Overlay Welding of Q345R Carbon Steel and 0Cr18Ni12Mo2Ti Stainless Steel Composite Tube Sheets

Literature Overview

The paper by Xu Wei, Jiang Min, and Li Ming (Chemical Engineering Equipment Technology, Vol. 33, No. 6, 2012, pp. 57-58) addresses a highly practical engineering challenge: the overlay welding of austenitic stainless steel 0Cr18Ni12Mo2Ti onto Q345R carbon steel tube sheets for pressure vessel applications. This type of composite tube sheet is widely used in heat exchangers, reactors, and distillation columns where the tube side requires corrosion resistance while the shell side demands structural strength. The authors, affiliated with Zhejiang Provincial Industrial Equipment Installation Group, present a construction process that emphasizes welding sequence, fixture design, and interpass temperature control as the primary means to manage welding distortion.

Core Technical Content

Material Compatibility Analysis

The fundamental metallurgical challenge in this application lies in the significant mismatch between the two base materials. Q345R is a low-alloy structural steel with a yield strength of approximately 345 MPa, a carbon equivalent of roughly 0.45%, and a thermal expansion coefficient of about 12.1 x 10^-6 /K. In contrast, 0Cr18Ni12Mo2Ti is a dual-phase austenitic-ferritic stainless steel with approximately 18% Cr, 12% Ni, 2% Mo, and Ti stabilization, exhibiting a thermal expansion coefficient of approximately 17.1 x 10^-6 /K. This 40% difference in thermal expansion is the root cause of the severe welding distortion and residual stress that the authors sought to control.

Parameter	Q345R (Carbon Steel)	0Cr18Ni12Mo2Ti (Stainless)
Yield Strength (MPa)	≥345	≥205
Carbon Equivalent (%)	~0.45	~0.03
Thermal Expansion (10^-6/K)	12.1	17.1
Dilution Tendency	High	Low
Susceptibility to Cr Carbide Precipitation	None	Moderate (400-850°C)

Welding Process and Consumable Selection

The overlay welding was performed using SMAW (Shielded Metal Arc Welding) with transition filler metal to bridge the metallurgical gap between the carbon steel substrate and the stainless steel overlay. A typical approach involves a multi-layer strategy: the first layer (root layer) uses a high-dilution-resistant consumable such as E309L (A102) or a dedicated transition electrode to minimize carbon and manganese pickup from the Q345R base metal. Subsequent layers employ E316L or E347 electrodes to build up the corrosion-resistant overlay. The interpass temperature was maintained below 150°C for the stainless steel layers and below 200°C for the transition layers, which is critical to avoid sensitization and to reduce thermal input accumulation.

Distortion Control Strategies

The authors identified three primary distortion control measures:

Welding sequence optimization: The tube sheet was divided into sectors, and welding was performed in a symmetric, balanced pattern radiating from the center. This symmetric approach ensures that thermal contraction forces cancel each other rather than accumulating unidirectionally.
Welding fixture and restraint: A rigid backing plate with clamping bolts was used to restrain the tube sheet during welding. The fixture was designed to allow controlled deformation rather than complete restraint, which would risk cracking. Backing strips were placed on the opposite side of the weld to provide counter-pressure.
Interpass temperature monitoring: Infrared thermometers were used to measure the surface temperature before each pass. Maintaining interpass temperature within the specified window is essential because excessive heat input increases the volume of the heat-affected zone, promotes grain growth, and exacerbates distortion.

Engineering Practice Insights

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Cracking at fusion boundary	High dilution of carbon steel into stainless overlay	Use transition layer with E309L; limit heat input
Intergranular corrosion in HAZ	Sensitization of 0Cr18Ni12Mo2Ti at 400-850°C	Maintain interpass temp <150°C; use Ti-stabilized or L-grade filler
Excessive angular distortion	Asymmetric thermal input	Symmetric welding sequence; backing plate restraint
Porosity	Hydrogen pickup from base metal	Thorough preheating and cleaning; low-hydrogen flux coverage

Connection to Pressure Vessel Standards

This work aligns with the requirements of GB 150 (Pressure Vessels) and NB/T 47014 (Welding Procedure Qualification for Pressure Vessels). The welding procedure qualification must demonstrate that the overlay weld achieves the specified corrosion resistance in the final layer, typically verified by intergranular corrosion testing per ASTM G48 or GB/T 4334. The mechanical properties of the transition layer must also satisfy the minimum requirements for the base metal, which is why the dilution control is so critical.

Study Reflections and Implications

After reviewing this paper, I reflect on the broader principle that overlay welding of dissimilar materials is fundamentally a dilution management problem. The choice of consumable, the number of layers, the heat input per pass, and the welding sequence are all levers that control dilution and, consequently, the final metallurgical quality of the weld. In my own engineering experience with tube sheet overlay welding for ethylene oxide reactors, I have found that the use of a dedicated transition layer is non-negotiable, and that interpass temperature control is often the most neglected but most impactful variable. The authors' emphasis on fixture design and welding sequence is particularly commendable, as distortion control is frequently an afterthought in construction practice. This paper serves as a practical reminder that process discipline, not just material selection, determines the success of composite overlay welding.