nickel-molybdenum alloy B-3 Hex Bar: Cold Drawing and Residual Stress

How nickel-molybdenum alloy B-3 Hexagon Bar Is Produced

nickel-molybdenum alloy B-3, designated UNS N10675, is a nickel-molybdenum alloy developed for resistance to hydrochloric acid at all concentrations and temperatures, together with other reducing media. Hexagon bar is typically produced from hot-finished round bar supplied to ASTM B335, which is first solution annealed to a soft, uniform microstructure. Two manufacturing routes then differ fundamentally. Cold drawing pulls the round bar through a series of tungsten carbide dies, the last of which has the exact hexagon profile. This is a cold-working operation: it raises yield and tensile strength by work hardening, reduces elongation, and leaves tensile residual stresses in the surface and near-surface regions. Centerless grinding, by contrast, removes material with an abrasive wheel to form the hexagon flats without deforming the bulk; the core remains in the annealed, essentially stress-free condition, and tolerances are typically 0.05 mm or better with a fine surface finish.

Composition and Metallurgical Condition

The high molybdenum content is the source of the alloy resistance to reducing acids. Properly solution annealed nickel-molybdenum alloy B-3 has a hardness in the 15 to 25 HRC range, which is the condition normally required for maximum corrosion resistance and for forming operations such as thread rolling. A cold-drawn bar without subsequent annealing is harder and stronger but carries residual stress and a worked surface, so for machined fasteners the annealed condition is usually preferred.

Residual Stresses and Their Practical Consequences

Residual stress matters when the bar is machined. A cold-drawn bar has stresses locked in from drawing; when material is removed during threading or drilling, the stress balance is disturbed and the part can warp or the finished dimensions can shift. A centerless-ground bar from annealed stock is essentially stress-free, so machined parts hold their shape. When ordering hex bar, always ask whether the product is supplied as-drawn or drawn and re-solution annealed. If it is drawn and then annealed, the buyer gets accurate geometry plus a soft, corrosion-resistant, stress-relieved microstructure, which is the best combination for fasteners.

Machining and Threading nickel-molybdenum alloy B-3

nickel-molybdenum alloy B-3 work hardens rapidly, has low thermal conductivity and is far more difficult to machine than 316 stainless steel, so stainless parameters will produce torn threads, work-hardened surfaces and rapid tool wear. Use C2 or C3 grade carbide inserts, preferably coated with TiAlN or AlTiN to reduce heat at the cutting edge, at surface speeds of about 50 to 80 SFM (15 to 25 m/min) with an aggressive feed that cuts beneath the work-hardened layer. For single-point threading, use flank infeed with the compound rest at about 29 degrees and make the final pass a full-depth cut, typically 0.002 to 0.005 inch on radius, so the tool cuts clean material instead of burnishing a hard surface. Flood coolant at high volume is essential. Thread rolling is often preferred for B-3 fasteners because it produces compressive residual stresses at the thread roots that improve fatigue life, but the bar must be in the soft solution-annealed condition for rolling to succeed.

Sour Service, Fastener Corrosion and Stress Relief

For sour-gas service, nickel-molybdenum alloy B-3 is an acceptable nickel-based alloy under NACE MR0175/ISO 15156 Part 3, provided it is in the solution-annealed condition and meets the hardness limits, typically 35 HRC maximum. Annealed B-3 at 15 to 25 HRC is well within the limit, but a cold-drawn hexagon bar can easily exceed it, so the purchase order should state the annealed condition, NACE compliance and hardness testing per ASTM E18. In hydrochloric-acid service, wetted fasteners should be made from the same heat as the flange material, or from a heat whose chemistry matches as closely as possible, to minimize any galvanic difference between bolt and flange; mixing heats without an electrochemical review is not recommended.

Stress relief of machined B-3 parts is justified for thin sections, tight tolerances or heavily worked surfaces. The treatment is a full solution anneal at about 1060°C to 1120°C (1940°F to 2050°F) in a protective argon, hydrogen or vacuum atmosphere, followed by rapid cooling through the 550°C to 850°C embrittlement range to retain a soft, corrosion-resistant structure. Because heat treatment of thin parts carries its own distortion risk, the practical sequence is to start with centerless-ground annealed bar, machine with sharp tools and aggressive feeds, and anneal after rough machining only when the service demands it.

Frequently Asked Questions

What is nickel-molybdenum alloy B-3? nickel-molybdenum alloy B-3 (UNS N10675) is a nickel-molybdenum alloy with 27 to 32 percent molybdenum, developed for outstanding resistance to hydrochloric acid and other reducing media across a wide temperature range.

Cold drawn or centerless ground hex bar for machined fasteners? Centerless ground bar from annealed stock is generally preferred because it is stress-free and will not distort during machining; a cold-drawn bar can release residual stresses and warp the part.

Why should bolts and flanges be from the same heat of B-3? Wetted fasteners matched to the flange chemistry minimize galvanic differences between components in aggressive electrolytes such as hot hydrochloric acid; matching heats is standard best practice for critical bolted connections.

What hardness limit applies to B-3 in NACE sour service? NACE MR0175/ISO 15156 requires nickel-based alloys in the solution-annealed condition to stay within the stated hardness limit, typically 35 HRC maximum; annealed B-3 at 15 to 25 HRC complies, while cold-drawn bar may not.

What machining speeds work for nickel-molybdenum alloy B-3? Use C2/C3 carbide, coated with TiAlN or AlTiN, at about 50 to 80 SFM (15 to 25 m/min) with aggressive feed, flank infeed near 29 degrees for threading and a full-depth final pass, with flood coolant.

When should machined B-3 parts be stress relieved? For thin sections, tight tolerances or heavily work-hardened surfaces, solution anneal at about 1060°C to 1120°C in a protective atmosphere with rapid cooling through the 550°C to 850°C range to restore dimensional stability and uniform corrosion resistance.