Non-Magnetic Tungsten Carbide: Permeability & Grade Specs

Target Audience & Why This Matters

This guide assists downhole tool designers and battery engineers troubleshooting sensor drift or metallic particle clumping caused by ferromagnetism in carbide wear components.

Sensor Distortion and Particle Sticking from Residual Magnetism

In directional drilling bottom-hole assemblies, MWD/LWD probes rely on fluxgate magnetometers. Cobalt-bonded carbide sleeves produce ferromagnetic interference (relative permeability between 1.5 and 2.5), causing azimuth errors up to 2.8°.

In battery foil slitting and NdFeB magnet compacting, magnetized carbide attracts metallic dust. In battery slitting, fines pierce separators, risking micro-shorts. In magnet compacting, adhering powder scratches cavities under 600 MPa loads, ruining die sets.

Metallurgy of Austenitic Binders and Permeability Control

Pure tungsten carbide (WC) is non-magnetic; ferromagnetism originates from cobalt binders. Binder systems substitute cobalt with austenitic nickel or nickel-chromium matrices.

Studies in the International Journal of Refractory Metals and Hard Materials (doi:10.1016/j.ijrmhm.2019.01.014) show tungsten dissolved into nickel suppresses ferromagnetism while maintaining TRS above 2100 MPa between -40 °C and 175 °C.

Testing per ASTM A342 validates that stabilized WC-Ni grades (hardness 90.0 to 91.5 HRA, density 14.80 to 14.95 g/cm³ per ASTM B311) maintain permeability below 1.005, whereas sub-stoichiometric sintering forms ferromagnetic clusters degrading TRS below 1800 MPa.

Non-Magnetic Reference Grades and Physical Properties

Selecting non-magnetic grades requires balancing magnetic neutrality, hardness, and corrosion resistance:

Preform blanks require densification via vacuum Sinter-HIP at 6.0 MPa argon pressure, meeting ASTM B276 A02 B00 C00 porosity ratings.

Thermal Fit Assembly and Machining Tolerances

Non-magnetic carbide displays distinct physical characteristics compared to conventional WC-Co:

1. Thermal Expansion (CTE): WC-Ni has a CTE of 5.5 × 10−6/K versus 4.8 × 10−6/K for WC-Co. When shrink-fitting sleeves into stainless steel (AISI 316L), size retention fits to ISO H7/h6 or H7/g6 to prevent looseness at 150 °C.
2. Cylindricity and Runout: Grind OD and ID to total runout (TIR) within φ0.005 mm and surfaces to Ra 0.10 μm. Chatter marks induce micro-strains creating magnetic anomalies.
3. WEDM Recast Relief: Wire-cutting causes copper-zinc diffusion. Always lap away the 0.015 mm recast layer to restore magnetic neutrality.

Negative List: Prohibited Tooling & Conditions

• Prohibited High-Impact Blanking Applications: Never deploy WC-Ni in heavy-impact punches; nickel binder yield strength is 15% to 25% lower than cobalt, causing edge chipping.
• Prohibited Non-Verified Magnetic Callouts: Never accept non-magnetic blanks without certified ASTM A342 test reports; visual inspection cannot detect ferromagnetism.
• Prohibited Hydrochloric Acid on Pure Ni: Do not deploy un-alloyed WC-Ni in chlorides below pH 3; specify chromium-stabilized WC-Ni-Cr to prevent binder leaching.
• Prohibited Demagnetization Assumption: Never assume demagnetizers neutralize ferromagnetic WC-Co; magnetic neutrality requires an austenitic structure.

Sourcing Traps: Overseas Prints vs. Production Reality

1. Vague "Amagnetic" Drawing Notes: Drawings often state "material must be non-magnetic" without numeric limits. Mills may supply low-cobalt grades measuring permeability above 1.05. Mandate on prints: "Relative magnetic permeability ≤ 1.005 tested per ASTM A342 Method 3."

2. Unmonitored Carbon Porosity: Nickel binders possess a narrow carbon window. Excess carbon generates graphite flakes (C-porosity per ASTM B276) degrading wear life, while carbon deficits create brittle eta-phase. Specify: "Porosity ASTM B276 A02 B00 C00."

As an engineering sourcing partner for severe-service carbide components, Carbidea connects overseas OEMs with manufacturing partner mills operating under certified ISO 9001, ISO 14001, and ISO 45001 quality, environmental, and occupational safety management systems to manufacture non-magnetic carbide wear parts and tooling to your print, with quotes provided within 1–2 business days.

FAQ

Q1: Is tungsten carbide naturally magnetic or non-magnetic?

A: Pure tungsten carbide (WC) is non-magnetic. Conventional carbide is magnetic because cobalt is used as binder. Non-magnetic grades substitute cobalt with austenitic nickel binders to eliminate ferromagnetism.

Q2: What relative magnetic permeability is acceptable for MWD downhole tools?

A: Directional drilling MWD sensor housings mandate relative magnetic permeability below 1.005 measured per ASTM A342. Higher values create compass azimuth errors exceeding drilling tolerances.

Q3: How does the mechanical wear resistance of WC-Ni compare to WC-Co?

A: In sliding wear, WC-Ni delivers wear resistance within 90% to 95% of equivalent WC-Co of identical hardness (89–91 HRA). WC-Ni offers superior acid corrosion resistance, though with lower impact toughness.