Weld Overlay Process for Q345R and 0Cr18Ni12Mo2Ti Clad Plate

Overview and Background

The combination of Q345R (Chinese standard low-alloy steel for pressure vessels, equivalent to SA-516 Gr.70) and 0Cr18Ni12Mo2Ti (Chinese standard austenitic stainless steel, equivalent to 321) is one of the most common clad plate configurations in pressure vessel fabrication. This combination provides excellent economic efficiency: the carbon steel provides structural strength at low cost, while the stainless steel provides corrosion resistance for the internal service environment. The weld overlay process for joining clad plate sections involves critical considerations regarding dilution control, bond line integrity, and maintenance of the corrosion-resistant properties of the overlay layer.

Material Characteristics and Standards

Property	Q345R (Carbon Steel)	0Cr18Ni12Mo2Ti (SS Clad)
Yield strength	≥345 MPa	≥205 MPa
Tensile strength	490-630 MPa	520-750 MPa
Carbon content	≤0.20%	≤0.12%
Chromium	0.30-0.60%	17.0-19.0%
Nickel	0.30-0.60%	10.0-12.0%
Molybdenum	≤0.30%	2.0-3.0%
Titanium	≤0.030%	5×C to 0.70%
Applicable standard	GB/T 713	GB/T 24511
Clad plate standard	GB/T 19082	EN 10028-7

Bond Line Requirements

According to GB/T 19082 and NB/T 47015, the clad plate must demonstrate:

Minimum bond strength: 100% of the lower of the two base metals (i.e., ≥205 MPa for SS clad)
No delamination under hydrostatic pressure test (typically 1.5× design pressure)
Peel test: minimum 100 N/cm for 321/steel combination
Hydrogen-induced cracking (HIC) resistance per NACE MR0175 for sour service

Weld Overlay Process Design

Process Selection Matrix

Configuration	Recommended Process	Filler Metal	Key Consideration
SS-to-SS (clad face)	GTAW + GMAW	ER308L / ER347	Maintain Ti stabilization
Steel-to-steel (base)	SAW + FCAW	E7018 / E8010-D1	Full penetration
SS-to-steel (edge)	GTAW (root) + SAW (fill)	ER309L (root)	Dilution control
Clad-to-clad (edge)	GTAW only	ER347	Minimum dilution

Critical Welding Sequence for Clad Plate Vessels

Base metal preparation: bevel edges showing both layers; mark clad thickness clearly
Base metal welding: weld carbon steel layers using low-hydrogen process; grind flush to clad surface
Clad layer restoration: deposit 2-3 passes of austenitic stainless steel to restore clad thickness
Post-weld treatment: solution heat treatment at 1050°C for 1 hour (if required by design)
Final inspection: UT bond test, PT of weld overlay, dimensional verification

Heat Input and Dilution Control

For the critical edge weld where clad layers are joined:

Weld Position	Heat Input (kJ/mm)	Target Dilution	Acceptable Dilution
Root pass (GTAW)	0.3-0.6	<25%	<30%
Fill passes (GTAW)	0.6-1.0	<20%	<25%
Cap pass (GTAW)	0.4-0.7	<15%	<20%
Full weld (avg)	0.5-0.8	<20%	<25%

Defect Analysis and Engineering Challenges

Typical Defects in Clad Plate Welding

Defect	Location	Cause	Consequence	Detection
Cracking in HAZ	Steel side	High carbon equivalent, rapid cooling	Loss of strength	MT + UT
Intergranular corrosion	SS weld zone	Carbon migration, sensitization	Corrosion failure	Salt spray + metallography
Delamination	Bond line	Poor fusion, gas inclusion	Loss of containment	UT bond test
Excessive dilution	Clad weld	High heat input, wrong filler	Loss of corrosion resistance	Chemical analysis
Undercut	Clad surface	Poor technique	Stress concentration	Visual + PT

Engineering Practice Case Study

In the fabrication of a hydrogenation reactor (design pressure 15 MPa, design temperature 420°C, sour service) using Q345R/0Cr18Ni12Mo2Ti clad plate:

Challenge: The reactor required a 3.0 mm clad layer with 99.9% bond integrity and resistance to sulfide stress corrosion (SSC) per NACE MR0175
Solution: Implemented a three-stage welding approach:

Base metal: FCAW with E8010-D1, heat input 1.5-2.5 kJ/mm, preheat 100°C
Transition: GTAW with ER309L, 2 passes, heat input 0.5 kJ/mm
Clad restoration: GTAW with ER347, 3 passes, heat input 0.4-0.6 kJ/mm

Results:
Bond strength: 285 MPa (139% of minimum requirement)
HIC test: 0% crack area (passes NACE MR0175)
Corrosion rate: <0.05 mm/year in simulated sour environment
Dilution in final pass: 12% (well below 25% limit)

Study Insights and Reflections

The welding of Q345R/0Cr18Ni12Mo2Ti clad plate is fundamentally a dilution management exercise. Every process parameter — from base metal preheat to final cap pass current — must be optimized to minimize iron contamination of the stainless steel overlay while maintaining adequate fusion and mechanical integrity. The most common failure mode in field service is intergranular corrosion caused by excessive dilution or sensitization during welding, which is why the use of stabilized fillers (ER347 with titanium) is strongly preferred over ER308L for critical applications.

Furthermore, the inspection strategy must be comprehensive: ultrasonic bond testing is non-negotiable for verifying the integrity of the steel-SS interface, while intergranular corrosion testing per ASTM A262 Practice A or E is essential for confirming the metallurgical quality of the weld overlay. Engineers should never accept a clad plate weld based solely on visual and penetrant inspection — the hidden failure modes at the bond line and within the sensitized zone are invisible to surface methods.