I. Hastelloy Alloys Grade
Currently, Hastelloy alloys are primarily divided into B, C, and G series, which are mainly used in highly corrosive environments where iron-based Cr-Ni or Cr-Ni-Mo stainless steel and non-metallic materials cannot be utilized.
|Main alloy elements
|Typical usage scenarios
|Reducing media such as hydrochloric acid
|Mixed medium with both oxidation and reduction properties
|Phosphoric acid, sulfuric acid, sulfate, etc
In order to enhance the corrosion resistance and cold/hot workability of Hastelloy alloys, three major improvements have been carried out successively. Their development process is as follows:
- B series: B → B-2 (00Ni70Mo28) → B-3
- C series: C → C-276 (00Cr16Mo16W4) → C-4 (00Cr16Mo16) → C-22 (00Cr22Mo13W3) → C-2000 (00Cr20Mo16)
- G series: G → G-3 (00Cr22Ni48Mo7Cu) → G-30 (00Cr30Ni48Mo7Cu)
The second-generation materials N10665 (B-2), N10276 (C-276), N06022 (C-22), N06455 (C-4), and N06985 (G-3) are currently the most widely used.
II. Typical Hastelloy Chemical Compositions
Chemical composition of hastelloy materials
III. Mechanical Properties
The mechanical properties of Hastelloy alloys are exceptionally notable, featuring high strength and toughness. This implies some difficulties in machining, and its strain hardening tendency is extremely strong.
When the deformation rate reaches 15%, it is approximately twice that of 18-8 stainless steel. Hastelloy alloys also exhibit a mid-temperature sensitization zone, where the tendency for sensitization increases with the deformation rate.
At higher temperatures, Hastelloy alloys tend to absorb harmful elements, reducing its mechanical properties and corrosion resistance.
IV. Common Hastelloy Alloys
1. Hastelloy B-2 alloy
(1) Corrosion Resistance
The Hastelloy B-2 alloy is a Ni-Mo alloy with extremely low carbon and silicon content, reducing carbide and other phase precipitation in the weld and heat affected zone, ensuring good corrosion resistance even in the welded state.
It is well-known that Hastelloy B-2 alloy exhibits excellent corrosion resistance in various reducing media, capable of withstanding corrosion from hydrochloric acid at any temperature and concentration under normal pressure.
It shows good corrosion resistance in non-oxidizing sulfuric acid of medium concentration, phosphoric acid of various concentrations, high-temperature acetic acid, formic acid, bromic acid, hydrogen chloride gas, and also resists corrosion from halogen catalysts.
Hence, the Hastelloy B-2 alloy is often used in various harsh petroleum and chemical processes such as distillation and concentration of hydrochloric acid, alkylation of ethylbenzene, and low-pressure carbonyl synthesis of acetic acid.
However, during many years of industrial application of the Hastelloy B-2 alloy, it was found that:
(1) Hastelloy B-2 alloy has two sensitization zones that greatly affect its resistance to intergranular corrosion: a high-temperature zone of 1200~1300°C and a mid-temperature zone of 550~900°C;
(2) Due to dendritic segregation, intermetallic phase and carbides precipitate along the grain boundaries of the weld metal and heat affected zone of Hastelloy B-2 alloy, making it more sensitive to intergranular corrosion;
(3) The mid-temperature thermal stability of Hastelloy B-2 alloy is relatively poor. When the iron content in Hastelloy B-2 alloy is reduced to less than 2%, the alloy is sensitive to the transition to β phase (i.e., Ni4Mo phase, an ordered intermetallic compound). If the alloy stays slightly longer in the temperature range of 650~750°C, the β phase is instantly generated.
The presence of the β phase reduces the toughness of Hastelloy B-2 alloy, making it sensitive to stress corrosion, and can even cause cracking of Hastelloy B-2 alloy during raw material production (such as during hot rolling), equipment manufacturing (such as post-weld heat treatment of Hastelloy B-2 equipment), and in-service environment.
Nowadays, the designated intergranular corrosion resistance testing methods for Hastelloy B-2 alloy in China and other countries is the normal pressure boiling hydrochloric acid method, and the assessment method is the weight loss method.
Since Hastelloy B-2 alloy is a corrosion-resistant alloy against hydrochloric acid, the normal pressure boiling hydrochloric acid method is quite insensitive in testing the intergranular corrosion tendency of Hastelloy B-2 alloy.
Research institutions using the high-temperature hydrochloric acid method for studying Hastelloy B-2 alloy found that the corrosion resistance of Hastelloy B-2 alloy not only depends on its chemical composition but also on its thermal processing control process.
When the thermal processing technology is not properly controlled, Hastelloy B-2 alloy not only grows in grain size, but also high-Mo σ phase precipitates at the grain boundaries.
At this time, the intergranular corrosion resistance of Hastelloy B-2 alloy significantly decreases. In high-temperature hydrochloric acid tests, the grain boundary erosion depth of the coarse grain plate is about twice that of the normal plate.
(2) Physical Properties
Density: 9.2g/cm3, Melting point: 1330~1380℃, Magnetic permeability: (℃, RT)≤1.001
(3) Chemical Composition
(4) Manufacturing and Heat Treatment
For Hastelloy B-2 alloy, it is essential to keep the surface clean and away from contaminants before and during heating. If the alloy is heated in an environment contaminated with sulfur, phosphorus, lead, or other low melting point metals, it can become brittle.
The sources of these contaminants mainly include markings from markers, temperature-indicating paint, oils and liquids, and fumes.
These fumes must be low in sulfur; for example, the sulfur content in natural gas and liquefied petroleum gas should not exceed 0.1%, urban air sulfur content should not exceed 0.25g/m3, and fuel oil sulfur content should not exceed 0.5%.
The gas environment of the heating furnace should be neutral or slightly reducing, and should not fluctuate between oxidizing and reducing. The flames in the furnace should not directly impinge on the Hastelloy B-2 alloy.
Also, the material should be heated to the desired temperature as quickly as possible, which requires initially raising the furnace temperature to the desired level, and then placing the material in the furnace for heating.
2: Hot Working
Hastelloy B-2 alloy can undergo hot working within the range of 900~1160℃, after which it should be quenched with water. To ensure the best corrosion resistance, annealing should be carried out after hot working.
3: Cold Working
Cold-worked Hastelloy B-2 alloy must undergo solution treatment. Since it has a much higher work hardening rate than austenitic stainless steel, the forming equipment should be carefully considered.
If cold forming is carried out, it is necessary to perform intermediate annealing. When the deformation of cold working exceeds 15%, solution treatment is required before use.
4: Heat Treatment
The solution heat treatment temperature should be controlled between 1060~1080℃, followed by water-cooling quenching or rapid air cooling if the material thickness is over 1.5mm, to achieve the best corrosion resistance.
In any heating operation, the cleanliness of the material surface should be maintained.
When heat treating Hastelloy alloy materials or equipment parts, the following should be noted:
- To prevent deformation during heat treatment, stainless steel reinforcing rings should be used;
- Furnace temperature, heating and cooling times should be strictly controlled;
- Pre-treatment of the parts to be heat-treated should be carried out before they are loaded into the furnace to prevent thermal cracking;
- After heat treatment, 100% PT of the parts should be performed;
- If thermal cracking occurs during the heat treatment process, after the cracks are removed by grinding, welding should be carried out using a special welding process.
Oxides on the surface of Hastelloy B-2 alloy and stains near the weld should be thoroughly ground with a fine grinding wheel. Due to the alloy’s sensitivity to oxidizing media, a considerable amount of nitrogen-containing gas is produced during pickling.
Hastelloy B-2 alloy should be machined in the annealed state, and its work hardening should be clearly understood. For example, a slower surface cutting speed should be used compared to standard austenitic stainless steel, a larger feed amount should be used for the hardened layer on the surface, and the tool should be kept in continuous working condition.
The welding of Hastelloy B-2 alloy should be prudently planned and strictly controlled because the weld metal and heat-affected zone of the alloy are prone to the precipitation of β phase leading to Mo-depletion, thereby making it susceptible to intergranular corrosion.
The general welding process is as follows: ERNi-Mo7 filler metal should be used; GTAW welding method should be applied; the interpass temperature should be controlled to be no more than 120℃; filler wire diameters φ2.4, φ3.2; welding current 90~150A.
Simultaneously, before welding, the filler wire, the weld groove, and the adjacent areas of the workpiece should be cleaned and degreased.
Hastelloy B-2 alloy has a much lower thermal conductivity than steel. If a single V-groove is chosen, the groove angle should be around 70°, and a lower heat input should be applied.
Post-weld heat treatment can relieve residual stresses and improve resistance to stress corrosion cracking.
2. Hastelloy C-276 Alloy
1. Corrosion Resistance
The Hastelloy C-276 alloy belongs to the nickel-molybdenum-chromium-iron-tungsten-based nickel alloy family. It is one of the most corrosion-resistant metals in modern materials.
It primarily resists wet chlorine, various oxidizing chlorides, chloride salt solutions, sulfuric acid and oxidizing salts, exhibiting excellent corrosion resistance in low and medium temperature hydrochloric acid.
As a result, over the past thirty years, it has found extensive applications in harsh corrosive environments such as chemical industry, petrochemical industry, flue gas desulfurization, pulp and paper, and environmental protection sectors.
The various corrosion data of Hastelloy C-276 alloy are typical but cannot be used as standards. Especially in unknown environments, material selection must be done after testing.
The Hastelloy C-276 alloy does not have enough chromium to resist the corrosion of strong oxidizing environments like hot concentrated nitric acid. The production of this alloy is mainly aimed at the chemical process environment, particularly where mixed acids exist, such as the outlet pipe of flue gas desulfurization systems. (All welding samples are made using self-fusing tungsten electrode argon arc welding)
The Hastelloy C-276 alloy can be used for flue gas desulfurization parts in coal combustion systems, where C-276 is the most corrosion-resistant material. The table below shows the corrosion comparison test results of the C-276 alloy and typical 316 in a flue gas simulation system’s “Green Death” solution.
Corrosion Comparison Test in “Green Death” Solution
|Green Death Solution (Boiling)
|Corrosion rate (mm/a)
|7% sulfuric acid
|3% hydrochloric acid
2. Physical Properties
Density: 8.90g/cm3, Specific Heat: 425J/kg/k, Elastic Modulus: 205Gpa (at 21℃)
3. Mechanical Properties
The typical tensile test results of C-276 alloy are shown in the table below. The material was annealed at 1150℃ and quenched in water.
The formability of C-276 alloy is similar to that of common austenitic stainless steel. However, due to its higher strength than common austenitic stainless steel, it will have greater stress during cold forming.
Moreover, this material hardens faster than regular stainless steel, so intermediate annealing processing should be adopted during extensive cold forming.
4. Welding and Heat Treatment
The welding performance of C-276 alloy is similar to that of austenitic stainless steel. Before welding C-276 with any method, measures must be taken to minimize the decrease in corrosion resistance in the weld and heat-affected zone, such as Gas Tungsten Arc Welding (GTAW), Gas Metal Arc Welding (GMAW), Submerged Arc Welding or other methods that can minimize the decrease in corrosion resistance.
However, welding methods such as oxyacetylene welding, which may increase the carbon or silicon content in the weld and heat-affected zone, are not suitable.
For the selection of welding joint forms, refer to the successful experiences of C-276 welding joints in the ASME Boiler and Pressure Vessel Code.
It is best to use mechanical processing for the welding groove, but mechanical processing will cause hardening, so it is necessary to grind the mechanically processed groove before welding.
The welding process should adopt an appropriate heat input speed to prevent the generation of hot cracks.
In most corrosive environments, C-276 can be used in the form of a welded part. But in harsh environments, C-276 materials and weldments should undergo solution heat treatment to achieve the best corrosion resistance.
The welding of C-276 alloy can choose itself as the welding material or filler metal. If it is required to add certain elements to the welds of C-276, such as other nickel-based alloys or stainless steel, and these welds will be exposed to corrosive environments, then the welding rod or wire used for welding should have corrosion resistance equal to the parent metal.
The solution heat treatment of Hastelloy C-276 alloy material includes two processes:
- Heating at 1040℃~1150℃;
- Rapidly cool to a black state (about 400℃) within two minutes. The treated material has excellent corrosion resistance. Therefore, stress relief heat treatment for Hastelloy C-276 alloy alone is ineffective. Before heat treatment, clean the oil and other impurities that may produce carbon elements during the heat treatment process from the alloy surface.
The surface of C-276 alloy will produce oxides during welding or heat treatment, reducing the Cr content in the alloy, affecting the corrosion resistance, so it is necessary to clean the surface.
Stainless steel wire brushes or grinding wheels can be used, followed by pickling in a mixture of nitric acid and hydrofluoric acid in appropriate proportions, and finally rinse clean with water.
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