Deformation Behavior of Surfaced Bimetallic Materials During Cold Upsetting Processes

1. Definition and Fundamental Principles

Deformation behavior during the cold upsetting (cold heading) roughening process of surfaced bimetallic materials refers to the mechanical response of a weld overlay or cladding layer when subjected to compressive plastic deformation at or near room temperature. In this process, a base metal substrate—typically a structural steel or alloy steel—is first clad with a corrosion-resistant, wear-resistant, or chemically compatible overlay layer via TIG or MIG weld overlay technology. The resulting bimetallic blank is then subjected to cold upsetting, where axial compressive forces are applied to reduce length and increase cross-sectional area, forming bolt heads, stud ends, or other mechanically fastened geometries.

The fundamental principle governing this deformation is the compatibility of strain between two dissimilar metallic phases with differing flow stresses, elastic moduli, and ductility characteristics. During cold upsetting, the cladding layer experiences triaxial stress states that differ significantly from the base metal due to geometric constraints imposed by the tooling. The overlay layer, being typically thinner and often more ductile (e.g., austenitic stainless steel) than the base material, undergoes non-uniform strain distribution, creating a critical interface between the two materials where deformation mismatch can lead to cracking, delamination, or excessive thinning.

Key mechanical principles involved include:

Plastic strain compatibility: The overlay and base metal must accommodate similar volumetric strain during upsetting without generating interfacial shear stresses exceeding the bond strength.
Flow stress differential: Austenitic stainless steel overlays (e.g., 304, 316L) typically exhibit higher work-hardening rates compared to low-carbon or medium-carbon steel substrates, causing the overlay to resist deformation and thin at a different rate.
Triaxial stress state: Cold upsetting generates complex stress states at the free surface and at the tool-material interface, particularly affecting the cladding layer thickness reduction.
Temperature rise during deformation: Adiabatic heating during rapid cold upsetting can locally raise the temperature of the overlay layer, potentially altering its mechanical properties and affecting subsequent heat treatment requirements.

2. Category and Business Positioning

This technical competency falls within the company's advanced metallurgical process knowledge domain, specifically supporting the TIG/MIG weld overlay technology route. The understanding of deformation behavior during cold upsetting is a critical enabler for the company to deliver qualified bimetallic fasteners, studs, and hardware components to customers in the oil and gas, chemical processing, and power generation industries.

Within the company's three primary technology routes:

TIG/MIG Weld Overlay: This deformation behavior knowledge directly supports the design of overlay thicknesses, layer configurations, and heat input parameters that ensure the cladding layer survives subsequent cold forming operations without failure.
Hydraulic Explosive Bonding: While less directly applicable to cold upsetting scenarios, the principles of interfacial integrity under deformation inform the qualification of bonded plates and pipes that may undergo secondary forming operations.
Explosion Welding: The deformation mechanics knowledge contributes to understanding how explosion-welded interfaces respond to post-bonding cold working, ensuring that the metallurgical bond maintains integrity under service-relevant forming operations.

3. Technical Purpose and Value

The primary technical purpose of studying deformation behavior during cold upsetting of surfaced bimetallic materials is to establish process windows that guarantee reliable product quality. Specifically, this knowledge enables the company to:

Determine maximum allowable upset ratios: Establish the maximum reduction in height (upset ratio) that can be applied without causing overlay cracking, delamination, or excessive thinning below the specified minimum thickness.
Optimize overlay design for formability: Select appropriate overlay thickness, layer count, and alloy composition to balance corrosion resistance requirements with cold forming capability.
Define tooling and process parameters: Specify die geometry, lubrication requirements, upsetting speed, and temperature control to minimize adverse deformation effects on the cladding layer.
Establish qualification protocols: Develop WPS/PQR procedures that incorporate cold upsetting as a post-weld forming step with defined acceptance criteria for both metallurgical and dimensional quality.
Enable customer value delivery: Provide customers with bimetallic fastener components that combine the mechanical strength of a high-performance base material with the corrosion or wear resistance of a specialized overlay, all formed into complex geometries in a single manufacturing sequence.

4. Key Process and Implementation Points

4.1 Overlay Design Parameters Affecting Cold Formability

Parameter	Recommended Range	Rationale
Overlay thickness (single layer)	0.5 – 2.0 mm	Thinner overlays deform more uniformly; thicker overlays risk center cracking under biaxial tension
Number of overlay passes	2 – 4 passes	Multiple thin passes reduce residual stress and improve interpass fusion
Overlay/base thickness ratio	≤ 15% of base diameter	Higher ratios increase strain incompatibility and delamination risk
Overlay ductility (elongation)	≥ 30% (ASTM A370)	Higher ductility accommodates larger upset strains without cracking
Interface shear strength	≥ 40 MPa (ASME Section IX)	Ensures bond integrity under interfacial shear during upsetting

4.2 Cold Upsetting Process Parameters

Process Variable	Typical Specification	Effect on Overlay Deformation
Upset ratio (ΔH/H₀)	10% – 40%	Higher ratios increase overlay thinning; exceeds 40% requires pre-warming
Strain rate	1 – 100 s⁻¹	Higher rates increase adiabatic heating, reducing flow stress but potentially affecting overlay microstructure
Die temperature	Ambient – 200°C	Warm dies reduce surface friction and overlay strain localization
Lubricant	Synthetic oil-based or graphite	Reduces friction-induced strain concentration at overlay free surface
Blank pre-temperature	Room temperature – 150°C	Pre-warming improves overlay ductility and reduces cracking risk
Upset direction	Axial compression (die-upset or punch-upset)	Determines strain path in overlay layer; punch-upset generally produces more uniform overlay thinning

4.3 Critical Implementation Steps

Pre-upsetting metallurgical verification: Conduct hardness mapping across the overlay/base interface to confirm uniform weld penetration and absence of unmelted zones or cold shuts. Perform macrographic examination (NACE TM0177 if applicable for corrosion-critical overlays) to verify overlay continuity.
Strain monitoring: Apply strain gauges or use digital image correlation (DIC) on representative coupons during upsetting trials to map strain distribution in the overlay layer. Identify critical strain zones where overlay thinning is maximized.
Overlay thickness measurement: Measure overlay thickness at the pre-upset condition, at the post-upset condition, and at the thinnest point (typically at the die contact surface or at geometric transitions). Calculate thinning ratio: t_final / t_initial.
Interface integrity assessment: Perform shear testing (ASME Section IX, Appendix A) on upset specimens to verify that the overlay/base bond strength is maintained after cold forming. Target minimum shear strength of 40 MPa for pressure boundary applications.
Crack and delamination inspection: Conduct magnetic particle inspection (MT) or liquid penetrant inspection (PT) on upset surfaces to detect overlay cracking. Perform cross-sectional metallographic examination to assess interface integrity at microstructural level.
Post-upsetting heat treatment: Apply solution annealing (for austenitic overlays) or stress relief (for ferritic/martensitic overlays) as required by the applicable code to restore overlay ductility and relieve cold work-induced residual stresses.

5. Applicable Standards and Acceptance Criteria

5.1 Weld Overlay Qualification Standards

ASME Section IX, Part QW-400: Governs qualification of welding procedures for cladding applications. Requires demonstration of overlay thickness, hardness, and corrosion resistance after forming operations.
ASME Section IX, Appendix A: Specifies shear test requirements for weld overlay qualification. Minimum shear strength of 40 MPa (5,800 psi) for pressure-containing applications.
ASTM A240: Specifies chemical composition and mechanical properties of stainless steel overlay materials (304, 316, 316L, 904L, etc.).
ASTM A370: Standard test methods for mechanical testing of steel products, including tensile and hardness testing of upset specimens.
ASTM A923/A923M: Standard practice for weld overlay qualification testing, including corrosion resistance verification.

5.2 Cold Forming and Deformation Standards

ASTM A193: Specification for alloy steel and stainless steel bolt-type fasteners and stud hardware, including cold upset requirements for bolt head formation.
ASTM A194: Specification for carbon steel and alloy steel fasteners, including upset forming requirements.
ASME BPV Section II, Part D: Impact testing requirements for base material after cold working, including temperature correction factors for cold upset damage.
GB/T 3098.1: Chinese standard for mechanical properties of fasteners made of carbon steel and alloy steel, including cold heading requirements.
ISO 898-1: Mechanical properties of fasteners made of carbon steel and alloy steel, specifying cold forming acceptance criteria.

5.3 Non-Destructive Testing and Inspection Standards

ASTM E709: Standard practice for magnetic particle testing of welds and base metal, used to detect overlay cracking after upsetting.
ASTM E165: Standard practice for liquid penetrant inspection, used for surface defect detection on upset overlay surfaces.
ASTM E164: Standard practice for radiographic testing, used for volumetric defect detection in upset components.
GB/T 3323: Radiographic testing standard applicable to weld overlay qualification specimens.

5.4 Acceptance Criteria Summary

Criterion	Acceptance Requirement	Test Method
Overlay thickness after upsetting	≥ 90% of pre-upset thickness at thinnest point	Metallographic cross-section (ASTM E3)
Interface shear strength	≥ 40 MPa	ASME Section IX, Appendix A
Overlay surface defects	No cracks, no delamination	MT (ASTM E709) or PT (ASTM E165)
Base material hardness after upsetting	≤ 25 HRC (unless specified otherwise)	Astm E18 Rockwell hardness
Corrosion resistance after upsetting	No intergranular corrosion, no pitting initiation	ASTM A923 / ASTM G48

6. Common Risks and Controls

6.1 Overlay Cracking

Risk Description: During cold upsetting, the overlay layer is subjected to biaxial tensile stresses at the free surface (away from the die). If the overlay material's ductility is insufficient to accommodate the imposed strain, transverse or longitudinal cracks can initiate and propagate through the overlay thickness, compromising the corrosion protection function.

Control Measures:

Select overlay alloys with high elongation (≥ 35%) and low yield-to-tensile ratio.
Limit upset ratio to ≤ 30% for single-layer overlays; use multiple thinner layers for higher upset ratios.
Apply pre-warming (100–150°C) to increase overlay ductility during upsetting.
Use warm dies (150–200°C) to reduce strain rate sensitivity effects.
Apply appropriate lubrication to reduce friction-induced strain concentration.

6.2 Interface Delamination

Risk Description: The differential flow stress between the overlay and base metal creates interfacial shear stresses during upsetting. If the bond strength is insufficient (due to incomplete fusion, contamination, or residual stress from the welding process), delamination can occur at the interface, particularly at geometric transitions where strain localization is severe.

Control Measures:

Ensure full penetration of the first overlay pass into the base metal (verified by macrographic examination).
Control interpass temperature during overlay welding to ≤ 150°C to minimize residual stress buildup.
Apply post-weld stress relief annealing before cold upsetting to reduce residual stress at the interface.
Verify interface shear strength ≥ 40 MPa prior to upsetting via ASME Section IX Appendix A testing.
Design upset geometry to avoid sharp transitions that concentrate interfacial shear stress.

6.3 Excessive Overlay Thinning

Risk Description: Non-uniform strain distribution during upsetting can cause localized thinning of the overlay layer, particularly at the die contact surface where friction constrains material flow. Excessive thinning can reduce the overlay below the minimum specified thickness, compromising corrosion protection and potentially exposing the base metal.

Control Measures:

Design die geometry with adequate radii (R ≥ 2× overlay thickness) to promote uniform material flow.
Apply adequate lubrication to reduce friction-induced strain localization.
Monitor upset ratio and correlate with measured overlay thinning to establish process limits.
Design initial overlay thickness with adequate margin (≥ 20% above minimum service requirement).
Use punch-upset configuration rather than die-upset where more uniform overlay deformation is required.

6.4 Work Hardening of Base Material

Risk Description: Cold upsetting significantly increases the hardness of the base material, particularly in regions of high strain. If the base material hardness exceeds code limits, it may affect subsequent machining, thread rolling, or service performance (e.g., increased susceptibility to hydrogen embrittlement in high-strength fasteners).

Control Measures:

Limit upset ratio based on base material's cold work tolerance (typically ≤ 40% for medium-carbon steels).
Apply post-upsetting annealing or stress relief treatment to restore ductility.
Monitor base material hardness at critical locations (bolt shank, thread root) after upsetting.
Design upset sequence to distribute strain more uniformly across the component.

7. Application Scenarios Across Company Technology Routes

7.1 TIG/MIG Weld Overlay Route

This deformation behavior knowledge is most directly applicable to the TIG/MIG weld overlay route, where the company produces bimetallic fastener blanks, stud bolts, and hardware components. Typical applications include:

Corrosion-resistant bolt heads: A medium-carbon steel bolt shank is clad with 316L stainless steel via TIG overlay, then cold upset to form the bolt head. The deformation behavior study ensures the 316L overlay survives the upsetting without cracking, maintaining corrosion protection at the head-to-shank transition.
Wear-resistant studs: Studs with hardened overlay (e.g., Stellite 6 or D2 tool steel via MIG) are cold upset to form the bearing surface. The study establishes upset parameters that maintain overlay integrity while achieving the required bearing geometry.
Dual-metal fasteners for sour service: Base material (A193 B7 alloy steel) is clad with 310SS or Hastelloy C-276 via TIG overlay, then cold upset. The study validates that the overlay maintains its corrosion resistance after cold forming, meeting NACE MR0175/ISO 15156 requirements.

7.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding is primarily used for flat plate and pipe production, the deformation behavior knowledge informs the design of bonded components that undergo secondary cold forming. For example:

Bonded plate for cold-rolled components: A hydraulically explosion-bonded plate (e.g., 316L/CS) may be subsequently cold rolled or cold formed into sheet or strip for fastener manufacture. Understanding deformation behavior ensures the bonded interface maintains integrity during secondary forming.
Process parameter correlation: The strain compatibility principles derived from cold upsetting studies inform the selection of bonded plate thickness ratios and base material grades for downstream cold forming applications.

7.3 Explosion Welding Route

Explosion welding produces permanent metallurgical bonds with high shear strength, but the deformation behavior knowledge contributes to understanding post-bonding forming limitations:

Post-bonding cold forming: Explosion-welded plates or pipes that undergo cold drawing, cold rolling, or cold upsetting must be evaluated for interface integrity under deformation. The strain compatibility principles from this study guide the qualification of such processes.
WPS/PQR development: When developing welding procedure qualifications that incorporate explosion-welded base materials followed by cold forming, the deformation behavior data provides the technical basis for defining upset limits and post-forming inspection requirements.

8. Contribution to Qualification Building and Customer Value

8.1 Qualification Building

This technical competency directly supports the company's qualification building efforts in several ways:

WPS/PQR Development: The deformation behavior data enables the development of qualified welding procedures that incorporate cold upsetting as a post-weld forming step. This expands the company's qualified procedure database and enables acceptance of more complex bimetallic component orders.
Material Qualification: Understanding overlay deformation limits enables the qualification of new overlay/base material combinations for cold forming applications, expanding the company's product portfolio.
Code Compliance: The knowledge supports compliance with ASME Section IX, ASTM A193/A194, and other applicable codes that require demonstration of overlay integrity after cold forming operations.
Customer-Specific Qualifications: Many oil and gas and chemical processing customers require specific qualification of bimetallic fasteners that have undergone cold forming. This knowledge enables the company to develop customer-specific qualification packages.

8.2 Product Delivery

The deformation behavior knowledge enables reliable product delivery by:

Establishing process windows that minimize rework and scrap during cold upsetting of bimetallic components.
Providing the technical basis for defining first-article inspection requirements and in-process monitoring parameters.
Enabling the company to offer cold-formed bimetallic components (bolt heads, stud ends, bearing surfaces) that would otherwise require machining from solid bimetallic stock at significantly higher cost.

8.3 Customer Value

For customers, this technical competency delivers value through:

Cost reduction: Cold upsetting of pre-cladded blanks is significantly more cost-effective than machining from solid bimetallic bar stock, reducing material waste and machining time by 40–60%.
Performance assurance: The deformation behavior knowledge ensures that the overlay maintains its specified thickness and integrity after cold forming, guaranteeing the corrosion or wear resistance performance in service.
Design flexibility: Customers can specify complex geometries (tapered heads, flanged ends, contoured bearing surfaces) that are achievable through cold upsetting of pre-cladded blanks, enabling optimized component designs.
Quality confidence: The company's demonstrated understanding of deformation behavior provides customers with confidence that the bimetallic components will maintain their overlay integrity throughout their service life.

9. Conclusion

The study of deformation behavior during cold upsetting of surfaced bimetallic materials represents a critical technical competency that bridges the gap between weld overlay fabrication and subsequent cold forming operations. By understanding the mechanical response of the overlay layer under triaxial stress states, establishing process windows for upset ratios, and implementing rigorous quality controls, the company can reliably deliver high-performance bimetallic fasteners and hardware components that meet the demanding requirements of the oil and gas, chemical processing, and power generation industries. This knowledge directly supports qualification building, enables cost-effective product delivery, and provides significant value to customers who require corrosion-resistant or wear-resistant components in complex geometries.