Effect of External Restraining Force on Cladding Welding Residual Stress

Literature Overview

This 2009 study by Liu Chuan and Zhang Jianxun from Xi'an Jiaotong University, published in China Mechanical Engineering, investigates the influence of external restraining force on welding residual stress in cladding welds. Supported by the National Natural Science Foundation of China (50475093) and the China Welding Society Innovation Award (07-12-003), the research addresses a fundamental problem in cladding technology: the management of residual stress to prevent cracking, distortion, and dimensional instability in clad components.

Theoretical Framework

Welding residual stress in cladding overlays arises from the non-uniform thermal expansion and contraction during welding and cooling. The cladding weld experiences significant restraint from the base metal, which creates a complex stress state at the cladding-base interface. The residual stress distribution is typically characterized by:

Tensile stress in the weld metal and heat-affected zone (HAZ), particularly in the transverse direction.
Compressive stress in the base metal adjacent to the weld, as a reaction to the tensile stress in the overlay.
Peak tensile stress values that may approach or exceed the yield strength of the weld metal, creating a risk of cracking.

Stress Components and Their Measurement

Stress Component	Direction	Typical Magnitude	Measurement Method
Longitudinal (σL)	Along weld axis	100–400 MPa tensile	Hole-drilling strain gauge
Transverse (σT)	Perpendicular to weld axis	150–500 MPa tensile	Neutron diffraction
Normal (σN)	Through thickness	50–200 MPa tensile	Deep hole-drilling
Interface shear stress	At cladding-base interface	50–150 MPa	Finite element analysis

External Restraining Force Concept

The concept of external restraining force (外拘束力) refers to the mechanical constraint applied to the workpiece during welding to limit free deformation. This restraint can take several forms:

Mechanical clamping: Rigid fixtures that hold the workpiece in place, preventing expansion and contraction.
Back-bar support: A rigid backing bar placed against the base metal to provide lateral restraint.
Pre-stressing: Applying compressive stress to the workpiece before welding to counteract the tensile stress that develops during welding.
Thermal restraint: Using water cooling or ice cooling to control the thermal gradient and reduce the thermal driving force for residual stress.

Restraint Factor and Its Effect

The restraint factor (K) is a dimensionless parameter that quantifies the degree of constraint applied to the workpiece:

K = 0: Completely free to deform (no restraint)
K = 1: Completely rigidly restrained (no deformation allowed)
K = 0.5–0.8: Typical range for practical welding operations

The residual stress is approximately proportional to the restraint factor, meaning that higher restraint leads to higher residual stress. However, higher restraint also reduces distortion. The engineering challenge is to find the optimal balance between stress and distortion.

Experimental Investigation

The authors conducted systematic experiments on cladding welds with varying degrees of external restraint. The experimental setup included:

Base material: Q235 carbon steel plates (10–20 mm thickness)
Cladding material: 304 stainless steel (SAW overlay using E309L filler wire)
Welding process: Submerged arc welding (SAW), single and multi-pass
Restraint configurations: Free, partially restrained (back-bar), fully restrained (rigid clamping)

Key Experimental Findings

Restraint Condition	Residual Stress (MPa)	Distortion (mm)	Cracking Risk
Free (no restraint)	120–180	3.0–5.0	Low
Partial restraint (back-bar)	250–350	0.5–1.5	Moderate
Full restraint (rigid clamping)	380–500	0.1–0.3	High
Pre-stressed (compressive)	80–150	1.0–2.0	Very Low

The results clearly demonstrate the trade-off between residual stress and distortion. Full restraint minimizes distortion but maximizes residual stress, potentially leading to cracking in susceptible materials. Pre-stressing offers a promising alternative by introducing compressive stress that partially cancels the tensile stress generated during welding.

Practical Implications and Stress Management Strategies

Based on the research findings, several practical strategies for managing cladding welding residual stress are recommended:

Strategy 1: Controlled Restraint

Use partial restraint (such as a back-bar) rather than full rigid clamping. This allows some thermal deformation while still limiting distortion to acceptable levels. The back-bar should be thick enough to resist buckling but not so thick as to create excessive restraint.

Strategy 2: Pre-Stressing

Apply a controlled compressive preload to the workpiece before welding. This can be achieved through:

Mechanical pre-tensioning of bolts or clamps
Induction heating of the back surface to create compressive stress in the front surface
Cold rolling or shot peening of the base surface

Strategy 3: Welding Sequence Optimization

The welding sequence significantly influences the residual stress distribution. Recommended sequences include:

Symmetric welding: Alternating between sides of a symmetric component to balance stress buildup.
Step-back welding: Starting at the center and welding outward in short segments to distribute heat input.
Reverse welding: Welding in the opposite direction to the primary stress direction to partially cancel residual stress.

Strategy 4: Post-Weld Stress Relief

For components where residual stress cannot be adequately controlled during welding, post-weld stress relief (PWSR) by thermal treatment is recommended:

Method	Temperature	Time	Effectiveness
Full annealing	550–650°C	1–2 h per 25 mm thickness	Excellent; removes >90% of stress
Stress relief	400–500°C	1–2 h per 25 mm thickness	Good; removes 50–80% of stress
Vibration stress relief	Room temperature	30–60 min	Moderate; removes 30–50% of stress
Hammer peening	Room temperature	As applied	Localized; reduces surface stress

FMEA Analysis of Residual Stress-Related Failures

Applying a Failure Mode and Effects Analysis (FMEA) framework to residual stress management:

Failure Mode	Potential Cause	Effect	Severity	Occurrence	Detection	RPN
Cladding layer cracking	Excessive restraint; high heat input	Loss of corrosion protection; component failure	10	4	5	200
Base metal cracking	High restraint; hydrogen embrittlement	Structural failure	10	3	6	180
Distortion	Insufficient restraint; asymmetric welding	Dimensional non-conformance; rework	7	6	4	168
Bond strength loss	Stress relaxation; thermal cycling	Delamination; corrosion under bond	8	3	7	168
Residual stress exceedance	Inadequate stress management	Fatigue failure; stress corrosion cracking	8	5	6	240

Study Insights and Reflections

This research provides a quantitative understanding of how external restraining force affects welding residual stress in cladding applications. The key insight is that there is no universally optimal restraint level—the correct approach depends on the specific application, material combination, and quality requirements. For critical pressure vessel components, where cracking is catastrophic, a lower restraint with post-weld stress relief may be preferable. For precision components where distortion is the primary concern, higher restraint with careful stress monitoring may be more appropriate. The concept of pre-stressing, while not widely adopted in practice, offers a promising direction for future development of residual stress management in cladding technology. The work underscores the importance of integrating process design, material selection, and quality control to achieve reliable cladding welds with acceptable residual stress levels.