What material is B-2 Hastelloy - Knowledge

What Material is Hastelloy B-2?

Hastelloy B-2 (UNS N10665) is a nickel-molybdenum (Ni-Mo) based superalloy specifically designed for excellent corrosion resistance in strong reducing environments. It is a member of the Hastelloy family, optimized to resist aggressive chemical media that most other alloys (including stainless steel and common nickel-based alloys) cannot withstand.

Unlike Hastelloy C-series alloys (which focus on balancing oxidation and reduction resistance), Hastelloy B-2 is a "specialized corrosion-resistant alloy" with ultra-high molybdenum content. Its core advantage lies in resisting hydrochloric acid (HCl)-one of the most corrosive industrial acids-across a wide range of concentrations and temperatures. It is widely used in chemical processing, petrochemical, pharmaceutical, and metal pickling industries.

What is the chemical composition of B-2 Hastelloy?

The chemical composition of Hastelloy B-2 is strictly controlled to ensure its corrosion resistance and mechanical stability. The following are the typical composition ranges (by weight percentage, wt%) specified in standards such as ASTM B333 and ASME SB-333:

Element	Content Range	Role
Nickel (Ni)	≥ 65	Base metal ,provides structural stability and enhances molybdenum solubility.
Molybdenum (Mo)	26.0 – 30.0	Core alloying element,forms a dense passive film to resist reducing acids (e.g., HCl).
Iron (Fe)	1.0 – 2.0	Improves processability (e.g., hot working and welding) without reducing corrosion resistance.
Chromium (Cr)	0.4 – 0.8	Low content (to avoid reducing resistance to strong reducing acids), slightly enhances oxidation resistance.
Carbon (C)	≤ 0.02	Strictly limited to prevent the formation of brittle carbides (e.g., Mo₂C) during heat treatment, which can cause intergranular corrosion.
Silicon (Si)	≤ 0.10	Minimized to reduce the risk of forming brittle silicides and improve corrosion resistance in acid media.
Manganese (Mn)	≤ 1.0	Aids in deoxidation during smelting, ensuring alloy purity.
Phosphorus (P)	≤ 0.040	Impurity element,strictly limited to avoid reducing ductility and corrosion resistance.
Sulfur (S)	≤ 0.030	Impurity element,minimized to prevent hot cracking during welding and reduce corrosion susceptibility.

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What is the hardness of B-2 Hastelloy?

The hardness of Hastelloy B-2 varies with its heat treatment state and cold working degree, as hardness is closely related to the alloy's microstructure (e.g., grain size and dislocation density). Below are the key details:

1. Hardness in Annealed State

Annealing process: The standard heat treatment for Hastelloy B-2 is annealing at 1065 – 1175°C (1950 – 2150°F), followed by rapid cooling (water quenching). This process produces a uniform, fine-grained microstructure with optimal corrosion resistance and ductility.

Typical hardness values:

Brinell Hardness (HB): ≤ 230

Rockwell Hardness (HRC): ≤ 25

Vickers Hardness (HV): ≤ 240

This is the most common state for Hastelloy B-2 in practical applications, as it balances corrosion resistance and processability.

2. Hardness After Cold Working

Cold working (e.g., cold rolling, cold drawing, or cold forging) increases the dislocation density in the alloy, thereby significantly improving hardness (at the cost of reduced ductility). The harder the cold working degree, the higher the hardness:

10% cold working: HB ≈ 260 – 280; HRC ≈ 28 – 30

20% cold working: HB ≈ 300 – 320; HRC ≈ 32 – 34

30% cold working: HB ≈ 340 – 360; HRC ≈ 36 – 38

Cold-worked Hastelloy B-2 is rarely used in corrosion-resistant components, as cold working may increase stress corrosion cracking (SCC) risk in harsh media.

3. Hardness Variation with Temperature

At elevated temperatures, the atomic mobility of the alloy increases, leading to a gradual decrease in hardness:

At 200°C: Hardness remains almost the same as room temperature (HB ≈ 220 – 230).

At 400°C: HB ≈ 200 – 210 (slight decrease).

At 600°C: HB ≈ 180 – 190 (significant decrease, but still maintains basic structural strength).