Alloy 333 Forgings UNS N06333  /  W.Nr. 2.4608  /  NiCr26MoW  /  RA333

01 What is Alloy 333?

Alloy 333 is a wrought nickel-base heat-resistant alloy used above the temperature range in which austenitic stainless steels lose strength and begin to scale. Strength comes from solid-solution hardening by molybdenum, tungsten and cobalt, not from a precipitated second phase.

The properties that decide most Alloy 333 selections are:

• Oxidation resistance. The 24 to 27% chromium content forms a stable, adherent chromia scale that survives long exposure in oxidising atmospheres up to about 1204 °C (2200 °F).
• Carburization and metal dusting resistance. The high combined nickel and chromium content, supported by a deliberate silicon addition, resists carbon pick-up in reformer and carburizing-furnace atmospheres.
• Thermal shock resistance. Alloy 333 withstands repeated oil or water quenching of hot fixtures better than most heat-resisting stainless steels.
• Shutdown corrosion resistance. The alloy resists dew-point sulphuric acid attack and polythionic acid stress-corrosion cracking, which occur when a hot unit is taken offline and condensate forms.

Because Alloy 333 is not precipitation hardened, it does not soften or embrittle through the ageing reactions that limit age-hardening grades in long-term high-temperature service.

Note. Alloy 333 (UNS N06333) is solid-solution strengthened. It is supplied solution annealed and is not age hardened.

02 Chemical composition of Alloy 333

Chemical composition of Alloy 333 (UNS N06333) is given below. Jiangyin Jiangnan Metal Co., Ltd. controls melt chemistry to the specification range and reports the actual heat analysis on every certificate.

The silicon level is high for a nickel alloy and is deliberate. Silicon works with chromium to stabilise the protective scale and contributes directly to carburization resistance.

03 Mechanical properties

Room-temperature mechanical properties of Alloy 333 in the solution-annealed condition are shown below, with specified minima and values typically achieved on forged bar.

Alloy 333 retains useful load-bearing strength to approximately 1000 °C (1832 °F) and oxidation resistance to about 1204 °C (2200 °F). Above roughly 650 °C, design should be based on creep and stress-rupture data rather than short-term tensile values, with allowance for thermal cycling, stress concentration, thermal gradients and the service atmosphere. Elevated-temperature tensile and stress-rupture data for a given section size are available on request.

Note. Typical solution-annealed Alloy 333 forged bar reaches 738 MPa tensile strength, 324 MPa 0.2% proof strength and 48% elongation at room temperature.

04 Physical properties

Thermal conductivity is low compared with steel, so heating and cooling of thick forged sections must be controlled. Heavy Alloy 333 rings are heated on a ramp rate to limit thermal stress during forging and annealing.

05 Specifications, designations and equivalents

Note on ASTM B637. ASTM B637 does not cover Alloy 333. B637 applies to precipitation-hardening nickel alloys such as Inconel 718 and Inconel 725. Alloy 333 is solid-solution strengthened, so the applicable references are ASTM B718, B719, B722, B723 and B726, and AMS 5593 and 5717. Enquiries that call up B637 for N06333 should be clarified before the order is placed.

ASTM B718 covers UNS N06333 in plate, sheet and strip form. There is no dedicated ASTM product specification for open-die N06333 forgings, so forgings are normally ordered to the N06333 chemistry, with mechanical properties, heat treatment, grain size and NDT acceptance agreed in the purchase order. Jiangyin Jiangnan Metal Co., Ltd. also works to a customer specification, an end-user standard or a project technical delivery condition.

06 Forging, heat treatment and testing

Starting stock and melting route

Alloy 333 ingot and billet is produced by electric arc furnace melting, followed by AOD or VOD refining and electroslag remelting (EAF + AOD/VOD + ESR). The ESR stage controls sulphur, cleanliness and centre segregation, which governs ultrasonic response in heavy sections of a 45% nickel alloy containing tungsten and molybdenum.

Open-die forging and ring rolling

Alloy 333 has a narrow hot-working window and work-hardens quickly, so reduction is taken in controlled passes with frequent reheats rather than in a few heavy blows. Rings are produced by upsetting, punching and rolling on a radial-axial ring mill, which develops circumferential grain flow. A rolled ring therefore has higher tangential strength and toughness than a ring flame-cut from plate or trepanned from bar, which matters for flanges, seal rings and furnace retainer rings loaded in hoop.

Heat treatment

Alloy 333 forgings are solution annealed at approximately 1120 °C (2050 °F) and then rapidly cooled or air cooled. This dissolves carbides and restores full ductility and corrosion resistance. No ageing treatment is applied. A specification calling for solution plus age on N06333 has usually been copied from a precipitation-hardening grade and should be queried before production starts.

Testing, inspection and documents

• Chemical analysis by spectrometer on every heat, reported against the N06333 range.
• Room-temperature tensile and elongation testing. Elevated-temperature and impact testing on request.
• Ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388, to the quality class stated on the order.
• Dye-penetrant examination, grain-size determination and PMI when specified.
• EN 10204 3.1 certificate as standard. EN 10204 3.2 witnessed by TUV, SGS, BV or Lloyd's Register on request.
• Dimensional report against the customer drawing. Rough machining or finish machining available.

07 Forged shapes and size range

Alloy 333 is supplied in the open-die forged forms listed below. All forms are made to customer drawing or to finished dimensions. Minimum order quantity is one piece.

Surface condition options are as-forged with scale, shot blasted, pickled, rough machined with a defined stock allowance, or finish machined to drawing.

08 Applications for Alloy 333 forgings

N06333 is specified where a part must handle high temperature, a carbon-bearing or oxidising atmosphere, and repeated thermal cycling at the same time. The established application areas are as follows.

Heat treatment and thermal processing

• Radiant tubes, muffles and retorts in continuous and batch furnaces.
• Furnace fixtures, baskets, grids, trays and shaker-hearth components subject to oil or water quenching.
• Carburizing furnace conveyor chains, chain links, belt pins and roller shafts.
• Furnace rolls, hearth rolls and roller-hearth bearing sleeves.
• Thermocouple protection tubes and instrument wells in carburizing atmospheres.

Petrochemical, refining and power

• Refinery flare tips and flare-tip retaining rings.
• Refractory anchors, hangers and tie-backs in fired heaters.
• Dampers, damper shafts and expansion-joint hardware in hot flue-gas ducting.
• Calciner and rotary-kiln internals, lifters and discharge components.
• Sulphur-bearing service where polythionic acid stress-corrosion cracking on shutdown is a concern.

Other

• Gas turbine and combustion hardware where a solid-solution alloy is acceptable.
• Pickling racks and fixtures in hot acid service.
• Waste incineration and biomass boiler components exposed to aggressive ash and chloride.

Selection note. Alloy 333 is normally chosen where thermal shock and carburization occur together. For clean, purely oxidising service without quenching, a lower-cost grade such as Alloy 330 or Incoloy 800H is often sufficient.

09 Alloy 333 compared with alternative grades

Against Alloy 330, Alloy 333 has higher chromium plus molybdenum, cobalt and tungsten, which raises hot strength and improves thermal-shock and metal-dusting resistance. Against Inconel 601, Alloy 333 has lower nickel but better strength at temperature, while Inconel 601 relies on aluminium to form its protective film and is often preferred for purely cyclic oxidation duty. Against Haynes 230 and Hastelloy X, Alloy 333 is usually the lower-cost option and performs well in carburizing service, while Haynes 230 leads on long-term creep strength.

10 Machining and welding Alloy 333

Machining

Alloy 333 has a machinability rating of about 12 to 15% of free-machining steel B1112, comparable with Inconel 625 and Inconel 718. Turning with high-speed steel tooling is normally run at 20 to 25 surface feet per minute (6 to 8 m/min). Rigid setups, sharp positive-rake tooling, heavy positive feed to cut beneath the work-hardened layer, and flood coolant are all required. Dwelling or rubbing will work-harden the surface and damage the following pass. Forgings can be supplied rough machined so that only finishing stock is removed by the customer.

Welding

Alloy 333 is welded with matching UNS N06333 bare filler wire or matching coated electrodes. Preheat is not applied. Interpass temperature is held below approximately 100 °C (212 °F), and reinforced stringer beads are used in preference to wide weave passes. Heat input is kept low and the joint is allowed to cool between passes. On a critical assembly, a full solution anneal after welding restores oxidation and corrosion resistance in the heat-affected zone.

12 Frequently asked questions about Alloy 333