MTEK 25-35MA Micro Alloy

Application

MetalTek MTEK 25-35MA (Micro Alloy) is a contemporary cast micro-alloyed Fe-Ni-Cr based material in-tended for use in high temperature applications for tubes servicing in pyrolysis cracker furnaces and in reformers. The alloy possesses excellent creep rupture properties, good carburization and oxidation-resistance, and high structural stability. These unique properties render the material well suited for long life and good thermal fatigue resistance up to temperatures of approximately 80% of the absolute melting point.

Metallurgy

MTEK 25-35MA is an austenitic, face centered cubic (FCC) alloy suitable for elevated temperature service up to 1150 ÌŠC (2100 ÌŠF). This alloy is the result of microalloying MTEK 25-35Nb with additions of titanium and rare earth ele-ments which significantly increase creep rupture strength and improve oxida-tion and carburization resistance. Elevated temperature strength is obtained through solid solution strengthening and the presence of several complex carbide phases. Complex carbides found in this alloy include MC, M7C3, and M23C6 particles. The MC carbides are stable while the M7C3 carbide usually transforms on aging to M23C6 particles.

Weldability

MTEK 25-35MA is readily welded by manual and automatic and GTAW, GMAW, PAW, Shielded Metal Arc, and by EBW. The choice of any particular process depends on the joint configuration and on material thickness and other factors. Prior to welding the joint surfaces and adjacent areas should be thor-oughly cleaned. Preheat or post heat treatment is not required for as cast hardware as long as the base material is above 0ºC. Interpass temperatures should be kept low. The filler to be used is of matching material composition as the base material. It is available in straight length for manual welding, on reels for automatic welding, and as coated electrodes for Shielded Metal Arc welding.

Heat Treatment

MTEK 25-35MA is normally put into service in the as cast condition. Material can, however, be supplied to any requested heat-treatment condition. Welding operations should preferably be carried out with the material in the as cast condition.

Machining

The machinability of MetalTek MTEK 25-35MA is similar to that of heat resisting steels. In production, turning, boring and grinding of centrifugally cast tubes is regularly practiced .

Product Form

MTEK 25-35MA is available as centrifugally cast tubes, statically cast fittings and investment castings.

Carburization and Oxidation

MTEK 25-35MA has demonstrated excellent resistance to carburization, as determined in a hydrogen- propylene atmosphere with a controlled addition of steam in a concentration sufficient to oxidize both chro-mium and silicon, but not iron or nickel. After exposure for different times at isothermal conditions the sam-ples were sectioned and examined by light optical microscopy. Staining of the polished cross sections with an alcoholic KMnO4 or in a 5% FeCl3 revealed the internally precipitated carbide formation and the depth of the carburization zone. Results from laboratory tests are shown in Figures 2 and 3.

The resistance of MTEK 25-35MA to oxidation in flowing air is excellent as evaluated by dynamic labora-tory tests. The centrifugally cast test samples were exposed to air moving at 2 m/s for 1050 hrs at constant temperatures. In addition, the samples were cooled to RT every 150 hrs. The specimens were then evalu-ated for both metal loss and average internal oxide penetration. As shown in Fig. 11 the alloy can be used for long term exposure to air or combustion gases at temperatures up to 1150 ºC.

Mechanical Properties

Tensile Properties

The data given in Figure 4 are for centrifugally cast material in the as cast condition. The data were de-termined with a strain rate of 0.005/min to yield stress and 0.1/min thereafter. The elevated temperature data presented in Figure 7 are for centrifugally cast MTEK 25-35MA tubes in the as cast condition. The data were determined with a strain rate of 0.002/min to yield stress and 0.1/min thereafter.

Creep and Rupture Properties

Stress-rupture curves for centrifugally cast MTEK 25-35MA tubes and static castings are presented by Larson-Miller parameter in Figure 5. The data are based on 14 heats, tested in the as cast condition. Stress rupture tests were conducted at temperatures up to 1150ºC and test durations over 14,000 hours. The Larson Miller constant C was computed from the statistical regression analysis.