Application of Domestic Single-Layer Strip Electrode Electroslag Cladding Consumables in Hydrogenation Reactors

Literature Overview

This study documents the successful application of domestically developed single-layer strip electrode electroslag welding (ESW) cladding consumables in the fabrication of hydrogenation reactor inner surfaces. Hydrogenation reactors operate under severe conditions — high temperature (350–500°C), high pressure (10–30 MPa), and exposure to hydrogen-containing corrosive media — requiring high-performance nickel-based alloy overlay layers with excellent resistance to hydrogen embrittlement, corrosion, and thermal fatigue.

Core Technical Content

Material Selection and Specification

Component	Material Grade	Key Properties	Standard Reference
Reactor shell	16MnR / 15CrMoR	σs ≥ 345 MPa, good weldability	GB/T 713
Cladding layer	Alloy 625 / C-276 equivalent	Ni ≥ 55%, Cr 20–23%, Mo 8–10%	NB/T 47016
Strip electrode	Single-layer ESW strip	100% nickel-based alloy composition	GB/T 12469
Flux	Low-hydrogen ESW flux	H < 5 mL/100g, high slag fluidity	GB/T 12468

ESW Cladding Process Parameters

The single-layer strip electrode ESW process offers significant advantages over multi-layer SAW or GMAW overlay for thick cladding layers:

Parameter	Value	Rationale
Strip width	50–100 mm	Wide coverage reduces number of passes
Strip thickness	2.0–3.0 mm	Single-layer deposition of full cladding thickness
Current	600–1000 A	High current for deep penetration and good fusion
Voltage	30–40 V	Stable arc with proper slag coverage
Travel speed	300–600 mm/min	Controlled heat input for microstructure
Preheating	150–250°C	Prevent cold cracking in substrate
Interpass temperature	≤ 250°C	Control microstructure and residual stress
Post-weld heat treatment	720–780°C × 2–4 h	Solution treatment to dissolve carbides

Microstructure and Performance of Cladding Layer

The ESW cladding layer exhibits the following characteristics after proper heat treatment:

Grain structure: Fine equiaxed austenite grains (50–150 μm) with Laves phase (Mo₆Si) precipitates controlled to < 5% volume fraction.
Hardness: 200–280 HV after solution treatment; 280–350 HV after aging.
Bond strength: > 90% of the lower strength material (typically the base metal), verified by bond strength test per GB/T 12469.
Corrosion resistance: Excellent resistance to hydrogen embrittlement, sour gas corrosion (H₂S), and high-temperature oxidation.
Hydrogen permeability: Reduced by 50–70% compared to base carbon steel, providing effective hydrogen barrier.

Quality Control and Inspection Requirements

Inspection Method	Standard	Acceptance Criteria
Visual inspection (VT)	JB/T 4730.1	No surface defects, uniform color
Magnetic particle inspection (MT)	JB/T 4730.4	No linear indications > 3 mm
Ultrasonic testing (UT)	JB/T 4730.3	Bond quality Level I, no delamination
Radiographic testing (RT)	JB/T 4730.2	No porosity > 0.5 mm, no slag > 1 mm
Hardness test	GB/T 231.1	200–350 HV, uniform across surface
Chemical analysis	GB/T 223	Ni ≥ 55%, Cr 20–23%, Mo 8–10%
Intergranular corrosion	GB/T 4334	No intergranular attack after 120 h
Bond strength	GB/T 12469	> 90% of base metal tensile strength

Engineering Practice Challenges and Solutions

Challenge 1: Dilution Control

The single-layer strip electrode approach inherently has lower dilution (typically 5–15%) compared to multi-pass SAW overlay (20–40%). However, for thick cladding layers (> 5 mm), multiple ESW passes may be required, increasing cumulative dilution.

Solution: Use a dedicated low-dilution flux composition with high slag coverage and low thermal conductivity to minimize heat transfer to the substrate.

Challenge 2: Residual Stress Management

The high heat input of ESW welding generates significant residual stresses, which can compromise the hydrogen barrier function and promote stress corrosion cracking.

Solution: Implement a controlled cooling rate strategy with adequate post-weld heat treatment (PWHT) at 720–780°C for 2–4 hours, followed by slow cooling in the furnace.

Challenge 3: Hydrogen-Induced Cracking (HIC) and Sulfide Stress Cracking (SSC)

The base steel substrate is susceptible to HIC and SSC in hydrogen-containing environments. The cladding layer must provide an effective barrier, but any defects (porosity, lack of fusion) can create pathways for hydrogen ingress.

Solution: Strict control of substrate HIC resistance (per ASTM G178/G285), thorough NDT of the cladding layer, and consideration of a multi-layer approach with the first layer deposited by low-heat-input process (TIG) to ensure perfect fusion.

Key Reflections and Insights

The successful application of domestic single-layer strip electrode ESW consumables in hydrogenation reactor fabrication represents a significant advancement in China's capability to produce high-pressure hydrogenation equipment independently. The study demonstrates that domestically developed consumables can achieve performance equivalent to imported alternatives (such as those from ESAB, Lincoln, or Kemppi) while offering substantial cost advantages and supply chain security.

The single-layer strip electrode approach offers several practical advantages: reduced welding time (50–70% reduction compared to multi-pass SAW), lower labor costs, improved productivity, and consistent quality with reduced operator skill dependency. However, the approach requires careful attention to flux quality, arc stability, and post-weld heat treatment to ensure the microstructure meets the stringent requirements for hydrogen service.

From a regulatory perspective, the fabrication must comply with TSG 21 (China's pressure vessel safety regulation), NB/T 47002 (pressure vessel design), and NB/T 47014 (welding procedure qualification). The welding procedure qualification (WPQ) must include specific tests for the ESW cladding process, including bond strength, intergranular corrosion, and hydrogen permeability tests as applicable.