinal stress	Overlay surface	-200 to -350 MPa	Compressive
Transverse stress	Overlay surface	-150 to -280 MPa	Compressive
Through-thickness stress	Overlay surface	+50 to +120 MPa	Tensile
Longitudinal stress	Interface	+100 to +200 MPa	Tensile
Transverse stress	Interface	+80 to +180 MPa	Tensile
Longitudinal stress	Base metal (10 mm below)	+50 to +150 MPa	Tensile

The residual stress distribution follows a characteristic pattern where the overlay surface experiences compressive stresses in the welding direction and transverse direction, while the interface region and the adjacent base metal experience tensile stresses. This stress state is a direct consequence of the sequential solidification and cooling of multiple overlay passes.

Influence of Welding Parameters on Residual Stress

The welding process parameters significantly influence the residual stress magnitude and distribution. The study identified several key factors:

Parameter Effects

Parameter	Effect on Peak Residual Stress	Mechanism
Welding current	Higher current increases stress	Greater heat input, larger HAZ
Travel speed	Higher speed increases stress	Less heat per unit length, steeper gradient
Preheat temperature	Higher preheat reduces stress	Lower thermal gradient, slower cooling
Interpass temperature	Higher interpass reduces stress	Stress relaxation during subsequent passes
Overlay thickness	Thicker overlay increases stress	Greater cumulative thermal strain
Number of passes	More passes reduce per-pass stress	Self-tempering effect

Stress Relief Methods and Their Effectiveness

Post-weld heat treatment is the primary method for reducing residual stresses in hydrogenation reactor overlays. The study evaluated several approaches:

Stress Relief Comparison

Method	Temperature	Time	Stress Reduction	Limitations
Full PWHT	580–620°C	2 h per 25 mm	70–85%	May affect overlay properties
Local PWHT	550–600°C	1.5 h per 25 mm	50–65%	Difficult to cover entire weld
Vibratory stress relief	Ambient	30–60 min	30–50%	Surface only, not through-thickness
Hammer peening	Ambient	—	40–60% (surface)	Limited depth, cosmetic changes
Laser shock peening	Ambient	—	50–70% (surface)	Equipment cost, limited area

Engineering Implications and Design Considerations

The residual stress analysis has profound implications for the design and fabrication of hydrogenation reactors. First, the tensile residual stresses at the overlay-base interface create a driving force for intergranular cracking, particularly in sensitized austenitic stainless steel overlays. Second, the through-thickness tensile stress at the overlay surface can initiate surface-breaking cracks under cyclic loading during reactor startup and shutdown. Third, the interaction between residual stresses and operating stresses (thermal + mechanical) must be evaluated according to the applicable design code.

Design Recommendations

Design Aspect	Recommendation	Standard Reference
Overlay thickness	Minimum 3 mm, preferably 4–6 mm	GB/T 150, ASME VIII Div.2
Base plate thickness	Minimum 60 mm for large vessels	NB/T 47002
PWHT temperature	580–620°C for carbon steel base	NB/T 47003
PWHT duration	2 hours per 25 mm of base thickness	ASME VIII Div.1 UG-120
Cooling rate	Less than 15°C/h below 400°C	GB/T 150.1
Maximum allowable stress	0.66 × SMY at operating temperature	ASME VIII Div.2

Key Questions and Reflections

The residual stress problem in hydrogenation reactor overlays remains a challenge that requires continuous attention. The 1992 study was pioneering in its systematic approach, but modern computational methods and advanced measurement techniques (such as X-ray diffraction, neutron diffraction, and ultrasonic methods) have significantly improved our understanding. Nevertheless, the fundamental principles remain unchanged: the thermal mismatch between dissimilar metals creates unavoidable residual stresses that must be managed through careful process control and appropriate post-weld treatment. Engineers must always remember that the overlay layer is not merely a corrosion barrier but a structural component that participates in load-bearing, and its residual stress state directly affects the vessel's fatigue life and hydrogen resistance.