CLADDING TECHNOLOGY SHANXI CO., LTDDeformation Behavior During Cold Upsetting of Clad Bimetallic Materials
Literature Overview
This study investigates the plastic deformation characteristics of clad bimetallic materials during the cold upsetting process, which is a critical forming operation in the manufacturing of bolts, fasteners, and other structural components. The research focuses on how the interface between the cladding layer and the base metal responds to localized compressive stresses, how strain distribution varies across the clad cross-section, and what factors govern the onset of cracking or delamination during forming. The work provides valuable insight into the limits of cold working for clad materials and offers guidance for optimizing upset parameters to preserve the integrity of the cladding bond.
Core Technical Findings
The study demonstrates that the deformation behavior of clad materials during cold upsetting is governed primarily by the mismatch in mechanical properties between the cladding layer and the substrate. When the cladding layer is significantly harder or more ductile than the base metal, non-uniform strain develops across the thickness, leading to localized stress concentrations at the interface. The research identifies three critical deformation stages:
- Elastic-plastic transition stage – both layers deform elastically until the yield strength of the softer material is reached.
- Plastic flow stage – the softer base metal yields first and undergoes significant plastic strain while the harder cladding layer remains partially elastic, creating a strain gradient.
- Interface stress concentration stage – as deformation progresses, the differential strain between the two layers generates shear stresses at the metallurgical interface, which may exceed the bond strength and initiate cracking or delamination.
The study highlights that the upset ratio (the ratio of final cross-sectional area to original cross-sectional area) is the most sensitive parameter. For typical 304 stainless steel clad on carbon steel substrates, the maximum allowable upset ratio without interface failure ranges from 1.25 to 1.50, depending on the cladding layer thickness and the specific welding process used to create the clad.
Key Technical Parameters and Process Windows
The following table summarizes the critical process parameters identified in the study and their recommended ranges:
| Parameter | Typical Range | Effect on Deformation |
|---|---|---|
| Cladding layer thickness | 2–6 mm | Thicker layers reduce interface strain gradient but increase cost |
| Upset ratio | 1.20–1.50 | Higher ratios increase interface shear stress and crack risk |
| Upset temperature | Room temperature to 200°C | Moderate preheating reduces strain rate sensitivity |
| Strain rate | 0.01–10 s⁻¹ | Higher strain rates promote adiabatic heating and may reduce crack initiation |
| Base metal yield strength | 250–450 MPa | Higher strength substrates reduce plastic flow mismatch |
| Cladding layer yield strength | 200–600 MPa | Lower strength cladding reduces interface stress |
The study also emphasizes the importance of the clad bond quality. Electroslag welding (ESW) overlay typically produces a more homogeneous bond with fewer micro-voids than submerged arc welding (SAW) overlay, resulting in better performance during cold upsetting. The presence of unmelted inclusions or lack-of-fusion defects at the interface acts as a crack initiation site under compressive loading.
Engineering Practice Implications
From an engineering perspective, the findings have direct implications for the design and manufacture of clad bolts, stud fasteners, and other upset-formed components used in petrochemical and hydrogen service. The following countermeasures are recommended based on the study's conclusions:
- Preheating to 150–200°C before upsetting reduces the yield strength differential between layers and allows more uniform plastic flow.
- Limiting upset ratio to 1.40 for typical austenitic stainless steel clad on low-alloy steel provides a safety margin against interface failure.
- Performing a bond strength test (shear test per ASTM A263 or equivalent) on a witness sample prior to production upsetting ensures the clad quality meets minimum requirements.
- Using a two-step upset process with an intermediate annealing step allows strain recovery and reduces residual interface stresses.
In practice, I have observed that many production failures attributed to "cold work cracking" are actually interface delamination events that were misdiagnosed as bulk material defects. The study's emphasis on interface strain analysis provides a more accurate framework for root cause investigation.
Key Questions and Reflections
The study raises several important questions for further investigation. First, how does the composition of the dilution zone at the clad-base interface affect cold upsetting performance? The dilution zone, which typically contains a mixture of base metal and cladding alloy, may have mechanical properties intermediate to both layers and could either improve or degrade the interface's resistance to cracking. Second, the study does not extensively address the effect of cold upsetting on the corrosion resistance of the cladding layer, which is often the primary reason for using clad materials in the first place.
Reflecting on my own experience with bimetal fastener manufacturing, I find that the most practical approach is to combine the study's deformation analysis with a simple empirical rule: if the upset ratio exceeds 1.40 and the cladding layer thickness is less than 3 mm, perform a witness upset test on a pre-production sample and inspect the interface by sectioning and metallographic examination. This approach has prevented several costly production rejects in my experience.
Study Insights and Implications
The fundamental insight from this literature is that cold upsetting of clad materials is not simply a bulk deformation problem but an interface mechanics problem. The interface, which is often only a few micrometers wide, controls the overall forming limit. This shifts the quality control focus from purely mechanical property testing of the bulk material to detailed characterization of the interface itself. Engineers should invest in interface characterization techniques such as cross-sectional metallography, microhardness mapping, and bond strength testing to ensure that the clad material is suitable for the intended cold forming operation. Understanding the deformation behavior at the interface is essential for preventing costly failures in downstream forming operations.