Weld Overlay Process for Q345R and 0Cr18Ni12Mo2Ti Composite Plate

Literature Overview

This 2012 publication by Xu Wei, Jiang Min, and Li Ming from Zhejiang Industrial Equipment Installation Group Second Branch addresses the weld overlay fabrication of Q345R/0Cr18Ni12Mo2Ti composite plate for chemical equipment applications. The document represents a practical engineering approach to clad plate manufacturing, addressing the specific challenges of overlaying austenitic stainless steel onto low-alloy steel substrates for use in corrosive chemical processing environments.

Material System Analysis

The composite plate system combines two materials with fundamentally different properties:

Property	Q345R (Base)	0Cr18Ni12Mo2Ti (Overlay)
Classification	Low-alloy pressure vessel steel	Austenitic stainless steel (316Ti equivalent)
Carbon content	≤0.20%	≤0.08%
Tensile strength	≥490 MPa	≥520 MPa
Thermal expansion coefficient	12.2×10⁻⁶/°C	17.3×10⁻⁶/°C
Electrical resistivity	0.17 μΩ·m	0.74 μΩ·m
Primary corrosion mechanism	Uniform corrosion, HIC	Pitting, crevice, intergranular

The 0Cr18Ni12Mo2Ti grade, equivalent to ASTM 316Ti, contains titanium stabilization to prevent chromium carbide precipitation and intergranular corrosion. The addition of molybdenum provides enhanced resistance to pitting and crevice corrosion in chloride-containing environments. This makes it suitable for chemical equipment exposed to sulfuric acid, phosphoric acid, and other aggressive media.

Weld Overlay Process Design

The weld overlay process for this composite plate typically involves one of the following methods:

Process Method	Advantages	Limitations
SAW (Submerged Arc Welding)	High deposition rate; good penetration	Requires flux; limited to horizontal positions
ESW (Electroslag Welding)	Very high deposition rate; uniform microstructure	Limited to vertical/horizontal; high equipment investment
GMAW (Gas Metal Arc Welding)	Flexible; good for repair and small areas	Lower deposition rate; higher dilution
Oxy-acetylene	Simple equipment; good for small repairs	Low efficiency; high heat input

For production-scale fabrication, SAW or ESW are preferred due to their high deposition rates and consistent quality. The following process parameters are recommended for SAW overlay of 0Cr18Ni12Mo2Ti on Q345R:

Parameter	Value	Rationale
Welding current	400-600 A	Ensure full penetration of base metal
Arc voltage	30-35 V	Control bead width and profile
Travel speed	150-250 mm/min	Balance deposition rate and dilution
Flux	H10Mn2Si or similar	Provide adequate shielding and slag protection
Filler wire	0Cr18Ni12Mo2Ti or 0Cr19Ni11Mo2Ti	Match overlay composition
Number of layers	2-3	First layer for bonding; subsequent layers for composition control
Interpass temperature	≤150°C	Prevent sensitization of austenitic steel
Preheat	50-100°C	Reduce thermal gradient; prevent base metal cracking

Dilution Control and Composition Management

The most critical technical challenge in this application is controlling the dilution of the overlay layer by the base metal. The first overlay layer typically experiences 30-50% dilution, resulting in a martensitic or ferritic microstructure that lacks the corrosion resistance of the intended austenitic composition. The subsequent layers show progressively lower dilution, typically reaching 5-10% in the final layer.

To achieve a fully austenitic overlay with the required corrosion resistance, the following strategies are employed:

Multi-layer overlay (minimum 2 layers, preferably 3) to progressively reduce dilution
Use of a higher-nickel filler metal in the first layer to promote austenite formation despite dilution
Post-weld solution treatment at 1050-1100°C followed by water quench to homogenize the microstructure
Metallographic examination of the overlay layer to verify full austenite composition

Mechanical Properties and Quality Requirements

The composite plate must meet the following quality requirements per relevant standards:

Test Item	Requirement	Standard
Bond strength (shear)	≥140 MPa	NB/T 47014
Overlay layer hardness	HV 150-250	ASTM E92
Intergranular corrosion (ASTM A263 Method B)	No intergranular attack	ASTM A263
Chemical composition of overlay	Fe balance, Cr 16-20%, Ni 10-14%, Mo 2-3%, Ti 5×C	GB/T 24511
NDT (UT bond test)	100% area, no defects	NB/T 47014
NDT (RT or MT of overlay)	No cracks, porosity >2 mm	NB/T 47014

Residual Stress and Distortion Control

The thermal expansion mismatch between Q345R and 0Cr18Ni12Mo2Ti creates significant residual stresses during cooling. The austenitic overlay contracts less than the base metal upon cooling, resulting in compressive stresses in the overlay and tensile stresses in the base metal. These residual stresses can lead to:

Delayed cracking in the heat-affected zone of the base metal
Distortion of the composite plate, particularly for thin plates
Stress corrosion cracking of the overlay layer in service

Mitigation measures include:

Symmetric welding sequence to balance thermal input
Interpass temperature control to limit peak temperature
Post-weld stress relief at 425-450°C for 1-2 hours (with caution regarding sensitization)
Use of backing plates or clamping fixtures to control distortion

Engineering Practice and Quality Assurance

This literature provides a practical framework for the fabrication of clad plate pressure vessels and chemical equipment. The emphasis on dilution control, microstructural management, and quality assurance reflects the critical importance of these factors in ensuring the long-term reliability of composite plate components in corrosive service environments.

The work also highlights the importance of process qualification testing per NB/T 47014 or equivalent standards. Each production batch should include representative samples for metallographic examination, mechanical testing, and corrosion testing to verify that the overlay meets the specified requirements.