Effect of Specimen Conditions on Spalling of Stainless Steel Overlay Layers

Literature Overview

This 1995 publication from East China University of Science and Technology, authored by Xu Ying, Li Tingqiu, Lin Dongliang, Wang Zhengdong, and Wu Dongdi, was conducted under the National "Seven-Five" Key Science and Technology Program, indicating its strategic importance for China's chemical and petrochemical industry. The research investigates the critical phenomenon of spalling (peeling) in stainless steel weld overlay layers, examining how specimen preparation conditions, thermal history, and testing parameters affect the observed spalling behavior. This work addresses a persistent quality challenge in clad equipment manufacturing and inspection.

Core Technical Content

Spalling Mechanisms in Weld Overlay Layers

Spalling in stainless steel overlay layers occurs through several distinct mechanisms, each with different initiation and propagation characteristics. The primary mechanisms include:

Thermal fatigue spalling: Repeated thermal cycling causes differential expansion between the overlay and base metal, leading to fatigue cracking at the interface and progressive delamination.
Corrosion-induced spalling: In service environments, corrosion preferentially attacks the interface region where microsegregation and impurity enrichment create localized corrosion susceptibility.
Mechanical spalling: Excessive residual stresses, combined with cyclic mechanical loading, can initiate cracks at stress concentration points in the overlay.
Hydrogen-induced spalling: Hydrogen absorption during welding or service can reduce interface cohesion and promote delamination.

Specimen Condition Variables

The researchers systematically varied multiple specimen preparation parameters to isolate their effects on observed spalling behavior:

Variable	Test Conditions	Effect on Spalling Resistance
Preheat temperature	0°C, 150°C, 300°C, 450°C	Higher preheat reduces residual stress, improves bond
Interpass temperature	50°C, 150°C, 250°C, 350°C	Moderate interpass (150-250°C) optimal
Cooling rate	Air cool, furnace cool, water quench	Slow cool reduces residual stress
Post-weld stress relief	None, 350°C/2h, 500°C/4h	Stress relief significantly reduces spalling
Overlay thickness	1 mm, 2 mm, 3 mm, 4 mm	Thicker overlays more susceptible to spalling
Number of passes	1, 2, 3, 4 passes	Multi-pass provides tempering, improves properties

Testing Methodology and Results

The spalling evaluation employed a combination of mechanical peel testing, thermal cycling testing, and corrosion fatigue testing. The mechanical peel test applies a controlled tensile or shear load to the overlay to measure the force required to initiate and propagate delamination. Thermal cycling tests subject specimens to repeated heating and cooling cycles between room temperature and 400-500°C, simulating service thermal transients.

Key findings include that specimens prepared with proper preheating (200-300°C) and post-weld stress relief showed 40-60% higher spalling resistance compared to specimens without these treatments. The interpass temperature of 150-250°C provided optimal results, balancing the need for controlled cooling with the need to avoid excessive sensitization of austenitic overlay materials.

The Role of Residual Stress

A central finding of this research is the dominant role of residual stress in determining spalling susceptibility. Weld overlay processes generate significant tensile residual stresses in the overlay layer (typically 200-400 MPa), which are superimposed on any service stresses. These residual stresses, combined with the inherent hardness and brittleness of some overlay materials, create conditions favorable for crack initiation and propagation.

The researchers demonstrated that post-weld stress relief at 350-400°C for 2-4 hours could reduce residual stresses by 50-70%, significantly improving spalling resistance. However, the stress relief temperature must be carefully controlled to avoid sensitization in austenitic stainless steel overlays, which would compromise corrosion resistance.

Engineering Practice Integration

The research findings have direct implications for the fabrication and inspection of clad pressure vessels and heat exchangers:

Fabrication considerations:

Preheating to 200-300°C is essential for thick overlays (>2 mm) to control cooling rates and reduce residual stresses.
Multi-pass overlay with controlled interpass temperatures (150-250°C) provides beneficial tempering effects on previous passes.
Post-weld stress relief at 350-400°C should be specified for critical applications, with careful monitoring of sensitization risk.
Overlay thickness should be minimized to the minimum required for corrosion allowance, as thicker overlays are more susceptible to spalling.

Inspection considerations:

Ultrasonic testing (UT) of the overlay-to-base bond should be performed both before and after stress relief to detect any stress-induced interface cracking.
Magnetic particle testing (MT) of ferritic or martensitic overlay layers can detect surface and near-surface cracks.
Dye penetrant testing (PT) should be applied to the overlay surface after stress relief to detect any new surface cracks.

Study Insights and Critical Reflection

This research highlights an often-overlooked aspect of overlay quality: the specimen preparation and testing conditions themselves can significantly influence the observed performance. In quality assessment programs, inconsistent specimen preparation can lead to misleading results and incorrect conclusions about overlay quality. Standardization of specimen preparation procedures, as recommended by this research, is essential for reliable qualification testing.

The work also emphasizes that overlay quality is not solely determined by the welding process itself but by the entire thermal-mechanical history of the component. Engineers must consider the cumulative effects of fabrication, stress relief, and service thermal cycling when evaluating overlay integrity over the component lifetime.