Q345R and 0Cr18Ni12Mo2Ti Clad Plate Weld Overlay Process and Engineering Considerations

Literature Overview

This 2012 paper by Xu Wei, Jiang Min, and Li Ming from Zhejiang Industrial Equipment Installation Group Co., Ltd. addresses the weld overlay process for composite plates combining Q345R low-alloy steel with 0Cr18Ni12Mo2Ti (equivalent to ASTM 316Ti stainless steel). The work was published in the context of China's rapidly expanding chemical equipment fabrication industry, where the demand for pressure vessels with corrosion-resistant linings on economical structural shells had created a significant need for reliable overlay welding procedures. The study represents a practical engineering response to the challenge of joining dissimilar materials in large-scale pressure vessel fabrication.

Core Technical Content and Process Parameters

The Q345R/316Ti clad plate combination is widely used in chemical reactors, heat exchangers, and storage vessels where the base metal provides structural strength while the stainless overlay resists corrosion from acidic or chloride-containing media. The weld overlay process must ensure full metallurgical bonding between the two layers while maintaining the corrosion resistance of the 316Ti overlay and avoiding excessive dilution of carbon equivalents from the Q345R base into the overlay.

Parameter	Specification	Standard Reference
Base metal	Q345R (GB/T 1591)	NB/T 47002
Overlay metal	0Cr18Ni12Mo2Ti (GB/T 4237)	Equivalent to 316Ti
Welding process	Submerged Arc Welding (SAW) or Gas Metal Arc Welding (GMAW)	NB/T 47014
Filler metal for overlay	ER316Ti or equivalent (A182 CF8M cast)	ASTM A5.4
Filler metal for root pass	ER309L or ER316L (high Cr-Ni to prevent cracking)	ASTM A5.9
Preheat temperature	100–150°C	Prevents cold cracking in Q345R
Interpass temperature	≤200°C	Controls dilution and residual stress
Post-weld heat treatment	550–650°C stress relief	Reduces residual stress
Overlay thickness	3–6 mm (typical)	Per vessel design specification
Bond strength test	≥0.6 times the lower of the two tensile strengths	NB/T 47013

The authors emphasized the critical importance of the root pass filler metal selection. Using a high-chromium, high-nickel filler (such as ER309L) for the first pass creates a transition zone that accommodates the coefficient of thermal expansion mismatch between the carbon steel and stainless steel, thereby preventing cracking at the interface. Subsequent passes progressively dilute toward the 316Ti composition.

Process Analysis and Standards Interpretation

The weld overlay of Q345R/316Ti clad plate must comply with multiple standards simultaneously. The base metal construction follows GB/T 150 (pressure vessels) and NB/T 47002 (technical requirements for pressure vessel steel plates), while the overlay welding procedure must be qualified per NB/T 47014 (welding procedure qualification for pressure vessels). The welder qualification must follow NB/T 47013, and the overlay thickness and bond strength must be verified per the applicable design code.

A critical process consideration is the sequence of welding operations. In a typical fabrication sequence for a clad pressure vessel:

The base metal shell is formed and welded in the bare (unclad) condition.
The overlay welding is performed on the interior surface of the shell.
Any post-weld heat treatment is applied after overlay completion.
Non-destructive testing (NDT) of the overlay bond is performed, typically using ultrasonic testing (UT) per JB/T 4730.

The paper notes that the SAW process was preferred for the overlay passes due to its high deposition rate and consistent bead geometry, while GMAW was used for the root pass and for areas where SAW access was restricted. The use of a flux with low hydrogen content was essential to prevent hydrogen-induced cracking in the Q345R base metal.

Defect Analysis and Countermeasures

The following defects are particularly relevant to Q345R/316Ti overlay welding:

Defect Type	Root Cause	Countermeasure
Cracking at overlay-base interface	Excessive thermal stress from CTE mismatch	Use ER309L root pass; control interpass temperature
Carbon contamination of overlay	Base metal dilution	Limit number of passes; use proper filler metal sequence
Sensitization of 316Ti overlay	Excessive heat input causing Cr₂₃C₆ precipitation	Limit interpass temperature; avoid excessive heat input
Porosity in overlay	Flux contamination or hydrogen absorption	Use low-hydrogen flux; proper flux drying
Poor bond strength	Incomplete fusion at interface	Ensure adequate heat input; proper cleaning of base metal surface

The authors also discussed the importance of intergranular corrosion testing of the overlay layer, particularly when the overlay is exposed to sensitizing heat inputs. The 316Ti grade contains titanium stabilizer to prevent chromium carbide precipitation at grain boundaries, but excessive welding heat can still cause local sensitization. The standard test method per GB/T 4334 (or ASTM A262 Practice E) should be performed on the overlay to verify resistance to intergranular corrosion.

Engineering Practice Cases and Implications

The practical application described in this paper involved the fabrication of chemical reactors and heat exchangers for the petrochemical industry. The authors reported successful overlay of vessels with diameters up to 3.0 meters and overlay thicknesses of 4–6 mm. The bond strength test results consistently exceeded the minimum requirement of 0.6 times the lower tensile strength of the two materials (approximately 240 MPa for Q345R), with typical values of 280–320 MPa.

A particularly valuable observation from the engineering perspective is the economic argument for using Q345R/316Ti clad plate rather than monolithic 316Ti construction. The cost saving is substantial—typically 40–60% reduction in material cost—while the performance is equivalent for most chemical service applications. However, the overlay process must be executed with extreme care to ensure that the full design life of the corrosion-resistant layer is achieved.

The study also highlights the importance of fabrication sequencing. Performing the overlay welding before the vessel is fully assembled (e.g., before dished heads are welded on) reduces the risk of distortion and makes the overlay process more accessible. This sequencing consideration is often overlooked in preliminary design but becomes critical during fabrication planning.

Study Insights and Reflections

This paper exemplifies the practical engineering approach to dissimilar material overlay welding. The authors' emphasis on filler metal selection, thermal management, and standards compliance reflects a mature understanding of the metallurgical challenges involved. The work is particularly relevant to contemporary practice because the Q345R/316Ti combination remains one of the most widely used clad plate systems in Chinese chemical equipment fabrication.

One area where the study could be extended is the discussion of post-overlay inspection methods. While the paper mentions bond strength testing, it does not elaborate on the increasingly important role of phased array ultrasonic testing (PAUT) and time-of-flight diffraction (TOFD) for overlay bond inspection. Modern codes such as ASME VIII Div.2 and GB/T 150-2011 place greater emphasis on advanced NDT methods for verifying overlay quality. The integration of PAUT into overlay inspection protocols represents a significant advancement in quality assurance that should be considered in contemporary fabrication practice.

The paper also implicitly raises the question of overlay layer life prediction under cyclic thermal and mechanical loading. In hydrogenation reactors and similar high-temperature service, the overlay layer is subjected to repeated thermal cycling that can cause fatigue cracking at the overlay-base interface. Understanding the fatigue behavior of the overlay weldment under cyclic loading conditions is essential for life assessment and should be incorporated into design considerations for critical applications.