Strip Cladding Submerged Arc Welding Process for Nuclear Pressurizer Hemispherical Heads

Overview of the Topic

The topic, authored by Liao Guoping from Dongfang Boiler (Group) Co., Ltd. and published in 2008, addresses the strip cladding submerged arc welding (SAW) process technology applied to the hemispherical heads of pressurizers in nuclear power plants. Pressurizers are critical components in pressurized water reactor (PWR) nuclear power plants, responsible for maintaining the primary coolant system pressure within a safe operating range. The hemispherical heads of these pressurizers are subject to extremely demanding service conditions, including high temperature, high pressure, and aggressive corrosive environments. The application of strip cladding SAW on these components is a specialized technique that demands meticulous process control to ensure reliable metallurgical bonding and corrosion resistance.

Core Technical Content

Strip cladding SAW is one of the most efficient methods for depositing corrosion-resistant overlay layers on large-scale pressure vessel components. The process uses a continuous strip electrode fed directly into the weld pool, with the strip itself serving as the consumable filler material. The key advantages of this method include high deposition rates, good mechanical properties of the overlay layer, and the ability to achieve uniform thickness over large areas.

For nuclear pressurizer hemispherical heads, the typical base material is a low-alloy steel such as 15CrMoR or 18MnMoNbR, while the cladding layer material is usually austenitic stainless steel such as 304, 316, or 321, depending on the specific service environment and corrosion resistance requirements. The process must comply with stringent quality standards including ASME VIII Div.1, ASME IX, and relevant nuclear industry standards such as RBP-NB/T 47014.

Key Process Parameters

The following table summarizes the typical process parameters for strip cladding SAW on pressurizer hemispherical heads:

Parameter	Typical Range	Remarks
Welding current	400-600 A	Depends on strip width and thickness
Welding voltage	25-35 V	Maintains stable arc
Welding speed	80-150 mm/min	Adjusted for deposition rate
Strip width	20-40 mm	Commonly 25 mm or 30 mm
Strip thickness	1.0-2.5 mm	Thicker strips for higher deposition
Preheating temperature	150-250 °C	Prevents cold cracking
Interpass temperature	≤250 °C	Controls cooling rate
Flux type	Low-hydrogen or rutile	Protects the molten pool

Process Challenges and Solutions

The hemispherical geometry of pressurizer heads presents unique challenges for strip cladding SAW. The curved surface requires the welding equipment to be capable of following the contour, and the changing position during welding (from flat to vertical to overhead) demands excellent process stability. The key challenges include:

Maintaining uniform arc length and weld geometry on curved surfaces: Specialized welding fixtures and guide mechanisms are required to ensure the strip electrode remains properly aligned with the joint.
Controlling dilution rate: The dilution of base metal into the overlay layer must be carefully controlled to maintain the required corrosion resistance. Typically, the dilution rate should not exceed 30% for the first layer and 15% for subsequent layers.
Preventing cracking: The combination of high carbon equivalent of the base material and the thermal stress from welding can lead to cracking. Preheating, controlled interpass temperature, and post-weld heat treatment (PWHT) are essential mitigation measures.
Ensuring metallurgical bonding: The interface between the base metal and the overlay layer must have a sound metallurgical bond without defects such as lack of fusion, porosity, or slag inclusions.

Quality Control and Inspection

Quality assurance for strip cladding SAW on nuclear-grade components involves multiple levels of inspection. Visual examination (VT) is performed on all welds to check for surface defects. Magnetic particle testing (MT) or liquid penetrant testing (PT) is applied to the overlay layer surface to detect surface-breaking cracks. Ultrasonic testing (UT) or radiographic testing (RT) is used to examine the bond line between the base metal and the overlay layer for lack of fusion and internal defects. Additionally, hardness testing, chemical analysis of the overlay layer, and intergranular corrosion testing may be required depending on the specific application and regulatory requirements.

The FMEA (Failure Mode and Effects Analysis) approach is particularly valuable in identifying potential failure modes during the cladding process. Common failure modes include porosity due to inadequate flux coverage, slag inclusions from improper flux distribution, lack of fusion at the bond line due to insufficient penetration, and hot cracking in the overlay layer due to excessive sulfur or phosphorus content.

Engineering Practice Insights

From an engineering practice perspective, the application of strip cladding SAW on nuclear pressurizer hemispherical heads requires careful planning and execution. The process qualification procedure (PQR) and welding procedure specification (WPS) must be developed in accordance with the applicable standards and must cover the full range of process variables that will be used in production. The welder qualification procedure (WQP) must also be established to ensure that only qualified personnel perform the cladding operations.

One critical aspect that cannot be overlooked is the effect of the cladding process on the mechanical properties of the base material. The thermal cycles from multiple passes can affect the microstructure and mechanical properties of the base metal in the heat-affected zone (HAZ). This is particularly important for nuclear-grade components where the mechanical properties must remain within specified limits throughout the service life.

Summary

The strip cladding SAW process for nuclear pressurizer hemispherical heads is a highly specialized technique that demands rigorous process control, thorough quality assurance, and deep understanding of metallurgical principles. The successful application of this technology ensures the long-term reliability and safety of nuclear power plant pressurizers, which are critical components for maintaining the integrity of the primary coolant system. Engineers working in this field must continuously refine their process knowledge and stay updated with the latest standards and inspection technologies to meet the ever-increasing quality requirements of the nuclear industry.