MTEK Super 20-32Nb Steam / Hydrocarbon Reforming

Application

MetalTek’s MTEK Super 20-32Nb alloy is used in steam/hydrocarbon reforming for catalyst tubing, convec-tion tubing, pigtails, outlet manifolds, and quenching system piping. It is also used in ethylene production for transfer lines, pigtails, return bends, and outlet flanges. The mechanical properties combined with the resistance to corrosion at elevated temperatures make the alloy well suited for many applications in the hydro-carbon processing industries. These parts require high thermal shock resistance combined with sufficient creep rupture strength and post service ductility for exposure in the critical temperature range.

Composition

MTEKSuper 20-32Nb is a widely used cast alloy. It is a solid solution Fe-Ni-Cr material with a good combination of high temperature strength, ductility-, oxidation-, and carburization-resistance. The alloy has the basic low carbon Fe-32Ni-20Cr composition strengthened with the intermetallic forming element Nb. The creep rupture behavior of MTEK Super 20-32Nb is superior to ASTM A351 grade CT15C, alloy (Incoloy) 800H and alloy 800HT.

The well balanced Fe-Ni-Cr composition of the matrix in which the high per-centage of Ni in combination with Cr- and low carbon content maintains an aus-tenitic structure resulting in high ductility after long time exposure at elevated temperatures and imparting resistance to oxidation, scaling, internal oxidation, and carburization.

In addition to controlled carbon content, the Nb/C ratio is balanced for ductility and high strength for short and long term exposure at elevated temperature.

Metallography

MTEK Super 20-32Nb is a solid solution strengthened austenitic stainless steel alloy. Primary, niobium rich NbC carbide particles and fine, chromium rich M23C 6 are evident in the as cast microstructure. These NbC carbides are stable at all temperatures below solidus. Chromium rich M23C6 carbides precipitate in the alloy between 550ºC and 1075ºC. The precipitated carbides increase yield and tensile strength and decrease elongation and reduction of area to some extent. However, the aged ductility values remain relatively high with elongation values of ≥ 15 % observed after long term aging in the critical temperature range.

Product Form and Specifications

MTEK Super 20-32Nb is available as centrifugally cast tubing and statically cast fittings. Its product forms are covered by a number of engineering customer specifications and a full range of MetalTek International product specifications.

Heat Treatment

MTEK Super 20-32Nb is normally furnished in the as cast condition, however, solution annealing at 1200°C has been used for critical parts having cast wall sections greater than 7.5 cm to maximize ductility.

Machining

MTEK Super 20-32Nb is readily machined by standard methods for austenitic stainless steel. Turning operations can be performed under high metal removal rates resulting in good tool life and sufficient surface finish using CISC-grade C-2 cemented carbide tools.

Weldability

MTEK Super 20-32Nb exhibits excellent weldability by all welding processes. For shielded metal arc weld-ing (SMAW) use UTP-2133Mn or Inconel 117 (ENiCrCoMo-1) filler metal. For gas tungsten arc welding (GTAW) and plasma arc welding (PAW) Metrode 21.33Nb, Thyssen Thermanit 2133, UTP-A2133Mn or Inconel 117 (ENiCrCoMo-1) are acceptable. For maximum rupture strength, use Inconel 117 or Inconel 117 filler metal. Preheating or post-weld heat treatment of the welded joint is not required.

Oxidation and Carburization

MTEK Super 20-32Nb has good oxidation resistance at temperatures up to 1000ºC (See Figure 10). The data indicate that the material can be used for long term continuous operation in combustion gases or air. Exposure of MTEK Super 20-32Nb in a carburizing atmosphere of 2% methane and 98% hydrogen flowing at four furnace volumes per hour for 100 hours at 980ºCshowed an average weight gain of 5.3 mg/cm2. Based on this value, the alloy has an excellent resistance to carburization.

Mechanical Properties

Tensile Properties

MTEK Super 20-32Nb exhibits high tensile properties at room tem-perature and at elevated temperatures. Figure 6 shows the tensile properties up to 900ºC. The alloy exhibits good metallurgical stability for an alloy of this strength level. Studies involving exposure of as cast material at temperatures of 790ºC showed little variation in strength with some reduction in RT tensile ductility, but without formation of embrittling phases. The plot in Figure 2 shows the influence of aging time at 790°C on RT tensile strength and elongation.

Creep and Rupture Properties

In general, MTEK Super 20-32Nb is used for the combination of strength and ductility at service tempera-tures between 650ºCand 1000ºC. At these temperatures equipment design is typically based on creep and stress rupture properties. The centrifugal alloy displays a high level of creep and stress rupture strength up even to temperatures of 950ºC and above (Figure 4). These characteristics combined with the good resis-tance to oxidation atmospheres make the material especially suitable for service at elevated temperatures.