Selecting materials for high temp exhaust manifold applications.

Selecting Materials for High Temp Exhaust Manifold Applications

Proper material section for components exposed to elevated temperature service is complex. Although operating temperature and peak skin temperatures are the major factors limiting materials that can be used, many other variables contribute to the proper selection.

In addition to knowing operating working temperatures, the materials engineer should be aware of all mechanical and thermal loading conditions in the component's working environment. He should know if the loads are cyclic, if there are any specific areas of stress or other constraints on the part or if it is exposed to other conditions that should be factored into design consideration. With this type of information, he can evaluate the family of high temperature irons for their suitability for high temperature applications (see Table 1).

As the operating temperature of a component exceeds 600F, the range of service limitations begins, such as degradation of mechanical properties. The higher the temperature, the greater the number of degradation mechanisms at work, but alloying additions can be effective in sustaining many material properties.

The thermal conductivity of the matrix increases slightly from 100F up to 400F, then decreases as temperatures rise to 950F. Thermal conductivity data up to 950F have limited design utility since changes in microstructure at high temperatures influence thermal conductivity.

At about 600F, creep deformation occurs when a casting is under continuous stress. At 700F, creep deformation is measureable. Between 650-750F, strength and hardness begin to drop, and with long exposure slight growth and oxidation (scaling) appear.

Most cast irons are stable up to 800-900F, and expansion is reversed on cooling, so true coefficients of thermal expansion can be obtained. At 950F, a significant decrease in strength and hardness occurs, and growth and oxidation are significant. Pearlite decomposition to ferrite and graphite can result in linear growth up to 1%, but most alloy additions retard pearlite decomposition. Copper and tin additions are common in nodular iron as are combinations of Cr, Mo, Ni and Sn in flake cast irons.

Pearlite decomposition is unavoidable over extended exposure to 1000F, and pearlitic irons lose long-term strength and dimensional stability, leaving annealed and ferritic or austenitic irons for high temperature applications.

Ferritic grade nodular irons containing up to 3% silicon are usually specified to 1300F, increasing the silicon to 4-6% for applications above this temperature. Adding less than 1% Mo improves resistance to creep. These additions extend the useful operating temperatures of this cast material to 1650F.

Table 2 compares the relative performance of typical irons used for elevated temperature applications.