Residual Stress Analysis of Overlay Welding on Thick-Walled Hydrogenation Reactors

Literature Overview

This paper by Tian Hua and Wang Jiaxian from the Chemical Machinery Research Institute of East China University of Chemical Engineering, published in Chemical Equipment Technology in 1992 (Vol. 13, No. 3, pp. 13-18), addresses a critical engineering challenge in the design and fabrication of thick-walled hydrogenation reactors used in petroleum refining. The study focuses on residual stress analysis of austenitic stainless steel overlay welding applied to the inner surface of reactor shells made of 9Cr-2Mo steel, a material selected for high-temperature and high-pressure hydrogen service.

Core Technical Context

Hydrogenation reactors serve as core equipment in hydrotreating and hydrocracking units, where they operate under extreme conditions of high temperature, high pressure, and hydrogen-containing media. The base material, 9Cr-2Mo steel, provides adequate strength and creep resistance at elevated temperatures. However, it is susceptible to sulfide stress cracking (SSC) and sulfide stress corrosion cracking (SSCC) when exposed to wet H₂S environments. To mitigate this, an austenitic stainless steel overlay layer is applied to the inner wall surface to provide a corrosion-resistant barrier.

The fundamental problem is that the thermal expansion coefficients of the base material and the overlay material differ significantly, and the thick wall geometry introduces complex stress states during the overlay welding process. The residual stresses generated can compromise both the integrity of the overlay bond and the long-term fatigue performance of the reactor.

Key Technical Parameters and Analysis

Parameter	Base Material (9Cr-2Mo)	Overlay Material (Austenitic SS)	Implications
Thermal expansion coefficient (α, 10⁻⁶/°C)	~12.5	~17.0	Mismatch causes tensile stress in overlay upon cooling
Typical wall thickness	80-150 mm	2-4 mm	Thick wall amplifies constraint effects
Typical operating temperature	350-450°C	—	Elevated temperature reduces yield strength
Typical operating pressure	15-25 MPa	—	Hydrogen environment accelerates cracking
Welding process	SMAW/GTAW multi-pass	—	Heat input management critical

Residual Stress Mechanisms

The residual stress in the overlay weld arises from three primary mechanisms:

Thermal mismatch stress: Upon cooling from the welding temperature, the austenitic overlay contracts more than the 9Cr-2Mo base due to its higher thermal expansion coefficient, placing the overlay in a state of compressive stress while the base experiences tensile stress near the interface.
Phase transformation stress: The 9Cr-2Mo steel undergoes martensitic transformation in the heat-affected zone (HAZ), which is accompanied by volumetric expansion. This transformation-induced stress interacts with the thermal stress field.
Plastic deformation constraint: In thick-walled components, the interior material constrains the surface material from free contraction, leading to self-equilibrating stress distributions.

Engineering Practice and Countermeasures

The study emphasizes several practical measures to manage residual stresses:

Preheating: Applying a preheat temperature of 200-300°C reduces the peak cooling rate and minimizes martensitic transformation in the HAZ.
Interpass temperature control: Maintaining interpass temperatures between 250-350°C prevents excessive thermal gradients between passes.
Post-weld heat treatment (PWHT): A stress-relief annealing at 580-620°C for an extended duration is essential to relieve residual stresses below 50% of the yield strength.
Weld sequence planning: Strategic sequencing of overlay welds to allow uniform thermal distribution and minimize local stress concentrations.

Connection to Standards and Codes

The overlay welding of hydrogenation reactors must comply with ASME Section VIII Division 1 and Division 2, as well as specific requirements from API 941 for weld overlay cladding. The residual stress levels are evaluated against the allowable limits specified in these codes, particularly for fatigue assessment and stress corrosion cracking resistance.

Study Insights and Reflections

This 1992 study is notable for its early systematic approach to residual stress quantification in overlay welding of thick-walled pressure vessels. The authors recognized that the residual stress field is not merely a fabrication concern but directly impacts the service life of the reactor. In modern practice, we would supplement such analytical approaches with finite element method (FEM) simulations, neutron diffraction measurements, and X-ray stress analysis to validate predictions. The fundamental principles identified remain valid: managing thermal mismatch, controlling heat input, and ensuring adequate post-weld treatment are the pillars of successful overlay welding on thick-walled components. The challenge of maintaining metallurgical compatibility between the ferritic-martensitic base and the austenitic overlay, while simultaneously controlling residual stresses, continues to be a demanding engineering problem that requires careful process qualification.